太阳光热填料床放热过程动态特性分析

为了改善太阳能不能连续稳定输出的问题,针对填料床放热过程,采用数值模拟方法研究了传热流体流速,温度以及填料床的高径比对填料床动态放热特性的影响规律。结果表明：在计算条件下,提高传热流体流速和温度是改善放热速率的关键参数,而提高填料床高径比有利于降低放热速率。当传热流体的进口流速由0.000 5 m/s 增加至0.001 3 m/s,填料床的平均放热速率提高了150%,放热时间缩短了54%,对放热密度无影响;传热流体进口温度为473 K上升至513 K,填料床的平均放热速率下降了32%,放热时间延长了34%,放热密度下降了6%;填料床高径比由2.32增加至8.33,填料床的放热量无变化,放热完成时间增加了113%,填料床的放热速率下降了129%。     

基于响应面法的新型螺旋管传热性能优化研究

采用Box Behnken方法,对新型螺旋管的传热性能和阻力性能的模拟计算进行实验设计。设定螺旋管内径、进口流速、螺旋槽波高、螺距和单节螺旋管与光滑管长度比5个影响因素,应用响应面法建立综合性能评价指标（PEC）与5个因素之间的二次多项式模型,确定螺旋管的最佳参数,对螺旋管强化传热机理进行研究。结果表明：换热管中的螺旋槽增加了换热面积,同时使流体产生周期性扰动,加快热量的传递;在实验优化范围内,螺旋管内径是影响PEC最显著的参数;换热管的最佳参数为：换热管内径25 mm,进口流速0.4 m/s,螺旋槽波高3 mm,单节螺旋管与光滑管长度比为1,螺距15 mm;在该条件下,计算得到努塞尔数为395.8,阻力系数为0.044,综合性能影响因子为3.82。     

波纹板式换热器传热与流动特性分析

通过数值模拟的方式,对一种用于低粘度流体的波纹板式换热器的传热特性和阻力性能进行分析,以控制变量法分析了流体速度对传热特性及阻力性能的影响;搭建了板式换热器测试平台,验证了数值模拟结果的正确性和可行性;用等速法拟合了Nu-Re与Eu-Re的相关准则式,并采用JF因子评价换热器综合性能。结果表明：模拟结果与实验结果相比误差在10%以内;当流体流速小于1.0 m/s时,换热器传热系数和压降随着流速的增大而增大,但综合换热性能逐渐变差,在此流速范围内,总传热系数随冷流体进口温度升高而增大。     

熔融盐在蓄热混凝土中传热过程的数值模拟

用数值模拟的方法对熔融盐与蓄热混凝土组合式换热模型进行了研究．该组合模型以混凝土为蓄热介质,熔融盐（Hitec）为传热介质,分析混凝土的导热系数、流体管道长度、热流体进口温度与进口流速等因素对蓄热单元储热规律及其特性的影响。计算结果表明：当混凝土的导热系数在2．0～3．0W/（m·K）左右、管长为0．5—2m、热流体进口温度为420℃、进口流速为O．15～0．25m／s时,蓄热单元可获得较高的储热效率。     

熔融盐斜温层蓄热的热特性研究

对熔融盐高温斜温层蓄热过程进行较深入的理论与实验研究,建立熔融盐单相流体斜温层蓄热的瞬态热分析模型,模型考虑熔融盐的变物性。利用Fluent软件,通过求解N-S方程与能量方程,对熔融盐单相流体斜温层蓄热罐在各工况条件下的传热蓄热过程进行数值模拟。研究时间进程、初始条件以及结构尺寸等对蓄热性能的影响。结果表明:斜温层的厚度随时间的推移而增加,达到一定厚度后增加量趋缓;流体进口流速、长径比等是影响有效蓄热容量的主要因素,当进口速度为0.001m/s级、长径比为2∶1时,将减少斜温层厚度。     

