Experimental Investigation of Thermal Performance of ...

JOURNAL OF THERMOPHYSICS AND HEAT TRANSFER Vol. 29, No. 2, April–June 2015

Experimental Investigation of Thermal Performance of a Unique Heat Pipe Array Qiangqing Liang,∗ Xiaoxing Han,∗ and Yaxiong Wang† Inner Mongolia University of Science and Technology, 014010 Baotou, Inner Mongolia, People’s Republic of China

Downloaded by TIANJIN UNIVERSITY on May 22, 2015 | http://arc.aiaa.org | DOI: 10.2514/1.T4349

DOI: 10.2514/1.T4349 A novel heat pipe array with multi-evaporators and a shared horizontal exocentric tube condenser was developed and experimentally investigated on its thermal performance and reliability. This uniquely designed heat pipe array was expected to have extensive applications in the fields of solar heat collecting, multichip cooling, waste heat recovery systems, etc. This unique heat pipe array was fabricated with five evaporator tubes vertically arranged in parallel sharing an exocentric horizontal condenser tube. The test facility had been setup and experiments were conducted to investigate the thermal performance of the proposed heat pipe array systematically. Experimental results showed that the heat pipe array had a good thermal performance and an excellent operating stability. Furthermore, the uniquely designed configuration of an exocentric tube condenser and multi-evaporators not only improved vapor–liquid twophase flow and phase-change heat transfer in the horizontal direction, but also extended the area of condenser and evaporator. The average thermal resistance decreased and the maximum heat transport capacity increased when increasing the length of evaporators. The experimental results also indicated that a maximum thermal performance of the heat pipe array appeared with an evaporator length of 270 mm at an inclination angle of 60 deg and operating temperature of 80°C. In addition, the minimum average thermal resistance and maximum heat transport capacity of this heat pipe array could achieve up to 0.193 °C∕W and 1098 W, respectively.

max sn vs

Nomenclature Cpc

=

do;c , di;c do;e , di;e lc le Qc Qe Qin Qloss Qmax R Rav T c;i , T c;o

= = = = = = = = = = = =

T ic T i;e Wc

= = =

λ

=

η

=

specific heat capacity of cooling circulating water, J∕kg · °C outside, inside diameter of condenser tube, mm outside, inside diameter of evaporator tube, mm length of condenser tube, mm length of evaporator tube, mm heat flow of condenser, W heat flow of evaporator, W input power, W heat loss of the test fixture, W maximum heat transport capacity, W thermal resistance of heat pipe array, °C∕W average thermal resistance of heat pipe array, °C∕W inside, outside wall temperature of condenser section, °C inlet temperature of coolant through condenser tube, °C inside wall temperature of evaporator section, °C flow rate of coolant through condenser tube, kg∕ min thermal conductivity of AL6063 condenser tubing (W∕m · K) efficiency of electric heater, ∼0.96

= = = = =

boiling condenser capillary evaporator entrainment

maximum sonic viscous

Introduction

T

HE rapid development of a national economy results in huge energy consumption and serious pollutions to the environment [1–4]. Solving the conflict between demand of fusil energy consumption and energy risk becomes imperative [2]. Extensive industrial applications of the heat pipe equipment to recover waste heat have been approved to be an excellent way of saving energy and preventing global warming [5]. As a superthermal-conductor device, the heat pipe exhibits an extremely high effective thermal conductivity, allowing its extensive applications in a various fields such as the chemical plant power industry, aeronautics and aerospace, and nuclear, electronics, energy-saving, and renewable energy utilization. Heat pipes for the heat recovery could be currently classified as three types: the conventional heat pipe (CHP), two-phase closed thermosyphon (TPCT), and oscillating heat pipe (OHP). In the conventional heat pipe, circulation of the working fluid is predominantly completed by return flow of the condensate to the evaporator section through capillary action in the CHP, but few studies [5] have found that parameters, such as capillary, sonic, and entrainment limitations, confine heat transport in CHP and effectively avoid the dryout phenomenon. However, OHP delivers the heat, including convection heat transfer and phase-change heat transfer, via the oscillating motions of the liquid plugs and vapor bubbles [6]. Although the OHP has attracted considerable interest due to its unique features over the CHP and TPCT, a complete theoretical understanding of the operational characteristics of the OHP is not yet achieved [7]. Meanwhile, the mechanisms of dryout of OHP are not yet fully understood and only a few studies are available [8–10]. The thermosyphon or TPCT is considered to be a heat pipe with a wickless structure. There are many advantages of a thermosyphon (TPCT), such as simpler structure, higher thermal efficiency, lower thermal resistance, and low manufacturing cost [11]. Furthermore, TPCT transports heat exactly the same way as the conventional heat pipe, by evaporation followed by condensation, and the evaporator of TPCT must be located vertically below the condenser so that the gravity would ensure condensate returns to the evaporator [12,13]. A lot of experimental and theoretical works have been done on the

