Study On The Enhancement Of Boiling Heat Transfer Characteristics Of Microstructure Heat Transfer Surface

With the characteristics of low temperature difference and high heat flux, pool boiling heat transfer is widely applied in various engineering technical fields. Domestic theory research of boiling is still in the development stage,especially in theory mechanism of heat transfer in pool boiling with different wetting properties, different size of microstructures and fifferent working medium. It will have a great important practical significance to provide a reliable theoretical basis for the enhancement of boiling heat transfer performance by studing the mechanism of boiling heat transfer deeply. Boiling characteristics of heat transfer surfaces with different conditions were studied using theoretical and simulation analysis method based on the overview of basic theory of boiling heat transfer and influence of micro/nano structure, wetting properties, nanofluids to boiling heat transfer.Heat transfer surfaces with microstructures which have different size were selected as the main research object in the paper. Bubbles' behavior, temperatures on heat transfer surfaces and heat transfer coefficients were depicted and calculated to analyze the boiling characteristics under different wetting properties and working medium. The changes of wetting properties included hydrophilic properties and the mixture of hydrophilic and hydrophobic properties. By analyzing these parameters during the boiling process of each case, it can be found that boiling heat transfer performance of surfaces which modified by the combination of microsyructures and wetting properties(both hydrophilic property and the mixture of hydrophilic and hydrophobic property) were improved when compared with the boiling perfoamance of smooth flat. Under conditions of hydrophilic property and the mixture of hydrophilic and hydrophobic property, the maximum descent rate of temperature on heat transfer surface and maximum ascent rate of heat transfer coefficient were appeared on the surface with microstructures which the width and height of microstructures were 3 and 9, and the maximum descent rate of temperature and ascent rate of heat transfer coefficient under different wetting properties were6.75% and 7.07%, 243.60% and 244.88%, respectively. By analyzing and comparing boiling performance on surfaces with different wetting porperties can be found that the heat transfer performance on flat with hydrophilic property was better than it on flat with hydrophilic and hydrophobic property, and vice versa. Additionly, it had a significant advantage of average heat teansfer coefficient on surfaces with hydrophilic and hydrophobic property compared with surfaces with hydrophilic property when the heights of microstructures were 3 and 9. The range ofdifference between two wetting properties on ascent rates of heat transfer coefficient was from 30.56% to 71.05%.Under the same conditions of mathematical model and the mixture of hydrophilic and hydrophobic property on heat transfer surfaces, the boiling characteristics of Cu/water nanofluid(with 3% volume fraction) was also analyzed in this paper. The results showed that the lowest temperature on heat transfer surface was 391.38 K which appeared on the surface with microstructures and the width and height of microstructures were 3 and 9 due to the significant effect of the microstructures' size on temperatures. However, due to the collective effect of hydrophobic property and physical property of nanofluid, the contact area between solid and liquid was reduced and heat transfer was weaken when the height of mocrostructures was 9. The maximum value of average heat transfer coefficient was appeared at the surface with microstructures which the width and height of microstructures were 1 and 7, and the ascent rate of it was 136.74% when compared with the average heat transfer coefficient on smooth flat. Simultaneoously,under premise of appling Cu/water nanofluid as working fluid, boiling heat transfer performance of surfaces which modified by the combination of microsyructures and the mixture of hydrophilic and hydrophobic property were improved when compared with the boiling perfoamance of smooth flat. Because the viscosity of Cu/water nanofluid had a apparent increase, the bubble average departure size grew larger and departure frequency and velocity became relatively low, which caused the boiling heat transfer performance of Cu/water nanofluid were declined under the same conditions when compared with boiling heat transfer performance of pure water.