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. w 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.