D型集流管式微通道气冷器流量分配性能数值模拟

提出了一种D型集流管式微通道气冷器,采用CFD方法对其进行模拟研究,并比较分析了D型集流管、双圆筒集流管以及单圆筒集流管对微通道气冷器的流量分配性能的影响,同时还研究了D型集流管当量直径、CO2进口质量流量、CO2进口压力等参数对微通道气冷器流量分配性能的影响。设计制作了微通道冷模实验装置,对每排微通道内的体积流量进行测量,将实验测量结果与数值模拟计算结果进行对比。结果表明,实验测量结果与数值模拟计算结果变化趋势基本一致,两者相对误差最大为10.1%,由此验证了数值模拟计算的可靠性。     

集流管结构对微通道蒸发器流量分配均匀性影响

微通道蒸发器内制冷剂流量分配均匀性对换热器性能有着较大影响。本文在理想工况运行情况前提下,以水为流动工质,数值模拟了微通道蒸发器内流量分配特性,探讨了4种集流管形式(A型、B型、C型、D型)、不同入口速度(0.08 m/s~0.42 m/s)对换热器各扁管流量分配的影响。研究结果表明,集流管入口流速对换热器内各扁管流量分配具有较大影响,当速度从0.08 m/s增大到0.42 m/s时,换热器内流量分布从两侧高,中间低的分布转变为入口侧低,出口侧高的分布特性,且流量分配不均匀度随流速增加而显著增大;通过改变集流管结构能够在一定程度上改善流量分配特性。各扁管进口静压力分布与各扁管内流量分配具有相关性,可通过改进集流管结构保证静压力分布一致,使各扁管流量分配均匀,从而获得较好的换热性能。     

水冷套管式CO_2气冷器的设计及实验研究

本文设计了一台CO_2套管式气冷器并对其进行了换热特性的实验研究。该气冷器采用逆流三重套管,CO_2在内管流动,冷却水在内外管间流动。实验研究了不同CO_2质量流量、入口压力和冷却水温度对传热系数、换热量和换热器效能系数的影响。实验结果表明,随着CO_2质量流量的增加,传热系数和换热量均呈先增后减的趋势,换热器效能系数逐渐减小;CO_2质量流量不变时,传热系数、换热量和换热器效能系数均随气冷器CO_2入口压力的升高而逐渐增大;随着冷却水温度的升高,传热系数、换热量和换热器效能系数均逐渐减小。     

CO2热泵系统气冷器的热力学性能分析

针对CO₂热泵系统螺旋套管式气冷器,基于MATLAB建立了仿真模型,采用单因素分析方法,研究进水温度、CO₂压力和质量流量对气冷器换热量、■耗散、■损失、■效率以及出水温度的影响。经实验验证,在进水温度为24.5～35.0℃、CO₂压力为8.4～10.7 MPa、CO₂质量流量为0.032 6～0.047 6 kg/s工况下,气冷器模型制热量与实验数据相比总体误差在±10%以内。模拟结果表明：相比进水温度和CO₂质量流量,CO₂压力对气冷器性能的影响更为显著,且存在最优压力。在进水温度为20℃、CO₂进口温度为90℃工况下,当CO₂压力为10 MPa时气冷器■效率最高,当CO₂压力为11 MPa时气冷器换热量最大;当进水温度低于20℃时,CO₂压力为10.5 MPa时出水温度最高。     

小型CO_2制冷系统用气冷器的仿真研究

用有限单元法建立了气冷器稳态分布参数模型,并通过实验对模型的准确性进行了验证。运用该模型对小型CO_2制冷系统用气冷器的传热性能、系统内的假临界现象及平均密度进行了研究,结果表明:在质量流量相同时,进口温度的变化对气冷器换热的影响很小,对气冷器管内假临界状态的出现位置影响很小;在气冷器进口温度相同时,质量流量越大,假(准)临界点出现的位置越向后移,当质量流量足够大时,气冷器内可能不会有假临界现象发生;气冷器进、出口处CO_2的算术平均密度值与气冷器内真实平均密度值相近,而对数平均密度和平方平均密度与气冷器内CO_2的真实平均密度值相差较大。     

