Experimental Study And Analysis On Condensation Heat Transfer

This paper carries on experimental study on convective condensing heat-transfer effects of two kinds of heat exchangers’ structural form and further discusses the selecting principle of convective condensing heat exchanger’s structural type used in the flue gas of boiler on the basis of heat transfer enhancement mechanism of pure convective, pure condensing and convective condensing heat exchangers.In this paper, flue gas obtained by natural gas combustion was taken as research object, through controlling the ratio of the gas flow and air flow to guarantee the stability of flue gas components within the experimental requirements. To adjust humidity is realized by steam generator’s producing an adjustable mount of steam and injecting into flue gas. The experimental temperature at the inlet was kept around 100℃. The gas velocity, steam volume fraction, average temperature of cooling medium, influence of the flow type of cooling medium on convective condensation heat transfer coefficient and condensed water collection rate of vertical-cross arrangement smooth-tube heat exchanger were investigated, meanwhile the economy of heating surface arrangement when the water was recycled in this kind of heat exchanger was analyzed.Then the outer surface convective condensation heat transfer characteristics of horizontal arrangement finned-tubes was studied with different height of fin under the low flue gas velocity. The volume fraction of vapor in flue gas was controlled at 14%, and average flue gas temperature was controlled at 86.6-86.3℃. the average temperature of based tubes was controlled at 28.8-29.8℃. The finned-tubes include five types of 0, 3, 4, 5, 6mm fin height with the diameter of based tube was 12 mm.Experimental result shows that the condensation rate is increasing with the flue gas velocity decreasing in the staggered vertically arranged tube heat exchanger. The condensation rate increases as the volume fraction of vapor in flue gas increasing, under the 8m/s flue gas velocity condition. When water vapor share increased from 5.4% to 18.3%, the volume fraction of water vapor increases by 3.4 times, while the condensate flow rate from 0.2g / s increased eight-fold to reach 1.6g / s.The condensation rate of finned-tubes with 8.9 finned ratio is 6.2 times larger than that of tubes. There is an optimum flue gas velocity that makes condensation rate reaching largest in the convective condensation heat transfer process of outer finned tubes. The effect of flue gas velocity on convective condensation heat transfer coefficient decreases with increasing fin height. The condensate rate tends to be stable as increasing fin height. There is an optimum fin height that makes fin heat transfer enhancement efficiency reaching to maximum.