Mist cooling of hot metals coated with a thin insulating material

Mist cooling of hot metals coated with a thin insulating material Yoshihiro Kikuchi, Jingchun Min and Toshiki Yamanaka Department of Mechanical Engineering, Hiroshima University, 4-1, Kagamiyama 1-Chome, Higashi-Hiroshima, 724, Japan

Received 31 March 1994; accepted 15 September 1994 An experimental study was conducted to investigate transient heat transfer of mist (water-air) cooling of hot metals coated with a thin layer of insulating (low thermal conductivity) material. The test specimen selected for the present experiment was a silver disk (30 mm in diameter, and 10 mm thick) whose heat transfer surface was coated with a refractory paint. The mist flow impacted vertically onto the heat transfer surface. The paint coating produced a great enhancement in heat transfer since an earlier transition from film to transition boiling occurred. The minimum heat flux and temperature became higher with increasing coating thickness.

Keywords: mist cooling; heat transfer; coating thickness Nomenclature mp c

D G g hg

m mp

q T Tsat

t V A 6 )tc Pa

surface area of table-tennis ball drag coefficient specific heat of silver disk diameter of silver disk mass velocity of water gravitational acceleration gap conductance between coating and silver mass of silver disk mass of table-tennis ball heat flux temperature saturation temperature time linear velocity of air (or droplet) angle defined in Figure 4 difference thickness of coating thermal conductivity of coating density of air

Subscripts max point min point min,b point ms silver w wall

of maximum heat flux of minimum heat flux of minimum boiling heat flux surface

Introduction The process of cooling a high-temperature substance with a mist flow is widely encountered in metallurgy, cryogenic engineering, nuclear technology and other industrial applications. Heat transfer characteristics of mist cooling are affected by many factors, including

Correspondence to Yoshihiro Kikuchi 0261-3069/94/050269-06 © 1994 Butterworth-Heinemann Ltd

geometry, size, surface condition and thermal conductance of the surface material; properties, pressure, subcooling, velocity of coolant mixture; and time dependency of heat load 1. Attention should be given to the effect of thermal conductance of surface materials on the cooling process for controlling the rate of cooling, which is an important factor in the processing of metals. Several investigators2 8 have reported that for rapid cooling of metals coated with a thin insulating layer in boiling liquid an earlier onset of transition boiling occurs at a significantly higher minimum film boiling temperature (Train,b) than that predicted by the hydrodynamic model of Berenson9 or by the maximum liquid superheat theory of Spiegler et al. 1°. N o theory has yet been found, except the theoretical study of Kikuchi et al. 11 13, which explains the successful augmentation of Tm~n,b. Here, they assumed the occurrence of local and intermitten liquid-solid contacts in the film boiling regime in predicting the actual Train,b for the coated metal. The calculated results agreed well with the experimental data for Teflon-coated copper cooled in saturated liquid nitrogen and refractory paint-coated silver in saturated water, all under atmospheric pressure. In order to apply the intermittent liquid-solid contact model to mist cooling which is more useful for processing metals an experimental study has been conducted with a paint-coated silver disk to be cooled in a mist flow of a water-air mixture under atmospheric pressure. This paper gives the experimental results of the effect of coating thickness on the cooling process, with particular emphasis on heat transfer characteristics, minimum heat flux and its corresponding temperature. In addition, the effect of mass velocity of water on the cooling process will be discussed. Materials & Design Volume 15 Number 5 1994

269

Mist

cooling

of coated

metals:

Y. K i k u c h i

et al.

Experimental apparatus and procedure A schematic diagram of the experimental apparatus is shown in Figure 1. This contains a water system, an air system and a test section. Water is delivered from a reservoir (water tank) by a pump and is mixed with air at the spray nozzle. The air delivered by a compressor is regulated by a valve. A mist flow of a water-air mixture enters through an aperture plate and reaches a test specimen. In order to obtain a uniform velocity distribution of mist flow, the distance between the spray nozzle and the test specimen is maintained at approximately 500 mm. Figure 2 gives a detailed description of the test section. An aperture plate is set at the location 10 mm upstream of the test specimen because the mist flow does not directly impact onto the thermocouple leads. The test specimen is a silver disk (30 mm in diameter and 10 mm thick) whose heat transfer surface is coated with a thin refractory paint of a thickness of 3-20 ktm. The thermal conductivity and specific heat of the paint are 1.2 W/InK and 2.1 kJ/kgK, respectively. In order to measure the temperature of the silver disk two Chromel-Alumel thermocouples (1.0 mm in diameter) are buried with solder into holes, which are within 2.0 mm and 8.0 mm, respectively, of the surface in contact with the mist flow. An electric furnace is used to

