| In the background of "dual carbon",thermal energy storage technology plays an important role in solving the problem of unstable supply of renewable energy.With the advantages of high heat storage density and stable performance,latent heat thermal energy storage(LHTES)technology has been widely studied in the field of thermal energy storage.However,the thermal conductivity of phase change material(PCM)in the latent heat thermal energy storage unit(LHTESU)is generally low,so heat transfer enhancement is needed for the charging and discharging process of LHTESU.Among the various measures to enhance heat transfer,fins are used in industrial production in a large number of applications because of their low price and convenience of manufacture.Influenced by the different heat transfer mechanisms of LHTESU during the melting and solidification process,the fins should be non-uniformly arranged during the melting process,and uniformly arranged during the solidification process.For the complete melting-solidification process,uniformly arranged fins can achieve better enhancement results compared to non-uniformly arranged fins.However,the melting performance of the LHTESU needs to be further improved when the fins are arranged uniformly.In response to the above problems,this thesis proposes an active strengthening measure using rotating finned tube to further enhance the melting performance of the LHTESU when the fins are uniformly arranged.During the melting process,in order to avoid the collision between the fins and the solid PCM resulting in the increase of driving power to the rotation of finned tube,the finned tube will start to rotate to enhance heat transfer after PCM in the finned tube rotation domain melts;during the solidification process,the finned tube will remain static,which is similar to the solidification process of the unit enhanced by conventional static finned tube,therefore,this thesis focuses on the melting process.First,this thesis conducts an experimental study on the LHTES process under the strengthening of rotating finned tube in order to analyze the enhancement effect of rotating finned tube.Secondly,the melting performance of horizontal shell-and-tube LHTESU under the action of rotating finned tube is investigated numerically in this thesis to further analyze the heat transfer mechanism in the phase change region under the action of rotating finned tube.Finally,in order to further strengthen the melting performance of the PCM at the bottom of the unit,this thesis introduces the solution of flipping the whole unit by 180° on the basis of rotating finned tube,and conducts a numerical study on the melting process of the horizontal shell-and-tube LHTESU under the action of flipping coupled rotating finned tube.The main research contents and conclusions of this thesis are as follows:(1)In the experimental study on the strengthening of LHTES process by rotating finned tube,this thesis constructs a visualized experimental bench,photographs and analyzes the evolution pattern of solid-liquid interface,discusses the effects of different motion modes,and analyzes the strengthening effect of rotating finned tube.The results show that the stepwise mode does not contribute to the promotion of the PCM melting process because it weakens the heat transfer between the fin and the bottom PCM.The rotating mode is able to accelerate the melting process of the PCM.With the 4 rpm rotating finned tube,the total melting time of the PCM can be shortened by 23.66%based on the static finned tube.(2)Based on the experiment,the melting performance of the horizontal shell-andtube LHTESU under the action of rotating finned tube is investigated numerically in order to further analyze the heat transfer mechanism in the phase change region under the action of rotating finned tube.In the numerical simulation,the model of dynamic mesh technology coupled with enthalpy-porosity is used,the simulation and experimental results are compared to verify the accuracy of the employed numerical model,the evaluation index of TESR considering the driving power is introduced,and the melting process of PCM under different rotational speeds of finned tube and number of fins is discussed.The results show that the rotating finned tube improves the uniformity of the temperature field as well as the synergy of velocity and temperature gradient,and reduces the total melting time by 30.3% and increases the TESR by40.07%;the rotational speed of the finned tube should be not less than 2 rpm,and at 16 rpm,the total melting time of the LHTESU is shortened by 46.96% compared with the static finned tube,and the TESR can be increase by 79.02%;increasing the number of fins can allow the finned tube to start rotating at an earlier stage.When the number of fins is 10,the dimensionless start time of rotation can be advanced by 36% and the total melting time can be shortened by 33.49% compared to 4 fins.(3)The PCM at the bottom of the unit eventually melts under the reinforcement of the rotating finned tube.In order to further enhance the melting performance of the PCM at the bottom of the unit,this thesis introduces a scheme of flip 180° of the whole unit based on the rotating finned tube,and conducts a numerical study on the melting process of the horizontal shell-and-tube LHTESU under the action of the flipping coupled rotating finned tube,and discusses the effects of the dimensionless flipping time and the rotational speed of the finned tube.The results indicate that,compared with the rotating finned tube,the strengthening method of flipping coupled with rotating finned tube can reduce the total melting time by 23.65%;this thesis recommends that the unit should flip at the same time as the finned tube starts to rotate,in order to obtain a better thermal performance and to facilitate the implementation of reinforcement measures;increasing the rotational speed of the finned tube can further increase the melting rate of the PCM,and when the rotational speed is increased from 1 rpm to 8rpm,the total melting time can be reduced by 19.26%. |
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