PIV Measurement for Rayleigh Convection and Its Effect on Mass Transfer☆

The velocity distribution in Rayleigh convection caused by acetone volatilization in acetone-ethyl acetate binary system was observed in a vertical cross section of an initially quiescent liquid layer ...     

盐浴螺旋盘管式焦炉上升管余热回收装置传热性能试验

对盐浴螺旋盘管式结构的焦炉上升管高温荒煤气余热回收装置进行了以烟气代替荒煤气的传热性能模拟试验,得到了上升管余热回收装置螺旋盘管环形套筒的外壁温度分布、上升管内烟气侧的传热系数、环形套筒内螺旋盘管外盐浴的自然对流传热系数等试验研究结果。结果表明,装置螺旋盘管环形套筒的外壁温度分布并非均匀,随上升管内烟气温度的升高而波动增大;烟气侧对流传热系数在Re数高于2900后随Re数的增大而明显上升;螺旋盘管外盐浴的自然对流传热系数随熔盐温度的升高几乎不变。根据试验结果拟合出环形套筒内螺旋盘管外盐浴的自然对流传热关联式,为实际的工程应用提供参考。     

Heat/mass transfer from a neutrally buoyant sphere by mixed natural and forced convection in a simple shear flow

Building on the work of Yang et al. in 2011, the finite difference method and the Boussinesq approximation were applied to solve the time‐dependent Navier‐Stokes, convection diffusion and continuity equations in spherical coordinates. An idealized condition, the mass transfer from a neutrally buoyant sphere in a horizontal simple shear flow with natural convection was numerically simulated for the first time in this work. In the hybrid transfer case, the outwardly spiraling streamlines enhanced the transfer process, but the counter‐gravity spiraling streamlines near the sphere hindered the natural convection and the spatial dilution action weakened the natural convection transfer process. These competing effects led to nonmonotonic behavior of the Nusselt number with Reynolds number. Results from these previously undocumented cases were summarized into correlations for predicting Nusselt numbers at finite Reynolds numbers for various Grashof and Prandtl numbers. © 2018 American Institute of Chemical Engineers AIChE J, 64: 2816–2827, 2018     

Mixed convection mass transfer studies of opposing and aiding flow in a parallel plate electrochemical flow cell

Studies of combined natural and forced convection in a vertical parallel plate electrochemical cell in laminar conditions in cases of opposing and aiding flow are reported. In an ongoing project it was necessary to identify conditions in which natural convection had no significant influence on mass transfer rates at the cell walls so that data could be validly compared with purely laminar flow computational models. For the different electrode lengths investigated, natural convection dominated at low Reynolds number and there was no Reynolds number dependence. At high Reynolds number the data approached the laminar flow solution. At intermediate Reynolds number, however, there existed a distinct region where free and forced convection were significant. At high electrolyte concentrations data did not merge with laminar flow equations until Re=1000 and low electrolyte concentration data for the large plate could not be compared with numerical predictions below Re of 250. An attempt was made to compare the data with those of other workers on combined forced and natural convection heat and mass transfer.     

Optimization of natural convection heat transfer of Newtonian nanofluids in a cylindrical enclosure

This study characterizes and optimizes natural convection heat transfer of two Newtonian Al2O3 and TiO2/water nanofluids in a cylindrical enclosure. Nusselt number (Nu) of nanofluids in relation to Rayleigh number (Ra) for different concentrations of nanofluids is investigated at different configurations and orientations of the enclosure. Results show that adding nanoparticles to water has a negligible or even adverse influence upon natural convec-tion heat transfer of water:only a slight increase in natural convection heat transfer of Al2O3/water is observed, while natural convection heat transfer for TiO2/water nanofluid is inferior to that for the base fluid. Results also reveal that at low Ra, the likelihood of enhancement in natural convection heat transfer is more than at high Ra:at low Ra, inclination angle, aspect ratio of the enclosure and nanoparticle concentration influence natural convec-tion heat transfer more pronouncedly than that in high Ra.     

