energies Article
An Experimental Investigation on the NO and CO Emission Characteristics of a Swirl Convergent-Divergent Nozzle at Elevated Pressure Zhongya Xi *, Zhongguang Fu, Xiaotian Hu, Syed Waqas Sabir
ID
and Yibo Jiang
National Thermal Power Engineering & Technology Research Center, North China Electric Power University, Beijing 102206, China;
[email protected] (Z.F.);
[email protected] (X.H.);
[email protected] (S.W.S.);
[email protected] (Y.J.) * Correspondence:
[email protected]; Tel.: +86-10-6177-2361 Received: 8 May 2018; Accepted: 31 May 2018; Published: 31 May 2018
Abstract: The behavior of the pollutants NO and CO at elevated combustor pressure are of special importance due to the continuing trend toward developing engines operating at higher pressure ratios to yield higher thermal efficiency. An experiment was performed to examine the NO and CO emissions for a swirl convergent-divergent nozzle at elevated pressure. The NO and CO correlations were obtained. Meanwhile, the flame length, exhaust gas oxygen concentration, exit temperature and global flame residence time were also determined to analyze the NO and CO emission characteristics. The results showed that, with the increase in combustor pressure P, flame length decreased proportionally to P−0.49 ; exit O2 volume fraction increased and exit temperature was reduced. The global flame residence time decreased proportionally to P−0.43 . As pressure increased, the NO and Emission Index of NO (EINO) levels decreased proportionally to P−0.53 and P−0.6 respectively, which is mainly attributed to the influence of global flame residence time; the NO and EINO increased almost proportionally with the increase in global flame residence time. The EINO scaling EINO (ρue /d) was proportional to Fr0.42 , which indicated that compared with pure fuel, the fuel diluted with primary air can cause a decrease in the exponent of the Fr power function. At higher pressure, the CO and Emission Index of CO (EICO) decreased proportionally to P−0.35 and P−0.4 , respectively, due to the increased unburned methane and high pressure which accelerated chemical reaction kinetics to promote the conversion of CO to CO2 . Keywords: NO emission; CO emission; swirl convergent-divergent nozzle; combustion at elevated pressure; turbulent non-premixed flame
1. Introduction Pollutant emissions from the combustion of fossil fuels have become great public concern due to their harmful effects on human health and the environment [1]. The regulations for pollutant emissions are becoming increasingly strict, which has recently led combustion devices manufacturers to develop combustors that meet various regulatory requirements. Gas turbines are critical facilities in the gas and oil industry. The principal pollutants generated by gas turbines are NO and CO, and both emissions have drawn considerable academic interest, particularly in their formation mechanisms and influencing factors. NO could be formed by four different pathways: thermal NO, prompt NO, nitrous oxide NO, and fuel NO [2]. Concerning CO, it arises mainly from incomplete combustion of the fuel, due to inadequate reaction rates in the flame zone caused by very low equivalence ratio and/or insufficient residence time. The factors influencing both NO and CO emissions, e.g., equivalence ratio, flow velocity, fuel property, variable geometry, ambient air temperature have been investigated widely, as mentioned in [3]. Energies 2018, 11, 1410; doi:10.3390/en11061410
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Among various impacts, the influence of combustion pressure P on NO and CO formation is of particular significance, because of the continuing trend toward engines with higher pressure ratios (relative to the atmosphere) for achieving higher thermal efficiency and lower fuel consumption [4]. Pressure ratios of heavy-duty gas turbines (F/G/H level) have been increased to about 15~40-fold, such as GE 7HA/9HA (22), Siemens SGT6-6000G/SGT5-8000H (19), Mitsubishi M501G/M701G (20) and M501H (25), ALSTOM GT26 (33). However, owing to experimental difficulties, combustor measurement data under high pressures is extremely limited [3,5–7]. The vast majority of combustion tests were carried out at low pressure levels, and the results obtained then extrapolated to higher pressure levels. The extrapolation could be performed confidently assuming that the relationship between pollutant emissions and pressure were precisely known. Regrettably, the testing data obtained on various combustor categories are inconsistent in this regard. In the last few years, a few studies have been performed to explore the effect of combustion pressure on NO and CO emissions in gas turbine combustors. Correa [8] conducted a review to summarize NOx formation under gas turbine conditions (NOx consists of NO and NO2 , where the dominant component at high emission levels is NO). The results from his study showed that NOx and CO emissions scaled proportionally with a power function of the combustion pressure Pn . The exponent n could be swayed by some factors, for example, n approached 0 at low equivalence ratios and was greater than 0 for rich flames. Bhargava et al. [9] carried out experiments and PSR network simulations in the lean conditions range with two different nozzles; the pressure was varied from 100 psi to 400 psi. The results revealed that the NOx exponent n changed from −0.18 to 1.6 for the tangential nozzle and from −0.77 to 0.61 for the axial nozzle as the equivalence ratio varied from 0.43 to 0.65. Göke et al. [10] studied NOx and CO emissions for a premixed natural gas flame within a pressure range of 1.5–9.0 bar by performing experiments and simulations. They found that NOx increased and CO decreased with increasing pressure, the exponent for NOx emissions increased with equivalence ratio from 0.1 to 0.65 while the exponent for CO emissions was about −0.4 at lower temperatures and −0.5 at higher temperatures. The calculation results in Wang et al. [11] and Pillier et al. [12] also revealed that the NO has an increasing trend but CO has a decreasing trend with pressure. Nevertheless, when Rutar [13] studied NOx and CO emissions at an elevated pressure of 3.0–6.5 atm he concluded that NOx decreased a little with increasing pressure for a fixed residence time, which was also revealed by Steele et al. [14]. Biagioli and Güthe [15] analyzed the NO from 1–30 bar based on a model, and they found that the NOx pressure exponent is negative under fully premixed conditions. Leonard et al. [16] measured NOx emissions for well-premixed flames; they pointed out that NOx emissions were independent of pressure and it is possible to run a combustor from 1 atm to 30 atm without any noticeable change in NOx emissions. To investigate the parameters controlling NOx yields, Røkke et al. [17] proposed a leading-order scaling approach for buoyancy-dominated hydrocarbon non-premixed flames based on simplified expressions for the flame volume, simplified finite-rate reaction mechanism, and a flamelet description which includes hydrocarbon fuel components up to C3 . They proposed a theoretical expression to predict the NOx emission index EINOx , i.e., Equation (1): EINOx (ρue /d) ∝ Fr0.6
(1)
where ρ stands for the fuel jet density; ue represents fuel jet exit velocity; d stands for the diameter of nozzle exit; Fr represents the jet exit Froude number, which is defined by Equation (2): Fr = ue 2 /gd where g is the acceleration of gravity.
(2)
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The NOx emission index EINOx is defined as the ratio of the mass rate of NO produced to the mass of fuel supplied, as given by Equation (3) [18]: .
.
EINOx = m NOx /m F
(3)
Equation (1) was used to predict their experimental data as well as data from other researches for hydrocarbon fuels [19–22]. Generally, the predicted results were good for Fr less than 105 . Moreover, Szegö et al. [23] explored the impacts of diluent on EINOx for a laboratory-scale mild combustor. It was found that the exponential constant for Frn varied with the addition of N2 or CO2 into fuel compared to fuel with no diluents. To predict combustion emissions based on a set of operating parameters, e.g., fuel composition, combustor pressure, air flow rate, equivalence ratio, inlet air temperature, some empirical and semi-empirical models are widely employed. These models are developed by correlating test data on pollutant emissions in terms of all the relevant parameters. They serve in designing and developing low-emission combustors by reducing the complicated problems affiliated with emissions to forms which are more easier for the combustion engineers. These models can be found in the reviews by Tsalavoutas et al. [24], Chandrasekaran and Guha [25], as listed in Table A1 in Appendix A. For example, a general correlation proposed by Lefebvre [26] is showed by Equation (4): EINOx = 0.459 × 10−8 P0.25 · F · τ · exp(0.01Tst )
(4)
where F is the fraction of air employed in the flame zone; τ is the flame zone residence time, (measured in s); Tst is the stoichiometric flame temperature, (measured in K). Equation (4) considers the fact that in the combustion of heterogeneous fuel-air mixtures, the formation of NOx is determined by the stoichiometric flame temperature Tst , rather than the average flame temperature. However, the residence time in the flame zone τ is also important to NOx formation. Another example proposed by Røkke et al. [27] is displayed by Equation (5): .
