Phase Change Heat Transfer On Surfaces With Smartly-controllable Wettability

Researches of boiling and condensation phase change phenomena have a long history.Those phase change phenomena are widely existed in daily life,from the steam locomotive which uses boiler to generate steam to the thermodynamics cycle of power plant;from heat exchangers which are widely used in petrochemical and automobile industries to the treatment of sewage and seawater;from the concern of boiling crisis in nuclear industry to the need for higher heat transfer rate in energy storage system;from the conventional heat dissipation in refrigeration and air conditioning to the thermal control under ultra-high heat flux condition.The application of phase change heat transfer has been changing,but the research on phase change enhancement has never stopped,from the selection of heating material to the adjustment of working liquid and the control of solid-liquid interface.However,the surface property requirements are dynamically varying from the initial of nucleation to the final departure of bubbles/droplets,as well as at low heat flux and high heat flux,and the state of art technique is difficult to meet those requirements.In recent years,smart material with responsive wettability has received great attention,but its application in phase change heat transfer is seldomly studied.In this study,we proposed a new strategy to adaptively enhance phase change heat transfer by smart material.The thesis is arranged as follows,first,simulation by lattice Boltzmann method was conducted to investigate wettability effect on boiling;then,boiling performance on smart controlled wetting surface was studied numerically and experimentally;last but not least,we extend the application of smart material to condensation heat transfer enhancement.The main content of this thesis can be summarized as follows:1.Li-MRT model was adapted in this work and the temperature equation was added by hybrid method.A program which used C++ was made and parallel computation was achieved by adding MPICH library.The accuracy of the model and programming was verified by comparison with classical benchmark problems.Then,the influence of wettability on single bubble dynamics was studied by lattice Boltzmann method on both smooth and structured surfaces.For multi bubbles conditions,the effect of wall superheat and wettability on nucleation site interaction was investigated.The boiling curves on both hydrophilic and hydrophobic surfaces were obtained by stepwise heat flux increase and the influence of wettability on heat transfer coefficient and critical heat flux was analyzed.2.The boiling performance of smart controlled wettability surface was studied numerically.The results demonstrate that the relation between wettability and temperature could affect the boiling performance and there is an optimized temperature wettability relation.So,the best relation was derived base on classic boiling heat transfer theory,this relation could be a guidance for future smart material design.3.The numerical results were verified by experimental results.A series of boiling experiments were conducted on smart surface with and without structure.Visualization images were taken to investigate bubble behavior and analytical analysis was conducted to illustrate the enhancement mechanism on smart wettability control surface.Compared with simulation results,boiling performance in experiment was better that theoretical prediction.We attribute this increase to the initial hydrophobicity,since boiling is a successive process,the vigorous nucleation at low superheat will enhance subsequent boiling process on hydrophilic state.4.The application of smart material to condensation was also studied.First,wettability effect on thermal resistance and heat flux was investigated numerically and theoretically,the results show that heat flux rise as contact angle increase.While small contact angle,that is,the hydrophilic surface has a small nucleation energy barrier and it's difficult to nucleate on hydrophobic surface.The performance of condensation heat transfer on smart wettability surface show that the surface can integrate the merits of both hydrophilic and hydrophobic surfaces and enhance condensation heat transfer.Through the researches mentioned above,the feasibility and effectiveness of this smartly-controllable wettability strategy to enhance phase change heat transfer was verified.The mechanism of wettability effect on bubble dynamics and nucleation sites interaction was revealed and the principle of this smart controlled wettability on phase change heat transfer enhancement was clarified.Besides,an optimized relation between wettability and temperature was derived theoretically to provide guidance for future smart material design.This research opens a new method to heat transfer enhancement and smart control,and it could be combined with state of art enhancement method with structure to break through the technical bottleneck of phase change heat transfer enhancement and regulation,and to meet the requirements of variable heat flux load and ultra-high heat flux condition.