| The mismatch between energy supply and demand for the solar water heating system can be improved by coupling the phase change material(PCM)to its hot water tank.However,the practical application of PCM is still limited to its relatively low thermal conductivity.Existing enhancement methods for the heat transfer of PCM,such as adding fins and the expanded graphite,could decrease the energy storage density of the encapsulation.Additionally,the graphite may settle during the melting process of PCM.Accordingly,a new method for enhancing the heat transfer performance of the encapsulation through improving its structure is proposed in present study.Considering that contact melting modes can significantly enhance the melting rate of PCM,an inverted conical container,which has relatively lower ratio of the top area to the bottom area compared to cylindrical containers,has been proposed to encapsulate the PCM with the goal of establishing the contact melting mode between solid PCM and heated surfaces during the melting process of PCM.To evaluate its performance,a mathematical model was developed and validated by the results obtained from a visualization experiment.Based on this model,the melting performance of the PCM encapsulated in the inverted conical container was analyzed and compared to cylindrical containers.And a parametric analysis was conducted to investigate the total melting time of PCM encapsulated in different containers.In addition,an artificial neural network(ANN)model was developed to predict the liquid volume fraction of PCM during its melting process.The main conclusions of this study are as follows:(1)Compared to the cylindrical container,the inverted conical container benefited from the contact melting between the solid PCM and heated wall surfaces,and thus the melting performance of PCM was improved.Specifically,as the inverted conical and cylindrical containers have the same volume,the total melting time for PCM encapsulated in the former is 2520 s,while it decreased 21.5% compared to PCM encapsulated in the latter.(2)Apart from the contact melting,the natural convection occurred for liquid PCM during its melting process also significantly affected the melting performance of the PCM.It was found that the Rayleigh-Bernard convection could increase the thickness of liquid PCM in the side region,which resulted in the decrease in melting performance of the PCM.(3)For inverted conical containers,the increase of its aspect ratio weakened the contact melting at its side wall surface,while improved the natural convection at this surface.Meanwhile,it also led to a variety of top surface and side wall surface area,which affected the total heat transfer area and instantaneous heat transfer.Eventually,with the increase of its aspect ratio,the total melting time showed a trend of increasing first and then decreasing.It should be mentioned that the total melting time of PCM reach the maximum when the aspect ratio is 2 among the five aspect ratios selected in this paper.In practical application,the aspect ratio of inverted conical container should be selected according to the demand of the project.(4)The ratio of the bottom and top radius significantly affected the total melting time.Specifically,the total melting time of the encapsulated PCM increases first and then decreases with the increase of such ratio.When the radius ratio is 2,it would reach the maximum among the five kinds of radius ratio.Interestingly,a higher melting performance(i.e.,16.7%)was achieved for the PCM encapsulated in the conical container compared to that encapsulated in the inverted conical container.In this view,the combined usage of conical enclosure and inverted conical enclosure would be appreciated to improve both the heat storage capacity and the melting performance in practical applications.(5)The liquid volume fraction of PCM during melting process can be accurately predicted by ANN models.The mean square error(MSE)for the training data,validation data and testing data were 4.08e-05,4.05e-05 and 3.71e-05,respectively.Corresponding to 0.99968 、 0.99968 、 0.99971 for the correlation coefficient(R),respectively.Compared to the mathematical model,this method needs much shorter time and thus facilitates the practical applications.Based on this study,further research would focus on the application of PCM encapsulated by different containers in domestic hot water tank with influencing factors such as its layout position and water demand.Meanwhile,the control strategy should be developed to match the demand on the basis of the performance of the hot water tank,so as to provide theoretical design basis for its efficient application in practice. |
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