Natural Convection Heat Transfer Characteristics Of Magnetic Nanofluids In Sinusoidal Curved Cavities

Natural convection and forced convection are common heat transfer methods in the field of heat exchangers.Compared with forced convection,natural convection has the advantages of no pollution,safety and environmental protection.Therefore,cavity-based natural convection heat transfer is widely used in the fields of thermal management of electronic components,solar collectors and so on.Nowadays,with the increasing heat transfer intensity of the heat exchanger and the compact volume of the heat exchanger,the natural convection heat transfer based on the ordinary cavity is increasingly unable to meet the current needs,so it is necessary to find a more efficient solution to solve the current problems of the heat exchanger.At present,there are two common methods to improve the efficiency of heat exchangers.On the one hand,it is necessary to find a heat transfer medium with higher thermal conductivity to replace the traditional heat transfer medium such as oil and alcohol with lower thermal conductivity.On the other hand,the purpose of further enhancing heat transfer is achieved by designing and optimizing novel heat exchangers.Based on the above research background,this thesis mainly does the following work:Fe₃O₄-H₂O magnetic nanofluids with higher thermal conductivity were prepared to replace traditional heat transfer fluids with low thermal conductivity such as water and oil;Novel sinusoidal curved cavities were designed and their structures were optimized.Magnetic field and metal foam were applied to the heat transfer enhancement of the sinusoidal curved cavity,respectively.Effects of magnetic field and metal foam on the heat transfer characteristics of magnetic nanofluids in the sinusoidal curved cavity were studied.Finally,the coupling heat transfer enhancement mechanism of magnetic field/metal foam/sinusoidal surface on magnetic nanofluids is revealed.Firstly,Fe₃O₄-H₂O nanofluids with mass fractions of wt%=0.1%,wt%=0.3%and wt%=0.5%were prepared,and the sedimentation observation method was used.It was found that after 20 days of preparation,Fe₃O₄-H₂O nanofluids still did not precipitate,indicating that the stability of the prepared Fe₃O₄-H₂O nanofluids met the experimental requirements and could be used for further experiments.A new type of heat transfer square cavity-sinusoidal curved cavity was developed,and the cold and hot walls of the square cavity were symmetrical sinusoidal surfaces.The natural convection heat transfer experimental platform of the sinusoidal curved cavity was built,and the structure of the sinusoidal curved cavity was optimized.Effects of amplitude（1 mm,2 mm,3 mm）and wave number（1,2,3）on heat transfer were analyzed.The experimental results showed that when the amplitude is 3 mm and the wave number is 2,the Nusselt number is relatively high and the heat transfer effect is better.The maximum Nusselt number can be increased by 9.58%.Secondly,the natural convection heat transfer experimental platform of sinusoidal curved cavity under magnetic field excitation was built,and the influence of horizontal magnetic field and vertical magnetic field on heat transfer was explored.It was found that the horizontal magnetic field has a weakening effect on the heat transfer effect,and the Nusselt number is reduced by 2.57%at most.For the vertical magnetic field,the effects of three different arrangements of unilateral vertical,bilateral vertical correspondence and bilateral vertical staggered on heat transfer were mainly studied.Results showed that the bilateral vertical staggered magnetic field has a stronger heat transfer effect,and the Nusselt number can be increased by 5.37%at most.Thirdly,a natural convection heat transfer test bench for a sinusoidal curved cavity partially filled with metal foam was built.The effects of three different metal foam on heat transfer were studied when the pore density was 20 PPI,30 PPI and 40PPI,respectively.Results showed that the sinusoidal curved cavity filled with 40 PPI metal foam has the highest Nusselt number and the best heat transfer effect,and the highest heat transfer performance can be increased by 2.8%.Finally,on the basis of the previous chapters,the optimal magnetic field arrangement（vertical mode）and the optimal metal foam pore density（40 PPI）were selected.The natural convection heat transfer test bench of the sinusoidal curved cavity partially filled with metal foam under the excitation of vertical magnetic field was built.The influence of vertical magnetic field on heat transfer enhancement was studied when the pore density of metal foam is 40 PPI.The influence of unilateral vertical magnetic field,bilateral vertical corresponding magnetic field and bilateral vertical staggered magnetic field was mainly considered.Results showed that the Nusselt number can be increased by up to 9.4%under the excitation of bilateral vertical staggered magnetic fields.There are 73 figures,4 tables and 96 references in the thesis.