EINOx = 1.46P1.42 m air 0.3 f f ar 0.72
(5)
˙ air is air mass flow rate, ffar is fuel to air ratio. The influence of combustion temperature on where m NOx is considered by the inclusion of the fuel to air ratio ffar . Recently, several correlations were obtained for specific industrial combustors: a correlation for a model combustor of an aero gas turbine by Li et al. [28], a correlation for fuel staged combustion employing laboratory-scale gas turbine combustor by Han et al. [7], and another correlation for a 10 MW non-premixed gas turbine combustor utilizing high hydrogen fuels by Kroniger and Wirsum [29]. It can be concluded from the above that the NO and CO behavior at high pressure is variable and even contradictory in different tests. NO increased, decreased or even remained unaffected by pressure for different burners/combustors, correspondingly, the pressure exponent for NO was positive, negative or even zero. The emissions test data at high-pressure were rare, and the effects of pressure on them are comparatively unknown. Meanwhile, the proposed NO correlations are strongly dependent on the specific burners/combustors and combustion technology. An individual NO correlation could not be used to estimate NO emissions accurately for a particular combustion device. Lastly, the EINO scaling with Fr was not yet examined with the addition of primary air in fuel. Therefore, further exploration is needed to reveal the NO and CO characteristics at elevated pressure. In this paper, a swirl convergent-divergent nozzle designed for high flame stability and low pollutants emissions was examined experimentally regarding NO and CO emissions at elevated combustor pressure for turbulent non-premixed combustion. The flame zone, exit O2 volumetric fraction, exit temperature were also measured to analyze the emissions characteristics. The CO and NO correlations were obtained by fitting the test data, the EINO scaling with Fr was also presented.
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2. Experimental Experimental Setup 2. Experimental Setup 2.1. Experimental Apparatus 2.1. Experimental Apparatus convergent-divergent nozzle nozzle design design used used in in our our efforts was was mainly mainly based based on the The swirl convergent-divergent The swirl nozzle design used inconical our efforts was mainly based on the combination of convergent-divergent swirl combustion combustion technology technology and divergent divergent conical nozzle technology, combination of swirl and nozzle technology, as shown in combination of swirl combustion technology and divergent conical nozzle technology, as shown in Figure 1a, with an endeavor to enhance the flame stabilization and lowering pollutants emission. 1a, with an endeavor to enhance the flame stabilization and lowering pollutants emission. The Figure 1a, with an endeavor to enhance the flame stabilization and lowering pollutants emission. The The swirl generated by the swirler strengthen flame stabilizing,control controlflame flamesize sizeand and combustion combustion swirl generated by the swirler cancan strengthen flame stabilizing, swirl generated by the swirler can strengthen flame stabilizing, control flame size and combustion intensity, decrease NO formation by improving the mixing of fuel and air [30–32]. The divergent conical intensity, decrease NO formation by improving the mixing of fuel and air [30–32]. The divergent intensity, decrease NO formation by improving the mixing of fuel and air [30–32]. The divergent nozzle technology can obtain flames which be seen in previous studies [33–36]. conical nozzle technology canhighly obtain stabilized highly stabilized flamescan which can be seen in previous studies conical nozzle technology can obtain highly stabilized which can be seen in previous studies Moreover, the convergent passage can further enrich theflames mixing ofmixing fuel and toand lower pollutant [33–36]. Moreover, the convergent passage can further enrich the of air fuel airthe to lower the [33–36]. Moreover, the convergent passage can further enrich the mixing of fuel and air to lower the emissions. The nozzle mainly consists of three components: the central fuel tube, the swirler, and the pollutant emissions. The nozzle mainly consists of three components: the central fuel tube, the swirler, pollutant emissions. The nozzle mainly consists of three components: the central fuel tube, the swirler, convergent-divergent passage.passage. The swirler the radial type andtype has 10 straight as vanes, shown as in and the convergent-divergent Theisswirler is the radial and has 10vanes, straight and the1b, passage. is the radial type and the hasside 10 straight vanes, as Figure the setting angle of vanes is The 50◦ . swirler The between divergent face ofside the nozzle shown inconvergent-divergent Figure 1b, the setting angleαof vanes α isangle 50°. The anglethe between divergent face of ◦ . setting angle of vanes α is 50°. The angle between the divergent side face of shown in Figure 1b,42the and vertical linevertical is the nozzle and line is 42°. the nozzle and vertical line is 42°.
(a) (a)
(b) (b)
Figure Figure 1. 1. (a) (a) Swirl Swirl convergent-divergent convergent-divergent nozzle; nozzle; (b) (b) swirler. swirler. Figure 1. (a) Swirl convergent-divergent nozzle; (b) swirler.
For the present investigation, a 20 kW high-pressure combustion test rig was installed, which For the present investigation, a a2020kW high-pressure combustion testtest rig rig waswas installed, which can the investigation, kW high-pressure installed, which can be run atpresent pressures less than 2 Mpa, as shown in Figurecombustion 2. be at pressures less less thanthan 2 Mpa, as shown in Figure 2. 2. canrun be run at pressures 2 Mpa, as shown in Figure
Figure 2. The schematic diagram of high-pressure combustion test rig. Figure combustion test test rig. rig. Figure 2. 2. The The schematic schematic diagram diagram of of high-pressure high-pressure combustion
It is composed mainly of three sections: the supply section, the combustion chamber and the It issection, composed mainly of three as sections: the supply section, the combustion chamber and the exhaust which are described follows: exhaust section, which are described as follows:
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5 ofthe 19 sections: the supply section, the combustion chamber and exhaust section, which are described as follows: (1) The supply supply section, the air air mass mass flow rate (1) The section, in in which which an an air air compressor compressor is is used used to to supply supply air, air, the flow rate has a maximum of 2.5 kg/min. The air is regulated electrically through a control valve and measured has a maximum of 2.5 kg/min. The air is regulated electrically through a control valve and measured using meter. A A gas gas holder holder is is employed employed to to provide provide the the fuel, fuel, which which is is methane. methane. The using an an air air flow flow meter. The fuel fuel is is regulated and measured by a mass flow controller (MFC), the fuel mass flow rate has a maximum of regulated and measured by a mass flow controller (MFC), the fuel mass flow rate has a maximum of 36 36 g/min. g/min. (2) The combustion combustion chamber, chamber,which whichisisan anaxisymmetric axisymmetriccylindrical cylindricalchamber, chamber, diameter is 0.3 (2) The itsits diameter is 0.3 m m and height is 1.35 m, as shown in Figure 3. This combustion chamber is mainly composed of four and height is 1.35 m, as shown in Figure 3. This combustion chamber is mainly composed of four components: the components: swirl swirl convergent-divergent convergent-divergent nozzle, nozzle, wind wind distributing distributing plate, plate, cooling cooling slot slot near near to to the inner wall, and combustor. The swirl convergent-divergent nozzle has been previously displayed inner wall, and combustor. The swirl convergent-divergent nozzle has been previously displayed in in Figure 1b. A Figure 1b. A part part of of the the air air flows flows tangentially tangentially through through the the swirler swirler to to bring bring the the fuel fuel provided provided with with the the central central fuel fuel tube tube into into the the combustor combustor in in which which burning burning occurs occurs (initiated (initiated by by an an igniter igniter near near the the nozzle). nozzle). This part of air is referred as primary air, which is 5% of total inlet air by mass fraction, it is used to to This part of air is referred as primary air, which is 5% of total inlet air by mass fraction, it is used mix with methane before combustion for controlling flame length. The mixing degree of primary air mix with methane before combustion for controlling flame length. The mixing degree of primary air and and methane methane is is incomplete incomplete since since the the distance distance for for mixing mixing is is relatively relatively short. short. The The rest rest of of the the air air flows flows through air wind distribution plate to a cooling slot near to the inner wall, and then enters the through air wind distribution plate to a cooling slot near to the inner wall, and then enters into into the combustor. This part partof ofthe theair airisisreferred referred secondary is used to support combustion and combustor. This asas secondary air,air, andand is used to support combustion and cool cool the wall to avoid heat damage to the metal material. An optically accessible window is installed the wall to avoid heat damage to the metal material. An optically accessible window is installed on the on the combustor wall to observe thestructure. flame structure. combustor wall that canthat be can usedbetoused observe the flame
Figure Figure 3. 3. The The combustion combustion chamber. chamber.