板式换热器单边流动与对角流动数值模拟

基于传热学控制方程,采用数值计算方法,对板式换热器单边流动和对角流动时的流动与换热特性进行分析。在分析过程中保持换热器的结构参数不变,只改变进出口的流动方式,结果发现:在相同的流速下,单边流动的总对流换热系数要高于对角流动,而总压降单边流动要低于对角流动,在流速u=0.6 m/s工况下,努谢尔数单边流动比对角流动高出10.87%,压降对角流流动比单边流动高出5.13%。随着进口流速的增大,单边流动与对角流动的冷热流体进出口温差均减小,而且减小的趋势对角流动要大于单边流动,摩擦因子f和传热因子j逐渐减小。单边流动的流动和传热特性要优于对角流动。     

立管内椭球颗粒群强化传热特性模拟研究

为提高液固两相流传热效率,通过添加椭球形颗粒改善立式换热管内液固两相流分布,采用CFD-DEM数值模拟颗粒长径比β、颗粒进口体积分数φ以及流体进口速度v与管内椭球形颗粒群传热的关系,并根据PEC指数评价颗粒强化传热能力。模拟结果表明：在1.50≤β≤2.50,2.00%≤φ≤4.00%,1.0 m/s≤v≤1.5 m/s时,椭球形颗粒PEC指数总高于球形颗粒,即前者强化换热效果优于后者。     

基于S-CO2的强化壳管式蓄热单元传热性能优化

为了了解壳管式蓄热单元的传热性能,提出了一种强化壳管式蓄热单元传热性能的优化方法。对不同的换热流体(HTF)进行优质选择,并基于回归正交试验对相变蓄热单元结构参数进行优化。结果表明：回归方程满足试验的显著性和失拟性检验,验证结果与计算结果的相对误差在1%以内;采用800℃、20 MPa的超临界二氧化碳(S-CO₂)和管径比为0.666 3的组合形式最优;优化后的蓄热单元获得的最高储热量为2 674.52 kJ,比优化前增加约39.5%,传热速率范围从1.76 kW降到0.007 kW。     

单/双U形地埋管换热器传热特性模拟研究

为了研究不同因素对单/双U形地埋管换热器传热特性的影响,建立单U形和双U形地埋管换热器传热物理数学模型,分析流体进口流速、流体进口温度、回填材料和土壤竖向温度梯度对地埋管周围土壤温度分布规律和单位管长换热量的影响。结果表明：地埋管周围土壤温度随流速的增加而增加且双U形大于单U形,对径向距离大于1 m的土壤基本无影响;双U形地埋管换热器的单位管长换热量大于单U形,双U形地埋管换热器的热短路现象更严重;增加流体进口温度时,土壤温度、单位管长换热量均增加,但热作用范围基本不受影响;回填材料为砾砂时的土壤温度增加幅度大于黏土,单位管长换热量大于黏土,但增加比例小于回填材料为黏土时的情况;土壤竖向温度梯度越大,土壤温度和单位管长换热量波动幅度越剧烈,且双U形的波动幅度大于单U形;钻井深度和土壤竖向温度梯度较大时,地埋管不宜采用U形管作为换热器。研究结果对单/双U形地埋管换热器的设计及地源热泵系统的运行提供参考。     

应用废弃油气井获得地热能

为了有效的利用现有的废弃油井和气井中存在的大量热能,建立了废弃油气井中循环流体的流动换热方程以及循环流体和岩石的热交换方程,运用数值模拟进行了求解,计算结果表明:循环流体的流量和地温梯度是影响换热量的两个最主要因素。地温梯度Tg分别为25、45和65℃/km时,最大换热量分别为184.12,394.22和604.96 kW。对于确定的地温梯度,总存在最优的流速使换热量或发电量最大;地热能采出系统能够长期稳定运行。对于Tg=45℃/km的工况,第一年和第八年末采出井口流体的温度分别为100.38和99.48℃;两井之间的最小间距应不小于20 m。     

Experimental investigation on a combined sensible and latent heat storage system integrated with constant/varying (solar) heat sources

