Experimental Investigation Of Pulsed Spray Cooling On A Hot Surface

Both the thickness of liquid film and the reverse movement of steam and liquid droplet influence the effect of heat transfer during spray cooling. For previous research without considering solving the problem, a new project pulse spray cooling as a feasible project is brought forward. First of all, the mechanism of nucleus boiling is expounded based on the theory of boiling heat transfer and lay emphasis on the theory of bubble dynamics. Based on that, the heat transfer mechanism of spray cooling is provided, and then the theoretical gist of pulse spray cooling is also provided. Considering the possible project, an experimental system is designed, which is used to find the rule of improving heat transfer by pulse spray. In order to improve heat transfer, the width and duty cycle are adjusted to search for the proper thickness of liquid film. The surface with high heat flux density is achieved by a copper pyramid. The control and adjustment of pulse is carried out by Electronically Controlled nozzle and an integrate circuit plate. The experimental results show that the heat transfer coefficient is distributed as the function of parabola with the variety of pulse frequency. As the pressure of 0.5MPa, the distance between nozzle and filtration net of 60mm, the spray period of 50ms, and the duty cycle of 0.67, the maximum value of experiment is improved by 4%, compared with the continuous spray cooling. The heat transfer coefficient is improved by 10% compared with continuous spray cooling, when the pressure is reduced to 0.46 MPa and the duty cycle is at 0.65. The enhancement heat transfer effect is very obvious. The main reason is that the thickness of liquid film becomes thin during the paused period, and it results in the heat resistance be small, on the other hand, the time bubbles deviate the wall becomes short and the liquid film becomes thin. That enhance the turbulence in the liquid film, and more bubbles which do not achieve the deviated diameter have arrived at the liquid film surface and break.