Study On Single-phase Flow And Subcooled Flow Boiling Heat Transfer Characteristics Of R134A In Horizontal Helically-coiled Tubes

Helically-coiled tubes have high heat transfer efficiency, compact structure, small fouling resistance, and it is widely used in efficient steam generator and cooler, nuclear reactor, petrochemical industry, aviation and space flight, microelectronics devices cooling, advanced fuel cell system cooling, refrigeration and low temperature technology. Since the centrifugal forces and the secondary flow resulted from the curved structure, subcooled flow boiling heat transfer in helical pipes is much more complex than that in the straight pipes. So subcooled flow boiling heat transfer and mechanism in helically-coiled tubes becomes one of the most important subjects in the two-phase flow and heat transfer at present.R134a is a new refrigerant, and belongs to the role of the CFC type. Its thermal property approximates to R12. The ODP is zero, the GWP is0.26, and is not ease to burn, no poisonous, no corrosive. R134a is now being widely used in refrigerators, freezers, automotive air conditioning systems to replace R12So it is significant in both theoretical research and practical applications to study on subcooled boiling in horizontal helically-coiled tubes.This article investigates heat transfer characteristics of R134a single-phase flow and subcooled flow boiling in one horizontal helically-coiled tube, uses fluid flow characteristics in helically-coiled tubes, bubble dynamics, bubble thermodynamics and subcooled boiling heat transfer mechanism, analyses the heat flux, mass flow rate and inlet subcooled temperature to the impact of heat transfer coefficient.An investigation was performed to study heat transfer characteristics of R134a single-phase flow in one heated horizontal helically-coiled tube. The experimental parameters are the pressures of0.42～0.46MPa, heat fluxes of1900～3000W/m2, mass fluxes of300～450kg/(m2· s) and inlet subcooled temperature of7.2～8.9℃. It was found that the wall temperatures around the circumference of one section were the highest at the underside(270°) and the lowest at the top(90°). The heat transfer coefficient increases with increasing heat flux and mass flux, but it is not obvious variation with reducing or increasing inlet subcooled temperature.Experiments were carried out to investigate the heat transfer characteristics of R134a subcooled flow boiling in one heated horizontal helically-coiled tube at low mass flux and low pressure conditions. The experimental parameters are the pressures of0.44～0.46MPa, heat fluxes of2500～5000W/m2, mass fluxes of300～450kg/(m2· s) and inlet subcooled temperature of7.0～9.2℃.Based on the analysis of the data, it was found that the wall temperatures around the circumference of one section were the highest at the underside(270°) and the lowest at the top(90°), especially with reducing inlet subcooled temperature. In the same section place, the heat transfer coefficient increases with increasing heat-flow rate and mass flux, especially with increasing heat-flow rate, but it reduces with increasing inlet subcooled temperature.