Subscripts bl c cp e en

= = =

Received 16 December 2013; revision received 26 November 2014; accepted for publication 28 November 2014; published online 17 February 2015. Copyright © 2014 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved. Copies of this paper may be made for personal or internal use, on condition that the copier pay the $10.00 per-copy fee to the Copyright Clearance Center, Inc., 222 Rosewood Drive, Danvers, MA 01923; include the code 1533-6808/15 and $10.00 in correspondence with the CCC. *School of Chemical Engineering. † Professor, School of Chemical Engineering, 7# A Er Ding Avenue. Member AIAA (Corresponding Author). 346

347

LIANG, HAN, AND WANG

400mm

Inner Tube

Case Tube

Working Fluid

Cooling Water

Case Tube Inner Tube

Working Fluid

2#

3#

4#

5#

VAPOR

1# Evaporator

320mm

Liquid Phase

Evaporator

Fig. 1 Developed heat pipe array with multi-evaporators and shared horizontal condenser.

structure and design modification for further improving the thermal performance of thermosyphons, avoiding the dryout phenomenon and exacerbating total stability in the waste heat recovery system and multichips cooling [14–20]. In an effort to develop a high thermal performance with larger evaporation area and smaller condenser area for the special applications in solar heat recovery, multichips cooling, waste heat recovery systems, etc., a heat pipe array with multi-evaporators and a shared horizontal condenser was developed, fabricated, and investigated in this work. The novel structure of the heat pipe array developed here, as shown in Fig. 1, was constructed with five parallel evaporating tubes that were welded onto a horizontal exocentric condenser tube. All the specifications and geometric parameters of the investigated heat pipe array are listed in Table 1. The mechanism of vapor–liquid phase flow of the heat pipe array is shown in Fig. 2. When heat flux is added to the evaporators, the working fluid in the evaporator tube evaporates immediately and the vapor goes up to the condenser tube and condenses into the liquid phase. Latent heat is transferred to the flow through the outside tube. The condensed working fluid goes back to each evaporator under the gravity force. As previously mentioned, the working mechanism of the heat pipe array is similar to a conventional single thermosyphon [21,22]. The novel exocentric horizontal tubing condenser structure not only increases the condensation area but also reduces the vapor and liquid reflow distance and liquid film thickness, therefore enhancing the heat transfer performance. Theoretically, the maximum heat transport capacity of the heat pipe array could be extended unlimitedly by adding more evaporators and changing the size of the condenser. Thus, this developed unique heat pipe array presents a

Table 1

Specification of the studied novel heat pipe array

Parameters Condenser tubing Thermal conductivity of condenser Length of condenser Outside diameter of condenser Wall thickness of condenser Inside diameter of condenser Outside diameter of evaporator Wall thickness of evaporator Length of evaporator Liquid filled volume Inclination angle

Specifications AL6063 234 W∕m · k 400 mm 40.0 mm 2.0 mm 16.0 mm 10.0 mm 1.1 mm 170, 220, 270 mm 19% 15, 30, 45, 50, 75, 90 deg

VAPOR

Uniform Heat Flux


4x80=320mm

Uniform Heat Flux

Vapor Phase

Working Fluid

Fig. 2 Cross section of the unique heat pipe array.

lower thermal resistance and higher maximum heat transport capacity. To better understand the thermal behavior and operation limitations, as well as to optimize the heat transfer performance of the heat pipe array, a detailed experimental investigation was conducted. Temperature distribution, maximum heat transport capacity, and thermal resistance of the device are the main operating parameters that are qualitatively studied and analyzed in this current investigation.

