When a liquid undergoes sudden reduction in the surrounding pressure below its saturation pressure, the liquid then enters in a metastable state. In order to regain equilibrium, part of the liquid evaporates quickly i...When a liquid undergoes sudden reduction in the surrounding pressure below its saturation pressure, the liquid then enters in a metastable state. In order to regain equilibrium, part of the liquid evaporates quickly in a phenomenon called ＂flash evaporation＂, and the excess sensible heat contained in the liquid is converted into latent heat of vaporization. Therefore, temperatures of both the liquid and the generated vapor decline to the saturation temperature for the reduced pressure. As the heat and mass transfer occur in direct contact between the liquid and its own vapors, the process involves a very high heat transfer rate which makes it suitable for exchanging heat between sources of relatively small temperature difference. Moreover, dispensability of the heat exchange surfaces in this process is a considerable advantage as these surfaces constitute major part of the total system expenses in addition to the associated maintenance problems, especially when dealing with corrosive fluids such like seawater in the thermal desalination processes and in the OTEC （ocean thermal energy conversion） systems. This paper reports on the heat flux variation profiles during the flash evaporation of superheated water jets at various flow conditions. Heat flax was found to grow with time attaining a peak value before it starts to decrease monotonically.展开更多
Experimental results on the thermal characteristics of air-water spray impingement cooling of hot metallic surface are presented and discussed in this paper.The controlling input parameters investigated were the combi...Experimental results on the thermal characteristics of air-water spray impingement cooling of hot metallic surface are presented and discussed in this paper.The controlling input parameters investigated were the combined air and water pressures,plate thickness,water flow rate,nozzle height from the target surface and initial temperature of the hot surface.The effects of these input parameters on the important thermal characteristics such as heat transfer rate,heat transfer coefficient and wetting front movement were measured and examined.Hot flat plate samples of mild steel with dimension 120 mm in length,120 mm breadth and thickness of 4 mm,6 mm,and 8mm respectively were tested.The air assisted water spray was found to be an effective cooling media and method to achieve very high heat transfer rate from the surface.Higher heat transfer rate and heat transfer coefficients were obtained for the lesser i.e,4 mm thick plates.Increase in the nozzle height reduced the heat transfer efficiency of spray cooling.At an inlet water pressure of 4 bar and air pressure of 3 bar,maximum cooling rates670℃/s and average cooling rate of 305.23℃/s were achieved for a temperature of 850℃ of the steel plate.展开更多
A theoretical study was conducted to characterize hydraulic jump of laminar circular liquid jets. The objective of this research was to determine the jump location in a simple explicit form. The effects of many factor...A theoretical study was conducted to characterize hydraulic jump of laminar circular liquid jets. The objective of this research was to determine the jump location in a simple explicit form. The effects of many factors were investigated including nozzle diameter, jet exit pressure, nozzle-to-plate spacing, jet velocity and Reynolds number. It was found that the theoretical data collapsed well over the range of jet Reynolds number 500 Re 1800 with previous data.展开更多
文摘When a liquid undergoes sudden reduction in the surrounding pressure below its saturation pressure, the liquid then enters in a metastable state. In order to regain equilibrium, part of the liquid evaporates quickly in a phenomenon called ＂flash evaporation＂, and the excess sensible heat contained in the liquid is converted into latent heat of vaporization. Therefore, temperatures of both the liquid and the generated vapor decline to the saturation temperature for the reduced pressure. As the heat and mass transfer occur in direct contact between the liquid and its own vapors, the process involves a very high heat transfer rate which makes it suitable for exchanging heat between sources of relatively small temperature difference. Moreover, dispensability of the heat exchange surfaces in this process is a considerable advantage as these surfaces constitute major part of the total system expenses in addition to the associated maintenance problems, especially when dealing with corrosive fluids such like seawater in the thermal desalination processes and in the OTEC （ocean thermal energy conversion） systems. This paper reports on the heat flux variation profiles during the flash evaporation of superheated water jets at various flow conditions. Heat flax was found to grow with time attaining a peak value before it starts to decrease monotonically.
文摘Experimental results on the thermal characteristics of air-water spray impingement cooling of hot metallic surface are presented and discussed in this paper.The controlling input parameters investigated were the combined air and water pressures,plate thickness,water flow rate,nozzle height from the target surface and initial temperature of the hot surface.The effects of these input parameters on the important thermal characteristics such as heat transfer rate,heat transfer coefficient and wetting front movement were measured and examined.Hot flat plate samples of mild steel with dimension 120 mm in length,120 mm breadth and thickness of 4 mm,6 mm,and 8mm respectively were tested.The air assisted water spray was found to be an effective cooling media and method to achieve very high heat transfer rate from the surface.Higher heat transfer rate and heat transfer coefficients were obtained for the lesser i.e,4 mm thick plates.Increase in the nozzle height reduced the heat transfer efficiency of spray cooling.At an inlet water pressure of 4 bar and air pressure of 3 bar,maximum cooling rates670℃/s and average cooling rate of 305.23℃/s were achieved for a temperature of 850℃ of the steel plate.
文摘A theoretical study was conducted to characterize hydraulic jump of laminar circular liquid jets. The objective of this research was to determine the jump location in a simple explicit form. The effects of many factors were investigated including nozzle diameter, jet exit pressure, nozzle-to-plate spacing, jet velocity and Reynolds number. It was found that the theoretical data collapsed well over the range of jet Reynolds number 500 Re 1800 with previous data.