The objective of the present work is to investigate experimentally the thermal behavior of a packed bed of combined sensible and latent heat thermal energy storage (TES) unit. A TES unit is designed, constructed and integrated with constant temperature bath/solar collector to study the performance of the storage unit. The TES unit contains paraffin as phase change material (PCM) filled in spherical capsules, which are packed in an insulated cylindrical storage tank. The water used as heat transfer fluid (HTF) to transfer heat from the constant temperature bath/solar collector to the TES tank also acts as sensible heat storage (SHS) material. Charging experiments are carried out at constant and varying (solar energy) inlet fluid temperatures to examine the effects of inlet fluid temperature and flow rate of HTF on the performance of the storage unit. Discharging experiments are carried out by both continuous and batchwise processes to recover the stored heat. The significance of time wise variation of HTF and PCM temperatures during charging and discharging processes is discussed in detail and the performance parameters such as instantaneous heat stored and cumulative heat stored are also studied. The performance of the present system is compared with that of the conventional SHS system. It is found from the discharging experiments that the combined storage system employing batchwise discharging of hot water from the TES tank is best suited for applications where the requirement is intermittent.     

Numerical analysis of latent heat thermal energy storage using encapsulated phase change material for solar thermal power plant

Thermal energy storage improves the load stability and efficiency of solar thermal power plants by reducing fluctuations and intermittency inherent to solar radiation. This paper presents a numerical study on the transient response of packed bed latent heat thermal energy storage system in removing fluctuations in the heat transfer fluid (HTF) temperature during the charging and discharging period. The packed bed consisting of spherical shaped encapsulated phase change materials (PCMs) is integrated in an organic Rankine cycle-based solar thermal power plant for electricity generation. A comprehensive numerical model is developed using flow equations for HTF and two-temperature non-equilibrium energy equation for heat transfer, coupled with enthalpy method to account for phase change in PCM. Systematic parametric studies are performed to understand the effect of mass flow rate, inlet charging system, storage system dimension and encapsulation of the shell diameter on the dynamic behaviour of the storage system. The overall effectiveness and transient temperature difference in HTF temperature in a cycle are computed for different geometrical and operational parameters to evaluate the system performance. It is found that the ability of the latent heat thermal energy storage system to store and release energy is significantly improved by increasing mass flow rate and inlet charging temperature. The transient variation in the HTF temperature can be effectively reduced by decreasing porosity.     

柱状颗粒填充床的储能性能分析

填充床储能是一种很有发展前景的热能储存技术,它具有可降低存储成本和提高太阳能热系统开发效率等优点。研究人员多采用球形的储能单元,而圆柱体在储能填充床换热中有其独特的优势,因此基于圆柱形和拉西环形两种柱状颗粒,建立了一种潜热储能填充床的三维模型,采用数值模拟的方法分别研究两种柱状颗粒组成的填充床的储能性能,分析了储能填充床的直径比对其性能的影响。研究表明,填充床直径比越大,其储能性能越好。同时研究了圆柱形储能单元高度和拉西环形储能单元孔径对储能性能的影响。结果表明,在研究范围内,由高度为3 mm的圆柱形储能单元和孔半径1.50 mm的储能单元分别组成的填充床储能速率最高。     

Dynamic discharging characteristics simulation on solar heat storage system with spherical capsules using paraffin as heat storage material

The dynamic characteristics of solar heat storage system with spherical capsules packed bed during discharging process are studied. According to the energy balance of solar heat storage system, the dynamic discharging processes model of packed bed with spherical capsules is presented. Paraffin is taken as phase change material (PCM) and water is used as heat transfer fluid (HTF). The temperatures of PCM and HTF, solid fraction and heat released rate are simulated. The effects of inlet temperature of HTF, flow rate of HTF and porosity of packed bed on the time for discharging and heat released rate are also discussed. The following conclusion can be drawn: (1) the heat released rate is very high and decreases rapidly with time during the liquid cooling stage, it is stable at the solidification cooling stage, then it decreases to zero at the solid cooling stage. (2) The time for complete solidification decreases when the HTF flow rate increases, but the effect is not so obvious when the HTF flow rate is higher than 13 kg/min; (3) compared to the HTF inlet temperature and flow rate, the influence of porosity of packed bed on the time for complete solidification is not so significant.     

