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Thermal radiation is a significant mode of energy transmission besides conduction and convection in most combustion systems. Accurate treatment of thermal radiation is a crucial element for predicting the flame structure and pollutant formation. However, thermal radiation to fuel droplet evaporation is often neglected in the numerical simulations of two-phase spray combustion systems. In this paper, computational analysis on the thermal radiation to evaporation and combustion of fuel droplets in two-phase spray combustion is conducted. The influence of flame radiation on droplet evaporation and combustion is studied by adding radiation source term to the energy equation describing droplet evaporation process. Radiation transfer equation is solved, in which the radiation of CO 2 , H 2 O and soot is imposed to consider radiation effect of the flame. The partially premixed combustion model is employed, in which radiation heat transfer between droplet and ambient gases is taken into account. A reduced chemical reaction mechanism for n-heptane consisting of 76 species and 349 reactions is used. Four different numerical setups have been used and compared, and the simulation results coincide with the experiment data in terms of spray statistics and flame shapes. The peak flame temperature increases by 28 K when the radiative heat source term in the evaporation process is considered. The droplet evaporation is significantly enhanced by thermal radiation, leading to a promotion of 34.8% in C 7 H 16 concentration in the center region of combustor. The concentration of combustion products, such as H 2 O and CO 2 , increases by 13.2% in the reaction region. [ABSTRACT FROM AUTHOR]

In this paper, a numerical model describing the heat transfer and air flow inside and outside the casing of the permanent magnet brushless direct current (PM BLDC) motor is presented. In the model, a conjugate heat transfer including the heat conduction with heat sources in windings, magnets, bearings, natural and forced convection and radiation phenomena within internal and external air domain were analysed. The complex geometrical model included windings, a magnet circuit, a rotor with neodymium magnets, a PCB electronic plate, plastic covers, internal air between solid parts as well as a portion of the external air. The validation process of the developed model was realised on the basis of the velocity and temperature fields. The experimental data were recorded by 7 constant temperature anemometers at 4 levels above the motor housing, Laser Doppler Anemometry sensors in two axes inside the rear portion of the motor and 25 thermocouples fixed inside and outside the motor. The experimental and numerical tests were performed for 4 motor loadings and 3 rotational speeds. The average temperature error for the internal point was of 9 K, while that of the external points was of 2 K. The velocity field was very well predicted close to the housing wall in terms of horizontal and vertical components. A slightly worse agreement was found for the vertical component, especially in the vicinity of the shaft. The results were consistent for all the considered motor loadings and rotational speeds. [ABSTRACT FROM AUTHOR]

Artificial neural networks (ANN) have been widely used for thermal analysis of heat exchangers during the last two decades. In this paper, the applications of ANN for thermal analysis of heat exchangers are reviewed. The reported investigations on thermal analysis of heat exchangers are categorized into four major groups, namely (i) modeling of heat exchangers, (ii) estimation of heat exchanger parameters, (iii) estimation of phase change characteristics in heat exchangers and (iv) control of heat exchangers. Most of the papers related to the applications of ANN for thermal analysis of heat exchangers are discussed. The limitations of ANN for thermal analysis of heat exchangers and its further research needs in this field are highlighted. ANN is gaining popularity as a tool, which can be successfully used for the thermal analysis of heat exchangers with acceptable accuracy. [ABSTRACT FROM AUTHOR]

This paper presents the stability, convergence and optimization characteristics of interface treatments for steady conjugate heat transfer problems. The Dirichlet-Robin and Neumann-Robin procedures are presented in detail and compared on the basis of the Godunov-Ryabenkii normal mode analysis theory applied to a canonical aero-thermal coupling prototype. Two fundamental parameters are introduced, a “numerical” Biot number that controls the stability process and an optimal coupling coefficient that ensures unconditional stability. This coefficient is derived from a transition of the amplification factor. A comparative study of these two treatments is made in order to implement numerical schemes based on adaptive and local coupling coefficients, with no arbitrary relaxation parameters, and with no assumptions on the temporal advancement of the fluid domain. The coupled numerical test case illustrates that the optimal Dirichlet-Robin interface conditions provide effective and oscillation-free solutions for low and moderate fluid-structure interactions. Moreover, the computation time is slightly shorter than the time required for a CFD computation only. However, for higher fluid-structure interactions, a Neumann interface condition on the fluid side presents good numerical properties so that no relaxation coefficients are required. [ABSTRACT FROM AUTHOR]

