JET-25 SPRAY COOLING OF ROTATING ROLL M. Raudensky, J. Horsky, P. Kotrbacek Brno University of Technology, Czech Republic This paper presents the results from a program of extensive laboratory experiments. A rotating cylinder is cooled by a set of spray nozzles. The study concerns an initial cylinder temperature at 350 C and cooling is done using water at 20 C. Circumferential velocity of the roll varies within the range 0.1 to 10 m/s. The following parameters of spray are studied: coolant pressure, flow-rate, and spray height and impact angle. The achievable result is the heat transfer coefficient distribution on the roll surface. This paper deals in particular with the questions of both the influence of circumferential velocity and surface temperature of the rotating cylinder. The results of experimental study of mutual influence of neighbouring sprays are also discussed. The research was motivated by its useful application in the spray cooling of rolls in the rolling process, where characteristics of the cooling system can strongly influence the quality of production as well as the working life of rolls, mentioned by Downey (1997) and by Saha et al (2005). Cooling medium transport to the roll surface is achieved by a series of nozzles positioned in rows thereby forming the shape of coolant jet in the impact area. Cooling intensity can be controlled by coolant pressure and by the number of manifolds in operation. When the roll surface is overflowed, fresh coolant supplied by the bottom rows of nozzles cannot penetrate the coolant layer caused by the upper rows of nozzles, and the heat transfer mechanism becomes saturated. 1. Introduction Spray cooling of rotating cylinders is an area of heat transfer science where it is difficult to apply an analytical or numerical investigation of heat transfer. The spraying jet is in most cases described by "external parameters" such as pressure and flow-rate of coolant, type of nozzle and geometrical configuration of nozzles in relation to the cooled object. The inherent spray parameters such as droplet size and velocity and volume distribution of droplets are not known. Surface temperature of the cylinder plays an important role in the heat transfer mechanism, especially for higher temperatures where boiling must be considered. The rotational aspect of the cooled object makes the problem even more complex. The only reliable method of how to describe heat transfer under the above complex conditions is to use experimental methods. The experimental study described in this paper is restricted by the following geometrical and physical parameters: diameter of the cylinder from 250 to 1000 mm, surface temperature up to 400 C, circumferential velocity of rotation from 0.1 to 10 m/s, coolant pressure up to 400 bar (but typically from 1 to 10 bar), maximum coolant flow-rate 30 l/s and arbitrary nozzle number and nozzle configuration. The major application of obtained results is in the field of heat treatment of large objects and cooling of rolls in rolling mills. The goal of the experiment is to obtain the distribution of heat transfer coefficient (HTC) at the roll surface for a given spray configuration and spray condition. The term distribution means the HTC in both longitudinal and circumferential directions of the cylinder and a function of surface temperature.
2. Experimental Device 1 4 2 3 Figure 1 Scheme of test bench, 1- test roll, 2 spray nozzles, 3 velocity control unit, 4- datalogger The test bench is schematically shown in Figure 1. The basic part is an instrumented roll (1). Nozzles (2) are placed around the roll in arbitrary configuration. A mechanically opened deflector is placed between the nozzles and the roll surface with the purpose of reflecting sprayed coolant during the initial stages of the experiment. The roll is driven by a frequency control motor which allows for the setting of initial circumferential velocity. The test roll is hollow and the surface is formed by a steel shell. A part of the test roll surface is formed by a test plate cast from stainless steel. Special thermocouple-based sensors, individually calibrated, are built into the test plate. The result of calibration is used in the numerical model of the test plate with sensors. This model is the basic part of the inverse heat conduction task used for the data evaluation. Temperature data is stored together with instant roll position data in a datalogger (4) rotating within the roll. The cooling experiment is transient. The preparation of an experiment starts by heating the test plate with an electric heater. Only the test plate itself is heated, the rest of the tested roll remains at room temperature. The roll is stationary during test plate heating. The experiment starts as soon as the temperature of the test plate reaches a uniform start temperature. The heater is then removed, rotation starts, the pump is switched on and the pressure (or flow-rate) is set. The closed deflector protects the cylinder surface from spray. The computer controls the movement of the deflector (the deflector is moved by pneumatic units). The movement of the deflector is linked with the position of the test plate. The deflector is opened at the moment when the rotating test plate is on the opposite side of the cylinder from the sprays, away from water impact. This mechanism ensures that deflector opening and closing is at precisely measured timing for all experiments in the same way. Temperatures are not measured at the surface but at a measured point located under the surface. An example of the measured temperature history at one point is in Figure 2, the solid line representing measured temperature. Temperature drops shown on the curve represent the runs under the spray. Each drop and increase on the curve represents one revolution of the roll. Variation of the surface temperature is higher than variation of the measured temperature. The computed surface temperature is plotted in Figure 2 by a dashed line.