heat the test disk to a required temperature of approximately 750°C. The heated disk is plunged vertically into the mist flow and is cooled to the saturation temperature of water under atmospheric pressure. The output signals from the thermocouples are recorded by a personal computer through an A/D converter. During the runs the mist is maintained at 20+_2°C under atmospheric pressure. An immersion method is used to measure the diameter of the water droplets which collect in a small oil bowl within a few seconds. A photograph of droplet population is shown in Figure 3. Several large droplets can be observed in a population of many small ones. The average diameter is estimated to be 35 l~m, which is obtained from a population of approximately 1000 droplets. The mass velocity of water is measured by accumulating it for a certain period with a glass tube of 28 mm inside diameter. The linear velocity of air (or water droplet) is obtained by the following method. A tabletennis ball is suspended by a thread in the mist flow, as shown in Figure 4. According to the balance of forces acting on the ball, the velocity of air may be derived as

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Materials & Design V o l u m e 15 N u m b e r 5 1994

mp9

Figure 4 Measurement of the linear velocity of the mist flow

.

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M i s t cooling o f coated metals: Y. Kikuchi et al. 800 , .

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(2)

where I dT/dtl is the temperature change per unit time. Because the coating is extremely thin (less than scores of micrometres) the heat capacity of the coating layer is negligible and the heat flux at the coating surface is therefore equal to that passing through the silver surface in contact with the coating layer. The temperature distribution in the coating layer is assumed to be linear. The heat transfer surface temperature, Tw, can be calculated by

w: ms

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The heat flux, qms, at the silver surface is obtained using the lumped-parameter method since the thermal conductivity of silver is very high and the heat flux is comparatively low, i.e. the Biot number is very small (Bi
7rD2

1

600

Coating thickness, 8:0-20 gm Average diameter of droplet, d32:2043 gm Mass velocity of water, G: 0.01-0.1 kg/m2s Linear velocity of air, V: 1.0-2.0 m/s

_ 4mc d__~t

i

(1)

where the air velocity, V, is assumed to be the linear velocity of the water droplet since the average diameter of the droplet is very small. The drag coefficient, CD, is also assumed to be equal to the measured value in single-phase air flow because the liquid fraction (10_5 to 104) of the mist flow is extremely low. The experimental conditions are as follows:

qms

I

(3)

where hg is the gap conductance between coating and silver and is taken as 5.0 x 104 W/mZK1]A2.

Experimental results and discussion Figure 5 shows typical measured temperature changes during cooling of the silver disk for four kinds of coating thickness, 8. The abscissa is the elapse of time, t, and the ordinate is the temperature of the silver, T. The mist flow conditions are V = 1.1 m/s and G -- 0.03 kg/mZs. The paint coating produces a great enhancement in heat transfer since the cooling time for coated silver is shorter than that for uncoated silver (8 = 0). This tendency is more accentuated by increasing the thickness of the coating. Figure 6 shows the effect of coating thickness on heat transfer characteristics of mist cooling. The mass veloc-

ities of water are (a) G -- 0.03 kg/m2s and (b) G = 0.09 kg/mZs. The abscissa is wall superheat, ~ T s a t ( = Tw-Tsat) and the ordinate the wall heat flux, qwFor uncoated or thinly coated silver the heat transfer characteristics are similar to so-called pool-boiling curves and have a maximum heat flux point (qmax and ATmax) and a minimum heat flux point (qmin and ATtain). In the nucleate boiling region of ATsat8 gm. In addition, the qw increases over all the range of AT,at with increasing the mass velocity of water, G. The solid line in Figure 6 indicates the experimental results of air-cooling heat transfer which are obtained with no water (G = 0). It can be seen that the contribution of air cooling to total heat transfer is small. Local turbulence due to the introduction of water droplets into the air flow is assumed to be negligible because the volume fraction of liquid is extremely low M a t e r i a l s & Design V o l u m e 15 N u m b e r 5 1994

271

Mist cooling of coated metals: Y. Kikuchi et al. 10s

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Effect o f coating thickness on the heat transfer characteristics of mist cooling. (a) G = 0.03 kg/m2s; (b) G = 0.09 kg/m2s

duration and larger area of local L-S contacts which are accompanied by droplet impingement. As mentioned above, the effect of paint coating on the enhancement in heat transfer may be attributed to the fact that the qmin and Tmin become higher with an increase in the coating thickness, 8, and that the transition from film to transition boiling occurs earlier during cooling. The qmin and Tminare therefore indicative of the general level of augmentation in heat transfer. The qmin and ATtain, which are obtained for various coating thicknesses, 8, and mass velocity of water, G, are combined

in the mist flow. We therefore introduce the boiling heat flux, qw,b which is defined as the difference between the total heat flux, qw, of mist cooling and the heat flux of air cooling (the solid line in Figure 6). Figure 7 shows plots of the calculated value of qw,b against the superheat, ATsat. It can be seen that the dependency of qw,b o n ATsa t is stronger in the transition boiling region and weaker in the film-boiling one. In film boiling the coated silver has a much larger qw,b than the bare silver since the low thermal conductivity and good wettability of the paint coating cause the longer ........

