竖直套管内带热源的多孔介质中的自然对流

为研究内热源存在时多孔介质中流动和传热的特性,采用数值方法对上下底绝热、内外壁存在温差时,竖直套管中带均匀容积热源的多孔介质内的自然对流进行了分析。考查了内、外Rayleigh数之比Rai／Ra、高宽比A及半径比k对流场、温度场以及局部和平均Nusselt数的影响。研究结果表明,Rai／Ra和A较大时,内、外壁面附近分别形成两个逆向环流,温度场出现θ>1的高温区和θ<1的低温区,高温区的范围随Rai／Ra和A的增大而扩大。内壁面局部Nusselt数沿管长先增大后下降,分界点位置随k的减小和A的增大下移;外壁面局部Nusselt数在管上部区域迅速增大。内壁面平均Nusselt数Nui与Rai／Ra的增加呈现负的线性关系。     

Effect of Helical Surface Disc Turbulators on Heat Transfer and Friction Factor Characteristics in the Annuli of a Double‐Pipe Heat Exchanger

The heat transfer and pressure drop characteristics in annuli of a double‐pipe heat exchanger (DPHE) using helical surface disc turbulators (HSDTs) are experimentally investigated. The effect of a helical surface disc turbulator is studied for three pitch ratios, three diameter ratios, and varying Reynolds numbers. Water flows in the inner tube and air through the annulus. The tests are conducted for air with uniform wall temperature condition. The heat exchanger with the least pitch ratio and least diameter ratio was found to exhibit the highest Nusselt number and pressure drop. The thermal performance factor turned out to be greater than unity for all cases. Correlations were developed for Nusselt number, friction factor, and thermal performance.     

Laminar mixed convection in the thermal entrance region of horizontal rectangular channels with uniform heat input axially and uniform wall temperature circumferentially

This paper presents a numerical study on laminar mixed convection in the thermal entrance region of horizontal rectangular channels with uniform heat input axially and uniform wall temperature circumferentially. A relatively novel numerical method of solution is developed to obtain the developing velocity and temperature fields. The values of Prandtl number are 0.7 and 7.2, corresponding to air and water, respectively. The values of Rayleigh number are 0, 10⁴, 3 × 10⁴ and 10⁵. The channel aspect ratios considered are 0.2, 0.5, 1, 2 and 5. Variations in local friction factor ratios and local Nusselt numbers are presented. It is found that the circumferential boundary condition of uniform wall temperature significantly increases the value of local Nusselt number as compared to that found in earlier works under the boundary condition of uniform wall heat flux. But the boundary condition effect on the friction factor is shown to be comparatively minor. The asymptotic solutions at z → are compared to the existing numerical data with good agreement.     

STEADY LAMINAR FLOW THROUGH TWISTED PIPES: FLUID FLOW AND HEAT TRANSFER IN RECTANGULAR TUBES

Fluid flow and heat transfer characteristics in a twisted rectangular tube having an aspect ratio of two were studied using a numerical solution to the momentum and energy equations. Fluid flow solutions are presented for a fully developed laminar flow of a Newtonian fluid. Heat transfer results are presented for the case of axially uniform wall temperature. For the case of peripherally uniform wall temperature, the overall Nusselt number in a twisted rectangular tube was found to be higher than a straight tube by up to 30 percent over certain ranges of twist lengths. However, for the case of non-uniform wall temperature, the overall absolute Nusselt number increased very rapidly with decrease in the twist length     

横向大剪切干涉应用于水平圆管自然对流换热

将横向大剪切干涉技术扩展到温度检测以及传热分析等应用场合中,得到了大空间自然对流水平圆管壁温从400℃降至13℃的整个过程的实时干涉条纹,然后根据一维无限长假设反演出相应的温度场,最终得到了多个壁温下沿圆周各个方向的温度梯度值,并拟合了Rayleigh数和Nu_ave之间的准则关系式。实验结果表明虽然壁面0°附近部分真实条纹可能被圆管竖直倾角引发的阴影遮挡而造成条纹级数检测值有轻微误差,但这不会对换热分析造成显著影响。与文献中主流准则关系式对比可知,利用横向大剪切干涉技术仅需简单的后处理即可得到具有很高的时间、空间分辨率的自然对流换热的可信数据。研究结果为今后热管换热器的理论研究和工程应用提供了参考。     

