| Gas boilers are widely used to achieve clean combustion.The main component of natural gas is methane.The volume fraction of water vapor in the flue gas is large after combustion.Directly discharged into the atmosphere will take away a lot of latent heat,reducing boiler combustion efficiency,causing energy waste and aggravating greenhouse effect.Therefore,the thermal recovery of high temperature flue gas from gas boiler has important engineering application value.In the paper,the system for gas boiler flue gas heat recovery is an integrated system of finned-tube heat exchanger,spray device and direct expansion heat pump device.Indirect heat transfer occurs between flue gas outside the tube and the refrigerant inside the tube when the flue gas flows through the evaporator.The temperature of flue gas is lower than the dew point temperature,thus the flue gas is wetted on the outer surface of the evaporator,which makes the condensation heat transfer process complicated.The experimental platform can’t show the distribution of temperature field,pressure field and velocity field in the condensation heat transfer process of the evaporator embedded in the internal intuitively.The numerical calculation method can be used to simulate and analyze the process,thus we can deepen the understanding of the condensation heat transfer process and provide guidance and reference for the optimization design and system performance analysis of the evaporator.The paper mainly studies the condensation heat transfer characteristics of the evaporator of the direct expansion heat pump flue gas heat recovery system under the condition of moisture condition.Through the rationalization assumption for the phase transition process of the working fluid on both sides of the evaporator,the physical model of the heat and mass exchange process of the hot and humid flue gas flows through the evaporator is established.The experimental data obtained by the experimental platform under same working conditions are used to verify the heat balance and the accuracy of the model.The influence of evaporator structure parameters and physical parameters of wet flue gas on the heat transfer process were analyzed by simulation,including the distribution of flow field temperature,pressure,velocity,heat transfer coefficient and condensation volume fraction under different rib thickness,pipe diameter,longitudinal pipe center distance and flue gas inlet velocity,water vapor volume fraction,inlet temperature.Results show that under different evaporator structural parameters,the turbulence intensity near the pipeline is different,the intensity of convective heat transfer and condensation heat transfer of flue gas between pipelines is also different.The pipeline diameter influences heat transfer obviously.With the increase of fin thickness,the change of condensation volume fraction is small,while the convective heat transfer coefficient is the largest when the thickness increases to 0.75 mm.With the decrease of pipe diameter,the heat transfer characteristics of wet flue gas in the evaporator side shows a trend of increasing first and then weakening.When the pipe diameter is 7 mm,the heat transfer effect is the best.With the increase of longitudinal tube center distance,the heat transfer characteristics of wet flue gas in the evaporator side is strengthened.When the tube center distance increases to 29.4 mm,the convective heat transfer coefficient and condensation volume fraction are the largest.Physical parameters of wet flue gas influence the process of heat transfer obviously.With the increase of flue gas flow rate,the heat transfer intensity increases first and then decreases.The heat transfer effect of the whole heat transfer process is best when the velocity is 0.1m/s.With the increase of water vapor volume fraction in flue gas,the convective heat transfer coefficient increases,and the condensation volume fraction increases first and then decreases.When the water vapor volume fraction is 20%,the condensation volume fraction is the largest.With the increase of flue gas temperature at the inlet,the convective heat transfer coefficient increases,the convective heat transfer is strengthened,and the condensation volume fraction decreases. |