Experimental Setup As illustrated in Fig. 3, the experimental testing system was composed of the heat pipe array, heating system, data acquisition system, electromagnetic flowmeter, and temperature control system (low-temperature pump). The experimental system was set up to figure out the heat pipe array’s thermal performance through measuring the temperature distribution along the multi-evaporators, condenser, and power input, as shown in Fig. 3. The power was generated from 10 copper electric heating blocks coated with conductive silicone grease (HZ-KS101, >0.21 W∕m2 · K). Each block was measured 300 mm long and 40 mm wide with a semicircle groove in it to more easily accommodate the evaporator tubes. The power added to the evaporators was adjusted from 300 to 1100 W. The temperature distribution on the wall of the heat pipe array was measured with T-type thermocouples with a maximum uncertainty of 0.05 °C. Thirty-seven thermocouples were attached on the surface of the heat pipe array as demonstrated in detail in Fig. 4. There were three thermocouples in each evaporator, respectively, one in each adiabatic section, two in each condensation area, and five on each outer surface of the insulation blocks. Except for the thermocouples attached on the heat pipe array, two thermocouples were placed in the inlet and outlet of the cooling water through the horizontal condenser. T e1;j , T e2;j , and T e3;j represent temperature measurement points on each evaporator, j 1, 2, 3, 4, 5. T c1;j and T c2;j represent the temperature measurement points on the condensers, j 1, 2, 3, 4, 5. T adi;j represents the temperature of the adiabatic section of the heat pipe array, j 1, 2, 3, 4, 5. T cw;i and T cw;o denote the inlet and outlet of the cooling water, respectively. The cooling temperature of the circulated coolant (water) was set to 20°C through a thermostatic bath. The uncertainty in temperature

348


Data Collector

Cooling Water Tank

A

PC

V

Electric heater

Computer

Fig. 3

Linear Power

Schematic of experimental setup facility.


Data Processing

Electric Heater Te1

Te2

Evaporator Te3

Tad Tc1

Condenser Cooling Water

The surface temperature T i;e and T i;c were needed to determine the isothermal characteristics, thermal resistance, and maximum heat transport capacity of the heat pipe array. However, the inside-wall temperature could not be measured, and so inside-wall temperature of the evaporator section can be yielded by a modified equation given by T i;e T o;e −

Electric Heater

Fig. 4

Ts

Insulation Block

Tc2

Tcw,i Tcw,o

Thermocouple distribution along the heat pipe array.

measurements was 0.1 °C. The flowrate of cooling water was measured using a rotameter with a flowrate of 0.08 kg∕ min. The novel heat pipe array was insulated by foamed polystyrene of 20 mm thickness to reduce the heat loss. The heat pipe array was tested in a series of inclination angles of 15, 30, 45, 50, 75, and 90 deg (vertical). Acetone was used as the working fluid, and its major thermophysical properties at standard atmospheric pressure are as follows [23]: boiling point, 56.2°C; liquid density (20°C), 792 kg∕m3 ; liquid specific heat (20°C), 2.35 kJ∕kg · °C; thermal conductivity (20° C), 0.17 w∕m · °C. Before filling the working fluid in, the heat pipe array was evacuated using the vacuum pump to remove the air inside. The heat pipe array was initially filled with 42 ml which was 19% of the overall volume of the heat pipe array.