In this paper, dispersion effects on forced convection heat transfer in a gaseous slip flow through a microtube filled with anisotropic porous medium are investigated semi-numerically. Microtube is subjected to constant heat flux and a local thermal non-equilibrium condition is assumed. Rarefaction effects are taken into account by applying first-order velocity slip and temperature jump boundary conditions. Dispersion is proven to have a significant impact on heat transfer in rarefied gases. Temperature distributions for both solid and fluid phases are obtained. Nusselt number variation with respect to porous shape factor and Biot number is illustrated, indicating enhancement up to 10% in predicted heat transfer due to dispersion. The influence of Reynolds number on thermal dispersion is proven to be insignificant. Dispersion plays a significant role as gas becomes more rarefied. It is seen that an increase in shape factor of a porous media can improve Nu for a non-rarefied gas, but as rarefaction effects grow, this can affect heat transfer negatively. Also, effects of dispersion on heat transfer diminishes for higher values of Biot Number due to the overcoming of interstitial heat transfer to thermal dispersion effect. [ABSTRACT FROM AUTHOR]

Partitioned approaches for the simulation of coupled conjugate heat transfer are gaining popularity in fields that require accurate thermal predictions. Considerable efforts have been put into determining stable coupling schemes, but performance enhancements have been neglected. This paper presents, for the first time, a detailed and comprehensive study of the numerical properties of Dirichlet-Robin coupling procedures, used in conjugate heat transfer simulation, with emphasis put on the optimal local coupling formulation that was recently derived from a stability analysis. This all-new optimal coupling approach provides local adaptability and has never been tested on a complex setup. This investigation looks to determine the relevance and the limitations of the theory when applied to complex conjugate heat transfer setups. The stability theory of the optimal Dirichlet-Robin coupling scheme is first recalled, then, a realistic 3D application, with complex geometry and flow structures, is used to evaluate the performance and sensitivity of Dirichlet-Robin couplings, with respect to various numerical parameters. This detailed study allows, for the first time, to evaluate the advantages and limitations of the recently proposed optimal procedure, when used on realistic 3D CHT problems. It turns out that the local optimal Dirichlet-Robin formulation outperforms all what is found in literature, and insures unconditional stability with monotone convergence for all considered setups of the 3D model. [ABSTRACT FROM AUTHOR]

In this paper, the heat transfer rate and the pressure drop of metal porous media inserts are analyzed in the single phase. The testing range of the Reynolds number is between 3654 and 14617. Tap water is used as the working fluid. The effect of particle size and particle shape on the heat transfer performance is studied. The results show that an optimal value existed among the testing samples for dendritic particle. The best heat transfer rate is resulted by the sample D-125 instead of by the sample D-25 with the small particle size, which is approximately 2.2–3.0 times as that of the smooth tube. This is mainly because the velocity at the inlet of inserts increases greatly and the fluid mixing intensity at the annular is also high. Two factors work together to bring about a best heat transfer coefficient. The friction factor increases with the decrease in the particle size. The best Performance Evaluation Criterion (PEC) value is resulted by sample D-125, which is about 1.26–1.71. The particle shape also has an impact on the heat transfer rate. The heat transfer coefficient of the spherical particle is higher than that of dendritic particle. However, the higher heat transfer rate is paid at the cost of high flow resistance. This high flow resistance finally deteriorates the heat transfer performance. As a result, the best heat transfer performance is achieved by the dendrictic particle, which brings a high heat transfer rate as well as a low flow resistance. Therefore, the PEC values have to be taken into consideration in the design of heat exchanger. [ABSTRACT FROM AUTHOR]

