Investigation On Convective Boiling Heat Transfer Of R-134a In Helically-coiled Pipes

Helical tubes are widely used in energy and power, petrochemical engineering, nuclear reactor, refrigeration and air conditioning, etc by its higher heat transfer efficiency, compact in structure and ease to manufacture. Because of the complexity of the two-phase flow and short of the micro-knowing and quantitative analysis of the heat and mass transfer and dynamical action on the interface of the vapor and liquid phases, two-phase flow boiling heat transfer in helical tubes becomes one of the key subjects on two-phase flow. CFC refrigerants have the quick destruction of the ozone layer in the outer atmosphere around the earth, its use will be forbidden. R134a is currently considered to be the main replacement to CFC-12 by the world and it has begun to be used in commerce. Its ODP is zero, GWP is 0.26. And it is not ease to burn, no poisonous, no corrosive. It is the most practical replacement refrigerant at present. It is of significant importance to research the boiling heat transfer of R134a in helical tubes in both theoretical research and practical applications.The experimental unit for R134a on flow boiling heat transfer in the helically tube is developed and the flow boiling heat transfer characteristics is studied . The test section is made of a stainless steel tube with the length of 7.544m and inside and outside diameters of 8.1mm and 10mm respectively. The diameter of coil is 300mm The experiment range is as follows: the average saturated evaporating temperatures:t_s = 5 ～ 20℃, the mass fluxes:G=100 ～ 400kg/(m²s] , the heat fluxes:5 ～ 20kw/m² ,the vapor quality: x = 0.1～ 0.8 .A data-processing program is compiled with the computer language of visual basic.and it includes the input page of measured data, the input page of table-lookup data and the output page of the results. The program is convenient for the batch processing of the experimental data. Experimental data at various experimental condition is acquired and the error analysis is done..The experiments show that: the average heat transfer coefficient increases with the increasing of the mass flux and the evaporating temperature. The local heat transfer coefficient increases with the increasing of vapor quality. When heat flux is low, the local heat transfer coefficient increases monotonically with the increasing of the vapor quality. When the heat flux is high enough, the local heat transfer coefficient increases at first, but when the vapor quality reaches 0.71, it begins to decrease slightly. The vapor quality when the local heat transfer coefficient begins to decrease is lower as the heat flux is higher. Comparing the variations of the local heat transfer coefficient versus the vapor quality in helical tubes with that in the horizontal strait tubes, the vapor quality when the local heat transfer coefficient begins to decrease in strait tubes is lower than that in helical tubes, and decreases faster than that in helical tubes.Based on the experimental data in this paper and the correlation proposed by Jung et al. and Cavallini and Zecchin, a new correlation for predicting the two-phase flow heat transfer coefficient in helical tubes is proposed. The correlation expressed as a function of the equivalent Dean number, De_Eq; Prandtl number, Pr₁; Martinelli parameter, X_tt ; and boiling number, Bo . And experiment heat transfer coefficient is compared with the predicted values by the correlation. It can be seen that the experimental results agreed well with those predicted by the correlation with a deviation of ±15% .