Pool boiling heat transfer to liquid nitrogen from porous metallic coatings of tube bundles and experimental research of hysteresis phenomenon

Pool boiling heat transfer to liquid nitrogen from porous metallic coatings of tube bundles and experimental research of hysteresis phenomenon Y. Xiulin, X. Hongji, Z. Yuweng and Q. Hongzhang Xi'an Jiaotong University, Xi'an, People's Republic of China Received. 30 June 19"88 According to heat and mass transfer mechanism in porous layers, the knowledge of nucleate pool boiling from sintered porous surfaces is reported. The boiling heat transfer coefficient of porous surfaces is enhanced approximately 3-10 times that of smooth surfaces. Through experiment, we obtained results showing that the distance between two tubes makes impact on boiling heat transfer of porous surfaces. We observed that there is a hysteresis phenomenon when porous surfaces are submerged in liquid nitrogen.

Keywords: heat transfer; nitrogen; hysteresis The augmenting of boiling heat transfer plays an important part in the utilization of low potential energy and the saving of energy. To enhance heat transfer behaviour, using a porous metallic matrix surface, is an active method in augmenting heat transfer rates. In recent years, high-performance sintered porous surfaces have been developed. They are used widely in industry. This paper reports on theoretical analysis and experimental research to enhance pool boiling heat transfer of cryogenic liquid with a sintered porous layer on the outside of a copper tube. We analysed the influence of some parameters on characteristics of heat transfer, and studied the hysteresis phenomenon when porous surfaces were submerged in liquid nitrogen. We also analysed the influence of distance between porous surfaces tubes on boiling heat transfer.

meter of PZ12a type with sensitivity of 0.1 /~V. The sensitivity of the voltage meter and amp meter were 0.3 mV and 0.5 mA, respectively. The measurements were taken in steady operation. Figure 1 shows heat flux q (W m-2) plotted over temperature difference A T (K) for porous and smooth test tubes. The liquid nitrogen used had a purity of 90.1590.18%, the principal impurities being oxygen, argon, CO2 and H2 O. From experimental results of Figure 1, the boiling heat transfer coefficient of the porous tube is higher than that of the smooth tube. With transporting the same heat flux, the superheat degree of the porous tube is smaller than that of the smooth tube. Especially in high flux, this is very significant. Because the numbers of bubble columns

Experimental results and analysis

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Characteristics of heat transfer with sintered tube surfaces compared with that of smooth tube.

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Saturated liquid nitrogen at normal pressure was used as medium. Test tubes with porous and smooth surfaces were vertical in pool boiling nitrogen. The heat transfer surface was sintered copper porous layer, the thickness of which was 0.4-0.5 mm. The porosity was approximately 40-50%, the mean pore diameter approximately 120/zm.

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Measurement of main parameters Temperatures of wall and fluid were measured with copper-constantan type thermocouples. Wall thermocouples were fixed on the wall of the tube and the thermocouples were calibrated. The electromotive force of the thermocouples was measured by a digital voltage 0 0 1 1 - 2 2 7 5 / 8 9 / 0 4 0 4 6 0 - 0 3 $03.00 © 1 9 8 9 Butterworth & Co (Publishers) Ltd

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Pool boiling heat transfer to liquid nitrogen: Yuan Xiulin et al.

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Figure 3 Heat flux q in pool boiling with nitrogen versus AT. /k, Distance between tubes 3 mm; O, distance between tubes 6 mm; x , distance between tubes 9 ram; ©, distance between tubes 12 mm

per sintered surface are larger than those of smooth tubes at the same heat fluxes, in the small temperature difference porous tube surfaces can form bubbles and start to microcircuit. This causes the heat boundary layer to become thin and decrease resistance of heat transfer. Furthermore, the evaporation of thin liquid layer with porous surface restrains the rise of wall temperature. The coefficient of boiling heat transfer of the porous tube is much higher than that of the smooth tube.

From the experimental results of Figure 3, in the certain distance ranges the smaller the distance between tubes, the larger the boiling heat transfer coefficient. Because of the small distance between tubes, the collision between the bubbles is violent and enhances convection heat transfer. The result of liquid brushing the wall surface makes the temperature of the porous layer bottom decrease. Thus, the mean boiling heat transfer coefficient is enhanced. If the distance between tubes is very small because of limited boiling space, this forms a vapour-thin layer of great resistance. It results in weakening the boiling heat transfer.

Influence of the distance between two tubes on boiling heat transfer of porous surfaces Characteristics of heat transfer on tube bundles compared with that on a single tube. Two porous tubes

and a single porous tube, respectively, were vertical in pool boiling nitrogen. Figure 2 shows heat flux q (W m - 2) plotted over temperature difference AT (K) for two porous tubes and a single porous tube. The liquid nitrogen used had a purity of 98.12-98.14%, the principal impurities being oxygen and argon. From the experimental results of Figure 2, the boiling heat transfer coefficient of the two porous tubes was higher than that of the single test tube. This was explained as follows: when the bubbles left the two porous tube surfaces, they collided with each other. This caused the bubbles to break; the liquid was agitated violently and brushed the wall surface, thus enhancing boiling heat transfer.