翅片管式CO2气体冷却器模拟与优化

为了研究CO2在翅片管式气体冷却器内的流动特性,建立了稳态分布参数模型,并进行了实验验证。结果表明:CO2侧换热系数受入口压力和质量流量的影响较大,但入口温度对其影响很小。换热量随着入口压力的变化有一个最大值;且随着流量的增大,最大换热量所对应的入口压力值逐渐增大。压降和换热量均随入口温度的增加而线性增加。适当增加管程数,采用较小管径的气冷器性能更高。     

跨临界CO_2气冷器耗散分析

跨临界CO_2系统已成为热泵及空调领域的研究热点,本文以CO_2气冷器为研究对象,管内外两种流体因温差传热与流动阻力引起系统(火积)耗散,通过建立的CO_2气冷器跨临界区二维分布参数模型求解系统(火积)耗散数ΔE*。分析系统(火积)耗散数产生的主要原因及沿程分布,讨论CO_2、水入口状态参数对系统(火积)耗散数的影响。结果表明系统(火积)耗散数主要由温差传热引起,温差越大,系统(火积)耗散数越大。各微元段(火积)耗散数与CO_2温降幅度呈反比关系,在临界点ΔEj*达到最大值。随着CO_2质量流率、压力的增大,系统(火积)耗散数逐渐增大;随着水质量流量的增大,系统(火积)耗散数逐渐减小,减小幅度随着压力的增大而减小。系统(火积)耗散数随着CO_2入口温度的增大而减小,CO_2入口温度越大,减小幅度逐渐降低。水入口温度对系统(火积)耗散数的影响非常小。(火积)     

微通道散热器新型结构设计及流动与散热分析

为了提高微通道散热器的散热性能,采用Fluent对散热器内部流场流动和换热特性进行了数值仿真分析。同时通过调整通道和翅片截面的宽度,添加肋柱与孔洞,设计了一种新型微通道散热器结构。结果表明:在不同入口体积流量(0.25—1.5 L/min)下,新型微通道散热器结构的各通道内质量流量的极差为初始结构的1/8,流体不均匀分配因子为初始结构的9%—56%,基底最大温度降低6.4—8.8℃,基底平均温度降低7.1—10.1℃,高换热量区内换热量的极差为初始结构的17%—24%,平均Nu数是初始结构的2.1—2.4倍。Rth值较初始结构减少了7%—26%。说明该设计结构下的内部流场分配均匀且换热型性能优越。     

气体导流装置对微通道蒸发器性能的影响

针对微通道蒸发器制冷剂流量分配不均匀造成的换热性能恶化和干蒸现象,本文搭建了双流程微通道蒸发器性能测试实验台,研究导气装置对蒸发器换热性能及扁管中制冷剂分配均匀性的影响,并与常规的双流程微通道蒸发器进行对比。结果表明：由于入口制冷剂流量不变,液相制冷剂蒸发为气体的最大相变潜热不变,导致二者换热量和传热系数差值较小,最大值仅相差0.5%和6.9%。但加导气装置后流动阻力降低,两相段长度较常规结构增幅为87.3%,过热度显著降低,风速为3 m/s时两种结构的过热度降幅为44.4%。各扁管间制冷剂分布趋于一致,均匀性得到提升,干蒸现象得到缓解。     

基于分布参数模型的微通道平行流环路热管换热性能研究

基于分布参数模型建立了微通道平行流环路热管的稳态传热模型,并通过实验验证模型可行性,最大相对误差为10.5%。模拟研究分析了充液率、蒸发器和冷凝器高度差等因素对环路热管传热性能的影响。结果表明：环路热管的最佳充液率为80%~105.4%,换热量为1.27~1.36 kW;蒸发器和冷凝器的高度差从0.4 m增至1.0 m时,换热量约提升8.7%。同时,模型能够预测蒸发器流量分配的不均匀性及扁管内部的两相状态,使模拟结果更加精确,对微通道平行流环路热管的结构设计具有一定的参考价值。     