绝热圆腔内4根冷热微管阵列振动强化传热实验

目前针对微细管管外同时有自然对流和周期横向振动存在情况下的混合对流问题鲜有研究。为了探究这种现象, 进而开发一种新型的换热器, 本文提出了一种由4根冷热微细管组成的微管阵列, 将其平行放置于有中间流体的圆形腔体内, 并对管束施加微小横向振动的混合对流传热机理进行研究。详细描述了实验系统及实验方法, 以及后期对实验数据的处理方法, 得到了传热Nu数随管间自然对流强度Ra数, 强迫振动Re_v数等量纲为1参数的变化曲线;揭示了该结构形式下传热特性随冷热管温差、振动频率、微管振幅等传热规律。结果表明, 横向振动在小温差情况下传热强化效果更为明显。将本文实验数据与Lemlich单管振动实验中的数据进行对比, 表明了振动对4根微管阵列的强化传热效果比单根管更明显。     

CHARACTERISTICS OF HEAT TRANSFER FROM A SPHERICAL SOLID BODY UNDER PULSATING FORCED CONVECTION

Heat transfer from a solid sphere having a Biot number Bi ? 0.1 to a flow medium in a cooling process under pulsating forced convection is experimentally examined. In the experiment, two kinds of pulse modes, which are the type of pulsating forced convection with continuous feed and the feed type mixing forced and natural convection, are considered as a pulse feed technique. Initially, the evaluation method of the Nusselt number Nu is derived in the cooling process and the validity of that method is verified by comparison with the experimental results. With regard to the effect of pulsating feed, it is shown that the pulsating feed conditions have a great influence on the characteristics of the heat transfer. The enhancement of heat transfer under the condition of the forced convection pulsatively fed with continuous feed is recognized, and the characteristics of heat transfer for the pulsating feed including natural convective heat transfer region are aggravated. Besides, it is clear that the decrease in the heat transfer characteristics is dependent on the duration of the natural convective heat transfer. Furthermore, Ranz-Marshall correlations for each pulsating feed condition are presented.     

空气横掠沉没于多孔泡沫金属中错列管束的对流传热

对空气横掠沉没于多孔泡沫金属中正三角错列传热管束的对流传热进行了试验研究。结果显示,多孔泡沫金属使得流体的流动阻力明显增加,但阻力增加的倍数随Reynolds数的增大而减小;多孔泡沫金属对对流传热具有明显的强化作用,强化效果随Re的增大而减小。与横掠错列光管束相比,本试验范围内,流动阻力增加的最大倍数为12.727,最小倍数为10.109;对流传热的最大强化倍数为2.78,最小强化倍数为2.08。总结出了适合于试验范围的对流传热的气流量纲1阻力系数的表达式和传热计算关联式。综合强化对流传热及增加流动压降两方面可知,多孔泡沫金属较适合于流体较小流速的场合。     

窄缝环形管内流动与传热实验研究Ⅰ.单相强制流动换热

对垂直窄缝 (窄缝尺寸规格分 1、1 5和 2 5mm 3种 )环形管内氟 1 1 3单相受迫对流换热进行了研究 ,环缝的加热方式采用热水逆流双面加热。详细介绍了实验装置、过程和方法。实验结果表明 ,窄缝环形管内单相流动换热的Nu数与普通圆管内层流公式计算值相差较大 ,不能套用传统的单相流动换热模型来计算管内的对流换热系数。根据实验数据 ,将修正的Dittus Boelter公式用于计算窄缝环形管内的Nu数 ,平均误差在 1 8 6%以内。     

界面传质中Rayleigh对流的定量分析

通过纹影光路观察了特定气液传质装置中乙醇吸收CO₂过程所引发的Rayleigh对流在垂直界面方向上的发展过程。随着溶质吸收的进行,液层的流体稳定性变弱,扰动加剧气液界面失稳并发生湍动,进而发展为羽状流并逐步向液相主体发展,在此过程中伴随着对流胞的融合与增长。液层的浓度分布可通过对相应液层纹影图像进行定量分析获得。液层浓度分布和瞬时传质系数变化表征了Rayleigh对流的引发与发展及其对传质过程的强化效果,界面浓度分布及临界Rayleigh数解释了非均匀传质对湍动的引发机理。羽状流将高浓度液体快速带入主体,加速了近界面液层与主体液层的混合,增强了气液传质。     