Experimental Procedure Before each experiment, a preliminary test of the system was conducted to keep an equilibrium state to obtain the precise data in the following testing. First, turn on the power supply and set the power output at a given rate. The preliminary experiment took approximately 15–20 min to reach the steady operation state of the heat pipe array with a temperature fluctuation of less than 0.5 °C. Temperature distribution along the heat pipe array, coolant temperature, flowrate, and ambient temperatures, as well as current and voltage of the heater, were recorded automatically by data log when the steady-state condition was reached. The power input to the heater was then increased to obtain another steady state, and the process was repeated and the system was stopped until dryout appeared. Once the dryout was reached, the temperature difference between the evaporator and condenser increased rapidly. The power input at this point was assumed to be the maximum heat transport capacity at the setting operating temperature, which is defined as the adiabatic temperature of the heat pipe array. Changing the condensation temperature by adjusting the setting temperature of the coolant through the condenser tube could control the operating temperature of the heat pipe array. Repeating the procedure described earlier, maximum heat transport capacity and thermal resistance of the heat pipe array could be determined for various operating temperatures. The experimental uncertainties of the operating parameters are given in Table 2.

Qe lndo;e ∕di;e 2πλle

(1)

Similarly, the inside-wall temperature of the condenser section can be expressed as T i;c T o;c −

Qc lndo;c ∕di;c 2πλlc

(2)

where Qe is the heat flux through the evaporator section of heat pipe array, which is determined as follows: Qe ηQin − Qloss

(3)

Qin is the power input to the heat pipe array, which can be calculated as Qin U · I

(4)

where U and I represent the applied voltage and current, respectively; η is the efficiency of the electric heater, and Qloss is the heat loss through the test fixture. The heat transfer through the heat pipe array can be calculated by the heat taken out by coolant (water), which is expressed as Qc W c Cp;c T c;o − T c;i

(5)

The thermal resistance is an important parameter to measure the thermal performance of the heat pipe array, which is generally defined as Rav

Table 2

T e;av − T c;av Qe

(6)

Operating parameters and uncertainties

Variable Water flowmeter Temperature indicator Power transducer Heat input Heat output Evaporation temperature Condensation temperature Thermal resistance

Unit kg∕s °C W W W °C °C °C∕W

Uncertainty 1.0% 0.5 °C 1.0 W 1.25% 1.5% 1.65% 1.65% 2.35%

349


where T e;av is the average temperature of the multi-evaporators, and T c;av is the average temperature of the condenser. Maximum heat transport capacity of a heat pipe device is constricted by various limitations such as capillary limitation, boiling limitation, entrainment limitation, sonic limitation, viscosity, etc. Any limitation occurring means the maximum capacity has been reached: Qmax minQcp ; Qbl ; Qen ; Qsn ; Qvs

(7)

where Qcp is the capillary limitation of the heat pipe array, Qbl is the boiling limitation of the heat pipe array, Qen is the entrainment limitation of the heat pipe array, Qsn is the sonic limitation of the heat pipe array, and Qvs is the viscous limitation of the heat pipe array.


Results and Discussion A thorough investigation of the heat transport capacity under various operating temperatures and the impact factors on the developed heat pipe array were carried out and analyzed. Experimental results are discussed in this section. Temperature Distribution

The temperature distributions on the surface of the heat pipe array were obtained under various operation conditions. Isothermality is considered to be an important parameter to present the thermal performance of the heat pipe array. The more uniform temperature distributes in the axial and radial direction, the better thermal performance of the heat pipe array under the operating conditions. As shown in Fig. 5, the temperature distribution along the heat pipe array continually changed from the evaporation section to the condensation section. Through observing and analyzing the temperature change, startup point, maximum heat transport capacity, dryout point, and thermal resistance of the heat pipe array could be obtained. Theoretically, a smaller change of the temperatures means better thermal performance, and the experimental result showed a more even temperature distribution through all the tests in general. It was clearly seen in detail (Fig. 5) that the temperatures on each evaporator were slightly higher at the beginning of the evaporator section and nearly uniform in the center of the adiabatic section, and a small decrease was seen at the end of the condenser. The mean temperature of the evaporator tube was 83.3, 83.0, 82.6, 83.5, and 82.9°C from evaporator 1–5, respectively. The average temperature for the evaporator, adiabatic, and condenser sections of the heat pipe array was 87.6, 82.1, and 77.1°C, respectively. Maximum Heat Transport Capacity