(3) The exhaust section, which is mainly composed of a pressure controlling valve and induced (3) The exhaust section, which is mainly composed of a pressure controlling valve and induced draught system. The pressure controlling valve is used for regulating the combustor operating draught system. The pressure controlling valve is used for regulating the combustor operating pressure. pressure. The induced draught system is used for expelling the exhaust gas to the atmospheric The induced draught system is used for expelling the exhaust gas to the atmospheric environment. environment. Besides, there are some mounting bases installed on the exhaust section to allow the Besides, there are some mounting bases installed on the exhaust section to allow the measurements of measurements of exit gas composition and exit temperature. exit gas composition and exit temperature. 2.2. Measurement Methods 2.2. Measurement Methods Flame appearance was determined by measuring the CO2* chemiluminescence image through Flame appearance was determined by measuring the CO2 * chemiluminescence image through the optical window, with a high-speed camera (i speed 3, Olympus, Essex, UK) coupled to an optical the optical window, with a high-speed camera (i speed 3, Olympus, Essex, UK) coupled to an optical filter (BG 38, HB-OPTICAL, Shengyang, China) which has a bandwidth of 340–600 nm, as displayed filter (BG 38, HB-OPTICAL, Shengyang, China) which has a bandwidth of 340–600 nm, as displayed in in Figure 4. Figure 4.
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(a) (a)
(b) (b)
Figure Flamezone zonerecording recording instruments: instruments: (a) filter. Figure 4.4.Flame (a) high-speed high-speedcamera; camera;(b) (b)optical optical filter. Figure 4. Flame zone recording instruments: (a) high-speed camera; (b) optical filter.
The CO2* method is often used for determining flame zones, which can be seen in the previous
The CO * method oftenused used for determining flame zones, which can bebeseen inin the previous The CO isisoften determining flamethe zones, which can seen the previous 2 *2method studies reviewed by Samaniego et al.for [37]. Although within 340–600 nm wavelength range several studies reviewed by Samaniego et al. [37]. Although within the 340–600 nm wavelength range several studies reviewed by Samaniego et al. [37]. Although within the 340–600 nm wavelength range several species such as PAH, CH, C, etc., exist, their influence can be neglected as CO2* is the main emitter in species such as PAH, CH, C, etc., exist, their influence can be neglected as CO 2 * is the main emitter in in species such as flames. PAH, CH, C, etc., exist, influence be neglected as CO2 * is the main hydrocarbon As the review [37]their indicated, the amount of CO2* emissions is more thanemitter 95% of hydrocarbon flames. As the review [37] indicated, the amount of CO 2 * emissions is more than 95% of hydrocarbon flames. As the review [37] indicated, the amount COrange, is more than of 95% the total chemiluminescence integrated over the whole 340–600ofnm therefore, the effect 2 * emissions the integrated over the whole 340–600 nm range, therefore, the effect of of chemiluminescence the total total chemiluminescence chemiluminescence integrated over the whole 340–600 nm range, therefore, the effect signals from other species could be ignored in comparison with CO2* emissions. of chemiluminescence signals from other species could be with CO 2* emissions. As an indicator of signals the flame zone, thespecies CO2* formation consists of three elementary steps, in chemiluminescence from other could be ignored ignoredin incomparison comparison with CO emissions. 2 * given As an indicator of the flame zone, the CO 2* formation consists of three elementary steps, given in (6) [37]: AsEquation an indicator of the flame zone, the CO2 * formation consists of three elementary steps, given in Equation (6) [37]: Equation (6) [37]: CO + O + M → CO 2 ** + M CO ++OO++ M CO22 ∗++ CO M→ → CO MM * CO → (6) (6) 2* 2 2 + hv CO ∗ CO → CO + hv 2 CO 2* → CO 2 + hv (6) MM→→CO CO22 *∗++ CO +M 2M 2 + CO 2 + M → CO 2 + M The CO whichisisaathree-body three-bodyreaction. reaction.InIn the second and The CO * isproduced producedby bythe thefirst first step step reaction, which the second and 2 * 2is The CO 2* is produced by the first step reaction, which is a three-body reaction. In the second and third step reaction, CO The second secondstep stepreaction reactionisisthe thecause cause CO third step reaction, CO returnsto toits itsground ground state. The ofof thethe CO 2* 2 * 2 *2*returns third step reaction, CO2* returns itsthird ground state. The second step reaction isstep. the cause of the CO2* light emission andcompetes competes withto the third step reaction which light emission and with the step reaction which is isaaquenching quenching step. light emission andframe competes thecamera third step which is second. asecond. quenching step.case, The recording framerate ratewith the camera wasreaction frames per At anan averaged The recording ofofthe was 400 frames per Ateach each case, averaged The recording frame ratebyofaveraging the cameraa was 400 frames per second.instantaneous At each case, images. an averaged flame image was acquired total of 800 consecutive The flame image was acquired by averaging a total of 800 consecutive instantaneous images. The averaged flame image was acquired by averaging a total of 800 consecutive instantaneous images. The averaged was then employed to determine theshape flame shape and size. combustor pressure image was image then employed to determine the flame and size. The The combustor pressure was averaged image was then employed to determine the flame shape and size. The combustor pressure was measured with a pressure transmitter mounted on the chamber wall. Exit temperature was measured with a pressure transmitter mounted on the chamber wall. Exit temperature was measured was measured a pressure transmitter mounted onradius the chamber wall. Exit at temperature was thewith K-type thermocouples along(for the (for mean values) the gas bymeasured the K-typebythermocouples placed alongplaced the radius mean values) at the exhaust gasexhaust exit. Exhaust measured by the K-type thermocouples placed along the radius (for mean values) at the exhaust gas exit. Exhaust gas was extracted with a probe located downstream of the exhaust nozzle; NO, CO, O2 gas was extracted with a probe located downstream of the exhaust nozzle; NO, CO, O2 were measured exit. Exhaust gas was extracted with a probe located downstream of the exhaust nozzle; NO, CO, O2 were measured with a testo350 gas analyzer (TestoSE & Co. KGaA, Badenia-wirtembergia, Germany), with a testo350 gaswith analyzer (TestoSE & Co. KGaA, Badenia-wirtembergia, Germany), as displayed in were measured a testo350 gas analyzer (TestoSE & Co. KGaA, Badenia-wirtembergia, Germany), as displayed in Figure 5. Figure 5. as displayed in Figure 5.
Figure 5. Gas analyzer. Figure 5. Gas analyzer. Figure 5. Gas analyzer.
