Towards a Thermal Optimization of a Methane/Steam Reforming Reactor

Plug-flow reactors are very common in methane/steam reforming applications. Their operation presents many challenges, such as a strong dependence on temperature and inlet composition distribution. The strong endothermic steam reforming reaction might result in a temperature drop at the inlet of the reactor. The strong non-uniform temperature distribution due to an endothermic chemical reaction can have tremendous consequences on the operation of the reactor, such as catalyst degradation, undesired side reactions and thermal stresses. One of the possibilities to avoid such unfavorable conditions and control thermal circumstances inside the reforming reactor is to use it as a fuel processor in the solid oxide fuel cell (SOFC) system. The heat generated by exothermic electrochemical SOFC reactions can support the endothermic reforming reaction. Furthermore, the thermal effects of electrochemical reactions help to shape the uniform temperature distribution. To examine thermal management issues, a detailed modeling and corresponding numerical analyses of the phenomena occurring inside the internal reforming system is required. This paper presents experimental and numerical studies on the methane/steam reforming process inside a plug-flow reactor. Measurements including different thermal boundary conditions, the fuel flow rate and the steam-to-methane ratios were performed. The reforming rate equation derived from experimental data was used in the numerical model to predict gas composition and temperature distribution along the steam reforming reactor. Finally, an attempt was made to control the temperature distribution by adopting locally controlled heating zones and non-uniform catalyst density distributions.     

Heat transfer characteristics of water and APG surfactant solution in a micro-channel heat sink

The steady increase in internal heat production of cost and high performance electronic components has lead researchers to seek improved ways to remove the heat generated. Single-phase liquid flow has been considered as a potential solution for solving this cooling problem. However, when considering that any solution needs to be of low cost and low mass fluxes and yet retain low temperature gradients across the electronic components, it seems that two-phase boiling flow is preferred. Surfactant solutions have been introduced in connection with enhancement of the boiling processes. We investigated the effects of surfactant solution flows through a micro-channel heat sink. The experimental setup included a high-speed IR radiometer and a CCD camera that were used to characterize the test module. The module consisted of inlet and outlet manifolds that distributed surfactant solutions through an array of 26 parallel micro-channels. The experimental results have shown that there exists an optimal solution concentration and mass flux for enhancing heat removal. Surfactant solution boiling flows were also found to stabilize the maximum and average surface temperatures for a wide range of applied heat fluxes. In addition, the use of surfactant solutions at low mass fluxes has led to CHF enhancement when compared to regular water flows. In the last part of this work, possible explanations for the observed non-ionic surfactant effects are presented.     

电子封装、QFP和MCM热变形的实验及数值分析

0Introduction Anelectronicpackageisgenerallyconstructedwithanactivesiliconchip,mountisland,gold wires,leadframesandsoldersasshowninFig.1(a).Toprotectfromtheenvironment,thesilicon chipisusuallyencapsulatedinresin.SincethesematerialshavedifferentCTE(coeffic…     

Enhanced convection from horizontal printed-circuit board with lifted electronic components

Convective heat transfer from the electronic component on a simulated horizontal printed-circuit board (PCB) assembly to the ambient air was found to be enhanced significantly by lifting the electronic components a vertical protrusion off from the board's surface. Effects of the gap between the lifted electronic component and the PCB's surface, the electronic component's pitch-to-height ratio and the Reynolds number of the air-flow on the heat dissipation from the PCB assembly were investigated experimentally. Heat transfer measurements were obtained for duct clearance-based Reynolds numbers from 400 to 19000. Enhanced convection was obtained by increasing the vertical protrusion of the lifted component, especially when the Reynolds number is high. When Reynolds number is lower than 2100, a maximum vertical protrusion of 6mm is recommended. Correlations for the prediction of the average heat transfer coefficient of the simulated horizontal PCB assembly by adopting the lifted electronic components were proposed. Received on 22 May 1997     

TEC结构的三维非线性瞬态温度场分析

热电制冷器(TEC)以其体积小、作用速度快及无噪音等机械制冷无法替代的优点在航空航天和电子工业等领域得到了越来越广泛的应用。本文根据TEC的导热特点,推导了TEC结构稳态温度场的解析解,建立了其瞬态非线性温度场分析的微分方程。利用伽辽金法导出TEC结构热分析的有限元方程,对非线性热分析的有限元方程进行了求解,得到了TEC的稳态温度场和瞬态响应温度场。算例结果表明,本文提出的TEC结构热分析有限元模型具有较高的精度,能够有效地分析TEC的非线性瞬态温度场。     

