Study On Internal Flow Mechanism And Heat Exchange Performance Of Biomass Fuel Heat Exchanger

Biomass renewable energy has huge development potential in our country,among which biomass fuel particles are an important aspect.The heat exchanger in the structure of the biomass fuel heat exchange system is the basic core component of the development and application of biomass fuel particles.The heat exchanger’s performance directly affects thee exchange and utilization of heat.This paper launches the internal flow mechanism and the heat exchange performance in the biomass fuel heat exchange system research,and setting up the different tube structure(straight,serpentine,spiral)heat exchanger of three-dimensional geometric model.Meanwhile,numerical calculation was used to study the influence of the flow heat transfer characteristics of the structural heat exchanger,and the serpentine coiled tube biomass fuel heat exchanger test bench was built.The research results are as follows:(1)A serpentine tube biomass fuel heat exchanger test bench was set up for testing,and based on the parameters from the test bench,a three-dimensional model is built for the numerical calculations.The comparison and analysis between the test bench and the model carry out the heat exchanger performance research.The results show that the data calculated by the Realizable k-εmodel is closer to the experimental test value,the fit is good,and the error is within 5%.The increase in the inlet flow rate of the flue gas on the tube side has a greater impact on the temperature in the heat pipe,but has less effect on the temperature on the shell side.With the increase in the flue gas flow rate on the pipe side,the pressure drop and PEC index on the pipe side increase,the heat transfer performance is improved,and the friction coefficient decreased,and the magnitude of the decrease gradually decreased.(2)Three-dimensional geometric models of biomass fuel heat exchangers with three different forms of inner tubes(straight,serpentine,and spiral)were established.Numerical calculation is used to study the internal flow mechanism and heat transfer characteristics of different tubular heat exchangers.After comparing and analyzing the comprehensive heat transfer indices of Nu,f,and PEC,it is concluded that the heat transfer coefficient of the serpentine tube is 16%and 5%higher than that of the straight tube and the spiral tube.The existence of buoyancy makes the high temperature mixed flue gas in the tube.The phenomenon of heat exchange deterioration will occur.In the serpentine tube and the spiral tube,the buoyancy force caused by gravity has little effect on the heat exchange of the high temperature mixed flue gas.When the wall heat flux density increases by 10k W/m~2,the overall heat transfer performance of the serpentine,helical and straight tubes decreases by 17%,22%,and 28%.In the shell side of the heat exchanger,the temperature boundary layer at the edge of serpentine tube and straight tube shell side is very obvious.The temperature distribution of the air temperature at the edge of the shell side of the spiral tube heat exchanger is not uniform,and there is a low temperature area.The convective heat transfer coefficient of the serpentine heat exchanger is the largest.As the air inlet velocity increases,the convective heat transfer coefficient,the Nusselt number and heat transfer factor of the three heat exchangers will increase,and the PEC and the shell-side air inlet velocity are a linear increase relationship.(3)To explore the influence of(Re)and characteristic parameters on the heat exchange performance of the serpentine biomass fuel heat exchanger,five groups of characteristic parameter serpentine biomass fuel heat exchangers were designed for numerical calculation.The results show that there is a vortex and heat transfer dead zone at the outlet of the serpentine biomass fuel heat exchanger,which will have a certain impact on the heat transfer performance.With the increase of the air inlet velocity,the air distribution on the shell side is more uniformed,and the heat transfer dead zone area is reduced.The vortex mixing in the shell-side flow channel is more sufficient,the turbulent effect is enhanced,and the heat transfer performance is improved.The tube diameter(d)and the serpentine protrusion spacing(h)have a great influence on the Nu of the heat exchanger.The control h remains unchanged.With the increase of the speed,the Nu of the five groups of tube diameters increases.When d is small,the Nu increases.The amplitude is small,with the increase of d,the increase of Nu becomes larger;the control d remains unchanged,with the increase of speed,the Nu of five groups of h increases,and the increase range is basically the same,the larger the h,the larger the Nu.(4)ANSYS Workbench was used to optimize the multi-objective characteristic parameters of the serpentine inner tube biomass fuel heat exchanger.Focus on the effects of multiple factors such as the diameter d of the heat exchange tube,the distance between the serpentine protrusions and the height r of the serpentine protrusions on its turbulent heat transfer resistance and the heat transfer performance of the serpentine heat exchanger.The results show that the influence of h on Nu is greater,followed by r,and the influence of d is relatively small.The size of the drag coefficient f is determined by d,followed by h,and the influence of r is relatively small.For the PEC value,d has the greatest influence on it,followed by h,and r has almost no influence on it.Considering Nu,f,and PEC comprehensively,when optimizing the shell side of the serpentine biomass fuel heat exchanger,d and h are given priority.