Study On Boiling Heat Transfer Characteristics And Critical Heat Flux Characteristics Of R134a In Horizontal Helically-coiled Tubes

Two-phase flow is widely used in science technology and industry fields, especially in boiler, nuclear reactor industry, energy, power plants, chemical, refrigeration and low temperature, mining of gas and oil, etc. Two-phase flow is one of the most important subjects in study over the world. Many scientists are working on this issue. Because of the widely use in industry, two-phase in helical tubes now is becoming key subject.As the increase of people's awareness on ozone layer protection, the new environmentally friendly refrigerants are gradually replacing the role of the CFC type (CFCs) refrigerants which is serious damage to the ozone layer in refrigeration industrial applications. R134a is a new type of alternative refrigerant with zero ozone-depleting potential. The property of R134a is as follows. The ODP is zero, the GWP is 0.26, and is not ease to burn, no poisonous, no corrosive. And now R134a has being used in industry field. So it is necessary to research the boiling heat transfer of R134a in helical tubes in both theoretical research and practical applications. And this study is of significant in both theoretical research and practical applications.This article includes the construction, test and operation of experiment system using alternative refrigerant R134a as working material, acquisition and processing of the experiment data, the boiling heat transfer characteristics and critical heat flux characteristics of the refrigerant R134a, The experiment range is as follows:the mass fluxes: G=50～450kg/(m2·s), the pressure of the system is p= 0.3～0.9MPa, the heat fluxes:q= 5～25kw/m2, the vapor quality:x=-0.2～1.The boiling heat transfer characteristics of helical tube is the focus of this article. The result of the experiment are as follows:the boiling heat transfer coefficient of helical tube is closely relation to the pressure of the system, the mass flux, the heat flux. It increases along the direction of helical tube length, and it increases in the ascending segment and decreases in descending segment, the influences of pressure, heat flux and mass flux on the heat transfer characteristics are analyzed on the basis of experiment data, the results show that:the heat transfer coefficient increases with the increasing of mass flux, the influences of other factors are similar. Based on the analysis of the data, a new correlation for predicting the two-phase flow heat transfer coefficient in helical tubes is proposed. The error between the correlation and experiment data is small. The correlation can reflect the result well.The critical heat flux of helical tube is studied on basis of experiment. It is noted that the CHF (critical heat flux) value have an approximately linear increasing trend with mass fluxes and it seems like that this trend slightly moves up at lower inlet vapor qualities. It is also noted that CHF values decrease linearly with increasing inlet vapor qualities. And the system pressure has less effect on CHF value than mass flux and inlet vapor quality.