In experimental study, the maximum heat transport capacity of the heat pipe array could be determined by observing the temperature or temperature difference variation along the heat pipe array. If any of the evaporators experience dryout, this means the heat pipe array begins dryout [24]. When the heat pipe array began dryout, the

Fig. 5 Temperature distribution along the heat pipe array.

Fig. 6

Evaporation temperature variation with input power.

temperature on the evaporator increased rapidly and the maximum heat transport capacity was reached at this point, as demonstrated in Fig. 6. The maximum heat transport capacity depends on the diameter and length of the evaporator, condenser size and configuration, working fluid, operating temperatures, and the inclination angle. Investigation of these factors could help better understand the mechanism of the heat pipe and optimize the design parameters of the heat pipe array. Thermal Resistance

The thermal resistance is a convenient parameter to judge the performance and to track performance variation of the heat pipe array. The thermal resistance is the ratio of the temperature difference between the evaporator and condenser and the heat flux through the evaporator section of the heat pipe array, as expressed in Eq. (6). Typical thermal resistance of the heat pipe array is shown in Figs. 7a and 7b. Effects on the Thermal Performance

As mentioned earlier, configuration of the heat pipe array, such as structure, diameter and length of the evaporator and condenser, working fluid, operating temperature, and the inclination angle, have direct relationships with the thermal performance of the heat pipe array. Investigation of these factors could help better understand the mechanism of the heat pipe array and optimize the design parameters of the heat pipe array. Figure 8 shows the influence of the inclination angle on the temperature difference between the evaporator and condenser sections in the operating temperature (adiabatic temperature) of 40–80° C. It can also be seen from Fig. 8 that temperature difference between the evaporator and condenser could achieve a minimum value of 15.1° C, which occurred with the inclination angle in the range of 60 ∼ 75 deg. Meanwhile, the maximum temperature difference was 29°C with inclination angle of 15 deg in the operating temperature of 40°C. Therefore, it was summarized that the best inclination angle for the isothermality of the heat pipe array was 60 ∼ 75 deg in the measured inclination angle range. Figure 9 indicates the impact of heat flux on the temperatures of the all evaporator tubes. Similar to Fig. 8, the average temperature of the evaporator section clearly showed two stages when the heat flux was increased continuously. The temperature slowly escalated at the first stage and wound up very fast in the second stage when the value was over a specific point. This critical heat flux is the maximum heat transport capacity. The mean temperature of each evaporator 1–5 was 89.3, 94.0, 90.6, 94.0, and 91.4°C, respectively, and the maximum heat transport capacity of three evaporators in the multi-evaporators was approximately 892, 921, and 956 W, respectively, with an evaporator length of 270 mm, an inclination angle of 60 deg, and an operating temperature of 80°C. Figure 10 demonstrated the impact of inclination angle on the maximum heat transport capacity in various operating temperatures. The results showed the heat pipe array had a higher maximum heat transport capacity at higher operating temperatures and there existed an optimum inclination angle. The optimum inclination angle was identified around

350



a) Le = 270 mm, = 75º

Fig. 9 Influence of heat flux on temperature distribution of evaporators.

b) Le = 270 mm, = 60º Fig. 7 Influence of heat flux on average thermal resistance at inclination angle of a) 75 and b) 60 deg.

Fig. 10 Influence of inclination angle on maximum heat transport capacity in various operating temperatures.

Fig. 11 Influence of inclination angle on average thermal resistance. Fig. 8 Influence of inclination angle on temperature difference between evaporator and condenser.

