The Experimental Study And Numerical Simulation Of Multi-Nozzle Spray Cooling

With the continuous improvement of cooling technology requirements of the industry, the spray cooling emerges as the times require. In the past30years, domestic and foreign scholars focus more on the experimental study on the influence factors of single nozzle spray cooling. In recent years, due to the more uniform temperature distribution and higher heat flux, experimental study on multi nozzle spray cooling increases. The experimental studies on the effects of the nozzle number and injection parameters on the atomization and heat transfer characteristics are a lot, but the overall theoretical research combined jet and wall heat transfer is less, and the understanding of heat transfer enhancement is not clear.This paper studied atomization and heat transfer characteristics with different nozzle numbers and working conditions using commercial software, obtained the effects of the wall superheat, inlet pressure, inlet flow rate, nozzle height, nozzle number on the heat flux, reveals the atomization characteristics of multi nozzle spray cooling under different conditions, and provides reliable data for optimization of multi nozzle spray cooling and the study of heat transfer enhancement mechanism. The simulation result shows that the optimal injection height corresponding to the value when the spray directly impact zone cuts the heated surface. With the increasing of the nozzle number, averaged heat flux increases, but the increase in reducing. In this paper, the optimal nozzle number is eight.Spray cooling process will form a layer of thin film on the wall surface, thin liquid film in the complex flow and heat transfer process directly affect the final effect of heat transfer. In order to obtain the detailed liquid flow and heat transfer in thin liquid film. This paper uses a high-speed camera and microscope image capturing system to observe the formation and flow of multi nozzle spray cooling film layer, uses image gray value matrix to conduct quantitative analysis of the forming process, studies the film thickness and the fluctuation characteristics of liquid film under different conditions, analyzes the influence of bubble deformation on the local film thickness. The experimental results showed that with the increase of the Weber number, the dimensionless averaged film thickness has decreased, the surface wavelength and amplitude was decreased. With the increase of the surface temperature, the dimensionless film thickness increased rapidly at first, and then slowly change, and finally increased, thin film surface wavelength and amplitude had no obvious change. The local film thickness in bubble growth and rising process is thicker than that in the bubble formation and accumulation process. First discovered in different temperature, there is a local film thickness peak in the film forming process. At the same time, this paper studies the thin liquid film flow field and temperature field by numerical method. The mathematical model of gas-liquid two-phase flow, using the VOF method to capture the gas liquid interface, was used to study the deformation of bubbles and heat transfer characteristics of thin film. We analysed the heat transfer enhancement mechanism of the continuous subcooled droplets impacting on thin film and the influence of droplet parameters on bubble deformation, obtained the variation law of thin film thermal boundary layer and the bubble deformation under different conditions. The results show that the droplet impact makes the thicknesses of thin liquid film and thermal boundary layer are greatly reduced,the subcooled droplets reduces the temperature difference nearby the wall, both of which enhance convection. The initial droplet position and continuous impact makes the bubble detached from the wall ealier, and decreases the dry area of the wall.In addition, people are not satisfied with the spray cooling capacity at present, are seeking effective methods to enhance the spray cooling heat transfer. Combined the multi nozzle spray cooling and micro-structured surface is a new type of heat transfer enhancement technique. In order to study this method, with smooth surface as reference, this paper investigated the heat transfer enhancement and temperature distribution uniformity of micro-sturctrued surface with different sizes and shapes experimentally, obtained the heat transfer characteristic curves of the micro-structured surfaces, heat transfer coefficient and the temperature distribution under different conditions. A dimensionless number was proposed to characterize the heat transfer enhancement of micro-structured surfaces.