新型吸附床的研究进展

吸附床是吸附式制冷装置的核心部件,其性能的优劣直接关系到吸附式制冷装置的制冷效果.通过吸附床的热阻分析,得出强化吸附床传热的措施.文中重点介绍了提高吸附剂与换热壁面的换热系数和吸附剂间的导热系数来强化吸附床传热的方法,详细介绍了填充式吸附床、涂抹式吸附床以及固化式吸附床的结构特点,比较分析3种吸附床的传热传质特点,并阐...     

吸附器中导热胶热传导系数的研究

本文在研究吸附式热泵吸附器传热性能的特性基础上，发现导热胶能有效降低吸附剂与吸附换热器表面间的接触热阻，阻后研制并测试了导热胶对颗粒吸附剂与吸附换热器表面之间以及成型吸附剂与吸附换热器表面之间接触热阻的影响。实验结果表明，导热胶是降低吸附剂与金属表面之间接触热阻的一种有效方法，可以提高吸附剂与金属表面之间的传热性能，而对吸附剂的吸附性能影响不大，为强化两种不同介质之间的传热提供了新的技术途径。     

硅胶粒径对太阳能吸附制冷系统性能的影响

针对以硅胶-水为工质对的太阳能吸附式制冷系统,实验研究了不同粒径的硅胶材料对吸附床传热传质特性及系统制冷能力的影响。三组对比实验的硅胶材料平均粒径分别为1mm、3mm和5mm,在太阳辐射接近的条件下完成实验测试,结果表明粒径中等的硅胶材料表现最优,其制冷系数COP和按照循环周期定义的比制冷功率SCP2均最高。小粒径材料虽然使吸附剂填充量有所增加,但是会导致吸附床轴向传质阻力增加,影响其末端吸附能力的发挥。而材料的粒径过大,则会降低吸附床的吸附剂填充量及其传热性能,从而导致预热脱附和冷却过程的时间延长,不利于系统的制冷性能改善。实验结果表明,吸附剂粒径是影响太阳能吸附式制冷系统工作性能的一个重要因素,在系统设计中需要给予重视。     

Optimization of the cycle durations of adsorption heat pumps employing zeolite coatings synthesized on metal supports

In order to remove the limitations originating from inefficient heat and mass transfer in adsorption heat pumps an arrangement involving zeolite 4A coatings synthesized on stainless steel heat exchanger tubes was recently proposed and a related mathematical model was presented. In this study, the same mathematical model is employed to optimize the cycle durations in order to obtain the maximum amount of power from the adsorption heat pumps. Accordingly, the temperature and concentration profiles across the adsorbent layer are computed for distinct cases employing various cycle durations. The optimum cycle durations that maximize the energy obtained during a specific period of time for a given adsorber volume, corresponding to the employment of various zeolite layer thicknesses as well as two distinct wall thicknesses of the heat exchanger tubes are determined. The ratios between the optimum cycle durations and those obtained previously by allowing the operation of a cycle to be completed only when the temperature difference between the heat exchange fluid and the surface temperature of the adsorbent layer decreases to 1°C, remain around 0.2–0.4 for the cases investigated. The employment of the optimum cycle durations results in an increase of almost twofold in the power obtained from the adsorption heat pumps while the optimum zeolite layer thickness value obtained in this case is observed to increase by about 50% and is found to be in the range 75–150 μm depending on the wall thickness of the heat exchanger tubes utilized in this study.     

高温气体与过冷液直接接触凝结制冷循环的性能分析

提出制冷压缩机排出的高温高压制冷剂气体与制冷剂过冷液体直接接触凝结换热的新型制冷循环,结合自然工质氨的热力特性,分析直接接触凝结制冷循环的热力性能,并与常规双级压缩和单级压缩制冷循环的性能进行对比,得出：随着主循环饱和液温度的升高,直接接触凝结制冷循环的性能系数先增大后减小存在最大值,冷凝器散热量先减小后增大存在最小值,流过蒸发器的制冷剂质量流量逐渐增大。在相同蒸发温度和冷凝温度下,当过冷液体的过冷度为20℃时,较常规双级压缩制冷循环,直接接触凝结制冷循环的性能系数提高4.92%,冷凝器散热量减少6.65%,蒸发器的制冷剂质量流量减少7.2%～7.9%;当过冷液体的过冷度为5℃时,较常规单级压缩制冷循环,直接接触凝结制冷循环的性能系数提高6.52%,冷凝器散热量减少3.32%,蒸发器的制冷剂质量流量减少8.58%～8.91%。结果表明氨直接接触凝结制冷循环较常规制冷循环具有明显的优势。     

