Numerical Model for Heat Transfer in Dilute Turbulent Gas-Particle Flows

The heat transfer between a vertical pipe wall and turbulent gas-particle flow is numerically investigated according to the Eulerian-Lagrangian approach and the k-ε turbulence model. The particles are introduced homogeneously into the simulation volume by a unique technique referred to as an artificial feeding volume. The numerical code using additional computer programs is validated with available experimental results for the constant heat flux boundary condition. An average deviation of about 4% and a maximum deviation of about 7% were attained from the numerical predictions for various particle and pipe diameters. The effect of the geometrical parameters and the flow parameters on the gas/particle temperature, the convection heat transfer coefficient between the wall and the gas-particle mixture, and the thermal entry length were studied. An increase in particle diameter (loading ratio ≈ 0.5) extended the thermal entry length and decreased the bulk mixed temperature, particle temperature, and convection heat transfer coefficient. Increasing the pipe diameter led to a significant reduction in bulk mixed temperature and thermal entry length, in addition to a decrease in particle temperature and Nusselt number. Increasing the loading ratio up to 2.36 led to a reduction in wall temperature and bulk mixed temperature, in addition to an increase in the convective heat transfer coefficient and thermal entry length.     

Investigation of turbulent heat transfer and nanofluid flow in a double pipe heat exchanger

In present study, heat transfer and turbulent flow of water/alumina nanofluid in a parallel as well as counter flow double pipe heat exchanger have been investigated. The governing equations have been solved using an in-house FORTRAN code, based on finite volume method. Single-phase and standard k-ε models have been used for nanofluid and turbulent modeling, respectively. The internal fluid has been considered as hot fluid (nanofluid) and the external fluid, cold fluid (base fluid). The effects of nanoparticles volume fraction, flow direction and Reynolds number on base fluid, nanofluid and wall temperatures, thermal efficiency, Nusselt number and convection heat transfer coefficient have been studied. The results indicated that increasing the nanoparticles volume fraction or Reynolds number causes enhancement of Nusselt number and convection heat transfer coefficient. Maximum rate of average Nusselt number and thermal efficiency enhancement are 32.7% and 30%, respectively. Also, by nanoparticles volume fraction increment, the outlet temperature of fluid and wall temperature increase. Study the minimum temperature in the solid wall of heat exchangers, it can be observed that the minimum temperature in counter flow has significantly reduced, compared to parallel flow. However, by increasing Reynolds number, the slope of thermal efficiency enhancement of heat exchanger gradually tends to a constant amount. This behavior is more obvious in parallel flow heat exchangers. Therefore, using of counter flow heat exchangers is recommended in higher Reynolds numbers.     

Transient mixed convection with internal heat generation and oscillating plate temperature

Summary. Oscillating plate temperature effects on transient mixed convection heat transfer from a porous vertical surface with internal heat generation or depletion are considered. The governing equations are transformed into dimensionless form by a set of variables and then solved using the finite element method. It is found that the velocity inside the boundary layer increases and the temperature decreases as time passes. In addition, it is found that when the energy generation is increased, the temperature near the wall will be higher than the wall temperature, and the velocity inside the boundary layer will increase due to an increase of buoyancy forces. Increasing the energy depletion term decreases the velocity inside the boundary layer and increases the heat transfer rate. Different temperature and velocity profiles are drawn for different dimensionless groups. Numerical values of Nusselt number as well as local skin friction coefficient are tabulated. Comparison with previous works shows complete agreement.     

Non-Darcy mixed convection flow with thermal dispersion on a vertical cylinder in a saturated porous medium

Summary The non-Darcy mixed convection flow on a vertical cylinder embedded in a saturated porous medium has been studied taking into account the effect of thermal dispersion. Both forced flow and buoyancy force dominated cases with constant wall temperature condition have been considered. The governing partial differential equations have been solved numerically using the Keller box method. The results are presented for the buoyancy parameter which cover the entire regime of mixed convection flow ranging from pure forced convection to pure free convection. The effect of thermal dispersion is found to be more pronounced on the heat transfer than on the skin friction and it enhances the heat transfer but reduces the skin friction.     