Large eddy simulation in Code_Saturne of thermal mixing in a T junction with brass walls

Following on from Kuhn et al (2010) we study the capability of large eddy simulation with conjugate heat transfer to predict thermal fluctuations with thermal mixing. Wall functions are used to model the wall heat transfer. Comparison with experimental results show that the temperature variance on the outer skin of the solid is well predicted by the simulation. It is shown that the variance of thermal flux in the fluid closely maps the temperature variance at the outer boundary of the solid. Since the variance of thermal flux is closely related to the dissipation of temperature variance it can be concluded that the dissipation of temperature variance in the fluid is linked to temperature variance in the solid. Analysis of the equation of the temperature variance in the solid confirms this is indeed the case. It is the dissipation of temperature variance in the fluid that characterizes how the temperature variance penetrates the solid. Thus RANS modelling can be used to predict thermal variance in solids provided that there is an accurate model for the dissipation of temperature variance at the wall and an equation for the thermal variance in the solid is solved.     

L- and U-shaped heat pipes thermal modules with twin fans for cooling of electronic system under variable heat source areas

This study utilizes a versatile superposition method with thermal resistance network analysis to design and experiment on a thermal module with embedded six L-shaped or two U-shaped heat pipes and plate fins under different fan speeds and heat source areas. This type of heat pipes-heat sink module successively transfer heat capacity from a heat source to the heat pipes, the heat sink and their surroundings, and are suitable for cooling electronic systems via forced convection mechanism. The thermal resistances contain all major components from the thermal interface through the heat pipes and fins. Thermal performance testing shows that the lowest thermal resistances of the representative L- and U-shaped heat pipes-heat sink thermal modules are respectively 0.25 and 0.17 °C/W under twin fans of 3,000 RPM and 30 × 30 mm² heat sources. The result of this work is a useful thermal management method to facilitate rapid analysis.     

Numerical and Experimental Evaluation of Dispersion Coefficient for Resin Transfer Modeling

Resin transfer molding (RTM) is a composite manufacturing process. A preformed fiber is placed in a closed mold and a viscous resin is injected into the mold. In this article, a model is developed to predict the flow pattern, extent of reaction, and temperature change during the filling and curing in a thin rectangular mold. A numerical simulation is presented to predict the free surface and its interactions with heat transfer and cure for flow of a shear-thinning resin through the preformed fiber.To simulate this process, using local thermal equilibrium assumption, it is essential to include the thermal dispersion term in energy equation. The best method to achieve this result is experimental simulation and preparing proportionate system at simple conditions without curing. By comparison of recorded temperature values (using installed instruments at various locations), and the corresponding results from numerical solution for different estimated values of dispersion coefficient, this coefficient has been evaluated based on the best matching estimate. The results show that, to simulate composite manufacturing process by RTM method, the effect of dispersion term in energy equation shall not be neglected.     

Natural convection of a viscoelastic fluid

The influence of elasticity and shear thinning viscosity on the temperature distribution and heat transfer in natural thermal convection is discussed. The numerical investigations are based on a four-parameter Oldroyd constitutive equation, which represents the typical fluid response of dilute solutions and melts. It was found that especially the second normal-stress difference affects the heat transfer mechanism.     

Topology optimization and heat dissipation performance analysis of a micro-channel heat sink

Junction temperature in the electronic packaging process is one of the critical factors affecting the service life of electronic devices. A micro-channel heat sink is a common heat dissipating device used to reduce the thermal resistance between components and substrate. In order to maximize the heat dissipation while minimizing the pressure drop, this paper adopts a topology optimization method. A material interpolation method based on variable density principle is used together with a moving asymptote algorithm for the optimization. The physics is governed by the heat and mass transfer, coupled with the momentum conservation in the fluid. Four parameters are varied in order to investigate their influence on the optimization process. A three-dimensional geometry has been constructed to study the flow field and the results are compared to a reference case to verify the temperature uniformity and thermal performance of the model. It is demonstrated that the optimized design of the micro-channel heat sink is reliable and effective.     

