Study On Phase Change Heat Transfer Characteristics Of Porous Media Composite Phase Change Material Under A Magnetic Field

Paraffin wax has the advantages of large latent heat and high heat storage density.It is an ideal phase change material and has broad market prospects in the fields of building energy,solar energy,cooling storage systems,and battery thermal management.The low thermal conductivity affects the flow heat transfer of paraffin in the melting process,limiting its thermal management effect in practical applications.This thesis aimed to improve the melting heat transfer performance of paraffin,adding Fe₃O₄ nanoparticles to paraffin to enhance its thermal conductivity,and exploring the effect of external uniform magnetic field on the melting rate and convective heat transfer of magnetic composite PCM.The magnetic field force source term was defined in Fluent software by user-defined method(UDF),and the mathematical model of the mutual coupling of thermomagnetic convection and melting heat transfer was constructed.The effects of nanoparticle mass fraction,magnetic field strength and orientation on the melting process of PCM under different Rayleigh numbers were investigated by numerical calculation.Selecting the best magnetic field conditions,adding foam copper to further improve the heat transfer performance of PCM,and finally summarizing the effect of magnetic field on foam copper composite PCM,providing a way to optimize the phase change heat transfer characteristics of PCM.The main research contents and conclusions are as follows:Fe₃O₄ nanoparticles can improve the thermal conductivity of paraffin wax.It was measured by experiments that when the mass of Fe₃O₄ nanoparticles was 1%,the thermal conductivity of paraffin wax increased by 18.7%,and when the mass added was 3%,the thermal conductivity of paraffin wax increased by 31.9%.But it also reduces the latent heat value of paraffin.CTAB is a good dispersant.Adding CTAB can effectively prolong the settling time of composite PCM when preparing Fe₃O₄ and paraffin composite phase change material by melt blending method.The effect of the magnetic field on the nanoparticle composite PCM with different mass fractions was explored.The results showed that when the magnetic field strength was 0.001T,under the conditions of Ra=3e5,4e5 and 5e5,the melting rate of the composite PCM with a mass fraction of 1%was 3%composite PCM decreased at 1.38%,27.52%and 12.95%.Under the same magnetic field conditions,the mass fraction has little effect at low Ra number,but it is more obvious at high Ra number.The magnetic field strength is an important factor affecting the melting process of PCM,and is related to the Ra number.When Ra=3e5,the melting rate of PCM decreased by 8.27%and 13.91%under the magnetic field of 0.001T and 1T.When Ra=4e5 and 5e5,the melting rate of PCM decreased by 4.45%,14.18%and 7.20%,14.53%,respectively.The influence of the magnetic field strength on the melting of PCM is mainly due to the increase of the Kelvin force,which inhibits the natural convection of the melting process,and the greater the magnetic field strength,the more obvious the inhibition effect.When Ra=3e5,4e5 and 5e5,the average number of Nu in the melting process of PCM under 0.001T magnetic field was34%,52%and 64%of that without magnetic field,and the total heat flux was reduced by46.58%,42.4%and 48.2%.However,the average Nu number in the melting process of PCM under the corresponding Ra number in the 1T magnetic field was 15%,48%and 55%of that without the magnetic field,and the total heat flux was reduced by 49.49%,40.95 and 45.7%.It can be seen that the effect of magnetic field strength on melting rate and convective heat transfer is more obvious at high Ra number.Since the Kelvin force is related to the direction of the magnetic field,the effect of the reverse magnetic field on the melting of PCM was explored.It is found that the effect of the reverse magnetic field has a duality.When the magnetic field strength is small,it inhibits the melting process.Under the conditions of Ra of 3e5,4e5 and 5e5,the magnetic field of 0.001T reduces the melting rate of PCM by 11.8%,3.3%and 11.9%,respectively.The average Nu number decreased by 14.2%,20.2%and 22.5%,and the total heat flux decreased by 9.3%,13.19%and 14.32%,respectively.The 1T magnetic field accelerated the advancing speed of the bottom interface and promotes the melting process of the PCM.Under the same Ra number,the melting speed of the composite PCM is increased by 16.78%,8.28%and 14.24%,and the average Nu number is increased by 9.1%,24.89%and 3.89%,respectively.The total heat flux increased by 1.64%,37.41%and 9.26%.A certain strength of the reverse magnetic field canimprove the heat transfer performance of the PCM melting process.When the Ra number is large,the magnetic field has a more obvious effect on promoting meltingThe excellent thermal conductivity of metal foam accelerates the melting speed.When copper foam with a porosity of 0.92 is added to the nanocomposite PCM with a mass fraction of 3%,when the melting time was 60%of the original,the average liquid phase fraction was the same as that without copper foam.3 times,the total heat flux was 8.13 times.In order to further optimize the heat transfer performance of phase change,according to the results in Chapter 3,a reverse magnetic field was added.The results showed that the effect of the magnetic field on the foamed copper composite PCM was related to the magnetic field strength.The Nu number decreased by 17.40%,the total heat flux decreased by 1.26%,and the upward Kelvin force at the interface suppressed natural convection and slowed down the melting rate.At the same time at 1T,the melting rate of PCM increased by 4.56%,the number of Nu decreased by 26.37%,and the total heat flux increased by 1.85%.The influence of the magnetic field on the foamed copper composite PCM is also related to the magnetic field strength.Only when the magnetic field strength reaches a certain level,the melting of the PCM can be promoted,mainly relying on the promotion of the melting interface.