Active pipe-embedded building envelope can allow for substantial heat flow for relatively small temperature difference due to the embedded pipes in the external wall/roof. This structure can directly utilize the low-grade sources for reducing building cooling/heating load and improving indoor thermal comfort by intercepting the heat gain/loss through the building envelope due to the circular water inside the embedded pipe. This paper presents the experimental validation of a semi-dynamic simplified model of the active pipe embedded building envelope. The experiment test rig consists of two environment chambers with one for simulating the ambient environment and one for indoor environment with a pipe-embedded building envelope sample separating both chambers. A small chiller and a buffer tank are installed to provide stable water temperature for the test sample. The temperatures and heat fluxes on both sides of the test sample, the inlet and outlet water temperature as well as flow rate are measured. The measured boundary conditions are used as the inputs of the semi-dynamic simplified model for calculating the thermal performance of the active structure. The results show the semi-dynamic simplified model can predict the semi-steady or pure dynamic thermal performance of the pipe-embedded building envelope very well by comparing with the measurement. [ABSTRACT FROM AUTHOR]

This paper presents an investigation of the thermal performance of axial flow air cooling for lithium-ion battery pack. A thermal model for a battery pack with 32 single cells is developed and is numerically solved with coupling the fluid governing equations. The heat generation rate of the battery is calculated by a UDF (user defined function) program, which is based on a pseudo 2D model of the lithium-ion batteries. The effects of the radial interval between cells and air flux on the thermal performance of the axial flow air cooling system are investigated by numerical simulations. The results show that increasing the radial interval leads to a slight average temperature rise but benefits to the temperature uniformity of the battery pack. A larger radial interval can significantly reduce the power cost of the cooling system; however it causes a decrease of space efficiency mildly. A larger air flux is found to be benefit to the temperature uniformity within battery pack. An optimal region based on the assessment of the power cost, space efficiency and temperature homogeneity for the parameter designs of the battery cooling system is presented. [ABSTRACT FROM AUTHOR]

Abstract: Thermal actuators are extensively used in microelectromechanical systems (MEMS). Heat transfer through and around these microstructures are very complex. Knowing and controlling them in order to improve the performance of the micro-actuator, is currently a great challenge. This paper deals with this topic and proposes a dynamic thermal modelling of thermal micro-actuators. Thermal problems may be modelled using electrical analogy. However, current equivalent electrical models (thermal networks) are generally obtained considering only heat transfers through the thickness of structures having considerable height and length in relation to width (walls). These models cannot be directly applied to micro-actuators. In fact, micro-actuator configurations are based on 3D beam structures, and heat transfers occur through and around length. New dynamic and static thermal networks are then proposed in this paper. The validities of both types of thermal networks have been studied. They are successfully validated by comparison with finite elements simulation and analytical calculations. [Copyright &y& Elsevier]

For considering the non-equilibrium effect, the present paper uses the generalized dual-phase-lag model of bioheat transfer to describe the thermal behavior in a laser irradiated living tissue. The thermal damage is also estimated with the Arrhenius equation. The hybrid application of the Laplace transform and the modified discretization technique is employed to solve the present problem. The non-reduction of the generalized dual-phase-lag model to the Pennes equation is further explored. The effects of that the phase lag times depend on the porosity, heat capacities of blood and tissues, coupling factor, and the ratio of thermal conductivity of tissue and blood are taken into account. The results show that the generalized DPL bioheat transfer equation cannot reduce to the Pennes and classical DPL bioheat transfer equations, even with the effect of spatial heating source. Porosity and coupling factor are particularly important to the generalized DPL model. The relation between the phase lag times would demonstrate the characteristics of thermal response in tissue. [ABSTRACT FROM AUTHOR]

