Study Of Interfacial Heat Transfer On Droplet Impacting Heated Surface Evaporation Using Temperature Sensitive Paint

Droplet impacting can be seen everywhere in life,which can enhance the heat transfer effect between droplet and wall,and has high research value in many fields.However,the evaporation after impacting contains complex fluid motion law and heat and mass transfer mechanism,which brings difficulties for further research.As a new non-contact temperature measurement method,temperature sensitive paint(TSP)has the advantages of fast response,simple calibration,no requirement for working medium and low cost,which is an effective method to study interfacial heat transfer during droplet impacting heated surface.In this paper,TSP was made and applied to measure the temperature distribution between the droplet and the wall.Furthermore,the effects of different factors on the heat transfer characteristics during droplet evaporation process were discussed.Firstly,the evaporation of sessile droplets on the heated wall was carried out.The temperature distribution of the contact surface was recorded by using TSP.Combined with the established heat transfer model,the heat flux distribution of the contact surface was calculated and observed by using infrared thermal imager.The results show that sessile droplet evaporation can be divided to three stages: initial droplet heating stage,convective cell evaporation stage and thin film evaporation stage.Among them,the convective cell evaporation stage is the main stage of evaporation process.There is obvious temperature gradient in the temperature distribution captured by the TSP,and the convection unit can be observed by infrared thermal imager.Then,the experiments were carried out on three droplets: pentane,HFE7100 and hexane,among which the boiling point of pentane was the lowest,the heat flow movement in the droplet was the most intensive;the viscosity of HFE7100 was the lowest,the heat flow inside the droplet was the strongest;the latent heat of HFE7100 vaporization was the smallest,and the maximum heat flux density was the lowest during evaporation process.Based on the experiment of sessile droplet evaporation,the TSP was applied to study the heat transfer characteristics of droplet impingement heated wall during evaporation process.Two fluids,pentane and HFE7100,were used in the experiments.The droplet volume was 10?L,the initial surface temperature were 50?and 60?,and the droplet impacting velocities were 0.8m/s and 1.2m/s respectively.The distribution and variation of heat flux were calculated by measuring the temperature distribution of contact surface during impact.Because the boiling point is lower than the initial surface temperature,the nucleation occurred inside the droplet,and the heat flux was higher than HFE7100;with the We number increased,the droplet was easy to broken,the effective heat transfer area increased,and the total evaporation time decreased;with the initial wall temperature increased,and the droplet fragmentation was more intensive.However,splash occurred after the HFE7100 droplet hit the surface,and the heat flux and effective heat transfer area oscillated;with the We number increased,the oscillation duration increased;and with the initial wall temperature increased,the total evaporation time of HFE7100 became shorter,the larger the droplet spreading size was,the larger the effective heat exchange area was.