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O2 was employed to correct the NO and CO emissions under standard 15% oxygen concentration. The delay between the sample probe and analyzer required a steady flame be held in each test case for approximately 60 seconds before a steady measurement could be attained. The instrument specifications and measurement uncertainty according to the manufacturer are shown in Table 1. Table 1. Specifications of the instrument and uncertainty. Measurement
Instrument
Range
Accuracy
Uncertainty
Temperature O2
K-type thermocouple Testo 350 gas analyzer
273–1273 K 0–25 vol %
±1K 0.01 vol %
CO
Testo 350 gas analyzer
0–500 ppm
NO
Testo 350 gas analyzer
0–300 ppm
±1 K ±0.8% FSR ±2 ppm (0–39.9 ppm) ±5% (40–500 ppm) ±2 ppm (0–39.9 ppm), ±5% (40–300 ppm)
0.1 ppm
± 0.1 ppm
2.3. Experimental Conditions Test cases are shown in Table 2. Operating pressure P was increased from 3 bar to 6 bar; fuel mass ˙ F was varied from 12–16 g/min at a fixed pressure, air mass flow rate and primary air mass flow rate m ˙ pri were kept constant for all cases. flow rate m Table 2. Operating conditions of the experiment. Case
Combustor Pressure, P (bar)
Methane Mass Flow Rate, m ˙ F (g/min)
Air Mass Flow Rate, m ˙ A (kg/min)
Primary Air, m ˙ pri (g/min)
m ˙ F /m ˙ pri
1 2 3 4 5 6 7 8 9 10 11 12
3 3 3 4 4 4 5 5 5 6 6 6
12 14 16 12 14 16 12 14 16 12 14 16
2.1 2.1 2.1 2.1 2.1 2.1 2.1 2.1 2.1 2.1 2.1 2.1
105 105 105 105 105 105 105 105 105 105 105 105
0.114 0.133 0.152 0.114 0.133 0.152 0.114 0.133 0.152 0.114 0.133 0.152
3. Results and Discussion 3.1. Flame Shape and Length ˙ F /m ˙ pri ratios; Figure 6 displays flame appearances for combustor pressure P under varying m it should be noted that the flame base is the nozzle divergent end. One can see that the flame shape ˙ F /m ˙ pri ratio, became shorter and the flame volume decreased with the increment of P at a fixed m the same phenomenon was also detected in [38]. It could be associated with the declined axial diffusion of fuel under higher pressure. It can also be seen in Figure 6 that the flame zone became longer and ˙ F /m ˙ pri ratio increased at a given P, which could be attributed to the augmented ambient wider as the m air that needed to be entrained to reach stoichiometric proportions. The flame length Lf is defined as the distance from nozzle throat to flame tip, since the throat is treated as the nozzle injection exit, so, the Lf can be obtained by summing the measuring length and the length between the nozzle divergent end and the throat.
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Figure6.6.Flame Flameshape shapefor forcombustor combustorpressure pressureunder underdifferent differentm ratios. ˙ṁFF//ṁ ˙ pri Figure m pri ratios. Figure 6. Flame shape for combustor pressure under different ṁF/ṁpri ratios.
Figure 7 shows Lf shortened with the increasing P at a fixed ṁF/ṁpri ratio but enlarged with the ˙ F /m ˙ pri ratio but enlarged with the Figure 7 shows Lf shortened with the increasing P at a fixed m increment of ṁ7F/ṁ pri ratio. The Lf was influenced significantly by P and ṁF/ṁpri with an opposite effect. Figure shows L f shortened with the increasing P at a fixed ṁF/ṁpri ratio but enlarged with the ˙ F /m ˙ pri ratio. The Lf was influenced significantly by P and m ˙ F /m ˙ pri with an opposite increment of m Byincrement fitting theofLfṁ datapri againstThe P and ṁF/ṁ pri, a correlation was achieved which is shown in Equation (7): Lf was influenced significantly by P and ṁF/ṁ pri with an opposite effect. ˙ ˙ effect. By fittingF/ṁ the Lratio. data against P and m / m , a correlation was achieved which is shown in F f pri −0.49 0.73 By fitting the Lf data against P and ṁ F/ṁpri, a correlation was achieved which is shown in Equation (7): L = 0.91 P (m / m ) Equation (7): (7) f F pri . 0.73 0.49 . 0.73 = 0.91P 0.91P−−0.49 (m mpri ) LLf f = (mFF // m ) (7) (7) pri The values for the exponent of pressure and ṁF/ṁpri ratio were −0.49 and 0.73, respectively, the ˙F/ṁ ˙ ratio The values of pressure pressure andṁm m were 0.490.73, and 0.73, respectively, pressure exponent −0.49 approaches the exponent −0.67 given in − [39]. A comparison ofthethe The valuesfor forthe theexponent exponent and were −0.49 and respectively, F /pri pri ratio the pressureexponent exponent −0.49 approaches exponent −0.67 given [39]. comparison measurement and prediction of Lf with the thethe correlation Equation (7)inisin presented in Figure of 8. of Good pressure −0.49 approaches exponent −0.67 given [39]. A A comparison thethe measurement and prediction prediction of f with the correlation Equation (7) is presented in Figure 8. Good measurement ofLmeasured L with the correlation Equation (7) is presented in Figure 8. agreement wasand observed between and the calculated results, which implied the fitting for f agreement observed between measured and the calculated results, which implied the implied fitting forthe Good agreement was observed between and the calculated results, which obtaining thewas flame length correlation wasmeasured correct. obtaining the flamethe length correlation was correct. fitting for obtaining flame length correlation was correct.
Figure Flame length for combustor pressureunder underdifferent differentṁ F/ṁ ratios. Figure Flame lengthfor forcombustor combustorpressure pressure under different F/ṁ pripri ratios. ˙ṁ ˙ pri Figure 7.7.7. Flame length m ratios. F /m
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Figure 8. Prediction of of flame length from its correlation. Figure length from fromits itscorrelation. correlation. Figure8.8.Prediction Prediction of flame flame length
3.2. Exit Gas Oxygen Concentration and Temperature
Exit Gas OxygenConcentration Concentrationand andTemperature Temperature 3.2.3.2. Exit Gas Oxygen
As shown in Figure 9a, combustor exit gas O2 volume fraction increased with the increment of shown Figure9a, 9a,combustor combustorexit exit gas gas O O22 volume the increment of of AsAs shown ininFigure volume fraction fractionincreased increasedwith with the increment pressure when keeping the ṁF/ṁpri ratio constant. This indicates that less O2 was consumed at higher pressure when keeping the ṁ F /ṁ pri ratio constant. This indicates that less O 2 was consumed at higher ˙ F /m ˙ pri ratio pressure when keeping m constant. indicates as thatpressure less O2 was higher pressure, and thus the combustion became moreThis incomplete wasconsumed increased.at This pressure, and thus combustion became more incomplete as pressure was increased. This pressure, and thus combustion became more incomplete as pressure was increased. This phenomenon phenomenon corresponded to the decrease in exit temperature with increasing pressure, as shown phenomenontocorresponded tointhe decrease in exit with temperature with increasing asFigure shown9b, corresponded thetodecrease exit temperature aspressure, shown in Figure 9b, due the lessened heat release since theincreasing O2 amountpressure, taking part in globalinchemical in Figure 9b, due to the lessened heat release since the O2 amount taking part in global chemical due to the lessened heat since the O2 amount taking part in global chemical reactions (8) and/ reactions (8) and/or (9) release decreased: reactions (8) and/or (9) decreased:
or (9) decreased:
2CH+ + 3O 3O2 → → 2CO + + 4H 4H O 2CH 4H22O 2CH 22 → 2CO + 2O 4 44 +3O
(8) (8) (8)
2CO+++O O22 → → 2CO 2CO 2CO 2CO22 2CO O →
(9) (9) (9)
2
2
It alsocan can be observed observed that the exit O2 volume fraction became smaller with anwith increase in ṁF/ṁpri It It also thatthe the exit fraction became an increase 2 volume also canbe be observed that exit O2 O volume fraction became smallersmaller with an increase in ṁF/ṁpri in at given P, which implied moremore O2 was consumed through the aforementioned reactions byby ˙ Fat ˙aaprigiven m /m at a given P, which implied O was consumed through the aforementioned reactions 2 consumed through the aforementioned reactions by P, which implied more O2 was increasing the fuel. This trend can also be reflected in Figure 9b, which showed the exit temperature increasing the fuel. in Figure Figure9b, 9b,which whichshowed showedthe theexit exit temperature increasing the fuel.This Thistrend trendcan canalso also be be reflected reflected in temperature increased as the ṁ/Fm /ṁ pri ratio was increased because more reaction heat was released due to the ˙ ˙ increased as the m ratio was increased because more reaction heat was released due increased as the FṁF/ṁpri pri ratio was increased because more reaction heat was released due to to thethe increasing fuel and O 2 participating in reactions. increasing fuel and increasing fuel andOO participatingin in reactions. reactions. 2 2participating
(a) (a)
(b) (b)
Figure 9. (a) Exit gas O2 volume fraction; (b) exit temperature Figure 9. (a) Exit gas O2 volume fraction; (b) exit temperature Figure 9. (a) Exit gas O2 volume fraction; (b) exit temperature.
