Melting Heat Transfer Performance Of Organic Phase Change Materials With Electric Field

Latent heat storage is also named as phase-change energy storage,which utilizes phase change materials(PCMs)absorbing or releasing a amount of latent heat when the phase change take places.Latent heat storage can effectively alleviate the contradiction between energy supply and demand in time and space.It is widely adopted in solar thermal utilization,industrial waste heat recovery,power peak shaving and other fields.Organic PCMs,such as paraffin,fatty acids,and alcohols,have been widely adopted in the field of medium-low temperature latent heat energy storage due to their low cost and good stability.However,the low thermal conductivity of organic PCMs severely limits the performance of the latent heat storage system.At present,passive heat transfer enhancement technologies such as adding nanoparticles,fins,and foam metal have been widely studied to improve the performance of the latent heat storage system.The heat transfer enhancement technology based on the active application of electric field has the inherent advantages of intelligent control,no mechanical moving parts,simple design,and so on.However,melting characteristics of organic PCMs based on the active application of electric field are lack of systematic and deep investigation.Based on this,this thesis has carried out the following studies on the melting characteristics of organic PCMs with electric field:First,the space charge generation mechanism in dielectric liquid with different voltage amplitude,polarity,and waveform is obtained through the velocity field measured by particle image velocimetry(PIV)and electric current-voltage characteristic curve under the needle-plate configuration electro-convection experimental platform.When the electric field magnitude is lower than 10~6V/m,the space charge is produced by the conduction mechanism,and the voltage polarity does not affect the flow intensity in the conduction region.When the electric field intensity is higher than 10~6V/m,the space charge is generated by the injection mechanism.At this time,applying a negative voltage produces a stronger flow intensity than applying a positive voltage.The electric plume presents a three-dimensional axisymmetric structure.When the duty cycle of 50.0%,-2.0k V to0 pulse AC voltage is applied,the electro-convection still remains the plume shape.But the maximum velocity of the electric plume is less than that generated by the application of DC-2.0k V.When 0 to 2.0k V pulse AC voltage is applied to the needle electrode,the dielectric liquid hardly flows.When±2.0k V AC square wave is set to the needle electrode,the velocity of dielectric liquid decreases with the increase of square wave frequency.At this time,the weak flow near the needle electrode is produced by the conduction mechanism.Furthermore,the needle electrode is replaced by the blade electrode with the aim to increase the charge injection area.The dielectric liquid immersion jet cooling experiment under the blade-plate configuration is carried out.It is found that increasing the voltage amplitude and decreasing the electrode spacing would increase the magnitude of electric field between blade-plate electrodes and thus enhance the heat transfer effect.The submerged jet cooling effect generated by applying negative voltage to the blade electrode is more obvious than that generated by applying positive voltage.The Coulomb force is the main component of electric force.Compared to the case without applied voltage,the average Nusselt number is enhanced by 82.5%when-35.0k V is set.Then,the melting characteristics of organic PCMs in symmetric square cavity,asymmetric tube-square cavity,and asymmetric multi-wire square cavity configuration with the application of electric field are studied.For the melting of octadecane in the symmetrical square cavity configuration with electric field,the heat is transferred in the form of heat conduction at the initial stage.The effect of electric field is not obvious at this stage.As melting proceeds,heat is transferred in the form of convection.The electric field significantly influences the melting process.When positive or negative voltages are applied to the cavity’s left wall,space charges are generated by the conduction mechanism.As the liquid phase is attracted to the left,the contact time between the heated liquid and the cold solid is reduced.The melting process is inhibited.When a voltage is applied to the cavity’s right wall,the direction of electro-convection is the same as that of thermal convection.The flow in the liquid region is enhanced.At this time,the electric field significantly promotes the melting process.The liquid fraction is increased by44.4%when 20.0k V is applied.For the melting of organic PCMs in the asymmetric tube-square cavity configuration with electric field,the melting time is saved by68.0%when the tube is applied with-25.0k V.The electric current-voltage curve and the electric field distribution in the experimental cavity further confirmed the critical voltage dividing the conduction region and the injection region.For the melting of organic PCMs in the asymmetric multi-wire square cavity configuration,octadecane melting time is reduced by 54.5%when wire electrodes are applied with-10.0k V.Due to the high viscosity of liquid paraffin,the application of-10.0k V would only accelerate the melting locally,rather than significantly accelerate the overall melting process.Subsequently,the melting experiments based on the combined effect of actively applying electric field and passively adding fins or nanoparticles were conducted.For the melting of composite PCMs,passive addition of nano graphene can increase the composite PCMs thermal conductivity and electric conductivity.The space charge in the horizontal cavity is generated by the conduction mechanism.The effect of active heat transfer enhancement effect increases with the increase of the applied voltage amplitude.The octadecane’s absorbed energy is enhanced by153.5%when 25.0k V is set.Due to negatively charged nature of nano graphene,the deposition would be taken place when the cavity’s bottom wall is applied with the positive voltage.The composite PCMs’absorbed energy is extended by 111.2%when+10.0k V is applied.When the cavity’s bottom wall is applied with negative voltages,nano graphene keeps suspended.The composite PCMs’absorbed energy is enhanced by 124.8%when-10.0k V is applied.For the melting experiment with passively adding a fin,the melting effect with electric field is not obvious when the gradient of electric field and temperature field is the same.When the gradient of electric field is opposite to that of temperature field,the electric field inhibits n-octadecane melting within the cavity with a fin.Finally,the open source Open FOAM platform based on the finite volume method has been adopted to simulate PCMs melting with electric field in the concentric ring configuration under the unipolar charge injection assumption.The melting of octadecane with no electric field goes through the stages of heat conduction,natural thermal convection,and solid shrinkage.The numerical results agree well with the experimental results.When the electric field is applied,the electric force in the liquid phase is dominant than the buoyancy force,which causes the solid-liquid interface being parallel to the inner heating tube during the melting process.The maximum relative error between the numerical results and the experimental results is around 20.0%.The melting time decreases with the increase of voltage and temperature applied to the inner electrode.The melting time can be shortened by 1.26 to 1.84 times by adopting four tube electrodes compared to a single electrode with the same cross-sectional.The results of present thesis can provide theoretical reference and technical guidance for manipulating the performance of the latent heat energy storage system based on the active application of electric field.