The paper focuses on modeling of heat, momentum and concentration transport in stable film boiling of a nanofluid over a vertical surface. An approximate analytical model of the transport processes in the vapor film was employed in simulations. The model takes into account effects of the Brownian and thermophoretic diffusion. The novelty of the model consists in pinpointing six major non-dimensional parameters, which describe effects of the nanoparticles on heat transfer and fluid flow in the vapor film. They include: (i) parameter A that accounts for the relation between the thermophoretic and Brownian diffusion; (ii) the nanoparticle concentration φ ∞ in the vapor; (iii and iv) the normalized densities of the nanoparticles R pυ and R pf ; (v) the relative thermal conductivity of the nanoparticles K ; and (vi) parameter m that characterizes the viscosity of nanofluids. Novel analytical solutions resulting from this model characterize velocity profiles, the mass flow rate, the thickness of the vapor film and the Nusselt number as the functions of the aforementioned parameters. It was demonstrated that an increase in the nanoparticles concentration fosters the processes of momentum, mass and heat transfer. [ABSTRACT FROM AUTHOR]

Active pipe-embedded building envelope is a new kind of pipe-embedded building external wall/roofs. Usually, pipes are embedded in the wall slab or roof slab to allow water circulating in these pipes for heat transfer. Low-grade energy sources can be used for this structure since the enlarged heat transfer surface between the slab mass and water in the pipe allowing for substantial heat flow for relatively small temperature difference. This structure may reduce building cooling/heating load and improve indoor thermal comfort by intercepting the heat/coolth from the ambient air to indoor space. This paper presents a semi-dynamic model of this structure for thermal performance evaluation for system design or indoor environment control etc. This semi-dynamic model is a coupled model. The first part is a simplified dynamic RC model (i.e., resistance and capacitance model). The heat transfer along the width of this structure can be predicted easily by the RC model. The second part is a classical NTU model for evaluating the heat transfer along the pipe and total heat transfer on both surfaces of this structure. A CFD model is developed to act as a virtual experimental test rig to simulate the thermal characteristics of this structure. The simulated results are used to validate this semi-dynamic model and its performance prediction. [ABSTRACT FROM AUTHOR]

In this paper, an uncertainty propagation methodology considering correlations among the input variables for heat transfer analysis through fibrous composite insulation material subjected to severe aerodynamic conditions is presented, based on Monte Carlo (MC) simulation with Cholesky decomposition. The effects of correlation coefficient on the thermal evolution uncertainty, failure probability, and Sobol sensitivity indexes are discussed. The results show that the thermal response outputs become more scattered after taking into account the dependence of variables. The failure probability increase with the rise of the correlation coefficient, and nearly 15% increase is observed when the temperature limit is 470 K under the current uncertainty level. Sobol sensitivity analysis demonstrates that the correlated group variables of aerodynamic thermal and pressure load have the most significant first order and total effect on the thermal response simulations. However, the mean fiber diameter and the correlated group variables of density, specific heat and thermal conductivity of virgin material have weak contributions to the first order effect, but have relatively strong contribution to the interaction effect with other variables. The correlation effect of input parameters causes different influences on the sensitivity indexes of different variables. In all cases, the weakest correlation gives the highest total sensitivity index. The provided analyses demonstrate that incorporation of correlation information of stochastic variables is essential in random variable characterization, thermal evolution uncertainty level quantification, parameter importance identification and risk assessment, and it is especially imperative for thermal protection components with high reliability requirement to prevent the occurrence of catastrophic event. [ABSTRACT FROM AUTHOR]

