| As an effective means to solve the problem which the energy supply and demand in time and space does not match,phase change energy storage technology is widely studied and applied to power peak peaking,building energy saving,industrial waste heat recovery and solar energy storage systems and other fields.The core of the energy storage system is the energy storage unit.Therefore,it is an important prerequisite to grasp the phase change mechanism and heat transfer characteristics of energy storage unit to design and optimize the energy storage system and realize the wide application of phase change energy storage technology.In this paper,lauric acid was chosen as the phase change material.First,the numerical model and calculation method of the heat transfer process of fatty acid were established.The FLUENT software was used to simulate the melting heat transfer process of the capric acid in the rectangular unit,then the numerical calculation results are compared with the experimental data in the literature,which proves the correctness of the established enthalpy-porous media model and calculation method.On the basis of this,the sensitivity of the cavity heat transfer temperature difference,aspect ratio,inclination angle and its interaction to the melting time of phase change material were studied by orthogonal test.The research results indicate when the rectangular cavity containing phase change materials is designed and optimized(heat transfer temperature difference 20-40 ?,aspect ratio 1-4,and angle 0-60°),the heat transfer temperature difference,angle and the interaction of aspect ratio and angle should be considered emphatically,and the aspect ratio should be given due consideration.The interaction of aspect ratio and angle with heat transfer temperature difference can be disregarded.At the same time,for the most widely used annular unit,from two cases of the fixed external heating surface size and inner heating surface size,the melting heat transfer characteristics of the annular units with with different inner and outer diameter are analyzed.Discussion on comprehensive heat transfer mechanisam of heat conduction and natural convection,the influence of the inner and outer diameter of units on the heat transfer is studied which contains the average heat storage rate,the melting fraction,the melting time and the average Nusselt number,and the expression of melting fraction of two kinds of units and its application range are obtained.Under the premise of ensuring the heating surface size and phase change material quality fixed,two annular unit design schemes are compared.It is found that the outer ring heating unit and the inner ring heating unit have the melting fraction intersection,and the melting time of inner ring heating unit is shorter when the same mass of PCM is melted below the intersection,but the melting time of outer ring heating unit is shorter when the melting fraction is above the intersection point.The higher the boundary temperature,the more obvious the advantage of outer ring heating unit in PCM full melting time.The eccentricity optimization of the two units is based on the enhanced heat transfer for the annular unit melting characteristics of the outer ring heating and the inner ring heating:for the outer ring heating unit,respectively,up and down 10%40%of the eccentric setting;for the inner ring heating unit,only down 10%-80%eccentric setting.The results show that whether upward or downward eccentric,it has no positive effect on the melting and heat transfer of the unit,therefore,the outer ring heating ring unit without the need to set eccentric,inner ring and outer ring concentric is the optimal design of the unit.After the inner ring heating annulus is eccentric,the unit melts faster and has an optimal eccentricity of 70%in the range of 10%-80%of the downward eccentricity,and obtains the Fourier number expression of different size eccentric units and its scope of application. |
PDF Full Text Request