Experimental Investigations On R245fa Flow Condensation Heat Transfer In Inclined Tubes

Condensation heat transfer of refrigerants in inclined tube is widely used in industry. It is necessary to understand the heat transfer characteristic for the purpose of optimizing the heat exchangers and saving energy. The investigations focus on the condensation heat transfer in small diameter tubes such as air conditioner tubes. However, as the low grade energy such as ORC system is utilized more widely, the diameter of condenser tube becomes larger. Thus, this paper investigated the condensation heat transfer of R245fa in larger diameter inclined tube with the financial aid of Beijing key laboratory of low grade energy and multi-phase flow heat transfer.The condensation heat transfer of R245fa in a tube of 14.81 mm diameter and 1200 mm length was investigated experimentally. The R245fa mass fluxes, inlet vapor mass qualities and relevant Froude numbers had the ranges of 191.3～705.4 kg/m2s, 0.191～0.947 and 0.174-2.153, respectively. Inlet temperature of test section was 55.5℃, and the inclination angles ranged from vertical down-flow to vertical up-flow including θ=0°、±4°、 ±8°、±15°、±30°、±45°、±60°、±90°. The main flow patterns were stratified flow, intermittent flow and annular flow.The condensation heat transfer coefficients behaved the general increase trend versus inclination angles and the trend was obvious when Fr>>1. The heat transfer coefficient increased compared with that of horizontal tube whenever the inclination angles increased or decreased. The heat transfer coefficients behaved non-mono tonic periodic variation in the full inclination angle range.The heat transfer mechanism was analyzed by visualization experiment. When the inclination angle increased, intense interfacial wave occurred to increase the heat transfer and when the inclination angel decreased, liquid height decreased to lessen the thermal resistance, enhancing heat transfer. Thus, different factors such interfacial wave, liquid height, flow pattern, different force competed with each other to yield the non non-monotonic periodic variation characteristic. Finally, heat transfer coefficients were well correlated with the general increase term multiplied by the non-monotonic variation term, and the average error and root mean square error were 6.36%,5.97%, respectively.