Dynamics of explosive boiling and third heat transfer ... - Springer Link

2 downloads 0 Views 453KB Size Report
Results of experimental studies on dynamics of explosive boiling and third heat transfer crisis under the conditions of liquid subcooling are presented for the ...
Thermophysics and Aeromechanics, 2017, Vol. 24, No. 4

DOI: 10.1134/S0869864317040047

Dynamics of explosive boiling and third heat transfer crisis at subcooling on a vertical surface 1

B.P. Avksentyuk and V.V. Ovchinnikov

2

1

Vinnytsa Institute of Trade and Economics of Kyiv National University of Trade and Economics, Vinnitsa, Ukraine

2

Kutateladze Institute of Thermophysics SB RAS, Novosibirsk, Russia

E-mail: [email protected] (Received October 5, 2016; in revised form November 17, 2016) Results of experimental studies on dynamics of explosive boiling and third heat transfer crisis under the conditions of liquid subcooling are presented for the vertical arrangement of the heat-transfer surface. Acetone was used in experiments at the pressure in the working volume from 20 to 46 kPa and subcooling from 0 to 20 K. The studied processes were recorded. Data on the velocity of evaporation front propagation at liquid subcooling were obtained. These data are compared with the results of calculations according to the models available in the literature. The effect of liquid subcooling on the regions of regime parameters corresponding to explosive boiling and third heat transfer crisis is studied. Key words: boiling, subcooling, evaporation front, heat transfer crisis.

Introduction The homogeneous theory of boiling [1, 2] determines the upper limit for liquid superheating before boiling up. This theory considers only the stage of vapor bubble nucleation. Under the real conditions, as a rule, heterogeneous nucleation takes place. Even when the experimental values of superheating before boiling up agree with the results of calculations based on the homogeneous boiling theory, the photographs of boiling process argue about the heterogeneous nature of nucleation [3]. At heterogeneous boiling, the vapor bubbles are formed on the ready vaporization centers, i.e., depressions on the heat-transfer surface filled with gas. The ready vaporization centers cause a significant decrease in the values of liquid superheating in the case of heterogeneous boiling in comparison with limiting superheating. Vaporization on the ready sites occurs in the form of bubbles, i.e., there is nucleate boiling regime on the heat-transfer surface. Nucleation boiling is widely used in various fields of industry (power engineering, rocket and laser technology, chemical and refrigerating industry, cryogenic engineering, microelectronics, metallurgy, etc.) as the most effective way of removing high heat flux densities at relatively low temperatures of the heat-transfer surfaces. However, high intensity of heat removal at nucleate boiling is kept only up to certain (critical) values of heat flux density, when a vapor film is formed on the heat-transfer surface. Violation of the contact between liquid and heating surface leads to sharp deterioration in heat transfer with subsequent destruction of the heat-transfer  B.P. Avksentyuk and V.V. Ovchinnikov, 2017

