Investigation On Melting And Heat Transfer Characteristics Of Fatty Acid In Annulus And Optimum Design

Thermal energy storage technology provides a reservoir of energy to adjust the mismatch between energy demand and energy availability, which is an effective means of rational using of energy and reducing of pollution. Phase change energy storage technology have attracted extensively attention due to its advantages of storing a large amount of energy in a small mass/volume, phase transition at constant temperature and easy to control, and have been widely used in the fields of shifting peak electrical loads, application of solar energy, aerospace, energy saving of building, waste heat recovery and infrared camouflage in military use. The phase change mechanisms, the heat transfer characteristics of storage devices and the enhancement of phase change heat transfer are important subjects in the field of phase change energy storage.Lauric acid was taken as the phase change material, and the experimental set up was builded. The experimental research and numerical simulation were combined to study the melting process, heat transfer characteristics and the optimized design of annulus unit. Firstly, the accuracy of the numerical simulation was verified by experiment. The melting phase interface and melt fraction were compared of the feature size of40mm annular unit at the border condition of80℃isothermally. The result proved that enthalpy-porous model can be used to simulate the melting process of fatty acids.Secondly, the melting process and heat transfer characteristics of the annulus unit were studied from the following aspects: melting phase interface, melt fraction, temperature field and velocity field and average Nusselt number, such criteria were used to analyze the role of natural convection in melting process. The melting process under the action of pure heat conduction was compared with the process under natural convection, which lead to the result that the influence of natural convection cannot be ignored. The method to determine the transition point from conduction to natural convection is proposed by the change of average Nusselt number and the change of temperature field and velocity field in annulus. Based on this, the heat transfer model of the whole melting process coupled with conduction and natural convection is established.In addition, the phenomenon that the phase change material at the bottom of annulus is more difficult to melt lead to the thought of setting eccentric. By studying the melting process and heat transfer characteristics of the eccentric, we found that the melting is faster, the cell temperature is lower, the average convective heat coefficient is higher and the latent heat storage effect is better. And thus the melting process of different units within different eccentric distance was investigated, so the optimum eccentric distance was derived by analyzing the melting rate, the average convective heat transfer coefficient and the heat absorbed(The optimal eccentric distance is7-8mm for the unit of feature size of20mm). The same results were obtained at the constant border temperature of70℃and80℃.Finally, according to the findings above and consider the practical engineering needs, this paper carried out some optimal design for exchangers or other heat storage units within space limitations, i.e., reducing the in and outer diameter of annulus and setting eccentric simultaneously. By analyzing the strengths and weaknesses of different optimization schemes, determining the weight of the main evaluation index weight, the optimum phase change unit and melted condition were obtained.