Research On The Enhancement Of Heat Transfer Characteristics Based On The Drainage Technology Outside The Tubes

Condensation and evaporation are the most common heat transfer modes in nature and industrial processes. Film condensation is one of the most common condensation methods in engineering practice. And falling film evaporation is widely used in industry. A common feature of the two mode of heat transfer is the formation of a layer of continuous spreading film outside tubes, and the liquid film is the main factor of heat transfer. One can reduce the liquid film and accelerate liquid flow outside the tubes by drainage technology to strengthen the heat transfer.When the liquid film flow is stable, the heat transfer between the liquid film and the tube wall is the heat conduction mode, and the film thermal resistance is the main factor affecting the heat transfer. Using drainage technology has two benefits. Firstly,can accelerate the discharge of liquid and reduce the thickness of the liquid film,reduce the thermal resistance of the film, strengthen the heat transfer in the liquid film.Secondly, can improve the flow velocity and the wall of the fluid flow velocity gradient, and strengthen the liquid film and tube wall surface convection heat transfer.In this paper, the heat transfer characteristics of a drop-shaped tube and elliptical tube with different structural parameters were studied based on the boundary layer theory and the heat balance of the condensation heat transfer.The results show that the ovality e and Bond number play an opposite effect on the critical angle φc of liquid film drop of the tube wall. That is when the ellipticity e is at a certain value, the critical angle φc increases with the increase of Bond number, while Bond number is at a certain value, the critical angle φc decreases with the increase of ellipticity e. Drop-shaped tube and elliptical tube compared with the circular tube provided with a good drainage capacity and can effectively reduce the thickness of liquid film under the condition of the effective heat transfer area is same, which can enhance heat transfer performance.The ovality e and Bond number were the main factor affecting its heat transfer performance. In the design of drip-shaped tube and elliptical tube, one should according to the Bond number to determine the optimal ellipticity e. Therefore, the conclusion in the paper provides a reference for the optimization of tube type and heat transfer enhancement. Elliptical tubes have better discharge capacity than drop-shaped tubes in the same heat exchange area and the same ellipse e.Based on the VOF model, the numerical simulation of the different tube type of the tube with the liquid plate is carried out. The results show that circular tube with a strip compare with it, the average film thickness was more thinner, the local Nu number more greater. It was showed that the drainage plate can accelerate drainage and reduce the thickness of liquid film, which is advantageous to the enhancement of heat transfer. The higter of the strip, the thinner of the liquid film and the greater of local Nu when at the some circumferential angle. The width of the strip is small or large can not play a good role for accelerating drainage. Drop-shaped tube, oval-shaped tube,elliptical tube E=1.5 and E=2 were researched by Numerical simulation software Fluent. The results show that the fluid flow velocity increases, the dimensionless temperature decreases, the liquid film decreases, and the heat transfer ability is enhanced after the installation of the liquid plate. The film distribution of elliptical tube E=1.5 and E=2 were relatively uniform, and can reduce the possibility of ‘dry spots’ with drainage strips.