Study On Condedsation-convection Heat Transfer Of Flue Gas

Currently, natural gas account for 5.7% of the proportion of China’s primary energy consumption, while the international average is 23.7%. Chinese government is working to adjust the energy structure, increasing the proportion of natural gas in energy consumption. With the number of natural gas fired boilers increasing, how to improve its efficiency is deeply concerned by researchers. Natural gas fired boilers has great potential to achieve higher thermal efficiency. Natural flue gas with high moisture.When it goes through recuperative heat exchanger, water vapor will be condensated to release latent heat. In recuperative heat exchanger, there is a complex heat and mass transfer process during the convection and condenstion process. Due to condensation of water vapor in recuperative heat exchanger, flue gas temperature can not be accurately mesured, and we can not test the efficiency by reverse thermal balance method. The positve balance method affected by quality of natural gas and accurate measure flue gas temperature. Therefore, it is difficult for us to test condensation liquid mass by experiment.Fluent new version integrated evaporation-condensation model. This paper proved that the evaporation-condesation model is feasible by test, and then studied the process of condensation and convection in recuperative heat exchanger of flue gas by the method of experiment and numerical simulation.By designing and building the experiment platform: 30 k W condesation heat balance exchanger experimental platform, and using thermal imaging cameras capture flue gas condensation heat transfer phenomenon on the vertical wall online, and by the thermocouple under the copper board, temperature of the board is measured, we observe and analyse flue gas condensation heat transfer on the vertical copper wall. Using VOF model and evaporation-condensation model, based on the test results, we confrim and improve the key parameters of numerical simulation, include: unsteady calculation time step, method of dividing grid near the wall. Using numerical simulation to analyse the condensation phenomenon and heat transfer characteristics on the vertical copper board.For flue gas convection and condensate on vertical wall, Nu number related to flue gas temperature and wall temperature are derived. By combining experimental and numerical simulation method, we observe and analyse flue gas condensation heat transfer on the vertical wall, and summarize the condensation phenomenon: on the surface of vertical copper board, first, flue gas condensation form small dew, then collected dew become droplet, droplet will fall down under gravity, forming dozen of condensation liquid ban, in this process, non-condensable gases affect the local heat transfer.And on horizontal tube outside, the formula of convection and condensation heat transfer and condenstion rate are derived. Using numerical simulation to analyse the condensation phenomenon and heat transfer characteristics outside horizontal pipe and analyse single-tube outflow field to obtain water vapor volume proportion distribution near the tube wall, we find that water vapor is mainly concentrated on underside of tube to condensation. Increasing flow rate will change condensation heat transfer proportion in total heat transfer.