Theoretical And Experimental Investigations Of Enhanced Heat Transfer In A Heat Exchanger Based On Single-phase Natural Circulation Mini-loops

Fluid flow and heat transfer widely exists in all kinds of engineering and technology fields.The theory of convective heat transfer and enhanced heat transfer technology have attracted more attention.At present,the theoretical investigation of convective heat transfer mainly includes field synergy principle based on energy conservation equation,entropy generation of heat transfer process and entransy dissipation,while only a few investigations based on exergy loss have been reported.Moreover,for the conventional enhanced heat transfer technologies,different types of fins,disturbing components or the enhanced methods of fluids physical properties(such as nanofluids)have been used to improve the convective heat transfer coefficient of one-side fluid to achieve the purpose of enhanced heat transfer.It is known that in the heat transfer process of a dividing wall-type heat exchanger,the velocity of cold and hot fluids will be affected by wall surface,which can bring the irreversible exergy loss due to shear friction.On the other hand,the heat transfer process between cold and hot fluids depending on their temperature difference is also an irreversible process with exergy loss.Thus,differential equation which can be responsible for local exergy loss in convective heat transfer has been derived,and an idea to improve the heat transfer performace by recovering or resuming a part of the exergy loss resulting from the the temperature difference is raised in this paper.Based on the theoretical investigation,a passive heat transfer mini-device,singe-phase natural circulation loop as an enhanced heat transfer device,is designed and investigated.The heat transfer mechanism of this enhanced method is experimental investigated through the fluid flow and heat transfer of the intermediate third fluid in a loop driven by buoyancy in the gravity field subjected to the hot and cold fluids at different sides.In the present paper,based on the local entropy generation equation proposed by Prof.Bejan,the local exergy loss differential equation in convective heat transer is derived and used to analyze the fundamental Poiseuille and Couette heat transfer problems.Moreover,the 3-D numerical simulation,theoretical and experimental investigations for the single-phase natural circulation loop driven by cold and hot fluids are performed.The effects of temperature difference and fluid velocity of cold and hot fluids on transient and steady natural circulation are discussed.Meanwhile,the natural circulation mini-loops are appied to the heat exchanger to form the loop array at the heat exchange plate.A comparison of the heat transfer performance between smooth plate and the plate with loop array is investigated in a designed high temperature and low speed wind tunnel.A numerical calculation for the heat transfer of a loop fin identical with the natural circulation loop is also conducted.The specific research contents and conclusions are as follows.Firstly,the local exergy loss equation in convective heat transfer is derived based on the exergy balance equation of a 3-D fluid micro-element control volume.It is found that when the local entropy generation proposed by Prof.Bejan is substituted into the local exergy loss equation,the total exergy loss includes three parts.The first term is the exergy loss caused by entropy generation,which equals to the negative of the product of local entropy generation and the reference temperature.The minus represents the exergy loss.The second term is exergy generation produced by viscous dissipation and internal heat source,which is positive.The third one is the exergy loss due to the pressure drop in the process of fluid flow.Meanwhile,the dimensionless forms of entropy generation and exergy loss are proposed and used to analyze the Poiseuille flow and Couette flow.The results show that(1)for Poiseuille flow,the maximum local exergy loss occurs at the middle of the channel.The dimensionless temperature difference does not show much effect on the totoal local exergy loss in this case.The local exergy loss increases with the increase of Pr Ec.(2)for Couette flow,since there is no pressure drop,temperature difference and viscous dissipation have an influence on local entropy generation and exergy loss.The distribution between exergy loss and entropy generation is similar when the Pr Ec is lower.With the increase of Pr Ec,the effect of viscous dissipation becomes more significant.Secondly,the physical model of a single-phase natural circulation loop driven by cold and hot fluids is built and a three-dimensional numerical simulation is carried out.The effects of temperature difference and fluid velocity of two streams on the heat transfer of the natural circulation loop is analyzed.Based on the numerical results at steady state,the entropy generation and its stability of natural circulation are analyzed.The results show that(1)due to the asymmetry of thermal boundary conditions,the clockwise natural circulation in the loop can be always generated.(2)With increase of driven temperature difference,the time of start-up and from static state to steady state becomes shorter.However,the effect of fluid velocity on natural circulation flow and heat transfer is relatively low.(3)The total entropy generaion and dimensionless entropy generation of natural circulation at steady state increase with the increase of dirven temperature difference.Any given small disturbances for the stable natural circulation,circulation flow could achive the steady state through damped oscillation.Thirdly,in order to verify and reveal the thermo-hydraulic characteristics of the natural circulation loop,an experimental rig of a single-phase natural circulation loop is set up with the wall thermal resistance considered.Meanwhile,a one-dimensional mathematic model is built and a good agreement between experimental and theoretical results can be seen when the heat transfer correlation fitted by experimental data is used in the mathematic model.The results show that(1)under the experimental conditions,the stable natural circulation can be reached from stationary state and the flow direction is clockwise in our cases.(2)The Reynolds number Re_ss and heat transfer rate Q of natural circulation at steady state are proportional to the driven temperature difference T_h-T_c.(3)Since the thermal resistance at the side of driven fluids is far lower than the thermal-conduction resistance and thermal resistance of loop fluid,the mass flow rate of driven fluids has a little effect on the thermal performance of natural circulation.(4)Based on the obtained experimental data,a new heat transfer correlation of loop fluid is proposed,which is Nu=0.2702Re^0.4713.Based on the above simulations and experiments of natural circulation,it is found that the stable natural circulation in a mini-loop can be generated subjected to cold and hot fluids.Since this passive heat transfe device can be regarded as an enhanced heat transfer component aimed at making use of the temperature difference between the two streams and recovering or resuming a part of the exergy loss,a loop array can be designed and installed on a flat heat transfer plate.By doing this,we can see the possibility for its application of the loop array platein a prarctical heat exchanger.Therefore,the enhanced heat transfer performance of the loop array plate is experimentally investigated and compared with a smooth flat plate.Meanwhile,the heat transfer rate of a single loop and fins at the same condition is calculated and compared.The results show that(1)under all the experimental conditions,the heat transfer rate,number of heat transfer unit and effectiveness of the loop array plate are always higher that those for smooth flat plate.Therefore,the enhanced heat transfer ratio is always higher than 1.Also,the heat transfer rate of these two plates increases with the increase of the hot-air temperature and pump power.(2)The application of SPNCL arrays to a heat exchanger can bring extra exergy loss.However,in the view of the novel performance evaluation based on the second law of thermodynamics,the second-law enhanced number of SPNCLs array plate is always higher than one,which is between1.03 and 1.23.The maximum second-law enhanced number occurs at fre=8Hz and T_h=100?.(3)Under the same hot-air temperature and pump power,the heat transfer rate of single loop is always higher than loop fin.And the heat transfer rate of single loop and lop fin increases with the increase of hot-air temperature and pump power.The heat transfer ratio is between 1.07 and 1.46.