Cooling Characteristics and Heat Transfer Coefficients. during Fog Cooling of Hot Steel Plates*

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1 UDC :536.24: Cooling Characteristics and Heat Transfer Coefficients during Fog Cooling of Hot Steel Plates* By Masashi MITSUTSUKA** and Keiji FUKUDA** Synopsis Air-atomized fog cooling is suitable for the forced cooling of hot steel products, particularly in the soft cooling region where the cooling rate is between those of forced air cooling and water-spray cooling. In fog cooling, both water and air are considered to effect the cooling. It is well known that when fog impinges against the surface being cooled at high speed, the heat flux transferred from the surface to the fog increases with increases in fog impinging speed because of unstable vapor films on the surface. It has not yet been clarified whether or not the heat flux increases when fog impinges against the surface at lower speeds. The heat transfer phenomena have been studied when hot steel specimens are cooled with fog atomized by low-speed air. The major results obtained are as follows: (1) When fog impinges against the surface at a low speed, the air functions mainly to atomize water, and hardly contributes to heat transfer. (2) The cooling capacity of fog is solely dependent on the water flux being impinged onto the surface. (3) The cooling capacity of air-atomized fog is nearly equal to that of pressure-atomized fog insofar as water flux is the same. In this paper, in addition to the above results, the differences in heat transfer mechanism between the upper and lower surfaces of a horizontally placed steel plate and the influences of surface conditions of a steel plate on heat transfer are discussed. I. Introduction Ejection of atomized water droplets onto the steel surface being cooled is the most common method of force-cooling hot steel products. Pressure atomization and gas atomization are widely used to atomize water. In the pressure-atomizing method, the water pressurized to 1 to 10 kg/cm2-g is ejected at high speed through a nozzle. Because the spray pattern can be controlled over a wide range by the nozzle shape, the nozzle size, the ejection rate and the ejection pressure, the pressure-atomized water-spray cooling method has been widely used to cool hot steel products. In the gas atomization method, on the other hand, water is atomized by a gas jet of the order of 101 to 102 m/s. With this method, violent turbulence of atomized water flow on the surface being cooled facilitates uniform contact of water droplets and a steel surface (uniform cooling), and the atomized water flow impinging at high speed against the surface being cooled makes the vapor films over the surface unstable and facilitates heat transfer (intensified cooling). Furthermore, by reducing the water amount of atomized water flow, the cooling capacity is decreased in proportion to the water flux (softer cooling). Furthermore, the reduction of the water amount to zero would result in gas-impinging cooling. However, when used in steel processing lines, the conventional gas-atomized fog nozzle tends to clog because of its small diameter (1 to 2 mm). In addition, this method requires two separate piping systems for water and gas. For these reasons, gas-atomized fog cooling has hardly been used in steel processing lines despite its excellent cooling performance. As technology for the rolling and heat treating of steels have become more sophisticated, new water cooling methods capable of covering the soft cooling region between gas-impinging cooling and waterspray cooling are required. For this reason, the research and development of soft water cooling methods are being actively pursued."2~ In this soft water cooling, a jet of gas/water fog (hereinafter referred to as fog) is used. The principal focus of the current research and development, however, is on the gasatomized fog nozzle which offers stable fog flow.'