3.3. Global Flame Residence Time 3.3. Global Flame Residence Time As mentioned before [26], flame residence time tR has an important influence on NO formation. As mentioned before [26], flame residence time tR has an important influence on NO formation. The flame residence time tR can be measured with a tglobal flame residence time tg, which isformation. defined As mentioned before flame residencewith time an important on NOis R has flame The flame residence time[26], tR can be measured a global residenceinfluence time tg, which defined asflame Equation (10). The tg thas been used in some previous analyses for NO x formation [22,40,41], its The residence time can be measured with a global flame residence time t , which is defined g R been used in some previous analyses for NO x formation as Equation (10). The tg has [22,40,41], its
3.3. Global Flame Residence Time
as Equation (10). The tg has been used in some previous analyses for NOx formation [22,40,41],
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3 its definition based theconsideration considerationthat thatthe theflame flamevolume volume V Vff is is proportional proportional to definition is is based onon the to LLff3,, as as indicated indicated by Equation (11): by Equation (11): t g = L f 3 /ue d2 (10) 2 t g =2 L f 3 / u d (10) e t R ∝ Vf /ue d ∝ L f 3 /ue d2 = t g (11)
tR V f / ue d L f / ue d = t g (11) To calculate the tg in present work, the exit velocity ue needed to be determined. For the sake of convenience and simplicity of analyses in the present situation, the mixture of fuel and primary air is To calculate the tg in present work, the exit velocity ue needed to be determined. For the sake of ˙analyses ˙ ein ˙ F +present ˙ pri , and regarded as aand nozzle exit fuelofm =m m the throat treatedof asfuel the nozzle injection e , i.e., m convenience simplicity the situation, theismixture and primary air exit as stated previously. Then the nozzle exit u is obtained by Equation (12): e is regarded as a nozzle exit fuel ṁe, i.e., ṁe = ṁF + ṁpri, and the throat is treated as the nozzle injection 2
3
2
exit as stated previously. Then the nozzle exit ue is . obtained by Equation (12): ue = 4me /ρe πd2
(12) ue = 4me / ρeπd (12) where π stands for circular constant, d is nozzle throat diameter, d = 0.01 m, ρe is the density of nozzle where π stands constant, d is nozzle throat diameter, exit fluid, whichfor cancircular be obtained by ideal gas state Equation (13):d = 0.01 m, ρe is the density of nozzle exit fluid, which can be obtained by ideal gas state Equation (13): ρe = P/R g T (13) ρe = P / RgT (13) 2
where Rg is gas constant, and T represents inlet air temperature, T = 288 K. where Rg is gas constant, and T represents inlet air temperature, T = 288 K. The calculation result for ue is shown in Figure 10a; the ue decreased gradually as pressure The calculation result for ue is shown in Figure 10a; the ue decreased gradually as pressure ˙ /m ˙ increased at a fixed m ratio, but remained nearly unchanged under a given pressure since increased at a fixed ṁFF/ṁpripriratio, but remained nearly unchanged under a given pressure since the ˙ F was very small compared to m ˙ . Exit velocity ue was combined with Lf the variation value of m variation value of ṁF was very small compared to ṁe. Exit evelocity ue was combined with Lf through through Equation (10) to obtain the global flame residence time tg , as displayed in Figure 10b. One can Equation (10) to obtain the global flame residence time tg, as displayed in Figure 10b. One can observe ˙ F /m ˙ pri ratio was fixed, observe that tg had a decreasing tendency with the increment of P when m that tg had a decreasing tendency with the increment of P when ṁF/ṁpri ratio was fixed, which is which is attributed to the decreasing Lf (as discussed in the previous section) although ue decreased as attributed to the decreasing Lf (as discussed in the previous section) although ue decreased as well. ˙ F /m ˙ pri ratio when P was kept constant, which was well. However, tg extended with an increase in m However, tg extended with an increase in ṁF/ṁpri ratio when P was kept constant, which was explained explained by the increasing Lf presented in Figure 7. A correlation was obtained on the effects of P by the increasing Lf presented in Figure 7. A correlation was obtained on the effects of P and ṁF/ṁpri ˙ F /m ˙ pri on tg by fitting the experimental data with a power function and m y = Cx1 a x2 b , as shown in on tg by fitting the experimental data with a power function y = Cx1ax2b, as shown in Equation (14): Equation (14): . 0.43 . 2.07 2.07 269P−−0.43 m (14) t gtg==269P ((m m FF // m (14) pri )) pri ˙ FF/ṁ ˙ pri From of P and ṁ m /m tg decreased with From Equation Equation (14), one can clearly see the influence of pri onon tg,ttgg, decreased with an −0.43 ), but increased significantly with the increasing of m −0.43 2.07 ˙ṁFF//ṁ ˙pripri ((ṁ ˙FF/ṁ ˙ )pri an increase m ((m /m )2.07 ). increase in in P P(P(P ), but increased significantly with the increasing pri ). A A comparison of the measurement and prediction of t with correlation Equation (14) is presented in comparison of the measurement and prediction of tggwith correlation Equation (14) in Figure waswas observed between measured and the calculated result. Which Figure 11. 11.Good Goodagreement agreement observed between measured and the calculated result.implied Which the fittingthe forfitting obtaining the globalthe flame residence time correlation was correct. implied for obtaining global flame residence time correlation was correct.
(a)
(b)
Figure 10. 10. (a) (a) Nozzle Nozzle exit exit velocity; velocity; (b) (b) global global flame flame residence residence time. time. Figure
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Figure 11. 11. Prediction Prediction of of global global flame flame residence residence time time from fromits itscorrelation. correlation. Figure
3.4. NO NO Emission Emission 3.4. Thermal NO NO [42] [42] is is the the significant significant pathway pathway for for the the formation formation of of NO, NO, and and its its mechanism mechanism is is Thermal described by Equation (15): described by Equation (15): NN N 2 ++O O→ → NO NO + +N 2 (15) N + O2 → NO + O (15) N + O 2 → NO + O N + OH → NO + H
N + OH → NO + H
Thermal NO formation is controlled largely by flame temperature and increases dramatically with Thermal in NO formation temperature is controlled when largely flame temperature dramatically the increment combustion thebytemperature is larger and thanincreases 1800 K [2]. Therefore, with the increment in combustion temperature when the temperature is larger than K [2]. it always dominates the NO formation in high-temperature flames. The residence time in1800 the flame Therefore, it always dominates the NO formation high-temperature The residence time in zone also has a significant influence on the thermalinNO formation, sinceflames. the reaction rate of thermal the flame zone also has a significant influence on the thermal NO formation, since the reaction rate NO is relatively slow [2]. of thermal NO is relatively slow [2]. In non-premixed flames, flame surfaces exist where fuel and oxidizer meet in stoichiometric In non-premixed flames, flame surfaces existzone where fuel and oxidizer meetprimary in stoichiometric proportions, and thus the temperature of the flame is considerably high. The formation proportions, and thus the temperature of the flame zone is considerably high. The primary pathway of NO is thermal NO, which is affected by stoichiometric flame temperature Tstformation and the pathway of NOinisthe thermal NO, which by stoichiometric flame Tst and the residence time flame zone tR [26],isasaffected can be seen in Equation (4). Thetemperature Tst can be varied with residence time in the flame zone tR [26], can be type, seen in Equation (4). The TstFor canall be cases variedinwith some kinds of operating parameters suchasas fuel inlet air temperature. the some kinds of operating parameters such as fuel type, inlet air temperature. For all cases the present experiment, Tst was fixed and approached 2300 K, so the effect of Tst on NO formationinwas present experiment, T st was fixed and approached 2300 K, so the effect of Tst on NO formation was not considered. not considered. The measured combustor exit NO volumetric fraction and NO emission index EINO are displayed The 12. measured exitNO NOand volumetric and trend NO emission index EINO atare in Figure It can becombustor found that the EINO hadfraction a decreasing as pressure was raised a displayed in Figure 12. It can be found that the NO and EINO had a decreasing trend as pressure was ˙ F /m ˙ pri ratio, however, increased with the increment of the m ˙ F /m ˙ pri ratio at a given pressure. fixed m raised at mainly a fixed attributed ṁF/ṁpri ratio, however, increased with theresidence incrementtime of the ṁF/ṁ pri ratio at a given This was to the effect of the global flame tg , as shown in Figure 13, pressure. This was mainly attributed to the effect of the global flame residence time tg, as in the NO and EINO increased since tg was increased. Combining the results in Figures 10bshown and 13, Figure 13,ofthe NOm increased since tg was increased. Combining the results in Figures 10b ˙and ˙EINO the effect P and F /m pri ratio on NO and EINO can be explained as illustrated in Figure 12. and 13, the effect of P and ṁF/ṁpri ratio on NO and EINO can be explained as illustrated in Figure 12.