微细铜丝在水中自然对流换热

采用实验与数值模拟相结合的方法分别研究了封闭空间内水平放置的直径为39.9、65.8、119.1 μm的微细铜丝（微丝）在水中的对流换热,分析了微丝表面自然对流换热特性及机理。实验通过焦耳加热的方法测量了不同直径微丝在水中自然对流的传热系数及Nusselt数。同时建立三维不可压数学模型对微丝在水中的自然对流进行数值模拟,并将计算结果与实验值进行了对比。研究表明,数值模拟结果与实验值基本吻合,微丝在水中自然对流的传热系数随直径减小而显著增大,Nu则明显减小,且Nu随热通量增加的变化率也随直径减小而明显降低;微丝表面边界层厚度随直径减小而变薄,但边界层厚度与微丝直径的比值则逐渐增大;另外,对比微丝与常规尺度圆管表面自然对流的流场、温度场以及边界层分布,发现相同温差下微丝表面自然对流换热的边界层与常规尺度下沿壁面由底部向上发展的形状不同,而是沿微细丝表面呈椭圆形包裹于其上,因此削弱了表面对流换热强度,导致温度场呈现出较明显的导热特征。     

Analysis of combined forced and free convection across a horizontal cylinder

A new analysis is performed to study the flow and heat transfer characteristics of laminar combined forced and free convection about a horizontal cylinder. The transformed conservation equations of the nonsimilar boundary layers are solved by a finite difference method. Numerical results for gases having a Prandtl number of 0.7 are presented for buoyancy parameters which cover the entire domain of the mixed convection, ranging from pure forced convection to pure free convection. It is found that both the local wall shear and the local heat transfer rate (1) increase with increasing buoyancy force for assisting flow, (2) decrease with increasing buoyancy force for opposing flow, and (3) depend very strongly on the variation of the local free stream velocity, particularly for small to moderate buoyancy forces. With regard to the heat transfer results, the buoyancy force effects on forced convection are found to be important for Gr/Re² ≥ 1.93 and ≤ –1.60, respectively for assisting and opposing flows, whereas the inertia force effects on free convection become significant for Re²/Gr ≥ 0.012.     

MIXED CONVECTION BOUNDARY LAYER FLOW OF A MICROPOLAR FLUID ON A HORIZONTAL PLATE

An analysis is performed to study the heat transfer characteristics of laminar mixed convection boundary layer flow of a micropolar fluid past a semi-infinite horizontal fiat plate with variable surface heat flux. A nonsimilar mixed convection parameter £ and a pseudosimilarity variable v are introduced to cast the governing boundary layer equations into a system of dimensionless equations, which are solved numerically using the finite difference method. A single mixed convection parameter is used to cover the entire regime of mixed convection from the pure forced convection limit to the pure free convection limit. The effect of material parameters, the power-law variation of surface heat flux, nonsimilar mixed convection parameter and Prandtl number are considered. The micropolar fluids are observed to display drag reduction and reduced surface heat transfer rate when compared to Newtonian fluids. The effect of the buoyancy force results in the enhancements of friction factor, heat transfer rate and wall couple stress.     

幂律流体自然对流传质的研究

本文从解扩散微分方程组的角度研究非牛顿流体的传质问题,本构方程使用幂律模型,获得近似解,并利用电化学实验进行模拟和验证.在特定条件下,近似解可回复至牛顿流体的解.     

水吸收CO2过程界面对流的激光诱导荧光观测

建立了采用激光诱导荧光技术观测水吸收CO₂过程中界面对流现象的实验装置,利用荧光素钠作为荧光剂,通过标定获得了CO₂在水中溶解浓度与荧光强度的关系,进而获得了定量测量水中CO₂浓度分布的实验方法。通过建立的实验装置和方法,测量了水吸收CO₂过程中液体近界面Rayleigh对流发生时的浓度分布演化过程。利用实验结果对文献中报道的临界Rayleigh数进行了验证;并对上述吸收体系的界面传质由分子扩散主导逐步转变为对流传质主导的过程,以及Rayleigh对流对吸收过程界面传质的强化进行了定量分析。     

