Local heat transfer coefficients and pressure gradients for R-134a during flow boiling at temperatures between -9 °C and +20 °C

This study presents new flow boiling heat transfer and pressure gradient results of R-134a flowing inside a 6.00 mm internal diameter, smooth and horizontal stainless steel tube. The measurements were made over a wide range of test conditions, where there is a lack of data in literature for the investigated geometry: evaporating pressure from 2.1 to 5.7 bar (saturation temperature between −8.8 °C and 19.9 °C), refrigerant mass flux from 197 to 472 kg m−2 s−1 and heat flux from 8.5 to 20.1 kW m−2. The experimental results allow to evaluate the dependence of the heat transfer coefficients and pressure gradients on the vapour quality, the saturation temperature, the refrigerant mass flux and heat flux. The new database of 254 points it is used determine the best predictive methods for heat transfer coefficients and pressure gradients; indeed, it is presented an analysis of the influence of the saturation pressure and of the mass flux on the error of these predictive methods.