-4) The characteristics of fog cooling have already been thoroughly investigated by a number of researchers"2,5-7~ to such an extent that its heat transfer mechanism and cooling capacity are well known. A number of problems, however, still remain to be solved, including the effect of nozzle construction on cooling capacity, comparison of cooling capacity between fog and water-spray cooling methods, the difference of heat transfer coefficient values between the upper and lower surfaces of a horizontally placed wide plate on which fog is impinged, and the effect of scale deposit on heat transfer. To clarify these phenomena, tests using air-atomized fog to cool hot steel products were conducted. II. Tests In the first series of tests using small specimens (Test 1), the effect of nozzle shape on cooling capacity was investigated and cooling capacities of fog cooling and water-spray cooling methods were compared. In the second series of tests (Test 2), the cooling characteristics of the fog ejected through the crossflow type fog nozzle were evaluated and the heat transfer coefficient between a horizontally placed steel plate and fog was measured. 1. Test Apparatus 1. Test 1 Tests were conducted by ejecting fog under the same conditions onto both surfaces of a small vertically placed specimen, as shown in Fig. 1 (a). To ensure uniform cooling of the entire surface, the * ** Originally published in Tetsu-to-Hagane, 65 (1979), 608, in Japanese. Process Technology R & D Laboratories, Nippon Steel Corporation, English version received July 16, Edamitsu, Yawatahigashi-ku, Kitakyushu 805. Research Article (689)

2 ( 690 ) Transactions ISIJ, Vol. 21, 1981 specimen was moved along its vertical plane. In the tests, three types of nozzles, as shown in Fig. 2, were used. (1) Crossflow Type Fog Nozzle (Type A) : This is a new nozzle developed by the authors. For the fog flow characteristics obtained with this nozzle, refer to our previous report.3) (2) A Commercially Available Nozzle (Type B): In this nozzle, water is ejected through a 2 mm central hole while air is ejected through a 3 mm hole coaxially arranged around the central hole. Water droplets in the fog are very fine. (3) Full Cone Spray Nozzle (Type C): This Fig. 1. Outline of test apparatus. (Unit: mm) type has a full cone spray nozzle at the center of a straight pipe of 52 mm ID. The water droplets ejected under high pressure through the full cone spray nozzle are mixed with the air flow ejected from the straight pipe to form fog. 2. Test 2 Tests were conducted by ejecting fog onto the upper and lower surfaces of a large horizontally placed specimen, as shown in Fig. 1 (b) (Nozzle pitch: 100 mm in transverse direction, 210 mm in longitudinal direction). The nozzles used are crossflow type fog nozzles consisting of an air pipe 16.1 mm in ID and a water pipe 3 mm in ID and 5 mm in OD. Taking into consideration that the edges of the specimen tend to cool rapidly, the nozzles were placed approximately 50 mm away from the longitudinal edges of the specimen so as to prevent the fog from impinging directly onto the parts. The specimen was reciprocated in the longitudinal direction to ensure uniform cooling of the entire surface. 2. Specimens 1. Specimen for Test 1 (Specimen 1) The chemical composition (Table 1) of the specimen used in Test 1 was essentially the same as those used in a series of cooling tests conducted by the authors, such as water-spray cooling, water jet cooling and water-immersion cooling tests. Three pairs of thermocouples (CA of 0.6 mm wire diameter) were attached at the center in through-thickness direction (at 1/2- t position) and one pair each at positions 7 mm deep from the surface (at 1 /4-t positions), as shown in Fig. 3 (a). The hot junctions of thermocouples were securely spot-welded to the specimen. 2. Specimen for Test 2 (Specimen 2) The specimen used in Test 2 was prepared by laminating two steel plates of 14 mm thickness, 400 mm width and 800 mm length to measure the Fig. 2. Atomization nozzles cooling tests. (Unit: used for fog mm)

3 Transactions ISIJ, Vol. 21, 1981 (691) Table 1. Chemical composition of specimens. (wt%) thickness. Three pairs of thermocouples were attached at the center in through-thickness direction of each steel plate (corresponding to l /4-t positions), and one or two pairs along the heat insulated plane (corresponding to l /2-t position). Their lead wires were drawn out for external connection through the gap (4 to 5 mm) of the laminated plates. The edges of the steel plates were welded to prevent cooling water from entering into the gap. 3. Test Method 1. Test 1 Specimen 1 was heated to approximately 930 C in an electric furnace without atmospheric control. After the specimen was extracted from the furnace, loose scale was removed by wire brushing. The specimen was then placed vertically in between two horizontally-opposed fog flows to continuously or intermittently cool it down to ambient temperature under the predetermined conditions (See Table 2). 