吸附式制冷新技术

固体吸附式制冷技术是充分利用低品位热能的一种有效手段 ,本文结合国际上吸附制冷研究的最新动态以及作者实验研究成果 ,对吸附式制冷系统及其在能量综合利用中的一些新技术进行了总结及探讨 .     

π型向心径向流吸附器变质量流动特性研究

对径向流吸附器内变压吸附(PSA)制氧的变质量流动规律进行研究,有助于准确掌握吸附过程及床层内的变量因素对制氧性能的影响。对π型向心径向流吸附器建立气固耦合的两相吸附模型,并对其PSA制氧过程进行了数值模拟研究,得到了床层内氧气浓度分布、温度分布以及产品气浓度的变化规律。结果表明：首次循环结束时床层内氧气最高摩尔分数可达66.02%,回收率29.2%。非稳定循环期间,氧气摩尔分数从66.02%升高至 97.5%,回收率从29.2%提高至38.5%。循环达到稳定后,床层内氧气摩尔分数最高可达98.6%,回收率38.9%左右,且达到稳定状态后床层内气固两相温差减小,逐渐达到热平衡。获得了吸附器内部气体与吸附剂两相间的传质、传热过程,为π型向心径向流吸附器用于PSA制氧提供技术支持。     

A method to improve the barrier performance of flexible riser internal pressure sheath—Heat transfer experiment and simulation

A considerable amount of acidic gas penetrates into the annulus between the internal pressure sheath and the outer protective sheath during the service of flexible risers, which will inevitably lead to corrosion of the metal functional layer. Many researchers have modified nanocomposites from the mass transfer perspective to reduce the materials' permeability coefficient. While permeation is an integrated process of heat and mass transfer process, heat distribution directly impacts gas permeation. Herein, the thermal conductivity of flexible riser liner materials is predicted for the first time by a combination of molecular dynamics and experiments, and then the radial temperature distribution under different seawater temperatures and internal fluid temperatures is investigated by finite element analysis. Finally, the effect of temperature distribution on the permeation coefficient was analyzed. The results demonstrate that the thermal conductivity can be predicted by molecular dynamics simulation, and the thermal conductivity of (polyvinylidene difluoride) PVDF/TiO₂ decreases, while the thermal conductivity of PVDF/carbon nanotube (CNT) increases compared with pure PVDF. The temperature distribution of the internal pressure sheath material decreases when condensate water is present. As the fluid temperature rises from 30 to 110°C, the maximum increase ratio in the permeability of PVDF/CNT over PVDF increased from 3.6% to 14.8%, and the maximum decrease ratio of PVDF/TiO₂ permeability coefficient compared with PVDF is from 1.2% to 4.6%. The results present a new idea to improve the barrier properties of materials by decreasing thermal conductivity.     

化学聚合法强化吸附剂热传导

采用化学聚合法强化吸附剂——沸石颗粒的热传导性能,利用苯胺单体在沸石颗粒表面氧化聚合,使少量的聚苯胺形成均匀连续的导热性能好的高分子材料网,在实验条件下可使该种吸附剂的有效导热系数提高到原来的4倍多。同时,将强化后的吸附剂挤压成型,进行强化吸附床层的传热研究,结果表明堆积密度为原来的1.5倍的成型块,其有效导热系数比原来提高30％,对比混合法的实验和数据分析表明,化学聚合法较物理混合法效果更明显,是一种适宜吸附剂热传导强化的新途径。     

连续回热型吸附式空调/热泵循环特性与动态性能(Ⅱ)结果与讨论

根据非平衡吸附条件下系统的动态方程计算了系统各参数的动态变化规律 ,并与实验值进行了对比 ,验证了理论模型的正确性 ,同时通过计算确定了吸附床传热系数、吸附床滞留传热介质量、吸附床加热与冷却介质热容、热源温度、冷却水温度、循环周期等对连续回热型吸附式空调 /热泵系统运行 p -t-x图的影响以及它们对系统运行SCP与COP的影响 ,分析了产生这些影响的机理 ,为该类机型的优化设计与运行奠定了基础     

