Experimental Study Of Pool Boiling On Micro And Nano-structured Surfaces

Directed by Professor Tang Dawei and Associate Professor Liang ShiqiangAt present, there is a higher requirement of thermal management technologies with the rapid development of science and technology and the increasingly tense situation in the energy. Enhancing boiling heat transfer is of great significance in the fields of high and new-tech and traditional industry. On the one hand, involving working fluid’s phase transformation, boiling heat transfer has the characteristics of low temperature difference and high heat flux so that it has been hailed as one of the effective way to solve the problem of cooling high power device. On the other hand, as boiling is widely used in various energy power equipments in industry, enhancing boiling heat transfer performance is helpful to improve the efficiency of energy transformation, so as to achieve the purpose of saving energy and improving economy. However, the enhanced boiling heat transfer can still not meet the requirement of the heat dissipation of high power devices. This study intended to experimentally investigate and analysis the boiling heat transfer performance on micro and nano-structured surfaces, so as to provide theoretical basis for further enhanced boiling heat transfer.At first, an experimental system for testing boiling heat transfer performances of micro and nano-structured surfaces was built. And the system was further optimized in terms of quantifying the high/low heat flux and controlling the temperature and pressure.Then, a smooth surface, several surfaces with microchannels, a nano-structured surface, and a surface with micro and nano-structured surface were prepared. The wetting properties of the surfaces were determined. And the nano-structures on the surface were also analyzed by SEM.As to experimental tests, this paper carried out the pool boiling heat transfer experiments on both the smooth surface and the surface with microchannels heating with high heat flux. The experimental results showed that the microchannels improved both the critical heat flux and the nucleate boiling heat transfer coefficient. To be specific, the critical heat flux on the surface with the microchannels is1.28times as that of the smooth surface, while its maximum heat transfer coefficient is 1.69times. Compared with several predictions based on calculation models for critical heat flux, the experimental results of critical heat flux on the smooth surface is limited by the hydrodynamic instability, and the critical heat flux density on the surface with microchannels can be explained using macrolayer dryout theory.Finally, the boiling heat transfer performances of a smooth surface, a nano-structured surface, several micro-structured surfaces with the microchannels of different size, and a surface with both micro and nano-structures were also investigated within a period of low heat flux density. Experimental results showed that nano-structures can improve the heat transfer coefficient under the low heat flux, and with the increasing of the heat flux, the heat transfer coefficient ratio between the nano-structured surface and the smooth surface would decrease. Besides, it was found that increasing widths and (or) the depths of the micro microchannels, or (and) reducing the fins’ widths can improve the heat transfer coefficient of the micro-structured surfaces. In addition, compared with the heat transfer performance of the micro-structured surface, the surface with both micro and nano-structures possessed lower heat transfer coefficients. This poorer heat transfer performance could be attributed to the reduced bubble nucleation density because of nanostructure coating deposition.