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.     

Experimental research and numerical simulation on cryogenic line chill-down process

The empirical heat transfer correlations are suggested for the fast cool down process of the cryogenic transfer line from room temperature to cryogenic temperature. The correlations include the heat transfer coefficient (HTC) correlations for single-phase gas convection and film boiling regimes, minimum heat flux (MHF) temperature, critical heat flux (CHF) temperature and CHF. The correlations are obtained from the experimental measurements. The experiments are conducted on a 12.7 mm outer diameter (OD), 1.25 mm wall thickness and 7 m long stainless steel horizontal pipe with liquid nitrogen (LN₂). The effect of the lengthwise position is verified by measuring the temperature profiles in near the inlet and the outlet of the transfer line. The newly suggested heat transfer correlations are applied to the one-dimensional homogeneous transient model to simulate the cryogenic line chill-down process, and the chill-down time and the cryogen consumption are well predicted in the mass flux range from 26.0 kg/m² s to 73.6 kg/m² s through the correlations.     

Flow and heat transfer in outlet pipes of cryostatting systems

Flow and heat transfer at mixed convection in the vertical channel connecting a cryogenic vessel and a room temperature zone are considered. The two-dimensional problem of conjugated heat transfer in the metal wall of the channel and in its cavity is solved by the finite difference method. The calculated values of the heat flux into the cold zone and of the temperature of the hot pipe end at different channel wall thicknesses, lengths, diameters, helium flowrates, as well as at different constants of the interaction of heat with the environment are given.A. V. Lykov Heat and Mass Transfer Institute, Academy of Sciences of Belarus, Minsk. Translated from Inzhenerno-fizicheskii Zhurnal, Vol. 63, No. 6, pp. 665–672, December, 1992.     

重力铯热管等温性能研究

为了验证铯热管研制的关键技术,通过测量热管外壁温度,在定温条件下研究了重力铯热管的等温特性和启动性能;同时,分析了冷凝段长度对铯热管等温性能的影响。实验结果表明:当加热炉温度在330~630℃,铯热管均能正常启动;加热炉温度越高,启动越快;在该温区,铯热管具有优良的传热性能;然而,当冷凝段长度为300mm时,铯热管壁面温度出现锯齿状周期波动。从过热度的角度,分析了铯热管内部的沸腾相变传热机理;选择合适长度的冷凝段可避免周期性间歇沸腾的产生。实验结果同时也了证明铯热管在330~630℃温区可作为高效的传热元件,非常适合复现ITS-90国际温标锌凝固点。     

青藏铁路冻土重力热管传热特性的研究

应用重力热管是解决青藏铁路建设中冻土路基冻融问题的一个较为实际的方法.本文通过分析重力热管的热阻网络,提出了重力热管的传热模型,并利用现有的热管内部传热的经验关系式,对不同倾角、不同管径以及不同冷凝管长度条件下的传热量进行模拟计算.     

多壁碳纳米管水基纳米流体的对流换热特性

实验研究了纳米粉体浓度、雷诺数Re和热流密度对多壁碳纳米管水基纳米流体(MWNTs/H2O)对流换热性能的影响。纳米粉体浓度分别为0.05 g/L、0.1 g/L、0.2 g/L和0.4 g/L,雷诺数Re为500~900,热流密度为10~20 k W/m2。结果表明:1)纳米流体对流换热系数随着纳米粉体浓度、Re、热流密度的增加而增加。如在Re为631且纳米粉体浓度为0.4 g/L时,纳米流体对流换热系数比基液增大了17.6%;2)纳米流体对流换热系数的提高率明显大于对应的导热系数提高率,当纳米粉体浓度为0.05g/L时,其对流换热系数和导热系数的提高率分别为7.4%和0.15%;3)在Eubank-Proctor方程的基础上,建立了适合于低Re条件下的混和对流换热的实验关联式。     