细小尺度下潜热型功能热流体压降与传热特性

实验研究了相变微胶囊颗粒(囊芯材料为正十六烷,壳材为尿素-甲醛树脂)和去离子水混合制成的潜热型功能热流体流过等热流细小圆管的流动与传热特性,同时以去离子水作为传热工质在相同条件下进行了对比实验。得到了压降随质量流量的变化规律,实验段出、入口温度以及量纲1出口温度随Reynolds数变化规律,量纲1壁面温度沿轴向的分布规律,平均Nusselt数随Reynolds数的变化关系。结果表明,相变微胶囊颗粒的加入会导致流动压降增大,但随着流量增加,流动压降逐渐与单相液体的接近;出口温度及壁面温度要比相同条件下单相液体的低;含有较小相变微胶囊颗粒浓度的潜热型功能热流体的平均Nusselt数是相同条件下单相液体的2.0～4.0倍。     

EFFECTS OF SURFACE TENSION ON LAMINAR FILMWISE CONDENSATION ON A HORIZONTAL PLATE IN A POROUS MEDIUM WITH SUCTION AT THE WALL

The problem of two-dimensional, steady-state film condensation on an isothermal finite-size horizontal plate embedded in a porous medium is studied for the case in which the plate faces upward into a region of dry saturated vapor. Due to surface tension effects, a two-phase zone is formed between the liquid film on the horizontal plate and the vapor zone. The effects of surface tension are shown to reduce the thickness of the liquid film and hence to increase the heat transfer performance of the horizontal plate. Furthermore, the results show that the dimensionless heat transfer coefficient depends on the Darcy number Da, the Jacob number Ja, the effective Rayleigh number Ra_e, the effective Prandtl number Pr_e, the wall suction parameter Sw, and the surface tension parameter Bo_c. When the surface tension effects are neglected and there is no suction at the wall, a closed-form correlation for the Nusselt number can be established.     

Numerical Analysis of Thermal Development of Laminar Flow in a Concentric Multilayer Annulus

The thermal behavior of laminar flow in a concentric multilayer annulus is investigated numerically for varying dimensionless inner radii and a defined number of fluid layers. Under a hydrodynamically developed flow assumption within the fluid layers, the development of temperature profiles and heat transfer along the annulus are analyzed for two different boundary conditions. The mean temperature distribution, local Nusselt number, and mean Nusselt number are discussed in detail with an emphasis on the effects of the inner radius and number of fluid layers. The obtained results indicate that the change in heat transfer coefficient in a layered annulus is more pronounced at a small inner radius or larger radius ratio. A further insertion of more than ten layers has insignificant influence on the convective heat transfer in a layered annulus.     

Heat transfer to wires and cylinders in high-temperature surroundings

This paper presents an extension to the case of wires and thin cylinders of a theoretical analysis previously reported by the authors to account for the effects of large temperature differences and large variations of the fluid property on the heat transfer process to a sphere exposed to a gas at very high temperatures. Two concepts were used in this theoretical derivation: (i) the use of a limiting Nusselt number at Re = 0, as derived from the energy equation, and (ii) the use of a reference film temperature for the evaluation of the Reynolds number different from that used for the Nusselt number. The predictions of this analysis are shown to be in excellent agreement with published experimental data in the Re range 0.01 to 40, when the ratio of the wire to the free stream temperature exceeds 0.4. This range of operating conditions is frequently encountered in industrial plasma processing and in certain high-temperature chemical engineering operations. Measurements in argon and chlorine plasma tailflames with temperature differences exceeding 2000 K are described.     

错流降膜液体干燥剂除湿/再生传热传质数学模型及分析

分析了错流降膜液体干燥剂除湿及再生传热传质过程 ,建立了基于实际除湿系统的描述再生和除湿过程的数学模型 ,考虑到除湿过程中产生的热效应 ,以氯化钙溶液为除湿剂时 ,对气侧和液侧的传热传质系数进行了理论和数值求解 .计算结果表明 ,传热传质系数与气流流动状态、除湿剂的热物理性质等因素有关     

EFFECTS OF SURFACE TENSION ON LAMINAR FILMWISE CONDENSATION ON A HORIZONTAL PLATE IN A POROUS MEDIUM WITH SUCTION AT THE WALL

The problem of two-dimensional, steady-state film condensation on an isothermal finite-size horizontal plate embedded in a porous medium is studied for the case in which the plate faces upward into a region of dry saturated vapor. Due to surface tension effects, a two-phase zone is formed between the liquid film on the horizontal plate and the vapor zone. The effects of surface tension are shown to reduce the thickness of the liquid film and hence to increase the heat transfer performance of the horizontal plate. Furthermore, the results show that the dimensionless heat transfer coefficient depends on the Darcy number Da, the Jacob number Ja, the effective Rayleigh number Ra_e, the effective Prandtl number Pr_e, the wall suction parameter Sw, and the surface tension parameter Bo_c. When the surface tension effects are neglected and there is no suction at the wall, a closed-form correlation for the Nusselt number can be established.     

