Experimental Study On The Effect Of Expanded Graphite On The Storage And Release Performance Of Paraffin

Phase change energy ’storage technology is widely used in many fields,such as the utilization of industrial waste heat,the consumption of new energy,the thermal management of electronic device,the building energy efficiency and so on,due to the large heat storage and approximate constant temperature characteristics during the heat transfer.Pervious experimental researchs paid more attention to the temperature change inside the PCM,but the thermodynamic parameters such as heat flux density has been rarely reported.Beasuse of the few temperature measuring points and the single data display form,it’s difficult to reflect the changes in the temperature and flow fields inside the PCM.In this paper,expanded graphite/paraffin composite phase change materials were prepared by melt adsorption of paraffin and expanded graphite with good thermal conductivity.Then the thermal physical properties and heat storage and release characteristics of PCMs were tested by suitable data processing methods.The following research results are given:1.Various proportions of EG/paraffin composite PCMs by 5%,10%and 15%were prepared and their thermal properties were measured.The thermal conductivity of composite PCMs were increased to 10 times,17 times and 23 times of paraffin respectively.The main reason is that compact EG networks formed with the mass fraction increases of EG gradually.These networks provided thermal conduction paths which enhanced the thermal conductivity of the composite PCMs;the phase change temperature of the composite PCMs remained unchanged rarely,and the phase change latent heat of samples reduced by 7.4%、13.2%、19.2%.The main reason is that the EG and paraffin are physically combined.At the experimental temperature,EG didn’t undergo a phase change,and the latent heat of phase change came from paraffin completely.2.A vertical tube-type heat exchanger experimental setup was built to investigate the phase change laws of composite PCM with different EG mass fractions during heat storage and heat release under constant power and constant temperature heating conditions.The phase change laws are mainly reflected through the following two aspects:First,the temperature field described by the temperature changes,and the phase change and flow reflected by the temperature filed;second,the heat flux density curves fitted by the temperature changes of the temperature measuring points on the copper rod,which reflected the distribution of the energy transmission from the copper rod to the PCM.The experimental results show that under the same charging condition,the charging time of EG/paraffin composite PCM was at most 43.1%lower than pure paraffin,indicating that adding EG can greatly reduce the thermal resistance of the heat storage device.3.During paraffin heat storage,due to the effect of buoyancy,hot fluid flows upward and cold fluid flows downward,resulting in the temperature of the upper measuring point gradually exceeding the lower measuring point as the liquid paraffin increases.Adding EG could change the temperature and flow fields inside the PCM.The temperature of upper location was always higher than the lower location during the composite PCMs heat storage process,and when the inlet temperature or heating power was low,the increasing of the EG mass fraction made greater influence on the heat storage time.Under the heating conditions of 75℃ and 100℃,the charging time of 15%EG mass fraction composite PCM was 43.1%and 32.9%shorter than paraffin respectively,and the reason is that the strength of natural convection increases with the temperature increasing but the thermal conductivity does not change much.Under discharging conditions,the solidification time of composite PCMs gradually decreased with the EG mass fraction increases,which was at most 32.3%shorter than paraffin,mainly because heat conduction plays a major role in the solidification process.However,for the change of heat flux density,paraffin and composite PCM showed a big difference.The heat flux density on the copper rod decreased gradually from bottom to top in the late period of paraffin heat storage,whilefor composite PCMs heat storage,the heat flux density on the copper rod increased gradually from bottom to top.This paper analyzed the temperature and flow field inside the PCM and the thermodynamic parameters such as heat flux density during the heat storage and release process,showing the heat transfer laws inside the heat storage device in detail,which provide a theoretical basis for improving heat storage capacity of the phase change system.