Optimizing the Design of Space Radiators

A procedure for optimizing the configuration of a heat pipe/fin radiating element in terms of heat rejected per radiating mass has been developed. The optimization was carried out analytically by expressing the heat radiated per radiating mass in terms of a function involving a dimensionless heat transfer coefficient and the dimensionless thermal gradient at the root of the fin where it joins the heat pipe. The dimensionless Stefan–Boltzmann radiation equation was solved numerically to determine the value that maximizes the function that determines the heat transfer per radiating mass. Once this value is obtained, the optimum width and thickness of the fins as well as the heat radiated per mass can be specified in terms of the operating temperature, emissivity, diameter, and mass/length of the heat pipe, and the density and thermal conductivity of the fin material. The resulting analytical expression can then be used to determine the maximum heat radiated per radiating mass over a wide range of operating conditions, to optimize the design of a specific heat pipe/fin combination, and to conduct analyses of the influence of design and materials properties on the performance of the system. The optimization procedure was carried out for the case of uniformly tapered fins as well as for flat fins.     

土壤源热泵水平螺旋地埋管换热器换热性能研究

通过建立水平螺旋地埋管与回填材料以及土壤的三维换热数学模型,对水平螺旋地埋管换热器的换热性能进行研究。对影响水平螺旋地埋管与土壤之间换热性能的因素进行分析,包括水的流速、进口水温、螺旋环径、相邻环中心间距、回填材料等。研究结果表明这些影响因素对换热性能（单位管长换热量和单位土壤长度换热量）有着不同程度的影响。     

Thermal performance of cold storage in thermal battery for air conditioning

This article studies, experimentally and theoretically, the thermal performance of cold storage in thermal battery for air conditioning. Thermal battery utilizes the superior heat transfer characteristics of heat pipe and eliminates drawbacks found in the conventional thermal storage tank. Experimental investigations are first conducted to study the cold storage thermal performance in two experimental systems: the ratio of distance between heat pipes to outer diameter of heat pipe W/D=6 and 2. Different heat transfer mechanisms including nucleate boiling, geyser boiling and natural convection are identified in different experimental systems with various liquid fills. A theoretical model to determine the thermal characteristics of the thermal battery has also been developed. Comparisons of this theory with experimental data show good agreements in the nucleate boiling stage of cold storage process.     

基于超导磁储能(SMES)应用的低温热管冷凝段建模与分析

针对超导磁储能系统(SMES)磁体直接冷却G-M制冷机冷头与磁体之传导路径较长造成超导磁体轴向温差的问题,提出了采用低温热管进行均温的方案,以对流换热系数为目标函数建立了相应的热管冷凝数学模型,较之于传统的模型所建模型考虑了二相流界面间的摩擦切应力,所建模型适用于SMES冷却系统降温和失超阶段传递热流量较大工况的传热分析.     

Analysis of multi-layered filament-wound composite pipes under combined internal pressure and thermomechanical loading with thermal variations

The composite pipes manufactured by filament winding technology have anisotropic behavior owing to different reinforced ply angles. Composite pipes can be exposed to the thermomechanical loading due to hot fluid that flows into them. In this paper, based on the three-dimensional anisotropic elasticity, an exact elastic solution for thermal stresses and deformations of the pipes under internal pressure and a temperature gradient has been studied. Giving heat convection conditions the variation of temperature field within the pipe is obtained by solving the conduction equation at the wall. The influence of temperature field in the governing equations of thermoelasticity has been considered via a constitutive law. The shear extension coupling is also considered because of lay-up angles. Stress, strain and deformation distributions for different angle-ply pipe designs are investigated using the present theory.     

