Experimental Study On Transient Spray Cooling Surface Heat Transfer Characteristics

Around 55% of the electronic equipment failures are caused by excessive temperature.At the same time,stricter requirements for heat dissipation are imperative due to the higher integration of equipments.In this way,temperature control plays a vital role in ensuring the stable operational reliability of the electronic equipments.As an efficient heat dissipation technology for the high-power electronic equipment,spray cooling has many advantages such as good temperature uniformity,strong heat dissipation capacity and less demand for working fluid flow.And the key indicators of its cooling performance are the temperature,distribution and temperature gradient of the heat dissipation surface.However,current research on surface temperature and distribution during the spray cooling process are not sufficient.In this paper,a visual experimental platform for spray cooling was established,and the extremely thin copper foil and heated copper column were selected as heat source respectively.The spray flow rate was 400ml/min,700 ml/min and 900 ml/min,and the initial surface temperature was 75 °C,90 °C and 105 °C respectively.Surfaces of different wettability were used,which includes smooth,hydrophilic,hydrophobic and hydrophilic-hydrophobic combined surface.And the influence of spray flow,initial surface temperature and wettability on the heat transfer characteristics of spray cooling was analyzed.Based on the infrared thermal imaging technology as a non-contact temperature measurement,the change of average temperature and temperature difference ?T with time of the copper foil surface during spray cooling process were obtained.Experimental results show that the change of surface average temperature with time can be divided into two stages: the rapid cooling stage and the stable stage.When the solenoid valve is opened,the average surface temperature decreases rapidly,which is the rapid cooling stage.When the temperature is reduced to a certain extent,it remains basically unchanged,which is the stable stage.In addition,the surface temperature difference ?T with time amounts to the maximum value at first and decreases rapidly,then it becomes stable gradually.In application,the impact of the maximum value of the surface temperature difference ?T during the spray cooling process cannot be ignored.The larger the flow rate is,the faster the average surface temperature decreases,and the smaller the stability value becomes.The smaller the maximum value of the surface temperature difference ?T and its stability value get,and the more uniform the surface temperature distribution becomes.When the flow rate is the same,the average surface temperature and its stability value at the moment become larger as the initial surface temperature gets higher during the spray cooling process,and the greater the maximum value and its stability value of the surface temperature difference ?T become.Compared with the smooth surface,the contact angle of the droplets on the hydrophilic surface is small,and the spray droplets are rapidly spread on the heated surface and form a liquid film at first during the experiment,so the average temperature of the surface drops faster,and the maximum value of the surface temperature difference ?T is small.However,the hydrophilic surface characteristics enhances the adhesion of the working fluid,making the separation of the liquid film and the surface becomes more difficult.Besides,the liquid film elimination speed is reduced,leading to an increase in the thickness of the liquid film,which weakens the heat transfer capacity of the spray cooling.For the hydrophobic surface,the spray droplets are difficult to spread into a film and the wetting area of the heated surface is small at first,so the average surface temperature decreases slowly,and the maximum value of the surface temperature difference ?T is larger.In addition,the droplet rebound behavior appears when the spray droplets impact the hydrophobic surface,and it is not conducive to the heat transfer on the heated surface due to the strong effect of the droplet rebound sputtering.For the hydrophilic-hydrophobic combined surface,the hydrophilic area allows the spray droplets to spread faster on the surface without the rebound behavior,the average surface temperature drops rapidly,and the maximum value of the surface temperature difference ?T is small.There is no thicker liquid film formation on the surface because of the hydrophobic area.At the same time,the hydrophilic-hydrophobic combined surface has a strong influence in splitting the droplets,so the surface is filled with tiny drops.The stability value of the average surface temperature is small,while that of the surface temperature difference ?T is large.In order to obtain the heat flux and heat transfer coefficient of the heating surface during the spray cooling process,the experimental study of the spray cooling heat transfer characteristics of the heated copper column surface was conducted.The experimental results show that the heat flux increases with the increase of surface temperature.And the heat flux as well as the heat transfer coefficient increase as the the flow rate increases,and the spray cooling heat exchange capacity becomes larger.Compared with the smooth surface of the same temperature,the heat flux and heat transfer coefficient of the hydrophilic and hydrophobic surfaces are smaller,while the heat flux and heat transfer coefficient of the hydrophilic-hydrophobic combined surface are larger,which enhances the heat transfer capacity of spray cooling.