Experimental study on condensation of steam/air mixture in a horizontal tube

• Experiments were performed with a large range of NC gas fraction and inlet Re number.
• The effects of NC gas fraction, inlet mass flux and pressure were analyzed.
• The centerline temperatures and heat transfer coefficients was investigated locally.
• Two correlations respectively for stratified and annular flow had been developed.

The effect of noncondensable gas on condensation heat transfer inside horizontal tubes was investigated experimentally. Air was used as the noncondensable gas. The effects of inlet noncondensable gas mass fraction, inlet gas mixture mass flux and inlet pressure on overall heat transfer performance were analyzed. Moreover, local heat transfer performance was investigated by comparing centerline temperatures and local heat transfer coefficients. The results showed that the overall heat transfer coefficient decreased as inlet noncondensable gas fraction increased. Increasing inlet mass flux could enhance the heat transfer rate and overall heat transfer coefficient. Furthermore, the heat transfer rate showed an increasing trend, whereas the overall heat transfer coefficient decreased with increase of inlet pressure. The local heat transfer coefficient decreased along the condenser tube due to the decrease of gas mixture Reynolds number and the increase of liquid film thickness and local air fraction. For stratified flow, the local heat transfer coefficient at the top part was higher than that at the bottom owing to the asymmetrical liquid film. However, this gap gradually decreased along the tube, especially at higher inlet noncondensable gas fraction. Finally, two correlations respectively for stratified flow and annular flow regimes were developed, using the modified Froude number as the indicator of transition from stratified to annular flow.