矩形翅片椭圆管束性能研究及场协同分析

应用计算流体动力学（CFD）理论,建立了矩形翅片椭圆管的数学模型,阐述求解的数值计算方法。采用k—ε标准湍流模型、有限体积法（FVM）和SIMPLEC算法,运用FLUENT软件对管外扰流场进行三维数值仿真,得到其阻力特性与换热特性,仿真结果与实验结果相对比,验证了模型的正确性。在此基础上,采用场协同原理分析了温度梯度与速度矢量的夹角对矩形翅片椭圆管换热特性的影响。结果表明：对流换热的强度不仅取决于流体的速度和物性,还取决于速度与温度梯度之间的协同,尽管迎面风速增大,换热得到强化,但随着协同角的增大,换热效率变低,以致换热量非线性增加。     

空冷凝汽器蛇形翅片扁平管空气侧流动与换热特性研究

针对应用于电站直接空冷凝汽器的蛇形翅片扁平单排管,对其空气侧的流动及换热特性进行了数值模拟。给出了该管件基本结构的雷诺数和欧拉数随迎面风速的变化关联式;结果显示,同一迎面风速下,随着翅片间距、高度的增大,换热变差,流动变好。但当间距增加到一定程度时,风速与间距对换热流动性能的影响都变得很小。风速越大,换热性能对翅片间距、高度越敏感,而流动则相反,需根据实际工况来选择合适的翅片间距及高度。     

直接空冷系统椭圆翅片管管束传热与流动性能的数值分析

直接空冷系统的空冷元件中,大口径热浸锌椭圆钢管绕椭圆翅片管因其具有空气侧流动阻力较小、换热系数较高、使用寿命长、清洗效果好、技术成熟等优点,在西北地区直接空冷机组中广泛应用。本研究采用CFD技术对三排管直接空冷凝汽器空气侧流动及换热性能进行细致分析,讨论了不同迎面风速下的三排管空冷翅片间的压力、速度和温度分布特性。结果表明,随着迎面风速的增加使换热系数增大,阻力增大;后排管的换热较前排管弱。本文模拟的结果可为空冷凝汽器的优化设计提供参考依据。     

平行流冷凝器百叶窗翅片结构优化

建立平行流冷凝器百叶窗翅片的三维流动和换热模型,并对不同迎面风速下百叶窗翅片的流动和换热性能进行数值模拟,计算结果与试验结果具有较好的一致性,验证计算模型和方法的正确性。在此基础上,建立8种结构形式的百叶窗翅片模型,并对其空气侧的换热和流动过程进行仿真,分析结构形式和排列方式对百叶窗翅片换热量和空气阻力的影响,结果表明:增加开窗区域数能够有效提高翅片的换热性能,增加百叶窗的转向区数目能够有效降低进出口的压降。     

小管径椭圆管开缝翅片换热器的数值模拟

对小管径椭圆管开缝翅片换热器空气侧的流动与传热特性进行数值模拟,对影响其换热性能的2个主要参数椭圆管偏心率和开缝翅片开缝错列高度分布进行优化,与传统管翅式换热器换热性能进行比较。模拟结果表明:当椭圆管两轴之比Rx:Ry=2:3(偏心率),开缝高度分布为0.8 mm,0.6 mm和0.4 mm时,换热效果最好。与传统管翅式换热器相比,小管径椭圆管开缝翅片换热器换热系数提高10%~20%,而压降几乎相等,总体换热性能提高。     

椭圆管换热器对空调室内机声场影响的数值模拟

椭圆管换热器由于良好的流动和换热特性,在换热设备中有广泛的应用。本文利用计算流体力学方法,对椭圆管翅片换热器应用于空调室内机的流场和声场进行数值模拟,并与圆管作对比。计算结果表明,换热管形状对贯流风机内部偏心涡的形成位置和大小没有影响;与采用相同截面积的圆管翅片换热器的室内机相比,相同条件下,采用椭圆管换热器对于室内机增加风量,改善制冷性能,降低噪声尤其是低频噪声方面有良好的效果。其中,长短轴之比为2的椭圆管可以降低室内机噪声4 d B。     

攻击角对纵向涡错排椭圆管板式翅片强化传热的影响

利用萘升华传质/传热比拟实验方法,研究了纵向涡产生器攻击角对椭圆管板式翅片换热和阻力特性的影响,分析了纵向涡错排椭圆管板式翅片换热器在不同纵向涡产生器的攻击角时的传热与阻力特性,为换热器设计提供了一定的理论依据.     

