不同截面形状超疏水微肋阵内对流换热特性

通过在圆形、菱形以及椭圆形微肋阵表面固化含有微纳米颗粒涂层制备超疏水微肋阵,并对不同截面形状超疏水微肋阵内流动与对流换热特性进行实验研究,测量得到了超疏水处理前后各截面形状微肋阵内流动阻力系数f以及Nusselt数。此外,通过计算超疏水微肋阵内的综合传热强化因子ε,定量分析了超疏水处理对不同截面形状微肋阵内流动与换热特性的综合影响。研究结果表明,超疏水处理后圆形、菱形、椭圆形微肋阵内摩擦阻力系数与未处理前相比最大可降低72%、66%、70%;同时,3种截面微肋阵内Nu有所降低,且疏水前后微肋阵Nu偏差随加热功率的增加逐渐减小,高加热功率下3种微肋阵超疏水处理前后Nu偏差最大分别不超过44%、17%、47%;高加热功率下超疏水菱形微肋阵在Re1200范围内具有良好的综合传热强化性能,ε始终高于1.17。     

不同截面形状超疏水微肋阵内对流换热特性

通过在圆形、菱形以及椭圆形微肋阵表面固化含有微纳米颗粒涂层制备超疏水微肋阵,并对不同截面形状超疏水微肋阵内流动与对流换热特性进行实验研究,测量得到了超疏水处理前后各截面形状微肋阵内流动阻力系数f以及Nusselt数。此外,通过计算超疏水微肋阵内的综合传热强化因子ε,定量分析了超疏水处理对不同截面形状微肋阵内流动与换热特性的综合影响。研究结果表明,超疏水处理后圆形、菱形、椭圆形微肋阵内摩擦阻力系数与未处理前相比最大可降低72%、66%、70%;同时,3种截面微肋阵内Nu有所降低,且疏水前后微肋阵Nu偏差随加热功率的增加逐渐减小,高加热功率下3种微肋阵超疏水处理前后Nu偏差最大分别不超过44%、17%、47%;高加热功率下超疏水菱形微肋阵在Re<1200范围内具有良好的综合传热强化性能,ε始终高于1.17。     

浅析列管式换热器传热系数的影响因素

列管式换热器传热方式是热传导和对流传热同时进行的,本文通过Nusselt准数关系式、Reynolds准数关系式、Prandtle准数关系式等对影响传热系数因素-换热器结构、流体物理性质和污垢热阻等进行分析。以便在设计过程中合理调整结构参数使换热器获得最佳性能。     

疏水表面结霜初期液滴生长的理论分析

研究了自然对流条件下疏水表面结霜初期冷凝液滴的生长过程, 建立了考虑不凝气影响的液滴传热及生长模型, 分析了表面接触角和冷面温度对液滴生长的影响。结果表明, 液滴生长过程中的主要热阻为液滴内部导热热阻和相界面热阻, 随着表面接触角的增大, 这两个主要热阻均增大, 因此表面疏水性越好, 液滴生长越缓慢;而由于冷凝传热温差随冷面温度降低而增大, 因此冷面温度越低, 液滴生长越快。     

超疏水表面微通道内水的传热特性

微通道传热效率高但流动阻力大,超疏水表面因其与水具有滑移边界而表现出低流阻的特征,在微过程中具有应用前景。利用化学刻蚀法制备出具有微纳米阶层结构的铝基超疏水表面微通道(内径为0.68mm)。在超疏/亲水微通道内进行了水的流动传热实验研究,并将结果进行对比。研究发现存在于超疏水表面微纳米结构里的气泡层减小了水的流动阻力,也降低了表面传热系数,但降低程度明显小于流动阻力的降低,传热系数高于考虑纳米气泡层计算的传热系数。因此认为在水的滑移速度作用下,凹穴中微纳米级气泡内产生了气体的涡旋流动,一定程度上增强了传热效果。     

超疏水表面蒸汽及含不凝气蒸汽滴状冷凝传热实验分析

通过紫铜基表面上制备两种具有微纳米结构的超疏水表面以及相同化学修饰的光滑疏水表面,实验研究了各表面上空气环境下水的润湿特性以及在纯蒸汽、蒸汽-空气混合气体环境下,表面的滴状冷凝传热特性和冷凝液滴的运动和润湿特性。结果表明:纯蒸汽滴状冷凝条件下,超疏水表面的传热性能明显低于光滑疏水表面的传热性能;含低浓度不凝气蒸汽冷凝环境下,超疏水表面传热性能与光滑疏水表面相近;蒸汽冷凝环境中,超疏水表面上液滴的接触角明显低于其在空气条件下接触角,并且接触角滞后增大。分析得到,微纳米结构的存在使冷凝过程液滴的接触角滞后增大,微纳米结构中冷凝液滞留增加的壁面热阻等抑制了滴状冷凝传热性能;并提出了蒸汽及含不凝气蒸汽冷凝环境中液滴在超疏水表面上的润湿模式。     