2. Test 2 Specimen 2 was heated to approximately 700 C or 900 C in an N2-atmosphere electric furnace. After extraction from the furnace, the specimen was placed horizontally on a buggy and cooled continuously down to ambient temperature under the predetermined conditions. The fog ejection conditions onto the upper and lower surfaces were determined, using both the data of preliminary tests and those of waterspray cooling, so that the cooling rates of the two steel plates became nearly equal. If the peripheral zones of the specimen are in a wet state, the water droplets ejected onto the upper surface (which exist in Leidenfrost state) are entrapped in those areas, and accelerated cooling may result. In order to avoid this phenomenon, efforts were made to prevent the fog from impinging directly on the areas approximately 50 mm away from the longitudinal edges of the specimen. 4. Calculation to Heat Transfer Coefficient The heat transfer coefficient, a between the fog and the specimen, was calculated from the cooling curve obtained at the l /4-t positions. The calculating method was the same as that in our previous report.8~ Fig. 3. Specimens and mm) thermocouple installation. (Unit: cooling effects on the upper and lower surfaces independently (Fig. 3 (b)). In double-side cooling, this specimen is equivalent to a steel plate 28 mm in 5. Calculation of Cooling Rate The cooling rate, V was calculated from the time required to drop from 800 to 500 C at the 1/2-t position. Assuming that a is 200 to kcal/m2 h C and the thickness of the specimen is 28 mm, the ratio (R8/RZ) of the surface thermal resistance R8 to the internal thermal resistance Ri is 20 to 2. Ri is not dependent on the value of a, but keeps an almost con- Research Article

4 (692) Transactions ISIJ, Vol. 21, 1981 Table 2. Test conditions. stant value which corresponds to a specimen thickness. Consequently, with the specimen thickness of 28 mm, the evaluation of the heat transfer behavior of the steel surface from temperature changes at the l /2-t position can be considered adequate, though accuracy is slightly lowered. III. Results 1. Cooling Rate Changes in V at the l /2-t position of Specimen 1 are shown in Fig. 4. Two solid lines in the figure show the ranges of cooling rates in the waterspray cooling and the water jet cooling tests conducted by the authors using essentially the same specimens as those of the present tests.9~ The figure reveals the following points. (1) With water flux, W, being within a range of less than approximately 100 l/m2. min, the cooling rate V of fog cooling is virtually equal to those of water-spray cooling and water jet cooling. (2) V of fog cooling is strongly influenced by W. (3) V8oo-500 of fog cooling is little influenced by nozzle construction. 2. Heat Transfer Coefficient Changes in heat transfer coefficient, a, as calculated from the cooling curve at the if 4-t positions of Specimen 2 are shown in Fig. 5 (a in the figure includes the heat transfer coefficient ar due to radiation). Fig. 4. Relationship between impin cooling rate during fog coolin ged water g (Test 1). flux and

5 Transactions ISIJ, Vol. 21, 1981 (693) Fig. 5. Relation between imping heat transfer coefficient (Test 2). ed (a) water flux (W) and during fog cooling The value a, which is strongly influenced by the surface temperature 6S of the specimen, was plotted separately in graphs for every 50 C ranges of BS. The figure indicates that the values of a for both upper and lower surfaces increase in proportion to W. Next, the relationship between es and a was obtained by estimating the relationship of " W-08-a" from Fig. 5 and all other graphs not shown here (See Fig. 6). Figure 6 reveals the following : (1) The value a of fog cooling is strongly influenced by BS and W, as in water-spray cooling and water jet cooling. (2) The value a of fog cooling reaches its maximum in the range of BS : 100 to 200 C, and rapidly decreases in higher temperature regions as ds rises. (3) The value a of fog cooling reaches its minimum in the range of BS : 500 to 600 C and, in higher temperature regions, gradually increases as 8S rises. (4) The heat transfer coefficient au of the upper surface sharply decreases in the range of 8s : 200 to 400 C as 0, rises. (5) With 0, and W left uuchanged, au is larger than the heat transfer coefficient al of the lower surface over the entire temperature range. (6) a in higher temperature regions tends to be influenced more strongly by radiation than by fog as W decreases. The phenomenon as described in (4) can be explained by the fact that a in higher temperature regions is strongly influenced by radiation because a includes heat transfer by radiation. IV. Fig. 6. Heat transfer coefficient during fog coolin g (Test 2). Considerations 1. Effect of Nozzle Construction on Cooling The state of fog ejected from the air-atomized fog nozzle varies with nozzle construction, and the cooling capacity of fog cooling may be affected by the state of fog. Figure 4 shows the effect of nozzle construction on cooling. Although limited data make a definite conclusion difficult, a comparison of each nozzle suggests that the relationship " Type B>Type