537

B.P. Avksentyuk and V.V. Ovchinnikov

surface. The trouble-free operation of devices with a liquid coolant requires a range of operating parameters that exclude the possibility of heat transfer crisis. The heat transfer crisis that occurs after nucleate boiling has been termed the “first heat transfer crisis”. The heat flux density, when this crisis arises is called the first critical heat flux density qcr1. Many experimental and theoretical studies are devoted to this type of heat transfer crisis. Some time ago, it was believed that in liquids that wet the surface well, the heat transfer crisis occurs only after nucleate boiling, until it was found in the experiments on organic liquid boiling on the thin wires at pressures below 0.3 bar [4–6] that the heat transfer crisis appeared after single-phase convection. The authors of [4, 5] associated the absence of nucleate boiling with high superheating before boiling up and large diameters of bubble at subatmospheric pressures in comparison with the atmospheric pressure. According to these works, such heat transfer crisis occurs according to the following scheme. In liquids that wet surface well, the vapor bubble is separated from the heat-transfer surface by a liquid microlayer. A growing vapor bubble envelops a thin wire and isolates the microlayer of liquid from its main mass. As a result, liquid under the bubble evaporates. In the region of the formed dry spot, the temperature of heat-transfer surface increases due to deterioration of heat transfer, and this leads to an increase in the dry area. After dry area spreading over the entire heat-transfer surface, the regime of film boiling is formed in the working section. Thus, the primary cause of vapor film formation for this type of heat transfer crisis is heat diffusion in the heater. Experimental studies using high-speed video recording [7–11] have shown that at high levels of wall liquid metastability before boiling up, a different mechanism of stable vapor film formation occurs after the regime of single-phase convection due to the loss of stability by the bubble structure of vaporization. At superheating of the heat-transfer surface Tw relative to saturation temperature Tsat before boiling up ∆Tb = (Tw – Tsat ), exceeding threshold values ∆Tthr, perturbations are generated on the vapor bubble surface near its base due to instability development. These perturbations grow in the metastable wall liquid with a constant rate, which is an order higher than the rate of heat propagation in the heater and rate of liquid evaporation. To denote the frontal part of vapor formation growing at a high rate (up to tens meters per second) along the heat-transfer surface, the authors of this work introduced the term “evaporation front”. Due to the high velocities of evaporation front propagation, the process of vaporization at high levels of metastability has been termed “explosive boiling” [12]. A vapor cavern formed due to evaporation front propagation isolates the heat-transfer surface from liquid, which results in the heat transfer crisis. Formation of a vapor film is caused by the action of reactive force on the liquid-vapor interface. For this type of crisis, the term “third heat transfer crisis” was introduced [13]. The heat flux density corresponding to this crisis beginning is called the third critical heat flux density qcr3. Dynamics of explosive boiling was also investigated in [14–19]. Since the velocity of evaporation front propagation Vf exceeds the velocity of heat diffusion in the heater by the orders of magnitude, the transition from single-phase convection to film boiling according to the mechanism considered in [4, 5] is possible only at superheating before boiling up below threshold values ∆Tthr, when the evaporation fronts are not generated. The ranges of regime parameters, where the third heat transfer crisis arises under the saturation conditions, were determined in [7, 8]. The heat flux densities for this heat transfer crisis can be substantially lower than the critical densities of heat fluxes at nucleate boiling qcr1. The effect of liquid subcooling to the saturation temperature on this phenomenon was studied in [20]. The third heat transfer crisis was studied experimentally mainly with horizontal location of the heating surface. Only in [21], the studies of the third heat transfer crisis under the saturation conditions were carried out with vertical arrangement of the heating surface. The results of experimental studies of subcooling influence on dynamics of explosive boiling and occurrence of the third heat transfer crisis at vertical arrangement of the heat-transfer surface are presented below. 538

Thermophysics and Aeromechanics, 2017, Vol. 24, No. 4

1. Measurement methods A stainless steel tube with the external diameter of 2.5 mm, wall thickness of 0.5 mm, and length of 80.5 mm was used as the working section in experiments. The finish of the heattransfer surface corresponded to class 9–10. Inside the tube, there was a thermocouple, whose “hot” junction was in the central part of the working section. Based on indications of this thermocouple, the temperature of the heat-transfer surface was determined, taking into account the corrections for the temperature difference in the heater wall. To obtain data on a change in the average temperature of the working section over the tube volume in time, the working section was used as a resistance thermometer. Dependence of electrical resistance of the working section on the temperature was previously determined under the steady-state conditions. Calibration was carried out with the horizontal location of the tube. By changing the average temperature of the working section over the tube volume, the characteristic times of development of the third heat transfer crisis were determined, and its occurrence was recorded. Quasistationary heat release at the working section was performed by passing the direct current. In liquid and vapor region, the thermocouples were mounted to measure the temperatures of vapor and liquid in the working volume. The error in measuring the temperature by thermocouples did not exceed 0.5 K; the error of measuring the temperature through the electrical resistance of the working section was below 1.5 K. The experiments were carried out with acetone of the grade “chemically pure” under the conditions of natural convection with vertical location of the working section. To ensure the specified levels of liquid metastability before boiling, the boiling-up initiator was used, which was a platinum wire of 100 μm in diameter and 6 mm in length, located near the heattransfer surface at the distance of 10 mm from the lower edge of the working section. Before the start of evaporation process, the stationary regime of single-phase natural convection was achieved on the working section at a given value of the heat flux density. Then, the heat flux density, surface temperature of the working section, temperature of vapor and liquid, and pressure in the working volume were measured. The electric pulse was sent to the boiling-up initiator automatically, when the temperature of heat-transfer surface reached the programmed level. This temperature was determined by the readings of thermocouple placed at the center of the working section. The acoustic pressure sensor located in the working volume recorded vaporization beginning, when the voltage was applied to the boiling-up initiator. The time delay for vaporization on the initiator was of the order of millisecond. The measurements after boiling up were carried out using a 14-bit ADC with time resolution from 5 μs per channel. The heat flux density qb was measured before the initiator of boiling up was turned on. During the experiment, the video and photographs of the boiling process were made. The acoustic sensor signal at boiling up was used to turn on an impulse lamp, near which the photosensor was located. The signals of the photosensor and pressure sensor were used to determine the delay time of recording relative to vaporization beginning. The speed of video shooting was 600 fps; the exposure time was 0.4 ms. When the velocity of interface was less than 5 m/s, the accuracy of velocity determination was 5%; at the velocity of about 20 m/s, the error increased up to 15%. The pressure in the working volume was measured by a standard vacuometer of the class index number 0.6. The experiments were carried out at the pressure in the working volume from 20 to 46 kPa and liquid subcooling to the saturation temperature from 0 to 20 K. Liquid subcooling was achieved by adding air into the vapor region of the working volume. Saturation temperature Tsat was calculated by the pressure in the working volume with the accuracy of about 0.3 K. Subcooling ∆Tsub was defined as the difference between the saturation temperature and the temperature of liquid in the working volume: ∆Tsub = (Tsat – Tl ). In order to achieve the levels of liquid metastability required for studies, the vaporization processes were considered on the heat-transfer surface depleted by the ready vaporization centers. The method for producing such surfaces is presented in [20]. 539

