Study On Enhancement Law Of Condensation On A Vertical Fluted Wall And Structural Optimization

Heat exchanger is the typical equipment in the thermodynamic process and the key equipment to save energy and reduce consumption. With the work of energy-saving and emission reduction gradually deepening, it is imperative to further develop the heat exchangers with higher efficiency. Because of the advantage of simple structure and high efficiency, fluted tubes which are applied to the enhancement of condensation on a vertical tube become one of the most prospective high efficient heat exchange tube. However, there is not a convenient and reliable method to analyze the condensation on a fluted tube because of the complexity of the fluid flow and heat transfer.The law of condensation on a vertical fluted surface was systemically investigated using theoretical analysis, numerical calculation and experiment in this paper. A type of flute which has better comprehensive performance has been optimized and was applied to condensation on a fluted tube. The main research contents and conclusions are as follows:(1) The theoretical model of film condensation on a vertical fluted surface is established. The film thickness equation in orthogonal curvilinear coordinates and velocity expressions in s and z directions are deduced based on the boundary layer theory considering gravitational and surface tension effects. The equation for two-dimension thermal conduction including the wall and condensate film is set up using the heat transfer theory considering non-homogenous temperature distribution. The film thickness equation is solved coupled with the two-dimension thermal conduction equation using finite difference and finite element method, and then film thickness, velocity and temperature fields are obtained. The results show that the model of film condensation on a vertical fluted surface is reliable, and the condensate in the crest and trough mainly flow along horizontal and vertical directions respectively. Heat mainly transfers through the crest region in which the film thickness is thin, and condensate mainly discharges through the trough region in which the film thickness is thick.(2) The experiments of flow visualization and heat transfer are carried out to investigate condensation on fluted surface. The time and space data of the transport of tracers are recorded with the time intervals of 0.0025s and 0.0125s respectively using high-speed video camera. The relationship between velocity of condensate in the trough and z coordinate is obtained. The condensing heat transfer coefficient is surveyed. The results show that the flow direction of condensate, the velocity of condensate in the trough and the condensing heat transfer coefficient agree well with the analyses. The experiments prove that the theoretical model of film condensation on a vertical fluted surface is reliable.(3) The partition model of film condensation on a vertical fluted surface is set up after simplifying according to the distribution regularity of velocity field in the film. The heat transfer calculation method based on the algorithm of Runge-Kutta, iteration and analogy methods is proposed. The relationship between heat transfer rate and tube length for condensation on cosine fluted surface is obtained by programming. The results show that the partition model is reliable, the average heat flux decreases with the increase in flute length, and the heat transfer rate after use the skirts is (m+1)1-b times more than that before use the skirts.(4) A four-region model and heat transfer calculation method used for U shape flute are developed on the basis of the partition model. The selection principles of flute shape are established. A type of U shape flute which has better comprehensive performance is optimized aiming at the discharging ability, heat transfer performance and maximum effective length. Compared with V shape flute, the optimized U shape flute shows better comprehensive performance. The calculation proves that the temperature on flute surface is not uniformly distributed.(5) The flow directions of condensate on U shape flute are investigated by visualization experiment. It is proved that the assumption of flow field distribution in four-region model is correct. The heat transfer experiments on optimized fluted tube, UOP fluted tube and smooth tube are carried out respectively. It is showed that the four-region model and heat transfer calculation method are reliable, the heat flux of optimized fluted tube is 1.8 times that of smooth tube for a given temperatureâ–³Tfw, the optimized fluted tube only requires 43% of smooth tube temperature differenceâ–³Tfw for a given heat flux, the heat flux of optimized fluted tube is 9.5% over that of smooth tube for a given temperatureâ–³Tfw, and heat transfer coefficient hvw is 12% over that of smooth tube for a given heat flux.The optimized fluted tube has better performance in enhanced condensation.