Influence of the distance between two tubes. Two porous tubes were vertical in P0ol boiling nitrogen with different distances between the tubes. Figure 3 shows heat flux q (W m-2) plotted over temperature difference AT (K) for different distances between tubes.

Influence of some parameters on boiling heat transfer

Influence of porous layer material The thermophysical properties of liquid match well that of the material of the porous layer. First, for example, if the bubble contact angle is less than 90 ° the liquid is matchable with the wettable surface. Second, the porous layer material must have thermoconduction characteristics as well. Influence of surface construction parameters. Pore diameter, do, influences the characteristics of heat transfer, because there are many kinds of mediums of different thermophysical properties matching different optimal values of do. However, influence of large or small do is not significant for some mediums. The porous layer thickness 6 also influences the boiling heat transfer, but so far there have been no consistent ideas regarding this. While the porosity of the porous layer of surfaces may not be controlled easily in this process, in general 40-60% is reasonable. Among three qualities of expressing surface characteristics there are optimal combinations. If the particle diameter is small, many evaporating cores of the surface

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Pool boiling heat transfer to liquid nitrogen." Yuan Xiulin et al. and thin capillary tunnels are formed. The smaller the particle diameter, the smaller the diameter of tunnels of capillary pore. This increases the flowing resistance of vapour and liquid and prevents their circuit. The sintered layer's thickness must not be too large nor too small: if the thickness is too large, this can make capillary tunnels become curved and long and increases flowing resistance of the vapour and liquid. If the thickness is too small, this means that capillary tunnels will not connect with each other very well. If the particle's diameter and sintered layer's thickness reach optimization, the larger the porosity of porous layer, the better the boiling heat transfer, but this is limited by the particle's diameter and sintered layer's thickness.

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T (K) Studies on heat transfer in the range of developed bubble-boiling in liquid nitrogen under pressure give q-AT curves. A test tube with a porous surface is vertical in pool boiling nitrogen with normal pressure. At first AT rises with increasing q as normal. When q is not high, i.e., point 1, the heat flux density is reduced, and after 5 min point 2 has been reached. Then points 3 and 4 are passed, and the initial curve is reached again; Figure 4 shows the heat flux density as a function of temperature difference. Bergles 2 observed a quite similarly occurring phenomenon in pool boiling Rl13 with a porous tube; Figure 5 z shows this hysteresis phenomenon. From observing Figures 4 and 5, the direction of hysteresis curves is different. By thorough purification of liquid R 113 with increasing heat flux q, temperature difference AT increases, and active centres of bubbles increase. Due to the internal generation of vapour, there is less tendency for the vapour to spread over the surface and activate other sites. When heat flux q decreases, these active centres continue to act on heat transfer. Therefore, the direction of the hysteresis loop in Figure 5 must be anticlockwise. However, the direction of the hysteresis loop in Figure 4 is clockwise. In our tests, liquid nitrogen contained impurities such as CO 2 and H20. In certain conditions, these impurities formed a solid thin coating on the heat transfer surface, and caused heat resistance. When the wall temperature increased and reached a certain value, the thin coating vanished. When heat flux q decreased, the thin coating formed and the thickness of the coating and heat resistance also increased. Therefore, the direction of the hysteresis loop in Figure 4 is clockwise. This hysteresis influence, due to particles suspended in the liquid, may cause serious trouble in the case of long term applications.

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The coefficient of boiling heat transfer with porous tube surfaces is enhanced about four times that of smooth tube surfaces for liquid nitrogen. It is also well established that the average boiling characteristics of tube bundles are considerably different from those expected from single

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tube tests. In certain ranges of distance between two tubes, the smaller the distance between tubes, the larger the boiling heat transfer coefficient. There is a hysteresis phenomenon when heat flux q increases and then decreases. From observing Figure 4, when heat flux q is decreased AT is also decreased uniformly. The simplest way of avoiding the problem is to apply a high heat flux or large temperature difference to initiate boiling. References Bewilogua, L. etal. Heat transfer in liquid hydrogen Cryogenics (1974) 14 516-517 2 Bergles, A.E. et al. Characteristics of nucleate pool boiling from porous metallic coatings A S M E J Heat Transfer (1982) 104 279-285 3 Kosky, P.G. and Lynn, D.N. Pool boiling heat transfer to cryogenic liquids AICHE Journal (1968) 14 372-379 4 O'Neill, P.S. et al Novel heat exchanger increases cascade cycle efficiency for natural gas liquifaction Ado Cryog Eng (1972) 17 420-437 1

Conclusions

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