Transient spray cooling of top and bottom moving high temperature surfaces

High pressure water spray cooling, widely used in various industrial process applications due to high impact force and high dissipating ability which is required in various manufacturing processes such as scale removal from oil wells, scale removal from crude oil storage tanks, steel making, including continuous casting and descaling. The conditions and methods used in these processes are vital. There are number of available reports which are concerned with water spray cooling process to determine the most effective conditions and methods of cooling. However, there is a lack of comprehensive information regarding the fluid dynamics of spray characteristics upon their impaction on the hot moving surfaces. The objective of this experimental study is to use high-pressure water sprays impacting on the top and bottom surfaces of a moving hot plate, by using specially developed experimental techniques. For control of spray characteristics, three full cone atomisers were used with exit orifice diameters are 0.94, 1.19 and 1.70 (mm}, at water spray pressures of 0.69, 1.38 and 2.07 (MPa), impacting at 140 and 240 (mm) on a heated surface. For present work, a new transient cooling is developed to investigate upward and downward sprays characteristics impacting on hot moving surface. The apparatus uses a rotating steel disk, which is preheated by gas burners. The water spray then impacts on the hot disk through a rectangular orifice for providing a central and relatively homogeneous region. A slip ring commutator is used to measure the time dependent temperature by the direct contact method using K-type thermocouples. Heat transfer tests with heated surface temperatures 200, 300 and 400 °C, are described under transient conditions sprays on both sides of the heated surface, which rotates at 60 and 120 (rpm). It is found that the heat transfer coefficient increased linearly with increasing surface temperature and increasing flow rate. The three full cone 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 proved a convenient method for determining moving surface heat transfer. Generally the data are consistent qualitatively as expected. The local heat flux increases just after the impaction of the water sprays, and reaches a maximum around 75 (mm) downstream and increase with increasing nozzle orifice size, increasing supply water pressure and decreasing the nozzle surface distance, also it decreases significantly with increasing the rotational speed. The pertinent finding of this work also includes that, the difference in the local heat flux between the top and bottom of the moving surfaces is about 35% since the sprays on the bottom surface are subjected to gravity upon the impaction. Engineering correlation equations are developed by using Excel software and presented as maximum local heat flux. Finally, The tests show that there is need to modify the apparatus in order to reduce the noise especially through "LAB VIEW" software package. Replacement of more precision thermocouples and the test segment would be useful in order to use the ideal method of calculating the heat flux from the surface.