水溶解CO2过程界面对流现象的PIV/LIF测量及传质系数预测

建立了用于观测水溶解CO₂过程界面Rayleigh对流现象的粒子成像测速（PIV）和激光诱导荧光（LIF）技术联用方法,同时获得了该对流传质过程的液相速度场及浓度分布,定量观测了Rayleigh对流的发展过程。经界面对流发生条件下传质过程的分析,指出Rayleigh对流过程的涡量是影响传质过程的主要因素,给出了传质系数随涡量的变化关系和相应的关联式,进而为通过液相速度场的测量预测Rayleigh对流条件下的液相传质系数提供了一种有效的方法。     

降膜传质过程Marangoni效应稳定性分析

通过建立降膜流动传质模型，并考虑液相流动及非线性浓度分布的影响，利用线性微扰理论得到了降膜流动传质过程Marangoni效应发生的理论临界条件.用所建立的降膜流动传质实验装置得出实测传质系数的变化，由此获得Marangoni效应发生的实测临界条件，实测值与理论值基本符合，从而为工业上常见的降膜流动传质过程中Marangoni现象的预测和利用提供了理论依据.     

格拉斯霍夫数物理意义的种解释方法简

格拉斯霍夫数是将影响对流传热膜系数因素进行量纲分析得到的无量纲数。国内外几乎所有《化工原理》教材均将其解释为表征自然对流的影响,但解释过程均较为模糊。本文以一垂直平板与大空间液体的自然对流传热过程为例,将流体在垂直平板附近的热边界层内流动等效到圆管内流动,直接用泊稷叶公式计算出热边界层内流体流动为层流时的环流速度,取等效圆管的当量直径为特征尺寸,明确得到格拉斯霍夫数是雷诺数的一种变形,表征自然对流时惯性力与黏性力之比。     

Effect of ΔT- and Spatially Varying Heat Transfer Coefficient on Thermal Stress Resistance of Brittle Ceramics Measured by the Quenching Method

A study was conducted of the effect of a spatial variation in heat transfer coefficient during forced convection and the effect of temperature dependence of the heat transfer coefficient during free convection on the magnitude of thermal stress encountered during the thermal shock testing of brittle ceramics by the quenching method. For specimens with circular geometry, the results obtained by the finite element method indicate that, depending on the value of the Biot number, a factor of three spatial variation in heat transfer coefficient changes the tensile thermal stresses on cooling by a maximum of about 17% over the value for spatially uniform heat transfer. On heating, the corresponding tensile thermal stresses are lower by a maximum of about 6%. In free convection for a heat transfer coefficient, h , proportional to Δ Ti^1/4 , where Δ T_i is the instantaneous temperature difference between the specimen and quenching medium, the stresses are appreciably less than those for Δ T_i -independent heat transfer for the same initial value of h at the onset of the thermal quench. The validity of the dependence of h on Δ T_i was established experimentally. The relevance of this result to the interpretation of thermal quenching studies is discussed.     

THERMAL CONVECTIVE INSTABILITY IN TRANSLUCENT POROUS MEDIA WITH RADIATIVE HEAT TRANSFER

The onset of thermal convection in a translucent porous layer is considered. Attention is focused on the effect of radiative heat transfer on the critical Rayleigh-Darcy number and the convection cell shape. If we consider the contribution of radiative heat transfer, the basic temperature profile is non-linear and the thermal convective instability is influenced by the ratio of conduction to radiation heat flux, the temperatures at the boundary surfaces, and radiative parameters such as wall emissivity, scattering albedo and extinction coefficient as well as the usual Rayleigh-Darcy number. Effects of these parameters on the onset of convective instability are investigated with the help of linear stability theory employing the Darcy's law and the radiative transport equation simplified by the P₁ approximation. The increased effective thermal conductivity due lo the radiation inhibits the onset of convection and causes increased critical Rayleigh-Darcy number and decreased convection cell size. The results of the present work may be exploited to find out the optimal diameter of aerogel pellets and the air pressure in the double pane window filled with the translucent silica aerogel granules to suppress natural convection.