| The latent heat thermal energy storage(LHTES)is an important part of the energy storage technology system,which effectively mitigates the mismatch between energy supply and demand in time,intensity and location.LHTES unit have has many advantages,such as high energy storage density,stable input/output temperature and compact structure.LHTES unit can be used in building energy conservation,industrial waste heat recovery,aerospace and other fields.Thus,it is necessary to study of heat storage mechanism and characteristics for the LHTES unit in detail,which can support a foundation for its engineering application.The melting process of shell and single-tube LHTES unit is studied deeply in this paper,involving two methods of scale analysis and numerical calculation,two ways of vetical and horizontal placement,heat storage unit of multiple structural sizes and phase change material(PCM).Finally,the prediction of the main characteristics of the shell and tube bundle LHTES unit in engineering using a shell and single-tube unit is realized.The research contents of this paper are as follows,(1)Based on the scale analysis method,the variation of melting volume fraction(MVF)and Nusselt number with time in the melting process for rectangular cavity and vertical circular were studied.In order to investigate the general rules of the melting process for the heat storage unit,the melting process of rectangular cavity and vertical circular cavity was studyed by scale analysis method.The results show that the melting process considering natural convection can be divided into four stages,involving the conduction stage,the mix conduction plus convection stage,the convection stage Ⅰ and the convection stage Ⅱ.The conduction stage and the convection stage Ⅰ are the main stages.In addition,the scaling correlations of MVF and Nu in each stage are obtained,and the correctness of the correlations is verified by numerical simulation results.(2)Based on the natural convection model,the coupled heat transfer mechanism of the vertically placed shell and single-tube LHTES unit was analyzed.In the melting process for the shell and single-tube LHTES unit,the temperature distribution of heat transfer fluid(HTF)region,tube wall region and PCM region is influenced by each other.Natural convection model was used to simulate the coupled heat transfer process of three regions in the vertically placed shell and single-tube unit.The results show that the outer surface temperature of HTF tube first decreases nd then increases with the increase of height before 900 s,and then increases with the increase of height after 900 s.During the whole melting process,the local radial heat flux on the outer surface of HTF tube decreases gradually with heigh.To further quantify the influence of natural convection on the melting process,the convection influence factor was defined,and it is the ratio of melting volume fraction with and without natural convection.For the heat storage unit in the melting process,the convection influence factor was not completely greater than 1.This indicates that natural convection gradually increased during the melting process,but natural convection does not completely accelerate the melting process.(3)The melting performance of the vertically placed shell and single-tube LHTES unit is influenced by structural parameters.To analyze the influence of structural parameters on the performance of the vertically placed unit,melting processes of vertically placed units with different tube lengths and shell radii under uniform wall temperature were compared,which involves whether natural convection occurs,the influence of natural convection and the evaluation of the overall melting performance.The distance between the solid-liquid interface and the tube wall when natural convection occurs was defined as the critical vertical annular layer thickmess,which is compared with radial spacing in annular space to determine whether natural convection occurs during the melting process.The influence of natural convection on the melting process was evaluated by convection influence factor.The time average value of radial heat flux in the whole melting process was defined as as the time-average radial heat flux,which is used to quantify the overall melting performance of the unit.The results show that the correlation between critical vertical annular layer thickness and tube length(shell radius)is low,and the unit size can determine whether natural convection occurs during melting.When natural convection occurs in the unit,there is a critical tube length(critical shell radius)value for the unit with the same shell radius(tube length).When the tube length is greater than the critical tube length(When the shell radius is less than the critical shell radius),the natural convection accelerates the melting in the early stage and inhibits the melting in the late stage.When the tube length is less than the critical tube length(When the shell radius is lager than the critical shell radius),natural convection accelerates the melting at the whole phase change process.For the overall melting performance of the unit,the overall melting rate increases with the decrease of tube length,and it decreases first and then increases with the increase of shell radius.(4)The melting performance of the vertically placed shell and single-tube LHTES unit is influenced by thermal properties of PCM.Ga,n-octadecane,stearic acid and RT42 were selected as PCM to analyze the effect of thermal properties of PCM on the performance of the vertically placed unit.When PCM is Ga,the convection influence factor is close to 1 during the melting process,which indicates no natural convection occurs during the melting process.When PCM are n-octadecane,stearic acid and RT42,natural convection occurs which is judged by the critical vertical annular layer thickness.This shows that the thermal property of PCM determines whether natural convection occurs.When natural convection occurs,thermal properties of PCM alse affect the melting performace.According to the time-average radial heat flux,the whole melting rate of the unit is from high to low for RT42,stearic acid and n-octadecane.(5)Based on natural convection model,the couple heat transfer mechanism and parameter influence of horizontally placed shell single-tube LHTES unit were analyzed.Horizontal placement is another common placement way for heat storage unit.Due to the existence of natural convection,the melting process of horizontal placed unit is different from that of the vertical placed unit.The coupled heat transfer process of horizontally placed shell and single-tube unit was numerically simulated by natural convection model.The results show that the HTF tube surface temperature of the upper part is higher than that of the lower part due to natural convection,and the local radial heat flux in the upper part of the unit is less than that in the lower part of the unit.The influence of structural parameters and thermophysical properties of PCM on the melting performance was further analyzed.The influence of shell radius on the critical horizontal annular layer thickness is little,and the unit size can determine whether natural convection occurrs during the melting process.The critical shell radius exists,when the shell radius is less than the critical shell radius,natural convection accelerates the melting in the early stage and inhibits the melting in the late stage.When the shell radius is larger than the critical shell radius,natural convection accelerates the melting process.For the overall melting performance of the unit,the melting rate of the unit decreases gradually with the increase of the shell radius.In terms of thermophysical properties of PCM,the effect on melting performance is consistent with that of the vertically placed unit.(6)Geometric sizes cause whether the performance of vertical placement shell and single-tube LHTES unit is better than that of horizontal placement unit.In order to select a reasonable placement way for a single tube unit,the correlation between the melting performance of vertical and horizontal heat storage units and unit sizes was analyzed.According to the time-average radial heat flux,there is a critical tube length under the same shell radius.The overall melting rate of vertically and horizontally placed unit is consistent at critical tube length.When the tube length is greater than the critical tube length,the overall melting rate of the unit placed vertically is lower than that of the unit placed horizontally.Otherwise,it is contrary.In addition,the critical tube length tends to increase with the increase of the shell radius.(7)The heat storage characteristics of the multi-tube unit are predicted by using the single tube LHTES unit.The shell and multi-tube LHTES units are commonly used as heat storage units in engineering practice.However,due to the unsteady characteristics of phase transition heat storage process,its numerical analysis is a time-consuming work.In order to shorten the numerical calculation time,it is of great significance to select a single tube unit to predict the melting performance of the multi-tube unit.The results show that there is a great difference between the melting time in the middle region and the edge region of the shell and multi-tube LHTES unit and the melting time of the shell and single-tube unit under the adiabatic boundary.When reasonable boundary conditions are selected,such as uniform wall heat flux or uniform wall temperature,the main heat storage characteristics of the multi-tube unit model can be obtained by using the shell and single-tube unit model. |
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