一种回热回质循环吸附式制冷系统的仿真

吸附式制冷常采用回热回质循环来提升系统性能。研究了一种采用串联回热和类回质方式的回热回质循环吸附式制冷系统,并对其进行仿真。系统的主要部件（含作为储液器的蒸发器）采用3层换热法建立数学模型。仿真结果表明,随着制冷时间的延长,系统性能系数（COP）单调增大,单位质量制冷量（SCP）单调减小。随着回热时间的延长,COP和SCP是先增大后减小,最佳的回热时间为10 s。随着回质时间的延长,COP和SCP波动性下降,回质过程未提高系统性能。COP和SCP随着热水、冷冻水温度的升高以及冷却水温度的下降而增大。热水温度对SCP以及冷冻水、冷却水温度对COP和SCP的影响,呈现线性变化,而热水温度对COP的影响呈现二次变化。     

采用回质回热的活性炭-氨吸附制冷循环性能

回质是改善吸附循环性能的重要手段 ,回质过程大幅度提高了每循环单位质量吸附剂的制冷量 ;根据工况的不同 ,回质过程有可能在一定范围内提高或降低系统的性能系数 ;回质过程对循环的作用对金属及流体热容变化不敏感 ;回质回热复合循环具有较高的性能系数     

Wall-to-bed heat transfer in liquid—solid and gas—liquid—solid fluidized beds part I: Liquid—solid fluidized beds

Experimental work was conducted to investigate the effect of particle size and particle density upon the wall-to-bed heat transfer characteristics in liquid—solid fluidized beds with a 95.6 mm column diameter over a wide range of operating conditions. The radial temperature profile was found to be parabolic, indicating the presence of a considerable bed resistance. The effective radial thermal conductivity and the apparent wall film coefficient were obtained on the basis of a series thermal resistance model. The modified Peclet number of the radial thermal conductivity decreases upon the onset of fluidization, has a minimum at a bed porosity of 0.6 to 0.7 and increases with further increase of bed porosity. The modified Peclet number decreases considerably with decreasing particle size or increasing particle density. The apparent wall heat transfer coefficient can be represented well by a Colburn j-factor correlation over a wide range of data as follows: j′_H = 0.137 Re′^?0.271 A close analogy is found to exist between the modified j-factor for wall heat transfer coefficient and that for wall mass transfer coefficient, in liquid—solid fluidized beds.     

热泵吸附器中传热传质过程的研究

对吸附式热泵循环系统中的传热传质进行了理论分析和实验研究,建立了吸附器中热传导方程.计算求解值与实验结果相一致,说明了吸附器中的传热速率控制了热泵循环速度.据此,对吸附器中强化传热肋片进行了模拟分析,作为吸附器优化设计的理论依据.     

R1234ze(E)在泡沫金属管内的流动沸腾换热和压降特性

泡沫金属具有超大比表面积和高热导率,将其填充于换热管内可用于制冷空调系统的强化传热。研究了R1234ze(E) 在泡沫金属管内的流动沸腾换热和压降特性。实验工况为：干度0.1~0.9,质流密度90~180 kg·m^-2?s^-1,热通量12.4~18.6 kW·m^-2。测试样件为泡沫铜填充管,孔密度为10~40 PPI、孔隙率为90%~95%。实验结果表明,R1234ze(E) 比R410A的传热系数低2%~10%,两相压降低30%~42%;当干度大于0.8时,低质流密度下泡沫金属管内传热系数随干度的增加增幅更大;泡沫金属在强化流动沸腾换热的同时,造成压降显著增加,换热影响因子的范围为1.23~2.90,压降影响因子的范围为6~45。开发了适用于R1234ze(E) 的泡沫金属管内流动沸腾换热和压降关联式,传热系数和两相压降的预测值与95%的实验值误差分别在±15%和±25%以内。     