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Figure 12. 12. The The NO and EINO EINO for combustor pressure pressure under under varying ṁFF˙/ṁ /ṁpri pri ratios. Figure combustor ratios. ˙ pri ratios. Figure 12. The NONO andand EINO forfor combustor pressure undervarying varyingṁ m F /m
Figure 13. 13. The The NO NO and and EINO EINO for for the the global global flame flame residence residence time. time. Figure
Figure 13. The NO and EINO for the global flame residence time. The influence influence of of tg tg on on NO NO and and EINO EINO was was examined examined by by fitting fitting the the NO NO and and EINO EINO data data with with aa The a. The fitted results are indicated by Equations (16) and (17): power function y = Cx a. The The influence NO andresults EINOare was examined by fitting the NO and EINO data with a power function of y =tg Cxon fitted indicated by Equations (16) and (17): 0.99 power function y = Cxa . The fitted results areNO indicated by Equations (16) and (17): = 6.54tg0.99
NO = 6.54tg
EINO 0.28gtt0.99 NO = ==6.54t g1.15 EINO 0.28 g
1.15
(16) (16) (17) (16) (17)
Both equations equations showed showed that that NO NO and and EINO EINO increased almost proportionally proportionally with with the the increasing increasing Both almost EINO =increased 0.28t g 1.15 (17) ttgg;; the the increasing increasing rate rate of of EINO EINO is is larger larger slightly slightly than than that that of of NO. NO. Combining Combining the the Equations Equations (14), (14), (16) (16) and (17), (17), the NO NOshowed and EINO EINO correlation with P Pincreased and ṁ ṁFF/ṁ /ṁalmost pri ratio were derived and expressed as Both equations thatcorrelation NO and EINO with the increasing and the and with and pri ratio proportionally were derived and expressed as Equations (18) and (19): tg ; the increasing of EINO is larger slightly than that of NO. Combining the Equations (14), Equations (18) rate and (19): 0.47 ˙ 2.17 (16) and (17), the NO and EINO correlation with and m as F/ 0.47 pri ratio were derived and expressed NO = = 2299 2299 P−−P (mF m /˙ m (18) pri )2.17 NO P (m (18) F / mpri ) Equations (18) and (19): −0.54 . 2.52 . 2.17 −0.54 (m EINO = = 254 254− P0.47 m )2.52 (19) (18) pri ) EINO P NO = 2299P ((m mFFF /// m (19) pri pri )
Meanwhile, the the data data in in Figure Figure 12 12 was was also also directly directly fitted fitted. with with. the the 2.52 power function y = Cx1ax2b; the result Meanwhile, EINO = 254P−0.54 (m F /m pri ) power function y = Cx1ax2b; the result (19) is characterized characterized by by Equations Equations (20) (20) and and (21): (21): is
0.53 2.02 Meanwhile, the data in Figure 12NO was alsoP−−directly fitted with the power function y =(20) Cx1 a x2 b ; = 1831 1831 (mF // m m )2.02 pri ) NO = P 0.53 (m (20) pri the result is characterized by Equations (20) and (21): F 0.6 2.3 EINO = = 180 180P P−−0.6 (mF // m mpri ))2.3 EINO (m .F . pri
NO = 1831P−0.53 (m F /m pri )
2.02
(21) (21)
(20)
A comparison comparison of of the the measurements measurements and and predictions predictions of of NO NO with with the the derived derived correlation correlation Equation Equation A (18) and directly fitted correlation (20), respectively, is presented in Figure 14a. Good accordance was . . (18) and directly fitted correlation (20), respectively, Good accordance was −0.6is presented 2.3 EINO = 180P (for mF / m pricorrelation. ) in Figure 14a. (21) found between measured and the calculated result each The results with directly found between measured and the calculated result for each correlation. The results with directly
A comparison of the measurements and predictions of NO with the derived correlation Equation (18) and directly fitted correlation (20), respectively, is presented in Figure 14a. Good accordance was found between measured and the calculated result for each correlation.
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The results with directly fitted correlation (20) were closer to experimental data than that of derived correlation Equation(20) (18). This should be associated withthat theofaccumulated errorsEquation as each equation fitted correlation were closer to experimental data than derived correlation Energies 2018, 11, x FOR PEER REVIEW 13(18). of 19 during deriving processwith wasthe based on fittings. The comparison was for EINO, Thisthe should be associated accumulated errors assame each equation during theperformed deriving process fitted correlation (20) were closercomparison to experimental dataasthan that of derived correlation Equation (18). usingwas thebased measured data andsame Equations (19) and shown in Figure 14b, result of comparison on fittings. The was(21), performed for EINO, using thethe measured data and This should beand associated theinaccumulated errors as each equation during the deriving process Equations (21), as with shown Figure 14b, the result of comparison was similar to that of NO. was similar to (19) that of NO. was based on fittings. The same comparison was performed for EINO, using the measured data and Equations (19) and (21), as shown in Figure 14b, the result of comparison was similar to that of NO.