微纳结构超疏水表面参数影响含不凝气蒸汽冷凝传热的理论分析

超疏水表面结构参数对滴状冷凝传热性能、液滴生长和分布均会产生影响。首先利用竖直壁面液滴的受力平衡确定了不凝气条件下蒸汽冷凝过程中液滴的脱落半径,随后建立了含不凝气蒸汽滴状冷凝的传热模型,研究了不同不凝气浓度和过冷度下超疏水表面微柱的柱间距对滴状冷凝传热性能的影响,得到了使超疏水表面冷凝传热性能达到最佳的柱间距值,并对其随不凝气浓度和过冷度的变化规律进行了分析。结果表明在所研究的过冷度范围内,当不凝气浓度较低时（<20%）,最佳柱间距随不凝气浓度升高而增大;而当不凝气浓度高于20%时,最佳柱间距随不凝气浓度升高而减小。为驱动含不凝气蒸汽冷凝传热性能的超疏水微纳结构参数的优化提供了必要的基础数据。     

Y型三维微肋管管外池沸腾传热特性

为了提高换热管的传热效率，选用1根光滑管和2根不同肋间距的Y型三维微肋管进行了R410A的管外池沸腾传热实验。研究发现，受气泡行为的影响，Y型三维微肋管的管外传热系数随热流密度升高而降低，传热强化效果削弱并逐渐趋于光滑管；提高饱和温度和肋间距可以增大管外传热系数，但在高热流密度下，饱和温度对管外传热系数的影响不大；与其他商用强化管相比，2根Y型三维微肋管的传热系数在热流密度在低于25 kW/m²时更具优势，但在热流密度高于45 kW/m²时强化倍率较低。为了预测Y型三维微肋管的管外表面传热系数，拟合了一个传热关联式，该关联式92.62%的计算值与实验值相对误差不超过30%。     

低Reynolds数条件下水流纵掠长冰柱沿方位角传热特性

通过实验研究了低Reynolds数条件下水流纵掠长冰柱过程的沿方位角相界面移动规律及传热特性。改变水流速度、水流温度、冰柱尺寸及冰柱初始温度等控制参数,采用非接触测量方法获得了冰柱融化图像,基于影像实验数据与冰柱相界面传热系数之间的映射关系,分析了沿方位角对流传热系数等传热参数的变化规律。结果表明:各测量点的沿方位角Nusselt数不同,随着融化过程的进行,各测量点的沿方位角Nusselt数值均先增大后减小,整个融化过程中间某处存在最大值;其余实验控制参数不变的条件下,沿方位角对流传热系数均随着水流速度或水流温度的增大而增加;获得了水流纵掠冰柱融化过程Nu?与Gr、Re、Pr、Ste及?之间的关联式。     

异形整体式热管散热器传热实验与分析

针对电子电气设备散热和均温的需求,提出了一种新型结构形式的异形整体热管散热器：平板热管形式的蒸发段与具有高肋化比翅片的冷凝铜管集成。对该热管进行了传热实验与分析,研究热管在不同工况下温度数值及分布,探究影响热管性能的因素和规律,验证其传热能力。结果表明：在各种工况下热管温差始终在1.75℃之内,均温性能良好。加热功率和对流散热状况对热管启动性能、总体热阻、当量热导率、传热系数都有影响。随着加热功率和对流速度增加,热管启动时间和热阻均降低,当量热导率和传热系数则逐渐上升。热阻最低为0.189℃·W^-1,最佳当量热导率为20964 W·m^-1·K^-1。相比于同等尺寸的传统热管,热阻降低了37%,传热效率提升15%。     

微/小圆柱针肋热沉传热特性

圆柱形微/小针肋热沉热阻由底板及微/小肋热传导热阻、冷却液与肋间对流换热热阻以及冷却液吸热焓变热阻等组成。以去离子水为冷却液,恒定热流下,进行了叉排排布的不同尺寸的微/小圆柱形针肋热沉的单相强制对流传热实验,实验Re为100～1000,加热功率为50～300 W。实验表明：微/小圆柱形针肋热沉的Nu随Re和加热功率的增大而增大,当Re=600时对     

Natural convection on a vertical plate with variable heat flux in supercritical fluids

The present article investigates laminar free convection with uniform or non-uniform prescribed surface heat flux over a vertical flat plate in supercritical fluid, numerically. A new equation for thermal expansion coefficient in a supercritical fluid is derived based on Redlich–Kwong equation of state as a function of pressure, temperature and the compressibility factor. Calculated values of thermal expansion coefficient have been compared with the experimental results which show better accuracy in comparison with van der Waals ones. The governing systems of partial differential equations were solved numerically using the finite difference method. The local Nusselt number was calculated and plotted as a function of the local Rayleigh number. Using supercritical fluids in constant heat flux free convection, decrease wall temperature in comparison with under critical fluids. It was observed that positive and negative slopes of surface heat flux distribution increases and decreases the heat transfer coefficient, respectively.     