B.P. Avksentyuk and V.V. Ovchinnikov

2. Experimental results and discussion High-speed video recording made it possible to trace the effect of liquid subcooling in the working volume to saturation temperature on dynamics of explosive boiling and the process of vapor film formation at the third heat transfer crisis on the vertical heat-transfer surface. The video frames of the third heat transfer crisis beginning at subcooling of 9.6 K and 115 K superheating of the heat-transfer surface relative to the saturation temperature before boiling up, which is higher than the threshold value for the third heat transfer crisis, are shown in Fig. 1. Each frame shows the time from the moment of boiling-up initiator turning on to

Fig. 1. Video frames of the third heat transfer crisis beginning. 2

Acetone; Тsat = 304 K, ∆Тb = 115 K, qb = 58 kW/m , ∆Тsub = 9.6 K.

540

Thermophysics and Aeromechanics, 2017, Vol. 24, No. 4

shooting beginning. In the frames, the light background is liquid, vertical dark strip is working section with the scale marks on the left. The acoustic sensor recorded boiling up in 5.08 ms after the boiling-up initiator was turned on (frame 1). Frames 2 and 3 show explosive boiling — evaporation front propagation up along the heat-transfer surface with the constant velocity of 15.6 m/s. High rates of vapor formation growth determine the explosive nature of boiling. It can be seen in frames 4 and 5 that the vapor structure formed at explosive boiling consists of two parts: one part is formed as a result of the growth of the initial bubble, and another is formed due to evaporation front spreading. The vapor formation isolated the heat-transfer surface from liquid. Then, for about 5 ms, the transverse dimension of vapor formation grew (frame 4), then the stage of its decrease occurred (frames 5 and 6). The maximal reduction in the transverse dimension took place in the transition zone from the part of vapor formation originating from the initial bubble to the part that arose because of the evaporation front propagation. The process of partial destruction of the vapor film is traced in frames 7–14. It can be seen in frame 15 that on the lower and central parts of the heat-transfer surface, heat transfer was carried out in the regime of nucleate boiling, and on the upper part, it occurred in the regime of film boiling. When nucleate and film boiling regimes coexisted on the heat-transfer surface, it was difficult to register the heat transfer crisis by an increase in the average temperature of the working section, since the temperature increase in the film boiling zone was compensated for by its decrease in the zone of nucleate boiling. In these cases, the threshold values of the third heat transfer crisis were determined by the beginning of formation of the stable centers of film boiling. These processes were registered by high-speed video. The use of boiling-up initiator and working section with the heat-transfer surface depleted by the ready vaporization centers made it possible to investigate the effect of subcooling on vaporization dynamics at specified values of superheating before boiling up. Data on propagation velocity of evaporation front obtained at the pressures from 28 to 46 kPa in the working volume under the conditions of saturation and subcooling with vertical arrangement of the heattransfer surface, are shown in Fig. 2. Data of [11] for the case of horizontal arrangement of the heat-transfer surface are also given there for comparison. Data scattering is caused by the effect of pressure on evaporation front velocity and measurement error. A change in orientation of the heat-transfer surface relative to the gravitational forces and a change in liquid subcooling to the saturation temperature in the working volume did not have a significant influence on propagation velocity of the evaporation front. The results of calculations by the models for evaporation front suggested in [22–24] are also presented in Fig. 2. The calculated

Fig. 2. Evaporation front velocity vs. superheating before boiling up. Acetone; pressure 28−46 kPa; vertical working section: saturation (1), subcooling 4 (2), 8 (3), 10 (4) K; horizontal working section [11]: saturation (5), subcooling 18 K (6); calculations by models of [23] (7), [24] (8), [22] (9).