(a)
(b)
Figure 14. Predictions of NO and EINO from their directly fitted correlation and derived correlation:
Figure 14. Predictions of NO (a) and EINO from their directly fitted correlation (b) and derived correlation: (a) NO; (b) EINO. (a) NO; (b) EINO. Figure 14. Predictions of NO and EINO from their directly fitted correlation and derived correlation:
Moreover, quest of the EINO scaling, the EINO scaled with (ρue/d) is plotted in Figure 15 along (a) NO; (b) in EINO. with jet Froude number, Fr,EINO as defined in Equation (2).scaled It canwith be found thatisEINO (ρuin e/d)Figure increased Moreover, in quest of the scaling, the EINO (ρue /d) plotted 15 along Moreover, inin quest ofas thealso EINO scaling, the EINO scaled e/d) correlation isthat plotted inisFigure 15 along an increase Fr,Fr, and with anEquation increase in ṁFIt /ṁcan pri with ratio. The presented in withwith jet Froude number, defined in (2). be(ρu found EINO (ρu /d) increased e with jet Froude It can be found that EINO (ρue/d) increased Equation (22): number, Fr, as defined in Equation (2). ˙ F /m ˙ pri ratio. The correlation is presented in with an increase in Fr, and also with an increase in m with an increase in Fr, and also with an increase in4 ṁF0.42 /ṁpri ratio. The correlation is presented in Equation (22): EINO (ρue / d) = 1.23 10 Fr (mF / mpri )2.08 (22) Equation (22): . . 2.08
EINO (ρue /d) = 1.23 × 104 Fr0.42 (m /m
)
(22)
F pri 4 0.42 (22) was compared The prediction of EINO EINO (ρue/d)(ρu with/ correlation Equation to the measurement (mF / mpri )2.08 (22) e d) = 1.23 10 Fr as depicted in Figure 16. Good accordance was foundEquation between measured and the calculated results. The prediction of EINO (ρue /d) with correlation (22) was compared to the measurement Theinprediction of EINO (ρu e/d) with Equation (22) was the measurement This implied the fitting for obtaining thecorrelation correlation Equation (22) is compared appropriate. In calculated previous NO as depicted Figure 16. Good accordance was found between measured andtothe results. as depicted Figure Good accordance was foundtobetween measured and the calculated results. scaling with in pure fuel,16. EINO(ρu e/d) was proportional Fr0.6 [19–22], the present NO scaling with the This implied the fitting for obtaining the correlation Equation (22) is appropriate. In previous NO This implied the fitting the correlation (22) is appropriate. previous NO addition of primary air for to obtaining fuel indicated EINO (ρueEquation /d) was proportional to Fr0.42In , the exponent scaling with pure fuel, EINO(ρue /d) was proportional to0.6Fr0.6 [19–22], the present NO scaling with scaling with pure EINO(ρu e/d) wasindicated proportional Fr [19–22], the present NOfuel scaling with the decreased from 0.6fuel, to 0.42. This decline that to addition of primary air in the could cause the addition ofinprimary air to fuel indicated EINO (ρue/d) was proportional to Fr, 0.42 , exponent thewhen exponent e /d) 0.42 primary air to fuel indicated EINO (ρumay was proportional the aaddition decreaseof the exponent of Fr power function, which due to the decreasetoinFr flame length decreased from 0.6 to This decline indicated that addition ofprimary primary air the fuel could cause a decreased 0.60.42. toto0.42. This decline indicated that addition of inin the fuel could cause primary airfrom is added fuel [43], thus affecting the flame residence time andair NO. decrease in the exponent of Fr power function, which may due to the decrease in flame length a decrease in the exponent of Fr power function, which may due to the decrease in flame length whenwhen primary air isair added to fuel [43], thus residencetime time and NO. primary is added to fuel [43], thusaffecting affectingthe the flame flame residence and NO.
Figure 15. Variation of EINO (ρue/d) with Fr under varying ṁF/ṁpri ratios. Figure 15. Variation of EINO (ρue/d) with Fr under varying ṁF/ṁpri ratios.
˙ F /m ˙ pri ratios. Figure 15. Variation of EINO (ρue /d) with Fr under varying m
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Figure PredictionofofEINO EINO(ρu (ρue/d) /d) from correlation. Figure 16.16. Prediction fromitsits correlation. e Figure 16. Prediction of EINO (ρue/d) from its correlation.
3.5. CO Emission
3.5. CO Emission 3.5. CO Emission The formation reaction of CO is shown in Equations (8) and (9), where Equation (8) is the CO The formation reaction of CO is(9) shown in Equations (8)reaction. and (9), where Equation (8) in is the CO production reaction, and Equation is the CO consumption presence of the CO The formation reaction of CO is shown in Equations (8) and (9),The where Equation (8) is the the CO production andquantities Equationis (9) is the CO reaction.ofThe the CO in the exhaustreaction, gas in large a reflection of consumption incomplete combustion fuelpresence caused byof insufficient production reaction, and Equation (9) is the CO consumption reaction. The presence of the CO in the burning rates in flame zone, is inadequate mixing of fuel and air, the chillingofinfluence of liner exhaust gas in large quantities a reflection of incomplete combustion fuel caused bycoolant, insufficient exhaust gas in large quantities is a reflection of incomplete combustion of fuel caused by insufficient and/or insufficient time. burning rates in flame residence zone, inadequate mixing of fuel and air, the chilling influence of liner coolant, burning rates in flame zone, inadequate mixing of fuel and air, the chilling influence of liner coolant, In present measurement, the impacts of combustor pressure under different ṁF/ṁpri ratios on CO and/or insufficient residence time. and/or insufficient residence time. and EICO are presented in the Figure 17. TheofCO emission index EICO is defined as them of the mass ˙ ratio ˙ pri In present measurement, impacts combustor pressure under different ratios on CO F /m In present measurement, the impacts of combustor pressure under different ṁ F/ṁpri ratios on CO rate of CO produced to the mass of fuel supplied, as given by Equation (23) and EICO are presented in Figure 17. The CO emission index EICO is defined as the ratio of the mass and EICO are presented in Figure 17. The CO emission index EICO is defined as the ratio of the mass rate of CO produced to the mass of fuel supplied, EICO =as m given / m by Equation (23) (23) rate of CO produced to the mass of fuel supplied, asCOgivenF by Equation (23) . . gradually with the raise in P at a fixed The results showed that the CO and EICO decreased EICO == m m (23) F CO / EICO m / m ṁF/ṁpri ratio. This can be attributed to two reasons: CO one of Fthe reasons is that the flame volume got(23) shrunk at higher P, leading to less CH4 captured and burned in a timely way by the flame, therefore The results showed that the CO and EICO decreased gradually with the raise in P at a fixed resulting in the less CO formation through Equation (8). This also corresponded to the less O 2 ˙ FF/ṁ ˙pripriratio. m /m ratio.This This can attributed two reasons: of the reasons is that flame volume ṁ can be be attributed to to two reasons: oneone of the reasons is that the the flame volume got consumption at higher P as mentioned before. Another reason is that the increasing pressure got shrunk at higher P, leading to less CH captured and burned in a timely way by the flame, shrunk at higher P, leading to less CH 4 captured and burned in a timely way by the flame, therefore 4 accelerated chemical reaction kinetics to promote the conversion of CO to CO2 through the Equation therefore resulting inthe thedissociation less CO formation through Equation corresponded to the resulting in the less CO formation through Equation (8). This(8). alsoThis corresponded to Othe less O2 (9). Furthermore, was suppressed as high pressure favors COalso 2 over CO and 2 [44].
less O2 consumption at higher as mentioned before. Another reason thatthe theincreasing increasing pressure consumption at higher P as P mentioned before. Another reason is is that accelerated the conversion of of COCO to CO through thethe Equation (9). acceleratedchemical chemicalreaction reactionkinetics kineticstotopromote promote the conversion to CO 2 through Equation 2 Furthermore, the dissociation was was suppressed as high pressure favors CO2 CO over CO and O2 [44]. (9). Furthermore, the dissociation suppressed as high pressure favors 2 over CO and O2 [44].
Figure 17. The CO and EINO for combustor pressure under varying ṁF/ṁpri ratios.
It also can be observed that CO and EICO increased with the increment in ṁF/ṁpri ratio at a given P, which is explained by the escalating CO formation through Equation (8) as more CH 4 participated
Figure17. 17.The TheCO COand andEINO EINOfor forcombustor combustorpressure pressureunder undervarying varyingm ratios. ˙ṁF F//ṁ ˙ pri Figure m pri ratios.
It also can be observed that CO and EICO increased with the increment in ṁF/ṁpri ratio at a given P, which is explained by the escalating CO formation through Equation (8) as more CH 4 participated
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˙ F /m ˙ pri ratio at a given It also can be observed that CO and EICO increased with the increment in m P, which is explained by the escalating CO formation through Equation (8) as more CH4 participated 2018, 11, x FOR PEER REVIEW 15 of 19 in Energies the combustion. Fitting the data in Figure 17 with the power function y = Cx1 a x2 b , the correlations ˙ F /m ˙ pri ratio on CO and EICO are described by Equations for the influences of P and m (24) and (25): a b in the combustion. Fitting the data in Figure 17 with the power function y = Cx1 x2 , the correlations for the influences of P and ṁF/ṁpri ratio on CO and EICO by Equations (24) and (25): . are. described 0.72 −0.35
CO = 1196P
(m F /m pri )
(24)
CO = 1196P −0.35 (mF / mpri )0.72 .
.
