Pool Boiling Heat Transfer On Multi-scale Porous And Modification Structures Of Surfaces

Compared with single-phase heat transfer,boiling heat transfer has higher heat transfer coefficient and smaller heat transfer temperature difference,widely used in the high heat flux heat transfer equipment and devices,such as nuclear reactors,supercritical boiler,high power LED,supersonic aircraft combustion chamber.So how to enhance boiling heat transfer has become the focus research in the field of heat transfer.In view of the fact that boiling heat transfer is closely related to the micro-structure morphology and wettability,at the same time it is difficult to solve the contradiction between the steam overflow and liquid suction in the process of boiling heat transfer for traditional single scale structure surface.Therefore in present paper,boiling surfaces were prepared by two ways.Multi-scale mastoid structure surface can be prepared by sintering copper particles,and modification structures of surfaces were prepared by chemical method.A series of experiments were performed to investigate and analysis the boiling heat transfer performance on these surfaces,so as to obtain the mechanism and regularity of heat transfer enhancement.Firstly,we used copper sintering method to prepare micro-nano porous materials,in order to acquire the capillary performance of these materials.Some experiments were tested to study liquid capillary rise height and the suction process of a liquid droplet.The results showed that the capillary rise height was exponential growth with time,and related to working medium density,permeability and porosity of the porous materials.Compared with micro scale porous materials,the nano materials have larger capillary pressure,but lower permeability and liquid capillary rise height.Capillary performances of porous materials can be considered as the best balance of capillary pressure and permeability.The responsible preparation of micro-nano multi-scale pore is the key to improve capillary performance of porous materials.Then,the multi-scale mastoid surface was prepared,and a series of pool boiling experiments were conducted using deionized water as working fluid at atmospheric pressure.The effects of surface structure,liquid temperature,heat flux and surface superheat on the pool boiling heat transfer were investigated.The results showed that multi-scale surface can perfectly meet the different demands for pore size between vapor release and liquid suction.Compared with plain surface,the multi-scale surface superheat at the onset of nuclear boiling was decreased significantly,the heat flux was expanded which up to 196 W/cm2,and the boiling heat transfer coefficient can be improved 5~6 times.Finally,in order to study the effects of surface wettability on the boiling heat transfer,four kinds of surface wettability were prepared,which are super-hydrophilic surface,enhanced hydrophilic surface,hydrophilic surface and super-hydrophobic surface.And a series of experiments were performed using the water as the working fluid at atmospheric pressure.The results showed that with the increase of solid liquid static contact angle,the boiling curve had gradually shifted to the larger degree of superheat,that is to say the heat transfer effect weakened.In the four kinds of surfaces,super-hydrophobic surface was easy to generate bubbles at onset of nucleate boiling,which had lower surface superheat and better heat transfer performance.However,for super-hydrophilic surface the existence of micro-nano sheet structure,it not only increased phase change area and the number of nucleate sites,but also had larger capillary pressure which promoted the surface wetting.So the bubble departure frequency was large,the boiling heat transfer was enhanced greatly.The maximum heat transfer coefficient was nearly 1.7 times than that of hydrophilic surface and the CHF can be improved to 131 W/cm2,which showed better heat transfer performance in pool boiling.What's more,the CHF of four kinds of surfaces were increased with the degree of super-cooling.But the deviation trend of four kinds of surfaces' boiling curve is different with the degree of super-cooling.Under low heat flux,the heat transfer effect of super-hydrophobic surface was increased along with the degree of super-cooling,while the super-hydrophilic surface' boiling heat transfer is restrained on the larger degree of super-cooling.