Development of a modular zeolite-water heat pump

The working pair zeolite-water has very good characteristics for the heat pump application. It is non-poisonous, non-flammable and low-corrosive so that the use of a zeolite-water heat pump in the large field of domestic heating is very promising. The poor heat and mass transfer of the zeolite has to be considered by an appropriate design of the adsorber heat exchanger. Compact zeolite layers directly linked with the heat exchanger enable a high specific thermal output (thermal output related to the mass of zeolite) which is the main shortcoming of these machines. Additionally the coefficient of performance (COP) can be improved significantly by a modular design of the machine consisting of six to eight heat pump modules. Due to the periodical operating mode which is required by the zeolite-water pair the single module is built up in a simple way without any moving parts. The different modules, each of them operating in another phase of the sorption cycle, are connected in series by a heat transfer medium circuit so that a continuous thermal output together with high COP is achieved by this zeolite-water heat pump. First experimental investigations focus on the layout of the different components of the heat pump, e.g. the single module, the adsorber/desorber and the evaporator/condenser. The paper will present the design of these components as well as the design of the entire modular machine. Furthermore there will be a theoretical discussion of the COPs of the modular heat pump depending on the ambient temperature, on the number of modules and on the heating system. Received on 12 November 1998     

Thermo-mechanically coupled investigation of steady state rolling tires by numerical simulation and experiment

In this contribution, a numerical framework for the efficient thermo-mechanical analysis of fully 3D tire structures (axisymmetric geometry) in steady state motion is presented. The modular simulation approach consists of a sequentially coupled mechanical and thermal simulation module. In the mechanical module, the Arbitrary Lagrangian Eulerian (ALE) framework is used together with a 3D finite element model of the tire structure to represent its temperature-dependent viscoelastic behavior at steady state rolling and finite deformations. Physically computed heat source terms (energy dissipation from the material and friction in the tire–road contact zone) are used as input quantities for the thermal module. In the thermal module, a representative cross-sectional part of the tire is employed to evaluate the temperature evolution due to internal and external heat sources in a transient thermal simulation. Special emphasis is given to an adequate material test program to identify the model parameters. The parameter identification is discussed in detail. Numerical results for three different types of special performance tires at free rolling conditions are compared to experimental measurements from the test rig, focusing especially on rolling resistance and surface temperature distribution.     

Heat transfer mechanism of miniature loop heat pipe with water-copper nanofluid: thermodynamics model and experimental study

In order to ensure the normal work of electronic product, the thermal management is of key importance. Miniature loop heat pipe (mLHP) is a promising device of heat transfer for electronic products. Cu-water nanofluid with different concentration is used as working material in mLHP. Experiments are conducted to investigate its heat transfer performance. The heat flux owing to thermal diffusion is calculated. It is found that this heat flux and the boiling temperature are non-monotonic function of concentration of nanoparticle. Turning concentration appears at about 1.5 wt%. Differential equation of thermal diffusion produced by micro movement of nanoparticle is established in this paper. Average speed formula for nanoparticles is derived and slope of the curve of phase equilibrium is obtained. Based on the theoretical research in this paper, enhanced heat transfer mechanism of nanofluid is analyzed. The facts that heat flux owing to thermal diffusion and boiling temperature are all associated with nanoparticle concentration are also well explained with the aid of the derived theory in this paper.     

Dynamical temperature and generalized heat-conduction equation

By means of a dynamical non-equilibrium temperature we derive a generalized heat-conduction equation which accounts for non-local, non-linear, and relaxation effects. The dynamical temperature is also capable to reproduce several enhanced heat equations recently proposed in literature. The heat flux is supposed to be proportional to the gradient of the dynamical temperature, and the material functions are allowed to depend on temperature. It is also pointed out that the heat flux cannot assume arbitrary values, but it is limited from above by a maximum value which ensures that the thermal conductivity remains positive.     

Analytical model for melting in a semi-infinite PCM storage with an internal fin

The most PCMs with high energy storage density have an unacceptably low heat conductivity and hence internal heat transfer enhancement techniques such as fins or other metal structures are required in latent heat thermal storage (LHTS) applications. Previous work has concentrated on numerical and experimental examination in determining the influence of the fins in melting phase change material. This paper presents a simplified analytical model based on a quasi-linear, transient, thin-fin equation which predicts the solid-liquid interface location and temperature distribution of the fin in the melting process with a constant imposed end-wall temperature. The analytical results are compared to the numerical results and they show good agreement. Due to the assumptions made in the model, the speed of the solid-liquid interface during the melting process is slightly too slow.     

