Enhanced Heat Transfer Experimental And Simulation Research Of Nanocomposite Phase Change Materials

Thermal energy storage is one of the important methods which improve the energy efficiency and protect the environment. It is widely used in solar heat storage, power load shifting and industrial waste heat recovery. Among this technology, phase change energy storage has become the preferred method in reasonable utilization of energy and reducing environmental pollution for its high heat storage density, constant phase change temperature and easy control.Phase change material (PCMs) is the fundamental issue of the phase change energy storage technology. Inorganic PCMs have been investigated extensively in the past years, but, the limitations of corrosion, supercooling and phase separation et al., restricted its further application. However, the organic PCMs with optimum properties of low corrosion, chemical stability and cheap has been popular applied in some field of power load shifting, waste heat recovery and electronic equipment management. Futhermore, the main disadvantage of organic PCMs is the internal low thermal conductivity, which results in the poor heat transfer performance, and futher low utilization capacity heat storage efficiency and decreasing the system efficiency. Thus, one of the most important basic researches on energy storage technology is the development of phase change material.This study aims to the poor heat transfer problem of organic PCMs. A new kind of phase change material for thermal storage was developed incorporated with nanotechnology, so called"Nanocomposite phase change material (NC-PCMs)". The energy transfer efficiency including charging and discharging is improves significantly due to the presentation of nanoparticles.The thesis emphasizes on preparation, thermal storage and phase change characterization of NC-PCMs are as follows:1. Preparation of nanocomposite phase change materialNC-PCMs were prepared by directly mixing nanoparticles and pure phase change material. Through the visualization settlement experiments and the temperature-time tests, Cu nanoparticle was selected. Different dispersant types and concentration were considered to study the stability of liquid Cu-paraffin. The best mass ratio between nanoparticle Cu and Hitenol BC-10 dispersant is 2.6:1. The colloidal dispersion stability theory was used to analyze the mechanism of the dispersant on making Cu nanoparticle stably suspended in liquid paraffin. The scanning electron microscopy and Fourier transform infrared spectroscopy analysis was used to investigate the microstructure characteristic of NC-PCMs.2. Latent heat and specific heat of NC-PCMsPhase change temperature, latent heat and specific heat of Cu-paraffin were measured with Differential Scanning Calorimetry (DSC). The DSC results reveal that the melting and solidification latent heats of Cu-paraffin shift to lower values compared with those of pure paraffin, however, the melting and freezing temperatures keep almost the same as pure paraffin. After 100 thermal cycles, the maximum change value of latent heats is 3.2%, and the maximum change value of phase change temperatures is 1.9%. The result indicates that the NC-PCMs have a good energy storage characteristic. Meanwhile, the specific heat of Cu-paraffin is decreased with increasing the mass fraction of Cu nanoparticles, moreover, no apparent temperature dependence is observed. Theoretical ananlysis has been discussed to explain the possible mechanism for the decreasement of latent heat and capacity heat.3. Thermal conductivity measurement of NC-PCMsThe thermal conductivity of liquid and solid Cu-paraffin was measured by a Hotdisk Thermal Constants Anlyser. The effects of particle concertration, temperature and cylcle times on the thermal conductivity of Cu-paraffin were experimentally investigated.The thermal conductivity of liquid and solid Cu-paraffin is enhanced approximately nonlinearly with the mass fraction of the copper nanoparticle. For composites with 1.2 wt% Cu nanoparticle, the enhancement of liquid and solid thermal conductivity is 14.3% and 10%. The thermal conducitivity of Cu-paraffin in liquid and solid states are still stable after 100 thermal cycles. The traditional theoretical model, used to calculate the thermal conductivity of the two-phase mixture, is no longer to reflect the energy transfer process. The effects of interface interaction, lattice vibration, particle aggregation and Brownian motion of particles on the thermal conductivity of nanocomposites were discussed.4. The experimental study on the phase change of Cu-paraffinThe temperature-time tests were conducted in order to verify the improvement of heat transfer rate in the presence of Cu particles. The Infrared Heat Camera was used to observe the temperature distribution in melting and solidification process. A heat pump water heater for thermal storage was built to verify the excellent thermal properties of Cu-paraffin.The temperature-time curves indicate that the heating and cooling rates of PCMs were significantly improved upon the addition of Cu nanoparticles. For nanocomposites with the mass fraction of 1% Cu nanoparticle, the melting and solidification rates can be reduced by 30% and 28.2%, respectively. The temperature distribution shows that the temperature diffucivity is enhanced due to the addition of nanoparticles and increases with the increasing of the mass fraction of nanoparticles. There is an obvious mushy zone in the phase change process.The running experimental results of the heat pump water heater suggests the reliability of designment of the thermal energy storage and the feasibility of Cu-paraffin using in the heat pump water heater. The heat pump heat water with thermal storage has good stability in heat storage and release process.5. The numerical simulation on the phase change of Cu-paraffinThe Fluent software was used to simulate the melting and solidificaton process of Cu-paraffin. The results proved that the the melting and solidificaton rates of pure paraffin were enhanced due to the addition of nanoparticle. The heat transfer way for the melting process is mainly the natural convection. For the solidification process, the mainly heat transfer way is the heat conduction. Because the density of the solid PCMs is higher than the liquid PCMs, the solid PCMs will sink in the melting process, which result in the"close-contact-melting". Natural convection is initiated on the soild-liquid interface. Thus, the melting rate is quicker than the solidification rate.The theoretical analysis of the melting and solidificaton process is from the view of nucleating effect, thermal diffucivity and thermal conductivity.The method of adding Cu nanoparticle to oragnic PCMs was used to effectively enhance the heat storage and release rate. This study is not only to guide a new direction for preparation of composite phase change material, but also to provide the necessary experimental evidence and theoretical support for developing the heat pump water heater for thermal storage.