Abstract: This paper presents a numerical study of heat transfer from spheres settling under gravity in a box filled with liquid. Three-dimensional Lattice Boltzmann Method is applied to simulate fluid-particle interaction. Firstly the developed numerical model is validated in comparison with experimental and numerical data for a falling sphere in a box filled with liquid. Then the effects of Reynolds, Prandtl and Grashof numbers (Re, Pr, Gr) are investigated for a settling particle at fixed/varied temperature. The time variations of velocity, height and Nusselt number of the settling particle are also investigated. The results depict that the maximum settling velocity of the particle at varied surface temperature is higher at lower Reynolds numbers. As the Reynolds number increases, the settling velocity of the particle for both cases of constant and varied temperature is the same. However, the Nusselt number of the particle at varied temperature is lower compared with that for the particle at constant surface temperature. Increasing the Grashof number leads to slower settling and larger average Nusselt number. Finally sedimentation of 30 hot spherical particles in an enclosure and their hydraulic and heat transfer interactions with the surrounding fluid are studied. The results depict how the presence of heat transfer phenomena can significantly alter the behavior of settling particles. [Copyright &y& Elsevier]

Abstract: The present paper deals with CFD study of flow and thermal characteristics in air-conditioned rooms under turbulent mixed convection conditions. Air enters room from a sidewall slot flush with isothermal ceiling and leaves through a slot in opposite wall flush with floor. Velocity and temperature distributions are determined by a CFD model that has been previously validated by comparison with experimental data. Rigorous dimensional analysis shows that performance of the air-distribution system depends on: supply Reynolds number Re d , Grashof number Gr d (or Archimedes number Ar d ), room aspect ratio L/H, dimensionless heights of supply and return ports (d/H and t/H), where H is room height. For a room with fixed geometry, the effects of Re d and Gr d (or Ar d ) are investigated systematically by varying one parameter at a time. Results show that ceiling average Nusselt number (Nu d,av) increases with Re d and decreases with Gr d (or Ar d ). For a given Gr d , the heat removal index (HRI) increases with Re d up to Re d  ≈ 9000, then stays practically constant thereafter. For a given Re d , HRI decreases with increasing Gr d (or Ar d ). A criterion for distinction between forced and mixed convection situations is developed, resulting in a critical Archimedes number of 0.11. Results further indicate that Ar d alone, irrespective of the particular values of Re d and Gr d , determines to a reasonable extent Nu d,av and HRI when Ar d is above its critical value. Streamlines and temperature contours are found to be consistent with other results and helped explain them. [Copyright &y& Elsevier]

Abstract: This paper presents a numerical investigation for examining the secondary vortex motion and associated heat transfer process in fluid flow through curved passages. Overcoming current modelling limitations, the study formulates an improved simulation model based on 3-dimensional vortex structures for describing secondary flow and its thermal characteristics. For developing laminar fluid flow through curved rectangular ducts, the analysis performs a detailed parametric study involving the contours of helicity and outer duct wall pressure gradient for a range of flow rates, duct aspect ratios, duct flow curvatures and external wall heat fluxes. The flow conditions leading to hydrodynamic instability and Dean vortex generation in curved passages are carefully analysed, identifying the flow and geometrical parametric influences. Active interaction of buoyancy force on fluid motion arising from wall heating is considered where aspects of boundary layer separation is used in recognising thermal enhancement due to secondary flow. The simulated results are validated with the available experimental data. Surpassing precision and reliability of current practise, the study develops two numerical approaches offering improved capabilities for capturing the onset of hydrodynamic instability and resulting Dean vortices. These techniques are compared and assessed for their merits of application in determining the onset of secondary flow hydrodynamic instability with and without external wall heating. [Copyright &y& Elsevier]