6 (694) Transactions ISLE, Vol. 21, 1981 C =. Type A" holds, with a small difference between them. This can be explained as follows. In Type B, the distance H between nozzle and specimen is 300 mm with the exit velocity Ve of air being estimated at 100 to 200 m/s (pressure at the air header: 1 to 3 kg/cm2-g). In Types A and C, on the other hand, H is 750 to 800 mm and Ve 1s 23 to 31 m/s. Consequently, as far as the velocity Va at which fog impinges on the specimen is concerned, the relationship " Type B > Type C ' : Type A" holds. The difference in Va seems to be a major cause for the slight difference in V If H and Ti,, of Type B can be made equal to those of Types A and C, the aforementioned difference in V800_500 can probably be eliminated. It can be concluded from the reasons mentioned above that the cooling capacity of the fog having low Va is hardly influenced by nozzle construction. 2. Effect of Atomizing Air on Cooling According to the results of tests conducted by Kunioka et al.15~ (Va: 30 to 200 m/s), Yanagi et al.,'~ (Va: 5 to 9), and Shimada et al.,7~ (Va: 4 to 15), Va of fog has a significant effect on heat transfer. As Va increases, the momentum of water droplets increases, and at the same time the fog impinging on the specimen flows violently on the specimen surface, preventing the formation of stable vapor films on the surface. Consequently, the heat transfer from the specimen to the fog ought to increase in proportion to Va. Figure 4 indicates that the cooling rate of the fog with W: 390 l/m2 min is higher than that of waterspray cooling at the same water flux. This is attributable to higher Ve (pressure of air in header: 3 kg/cm2-g) and lower H (300 mm). The cooling rate of fog cooling at water flux other than W: 390 l/m2 min is essentially the same as that of water-spray cooling at the same water flux. Within a range where Va is low (less than a few m/s on an average), therefore, air flow has little effect on cooling, taking part only in water atomization. 3. Heat Transfer Coefficient of Fog Cooling In Section IV. 4. above, the fact that water atomizing air hardly effects cooling when fog impinges on the specimen at low speeds was discussed. It was expected, therefore, that the heat transfer characteristics of fog cooling in the present tests would be practically the same as those of water-spray cooling. Figure 5 (Test 2) indicates, despite a limited number of data and a large scatter in data, that the following relation exists between W and heat transfer coefficient a within a range of W : 5 to 50 (excluding the range of O,< 150 C). a cc Wo.s...(1) This relationship shows a good agreement with the results of the water-spray cooling tests10' conducted by the authors under essentially the same conditions as those of the present test. In contrast with this, the following relationship has been obtained from the test with high Va values conducted by Kunioka et a1.,15~ and the test with high BS values conducted by Yanagi et a1.1~ a cc (WVa)n...(2) where, n : 0.36 in the test by Kunioka et al., n : 0.75 in the test by Yanagi et al. A comparison of major test conditions for these tests is given below. In the test of Yanagi et al., using small-sized specimens, the entire surface of the specimen was cooled by the fog impinging on it while the specimen used in the present test was cooled by both the impinging fog and the fog flowing on the specimen surface because of a large specimen size. The test conducted by Yanagi et al., was concerned mainly with the film boiling region while the present test was concerned with both the nucleate boiling and transition boiling regions (partially including the film boiling region). It might be thought from the abovementioned reasons that the heat transfer mechanism of fog cooling is strongly influenced by Va in regions where Va is sufficiently high or in the film