Surface temperature [ C] 350.0 262.5 175.0 87.5 0.0 0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000 Number of record [-] Figure 2 Example of measured temperature history (solid line) and computed surface temperature (dashed line) 3. Evaluation of Measured Data All the measured temperature data goes through a standard inverse procedure, details of which can be found in Raudensky (1993). Surface temperature, HTC and heat flux are computed. Each data point carries information about the angle of cylinder rotation. The data collected in "time" order is converted into position order. This provides HTC distribution on the cylinder surface. 3.1 Velocity The first example shows how circumferential velocity influences heat transfer as the rotating cylinder is cooled by rows of nozzles. The configuration of sprays is schematically shown in Figure 3. Three rows of nozzles were used. Nozzle axes were perpendicular to the cylinder surface. Spray height of 100 mm, roll diameter of 600 mm and water pressure of 5 bar were used. The nozzles provided 10.7 l/s at each metre of the cylinder length. Five speeds of rotation were used: 10, 50, 100, 150, and 200 RPM. Figure 3 Side view schematic of the experiment
25000 20000 Average HTC 15000 10000 5000 0 10 50 100 150 200 RPM Figure 4 Average values of HTC for experiments with increasing velocity Average values of the heat transfer coefficient in the sprayed area for surface temperature below 100 C were computed and the results are shown in Figure 4. It was found that the heat transfer intensity drops with the increase of circumferential velocity. The effect can be explained by the growing centrifugal forces removing the impacting water from the roll surface. An interesting finding obtained is that cooling intensity decreases when very low velocities are used. The maximum cooling intensity was observed for 50 RPM, i.e. 1.7 m/s. Decrease of the heat transfer for low velocities can be explained by flooding of the slowly moving surface. 3.2 Surface Temperature The surface temperature determines the mechanism of heat transfer. Spray cooling is influenced by the presence of boiling. Significant points on the boiling curve known from pool boiling can be found in spray cooling. The most important point is the Leidenfrost temperature. Spray cooling efficiency strongly depends on surface temperature. Stable vapour layer can be formed at the cooled surface. The stable vapour layer protects the surface from direct contact with the coolant and the cooling becomes of lower intensity. Stability of the vapour layer is coupled to the surface temperature. When temperature decreases and the vapour layer collapses the cooling instantly grows. The cooling intensity can be ten times higher in a low temperature region in comparison to the intensity in a high temperature region. The border between these two temperature areas is the Leidenfrost temperature discussed by Raudensky and Horsky (2005) and by Bernardin and Mudawar (2002). Spray cooling experiments starting at surface temperatures of about 300 C are in the area below the Leidenfrost temperature. This is not to say that heat transfer is temperature-independent in this low temperature region. A simple experiment using two rows of nozzles shows the influence of surface temperature. Figure 5 shows heat transfer coefficient distribution on the cylinder surface. The diameter of the cylinder is 600 mm and circumferential velocity 1 m/s. Horizontal axis in Figure 5 is for a position on the cylinder circumference where zero represents the surface point on the level of roll axis. Position of the first row of nozzles is 100 mm and position of the second row is 500 mm (measured at the roll circumference). Figure 5 shows that the highest intensity of heat transfer was obtained for low
HTC [W/m 2 K] 50000 40000 200-300 C 100-200 C 30-100 C 30000 20000 10000 0-400 -200 0 200 400 600 800 1000 Position [mm] Figure 5 Study of the influence of surface temperature on heat transfer. surface temperatures, below 100 C. Temperature intervals of 100-200 C provide lower HTC values but the difference is not significant. Increasing surface temperature makes heat transfer less intensive. The above can be explained a negative role of vapour formation. Vapour forming by the contact of a hot surface with water prevents the surface from actual direct contact with coolant. The second effect is that impacting water sticks well to the cold surface and support cooling outside the direct impact of droplets. Water in mixture with vapour is easily sprayed-out due to centrifugal forces acting on the rotating surface. 3.3 Impact Angle There are different methods to intensify achieved cooling. The simplest method is to increase coolant pressure and flow. Experimental study showed that increasing flow-rate is proportionate to increased heat transfer only up to a saturation point. Further flow-rate increase causes a decrease in cooling intensity due to flooding of the roll surface by a thick layer of coolant which restricts the efficiency of impacting jets. A two-pressure system can be used for further increase in cooling intensity, discussed by Horsky et al.(2004). Heat transfer mechanism depends on impact angle. The parameters of experiments studying the roll impact angle are as follows: four flat jet nozzles at a spray angle of 60 were used. Each nozzle received a flow-rate of 25 l/min at a pressure of 5 bar. Spray height was 100 mm and the circumferential velocity of the 600 mm diameter roll was 1 m/s. Impact area of the nozzles is shown schematically in Figure 6. The first experiment used perpendicular impact. Nozzle axes were tilted by 30 in the second and third experiments. The inclination of nozzles was in-direction and against-direction of rotation as shown in Figure 7.