Abstract: This paper addresses the influence of thermal non-equilibrium state on fully developed mixed convection in a vertical channel filled with porous medium. The non-Darcy-Brinkman-Forchheimer extended model has been considered. The governing equations are solved numerically by spectral collocation method and analytically for a special case (form drag equal to zero). The governing parameters for this problem are Rayleigh number (Ra), Darcy number (Da), Forchheimer number (F), inter-phase heat transfer coefficient (H), and porosity-scaled thermal conductivity ratio (γ). Special attention is given to understand the effect of thermal non-equilibrium parameters: H and γ on the rates of heat transfer and fluid flow profiles, for both buoyancy assisted as well as opposed cases. A comprehensive investigation indicates that, in case of buoyancy assisted flow, for each Ra considered in this study and when γ ≤ 1, there exists a minimum value Ho of H such that the heat transfer rate of fluid (Nuf ) at the wall is an increasing function in [0, Ho ]. Furthermore, as H is varied beyond Ho , Nuf decreases. Initially the decay of Nuf is fast, but later on it becomes smooth and slow. Similar results are also observed in buoyancy opposed case for very small values of γ. Variation of velocity profile as a function of γ indicates that change in velocity profile is sudden and abrupt. For H > 6, when γ = 0.01, Da = 10−2, |Ra| = 103 and F = 102, flow separation and point of inflection are completely died out from the profile. Overall, for both the cases, on increasing H makes the flow profile smooth (stabilizes the flow) and recovers the system equivalent to equilibrium state, whereas, γ destabilizes the flow by inviting point of inflection or flow separation on the velocity profile. [Copyright &y& Elsevier]

Abstract: This paper is concerned with the exact solutions for thermal diffusion in a straight fin with varying exponential shape when the thermal conductivity and heat transfer coefficients are temperature dependent. The conduction and heat transfer terms constitute the strong nonlinearity given by power laws. Explicit analytical solutions are derived for the relevant parameters in terms of special functions of advanced mathematics. The effects of parameters of physical interest such as the fin tip temperature, the fin efficiency and the fin base heat transfer rate can be better captured from the presented formulas. Moreover, with the help of the analysis here, the best shape of the straight fin is identified under a variety of operating conditions. Analysis further shows that the efficiency and base heat transfer rate of the exponential profiles are higher than those of the rectangular fin. [Copyright &y& Elsevier]

Abstract: The wave-finned flat tube heat exchangers are widely used in air-cooled condensers of a power plant. Due to the oblique arrangement of the wavy fins on the A-frame tube bundles of the air-cooled condenser, the finned tube is prone to fouling thus the thermo-flow performances of air-cooled condensers are likely to become deteriorated. A new configuration of wave-finned flat tube, in which the fin surface rotates to be perpendicular, is proposed in this paper to prevent the fouling from gathering on the finned tube. The physical and mathematical models for the fluid and heat flows of the cooling air through the present and proposed wave-finned flat tube bundles are setup, and the thermo-flow characteristics are compared. For the proposed finned tube, the influences of the fin pitch, height and the Reynolds number on the flow and heat transfer performances are investigated, and the correlations of the friction factor and Nusselt number are recommended. The results show that the buoyancy effect should be taken into account for the correlation of the friction factor at low Reynolds numbers, but this buoyancy force can be ignored for the Nusselt number. [Copyright &y& Elsevier]

Abstract: Thermal performance in oil-immersed power transformers is governed by the flow of oil, acting both as an electrical insulator and a medium for the transfer of heat generated in the core and windings toward the tank and the surrounding air. This paper presents the development of an advanced three-dimensional (3D) finite element model for the coupled solution of heat transfer and fluid flow equations governing transformer thermal performance. The main advantages of the proposed method are: (i) no need to predefine the convection coefficients at the interfaces between the active part/tank walls and the circulating oil, (ii) detailed representation of specific transformer parts that play an important role in the accurate representation of oil flow and heat dissipation (such as winding cooling ducts and corrugated tank panels) through an automated design process, enhancing the model accuracy with the least possible computational effort and (iii) accurate definition of the transformer heat sources (core and windings loss). The proposed methodology provides an integrated tool for thermal simulation, able to predict detailed thermal distribution in a specific transformer, without requiring prior knowledge of nodal temperature or temperature gradient values. [Copyright &y& Elsevier]

