Effect Of Solid Particles On Pool Boiling Heat Transfer

With the development of-economy,the orientation of technology products and precision devices were toward to miniaturization.How to solve the problem of equipment heat dissipation had become the key to restrict the development of devices.The boiling heat transfer could effectively save energy and reduce costs that as a efficiently transfer method,and the wettability of surface and nucleation had a greater impact on boiling heat transfer.Therefore,in order to investigate the effect of particle wettability on boiling heat transfer,this paper firstly prepared different wettability of particles and different size of particle 0.5 mm,1.0 mm and 5.0 mm.Then combining experimens and theoretical analysis that particles collide bubble and droplet collide particles under cold conditions.Finally,this paper modified the wettability of partciles and carried out pool boiling experiments.First,acquire the unique behavior characteristics that droplets collide on the Janus particle(amphiphilicity)sphere surface under cold condition,the diameter of Janus particles and droplets were 5.0 mm and 2.0 mm.Droplets were used to collide hydrophbilic surface,hydrophobic surface and hydropholic-hydropholic boundary of Janus particle,under four Weber numbers which are 2.7,10,20 and 30,the corresponing Reynold numbers were 621.8,1191.9,1589.2 and 2185.1.The collision process can be divided into four stages:spreading-retraction,oscillation and rebound.Under different Weber number,the wettability of particle surface played an important role in behavior characteristics.On the hydrophbilic surface,as time increased,the diffusion coefficient gradually increases and eventually stabilized.When Weber number increased,the difference of spreading coefficient for droplet would gradually decrease under adjacent Weber number,indicating that droplets spreading were mainly affected by inertia.On the hydrophobic surface,the spreading coefficient on the figure appeared a "parabola" shape.However,when droplets collided on the hydropholic-hydropholic boundary,droplets showed spreading and rebound behavior characteristics simultaneously.At the beginning of droplets spreading,both sides of the hydrophbilic-hydropholic boundary had almost same the spreading coefficient same on With the increase of time,the part of droplets on the hydrophobic would be attracted by the hydrophbilic side surface and go into hydrophbilic side zone.In order to explain this phenomenon,the concept of line tension was introduced and found that the line tension on the hydrophilic side was less than that on the hydrophobic side by analyzing the force on both sides of the droplets.Secondly,an experimental platform for different wettability particle(0.5 mm and 1 mm)impacting bubble were set up and carried out.Through experiments and theoretical analysis,it was found that when hydrophilic particles collided on bubbles,the process could be divided into:particle collide to bubble process,collision and rebound.When superhydrophobic particles collided on bubbles,The process could be separated into four stages:particle collide to bubble process,particle collision and particle escape bubble process.The Janus particles could be divided into four processes:particle collide to the bubble,collision,particle rotation and particle sliding into the bubble.The results showed that the particle acceleration was 2.03 X 10-3 mm/ms2 in the process of particles collide to the bubble.Due to the acceleration was so small that can negligible,therefore,it could be considered that the Janus particles’ velocity are constant.In the process of rotation,rotating torque was calculated 1.14×103 μN·μm.By comparing 0.5 mm Janus and 1 mm Janus particles,it was found that fine particle had a smaller kinetic energy to potential energy ratio and was more likely to adhere bubble.Finally,under atmospheric pressure and deionized water,different wettability of particles were added to explore the effect on boiling heat transfer by pool boiling experiments.It was found that 1 mm hydrophilic particles and 1 mm Janus particles have better boiling heat transfer characteristics.The CHF and HTC of hydrophilic particles increased by 15.4%and 83%,and Janus CHF and HTC increase by 16%and 123.8%.On the contrary,single layer 1 mm superhydrophobic particles made heat transfer worse,CHF and HTC both deteriorated.Then,the influence of different wettability particle layers(N=1,N=3 and N=5)on boiling heat transfer were investigated,and it was found that both hydrophilic,superhydrophobic and Janus particles had the best heat transfer performance at N=3 layers.At the boiling start point,the superheat of N=3 Janus particles was 0.62 K,compared with the smooth surface to reduce 7.83 K.It has a great improvement compared with the smooth surface,CHF and HTC are 144.1 W/cm2 and 3911.4 kW/(m~2K),which are 1.3 times and 92.1 times of smooth surface.