60 deg for all the measured operating temperature ranges. The maximum heat transport capacity observed was approximately 562, 786, 924, 963, and 1098 W, respectively, in the inclination angle range of 60–70 deg and operating temperature range of 40–80°C. Figure 11

shows the variation of average thermal resistance at different inclination angles in various operating temperatures with an evaporator length of 270 mm. The average thermal resistance had the same trend as the maximum heat transport capacity, as demonstrated in Fig. 10. The average thermal resistance was observed to be lower in the inclination angle of 60–75 deg and lower in higher operating temperatures.

351


optimum inclination angle of 60 deg. The length of the evaporation section had a significant influence on the maximum heat transport capacity and average thermal resistance. Increasing the length of the evaporator resulted in an increase of the maximum heat transport capacity and a decrease of average thermal resistance. Compared with conventional heat pipe, the heat pipe array generally presented a better overall thermal performance, especially in the case of higher operating temperatures. Generally, the heat pipe array had a good isothermality, an excellent operating stability, and higher overall thermal performance. Despite the advantages, the evaporator section sometimes showed temperature fluctuations and internal oscillation during the course of the experiment. These phenomena had a negative impact on its thermal performance, which should be an important aspect in future study of heat transfer enhancement. The mechanism of dryout phenomena of the multi-evaporator should also be clarified in later study.


Fig. 12 Influence of length of evaporator on maximum heat transport capacity.

The maximum heat transport capacity of the heat pipe array also depended on the length of the evaporators. It can be seen from Fig. 12 that maximum heat transport capacity increased by increasing of the length of evaporators at an operating temperature of 80°C and inclination angle of 60 deg. Figure 12 also indentifies that an inclination angle of 60 deg benefits the maximum heat transport capacity of the heat pipe array at more than 75 deg. Further study of the effect of heat flux on the average thermal resistance of the heat pipe array is shown in Fig. 7 with an inclination angle of 60 and 75 deg. It can be seen that, for the heat pipe array with evaporator length of 270 mm, the average thermal resistance decreased initially at a lower heat flux and reached a minimum point and then went up slightly with further power input for all the operating temperature cases. The operating temperature also had a great influence on the average thermal resistance. Lower operating temperature not only resulted in higher average thermal resistance, but also lower maximum heat transport capacity. The thermal resistance and maximum heat transport capacity were 0.44 °C∕W and 441 W, respectively, in the operating temperature of 40°C and were 0.19 °C∕W and 896 W in the operating temperature of 80°C. Comparing Figs. 7a and 7b, it is obvious that, whether the inclination angle was 60 or 75 deg, the average thermal resistance at first decreased significantly as the heat flux of evaporator section increased in the operating temperature of 40–80°C, reached a lowest value, and then increased slightly. There was an overall downward trend of the thermal resistance by increasing the operating temperature. Also, the average thermal resistance in inclination angle of 60 deg was smaller than in the inclination angle of 75 deg. Generally, the average thermal resistance was evaluated on overall thermal performance of the heat pipe array. Even if one of the evaporator tubes dried out, the other four evaporator tubes can still work well. Further study is needed to clarify the mechanism of dryout phenomena of the multi-evaporator.

Conclusions A unique heat pipe array with a multi-evaporator and an exocentric horizontal tube condenser has been developed and experimentally investigated on its overall thermal performance in this work. The heat pipe array has a prospect to be used extensively in the applications of solar heat collecting, multiple chip cooling, waste heat recovery systems, etc. The experimental results showed that the developed heat pipe array has a good thermal performance. The operating temperature had an active effect on the thermal performance. The maximum heat transport capacity increased and the thermal resistance decreased when operating temperature rises. The maximum heat transport capacity could reach 1098 W and, simultaneously, the minimum thermal resistance was 0.193 °C∕W in the operating temperature of 80°C and inclination angle of 60 deg. Comparing the impact of inclination angle on the thermal performance of heat pipe array, it clearly showed an

Acknowledgments The authors would like to acknowledge the support of the National Natural Science Foundation of China (51066004), and the financial support of Science and Technology Innovation Award Fund Projects. Invaluable comments from the anonymous referees are also gratefully acknowledged.

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