Study On The Enhanced Heat Transfer Characteristics Of Shell-and-Tube Latent Heat Exchanger And Composite Phase Change Material

The latent heat thermal energy storage technology is an important technology for waste heat,waste heat recovery and solar energy and other renewable energy utilization,which is characterized by high thermal storage capacity and low temperature fluctuation.It can effectively improve the recovery and conversion efficiency of thermal energy.Nowadays,energy supply is becoming increasingly tight,as the development and application of latent heat energy storage technology leads to a reduction in the use of fossil energy,which results in less air pollution.The optimization research on high-efficiency latent heat exchanger and the development of environment-friendly phase change materials are powerful measures to promote the development and application of the latent heat thermal energy storage technology.In this paper,a shell and tube latent heat exchanger is designed,which employs sextant helical baffles on the shell side to keep the heat transfer fluid flowing in a spiral manner,and uses a paraffin-filled annular tube as a heat storage unit on the tube si.In order to optimise the structure of this novel latent heat exchanger,numerical simulations are applied to investigate it in terms of both heat transfer fluid side and phase change material side.On the heat transfer fluid side,the effect of the helix angle of the sextant helical baffle and the effect of the heat transfer fluid with different Prandtl numbers on the discharging performance of the latent heat exchanger are investigated.On the phase change material side,the effect of the structural parameters of the internal-external fins and the effect of paraffin with titanium dioxide nanoparticles on the discharging performance of the latent heat exchanger are investigated.The results show that:（1）The thermal performance（Nu）of the latent heat exchanger is inversely proportional to the helix angle of the sextant helical baffle,and the comprehensive performance（h/Δp）of the latent heat exchange is directly proportional to the helix angle of the sextant helical baffle;（2）At a fixed mass flow rate,the Prandtl number and thermal conductivity of the heat transfer fluid are important factors affecting the discharging characteristics of the latent heat exchanger.Therefore,the selection of a fluid with a small Prandtl number and a large thermal conductivity is conducive to improving the performance of the latent heat exchanger;（3）The rectangular longitudinal internal-external fin with a height of 7.5 mm and a thickness of 1 mm is determined to be the optimal structure,and the fins optimal arrangement is staggered and evenly arranged inside and outside;（4）The discharging rate of the latent heat exchanger tends to augment with increasing titanium dioxide nanoparticle content,while the total enthalpy of phase change（H）reduces with increasing titanium dioxide nanoparticle content.Conventional paraffin-based phase change materials are derived from petroleum derivatives,which have the disadvantages of being non-renewable and providing low thermal conductivity.Based on this,a composite phase change material is experimentally developed in this paper.It utilizes a binary eutectic mixture of lauric acid（LA）and 1-hexadecanol（HD）as the base phase change material,and high-temperature modified shell powder（SP）,which is obtained from waste shells with multiple treatments,as the thermal conductivity reinforcement.The experimental results indicate that high-temperature modified 80-mesh and 300-mesh shell powder mixed in a mass ratio of 3:7 improves the thermal conductivity of the base phase change material by 33.9%,and the thermal conductivity of the final composite phase change material reaches 0.336 W·m^-1·K^-1.In addition,the final composite phase change material is characterised for microstructure,thermal properties,thermal stability and thermal cycling stability.The characterisation results demonstrate that the composite phase change material prepared in this paper has good chemical stability,high latent heat(0.19 k J·g^-1),excellent thermal stability and thermal cycling stability.In this paper,the heat transfer characteristics of shell-and-tube latent heat exchangers and composite phase change materials are studied,which provides theoretical and technical support for the development and optimization of equipment and materials in the field of latent heat storage,while the application of latent heat storage technology is promoted.