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Materials & Design Volume

15 N u m b e r

5 1994

Mist cooling of coated metals: Y. Kikuchi et al. 3.0

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in Figure 8. Only the experimental data for ~<8 gm are shown in this figure since it is difficult to stipulate the qmin for g>8 I-tm. The qmin becomes higher with increasing G owing to an increase in the amount of droplets colliding with the heat transfer surface. The ATm~. also rises with increasing G. Moreover, both the qmin and

increase with increasing 5 since greater falls in temperature occur on the local surface due to L-S contacts and an earlier transition from film to transition boiling is caused during cooling. Figure 9 shows the minimum boiling heat flux, qmin,b and its corresponding superheat, ATtain,b which are ATmi n

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Figure 9 Effect of the mass velocity of water on the m i n i m u m boiling heat flux and temperature of mist cooling. (a) Mi ni mum boiling heat flux; (b) superheat corresponding to the m i n i m u m boiling heat flux

Materials & Design V o l u m e 15 N u m b e r 5 1994

273

Mist cooling of coated metals: Y. Kikuchi et al. obtained by subtracting the air-cooling heat flux from the total heat flux. The qrnin,b is always much smaller than the relevant qmin while the ATmin,b is usually higher than the relevant ATmin. Both qmin,b and ATmin,b increase with increasing g since the L - S contacts cause greater falls in temperature for longer periods, as described above. It is interesting that the ATmin,b is barely influenced by the change in G. M o r e detailed information on microcharacteristics will be needed for understanding this fact, especially the boiling mechanism of droplets.

Conclusions A paint-coated silver disk was cooled with a mist flow of an air-water mixture under atmospheric pressure. The effects of coating thickness and mass velocity of water on heat transfer characteristics, with particular emphasis on minimum heat flux, qmin, and its corresponding temperature, Train, have been investigated. The following can be concluded: 1.

2.

274

The paint coating produces a great enhancement in heat transfer since the qmin and Tmin become higher with an increase in the coating thickness. The Train increases as the mass velocity of water increases. However, the temperature Tmin,b, corresponding to the minimum boiling heat flux remains constant.

Materials & Design Volume 15 Number 5 1994

Acknowledgements The authors wish to acknowledge the technical contributions of Messrs M. Sako, T. K u r a m o t o and T. Tagawa at all stages of the experiments.

References 1 Carbajo, J.J. Nucl. Eng. Design 84 1985, 21-52 2 Sato, S. Kinzoku No Kenkyu 10 1933, 63-71 (in Japanese) 3 Cowley, C.W., Timson, W.J. and Sawdye, J.A. Adv. Cryogenic Eng. 7 1962, 385-390 4 Butler, A. P., James, G. B., Maddock, B. J. and Norris, W. T. Int. J, Heat Mass Transf 13 1970, 105-115 5 Moreaux, F., Chevrier, J. C. and Beck, G. Int. J. Multiphase Flow 2 1976, 183-190 6 Narasaki, M., Fuchizawa, S, Keino, T. and Takeda, N. Proc. 18th National Heat Transf Syrup. Japan 1981, 421-423 (in Japanese) 7 Kikuchi,Y., Hori, T. and Michiyoshi, I. Proc. 9th Int. Cryogenic Eng. Conf. 1982, 77-80 8 Nishio, S. Proc. 1st A S M E - J S M E Thermal Eng. Joint Conf. 1 1983, 103-109 9 Berenson, P. J. Trans. A S M E Set. C, J. Heat Transf 83 1961, 351-358 10 Spiegler, P., Hopenfeld, J., Silberberg, M., Bumpus Jr, C. F. and Norman, A. Int. J. Heat Mass Transf 6 1963, 987-989 11 Kikuchi, Y., Hori, T. and Michiyoshi, I. Int. J. Heat Mass Transf 28 1985, 1105-1114 12 Kikuchi, Y., Hori, T., Yanagawa, H. and Michiyoshi, I. Trans. ISIJ 26 1986, 576-581 13 Kikuchi, Y., Nogaki, T. and Matsumoto, R. Trans. J S M E Ser. B 56, 1990, 2038-2043 (in Japanese)