铜翅片换热器开发应用的分析

从翅片效率、单个换热单元的传热系数和整个换热器的换热量三个方面对不同翅片结构、不同管子结构和不同工况下的翅片管换热器由铝翅片换为铜翅片前后的换热特性和成本进行了分析比较;详细给出了各个结构和工况参数单独变化对换铜前后的翅片效率、单位制冷剂侧换热面积上的总传热系数和换热器换热量的影响规律.分析结果发现,翅片越薄、越高,管外径越小,风速越大时,换铜后的翅片管的换热能力增强越大,成本增加越小.在所选择的结构和工况范围内,铜管铜片换热器比铜管铝片换热器的翅片效率约提高0.938%～29.86%、总传热系数约提高9.88%～23.276%、总换热量约提高0.112%～22.3%;对于典型的1.1kW空调器的蒸发器,材料成本约增加8～42元,体积可最多缩小18%.     

内翅片管换热器换热效率降低分析

内翅片管换热器使用一段时间后,换热效率会降低。主要原因是:波纹型翅片与内管定位不好,引起管内堵塞;内管一端封头的头部很薄,使用一段时间后出现中孔现象,使流体短路,换热系数发生变化;内(外)管和翅片清洗不彻底,使内翅片换热管钎焊部位脱落,影响钎焊质量甚至堵塞通道。详细介绍在内翅片换热管加工过程中对上述三种问题的解决方法。     

风速与压缩机频率对两种回路蒸发盘管换热除湿性能影响的实验研究

通过对翅片管换热器流程与回路设计的研究,本文提出了一组6排管全逆流蒸发盘管流程与回路的改进方案,即并联型先顺后逆流蒸发盘管,利用焓差实验室测试了两组蒸发盘管在不同工况下的换热和除湿特性并进行了对比分析。结果表明：在相同迎面风速和压缩机频率下,并联型先顺后逆蒸发盘管流程与回路设计更有利于换热除湿,但同时空气侧压降增大。在相同压缩机频率下,随着迎面风速的增加,并联型先顺后逆蒸发盘管比全逆流蒸发盘管换热性能的提升较大,除湿性能的提升较小;两组蒸发盘管运行时迎面风速不得超过2.8 m/s。压缩机频率越大,两组蒸发盘管随迎面风速增加换热性能的提升越小,除湿性能的提升略有增大。在相同迎面风速下,随着压缩机频率的增加,并联型先顺后逆蒸发盘管比全逆流蒸发盘管的换热和除湿性能提升的更快,更适合在高压缩机频率下运行。     

不同强化管传热特性的数值模拟与实验研究

对横纹槽管、缩放管和螺旋槽管在夹套间进行了传热特性实验,研究了传热效率指标随雷诺数的变化规律.运用FLUENT软件,采用二维轴对称方法和k-ε模型对夹套间流体流动传热进行了数值模拟,并将模拟结果与实验结果进行对比.从场协同的角度研究了速度场与温度场夹角对传热膜系数的影响.     

水平管外沸腾强化换热的数值模拟与场协同分析

应用FLUENT软件对制冷剂R134a在光管和横纹槽管水平管外沸腾传热进行三维数值模拟,得到其饱和泡状沸腾过程中体积含气率的分布规律,并比较它们的换热系数。结果表明横纹槽管外侧能够很好地强化沸腾传热。此外,还通过改变边界条件分析质量流量、热流密度的变化对横纹槽管管外沸腾换热系数的影响。最后应用场协同理论,从局部换热角度分析其强化机制。研究表明,横纹槽管水平管外沸腾换热得到强化的原因是其凹槽前后的速度场与温度梯度场之间夹角较小,协同程度更好。     

Effect of tube diameter on boiling heat transfer and flow characteristic of refrigerant R32 in horizontal small-diameter tubes

This study investigated the effect of tube diameter on flow boiling characteristics of refrigerant R32 in horizontal small-diameter tubes with 1.0, 2.2, and 3.5 mm inner diameters. The boiling heat transfer coefficient and pressure drop were measured at 15 °C saturation temperature. The effects of mass velocity, heat flux, quality, and tube diameter were clarified. The flow pattern of R32 for adiabatic two-phase flow in a horizontal glass tube with an inner diameter of 3.5 mm at saturation temperature of 15 °C was investigated. Flow patterns such as plug, wavy, churn, and annular flows were observed. The heat transfer mechanisms of forced convection and nucleate boiling were similar to those in conventional-diameter tubes. In addition, evaporation heat transfer through a thin liquid film in the plug flow region for low quality, mass velocity, and heat flux was observed. The heat transfer coefficient increased with decreasing tube diameter under the same experimental condition. The fictional pressure drop increased with increasing mass velocity and quality and decreasing tube diameter. The experimental values of the heat transfer coefficient and frictional pressure drop were compared with the values calculated by the empirical correlations in the open literature.     

