Study On Flow And Heat Transfer Characteristics Of Natural Circulation Loop Driven By Heating And Cooling Fluids

As a passive heat transfer device,natural circulation loop（NCL）have been widely used in solar collector,electronic chip cooling,nuclear power plant.The working principle of NCL is to use the natural convection process to transfer the heat from the heat source to the heat sink through a closed loop.Due to the temperature difference between the source and the sink,buoyancy force is generated,and fluid flow is established under the actions of buoyancy force and friction.Under the synergistic action of the fluids inside and outside the loop,the heat transfer progress between the heating and cooling fluids is accelerated,thereby enhancing the heat transfer effect.Comparing with the heat transfer between the flat plate,natural circulation loop directly transfers the heat of the bulk heating fluid to the bulk cooling fluid through the heat and mass transfer process of the fluid in the loop.Thereby avoiding the influence of the cold and hot boundary layer on the heat transfer performance.Therefore,the total heat transfer resistance is decreased and the overall heat transfer performance is improved.At present,the previous researches on natural circulation loop were mainly focused on its start-up and stability features,and most of the thermal boundary conditions are convective cooling,electric heating and two-leg insulation,which are much different from the general boundary conditions of heat exchangers.In this study,the heat transfer performance of the water-based SPNCL with convective boundary conditions at both heating and cooling fluid sides was investigated.In order to discuss whether the natural circulation loop has the ability to enhance heat transfer,the same shaped and sized copper fin was set as a counterpart of comparison.The detailed contents of this study are as follows:First,a 3-D numerical simulation model of the natural circulation loop driven by heating and cooling fluids was constructed.The analytical solution of the 1-D temperature distribution of the natural circulation loop in the steady state was derived,compared and verified with the 3D model.In addition,the influence on the flow and heat transfer characteristics of natural circulation loop without adiabatic boundary are simulated.Such as the heating and cooling fluid temperatures,velocities,the geometry size of the NCL,the inclination angle of the NCL and so on.Secondly,the one-dimensional analytical solution for the heat transfer of the natural circulation loop and the copper fin are derived.Combining with 3-D numerical simulation,the heat transfer performance of NCL under different conditions was studied.In addition,the same shaped and sized copper fin was set as a reference to explore the most suitable size of the NCL.In practical applications,the wall axial heat conduction has a great influence on the heat transfer performance of the small-sized natural circulation loop.Therefore,the heat exchange experiment of NCL with a finite wall thickness was constructed.The influences of the heating and cooling fluid temperatures and velocities on the flow and heat transfer characteristics were analyzed.The heat transfer performance of the same shaped and sized copper fin was compared and analyzed to further reveal the enhanced heat transfer mechanism of the natural circulation loop.According to the above researches,some conclusions that can be drawn are as follows:1.The increase of temperature difference and velocities of heating and cooling fluids will increase the start-up and mass flow rate of the fluid in the loop.Then the convective heat transfer resistance in the loop and total amount of heat transfer resistance decreases,and the heat transfer rate increases in the final.The inclination of the NCL has a relative small effect on heat transfer comparing with the temperature difference and velocities of driven fluids.The increase of the loop size can result in an increase of flow rate and heat transfer area,therefore the heat transfer rate also increases.Besides,a correlation of Re_ss and Gr_m/N_Gfor the NCL is proposed.2.To analyze the heat transfer performance of the NCL,the same shaped and sized cooper fin is set as a reference.The 1-D heat transfer rate analytic solutions of NCL and copper fin are derived,respectively.It indicates that the heat transfer performance of NCL with a larger size and temperature difference is better than the same shaped and sized copper fin.The enhanced heat transfer ratioηwas defined by comparing the heat transfer rate of the NCL with that of the copper fin,which shows an increase with the increase of temperature difference between the heating and cooling fluids.At a large temperature difference,the enhanced heat transfer ratioηincreases with the increase of heating and cooling fluid velocities.3.The natural circulation loop heat exchanger was constructed.The results show that the heat transfer performance of the natural circulation loop with wall thickness has a strong influence,and that the fluid flow in the loop tends to be stable more easily.In addition,the influences of the temperature difference and flow velocities of the hot and cold fluid on the heat transfer performance are consistent with the simulation results.Finally,the experimental relationship between Nu and Re_ss in the natural circulation loop was obtained.