Experimental study on flow boiling of R134a and R410A in a horizontal microfin tube at high saturation temperatures

This paper presents an experimental study on vaporization of R134a and R410A inside a horizontal microfin tube at 30 °C and 40 °C saturation temperatures. A wide range of operating conditions is investigated: mass flux from 80 to 600 kg m−2 s−1, heat flux from 14 to 83.5 kW m−2 and vapour quality from 0.1 to 0.99. The experimental database includes measurements of heat transfer coefficient and dry-out inception vapour quality.The heat transfer data are compared with those for a plain tube at similar operating conditions. The experimental data are then used to assess the accuracy of models for the prediction of the heat transfer coefficient and dry-out vapour quality.The present data are measured at high evaporating temperature, covering values of reduced pressure between 0.19 and 0.49, which are higher than usual for air-conditioning applications with halogenated refrigerants. The main applications for the present study are related to heat pumps reaching high condensing temperature (above 70 °C) and high evaporating temperature (above 30 °C), although other applications of evaporators operating at high reduced pressure may also benefit from the present data.

► Flow boiling in a microfin tube at high saturation temperature is investigated.
► Primary application: heat pumps reaching high evaporating temperature (above 30 °C).
► The effects of heat flux, mass flux and vapour quality are measured.
► Available correlations for predicting the heat transfer coefficient are assessed.
► A new correlation for predicting the onset of dry-out is presented.