Study On The Mechanism And Influence Law Of Heat Transfer Enhancement Of Microparticle Multiphase Flow

Energy is an important material foundation for the survival and development of human society,and improving energy efficiency,protecting and improving the ecological environment are an urgent task for the development of our country.Due to the strong applicability and high reliability,shell-and-tube heat exchangers are widely used as an important equipment for heat transfer.However,because of the smooth tubes of the shell-and-tube heat exchanger,a conducting-heat viscous sublayer near the wall of the tube makes the heat transfer performance poor and energy efficiency low.Research on the new and higher efficient energy-saving technologies is imminent.So a microparticle multiphase flow with a good thermal conductivity as a medium is uesd to enhance the convective heat transfer in the tube.A heat exchanger experimental device is set up to study the Nusselt number,friction factor and thermal performance characteristics of the turbulent convective heat transfer of graphite,Al₂O₃ and Cu O microfluid with the particle volume fraction of 0.25%,0.5%,0.75%and 1.0%,the particle size of 5μm and the Reynolds numbers from 7000 to 24,000.The experimental results show that the Nusselt numbers of the three kinds of microfluids are significantly higher than that of water.The Nusselt number of graphite microfluid is higher than that of any other microfluid under the same condition,and the Nusselt numbers of A1₂O₃ and Cu O microfluid are similar.However,the micorparticles adding into the water increase the pressure loss of the tube,and the friction factor of microfluids are higher than that of water.The friction factor of Cu O microfluid is the highest,and the friction factor of the graphite microfluid is the lowest among the three kinds of microfluids under the same condition.Comparing the results of the thermal performance between the microfluids with the water,the thermal performances for all tested microfluids are greater than that of water.The thermal performance of graphite microfluid is higher than that of any other microfluid under the same condition,and the maximum value of the thermal performance is 2.23.The influence of the particle size on thermal performance is furtherly studied.Compared with the graphite particle size of 5μm,the Nusselt number and the friction factor of graphite particle size of 1μm are more than that of size of 5μm.The thermal performance of the graphite microfluid of particle size of1μm is higher than that of particle size of 5μm.According to the obtained experimental results of the graphite microfluid,the nonlinear correlation formulas are obtained by the least square method.A convective heat transfer model of microparticle multiphase flow is established,using a Eulerian-Eulerian two-fluid model based on the SST k-ωturbulence model.By analyzing the flow field of the microparticle multiphase flows,the law of influence of the particle volume fraction,particle size,and particle material on the velocity field,turbulent kinetic energy and thickness of the viscous sublayer are obtained.By analyzing the heat transfer of the microparticle multiphase flows,the effect of the particle volume fraction,particle size,and particle material on the temperature field,heat transfer coefficient and degree of the field synergy are obtained.The degree of the field synergy of the graphite particle flow are the best among three particle flows.The reason is that the thermal conductivity of the graphite particle material is much higher than that of Al₂O₃ and Cu O particle materials,and the thermal conductivity of microparticle has the highest weighting effect on the heat transfer.The mechanism of microparticle flow to enhance the convective heat transfer is mainly explained from three aspects:the velocity boundary layer,thermal boundary layer and degree of the field synergy.The irregular movements of the particles increase the turbulent kinetic energy of the flow field and enhance the exchange of the momentum,reduce the thickness of the viscous sublayer and increase the degree of turbulence in the velocity boundary layer,thereby the viscous sublayer is damaged and thinned,which reduces the thermal resistance of the heat transfer.At the same time,a good thermal conductivity of the microparticles increases the equivalent thermal conductivity of the microparticle multiphase flow and strengthens the heat transfer process.The essence of the heat transfer enhancement in the tube is the interaction effect between the velocity boundary layer and the thermal boundary layer,mainly the degree of the field synergy between the velocity field and the temperature gradient field.The micorparticles have a significant effect on the synergy between the two fields,making the velocity field and temperature gradient field have a higher degree of synergy,and the effective synergy between the two fields makes the the microparticle multiphase flow complete the enhanced process of the heat transfer.By the orthogonal design and signal-to-noise ratio analysis of the density,thermal conductivity,specific heat,volume fraction and size of the microparticle on the Nusselt number and pressure drop.It is concluded that the volume fraction and thermal conductivity of particle are the most important factors on the Nusselt number.The order of contribution from high to low is the volume fraction,thermal conductivity,size,specific heat and density.For increasing the Nusselt number,the higher volume fraction and thermal conductivity of the microparticle are choosed.The volume fraction and density of the micorparticle are the two most important factors on the pressure drop.The order of contribution from high to low is the volume fraction,density,size,thermal conductivity,specific heat.In order to reduce the pressure loss,the low density and volume fraction of the microparticle are choosed.By weighing the two objectives,for improving the overall heat exchange effect,the good thermal conductivity of the microparticle is choosed,and the volume fraction and microparticle size can be determined according to the requirements of the heat transfer and the pressure drop.In this paper,the experiments test the heat transfer and flow characteristics of the three kinds of microfluid with different particle materials,different particle volume fractions,and different particle sizes at the Reynolds numbers from 7,400 to 23,000;the non-linear fitting formulas of the Nusser number and friction factor are established.Combining the experiments and the numerical simulations,the mechanism of the enhanced heat transfer of the microparticle multiphase flow is illustrated.These provide data support and theoretical basis for the enhancing convective heat transfer of the multiphase flow with the microparticle.The contribution of the microparticle material properties on the heat transfer and flow resistance are obtained,and it can more effectively guide the industrial application of the microparticle multiphase flow.