不同空气侧风速下微通道蒸发器换热特性实验研究

搭建微通道蒸发器性能实验台,采用控制变量法研究不同空气侧风速下微通道蒸发器表面温度分布、制冷剂进出口压力的变化规律,计算换热量和换热系数,从而分析空气侧风速对微通道蒸发器的流量分配特性和换热效果的影响。结果表明,随着风速增大,微通道蒸发器制冷剂流量分配不均匀性增大,进出口压力波动振幅和周期增加,压降增大,风速2 m/s时微通道蒸发器换热效果最佳。     

套管结构对翅片管气化器结霜特性的影响

结霜会对不同类型气化器造成长效性能的衰减.为研究套管结构对强化传热与表面结霜效果的影响,本文搭建了实验台,在普通传热管内加装套管,以液氮为工质,根据不同套管内径(Φ6、Φ8、Φ10 mm)及入口流量,设计了12组实验工况进行研究,获得翅片管表面不同测点处温度、霜层厚度及翅片管出口流体温度等参数.结果表明:入口流量为2....     

基于Al2O3纳米流体的槽式太阳能热发电集热器传热建模及性能分析

基于计算流体动力学中的有限体积法,研究了Al₂O₃/Syltherm800导热油纳米流体作为传热介质时槽式太阳能热发电集热器的性能,建立了真空管集热器的三维模型,进行了光学模拟和传热数值模拟,并通过实验进行了验证。在非均匀热流密度分布的情况下,研究了进口温度、进口流速等运行参数对采用纳米流体的槽式集热器传热性能的影响规律。结果表明:随着Al₂O₃体积分数的增加,槽式集热器的换热性能及热效率均有所提高;进口温度、进口流速等运行参数对集热器的传热性能影响很大,随着进口温度的上升和进口流速的减小,纳米流体对传热性能的影响程度逐渐增大。     

Effects of tube flattening on the fluid dynamic and heat transfer performance of nanofluids

In the present article, numerical simulation of Al₂O₃–water nanofluid flow in different flat tubes are performed to investigate the effects of tube flattening on the fluid dynamic and heat transfer performance of nanofluids. The numerical simulations of nanofluids are performed using two phase mixture model by FORTRAN programming language. The flow regime and the wall boundary conditions are assumed to be laminar and constant heat flux respectively. The simulated results are compared with previously published data and good agreement is observed. The effects of tube flattening on different parameters such as heat transfer coefficient, wall shear stress, nanoparticles distribution, temperature distribution, secondary flow and velocity profiles are presented and discussed. The results show that with increasing the flattening, the heat transfer coefficient and wall shear stress increase. The rate of increasing is soft for all flat tubes except for the tube with the most flattening which has a severe increasing in heat transfer and wall shear stress values.     

Effect of inlet configuration on the refrigerant distribution in a parallel flow minichannel heat exchanger

The refrigerant R-134a flow distribution was experimentally studied for a round header/ten flat tube test section simulating a brazed aluminum heat exchanger. Three different inlet configurations (parallel, normal, vertical) were investigated. Tests were conducted with downward flow for mass flux from 70 to 130 kg m⁻² s⁻¹ and quality from 0.2 to 0.6. Tubes were flush-mounted in the test section, with no protrusion into the header. It is shown that normal and vertical inlet yielded similar flow distribution. As mass flux or quality increased, however, better results were obtained for normal inlet configuration. The flow distribution was worst for the parallel inlet configuration. Possible explanation is provided based on flow visualization results. Correlations were developed to predict the fraction of liquid or gas taken off by downstream channel as a function of header gas Reynolds number at immediate upstream.     