Effect of Shear Work on the Heat Transfer Characteristics of Gaseous Flows in Microchannels

The effect of shear work at solid boundaries for parallel plates and a micropipe is considered to analyze the heat transfer characteristics in the slip flow region for gaseous flow. The fluid flow is assumed to be laminar, incompressible, steady, and hydrodynamically and thermally fully developed. The effects of second‐order velocity slip, temperature jump, shear work at the solid surface, and viscous dissipation are considered. The constant heat flux boundary condition is used at the surface of the parallel plates and of the micropipe. Closed‐form expressions are obtained for the temperature distribution and Nusselt number as a function of various modeling parameters for both geometries. The results show that neglecting the shear work underpredicts the Nusselt number.     

MOMENTUM INTEGRAL METHOD FOR FORCED CONVECTION IN THERMAL NONEQUILIBRIUM POWER-LAW FLUID-SATURATED POROUS MEDIA

Forced convection heat transfer for power-law fluid flow in porous media was studied analytically. The analytical solutions were obtained based on the Brinkman-extended Darcy model for fluid flow and the two-equation model for forced convection heat transfer. As a closed-form exact velocity profile is unobtainable for the general power-law index, an approximate velocity profile based on the parabolic model is proposed by subscribing to the momentum boundary layer integral method. Heat transfer analysis is based on the two-equation model by considering local thermal nonequilibrium between fluid and solid phases and constant heat flux boundary conditions. The velocity and temperature distributions obtained based on the parabolic model were verified to be reasonably accurate and improvement is justified compared to the linear model. The expression for the overall Nusselt number was derived based on the proposed parabolic model. The effects of the governing parameters of engineering importance such as Darcy number, power-law index, nondimensional interfacial heat transfer coefficient, and effective thermal conductivity ratio on the convective heat transfer characteristics of non-Newtonian fluids in porous media are analyzed and discussed.     

三角形螺旋夹套内流体的湍流流动及换热模拟

对三角形螺旋夹套内流体的湍流流动及换热性能进行了模拟,得到了充分发展条件下恒定热流加热时釜内湍流流体的速度场,分析了雷诺数(Re)和无量纲曲率(k) 对流体阻力和换热性能的影响,并由模拟数据拟合出平均阻力系数及平均努赛尔数的关联式. 结果表明,湍流流动中,夹套内流体的二次流动为稳定的二涡结构,随雷诺数增大,二次流强度和湍动能均增强. 由于离心力的作用,外壁面的阻力系数远大于内壁面. 换热面上局部努塞尔数的峰值出现在靠近二次涡中心位置的换热壁面处,换热面中心处的局部努塞尔数约为峰值的85%. 随Re和k增大,峰值处的局部努塞尔数值增大最明显,流体的平均努塞尔数及阻力系数均增大. 在所模拟的范围内,三角形螺旋夹套的效率因子E>3.7,且随Re和k增大,E逐渐增大.     

石蜡乳状液高温潜热输送材料的传热特性

石蜡乳状液高温潜热输送材料是一种在80～90℃之间存在相变的流体。由于相变潜热的存在,石蜡乳状液在相变区间内的储热、载热密度远大于水,且集储热与潜热输送于一体,适用于余热利用、太阳能利用和集中供热等领域,具有广阔的应用前景。研究了定热流条件下石蜡乳状液在管内的强制对流换热特性。结果表明,与纯水相比,虽然石蜡乳状液的有效比热大,但其对流传热系数低,且浓度越高,对流传热系数越低。实验发现入口温度对石蜡乳状液对流传热系数的影响不大,但随着入口温度升高,对流传热系数有增大的趋势。最后获得了石蜡乳状液对流换热量纲1关系式。     

TRANSIENT MIXED CONVECTION ALONG A VERTICAL PLATE EMBEDDED IN POROUS MEDIA WITH INTERNAL HEAT GENERATION AND OSCILLATING TEMPERATURE

The effects of oscillating plate temperature on transient mixed convection heat transfer from a porous vertical surface embedded in a saturated porous medium with internal heat generation or absorption are studied. The governing equations are transformed into dimenionless form by a set of variables and solved using the Galerkine finite element method. As the energy generation increases, the temperature near the wall will be higher than the wall temperature, thus increasing buoyancy forces inside the boundary layer and consequently increasing the velocity. The increase of energy absorption term for either space or temperature dependence will decrease the velocity inside the boundary layer and increase heat transfer rates. Different temperature and velocity profiles are drawn for different dimensionless groups. Numerical values for Nusselt numbers as well as local skin friction coefficient are also tabulated.