Abstract: Thermal transport characteristics of electroosmotic flow of power-law fluids in the presence of pressure gradient through a slit microchannel are studied in this paper. Considering a fully developed flow with a constant wall heat flux as the boundary condition, the governing equations are numerically solved by means of the finite difference method. A complete parametric study is done in order to investigate the effects of different flow parameters on the thermal behaviors of the flow. The results show that the non-Newtonian characteristic of the fluid can influence the thermal behaviors of the flow by affecting the rate of heat convection and viscous dissipation; however, its influence diminishes at higher values of the dimensionless Debye-Hückel parameter. It is also found that the zeta potential, whose value is dependent on the channel wall and the electrolyte solution characteristics, can highly affect the thermal behaviors of the flow, especially at smaller values of the dimensionless Debye-Hückel parameter. [Copyright &y& Elsevier]

Abstract: This paper presents an analysis of the heat transfer characteristics and cooling performance of a microchannel heat sink with water–γAl2O3 nanofluids having different nanoparticle volume fraction. In view of the small dimensions of the microstructures, the microchannel heat sink is modeled as a fluid-saturated porous medium for problem solving. The Forchheimer–Brinkman–extended Darcy equation is used to describe the fluid flow and the two-equation model with thermal dispersion is utilized for heat transfer. Typical results for the temperature distributions of the fluid phase and the channel wall are presented for various values of nanoparticle volume fraction and the inertial force parameter. It is found that the temperature distribution of the channel wall is practically not sensitive to the inertial effect, while the fluid temperature distribution and the total thermal resistance change significantly due to the inertial force effect. In general, the effect of fluid inertia is to reduce the total thermal resistance and the temperature difference between the channel wall and the fluid phase. The total thermal resistances obtained from the present model with inertial effect match well with the existing experimental results, whereas the thermal resistance is overestimated as the inertial effect is neglected. [ABSTRACT FROM AUTHOR]

Abstract: Using air as heat transfer fluid for electricity generation offers some significant advantages for the development of Concentrated Solar Power (CSP): high conversion efficiency, low environmental impact and being used in deserts or other areas scarce of water resources. Silicon carbide ceramic foams have the characteristics of light weight, high strength, large specific surface areas, high porosity and excellent thermal shock resistance performance which make them particularly fit for absorber material in CSP. In this paper, thermal performance of silicon carbide ceramic foam as solar air receiver is investigated analytically based on the one dimensional physical model. The analytical results show that the air flow resistance increases obviously with increasing air outlet temperature, the air flow resistance while the air outlet temperature is equal to 1000 °C is nearly 3 times the one while the air outlet temperature is equal to 20 °C with air velocity range is between one and six meters per second. The results of one dimensional analysis of flow and heat transfer process of ceramic foams suggest that there exists an input solar energy flux limit for the unpressurized system, which will lead to limit the power capacity and the outlet air temperature enhancement. [ABSTRACT FROM AUTHOR]

Abstract: A two dimensional lumped parameter model (LPM) which provides the steady state solution of temperatures within axisymmetric single-sided, slotted axial flux generators is presented in this paper. The two dimensional model refers to the heat modelling in the radial and axial directions. The heat flow in the circumferential direction is neglected. In this modelling method, the solid components and the internal air flow domain of the axial flux machine are split into a number of interacting control volumes. Subsequently, each of these control volumes is represented by thermal resistances and capacitances to form a two dimensional axisymmetric LPM thermal circuit. Both conductive and convective heat transfers are taken into consideration in the LPM thermal circuit by using annular conductive and convective thermal circuits respectively. In addition, the thermal circuit is formulated out of purely dimensional information and constant thermal coefficients. Thus, it can be easily adapted to a range of machine sizes. CFD thermal modelling and experimental testing are conducted to validate the temperatures predicted from the LPM thermal circuit. It is shown that the LPM thermal circuit is capable of predicting the surface temperature accurately and potentially replacing the CFD modelling in the axial flux machine rapid design process. [Copyright &y& Elsevier]