New experimental data of CO2 flow boiling in mini tube with micro fins of zero helix angle

Heat transfer and pressure drop characteristics of CO₂ flow boiling in mini tube with micro fins of zero helix angle were experimentally investigated. The working conditions cover mass flux from 100 to 600 kg m⁻² s⁻¹, heat flux from 1.67 to 8.33 kW m⁻², vapor quality from 0 to 0.9 and saturation temperature from 1 to 15 °C. The results show that the heat transfer coefficient increases with increasing vapor quality, but sharply decreases at vapor quality around 0.2~0.4 under most conditions, and the dryout vapor quality decreases with the increasing heat flux and saturation temperature. Pressure drop increases with increasing mass flux and heat flux, or decreasing saturation temperature, and mass flux is the major influence factors. The enhancement ratio of heat transfer coefficient is higher than that of pressure drop, which shows potentials of using such kind tubes to enhance the overall heat transfer performance. A heat transfer coefficient correlation and a pressure drop correlation for 0° helix angle micro-fin tube were developed, and they agree well with the experimental data.     

结构特征对螺旋管换热器性能的影响

目的研究结构特征对螺旋管换热器换热性能的影响,为换热器设计和结构优化提供理论依据。方法在CFD软件Ansys的FLUENT模块中模拟螺旋管换热器内流体的流动换热过程。在相同边界条件下,不改变总换热面积,通过改变换热器的换热管直径和壳体长度,研究几何参数对换热性能的影响。通过对比单管单螺旋、单管双螺旋、双管双螺旋和内外双螺旋等4种不同缠绕方式换热器中流体的温度分布云图,研究螺旋管的结构对换热器换热性能的影响。结果保持总换热面积不变,减少换热器的管径,增大壳体的长度,都能有效提高换热效率。与单管单螺旋结构的换热器相比,单管双螺旋结构换热器的流体出口温度下降了9.74%,平行双螺旋结构换热器的出口温度下降了5.05%,内外双螺旋结构换热器的出口温度上升了10.11%。结论在螺旋管换热器的设计和优化过程中,可以通过减小换热管径,增大壳体长度,采用内外双螺旋结构,以实现提高传热效率的目的。     

开架式气化器竖直圆管内超临界氮换热实验研究

目前国内外对液化天然气(LNG)接收站的开架式气化器中超临界天然气的流动换热实验研究非常少,本文为了研究开架式气化器中竖直管内超临界流体的流动换热特性,搭建了竖直单管超临界流体换热实验平台。以液氮代替液化天然气,研究了氮入口压力、水温和水流量等不同参数对换热的影响。结果表明:在拟临界温度以下,表面传热系数随着压力的增大逐渐减小,但拟临界温度以后,这种趋势相反;当水流量足够大时,氮出口温度取决于管外水温而不是水侧流量。最后,基于实验数据拟合出了适用于竖直圆管内超临界低温流体流动换热的半经验关联式,关联式预测值和实验值的平均绝对偏差为8.42％,可以准确预测竖直加热管中超临界氮的表面传热系数。     

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.     

高温气冷堆蒸气发生器结构内石墨粉尘的运动行为

为研究高温气冷堆石墨粉尘的运动规律,采用数值模拟的方法,分析高温气冷堆一回路蒸气发生器由于特殊的结构特点对石墨粉尘颗粒沉积扩散特性带来的影响。数值模拟结果表明:蒸气发生器的结构特点造成的石墨粉尘沉积主要集中在外套壁面、中心管壁面与第一道换热管的迎风管壁上;在颗粒粒径确定的情况下,其沉积的效率随气流速度的增大而相应增大;在一定的气流进口速度下,颗粒沉积率随颗粒粒径的增大而增大。     

Heat transfer characteristics of cryogenic helium gas through a miniature tube with a large temperature difference

Heat transfer with a large temperature difference between the inlet and outlet flows is often encountered in many cryogenic systems. The heat transfer characteristics of cryogenic gas with temperature-dependent thermophysical properties (TDTP) are different from those in the ambient condition with constant thermophysical properties. In this paper, a combined experimental and numerical study has been conducted to determine the heat transfer characteristics of cryogenic helium gas with temperature-dependent thermophysical properties in a miniature tube. In order to validate the theoretical model, an experimental investigation was conducted. Utilizing the validated model with the FLUENT software, the temperature distribution and velocity profile in a miniature tube have been calculated, and a correlation predicting the temperature effect on the Nusselt number determined.     

Condensation of R407C in a horizontal microfin tube

This paper describes experimental results that show the effects of mass velocity and condensation temperature difference on the local heat transfer characteristics during condensation of R407C in a horizontal microfin tube. The experiments were performed at the saturation temperature of 40 °C, the refrigerant mass velocity of 50, 100, 200 and 300 kg m⁻² s⁻¹, and the condensation temperature difference of 1.5, 2.5 and 4.5 K. A superficial heat transfer coefficient for the vapor phase was obtained by subtracting the heat transfer resistance of condensate film estimated by using a previously developed theoretical model of film condensation of pure vapor from the overall heat transfer resistance. On the basis of the analogy between heat and mass transfer, an empirical equation for the superficial vapor phase heat transfer coefficient was developed. The heat transfer coefficient predicted by the combination of the previously developed theoretical model of film condensation of pure vapor and the empirical equation of the superficial vapor phase heat transfer coefficient agreed with the measured values with the r.m.s. error of 9.2%.