一种高效吸附床的设计及二维传热数值模拟分析

设计了一种特殊的高效换热的板翅式吸附床,用于硅胶-水连续循环的吸附式制冷系统中。建立了吸附床的二维数学模型。因硅胶-水吸附式制冷系统具有很快的吸附和解吸过程,热量的及时传递成为极其关键的因素。因此在该模型中,只考虑了快速加热和冷却的传热过程,以寻求提高传热速度的途径。对于吸附床的各关键尺寸对换热性能的影响进行了对比和分析,并对实验结果也进行了对比分析。理论研究表明,吸附床的各参数的最优尺寸为：翅片间距为3～6 mm,翅片高度为6～12 mm,翅片厚度为0.03～0.15 mm。实验结果表明,使用这种吸附床结构,系统的循环时间可短至420 s。在硅胶的热导率仅为0.175 W·(m²·℃)^-1的情况下,吸附床的总传热系数可提高至相似文献   

直管式直流蒸汽发生器不同运行参数下两相流压降预测

将B&W公司的直流蒸汽发生器进行简化,采用常热流边界条件进行不同运行参数下直流蒸汽发生器二次侧流动与换热过程数值模拟,并与经典摩擦压降经验关联式进行对比。结果表明：Martinelli-Nelson关联式更适用于预测蒸干发生时两相流的摩擦压降;摩擦压降随质量含汽率增加整体呈现上升趋势,蒸干发生时摩擦压降的变化率明显增大;管内气液两相流摩擦压降随质量流量和热通量增加而增大,随运行压力增大而减小。其中质量流量、运行压力对摩擦压降的影响较明显,热通量对其影响较小。     

周期性边界条件下埋地热油管道传热影响因素的分析

建立土壤多孔介质模型,采用有限容积法对地表温度周期性波动条件下埋地热油管道非稳态传热过程进行数值计算。考虑了土壤中水相、气相迁移对管道传热的影响,对比分析了有、无保温层及保温层厚度、保温层导热系数、土壤导热系数、土壤含水率、管径、埋深等因素对埋地管道非稳态传热规律的影响。研究表明：保温层厚度、导热系数、土壤导热系数对埋地热油管道非稳态传热的影响相对较大。管径、埋深对管道传热的影响相对次之,且埋深对管道的影响冬季远要大于夏季,而土壤含水率对管道传热的影响相对较小。     

R744直接接触冷凝制冷循环性能分析

对R717循环辅助过冷、R744主循环制冷压缩机排出的气体与R744过冷液直接接触冷凝的R717/R744-DCC制冷循环的热力性能进行分析,得出：R717/R744-DCC直接接触冷凝制冷循环存在最佳的R744主循环冷凝温度,并获得最优的性能系数和最低的R717冷凝器散热量。R744主循环过冷液体的过冷度增大,最优的性能系数降低,最低R717冷凝器散热量增大,对应的R744主循环冷凝温度升高,R744蒸发器的质量流量减少。与常规R717/R744复叠式制冷循环的热力性能比较,在相同的运行工况和最佳R744主循环冷凝温度下,R717/R744-DCC直接接触冷凝制冷循环最优性能系数提高了5.2%,最低R717冷凝器散热量减少了1.6%。R744主循环冷凝温度在-10～8℃范围内,R717/R744-DCC直接接触冷凝制冷循环R744蒸发器的制冷剂质量流量减少了1.75%～2.61%,R717冷凝器的制冷剂流量减少了0.51%～0.82%。     

Boundary layer flow past a stretching/shrinking surface beneath an external uniform shear flow with a convective surface boundary condition in a nanofluid

The problem of a steady boundary layer shear flow over a stretching/shrinking sheet in a nanofluid is studied numerically. The governing partial differential equations are transformed into ordinary differential equations using a similarity transformation, before being solved numerically by a Runge-Kutta-Fehlberg method with shooting technique. Two types of nanofluids, namely, Cu-water and Ag-water are used. The effects of nanoparticle volume fraction, the type of nanoparticles, the convective parameter, and the thermal conductivity on the heat transfer characteristics are discussed. It is found that the heat transfer rate at the surface increases with increasing nanoparticle volume fraction while it decreases with the convective parameter. Moreover, the heat transfer rate at the surface of Cu-water nanofluid is higher than that at the surface of Ag-water nanofluid even though the thermal conductivity of Ag is higher than that of Cu.