Experimental Studies On Flow Boiling Heat Transfer Inside The Three-dimensional Horizontal Enhanced Tubes

Recently,in view of the sustainable development of energy and environment protection,enhanced heat transfer technology has become an important method to improve energy efficiency.With the development of industry and technology,a number of higher requirements for enhanced heat transfer have been proposed.At home and abroad,the research on enhanced heat transfer is also abundant.At present,heat exchange equipment is gradually tend to miniaturize,reducing weight and volume,making it more compact,and greatly increasing the heat transfer coefficient.During the flow boiling,a large amount of latent heat of vaporization is absorbed when the fluid changes from the liquid phase to the gas phase,thereby achieving efficient heat exchange.This paper studies the effects of the patterns,heat transfer and pressure drop characteristics using different refrigerants inside the different enhanced tubes.Based on the experimental results,the main flow patterns involved in the flow boiling heat transfer was observed,which are suitable for semi-empirical prediction models of heat transfer coefficient and pressure drop inside the different enhanced tubes.It is expected that this model can not only verify the experimental data,but also better predict the variation tendency of heat transfer coefficient and pressure drop with different experiment parameters such as vapor quality,mass flux,and heat flux.On the one hand,this research can provide a way for the development of high-efficiency enhanced tubes and make it widely used in industrial production activities;on the other hand,it has an important significance for improving the efficiency of heat exchangers.The experimental research of the flow boiling heat transfer characteristics was studied in the three-dimensional enhanced tubes.The results show that the flow boiling heat transfer coefficient increases with the increase of mass flux,vapor quality.However,it decreases with the tube diameter and the saturation temperature increasing.The friction pressure drop increases with the mass flux and vapor quality increases gradually.The enhancement effect of the flow boiling heat transfer is very obvious in the three-dimensional enhanced tubes.The flow boiling heat transfer coefficient in the enhanced tubes is about 1.5 to 2.5 times than that of the smooth tube,and the friction pressure drop is only about 10% larger than that of the smooth tube.This is mainly due to the large number of nucleation sites produced on the dimpled surface,increased interfacial turbulence,disrupted the flow boundary layer,flow separation and secondary flow.During the flow boiling heat transfer process,the flow pattern was analyzed in the three-dimensional enhanced tube and smooth tube.The flow boiling pattern could be visualized by a high-speed camera,which included stratified,stratified-wave,slug,intermittent,annular and dryout.Based on the experimental data,a semi-empirical prediction model applied to the enhanced tubes for flow boiling heat transfer coefficients and friction pressure drop is proposed.The improved asymptotic model can predict 90% experimental results within an error range of ±20%.