Study On The Effect Of Gravity On Heat Transfer Performance In Two-phase Region In Spray Cooling

In order to solve the problem of heat dissipation in high-power and high heat flux components in aerospace field,the spray cooling in a single body can be removed from many cooling methods with the advantages of strong heat dissipation capacity,less working substance consumption,less thermal surface temperature gradient and no contact heat resistance on the surface of heat exchange.Due to the demand of unconventional gravity conditions in aviation field,it is necessary to explore the heat transfer in spray cooling system when the gravity condition changes.Because the experiments under microgravity are difficult to achieve,most studies on the influence of gravity conditions on spray cooling are still in the simulation stage,and there is no reliable experimental results.This paper will study from this aspect.Through the two ways of model analysis and simulation and experiment,taking water as the working substance,the spray cooling effect of gravity size and spray system relative gravity direction will be studied and analyzed respectively.First of all,from the dynamic analysis of various heat transfer states in the spray cooling process,it is found that the heat transfer process mainly affected by gravity is the process of heat transfer during the phase transition.Then,the FLUENT software is used to carry out the simulation,and the single bubble model with different gravity sizes and directions is calculated and analyzed.The results show that when the gravity direction is perpendicular to the normal angle of the surface,the liquid around the bubble will be subjected to a greater tangential force when the bubble escapes,resulting in a greater flow velocity,thus enhancing the heat transfer capacity.When the direction of gravity is not changed,the escape process of bubbles is affected to a certain extent by the magnitude of gravity,and the decrease of gravity will increase the critical diameter and time of bubble detachment;but when the liquid film is thin,the change of gravity size will not affect the time of bubble escape,and the Nu number of surface will decrease with the increase of gravity at the instant of bubble escape.Finally,the spray cooling experiment with the relative direction of gravity is carried out.The experimental results show that the change of the relative gravity direction will have a certain effect on the heat transfer in the spray process,and the heat transfer effect is better than that of the positive and backward spray.The maximum 10% can be achieved under the experimental conditions.Compared with side injection,the difference of heat transfer effect between forward and backward injection is small,and the law is not uniform.This may be due to the random error of the experiment.The difference of heat transfer caused by the change of relative gravity direction will also decrease with the increase of spray pressure,spray height and heat transfer area.However,the change of nozzle aperture and spray frequency will not affect the experimental results.