Comparison of energy and exergy efficiencies of an underground solar thermal storage system

In this experimental study, solar energy was stored daily using the volcanic material with the sensible heat technique. The external heat collection unit consisted of 27 m² of south‐facing solar air collectors mounted at a 55° tilt angle. The dimensions of the packed‐bed heat storage unit were 6 × 2 × 0.6 m deep. The packed‐bed heat storage unit was built under the soil. The heat storage unit was filled with 6480 kg of volcanic material. Energy and exergy analyses were applied in order to evaluate the system efficiency. During the charging periods, the average daily rates of thermal energy and exergy stored in the heat storage unit were 1242 and 36.33 W, respectively. Since the rate of exergy depends on the temperature of the heat transfer fluid and surrounding, the rate of exergy increased as the difference between the inlet and outlet temperatures of the heat transfer fluid increased during the charging periods. It was found that the average daily net energy and exergy efficiencies in the charging periods were 39.7 and 2.03%, respectively. The average daily net energy efficiency of the heat storage system remained nearly constant during the charging periods. The maximum energy and exergy efficiencies of the heat storage system were 52.9 and 4.9%, respectively. Copyright © 2004 John Wiley & Sons, Ltd.     

Thermal performance simulations of a packed bed cool thermal energy storage system using n-tetradecane as phase change material

The objective of this paper is to study the thermal performance of latent cool thermal energy storage system using packed bed containing spherical capsules filled with phase change material during charging and discharging process. According to the energy balance of the phase change material (PCM) and heat transfer fluid (HTF), a mathematical model of packed bed is conducted. n-tetradecane is taken as PCM and aqueous ethylene glycol solution of 40% volumetric concentration is considered as HTF. The temperatures of the PCM and HTF, solid and melt fraction and cool stored and released rate with time are simulated. The effects of the inlet temperature and flow rate of HTF, porosity of packed bed and diameter of capsules on the melting time, solidification time, cool stored and released rate during charging and discharging process are also discussed.     

两级填充床蓄热器式绝热压缩空气储能系统变工况特性研究

利用压缩机和透平的变工况模型及填充床蓄热器的非稳态模型,对两级填充床蓄热器式绝热压缩空气储能系统一次完整的充放电循环过程进行模拟,分析系统的热力学性能和各个部件的变工况特性.结果表明,系统的充放电效率可达62.4％.储能时,受洞穴内压力变化影响,两级压缩机压比、效率不断改变,从而引起下游填充床蓄热器进口温度变化;释能时...     

A numerical investigation into the charge behaviour of a spherical phase change material particle for high temperature thermal energy storage in packed beds

基于高温相变材料,对填充床储热系统中储热单元球体的储热性能进行了模拟研究.研究了不同传热流体温度和球体直径对球体储热性能的影响规律,对导热为主的相变储热过程与导热和自然对流共同作用的相变储热过程进行了比较分析,同时还探讨了高温辐射换热的影响.结果表明,相变时间随球体直径的增大而增大,随传热流体温度的增大而减小.当考虑相变区域自然对流时,总的相变时间显著减少,和单纯导热相比,完全相变时间缩短了近16%.在导热和自然对流的基础上加上辐射传热后可以看出,辐射换热强化了球体内的传热过程,加快了相变材料的熔化速度,强化了自然对流的作用.     

Exergy‐based performance analysis of packed‐bed solar air heaters

This paper presents an experimental investigation of the thermal performance of a solar air heater having its flow channel packed with Raschig rings. The packing improves the heat transfer from the plate to the air flow underneath. The dimensions of the heater are 0.9 m wide and 1.9 m long. The aluminium‐based absorber plate was coated with ordinary black paint. The characteristic diameter of the Raschig rings, made of black polyvinyl chloride (PVC) tube, is 50 mm and the depth of the packed‐bed in flow channel is 60 mm. Energy and exergy analyses were applied for evaluating the efficiency of the packed‐bed solar air heater. The rate of heat recovered from the packed‐bed solar air heater varied between 9.3 and 151.5 W m^?2, while the rate of thermal exergy recovered from the packed‐bed solar air heater varied between 0.04 and 8.77 W m^?2 during the charging period. The net energy efficiency varied from 2.05 to 33.78%, whereas the net exergy efficiency ranged from 0.01 to 2.16%. It was found that the average daily net energy and exergy efficiencies were 17.51 and 0.91%, respectively. The energy and exergy efficiencies of the packed‐bed solar air heater increased as the outlet temperature of heat transfer fluid increased. Copyright © 2004 John Wiley & Sons, Ltd.