新型定形板状相变材料的蓄/放热特性

对填充了新型定形板状相变材料的蓄热槽的蓄／放热特性进行了数值计算分析和实验比较。根据数值计算，对影响蓄热槽蓄／放热特性的主要因素——相变材料的几何尺寸、相变材料的导热系数、流体流速、对流放热系数、相变材料填充率等的影响规律进行了分析研究，计算出了蓄热槽内温度分布随时间的变化；并在实验台上测试了蓄热槽初始温度、流人和流出蓄热槽流体温度、作为蓄热体的相变材料测点温度随时间的变化，计算结果与实验结果具有较好的一致性。     

管壳式换热器蓄热单元数值模拟与优化

相变材料导热系数低,导致相变蓄热装置无法快速地进行热量储存和释放,文中建立了翅片管和光管式相变蓄热单元的三维计算模型,采用数值模拟方法,从蓄热速率、蓄热量以及温度场等方面比较分析了翅片管和光管结构对储热性能的影响。结果表明:在光管外壁添加翅片可以缩短相变材料完全熔化以及整个蓄/放热过程所需时间;与采用光管结构相比,采用翅片换热管时,完全熔化时间缩短32%,完全放热时间缩短14.5%。可见,在一定条件下添加翅片有助于提高蓄热体的蓄放热性能,所得结论对实际工程中相变蓄热系统的设计和优化具有一定的参考价值。     

中温相变蓄热装置蓄放热性能的数值分析与实验研究

以所研制的相变温度为76℃的相变蓄热装置为研究对象,通过数值模拟和实验研究,对该相变蓄热装置的蓄、放热性能进行了模拟分析与实验验证。研究结果表明:所研究的相变温度为76℃的中温相变蓄热装置具有良好的蓄、放热性能,为在太阳能利用、工业废热利用以及暖通空调蓄热等领域的工程应用提供了可能。     

太阳热泵蓄热系统蓄热过程的数值模拟

对以CaCl2.6H2O为相变材料的太阳热泵蓄热系统,采用焓法建立了以相变材料封装容器为控制体单元的相变传热模型,计算了蓄热水箱进出口温度,相变材料温度及水箱蓄、放热量等参数。通过对实验结果与模拟结果的比较分析可知,两者数据接近,变化趋势吻合,该模拟方法能较好地反映系统运行情况。     

相变蓄热式集热器蓄放热数值与实验分析

对一种相变蓄热式太阳集热器的蓄放热过程进行CFD软件数值模拟和实验测试。该集热器由全玻璃真空集热管、相变蓄热单元和U型换热器组成。选用相变温度在330~400 K的3种适合于太阳集热器的复合相变材料石蜡、Ba(OH)_2·8H_2O和赤藻糖醇,在蓄热式集热器中进行蓄放热性能测试。研究表明,填充Ba(OH)_2·8H_2O的蓄热式集热器在总蓄热量、材料内部温度均匀性等蓄放热方面性能优越。实验结果对蓄热式集热器的设计具有参考价值。     

相变蓄热单元蓄/放热过程的数值模拟研究

建立同心套管相变蓄热单元的二维模型,并与文献对比验证,完成网格无关性和时间步长独立性验证后,数值模拟分析模型结构和流体入口速度对蓄热特性的影响,探究导热系数和相变材料的熔点对放热特性的影响。研究结果表明:采用波节管代替光管可优化蓄热单元的蓄热特性,且3#波节管的换热性能最优,相比于光管,蓄热时间可缩短39%;其他条件不变时,增加传热流体(heat transfer fluid,HTF)的导热系数,蓄热单元的放热效率先增后降,存在最佳值;当相变材料(phase change materials,PCM)的导热系数大于HTF时,继续增加PCM的导热系数会使放热效率下降;随着PCM熔点的增加,有效放热时间先增后减,在熔点为603 K时,放热效率达到最大值0.82。     

太阳能热发电用相变蓄热器的数值模拟与优化

采用单级壳管式蓄热结构,运用FLUENT软件进行数值计算,对蓄、放热过程进行分析,分析了固、液界面前端位置的变化,通过对数据的曲线拟合得出熔化时间与入口温度的函数关系,讨论了蓄、放热熔化凝固时间以及管内温度的变化.通过拟合,得到的曲线显示出很好的一致性,得出最佳入口温度1025 K,流量0.05kg/s.对蓄热和放热阶段分析,得到前端位置随时间变化形状呈现锥形,管内温度的变化,显示了相变蓄热技术的优势,得出的结论对相变问题的数值模拟以及相变装置的结构设计具有重要的理论价值.     

