Flow boiling of halogenated refrigerants at high saturation temperature in a horizontal smooth tube

Several correlations are available in the open literature for computing the heat transfer coefficient during flow boiling inside plain channels. With respect to halogenated refrigerants, these correlations are usually compared to data taken in a limited range of evaporation temperature and reduced pressure. More recently, the adoption of new refrigerants, such as high pressure HFCs and carbon dioxide, requires to largely extend the pressure range of application of such correlations. Besides, the design of evaporators for some heat pumping applications, where temperatures are set at higher values as compared to usual evaporating temperatures in air-conditioning equipment, requires proper validation of the computing methods. The present paper aims at comparing four well-known predicting models to a new database collected during flow boiling of HCFC (R22) and HFC refrigerants (R134a, R125 and R410A) in a horizontal 8 mm internal diameter tube. This database is characterized by saturation temperature ranging between 25 C and 45 C, reduced pressure spanning between 0.19 and 0.53. Mass velocity ranges between 200 and 600 kg m2 s1 and heat flux between 9 and 53 kW m2. Evaporating heat transfer coefficients of halogenated refrigerants at such high temperatures have not been reported in the open literature so far. The discussion of the results will enlighten some similarities with experimental trends presented in the literature for evaporating carbon dioxide. Two models tested here show good prediction capabilities of the present experimental data, but not for all the data sets in the same way. For the purpose of practical use, a simple modification of the correlation by Gungor and Winterton [1] is proposed, showing that this is able to catch the experimental trends of the present database with good agreement.