A Visual Experimental Study On The Droplet Boiling Strengthened By Micro-pillar Structure

Spray and jet cooling are widely used in industrial and agricultural production because of its easy implementation and strong heat transfer ability.Droplet boiling is different from pool boiling.It has remarkable characteristics,such as uneven distribution of droplet film thickness and large heat flux at the three-phase contact line.The evaporation of gas-liquid interface in the droplet,the boiling of the solid-liquid interface on the wall and the condensation of boiling bubbles make the evolution process of bubble dynamics and it’s heat transfer more complex.In order to further enhance the thermal performance of droplets phase transformation and improve the controllability of its heat transfer performance,it is urgent to study the boiling heat transfer process and the dynamics of boiling bubble in droplets on smooth surfaces and micro-pillar surfaces.In this paper,silicon surfaces were used to text the droplet boiling.And two high speed cameras were used to monitor the boiling process of droplets on smooth surface and micro-pillar structure surface at the same time.The evolution of droplet morphology,bubble dynamic parameters such as nucleation distribution,nucleation density and bubble diameter were obtained.It revealed the mechanism of droplet boiling heat transfer enhanced by micro structure,At the same time,the effects of wall temperature,environment temperature,the height of droplet and the angle of boiling heat transfer surface on the boiling heat transfer performance of droplet boiling enhanced by micro-pillar structure were investigated.From the perspective of droplet morphology,the results show that the droplet deformation on the smooth surface decreases with the contact diameter and droplet thickness at the same time,,while the micro-pillar surface makes the droplet morphology experience a stable contact diameter due to the existence of the micro-pillar structure in the first stage.After the droplet’s thickness is reduced to a certain value,the droplet is quickly evaporated to dryness at the second stage.This process makes the droplet boiling process on the micro-pillar surface always have a larger heat transfer area.Under the condition that the heat transfer wall is inclined,the stable shape of droplets on the smooth surface after rolling mainly depend on the contact angle,and the effect of the lag angle of the micro-pillar structure on the droplet rolling process makes the increase of the heat transfer area more significant.From the perspective of the strengthening mechanism,in addition to increasing the heat transfer surface area of the droplet boiling,the micro-pillar structure significantly increases the nucleation density and reduces the diameter of the stable bubbles.When the droplet hits the wall,there is residual non-condensable gas on the structure surface.It makes the distribution of bubbles have a radial distribution law.From the perspective of influencing factors,when the environment temperature is high,the micro-pillar structure can only strengthen the heat transfer at certain wall temperatures.When the environment temperature is low,the micro-pillar surface can strengthen the heat transfer throughout the monitoring temperature range.And the enhanced heat transfer effect does not increase continuously with the wall temperature.The higher of the droplet drop height,the smaller of nucleation density and the smaller of the average diameter of stable bubble in the droplet.When the height of droplet is high,the micro-pillar surface shows better heat transfer enhancement effect.With the increase of the inclination of the wall surface,the boiling droplets on the micro-pillar surface have a more pronounced asymmetry.This asymmetry makes the droplets have a larger heat transfer area,while reducing the vaporization core density and bubble diameter,which makes the evaporation time reduced significantly.And the inclination of the wall surface can effectively improve the strengthening effect of the micro-pillar structure on droplet boiling heat transfer performance.The research in this paper reveals the micro-pillar structure enhanced droplet boiling heat transfer mechanism,which has strong academic value.And it is of practical guidance significance for the application of micro-pillar structure surface in spray cooling.