−0.4 EICO = 65P (m F /m0.75 pri ) −0.4
EICO = 65P
(24) 0.75
(25)
(mF / mpri )
(25)
The prediction of CO with the correlation in Equation (24) was compared to the measurement The prediction of CO with the correlation in Equation (24) was compared to the measurement as as shown 18a. Good Goodaccordance accordance is found between measured the calculated results. shownininFigure Figure 18a. is found between measured and theand calculated results. This This implied fitting obtaining correlation Equation is appropriate. same comparison implied thethe fitting forfor obtaining thethe correlation Equation (24)(24) is appropriate. TheThe same comparison was performed for EICO, using the measured data and Equation (25), as shown in was performed for EICO, using the measured data and Equation (25), as shown in Figure Figure 18b, the18b, thedifference difference between experimental data and predicting the result is reasonable. between experimental data and predicting the result is reasonable.
(a)
(b)
Figure 18. Predictions of CO and EICO from their correlations: (a) CO; (b) EICO
Figure 18. Predictions of CO and EICO from their correlations: (a) CO; (b) EICO.
The correlation Equations (20), (21) and (24), (25) are similar to Equation (5) in terms of their The correlation Equations (21) and arefunctions similar toof Equation (5) inThe terms of and theirCO forms, forms, they are composed of(20), a product of(24), the (25) power parameters. NO they are composed of a product of the power functions of parameters. The NO and CO correlations correlations obtained in present paper may help in understanding the emissions behavior of nonobtained in present paper may help in understanding the emissions behaviorpressure. of non-premixed flame premixed flame with the partial premixing of fuel and primary air at elevated
with the partial premixing of fuel and primary air at elevated pressure. 4. Conclusions
4. Conclusions
The NO and CO emissions for a swirl convergent-divergent nozzle were examined experimentally elevated combustor pressure; The NO and CO nozzle correlations obtained in terms The NO and at CO emissions for a swirl convergent-divergent werewere examined experimentally pressure P and fuel to primary ratio F/ṁpri . The flame were shapeobtained and length, exit gas oxygen P at of elevated combustor pressure; Theair NO andṁCO correlations in terms of pressure concentration and temperature, global residence timelength, were also determined to analyze the ˙ Fand ˙ pri and fuel to primary air ratio m /m . Theflame flame shape and exit gas oxygen concentration NO and CO characteristics. The main findings of the present paper include: and temperature, and global flame residence time were also determined to analyze the NO and CO
characteristics. main findings of theṁpresent paper include: (1) With the The increment of P at a fixed F/ṁpri ratio, flame volume became smaller, and flame length decreased proportionally to P−0.49; while increasing ṁF/ṁpri ratio at a given P, flame volume
˙ F /m ˙ pri proportionally With the increment P atlength a fixedincreased m ratio, flame volume became smaller, and flame length became larger, andof flame to (ṁF/ṁ pri)0.73. −0.49 ; while increasing m ˙ ˙ decreased proportionally to P / m ratio at a givenbecame P, flame volume F (2) With the rise in P at a fixed ṁF/ṁpri ratio, combustor exit gas Opri2 volume fraction larger, 0.73 ˙ F /m ˙ pri became larger, and flame length increased proportionally m . P, exit O2 volume and exit temperature decreased; with the increasing of ṁF/ṁto pri ( ratio at a )given
(1)
fraction became and increased. ˙ Fexit ˙ temperature With the rise in Psmaller, at a fixed m /m exit gas O2 volume fraction became larger, pri ratio, combustor (3) and With increasingdecreased; P at a fixed F/ṁincreasing pri ratio, the global residence time ˙ F /m ˙ priflame exitthe temperature withṁthe of m ratio at a given P, exitlessened O2 volume proportionally P−0.43; with increasing of ṁFincreased. /ṁpri ratio at a given P, the global flame residence fraction becameto smaller, andthe exit temperature time the extended proportionally to (ṁF/ṁ )2.07 . ratio, the global flame residence time lessened ˙ pri ˙ pri (3) With increasing P at a fixed m F /m (4) As P increased at a −fixed ṁ F /ṁ pri ratio, the NO and EINO decreased proportionally to P−0.53 and ˙ F /m ˙ pri ratio at a given P, the global flame proportionally to P 0.43 ; with the increasing of m P−0.6, respectively, but increased proportionally to (ṁ2.07 F/ṁpri)2.02 and (ṁF/ṁpri)2.3 separately as the ˙ F /m ˙ pri ) . residence time extended proportionally to (m
(2)
ṁF/ṁpri ratio increased at a given P. This is mainly attributed to the influence of global flame residence time; the NO and EINO nearly increased proportionally with the increasing global flame residence time.
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(4)
(5)
(6)
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˙ F /m ˙ pri ratio, the NO and EINO decreased proportionally to P−0.53 and As P increased at a fixed m − 0.6 ˙ F /m ˙ pri )2.02 and (m ˙ F /m ˙ pri )2.3 separately as P , respectively, but increased proportionally to (m ˙ F /m ˙ pri ratio increased at a given P. This is mainly attributed to the influence of global flame the m residence time; the NO and EINO nearly increased proportionally with the increasing global flame residence time. With the addition of primary air in fuel, the EINO scaling EINO(ρue /d) was proportional to Fr0.42 . Which indicated the dilution of primary air to fuel could cause a decrease in the exponent of Fr power function comparing with Fr0.6 for pure fuel. ˙ F /m ˙ pri ratio, the CO and EICO decreased proportionally to P−0.35 and As P increased at a fixed m − 0.4 P respectively, due to the increased unburned methane and high pressure which accelerated chemical reaction kinetics to promote the conversion of CO to CO2 . However, the CO and EICO ˙ F /m ˙ pri )0.72 and (m ˙ F /m ˙ pri )0.75 separately as m ˙ F /m ˙ pri ratio increased enlarged proportionally to (m at a given P, due to the increased fuel participating in combustion.
Author Contributions: Z.X. and Z.F. conceived and designed the experiments; Z.X., X.H., and Y.J. performed the experiments; Z.X. and X.H. analyzed the data; Z.X. and X.H. contributed reagents/materials/analysis tools; Z.X. and Syed Waqas Sabir wrote the paper. Acknowledgments: This research was supported by Beijing Natural Science Foundation (Grant No. 3162030). Conflicts of Interest: The authors declare no conflict of interest.
Appendix A Table A1. The NO and CO correlations in the open literature. Scale equations. Reference Rizk and Mongia [45] Lipfert [46] AECMA [46] Becker et al. [47]
EINOx =
15 × 1014
· t0.5 res
Model Equation · exp −71, 100/T f l · P3 −0.03 · (∆P3 /P3 )−0.5
EINOx = 0.17282 · exp(0.00676593T3 ) EINOx = 2 + 28.5 · ( P3 /3100)0.5 exp(( T3 − 825)/250) −6 0.5 NOx ( ppm) = 5.73 × 10 · exp(0.00833T f l ) P3
Model Parameter Tres , Tfl , P3 , ∆P3 T3 P3 , T3 Tfl , P3
Lefebvre [26]
EINOx = 29 · exp −21, 670/T f l ) P3 0.66 (1 − exp(−250t f orm ) EINOx = 0.459 × 10−8 · P3 0.25 · F · tres · exp 0.01 T f l + 273
P3 , F, tres , Tfl
Rokke et al. [27] General Electric [48] AERONOX [48]
EINOx = 1.46P1.42 m air 0.3 f f ar 0.72 EINOx = 2.2 + 0.1235 · P3 0.4 · exp( T3 /194.4 − hum · 1000/53.2) EINOx = 1.5 · ( P3 /100)0.6 · exp(−600/T4 ) · t0.7 res
mair , ffar = mfuel /mai P3 ,T3 ,hum P3 , T4 , tres
Odgers and Kretchmer [44]
.
Rizk and Mongia [49]
0.333×1010 ·exp(−0.00275· TPZ ) F · P3 1.5 ·(tres −0.55·tevap )·(∆P3 /P3 )0.5 0.18 · 109 exp(7800/TPZ )/P2 (t − 0.4te )(∆P/P)0.5
EICO =
Levebvre [26] EICO =
Tfl , P3 , tform
Tpz , F, P3 , tres , tevap , ∆P3 TPZ , P, t, te , ∆P
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