A numerical study of flow boiling in a microchannel using the local front reconstruction method

The rapid advances in performance and miniaturization of electronic devices require a cooling technology that can remove the produced heat at a high rate with small temperature variations, as is obtained in flow boiling. To obtain insight in flow boiling, we performed numerical simulations in a 200 μm square microchannel using the local front reconstruction method. Besides validation with literature results, a parametric study shows an increasing heat removal rate and bubble growth rate with increasing wall temperature, liquid mass density, and liquid heat capacity and decreasing inlet velocity indicating the importance of phase change compared to convective transport. Finally, the heat transfer in the liquid film is studied using a Nusselt number defined with the film thickness, which is comparable to Nusselt number for falling films on hot surfaces. It is observed that convective effects are more pronounced at the bubble rear compared to the bubble front.     

Emissions and wall temperatures for lean prevaporized premixed gas turbine combustor

Experimental studies are carried out for investigating emission and wall temperature for traditional gas turbine combustor converted to lean premixed prevaporized (LPP) combustor. Vortex chamber, air preheating system, flat flame burner and inlet temperature control system are designed. Vortex chamber was maintained at the main air inlet port for controlling secondary air flow rate and wall temperature. Kerosene/air mixture temperature at exit from burner and entering combustion chamber was kept constant at 650 K for all runs. Special considerations were given for measuring NO_X, UHC, CO, local A/F ratio, flame temperature, exhaust gases temperature and wall temperature. For swirl and non swirl cases, secondary air ratio and primary zone air/fuel ratio were varied. The different operating parameters affecting flame temperature through it is affecting on local A/F ratio which is the main parameter for controlling flame temperature, emissions and walls temperatures. Flat flame burner and vortex chamber are useful tools for reducing emission and controlling walls temperatures. The inner liner wall temperatures are more affected by primary zone equivalence ratio while the outer liner wall temperatures are more affected by secondary air flow rate. Semi empirical correlations for NO_X, UHC and CO concentrations, exhaust gases temperature and maximum inner liner wall temperature are carried out. Good agreement between the measured and the calculated results are obtained. The present results are useful for further development of the traditional gas turbine combustor converted to LPP combustor.     

Finite element analysis of natural convection in a triangular enclosure: Effects of various thermal boundary conditions

The phenomena of natural convection in a right-angled triangular enclosure is studied numerically. A penalty finite element analysis with bi-quadratic elements is used for solving the Navier-Stokes and energy balance equations. The detailed study is carried out in two cases depending on various thermal boundary conditions:

(a)

Vertical wall is uniformly or linearly heated while inclined wall is cold isothermal.

(b)

Inclined wall is uniformly or linearly heated while vertical wall is cold isothermal.

In all the cases horizontal bottom wall is adiabatic and the aspect ratio of the lengths of base and height is considered to be one. The present numerical procedure adopted in this investigation yields consistent performance over a wide range of parameters of Rayleigh number Ra(10³?Ra?10⁵) and Prandtl number Pr(0.07?Pr?1000) in all the cases mentioned above. Numerical results are presented in terms of stream functions, temperature profiles and Nusselt numbers. It has been found that at low Rayleigh numbers (Ra?10⁴), the isotherms are almost parallel near the bottom portion of the triangular enclosure while at Ra=10⁵, the isotherms are more distorted. This is because the heat transfer is primarily due to conduction for lower values of Rayleigh number. As Rayleigh number increases, there is a change from conduction dominant region to convection dominant region, and the critical Rayleigh number corresponding to on-set of convection is obtained. It has been shown that the average Nusselt number for vertical wall is times that of the inclined wall as the bottom wall is adiabatic and that verifies the thermal equilibrium of the system for all case studies.     

