INVESTIGATION OF ENHANCEMENT OF STEAM CONDENSATION HEAT TRANSFER ON FINNED TUBES WITH POROUS DRAINAGE STRIPS

摘要

Condensation of steam on horizontal finned tube with porous drainage strip was investigated experimentally and theoretically. The structure of finned tube with porous drainage strip is illustrated in Fig. A-1. Composite metal screens were used as porous drainage strip. Distilled water was taken in the experiments as working fluid. The experiments were carried out to find the structure effects of composite screen drainage strip on the condensation heat transfer on finned tubes with fin spacing of 0.5 mm. Influences of the mesh number of both outer and inner layers and of length and thickness of the drainage strip were investigated. The outer layers of the screens have the mesh numbers of 100, 150, 185 and 220, which lead to different effective pore radius; the inner layers have the mesh numbers of 20, 30 and 40, which lead to different permeability. Strip lengths are 15 mm, 20 mm and 30 mm, while the thickness is 2 mm, 4 mm and 6 mm. Experimental results show that the structure parameters of the porous drainage strip strongly influence condensation on the finned tube. A porous strip having small effective pore radius on the outer layer as well as large permeability, reasonably large length and thickness will lead to excellent performance. In the tested ranges, for steam condensation on a 0.5 mm spacing finned tube, the optimum structure of a composite screen drain-age strip is of 150 mesh outer layer screen, 20 mesh inner layer screen, 30 mm long and 4 mm thick. The enhanced condensation heat transfer coefficients are equivalent to 1.15～1.74 times that of the finned tube and 2.08～3.08 times that of the plain tube. A new prediction model of condensation on finned tube with porous drainage strip was established. The condensate flow in the interfin grooves and in the porous strip was treated as flow through two kinds of porous medium with different structures. The momentum equation of single phase flow in porous medium was used to solve the condensate flow. The equation was analyzed and several terms were neglected because of their little influences on the present problem. The non-Darcy effect caused by the high Reynolds number in the tiny channels was considered. As a result, micro-scale inertial force term was involved in the control equation. The effective driving force of condensate in the porous drainage strip was defined. It reflects the total effect of forces exerted on the condensate and is helpful to analyze influences of strip parameters. The length of pendant part which could only be obtained by previous experiments was determined by a model based on simulation experiments. The condensation model is validated by present and the other investigation experimental data.