Thermal-hydrodynamic Study On The Interplay Of Nucleation Sites And Heat Transfer Characteristics Of Nucleate Boiling

With the advancement of micro/nano fabrication and surface modification technologies,the enhancement technologies for boiling heat transfer have been developed rapidly in recent decades.However,due to the complexity of the process,the inherent mechanism of boiling heat transfer is not yet fully understood.In order to investigate the interaction between the bubble dynamics and heat transfer of the boiling surface in nucleate boiling,a thermal-hydrodynamic coupling method including two-phase hydrodynamics,microlayer evaporation,and heat transfer within the solid heater is established.The bubble growth and surface temperature fluctuations during the single-and twin-bubble boiling process are simulated.The leading contribution of the microlayer evaporation to the growth of vapor bubble is further confirmed by single-bubble boiling experiments.Moreover,the bubble coalescence and heat transfer characteristics in high heat flux boiling are also analyzed.The main innovative contents are as the following:A thermal-hydrodynamic coupling method for boiling simulation is first developed,which includes three sub-processes:the solid heat conduction,the microlayer evaporation,and the hydrodynamics of two-phase fluid.By this method,the spatial and temporal temperature distribution of the heating surface during the bubble growth and departure process can be modeled.In the boiling simulation,the vapor required for the bubble growth is produced by the evaporation of microlayer.The moving interface of the bubble is captured by the VOF(Volume of Fluid)model in the OpenFOAM framework.In the computation of heat transfer in the solid heater,three types of the boundary conditions on the boiling surface are employed as natural convection,microlayer evaporation,and adiabatic condition in the dry spot region.The bubble root obtained from the two-phase flow computation is used as the criteria for distinguishing the regions of natural convection and vapor evaporation.The coupling of heat transfer at the solidfluid interface and the bubble dynammics within the fluid pool is thus achieved via the boundary conditions at the boiling interface.The proposed method is well validated by simulating the single-bubble boiling experiment in the available reference,where the bubble growth process,heater temperature distribution,and microlayer thickness distribution are in a favorable agreement with the experimental results.The effects of boiling heat flux and wall contact angle on the bubble departure behaviors are subsequently studied.The results show that both the bubble departure diameter and frequency increase with increasing of the applied heat flux.Moreover,due to the stronger wake flow evoked by the rising bubble,higher heat flux can cause the earlier departure of the growing bubble and lead to irregular bubble departure periods for successive bubbles.It is confirmed as well that the bubble departure diameter increases with the increasing of contact angle.The reason is that a larger contact angle can form a wider microlayer area,leading to the removement of more heat from the surface.It's also shown that the size of cavity and heater material have a significant impact on the activation of nucleation sites.The critical activation superheat becomes higher when the open diameter of the cavity decreases,causing the appearance of a waiting time before the activation of a nucleation site.The duration of the waiting time is dependent on the matierials of the heater and input heat flux.Compared with the silicon heater with a poorer thermal conductivity,the fluctuation of surface temperature in the copper heater is found to be smaller during the bubble growth period.Meanwhile,the temperature recovery within the copper heater is more rapid,leading to a shorter waiting period.It's therefore implied that waiting time is the key factor for the influence of boiling heat transfer performance in different materials.Boiling simulation of water on a copper with twin nucleation sites are further performed using the thermal-hydrodynamic coupling method.The results show that two bubbles growing from different nucleation sites are observed to approach and attract each other,which should be attributed to the wake flow left by the neighboring rising bubbles.Moreover,hydrodynamic interaction between the adjacent bubbles can cause the alternative activation of the nucleation sites and lead to irregular fluctuations in the temperature of the heating surface.The activation of the nucleation site with a larger diameter can inhibit the nucleation of the smaller one,inducing intermittent activation of the smaller-size nucleation sites.The smaller the nucleation site is,the waiting time before its activation is longer,leading to a worse heat transfer performance.In the single-bubble boiling experiment,the sequential images of the bubble growth are obtained and the variations of diameter,volume,aspect ratio,as well as root radius of the growing bubble are measured.Through correlation analysis,it is found that the bubble growth rate is remarkably associated with the variation of bubble root radius,indicating that the evaporation of microlayer plays a dominant role in the phase change process of the singlebubble boiling.Used the extracted bubble root radii and departure period,the heat transfer process within the solid heater is numerically analyzed by matching the bubble growth rate and the temperature inside the heater,and then the microlayer thickness in the experiment is predicted.Regarding the high-heat-flux nucleate boiling with a larger density of active nucleation sites,we first experimentally observe the continuous coalescence process of the isolated bubbles,the formation,and the evolution of mushroom-type bubbles.Then a coupling model based on the evaporation of macro layer is proposed,which emphasizes the heat transfer process at the solid-fluid interface underneath the mushroom bubble.In order to capture the activation of the cavities at the scale of micrometer,the boiling surface is divided into several subdomains,where a few cavities are scattered randomly.When it is necessary to identify if the cavities inside the subdomains are activated or not,they are further divided into finer grids.When some cavities become active,coalesced bubble forms firstly within the subdomain Then coalescence occurs continuously between the coalesced bubbles at the neighboring subdomains,and a much larger coalesced bubble would appear.The local boiling heat fluxes calculated by the macrolayer evaporation model are then assigned as the boundary conditions of every grid at the solid heater surface.The activation,growth,and departure process at the boiling surface can be thus modeled by the mixed-dimensional coupling method.To investigate the influence of the roughness of the heater surface on boiling heat transfer,boiling simulations are performed in various surfaces with different numbers of randomly distributed cavities.A good agreement for the simulated boiling curves with the available experimental data is obtained.Results show that the surface with a higher roughness has a better heat transfer performace,hence the boiling curve moves to the left of that in a smoother one.Meanwhile,it is shown that nucleation site density is more sensitive to the wall superheat for the surface with a larger roughness,resulting in the appearance of different departure periods of successive coalesced bubbles.