Transient spray cooling of top and bottom moving high temperature surfaces
High pressure water spray cooling, widely used in various industrial processapplications due to high impact force and high dissipating ability which is required invarious manufacturing processes such as scale removal from oil wells, scale removalfrom crude oil storage tanks, steel making, including continuous casting and descaling.The conditions and methods used in these processes are vital. There arenumber of available reports which are concerned with water spray cooling process todetermine the most effective conditions and methods of cooling. However, there is alack of comprehensive information regarding the fluid dynamics of spraycharacteristics upon their impaction on the hot moving surfaces.The objective of this experimental study is to use high-pressure water spraysimpacting on the top and bottom surfaces of a moving hot plate, by using speciallydeveloped experimental techniques.For control of spray characteristics, three full cone atomisers were used with exitorifice diameters are 0.94, 1.19 and 1.70 (mm}, at water spray pressures of 0.69, 1.38and 2.07 (MPa), impacting at 140 and 240 (mm) on a heated surface. For presentwork, a new transient cooling is developed to investigate upward and downwardsprays characteristics impacting on hot moving surface. The apparatus uses a rotatingsteel disk, which is preheated by gas burners. The water spray then impacts on the hotdisk through a rectangular orifice for providing a central and relatively homogeneousregion. A slip ring commutator is used to measure the time dependent temperature bythe direct contact method using K-type thermocouples.Heat transfer tests with heated surface temperatures 200, 300 and 400 °C, aredescribed under transient conditions sprays on both sides of the heated surface, whichrotates at 60 and 120 (rpm). It is found that the heat transfer coefficient increasedlinearly with increasing surface temperature and increasing flow rate. The three fullcone atomisers are used to study the effects of mass flux (G) 0.98 to 12.5 (kgm' 2s' ! \median droplet diameter (D v0,5) 49.0 to 230.4 (jum) and the mean droplet velocity (U)9.8 to 32.32 (ms' 1 } on the surface cooling.However, analysis of the data shows that the rotating disk technique has been proveda convenient method for determining moving surface heat transfer. Generally the data are consistent qualitatively as expected. The local heat flux increases just after theimpaction of the water sprays, and reaches a maximum around 75 (mm) downstreamand increase with increasing nozzle orifice size, increasing supply water pressure anddecreasing the nozzle surface distance, also it decreases significantly with increasingthe rotational speed.The pertinent finding of this work also includes that, the difference in the local heatflux between the top and bottom of the moving surfaces is about 35% since the sprayson the bottom surface are subjected to gravity upon the impaction. Engineeringcorrelation equations are developed by using Excel software and presented asmaximum local heat flux.Finally, The tests show that there is need to modify the apparatus in order to reducethe noise especially through "LAB VIEW" software package. Replacement of moreprecision thermocouples and the test segment would be useful in order to use the idealmethod of calculating the heat flux from the surface.
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