Study On Two-Phase Flow And Heat Transfer Characteristics Of R134a In Helical Coils

Helical pipes are widely used in a variety of practical applications,such as in steam generators and coolers,nuclear reactors,power plant boilers,ship power equipment,petrochemical industry,aviation electronic device cooling,advanced cooling system for fuel cells,food and pharmaceutical,as well as refrigeration and cryogenics,due to their high efficiency in heat transfer,compact in volume,and easiness in manufacturing.Since the centrifugal forces and the secondary flow resulted from the curved structure,the two-phase flow and heat transfer in helical pipes is much more complex than that in the straight pipes.As a result,the two-phase flow and heat transfer in helical pipes is still an unsolved important problem in the two-phase flow area. Because the magnitude,direction,and working mechanism of the gravitational and centrifugal forces continuously vary due to the continuous variation of flow directions in the horizontal helical pipes,the two-phase flow and heat transfer in the straight helical pipe is much more complex than that in the vertical helical pipes.To date, most of the research activities have been conducted on the gas-water and water-steam two-phase flows in the horizontal helical pipes,not much effort has been devoted to the two-phase flow and heat transfer characteristics and mechanism of environmentally friendly refrigeration of R134a in the horizontal helical pipes. Therefore,the primary objective of this thesis is to establish a new experimental setup for investigating the two-phase flow and heat transfer characteristics in the horizontal helical pipe.The visualization experiment is performed for developing the flow pattern figures.The two-phase flow and boiling heat transfer experiments for R134a in the horizontal helical pipe are conducted for determining the flow frictional pressure drops and boiling heat transfer characteristics,and new correlations for predicting the frictional pressure drops and boiling heat transfer characteristics for R134a are developed based on the obtained experimental data.In addition,the condensation heat transfer characteristics for R134a in the horizontal helical pipe is experimentally determined. The flow patterns for two-phase flow in the horizontal helical pipe are obtained through visualization experiment and the mechanism for the flow pattern forming and change features are analyzed.Two new flow patterns,which are tentatively named the "wave annular flow" and "super slug flow ",are observed,in addition to the well-known flow patterns,such as the bubbly flow,plug slug flow,stratified flow, wave stratified flow,dispersed flow,annular flows.Since the flow patterns are different in the rising section and the declining section,two different flow pattern figures are proposed for the rising and declining sections,respectively.The flow frictional pressure drops are experimentally determined for the two-phase flow of R134a in the horizontal helical pipe.The effect of the vapor quality, mass flowrate,and system pressure on the flow frictional factorφ₁₀² is obtained.The flow frictional factorφ₁₀² increases distinctly with increase in the quality and decreases in the system pressure,and increases lightly with increase in the mass flowrate.The new correlation is developed for predicting the flow frictional factors through the regression analysis on the test data with a maximum deviation of 19.7% under an uncertainty of±15%and the confidence level of 95.3%.The wall temperature distribution during flow boiling of R134a in the horizontal helical pipe is experimentally determined along with a mechanism analysis.The experimental result show that the wall temperature continuously decreases along the pipe length during the flow boiling of R134a in the horizontal helical pipe.The highest and lowest temperature occurred at the inner and outside of the cross sections, respectively,and the difference in temperature of the top is higher than that of the bottom cross section.The flow boiling heat transfer characteristics of R134a in the straight helical pipe is experimentally determined and the mechanism is analyzed.The effects of vapor quality,mass flowrate,pressure,and heat flux on the local boiling heat transfer coefficients are obtained.The local boiling heat transfer coefficients of R134a in the horizontal helical pipe continuously increases along the pipe length and with increases in quality,heat flux,and mass flowrate.The effect of mass flowrate on the heat transfer coefficients is more distinct in the higher quality range than in the lower quality range.The new correlation is developed for predicting the local boiling heat transfer coefficients through the regression analysis on the test data with a maximum deviation of 22.1%under an uncertainty of±15%and the confidence level of 90.3%.The condensation heat transfer characteristics for R134a flowing in the horizontal helical pipe is experimentally investigated.The effect of vapor quality, mass flowrate,and average condensing temperature on the average condensation heat transfer coefficients is determined.The vapor quality and mass flowrate have a distinct effect on the average condensation heat transfer coefficients.The coefficient increases with increase in the mass flowrate,and the effect of mass flowrate on the coefficient increase with increase in the quality.The experimental results show that the average condensation heat transfer coefficient for R134a in the horizontal helical pipe is larger than that in the straight tubes.In summary,some innovative research results have been achieved in the flow patterns,flow frictional factors,and heat transfer characteristics for flow boiling of R134a in the horizontal helical pipe.The research work can provide useful knowledge and design basis for retrofitting the present systems and developing new equipment and systems for refrigeration and air-conditioning applications.It also has potential application for the development of innovative helical pipe heat exchangers.