541

B.P. Avksentyuk and V.V. Ovchinnikov Fig. 3. Threshold values of heat flux densities for the fronts of evaporation (3, 5) and third heat transfer crisis (1, 2, 4) depending on subcooling. Acetone; pressure 20−46 kPa; vertical working section: 1–3; horizontal working section [11]: 4 and 5; 6  first critical heat flux density (calculation by formula of [25]).

curves for each model were plotted for the pressures of 28 and 46 kPa. Depending on the presence of ready vaporization centers on the heat-transfer surface, explosive boiling and third heat transfer crisis occur in a wide range of superheating before boiling up  from the threshold value of superheating to the ultimate superheating of liquid. In this connection, the effect of subcooling was studied for the lower boundary of explosive boiling and third heat transfer crisis. Data on the effect of subcooling on the threshold values of heat flux densities for evaporation fronts (symbols 3) and third heat transfer crisis (symbols 1 and 2) with vertical location of the heat-transfer surface are shown in Fig. 3. Experimental investigation of the threshold values is a rather laborious process, since several experiments (in some cases up to ten) are required to measure one threshold value with the required accuracy. In addition, after each experiment, the superheating of the heat-transfer surface before boiling is significantly reduced because of an increase in the number of ready vaporization centers due to heterogeneous boiling. To achieve high values of superheating before boiling up, it is necessary to burn-in the surface after each experiment to reduce the number of ready vaporization centers. The threshold values of the heat flux density for evaporation fronts are below the threshold values of the heat flux densities for the third heat transfer crisis (Fig. 3). Explosive boiling is a necessary, but not sufficient condition for generation of the third heat transfer crisis, when a vapor structure is formed along the heat-transfer surface due to evaporation front propagation. This vapor formation isolates the heat-transfer surface from liquid in the working volume. A sufficient condition is the preservation of the vapor film formed because of evaporation front propagation after partial destruction of a vapor formation at subcooling. The threshold values for the third heat transfer crisis, diagnosed by formation of the stable sites of film boiling with the help of high-speed video recording, are marked by symbol 1 in Fig. 3. Symbol 2 indicated the threshold values for the third heat transfer crisis, diagnosed by an increase in the average temperature of the working section. At subcooling exceeding 5 K, both measurement methods gave close values. The figure also shows the results of calculations of the first critical heat flux density depending on subcooling by the formula of [25]. It can be seen that under the subcooling conditions, the threshold values of the heat flux densities at the third heat transfer crisis are significantly lower than the values obtained for the first heat transfer crisis. Thus, the studies on dynamics of explosive boiling and third heat transfer crisis on a vertical surface showed that a change in orientation of the heat-transfer surface relative to the gravitational forces does not significantly affect evaporation front dynamics and threshold values of superheating at saturation and subcooling. The lower threshold values of the heat flux density in comparison with the horizontal location are caused by peculiarities of heat transfer in the regime of single-phase natural convection with vertical location of the heat-transfer surface. 542

