The Mechanism Of Phase Change Cold Storage Based On High Efficiency Heat Transfer Of Foam Effect

The solid-liquid phase change cold energy storage has a high energy storage density and has developed rapidly in recent years.However,the problem of the gradual decay of the charging rate has been difficult to solve,which has seriously affected the turnover rate of the cold storage module in the cold chain logistics,making the cold storage capacity unable to meet the cooling load requirements of the central air conditioning system.To solve this problem,heat transfer enhancement of phase change material(PCM)is needed.In traditional heat transfer enhancement methods,it is difficult to optimize the energy storage density and cooling storage speed at the same time.In this paper,the method of direct contact between heat transfer fluid(HTF)and PCM is used to enhance heat transfer.The foam of HTF is used to increase the contact area to achieve efficient heat transfer between the HTF and PCM,which can effectively improve the charging rate without affecting the energy storage density.The formation of a large number of microchannels in the frozen body is the basis for producing foam.In this study,microchannels were formed by controlling the diameter and distribution density of the induced capillary,air flow rate,initial air temperature and initial water height.The critical forming temperature of microchannel was obtained by analyzing the attachment condition of ice crystal,and the growth of microchannels was numerically simulated based on the critical value.It is found that the temperature and flow rate of cold air flow have the greatest influence on the formation of microchannels;The microchannel is supported by ice beards,and the radial growth of ice beards makes the isolated ice beards fuse into a whole frozen body;The convective heat transfer resistance in the microchannel plays a decisive role in the growth of ice beards;The growth rate of ice beards and microchannels decreased gradually.Therefore,dense microchannels can be formed by controlling the parameters of cold air flow,so that the frozen body can become a foam generator growing with ice beards.The radius and gas phase ratio of bubbles in foam directly affect the heat transfer area between gas and liquid.In this study,the foam was realized by controlling the cold air flow parameters,and a clear image of the foam structure was obtained by using a uniform background light source.According to the morphological characteristics of bubbles at different positions,the foam was divided into three regions;The relationship between the rising velocity of the bubble group and the single bubble is analyzed.The motion model of bubble group is constructed in different regions.Using the invariance of the product of the movement velocity of the bubble group and the gas phase ratio,the subsection relationship of the gas phase ratio varying with time is established.It is found that the average radius and peak radius of bubbles in foam are both 2.5mm,and the bubble radius has uniformity;The bubble motion has independence at high Reynolds number and high gas phase ratio;Bubbles in foam are in uniform motion for most of the time.Therefore,the terminal velocity of bubble in water limits the its velocity,which causes the expansion of foam and increases the heat transfer area.The ripening phenomenon of foam will cause the continuous change of bubble radius.In this study,the fluctuating theory of active foam ripening is proposed.Based on this theory,a dimensionless ripening model of basic bubble group is established,and the model is verified by classical literatures;Based on the dimensionless ripening model,a prediction model of phase changing foam ripening is established by combining the subsection relationship of gas phase ratio with time,and the model is compared with the experimental image of foam.It is found that the ripening process of active foam is an fluctuating process,and the average bubble radius fluctuates up and down based on the separation radius;The fluctuating theory verifies the uniformity of bubble radius and accurately predicts the inhibitory effect of large bubbles in the critical region on the growth of bubbles in the other two regions;The inhibitory effect keeps the stability of radius distribution in the main region and foundation region.Therefore,the ripening phenomenon in active foam does not cause significant difference in bubble radius,and most of the bubble radius is still the separation radius.Foam freezing is a complex three-phase heat transfer process,which is difficult to solve,so it is necessary to simplify the heat transfer model.In this paper,based on the constant wall temperature boundary,the analytical heat transfer model of single bubble is established by using the lumped parameter method.According to the independence of single bubble heat transfer,the uniformity of bubble radius and the stability of radius distribution,the actual foam is equivalent to an equal diameter tangent bubble array in the study of phase change heat transfer.Based on this bubble array structure,the performance of foam freezing is studied.It is found that the convective heat transfer coefficient of bubbles with a radius of 2.5mm is 65 w/(m2.k);The maximum instantaneous heat transfer efficiency of foam freezing is up to 547 kw/m3.Therefore,foam freezing has a very efficient charging rate.Only when foam is sustainable can it reflect the superiority of its high-efficiency cold storage efficiency.In this paper,the heat transfer enhancement effect in the whole process of foam is verified by continuous cold storage experiment.The foam structure is compared to a virtual heat exchanger,and the foam freezing and traditional cold storage methods are compared from the instantaneous heat transfer performance;The phase change charging coefficient is proposed to compare the foam freezing with the traditional cold storage from the average heat transfer performance.It is found that foam has a significant persistence,which makes the foam freezing increase the charging rate by 4 times;The main reason why foam freezing can improve the charging rate is that it increases the heat exchange area between PCM and cold source,eliminates the heat conduction resistance,and strengthens the convection;In terms of instantaneous and average heat transfer performance,the corresponding indicators of foam freezing are significantly ahead.Therefore,compared with the traditional phase change cold storage methods,foam freezing has obvious advantages in charging rate.Foam freezing greatly improves the cold energy charging rate and solves the long-standing paradox between the charging rate and the energy storage density.It makes the cold storage process expand from one-dimensional unidirectional freezing to three-dimensional internal freezing.The phase transition takes place uniformly in all parts of the PCM,and the problem that the heat conduction thermal resistance gradually increase no longer exists.It has great research value and application prospects.