壳管式梯级相变蓄热装置内部结构参数的优化研究

梯级相变蓄热技术具有蓄放热平稳、储能密度大、易于控制、能有效克服相变材料导热系数低等优点,是最具应用前景的蓄热技术。壳管式蓄热装置是常用的蓄热装置,需要对其结构参数进行研究优化。采用焓法对壳管式梯级相变蓄热装置的相变过程进行了数值模拟,分析了梯级装置内管径和分管长对装置储热性能的影响。并以提高蓄热效率为优化目标,对参数进行了优化。对壳管式梯级相变蓄热装置的结构参数选择具有借鉴意义。     

微重力条件下相变材料容器的二维瞬态热分析

高温相变材料是空间太阳能热动力发电系统吸热器中普遍使用的蓄热介质。基于微重力状态下的导热控制微分方程，采用焓法对相变材料容器进行了二维数值分析，对计算结果给予了讨论。     

新型管壳式相变蓄热器的设计与数值模拟

设计了新型具有内外双螺旋结构的翅片管式蓄热换热器,通过Gambit软件建立三维蓄热体相变蓄、放热过程的物理模型和数学模型,运用Fluent软件对换热器的蓄、放热过程进行模拟。研究表明新型换热器的蓄、放热效率要比光管、仅有外螺旋翅片以及仅有内螺旋翅片的换热器高。最后对相变材料和外螺旋翅片的间距进行了优化,结果得出添加0.5%膨胀石墨/石蜡的复合相变材料要比纯石蜡蓄热时间缩短34%,翅片螺距对蓄热器的蓄热性能有重要的影响。     

集中供热热电联产

“集中供热 ,热电联产”是我国节能技术政策的一项重要内容。在国家计委、国家经贸委、国家科技局颁布实行的《中国节能技术政策大纲》中明确指出 :“2 0 0 0年实现城市集中供热普及率 2 5 %～30 % ,重点城市达到 4 5 %～ 5 0 %”“大力发展热电联产、区域锅炉房供热 ,合理选择集中供热方式 ,取代分散、小型工业锅炉供热 ,提高热电比重”。目前我国大约有 4 0万台小型工业锅炉在运行 ,我省约有 6 0 0 0台。集中供热是取代分散小锅炉 ,并在现有热负荷的基础上 ,建设区域性的集中供热。集中供热以高效、大容量的集中锅炉房取代小容量、低效的…     

具有热阻和热漏的联合普适和朗肯热机循环性能分析

用有限时间热力学的方法分析联合循环, 导出了存在热阻和热漏损失时,由两个绝热过程、一个加热过程和一个放热过程组成的空气标准普适循环和郎肯循环组成的联合动力循环的性能特性,并由数值计算分析了热阻和热漏对联合循环的输出功率和效率的影响,所得结果包含了顶循环为Carnot、 Otto、 Brayton、 Diesel、 Atkinson和 Braysson循环时联合循环的特性.当热源温度和工质流量相同时,联合热机的最佳功率、效率以及工作范围关系:Carnot和Rankine>Brayton和Rankine>Otto和Rankine;相同条件下如果换热器热导率为定值,联合热机的最佳功率、效率关系:Brayson和Rankine> Brayton和Rankine>Diesel和Rankine>Atkinson和Rankine>Otto和Rankine.     

Mass and heat transfer enhancement of chemical heat pumps

An inert additive, expanded graphite (EG), has been prepared and used to enhance the heat and mass transfer process of chemical heat pumps. The effects of mixing ratio and mixing method on the chemical reaction time are investigated.     

Numerical studies on flow and heat transfer in membrane helical-coil heat exchanger and membrane serpentine-tube heat exchanger

The flow and heat transfer characteristics of synthesis gas (syngas) in membrane helical-coil heat exchanger and membrane serpentine-tube heat exchanger under different operating pressures, inlet velocities and pitches are investigated numerically. The three-dimensional governing equations for mass, momentum and heat transfer are solved using a control volume finite difference method. The realizable k-ε model is adopted to simulate the turbulent flow and heat transfer in heat exchangers. There flows syngas in the channels consisting of the membrane helical coils or membrane serpentine tubes, where the operating pressure varies from 0.5 to 3.0 MPa. The numerically obtained heat transfer coefficients for heat exchangers are in good agreement with experimental values. The results show that the syngas tangential flow in the channel consisting of membrane helical coils is significant to the heat transfer enhancement to lead to the higher average heat transfer coefficient of membrane helical-coil heat exchanger compared to membrane serpentine-tube heat exchanger. The syngas tangential velocity in the membrane helical-coil heat exchanger increases along the axial direction, and it is independent of the gas pressure, increasing with the axial velocity and axial pitch rise and decreasing with the radial pitch rise.     