Thermophysics and Aeromechanics, 2017, Vol. 24, No. 4

References 1. M. Volmer, Kinetik der Phasenbildung, Steinkopf Verl., Dresden, Leipzig, 1939. 2. Ya.I. Frenkel, Kinetic Theory of Liquids, Nauka, Leningrad, 1975. 3. E.N. Sinitsyn, Concerning interpretation of experiments on superheated liquid boiling in the glass capillaries, Thermal-Physical Properties of Metastable Systems, Ural Branch of the Academy of Sciences of the USSR, Sverdlovsk, 1984, P. 61−67. 4. S.J.D. van Stralen, Heat transfer to boiling binary liquid mixtures at atmospheric and sub-atmospheric pressures, Chem. Engng Sci., 1956, Vol. 5, P. 290−296. 5. J.H. Lienhard and V.E. Schrock, The effect of pressure, geometry and the equation of state upon the peak and minimum boiling heat flux, Trans. ASME, Ser. C, J. Heat Trans, 1963, Vol. 85, No. 3, P. 261−272. 6. C.J. Rallis and H.H. Jawurek, Latent heat transport in saturated nucleate boiling, Int. J. Heat Mass Transfer, 1964, Vol. 7, No. 10, P. 1051−1068. 7. B.P. Avksentyuk, G.I. Bobrovich, S.S. Kutateladze, and V.N. Moskvicheva, The generation of nucleate boiling conditions under conditions of free convection, J. Appl. Mech. Tech. Phys., 1972, No. 1, P. 59−62. 8. B.P. Avksentyuk and N.N. Mamontova, Characteristics of heat-transfer crisis during boiling of alkali metals and organic fluids under free convection conditions at reduced pressure, Progress in Heat and Mass Transfer, Pergamon Press, Oxford, New York, 1973, Vol. 7, P. 355−362. 9. S.S. Kutateladse and B.P. Avksentyuk, Heat transfer crises in liquid helium, Cryogenics, 1979, Vol. 19, No. 5, P. 285−288. 10. B.P. Avksentyuk and V.V. Ovchinnikov, A study of evaporation structure at high superheating, Russian J. Engng Thermophysics, 1993, Vol. 3, No. 1, P. 21−39. 11. B.P. Avksentyuk and V.V. Ovchinnikov, Explosive boiling and transient regimes, J. Engng Thermophysics, 2003, Vol. 12, No. 2, Р. 99−130. 12. J.E. Shepherd and B. Sturtevant, Rapid evaporation at the superheat limit, J. Fluid Mech., 1982, Vol. 121, P. 379−402. 13. B.P. Avksentyuk and S.S. Kutateladze, Unstable conditions of heat transfer on surfaces depleted with respect to vaporization centers, High Temperature, 1977, Vol. 15, No. 1, P. 96–101. 14. S.A. Zhukov and V.V. Barelko, Dynamic and structural aspects of the processes of single-phase convective heat transfer metastable regime decay and bubble boiling formation, Int. J. Heat Mass Transfer, 1992, Vol. 35, No. 4, P. 759−775. 15. J. Mitrovic and J. Fauser, Propagation of boiling fronts along horizontally arranged heated tubes, Chemical Engng Research and Design, 2001, Vol. 79, Iss. 4, P. 363−370. 16. K. Okuyama, J.H. Kim, S. Mori, and Y. Iida, Boiling propagation of water on a smooth film heater surface, Int. J. Heat Mass Transfer, 2006, Vol. 49, Iss. 13, 14, P. 2207−2214. 17. A.N. Pavlenko, E.A. Tairov, V.E. Zhukov, A.A. Levin, and A.N. Tsoi, Investigation of transient processes at liquid boiling under nonstationary heat generation conditions, J. Engng Thermophysics, 2011, Vol. 20, No. 4, P. 380−406. 18. B.G. Pokusaev, D.A. Nekrasov, and E.A. Tairov, Modeling of boiling of subcooled water and ethanol under conditions of pulsed heat generation in a wall, High Temperature, 2012, Vol. 50, No. 1, P. 84−90. 19. V.E. Nakoryakov and S.Y. Misyura, Bubble boiling in droplets of water and lithium bromide water solution, J. Engng Thermophysics, 2016, Vol. 25, No. 1, P. 24−31. 20. B.P. Avksentyuk and V.V. Ovchinnikov, Third heat transfer crisis at subcooling, Thermophysics and Aeromechanics, 2008, Vol. 15, No. 2, P. 267−274. 21. B.P. Avksentyuk and V.V. Ovchinnikov, Investigation of the third heat transfer crisis on a vertical surface, Thermophysics and Aeromechanics, 2012, Vol. 19, No. 1, P. 101−107. 22. A.N. Pavlenko and V.V. Lel’, Approximate model for calculation of self-sustaining evaporation front, Thermophysics and Aeromechanics, 1999, Vol. 6, No. 1, P. 105−118. 23. B.P. Avksentyuk, Dynamics of Explosive Boiling, Int. J. Fluid Mechanics Research, 2001, Vol. 27, No. 5, P. 587−610. 24. S.P. Aktershev and V.V. Ovchinnikov, Model of steady motion of the interface in a layer of a strongly superheated liquid, J. Appl. Mech. Tech. Phys., 2008, Vol. 49, No. 2, P. 194−200. 25. S.S. Kutateladze, Fundamentals of Heat Transfer, Academic Press and Arnold, New York, 1963.

543