Optimization Study And Evaluation For Convective Heat Transfer

With the developing of society,energy issues have drawn the attraction from all the world,and energy conservation and emission reduction have also been one of important means to realize sustainable development for China.Heat exchanger is an important equipment for heat and mass transfer and it is widely used in industry.Improving the efficiency of heat exchangers is of great significiance to improve the enegryg utilization in industry.Heat convection is one of the basic physical phenomena in heat exchangers and correlational researches are siginificant for developing enhancement technology and improving energy utilization.Based on the existing theory and experience,the present thesis studied the optimization theory for heat convection,technologic development model,evaluation criteria and CFD algorithm.By taking available potential as a state variable for energy potential,the relation of available potential,Gibbs free energy and exergy is discussed in this thesis.Furthermore,available potential equilibrium equation is established.Heat transfer exergy destruction cased by temperature difficience and flow exergy destruction cased by pressure drop are obtained.Besides,available potential equilibrium equation can be used to describe the change of energy potential and irreversible loss when fluid flows in a tube or through a pump.Optimization method of exergy destruction minimization is proposed through functional calculus of variation on the Lagrange extremum model,in which heat transfer exergy destruction is set as optimization objective and constant power consumption as constraint condition.By solving the optimized equations,heat convection in a circular tube,an elliptical tube and a square cavity with a constant heat source are optimized.Longitudinal swirl flow with multi-vortexes is an optimized flow with great heat transfer and low increase of flow resistance.The distributions of temperature and exergy destruction are changed by this flow and the maximum EEC is 4.627 for circular tube,3.12 for elliptical tube.There are vortexes forming in the square cavity after optimization.The position and intensity of vortexes are in relation with the temperature boundary.The vortexes can improve the thermal homogentity and significantly descrease the average temperature and maximum temperature in the square cavity.Technologies of passive heat transfer enhancement are classified into two methods:surface-based and fluid-based.The former can destroy the flow boundary layer which brings an increase in flow resistance.The latter disturbs the flow in the core flow and makes the temperature uniformly distributed,and there is an equivalent thermal boundary layer with great temperature gradient forming near the wall,which enhances heat transfer with a low increase of flow resistance.Furthermore,a mathematical model is constructed with a method of zone optimization:heat transfer exergy destruction as optimization objective in the core flow and power consumption(flow exergy destruction)as optimization objective in the boundary flow.This method is applied in the optimization of heat convection in a circular tube.The results indicate that longitudinal swirl flow with vortexes is a flow with well comprehensive performance.In the flow with different Re,the comprehensive performances,EEC,are all over 3.Besides,several technologies are introduced in this thesis and a new model for technological development is proposed:technology of heat transfer enhancement is developed based on the optimized results and experience,and evaluated by a new comprehensive criterion.Based on the equilibrium equation of available potential,exergy efficiency is proposed as an evaluation criterion to evaluate the performance of heat convection from quantity and quality.The concept of transition section and enhanced fully development section are proposed in an enhanced tube.The value of criterion in enhanced fully development section is stable and can be used to evaluate the performance of different technologies.Through numerical calculation with periodicity condition,the exergy destruction distribution and exergy efficiency of circular tube,enhanced tube with short-width twisted tape and optimized tube are researched in this thesis.Among three tubes,the performance of optimized tube is the best and it can be regarded as an ideal case.For enhanced tube with short-width twisted tape,its exergy efficiency is higher than that of circular tube and its temperature distribution is improved,but there is still possibility of improvement.The ability to do work of fluid and relevant constitutive equation are stated in this thesis.The conservation equation of mechanical energy takes pressure as the variable,and a dot product law in the transfer of heat,pressure and mass is introduced with three kind of synergetic angle,β、θandα,reflecting the process synergy.By solving the mechanical energy equation and Navier-Stokes equation,CAPVE algorithm is developed and the flow in two dimensional parallel channel is calculated with Fortran language.There is no difference between the results from the calculating by CAPVE and SIMPLE,which shows the validity of mechanical energy equation.Otherwise,the factor in the constitutive equation is given by the method of numerical calculation,which is 0.15m~2/s for water and35.8m~2/s for air.