Boiling heat transfer and critical heat flux in liquid films falling on vertically-mounted heat sources

Boiling heat transfer measurements were obtained for an inert fluorocarbon (FC-72) liquid film injected over a vertical heated wall. Flow visualization revealed that vigorous boiling prior to burnout ruptured the liquid continuum, causing most of the film to separate from the heated wall leaving a thin liquid subfilm which maintained contact with the wall. The critical heat flux (CHF) was accompanied by dryout of the subfilm after total separation of the liquid near the upstream edge of the heater. A higher CHF was achieved by increasing the film velocity or by utilizing a shorter heater. Experimental data correlated favorably with predictions of a CHF model based on the Helmholtz instability and subfilm dryout.     

石墨烯-PFA复合材料换热器与金属换热器的传热性能对比

氟塑料换热器以其耐腐蚀、耐磨损等优点而备受关注,但氟塑料热导率较低,换热能力差,限制了其广泛应用。石墨烯-PFA复合材料兼具石墨烯优异的导热性和可熔性聚四氟乙烯(PFA)良好的耐酸碱腐蚀性,是新一代的换热器材料。搭建了余热回收测试实验台,对石墨烯-PFA复合材料换热器和金属换热器的传热性能进行对比。研究了不同烟气流速、不同进口烟气温度以及不同石墨烯配比对复合材料传热性能的影响。结果表明:对于金属换热器和复合材料换热器,当烟气流速从2.0增加到4.0 m/s时,传热系数分别增加到原来的1.19和1.34倍;随着进口烟温的升高,两种材质的传热系数分别降低了15.6%和14.7%;随着石墨烯含量增加,复合材料的导热系数以及传热系数均增加。     

“制冷、制热、卫生热水”型空气源热泵系统及分析

提出一种小型中央空调用“制冷、制热、卫生热水”型空气源热泵系统设计方案,能够利用空调部分冷凝热提供生活用卫生热水,该系统可与家用中央热水系统连接。     

Temperature Distribution and Heat Saturating Time of Regenerative Heat Transfer

In this paper, heat transfer of the ceramic honeycomb regenerator was numerically simulated based on the computational fluid dynamics numerical analysis software CFX5. The longitudinal temperature distribution of regenerator and gas were obtained. The variation of temperature with time was discussed. In addition, the effects of some parameters such as switching time, gas temperature at the inlet of regenerator, height of regenerator and specific heat of the regenerative materials on heat saturating time were discussed. It provided primarily theoretic basis for further study of regenerative heat transfer mechanism.     

Comparison of heat storage systems employing sensible and latent heat

This study makes an evaluation of the performances of heat storage systems destined for power plants with a discontinuous power source such as nuclear fusion. Two classes of heat storage systems are investigated: heat storage systems based on first sensible heat storage and second latent heat storage. Both classes of heat storage systems are evaluated both from a thermodynamic point of view, inquiring whether the irreversibility of the system stays limited or not, and from an economic point of view, examining whether the system makes proper use of the heat storage capacity present. It is shown that an unambiguous conclusion that one is superior to the other is not possible and that the operating conditions and the configuration of the phase‐changing materials play an important role. Copyright © 1999 John Wiley & Sons, Ltd.     

Fluid flow and heat transfer characteristics of cross-cut heat sinks

In this study, effects of cross-cuts on the thermal performance of heat sinks under the parallel flow condition are experimentally studied. To find effects of the length, position, and number of cross-cuts, heat sinks with one or several cross-cuts ranging from 0.5 mm to 10 mm were fabricated. The pressure drop and the thermal resistance of the heat sinks are obtained in the range of 0.01 W<P_p < 1 W. Experimental results show that among the many cross-cut design parameters, the cross-cut length has the most significant influence on the thermal performance of heat sinks. The results also show that heat sinks with a cross-cut are superior to heat sinks containing several cross-cuts in the thermal performance. Based on experimental results, the friction factor and Nusselt number correlations for heat sinks with a cross-cut are suggested. Using the proposed correlations, thermal performances of cross-cut heat sinks are compared to those of optimized plate-fin and square pin-fin heat sinks under the constant pumping power condition. This comparison yields a contour map that suggests an optimum type of heat sink under the constraint of the fixed pumping power and fixed heat sink volume. The contour map shows that an optimized cross-cut heat sink outperforms optimized plate-fin and square pin-fin heat sinks when 0.04 < log L1 < 1.