Smart Enhancement And Control Of Boiling Heat Transfer Over Mini Surface Structures Made Of Shape Memory Alloys

With the continuous progress of modern technology and the increasingly scarce energy resources,there is a higher requirement to enhance the heat transfer and develop the thermal control technologies.As a common method of heat transfer,boiling is applied to improve the heat transfer coefficient in the fields of high technology and traditional industry.At present,the micro-nano structured surface has become the main way of pool boiling heat transfer enhancement.It is known that after being processed,the geometry manufactured by traditional materials cannot change to adapt to the heat load of heating devices.However,the deformable surface fabricated by shape memory alloys can change its geometry with heat flux to achieve the adaptive temperature control of heating surface.Based on the above-mentioned idea,this study intended to experimentally investigate and theoretically analyze the boiling heat transfer characteristics of the undeformable surface structure and the deformable surface structure,so as to explore the boiling heat transfer law of the deformable structure and provide some reference and help for further research.An experimental system for testing pool boiling heat transfer performances was built and optimized from the airtightness of the experimental device and the stability of the experimental process.Then,four kinds of surfaces,including smooth surface,open-tunnel surface,closed-tunnel surface and deformable surface,were fabricated to compare the boiling heat transfer performance.Finally,four kinds of hydrophobic surfaces were prepared to study the effects of wetting characteristics on boiling heat transfer.As for experimental tests,this paper carried out the pool boiling heat transfer experiments on the undeformable surface structure and the deformable surface structure before and after surface treatment in the different fluids.The experimental results showed that the boiling heat transfer performance of the deformable surface was better than undeformable surface.Because the deformable structure has a strong adaptability,it can meet the heat transfer requirements at different boiling stages.At low heat flux,there are more nucleation sites seeded in the closed tunnel because of the increasing superheat.After the structure deformation at high heat flux,a number of nucleation sites are activated and the frequency of bubble growth and departure become faster to enhance the heat transfer performance.During the whole experiment,it was found that the deformable structure surface combines the merits of closed-tunnel surface at low heat flux and open-tunnel surface at high heat flux to achieve the thermal control.At the same time,this study combines the shape memory alloys and hydrophobic surface to improve the active nucleation sites density to enhance the boiling heat transfer.This paper compared the experiment data and classical theoretical model and found that the existing correlations for undeformable structures had not been fit for deformable structures,because large-scale deformation during the pool boiling process make the heat transfer mechanism different from the undeformable geometry structure.Thus,this paper deduces a new boiling correlation by analyzing the mechanism of heat transfer before and after deformation.And the correlation can make a better prediction to the heat transfer performance at different conditions.