STEADY FILMWISE CONDENSATION ON A FINITE-SIZE HORIZONTAL DISK IN A HOMOGENEOUS POROUS LAYER

This study employs a boundary layer approximation to solve steady filmwise condensation in a homogeneous porous layer near a cold finite-size horizontal disk. Numerical solutions are obtained using the fourth-order Runge-Kutta scheme with a shooting method. A theoretical analysis is performed to predict the influence of suction effects on the rate of condensation heat transfer. The dimensionless average Nusselt number and the thickness of the dimensionless liquid film on the disk surface are examined at various Darcy numbers, Jakob numbers, Prandtl numbers, modified Rayleigh numbers, and suction parameters. Finally, an algebraic correlation function is presented for the average Nusselt number based on the obtained numerical results.     

STEADY FILMWISE CONDENSATION ON A FINITE-SIZE HORIZONTAL DISK IN A HOMOGENEOUS POROUS LAYER

This study employs a boundary layer approximation to solve steady filmwise condensation in a homogeneous porous layer near a cold finite-size horizontal disk. Numerical solutions are obtained using the fourth-order Runge-Kutta scheme with a shooting method. A theoretical analysis is performed to predict the influence of suction effects on the rate of condensation heat transfer. The dimensionless average Nusselt number and the thickness of the dimensionless liquid film on the disk surface are examined at various Darcy numbers, Jakob numbers, Prandtl numbers, modified Rayleigh numbers, and suction parameters. Finally, an algebraic correlation function is presented for the average Nusselt number based on the obtained numerical results.     

Natural convection in a heat generating fluid bounded by two horizontal concentric cylinders

Convective flow and heat transfer of a Boussinesq fluid contained between two horizontal concentric cylinders is investigated under the effects of two driving mechanisms – an externally-imposed temperature gradient across the annulus, and a uniform internal heat generation. Numerical results for flow field and temperature distribution are obtained in terms of four dimensionless parameters, namely the radius ratio, R, the Prandtl number, Pr, the Rayleigh number, Ra*, and the ratio, S, between the characteristic temperature induced by internal heating and the applied temperature difference between the boundaries. Depending on the value of S, the flow pattern is made up of either one or two vortices in each half cavity, and heat is transferred into or out of the cavity through the hot wall. In particular, for a certain value of the applied temperature difference, the hot wall apparently acts as a thermally-insulated boundary, the internal heat is completely lost through the cold wall, and the fluid undergoes a transition from a bicellular to a unicellular flow regime.     

封闭腔内水自然对流换热数值模拟

为了揭示封闭腔内非Boussinesq流体在浮力驱动下所特有的流动换热现象和形成机理,采用CFD软件Fluent对封闭腔内水的自然对流进行数值模拟,得到矩形封闭腔高宽比、Rayleigh数、倾斜角度、壁面温度差对流动和传热的影响规律。研究结果表明：由于水的密度在3.98℃达到最大,两竖壁面温度跨越这一点时会引起流动图像反转;具有流动反转的双涡结构降低了对流换热平均Nusselt数;相同Rayleigh数下,高宽比为1对应对流换热平均Nusselt数最大值;倾斜角度对平均Nusselt数影响与Rayleigh数和温度边界条件有关。     

TRANSIENT BEHAVIOR OF NATURAL CONVECTION INSIDE A VERTICAL CYLINDER

The transient behavior for natural convection heat transfer within a vertical cylindrical enclosure was studied using the SIMPLER numerical algorithm. Initially, the temperature of the internal fluid was uniform. The unsteady state heat transfer was initiated by a temperature step change applied to all boundaries. The boundary temperature was then increased at the same rate as the center temperature until pseudosteady state was achieved. (Pseudosteady state is comparable to steady state convection for a fluid with uniform heat generation.) The transient heat transfer coefficient and the intensity of flow, along with the magnitude and the location of the minimum temperature, mean temperature, and maximum stream function are presented. Temperature and stream function contours are used to show the penetration of heat from the boundary and the development of the flow patterns. The local Nusselt number is also provided as supplementary information. A cylinder with HID = 1 was considered, with the Prandtl number of the fluid equal to 7 and a Rayleigh number of 10⁵. A characteristic length defined as 6 x (volume)/(surface area), which was found to produce the best regression results in our previous studies, was used in defining the Nusselt number and the Rayleigh number. The initial heat transfer rate was found to be correlated by

Nu =0.938 (Dimensionless Time) ^-0567

Oscillatory phenomena were found in support of the prediction made by Patterson and Imberger.