跨临界CO2气冷器火积耗散分析

跨临界CO2系统已成为热泵及空调领域的研究热点,本文以CO2气冷器为研究对象,管内外两种流体因温差传热与流动阻力引起系统火积耗散,通过建立的CO2气冷器跨临界区二维分布参数模型求解系统火积耗散数 。分析系统火积耗散数产生的主要原因及沿程分布,讨论CO2、水入口状态参数对系统火积耗散数的影响。结果表明系统火积耗散数主要由温差传热引起,温差越大,系统火积耗散数越大。各微元段火积耗散数与CO2温降幅度呈反比关系,在临界点 达到最大值。随着CO2质量流率、压力的增大,系统火积耗散数逐渐增大;随着水质量流量的增大,系统火积耗散数逐渐减小,减小幅度随着压力的增大而减小。系统火积耗散数随着CO2入口温度的增大而减小,CO2入口温度越大,减小幅度逐渐降低。水入口温度对系统火积耗散数的影响非常小。     

Comparative Investigation on the Heat Transfer Characteristics of Gaseous $$\hbox {CO}_{2}$$ and Gaseous Water Flowing Through a Single Granite Fracture

\(\hbox {CO}_{2}\) and water are two commonly employed heat transmission fluids in several fields. Their temperature and pressure determine their phase states, thus affecting the heat transfer performance of the water/\(\hbox {CO}_{2}\). The heat transfer characteristics of gaseous \(\hbox {CO}_{2}\) and gaseous water flowing through fractured hot dry rock still need a great deal of investigation, in order to understand and evaluate the heat extraction in enhanced geothermal systems. In this work, we develop a 2D numerical model to compare the heat transfer performance of gaseous \(\hbox {CO}_{2}\) and gaseous water flowing through a single fracture aperture of 0.2 mm in a \(\upphi 50\,\times 50\hbox { mm}\) cylindrical granite sample with a confining temperature of \(200\,^{\circ }\hbox {C}\) under different inlet mass flow rates. Our results indicate that: (1) the final outlet temperatures of the fluid are very close to the outer surface temperature under low inlet mass flow rate, regardless of the sample length. (2) Both the temperature of the fluid (gaseous \(\hbox {CO}_{2}\)/gaseous water) and inner surface temperature rise sharply at the inlet, and the inner surface temperature is always higher than the fluid temperature. However, their temperature difference becomes increasingly small. (3) Both the overall heat transfer coefficient (OHTC) and local heat transfer coefficient (LHTC) of gaseous \(\hbox {CO}_{2}\) and gaseous water increase with increasing inlet mass flow rates. (4) Both the OHTC and LHTC of gaseous \(\hbox {CO}_{2}\) are lower than those of gaseous water under the same conditions; therefore, the heat mining performance of gaseous water is superior to gaseous \(\hbox {CO}_{2}\) under high temperature and low pressure.     

Numerical investigation of supercritical LNG convective heat transfer in a horizontal serpentine tube

The submerged combustion vaporizer (SCV) is indispensable general equipment for liquefied natural gas (LNG) receiving terminals. In this paper, numerical simulation was conducted to get insight into the flow and heat transfer characteristics of supercritical LNG on the tube-side of SCV. The SST model with enhanced wall treatment method was utilized to handle the coupled wall-to-LNG heat transfer. The thermal–physical properties of LNG under supercritical pressure were used for this study. After the validation of model and method, the effects of mass flux, outer wall temperature and inlet pressure on the heat transfer behaviors were discussed in detail. Then the non-uniformity heat transfer mechanism of supercritical LNG and effect of natural convection due to buoyancy change in the tube was discussed based on the numerical results. Moreover, different flow and heat transfer characteristics inside the bend tube sections were also analyzed. The obtained numerical results showed that the local surface heat transfer coefficient attained its peak value when the bulk LNG temperature approached the so-called pseudo-critical temperature. Higher mass flux could eliminate the heat transfer deteriorations due to the increase of turbulent diffusion. An increase of outer wall temperature had a significant influence on diminishing heat transfer ability of LNG. The maximum surface heat transfer coefficient strongly depended on inlet pressure. Bend tube sections could enhance the heat transfer due to secondary flow phenomenon. Furthermore, based on the current simulation results, a new dimensionless, semi-theoretical empirical correlation was developed for supercritical LNG convective heat transfer in a horizontal serpentine tube. The paper provided the mechanism of heat transfer for the design of high-efficiency SCV.