boiling region. The values of a corresponding to W were obtained from typical curves in Fig. 5 and plotted against 0, in Fig. 6. The Fig. 6 reveals the following : (1) The effect of W and 0, on a is virtually the same as in water-spray cooling. (2) Despite relatively few data, it can be said that the heat transfer coefficient water droplets au, (excluding the heat transfer coefficient of radiation a,.) is little influenced by O, in the 0, range above approximately 600 C. (3) With W being the same, au is larger than al. (4) The values of a obtained in the present test are larger than the corresponding values obtained in the test using silver specimens (40 mm~i5 x 2 mmt) conducted by Shimada et al.,7> while the values of I4a/4BS obtained in the present test in the transition boiling region are smaller than those obtained by Shimada et al. The phenomenon as described in (1) can be considered a natural consequence in the present test where Va of fog was small (Refer to Section IV. 2.). As for (2), the water droplets ejected onto the upper surface of the specimen are heated, immediately after impinging against the surface, to their boiling point, run about over the specimen surface in the Leidenfrost state, and then fall from the specimen. The heat transferred from the specimen to the water droplets includes " sensible heat for heating up to boiling point + latent heat for evaporation ". The amount of heat transferred during the period from Research Article

7 Transactions ISIJ, Vol. 21, 1981 (695) the time the water droplets impinge on the hot specimen till the time they reach the Leidenfrost state is large. However, once the water droplets have reached the stable Leidenfrost state, the amount of heat transferred to them is small, though their evaporation rate slightly increases in proportion to OS." For these reasons, aw (=a-ar) would be expected to be little influenced by BS insofar as BS is sufficiently high. The test results by Muller and Jeshar,12~ where aw of water-spray cooling was measured at the steady state, indicate that within the BS range of 900 to 1200 C, the following relationship holds. aw = Ve+( Ve) W,...(3) where, Ve : 11 to 32 m/s H: 100 to 200 mm W: 19 to 5501/m2 min, specimen size : 20 to 60 mm. Equation (3) indicates that aw in the film boiling region is not affected by BS. The phenomena as described in (3) and (4) will be discussed in the next section. 4. Effect of Specimen Surface Properties on Heat Transfer When water droplets are ejected onto a hot metal surface, the evaporation behavior of water droplets is strongly influenced by the surface temperature and surface properties of the metal. To investigate this phenomenon, the values of a were measured using three types of specimens having different surface properties, the measurement results being shown in Fig. 7. Major conditions of each specimen are as follows : Fig. 7. Influence of specimen specimen arrangements water impinged cooling. on surface heat conditions transfer and during Investigator Cooling method Specimen material Surface finish deposit Scale Shimada et al. T Fog Silver Mirror Less finish Present test Fog Steel As- Little rolled Previous test by Water- Steel As- Much the authors10' spray rolled Figure 7 reveals the following. (1) BS corresponding to the maximum value amax of a is shifted toward the higher temperature side as the amount of scale deposit increases. (2) The value of amax decreases as the amount of scale deposit increases. (3) The value of 14a/4e81 i n the transition boiling region decreases as the amount of scale deposit increases. The temperature range of the transition boiling region increases as the amount of scale deposit increases. (4) The value of a in the transition boiling region increases as the amount of scale deposit increases. (5) The value of a in the film boiling region seems to be little influenced by the surface properties of the specimen. (6) With W being the same, au is larger than al,. The phenomena (1) through (5) can be explained qualitatively by using the evaporation curves of water droplets on metal surface. In fog or waterspray cooling, water droplets 0.05 to 0.5 mm in diameter continuously impinge against the specimen surface at a given speed while, in the evaporation curves, water droplets 2 to 3 mm in diameter fall gently on the upper surface of the specimen. It cannot be considered, however, that there is an essential difference in the evaporation behavior of water droplets and the Leidenfrost phenomenon between