Stagnation point heat transfer from laminar,high temperature methane flames

Combustion of methane-rich fuels frequently provides forced convective heating in industry, and the ability to predict the rate of heat transfer from such flames to solid surfaces is often desirable. Mathematical modelling of stagnation point heat flux has been achieved by numerical solution of the boundary layer equations, and by an analytical equation modified to include the effects of chemical reaction in the free stream flow and to allow for the enhancement in heat flux caused by the diffusion and exothermic recombination of reactive species in the boundary layer surrounding the heat receiving body. Predictions from these models have been compared with experimental data obtained in high temperature methane flames of various equivalence ratios. Within the equilibrium region of these flames, predictions from the modified analytical equation based on total Lewis numbers equal to and greater than one form a tight envelope around the experimental results, and hence provide a relatively simple method of predicting heat flux. Numerical solutions tend to slightly underestimate predictions from the analytical equation and experimental data, although agreement with the alternative prediction method increases with the surface temperature of the heat receiving body     

Thermal system identification using fractional models for high temperature levels around different operating points

This work aims at the preparation of an experiment for the thermal modeling of an ARMCO iron sample (iron of the American Rolling Mill COmpany) for small temperature variations around different operating points. Fractional models have proven their efficacy for modeling thermal diffusion around the ambient temperature and for small variations. Due to their compactness, as compared to rational models and to finite element models, they are suitable for modeling such diffusive phenomena. However, for large temperature variations, thermal characteristics such as thermal conductivity and specific heat vary along with the temperature. In this context, the thermal diffusion obeys a nonlinear partial differential equation and cannot be modeled by a single linear model. In this paper, thermal diffusion of the iron sample is modeled around different operating points for temperatures ranging from 400 to 1070?K, which is above the Curie point (In physics and materials science, the Curie temperature (T _C), or Curie point, is the temperature at which a ferromagnetic or a ferrimagnetic material becomes paramagnetic.) showing that for a large range of temperature variations, a nonlinear model is required. Identification and validation data are generated by finite element methods using COMSOL Software.     

Nonsimilar Boundary Layer Analysis of Free Convection Heat Transfer Over a Vertical Cylinder in Bidisperse Porous Media

This work presents a boundary layer analysis for the free convection heat transfer from a vertical cylinder in bidisperse porous media with constant wall temperature. A boundary layer analysis and the two-velocity two-temperature formulation are used to derive the nonsimilar governing equations. The transformed governing equations are solved by the cubic spline collocation method to yield computationally efficient numerical solutions. The effects of inter-phase heat transfer parameter, modified thermal conductivity ratio, and permeability ratio on the heat transfer and flow characteristics are studied. Results show that an increase in the modified thermal conductivity ratio and the permeability ratio can effectively enhance the free convection heat transfer of the vertical cylinder in a bidisperse porous medium. Moreover, the thermal nonequilibrium effects are strong for low values of the inter-phase heat transfer parameter.     

Interaction of surface radiation with combined conduction and convection from a discretely heated L-corner

Prominent results pertaining to the problem of multi-mode heat transfer from an L-corner equipped with three identical flush-mounted discrete heat sources in its left leg are given here. The heat generated in the heat sources is conducted along the two legs of the device before being dissipated by combined convection and radiation into air that is considered to be the cooling agent. The governing equations for temperature distribution along the L-corner are obtained by making appropriate energy balance between the heat generated, conducted, convected and radiated. The non-linear partial differential equations thus obtained are converted into algebraic form using a finite-difference formulation. The resulting equations are solved simultaneously by Gauss–Seidel iterative solver. A computer code is specifically written to solve the problem. The computational domain is discretised using 101 grids along the left leg, with 15 grids taken per heat source, and 21 grids along the bottom leg. The effects of surface emissivity, convection heat transfer coefficient, thermal conductivity and aspect ratio on local temperature distribution, peak device temperature and relative contributions of convection and radiation to heat dissipation from the L-corner are studied in detail. The point that one cannot overlook radiation in problems of this class has been clearly elucidated.     

A fitting, simple and versatile window program (HSHPTM) design using lumped parameters and one-dimensional thermal resistance models

A heat sink-heat pipes thermal module (HSHPTM) applies U-type or L-type heat pipes to transferring the total heat capacity from the heat source to the based plate and fins successfully, and then dissipates heat flow into the surrounding air. This article utilizes Visual Basic commercial software to develop a window program named HSHPTM V1.0 for proper design of the heat sink-heat pipes thermal module. The computing core of the HSHPTM program employs the theoretical thermal resistance analytical approach with iterative convergence stated in this study to obtain a numerical solution. The results show that this calculated error comparison with experimental results is within ±5 %. The embedded U-type heat pipes carry 46 and 63 percentages of the total dissipated heat capacity for one-pair and two-pairs embedded U-type heat sink-heat pipes thermal modules, and dissipate 87.2 % heat flow for six embedded L-type heat pipes, respectively. There has a benefit for HSHPTM V1.0 of rapidly and capably calculating the thermal performance of a heat sink-heat pipes thermal module installed with a processor horizontally by inputting simple and lumped parameters.