Condensation heat transfer characteristics of carbon dioxide in a horizontal smooth tube

The condensation heat transfer characteristics for R744 flowing in a horizontal smooth tube were investigated experimentally. The test section consisted of 2400 mm length with horizontal copper tube of 4.95 mm inner diameter. The experiments were conducted at refrigerant mass fluxes of 400–800 kg/m2 s, and saturation temperatures of 20–30 °C. The experimental results showed that the mass flux and saturation temperature have a strong effect on the condensation heat transfer coefficients of R744. The experimental data were compared against existing heat transfer correlations. Most of correlations failed to predict the experimental data. However, the correlation predicted by Kondou and Hrnjak showed relatively good agreement with experimental data within 20.7%. But this correlation was proposed for limited experimental conditions. Therefore, it is necessary to develop accurate and reliable correlation to predict the heat transfer coefficients of condensing R744 in the horizontal tube.

► Annular flow is almost dominated the major of condensation flow in the horizontal tube.
► The heat transfer coefficients have an effect on mass fluxes and saturation temperatures.
► The experimental data were higher than those of Kondou and Hrnjak.
► It is necessary to develop reliable correlation to predict the heat transfer of R744.