500 mm 500 mm 50 mm 50 mm Figure 6 Impact areas of the nozzles in tests of influence of impact angle 15000 13000 HTC 11000 9000 7000 5000 100 mm 50 mm 0mm 50 mm 100 mm Figure 7 HTC distribution along impact area for three tests with impact angle The results in Figure 7 show that the highest values of heat transfer coefficient were obtained for the case in which the water jet follows the direction of rotation. Less intensive cooling occurs with perpendicular impact. The presumption that spray aimed against the direction of rotation would cool most intensively due to high relative velocity was not confirmed. Research experiments showed that spraying against the direction of motion causes an indraft of heated water back to the impact area. Impacting coolant is partially reflected by the layer of coolant at the moving surface. Water spraying in-direction of movement is used more efficiently, coolant sticks to the surface and heat transfer per effective area is longer. 3.4 Interaction of Sprays Early experiments completed in the nineties were motivated by an effort to study heat transfer caused by single nozzle as a base for more complex spray configurations. Finding methods for heat transfer computation for complex problems using basic experiments was not successful. An example is given for cooling using a combination of spray bars. Three rows of nozzles with different sizes were used for a study of mutual interaction of sprays. All experiments were done using a cylinder diameter of 600 mm, circumferential velocity of 3 m/s and
Bar A Bar B Bar C Figure 8 Configuration of spray nozzles pressure of 5 bar. Spray bar A used the largest nozzles giving 15 l/s at 1 metre of roll length. Spray bar B used mid size nozzles with a flow-rate of 5 l/s/m and spray bar C used a flow-rate of 1 l/s/m. The positions of the axes of impact area on the roll were at distances of 200 mm measured at the roll circumference. The experiments using only one spraying bar of nozzles were made first. The results in Figure 9 show HTC distribution at the roll circumference. The positions of horizontal axis in Figs. 9 and 10 are connected with a roll coordinate system. It can be seen that axis of the impact point of the bar A is at position +100 mm, bar B is aimed at the position -100 mm and spray bar C is aimed to the -300 mm position. There is an HTC peak in the position at +350 mm. This peak is caused by a wiper cleaning the roll surface and the HTC increase at this position should not be considered for the purpose of this study. Figure 9 shows that heat transfer in the sprayed area is not proportional to coolant flow-rate. The portion of flow-rates for spray bars A, B and C is 15:5:1 while the portion of HTC is 5:3:1. The program of experiments was continued by experimenting using two rows of nozzles. The results in Figure 10 show the HTC distributions for the experiments A+B, A+C and C+D (see Figure 8). The most important finding is that the data usage for a single row of nozzles, where multiple rows of nozzles have been used, is only possible in cases when the impact areas are significantly far away from one another. HTC [W/m 2 K] 60000 50000 A B C 40000 30000 20000 10000-1000 -800-600 -400-200 0 200 400 600 800 1000 Position [mm] Figure 9 HTC distribution for tests using one spraying row of nozzles
HTC [W/m 2 K] 60000 50000 A+B A+C B+C 40000 30000 20000 10000-1000 -800-600 -400-200 0 200 400 600 800 1000 Position [mm] Figure 10 HTC distribution for tests using two spraying rows of nozzles Case A+B (see the solid line in Figure 10) is most enlightening. When the surface point receives spray A, the less intensive spray B can hold HTC at a constant level! A similar conclusion can be formulated when studying the experiment B+C in Figure 10. The comparison of experiments, described in Figures 9 and 10, shows that a given combination of sprays cannot be used for any superposition. 4. Conclusions Spray cooling of moving surfaces is a difficult heat transfer task which can only be precisely solved using experimental techniques. A large number of spray parameters influence heat transfer intensity. Experimental technique providing boundary conditions for numerical models, mentioned by Horsky et al. (1998) is described. Heat transfer in the sprayed area is seriously dependent on the surface temperature. The Leidenfrost temperature for spray cooling is typically higher than 500 C and for intensive sprays exceeds 1100 C. Even in the temperature interval below the Leidenfrost temperature there was found to be a serious dependence of the heat transfer coefficient on surface temperature. The experiments proved that decreasing values of heat transfer coefficient occurred with increasing temperature of the sprayed moving surface. The study of the velocity influence of the sprayed surface, proved a decrease in heat transfer intensity on a rotating cylinder proportional to the speed of rotation. Inclination of coolant jets against and along the direction of movement proved the surprising fact that higher HTC values are obtained when the jets follow the sprayed cylindrical surface. Any superposition cannot be used and only the experiment with experimental investigation into the mutual interference of several sprays can be used. The experiments made using a combination of several sprays proved the impossibility of finding a simple method to apply the results from simple configurations (for example experiments with single nozzle or single rows of nozzles) to those using complex spray configurations. Acknowledgement This research work was supported by the Czech Grant Agency within the project No. 106/06/0709.
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