fog or water-spray cooling and evaporation of water droplets. For these reasons, the evaporation behavior of water droplets was investigated, using specimens having surface properties corresponding to those of the abovementioned specimens, to elucidate the phenomena (1) to (5).13) The relationship between the specimen surface temperature BS and the evaporation time Z of water droplets is shown in Fig. 8. In the figure, the upper limit of BS in a region where r is very short, as corresponding to the nucleate boiling region, is defined as NP and the lower limit of temperature in a region where z is long, as corresponding to the film boiling region, as LP. The values of NP, LP, and z (zmax) in the vicinity of LP as obtained from the figure are as follows :

8 (696) Transactions ISIJ, Vol. 21, 1981 to the amount of scale deposit. The phenomenon (2) can be explained qualitatively by this reasoning. The region between NP and LP can be regarded as corresponding to the transition boiling region. The phenomenon (3) can therefore be explained qualitatively by the fact that the difference between as as corresponding to NP and BS as corresponding to LP increases as the amount of scale deposit increases. The fact that z is large is equivalent to the fact that a is small. The value of z between NP and LP decreases with increases in scale deposit. Consequently, the phenomenon (4) can be explained qualitatively by the fact that the value of z in the transition boiling region decreases with increases in scale deposit. The phenomenon (5) cannot be studied in this report since there are no measured values of z in the region above 500 C. The phenomenon (6) will be discussed in the next section. Fig. 8. Influence of metal droplet evaporation. surface conditions on water The temperature in the vicinity of NP can be regarded as the temperature corresponding to Amax. Consequently, the phenomenon (1) above can be explained qualitatively by the fact that NP is shifted toward the higher temperature side as scale deposit increases. In the vicinity of NP, water droplets are in very close contact with the specimen (base metal or scale). On the other hand, thermal resistance between water droplets and base metal surface is proportional to the amount of scale deposit. Consequently, the value a calculated from the temperature difference between water droplets, and steel surface and the heat flux transferred ought to be slightly lowered in proportion 5. Difference of Heat Transfer Coefficient between Upper and Lower Surfaces When water droplets are ejected onto the upper and lower surfaces of a horizontally placed wide steel plate, the behavior of water droplets after impingement on these surfaces differs. Accordingly, the heat transfer mechanism of the upper and lower surfaces must be different. According to Figs. 6 and 7, within the W range of 5 to 501/m2. min, a of the upper surface is larger than that of the lower surface over the entire temperature range. This can be explained as follows : 1. DS: in the High Temperature Region (Corresponding to the Film Boiling Region) The water droplets ejected onto the upper surface of specimen are heated, after impinging, to their boiling point, and then run about over the upper surface in the stable Leidenfrost state. Those on the lower surface, on the contrary, run about in the neighborhood of the lower surface after impingement, but fall from the surface before all of them are heated to the boiling point. The difference in the value of a between the upper and lower surfaces is attributable to the difference in the degree of temperature rise of the water droplets and the contact time of the water droplets and the specimen. 2. t1 : in the Intermediate Temperature Region (Corresponding to the Transition Boiling Region) The water droplets ejected onto the upper surface of the specimen run about over the specimen in the unstable Leidenfrost state after impingement. During the period, some of them are entrapped in the area, such as projections and scale deposit, which are liable to be wetted and in which the water droplets evaporate violently. Those on the lower surface fall from the specimen for the most part in liquid form as they run about in the neighborhood of the lower surface, though some of them are entrapped and evaporate in the areas liable to be wetted. Observation results reveal that the ratio of entrapment is larger on the upper surface than on the lower surface. These factors seem to help au to become larger than al.

