| Air cooler takes away heat from hot medium by passing air outside tube bundle,which plays an important role in the thermal cooling process of petrochemical industry.Finned tube bundles can effectively improve the heat exchange efficiency of air coolers by virture of their finned surfaces.At present,there are few publications on the research of finned tube bundles for evaporative cooling heat transfer.Especially,the design and calculation method of finned tube bundles for surface evaporative cooling needs to be further studied.In this paper,an experimental platform is built by using industrial air cooling device as a template,and a theoretical calculation method for analyzing the thermodynamic characteristics of evaporative cooling on the surface of finned tube bundles is presented.The influence of structural parameters on heat transfer of finned tube is analyzed by heat transfer enhancement ratio.Experiments on dry air cooling of finned tube bundles are done,and the correlations of heat transfer and pressure drop characteristics are given.The mechanism of heat transfer and pressure drop is further analyzed by numerical simulation of dry air cooling.Besides,Experiments of finned tube evaporative cooling are done,and the effects of water film and relative humidity on heat transfer are studied.The correlations,including heat and mass transfer and pressure drop,are given to make up for the research of evaporative cooling calculation on the surface of finned tube bundles.The finite difference equations of finned tube evaporative cooling are established,and the heat and mass transfer characteristics are analyzed.For the first time,the discrete phase model,Eulerian wall film model and mixture species transport model are coupled to simulate the effect of multi-parameters on three-dimensional finned tube evaporative cooling.The main research work and conclusions are as follws.(1)A theoretical calculation method for analyzing the thermodynamic characteristics of evaporative cooling on the surface of finned tube bundles is presented.The influence of structural parameters on heat transfer is analyzed by the heat transfer enhancement ratio.The heat transfer is enhanced with the increase of fin height,but it does not increase infinitely with the increase of fin height.Heat transfer is enhanced by the increase of fin thickness,fin density and fin thermal conductivity.Based on the experimental data as boundary conditions,the equations of fin heat transfer micro-element under dry and wet conditions is numerically solved.The numerical results of temperature of fin micro-element,temperature of water film,heat distribution,fin efficiency and fin thermal resistance are obtained.(2)An air-cooled experimental platform with finned tube bundles is built.The experiments show that high fin dry air cooling can replace normal plain tube evaporative air cooling at low ambient temperature,meet the technological requirements and achieve the goal of water saving.The correlations of finned tube air cooling are obtained by experimental fitting,which has industrial reference value(3)If the windward vertex of the fin is defined as 0°,the numerical simulation of finned tube dry air cooling shows that a tail reflux zone exists at 180°.And the highly unstable wake of bundles with standard tube spacing can promote heat transfer of downstream.The air turns at the top of the fin.And the vortices form and fall off continuously in the front of a single fin,which makes the temperature boundary layer renew continuously.And the heat transfer coefficient in the front of the fin is high.The temperature gradient between the fins at low inlet air temperature is larger,which explains the heat transfer enhancement menchanism at condition of low inlet air temperature(4)An experimental platform for finned tube evaporative cooling is built.Increasing the thickness of water film can increase heat transfer.The greater the relative humidity of inlet air,the smaller the partial pressure difference between the water film and air,which results in the decrease of mass and heat transfer.The correlations of finned tube evaporative cooling are given,which is of guiding significance to the design of evaporative air cooling(5)Based on the method of partial difference equation substitution and energy balance,the finite difference equations of evaporative cooling are established.And the parameters are obtained to analyze heat and mass transfer characteristics of evaporative cooling with finned tube bundles.The Lewis factor increases from top to bottom,and the relative renewal rate of the concentration and temperature boundary layer decreases from top to bottom.So the water film evaporation of upper tube bundle is stronger than that of the lower tube bundle.Latent heat exchange is the main way of heat exchange.The surface evaporative cooling has a high utilization efficiency of exergy at low ambient temperature(6)Three dimensional models of finned tube bundles with outer diameter 42 mm,50 mm and 57 mm are established.For the first time,the discrete phase model,Eulerian wall film model and mixture species transport model are coupled to simulate the effect of multi-parameters on three-dimensional finned tube evaporative cooling.Based on the field synergy theory and Fanning friction factor,a comprehensive performance factor is obtained to analyze the surface evaporative cooling performance of finned tube bundles with multi-structural parameters.The increase of air flow rate,spray intensity and fin height can improve the comprehensive performance of surface evaporative cooling.When the Re exceeds 2820,the increase of tube rows number makes the difference of pressure drop larger than that of heat transfer,so the comprehensive performance of finned tube bundle with more rows become worse.With the decrease of fin spacing,the gradient of temperature boundary layer between fins is small.The comprehensive performance of evaportive cooling decreases.Finned tubes with fin spacing greater than or equal to 3 mm should be selected for evaporative air cooling. |
PDF Full Text Request