Abstract: The logarithmic mean enthalpy difference, (LMED) method has been extensively used in evaluating the thermal performance of an air cooling coil under wet condition. The LMED method has been developed based on the assumption of unit Lewis Factor, i.e., Le2/3 =1. However, a number of previous studies have suggested that the Lewis Factor can actually deviate from being 1. Consequently, errors can be resulted in when calculating the total heat transfer rate of a wet cooling coil using the LMED method. Therefore, a modified LMED (m-LMED) method has been developed for calculating the total heat transfer rate under both unit and non-unit Lewis Factors and is reported in this paper. This m-LMED method has been validated by comparing its prediction of the total heat transfer rate to that from numerically solving the fundamental governing equations of heat and mass transfer of a wet cooling coil. The m-LMED method can therefore replace the LMED method for calculating the total heat transfer rate of a wet cooling coil under both unit and non-unit Lewis Factors. [Copyright &y& Elsevier]

Abstract: One of the major challenges in the study of thermal transport and its analysis, based on the hyperbolic model associated with Cattaneo equation, is the fact that it is necessary to determine the thermal relaxation time for the analyzed materials. This parameter has been an elusive physical quantity to be determined experimentally even though it is of crucial importance in heat transport. In this paper a system formed by a semi-infinite layer in contact with a finite one, that is excited by a modulated heat source is studied. It is shown that a frequency range can be found in which the amplitude and phase of the spatial component of the oscillatory surface temperature show strong oscillations when the thermal relaxation time of the finite layer is close to its thermalization time. When the thermal effusivities of the layers are quite different or their thermal relaxation times are similar, it is shown that simple analytical expressions for the values of the maxima and minima of the oscillations as well as for the frequencies, at which they occur, are obtained. These results were used to establish a methodology to determine the thermal relaxation time as well as additional thermal properties of the finite layer. [Copyright &y& Elsevier]

This paper presents the effect of roughness on heat transfer for semi-cylindrical cavity. Heat transfer measurements are performed on smooth and rough surfaces for different tilt angles. The results are compared with a rectangular cavity of the same surface area to study the effect of cavity shape on heat transfer. The roughness composed of 0.002 m diameter rods installed along the length of the cavity at equal spacing. Roughness shows a large effect on heat transfer for the semi-cylindrical cavities. Two competing effects are present with the existence of roughness. Roughness may increase the blockage effect on the flow that can cause the buoyancy force to decrease, but on the other hand it increases the turbulence intensity resulting in a higher heat transfer. Both effects are function of tilt angles. Heat transfer for the cylindrical cavity is higher than the rectangular cavity for all tilt angles. This increase in heat transfer for the cylindrical cavity is due to the absence of sharp corners that can slow the buoyancy driven convection mechanism. [Copyright &y& Elsevier]

Numerical simulation of heat conduction in the concrete structure with cooling pipes is meaningful and full of challenges. In this paper, the radial basis function finite difference method (RBF-FD) combined with a self-correcting prediction model (SCM) is proposed for thermal analysis of heat transfer in concrete structures with pipe cooling system. Using the Multiquadric radial basis function (MQ-RBF), the SCM is proposed to give full play to advantages of the localized meshless method. This technique allows use of more number of nodes in the whole domain to solve the ill-conditioning problem associated with multiple regions of rapid variation and increases the accuracy of models. A multiple-scale technique is applied to determine the shape parameter in the MQ-RBF and the fictitious nodes method is modified by using just one fictitious node outside the domain to deal with boundary conditions of the third kind in tiny holes. To show the flexibility and efficiency of the proposed scheme, we consider several examples with different number, size, and positions of pipes. Meanwhile, a guidance of determination of effected area near a tiny hole is provided and a simple optimization design of cooling pipe system is performed. The results demonstrate advantages of the proposed model which can effectively handle heat conduction problem with multiple holes subjected to the third kind boundary conditions. • The indirect scheme is simple, accurate and avoids the difficulty of adaptive collocation procedure. • The RBF-FD-SCM is accurate and efficient for the problem with many tiny pipes. • The proposed scheme is flexible to use just a set of fixed collocation points, even for multi-pipe cases. [ABSTRACT FROM AUTHOR]