9 Transactions ISIJ, Vol. 21, 1981 (697) 3. 8s: in the Low Temperature Region (Corresponding to the Nucleate Boiling Region) As specimen temperature drops and the entire specimen surface changes to the state liable to be wetted, the ejected water droplets violently evaporate on the upper and lower surfaces of specimen. However, as most of the water droplets on the lower surface fall in liquid form, the evaporation rate of water droplets becomes higher on the upper surface than on the lower surface. As a result, au seems to become higher than al. Using both the results of the present test and those of our previous report10~ (water-spray cooling, specimen size : 28 mm x 550 mm x mm), the conditions under which the cooling rates of the upper and lower plates of specimen become almost equal were determined. The results are shown in Fig. 9. The figure indicates that the ratio of water amount on the upper and lower surfaces at which the cooling rates of both plates become virtually the same as is shown below. With steel plates 3 to 5 m in width, the water droplets ejected onto the upper surface stay longer on the plate surface, gathering in relatively low portions to cool more intensively. This tends to cause deformation or residual stresses in the steel plate.l4~ With an excessively large amount of water, on the other hand, the upper surface of specimen is covered with a thick water layer, preventing ejected new water droplets from contacting the surface. As a result, it can be considered that au becomes slightly lower than al,16~ For the reasons described above, it is necessary to select the proper ratio of water amount on the upper and lower surfaces of plate in accordance with its size and the amount of water ejection to ensure uniform cooling of the upper and lower surfaces of a horizontally placed steel plate. V. Conclusion Fog cooling (cooling with an air-atomized fog flow) is most suitable for the forced cooling of hot steel products, particularly that in the soft cooling region between forced-air cooling and water-spray cooling. Tests were conducted to make clear the heat transfer characteristics during the fog cooling of hot steel products. Those tests have revealed the following: (1) At a low fog impinging velocity Va (Va < 5 to 10 m/s), air flow effects only water atomization. At higher Va, however, air flow effects both water atomization and heat transfer. (2) In the region where air flow does not participate in heat transfer, the cooling capacity of fog cooling is not influenced by water atomizing methods, but is a function of water flux W only. (3) The maximum and minimum values of the heat transfer coefficient a of fog cooling exist in the surface temperature ranges of 100 to 200 C and 500 to 700 C, respectively. With W: 201/m2 min, the maximum and minimum values of a are to and 300 to 500 kcal/m2 h C, respectively. Note that these values include the heat transfer by radiation. (4) In the W range less than 501/m2 min, the cooling capacity of fog cooling is virtually the same as that of water-spray cooling insofar as W is the same. (5) In the fog cooling of the upper and lower surfaces of a horizontally placed steel plate (28 mm x 400 mm x 800 mm) with the same W, the value a on the upper surface is higher than that on the lower surface in the region where W is small. (6) In the fog cooling of a hot steel plate with scale deposit, the value a increases in proportion to the amount of scale deposit in the region where W is small and its surface temperature is over about 200 C. Nomenclature a : Heat transfer coefficient (kcal/m2 h C) au : Heat transfer coefficient of upper surface (kcal/m2 h C) al : Heat transfer coefficient of lower surface (kcal/m2 h C) H: Distance between nozzle and specimen (mm) BS : Surface temperature ( C) W: Impinged water flux (1/m2 min) Wu : Impinged water flux on upper surface WL : (1/m2 min) Impinged water flux on lower surface (1/m2 min) Ve : Exit velocity of air (m/s) Va : Impinging velocity of air (m/s) REFERENCES Fig. 9. Comparison of and the lower horizontally. cooling surfaces effect between the upper of flat plates arranged 1) 2) K. Yanagi, K. Setoguchi and K. Hayashi: Mitsubishi Juko Giho, 9 (1972), 792. N. Kunioka, T. Noguchi, M. Takumi, K. Yago and K. Suga : Tetsu-to-Hagane, 62 (1976), A45. Research Article

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