方腔内Al2O3-H2O纳米流体热毛细对流的格子Boltzmann模拟

采用格子Boltzmann方法研究纳米颗粒形状影响下方腔内纳米流体热毛细对流的强化传热效果,主要分析了纳米粒子体积分数、颗粒形状以及Marangoni数Ma等相关参数对于纳米流体热毛细对流换热过程的影响。结果表明:长径比（长/半径）对纳米流体换热效果有影响,形状因子越大,平均Nu数Nu_ave越大。随着体积分数的增加,棒状、盘状和正方体状纳米颗粒均使热毛细对流的Nu_ave数减少,球状纳米颗粒条件下热毛细对流的Nu_ave数增加。Ma数越大,纳米流体热毛细对流的自由表面速度越大,对流换热效果也随之增强。      

Influence of developing flow on the heat transfer in laminar oscillating pipe flow

A theoretical analysis was performed to evaluate the influence of developing flow on the heat transfer associated with laminar oscillating pipe flow. Simplified analytical approaches are briefly discussed before an investigation based on the numerical solution of the conservation equations for mass, momentum and energy is presented, assuming constant wall temperature and an incompressible viscous fluid. Focusing on operating conditions as found in heat exchangers of regenerative thermal machines, like Stirling engines or Vuilleumier heat pumps, numerous calculations of the flow field and the heat transfer have been executed covering wide ranges of the characteristic dimensionless groups. The results are presented in tems of correlations of the mean Nusselt number averaged on the pipe length and a distribution function describing the local heat transfer. Furthermore, it is shown that the derived correlations are also applicable to compressible fluid flow, provided that the relative pressure amplitude is within the limits typical of regenerative thermal machines.     

套管式换热器结构变化对换热能力影响的模拟研究

运用ANSYS公司的CFX计算流体软件对内径分别为10mm、16mm,壁厚1mm,管段长1000mm的逆流式套管换热器在内管安置同心和偏心等3种状态在不同工况下进行了数值模拟,得到了管内流体的速度与温度分布,并比较了3种情况下的换热系数。从结果分析得到随着偏心距的增加,套管的换热效果逐步下降。     

Condensation of pure refrigerants R12, R134a and their mixtures on a horizontal tube with capillary structure: An experimental study

We experimentally studied free convection condensation heat transfer of pure refrigerants R12, R134a, and their mixtures on a horizontal single tube. Approximately equimolar mixtures of these refrigerants are azeotropic. The outside surface of the tube used had a capillary structure. The tube was integrated in an experimental set-up in a way that allowed its rotation around the axis. Movable thermocouples inserted in the tube wall enabled the determination of the average surface temperature. This temperature, the vapour bulk temperature, and the heat flux obtained from condensate collection served for the determination of the heat transfer coefficient. The condensation heat transfer of the pure refrigerants examined is observed to change with the driving temperature difference largely in accordance with the Nusselt theory. The experimental values of the heat transfer coefficient on the tube used, however, are by a factor of 2 larger than those on a smooth tube according to this theory. Under comparable conditions, the refrigerant R134a shows by 10 to 15% better heat transfer than R12. The heat transfer of mixtures decisively depends on the compositions of their phases. Basically, the stronger the compositions of the phases differ from each other, the lower the heat transfer coefficients; they always lie below those of R134a. In the range of low temperature difference, the heat transfer coefficient of mixtures increases with the temperature difference. This is the region of the so-called partial condensation. At a larger temperature difference, a local total condensation of the mixtures takes place and the heat transfer qualitatively follows the Nusselt theory.     

Combined convection in an annulus applied to a thermal storage problem

In a solar energy application involving thermal storage, one of the heat transfer situations is that of combined convection in vertical annuli for rather complex wall thermal boundary conditions. Predictive data of a high order of reliability are needed for incorporation within the suite of programs treating the whole problem. The program reported here treats the complete equations for combined free and forced convection in a vertical annulus. It allows for viscosity and density variation with temperature, and a variable heat flux or temperature at the walls. It was developed from a similar program for circular tubes. Comparisons are made with published data for velocity profiles and heat transfer performance. These are good, and show the step-wise energy balance method is necessary and valid. The strategy of generation of the required data is explained, together with sample output. These data are themselves analysed computationally; the performance equations agree with original predictions typically to within ± 11 per cent, with a standard deviation of around 2 per cent. The working fluid is water with 37 per cent ethylene glycol. Upward heated and downward cooled flows give aided combined convection. For the given design, laminar flow is predicted for the Reynolds number range 1800–2200.