Self-Excitation Of Phase Interface To Control The Motion And Heat Transfer Of Bubbles And Droplets

With the fast growth of advanced technologies such as the energy Internet,5G,and artificial intelligence,data centers are expected to play a key role for the development of next generation information technology.Heat dissipation is a bottleneck issue limiting the development of high-performance electronic equipment,and thus,it is imperative to find solutions for effective thermal management of electronics.Boiling heat transfer is an efficient energy transfer mode and is considered an efficient cooling method for electronics.However,due to the inherent randomness and diversity of the boiling process,it is challenging to understand the heat transfer mechanism of boiling and control the movements of vapor bubbles and droplets.In addition,during boiling heat transfer process,part of the energy(such as bubble expansion work and surface energy)is dissipated by the liquid and cannot be used.Therefore,this dissertation proposes a new idea of using self-excitation of the interface to regulate the motion of bubbles or droplets,thereby improving boiling heat transfer performance.The main research contents and findings of this dissertation are described as follows:The improvement of pool boiling heat transfer using liquid metal soft surface is studied.The liquid metal soft surface is applied in the boiling heat transfer system for the first time,which provides a new idea of using self-excited elastocapillary wave to enhance boiling heat transfer.It is found that the liquid metal soft surface can effectively decrease the wall superheat at onset of nucleate boiling.For saturated boiling of water,compared with smooth surfaces,the wall superheats at onset of nucleate boiling for liquid metal soft surfaces with thickness of δss,1～100 μm and δss,2～120 μm are decreased by 7 K and 12 K,respectively.Liquid metal soft surfaces with thickness of δss,1～100 μm and δss,2～120 μm show increased the heat transfer coefficient by up to 60.5%and 149%,respectively.During bubble growth,elastocapillary waves are generated on the soft surface under the action of the vertical component of the surface tension at the vapor-liquid interface γlvsinθ.The elastocapillary waves on the soft surface disturb the thermal boundary layer near the wall and promote bubble departure,thus enhancing boiling heat transfer.The easier bubble nucleation on soft surface agrees with the bubble nucleation theory for a system including two immiscible liquids.By introducing liquid metal soft surface,the boiling heat transfer is enhanced by elastocapillary wave self-excitation oscillation without extra equipment and energy consumption,which provides a new idea and method for boiling heat transfer.The droplet motion controlled by explosive boiling at the liquid-solid interface is studied.A new mechanism for controlling droplet dynamics by explosive boiling is proposed.The self-propelling Leidenfrost droplet system consists of two heterogeneous surfaces with different functions.One is a smooth surface made of brass,on which the droplet is heated by the surface to levitate and maintain in the Leidenfrost state,acting as the transport pathway.The other is a copper ring with a chamfered inner wall,which is used as a trigger for explosive boiling.The roughness of the inner ring surface not only enhances radiation heat transfer,but also provides sufficient nucleation sites to trigger explosive boiling to emit a droplet.The thrust force is large enough to overcome inertia force and drag force,generating a straight-line trajectory for the droplet to reach a target location.The droplet passes through a focusing area with a wide diameter range(0.259～1.350 mm),which accounts for about 1%of the Leidenfrost heat transfer surface area.The droplet velocity reached 85 cm/s,which is 2 times of that reported in the literature.The D-1 scale law of Fth/Fi explains the dynamic droplet behavior at different droplet diameters.This work provides a new method for manipulating droplet motion at high temperatures.It will play an important role in applications requiring droplet traveling speed and directionality.Furthermore,Leidenfrost droplet evaporation enhanced by explosive boiling at the liquid-solid interface is studied.A new Leidenfrost droplet heat transfer mode is proposed,designed to shorten the film boiling time and increase the probability of contact boiling.Through theoretical analysis,it is found that the key to enhancing droplet evaporation is to increase the droplet collision frequency f.Improving f can be achieved by reducing the ring diameter Dr and increasing the ring inner wall roughness.Reducing the diameter of the ring can significantly increase the evaporation rate of the droplet.Compared with the 28 mm diameter ring,the 13 mm diameter ring increases the droplet evaporation rate by 171%.The rougher the inner wall of the ring,the better the effect of enhancing droplet evaporation.When the temperature of Leidenfrost surface is Tw=260.0℃,compared with the ring with roughness Ra=0.35 μm,the ratio of droplet evaporation time to initial diameter r/D0 is decreased by 49.1%for the ring with roughness Ra=1.71 μm.The system is low-cost,efficient,and scalable.It can realize improved droplet evaporation rate and enhanced heat transfer when the Leidenfrost phenomenon on the main heating surface is unavoidable.This discovery provides a new idea for the structural design of high-efficiency heat exchange equipment,and can find important applications in energy transfer systems.Finally,the self-excited bouncing motion of the Leidenfrost droplet on a concave surface is studied.Leidenfrost droplets gently deposited on fully rigid surfaces experience self-induced spontaneous oscillations and bouncing.The study found that the Leidenfrost droplet bounces higher on the heating surface with a larger radius of curvature.Compared with heating surface with a radius of curvature Rc=9.0 mm,the droplet bounce height on heating surface with Rc=18.0 mm is increased by a maximum of 22%.Droplets on surfaces with different curvatures have similar dynamic behaviors,which are divided into five regimes:droplet vertical oscillation regime,large droplet low bouncing regime,medium droplet high bouncing regime,small droplet low bouncing regime,late stage blast or take-off regime.Theoretical analysis shows that the power of self-excited bouncing of Leidenfrost droplets on the heating surface mainly comes from the pressure Fp of the bottom vapor layer.The relationship between the pressure Fp and the thickness δof the vapor layer is Fp～δ-4,the capillary wave at the bottom interface of the droplet causes local overpressure,and the pressure instantly exceeds the gravity of the droplet itself,resulting in the self-excited bouncing motion of the Leidenfrost droplet.When Leidenfrost droplets impact the heating surface and spread in the circumferential direction,they will be restrained by the tangential gravity component Fgsinθ;when the radius of curvature is large,the value θ is small,and the restraining force is weak.Thus,the spreading area of the droplets on the heating surface is larger.The vapor pressure at the bottom of the droplet is proportional to the contact area SA between the droplet and the heating surface,and hence,the Leidenfrost droplet bounces higher on the surface with a larger radius of curvature.The complete description of the self-excited bouncing process of Leidenfrost droplets and the study of the effects of different surface curvatures on Leidenfrost droplet bouncing dynamics provide a new route for droplet manipulation and energy harvesting.