| Latent heat thermal enery storage(TES)is an energy-saving technology that utilizes the physical state change of phase change materials(PCMs)to realize the energy storage or release.It can be applied to systems where the supply is not continuous or where the supply does not match the instant demand,solving the problem of the continuity and stability of the energy supply.However,most PCMs have the drawback of low thermal conductivity,which strongly limits the heat transfer rate during both charging and discharging processes.This seriously restricts its application in actual engineering.In order to improve the energy storage/release efficiency of the PCMs,it is necessary to conduct research on the heat transfer enhancement of the energy storage module.The thermal conductivity of the storage module can be enhanced through material modification and structural reformation.However,the material modification will change the specific heat or latent heat of the PCM,and affect the energy storage and release of the storage module.This problem can be avoid by multi-tube structure reformation.The multi-tube heat exchanger has the advantages of simple struct ural design and easy processing.Meanwhile,it can increase the heat exchange area of the PCMs and enhance the thermal conductivity of the stourage modules.At present,there are few studies on the PCM based multi-tube heat exchanger under low temperature phase transition conditions,and lack of structural parameters optimization for such heat exchanger.Furthermore,as compared to the traditional data centers with water storage,the integration of such heat exchanger and loop thermosyphon can overcome the disadvantage of large area and small storage density,and extend the emergency cooling time of data center.In view of the above problems,this paper begins from two aspects of performance research and application of the PCM based multi-tube heat exchanger.A commercial paraffin RT11 HC with a melting temperature of 10–12 °C was employed as the PCM.Both experimental and numerical investigations were conducted to examine the thermal performance of the PCM based multi-tube heat exchanger,and also the structural parameters of such heat exchanger were optimized.In addition,a novel integrated cooling system comprising a loop thermosyphon and PCM based multi-tube heat exchanger was proposed.Moreover,the thermal performance of the integrated system was investigated under emergency conditions for a data center.The main research work of this paper is as follows:Firstly,according to the control volume method,the unsteady numerical heat transfer model of the PCM based multi-tube heat exchanger was established by using the enthalpy-porosity approach on the basis of the unstructured collocated grids.In the numerical experimental case of natural convection in a closed square cavity,the accuracy of the proposed model was verified by the distribution of fluid temperature and velocity.In the numerical experimental cases of isothermal phase change and nonisothermal phase change,by comparing the positions of melting front,the average liquid fraction and the hourly variations of temperature distribution,it concluded that the relative error between the simulated value and the experimental value is minor.This indicated the high accuracy of the developed numerical mothed;Secondly,the cold discharging/charing characteristics of the PCM based multitube heat exchanger with four inner tubes were studied experimentally.According to the experimental results,compaed with the flow rate of heat transfer fluid(HTF),the inlet HTF temperature played a more significant role during both cold charging and discharging processes.With the increase of temperature difference between the PCM and HTF,PCM temperature was varied dramatically with time and consequently the heat transfer rate was enhanced.Moreover,the increasing of HTF flow rate had less influence on the average PCM temperature,whereas the heat transfer rate and effectiveness were clearly affected by the HTF flow rate.In addition,in the case of the same inlet temperature and flow rate of the HTF,as compared to the counter-current flow mode(the cold and hot HTF were injected into the tubes from the opposite side),the heat transfer rate of parallel flow mode(the cold and hot HTF were injected into the tubes from the same side)was 25.2% higher and the average total thermal resistance was 34.3% lower;Thirdly,based on the enthalpy-porosity model,the PISO algorithm was applied to simulate the thermal performance of the PCM based multi-tube heat exchanger.For the discharging/charging condition,the heat transfer characteristics of PCM based multitube heat exchangers with three structures were examined.The variation trend of the temperature distribution,the liquid fraction distribution and the cooling capacity with time were analyzed.Simulation results showed that the increase of the number of inner tubes can improve the charging performance,whereas the increase of the number of inner tubes has little effect on the discharging performance.In addition,according to the orthogonal experimental aproach,the structural parameters of the PCM based multi-tube heat exchanger were optimized and configured,in which the mean charging rate was used as the experimental index.Variance analysis of the trial results showed that the number of inner tubes has a very significant impact on the mean charging rate,the ratio of eccentric distance to shell radius has a significant impact on the mean charging rate,while the angle ratio and the thickness of tube wall have no significant impact on the mean charging rate.Furthermore,the position distribution of the five inner tubes for a PCM based multi-tube heat exchanger was obtained by genetic optimization algorithm.Within the 120 min of charging,the optimal one reduced the complete melting time of the PCM by 61.11%,and increased the total charging amount by 5.21%,as compared with the case before optimization;Finally,a novel integrated cooling system comprising a loop thermosyphon and latent TES was designed for a data center.Experimental and numerical simulations were utilized to explore the thermal performance of the integrated cooling system under the emergency condition.The experimental results showed that in the first 6 min of emergency cooling,the minimum cooling capacity exceeded 1000 W,and the integrated system could maintain the servers operational during the time.The refrigerant flow rate decreased to zero after approximately 9 min,which implied that the loop thermosyphon stopped operating.In addition,the parametric analysis showed that the thermal conductivity of the PCM has a significant influence on the cold discharge rate and cooling capacity,followed by phase transition temperatue and latent heat.As the thermal conductivity of the PCM increased from 0.2 to 2 W/(m·°C),the minimum cooling capacity increased from 855.01 W to 1032.96 W,so that the cooling capacity met the emergency cooling demand within 15 min. |
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