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In numerical heat transfer, the model validation problem with respect to epistemic uncertainty, where only a small amount of experimental information is available, has been recognized as a challenging issue. To overcome the drawback of traditional probabilistic methods in dealing with limited data, this paper proposes a novel model validation approach by using evidence theory. First, the evidence variables are adopted to characterize the uncertain input parameters, where the focal elements are expressed as mutually connected intervals with basic probability assignment (BPA). In the subsequent process of predicting response focal elements, an interval collocation analysis method with small computational cost is presented. By combining the response BPAs in both experimental measurements and numerical predictions, a new parameter calibration method is then developed to further improve the accuracy of computational model. Meanwhile, an evidence-theory-based model validation metric is defined to test the model credibility. Eventually, the famous Sandia thermal challenge problem is utilized to verify the feasibility of presented model validation method in engineering application. [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]

The convection onset in a evaporation thermal model of a solute/solvent solution is numerically studied. The transient nature of the diffusion temperature profile requires to control the perturbations which will lead to the transition to a convective flow. Contrary to previous papers, we adopted here a perturbation mode, which is physical and experimentally reproducible, by accounting for a heat transfer at the lateral walls. Based on the Péclet number evaluated in the core flow field only, the critical viscosities and the corresponding times for the transition between a diffusion and convection regimes were calculated as a function of the fluid layer thickness, the aspect ratio and the convective heat transfer coefficient. The study of the spatio-temporal diagrams allowed us to characterize the way the convection propagates in the fluid domain as well as the evolution of the mean cell size and wavenumber. [ABSTRACT FROM AUTHOR]

The present paper presents fluid flow and mixed convective heat transfer characteristics past a semi-circular cylinder at incidence with a tandem square cylinder in cross flow. Using air ( Pr = 0.71 ) as an operating medium, numerical simulations are performed for the range of Reynolds numbers, 10 ≤ Re ≤ 45 and incidence angles, 0 0 ≤ α ≤ 180 0 . The effect of thermal buoyancy is brought about by varying the Richardson number in the range 0 ≤ R i ≤ 2 . The mathematical model is firstly validated with the experimental and numerical results from the literature and found to be in good agreement. The steady separated flow is observed to become unsteady periodic under the superimposed thermal buoyancy. Furthermore, apart from thermal buoyancy effect, angle of incidence is found to play a pivotal role in bringing hydrodynamic instabilities and thereby vortex shedding for such steady mixed convective flows. Functional dependence of drag ( C D ) , lift ( C L ) , and moment ( C M ) coefficients on the combined influence of α and R i , is explored and analysed in detail. Additionally, other global quantities, such as local and average Nusselt number distribution, Strouhal number ( S t ) are determined with respect to the various ranges of parameters considered in the present investigation. [ABSTRACT FROM AUTHOR]

Abstract: Along with the developments of lattice-Boltzmann method in simulation of fluid flow, its thermal features should be also improved. An important problem which is encountered in many cases is the conjugate heat transfer or heat transfer at the fluid–solid interface. In this case, a strict technique is required to calculate the heat exchange between fluid and solid. Only a few works have been published regarding this phenomenon. In this paper, we have presented a model for analysis of the conjugate heat transfer. The model has been validated by two benchmarks: (I) Fluid flow and heat transfer in a backward-facing step channel with heated thick wall, and (II) Fluid flow and heat transfer in a channel with infinite number of heated obstacles mounted on the wall. The numerical tips concerned in the simulation of these problems by lattice-Boltzmann method have been also represented in the paper. [Copyright &y& Elsevier]

Abstract: The inlet fog cooling scheme has been considered as an economic and effective means to augment gas turbine output power on hot or dry days. A previous paper developed a stage-by-stage wet-compression theory for overspray and interstage fogging using the equilibrium droplet evaporation model with given compressor and blade configurations. This paper extends the previous work by including the non-equilibrium droplet heat transfer model. The equilibrium model assumes that water evaporates to saturation according to psychrometry at each stage without considering water evaporation rates, whereas the non-equilibrium model considers water evaporation rates according to heat and mass transfer of droplets. The non-equilibrium model can predict the different evaporation from different droplet sizes, but equilibrium model can''t. An 8-stage, 2-D compressor airfoil geometry and stage settings at the mean radii are employed. Eight different cases including saturated fogging, overspray with different droplet sizes with both equilibrium and non-equilibrium heat transfer models have been investigated and compared. The results show saturated fogging increases the pressure ratio and reduces the compressor power consumption; however, overspray actually increases both the specific and total compressor power consumption. For small droplet size of 10 μm, the droplet evaporation rate is fast, so the non-equilibrium method predicts results close to the equilibrium method. Larger droplets lead to slower evaporation, reduction of pressure ratio, and less effective compressor performance than the smaller droplets. The equilibrium method predicts that wet compression increases axial velocity, blade inlet velocity, incidence angle, and tangential component of velocity. The non-equilibrium methods predict a similar trend except with lesser increments as the droplet size increases. In the present study, the equilibrium method predicts that all the water droplets evaporate completely at the end of stage 3, while the non-equilibrium approach predicts that the completion of evaporation delays, but all droplets completely evaporate in the compressor except the biggest droplets (30 μm). Saturated fogging increases air density; however, both equilibrium and non-equilibrium methods predict that overspray wet compression actually reduces air density in the earlier 70% of the compressor. The non-equilibrium model predicts that small droplets relax the load in the earlier stages but increase the load in the later stages. Larger droplets show less load changes. Detailed stage-to-stage performance and property value changes are analyzed and discussed in this study. [Copyright &y& Elsevier]

The ammonothermal crystal growth technique is one of the most effective methods used for growing Gallium Nitride (GaN) crystals in terms of the final product quality and manufacturing efficiency. Popular applications of GaN crystals include light emitting diodes (LEDs) and high-frequency electronic devices. A laterally-heated cylindrical enclosure with a top to bottom temperature gradient is considered in this study to investigate the fluid mechanics and heat transfer in an ammonothermal crystal growth reactor. Three-dimensional (3D) large eddy simulations (LES) results of natural convection in a laterally heated cylindrical reactor, using the commercial computational fluid dynamics (CFD) software, ANSYS FLUENT, are presented in this paper for a Rayleigh number ( R a ) of 8.8 × 10 6 . The R a is defined based on a length scale that is equal to the ratio of volume to the lateral area of the cylinder ( R / 2 ). In addition, an experiment of a geometrically- and dynamically-similar geometry is developed, and particle image velocimetry (PIV)-based flow visualizations are carried out for the purpose of validating the numerical model. Comparisons between experiments and numerical simulations showed that flow patterns were qualitatively similar, and Fourier transforms of velocity magnitudes at selected points in the domain matched reasonably well. An added interesting observation in the simulation was the existence of temperature inversion, which has potential implications on the choice of mineralizer (acidic/basic). [ABSTRACT FROM AUTHOR]

This study numerically examines three-dimensional natural convection of air induced by temperature difference between a cold outer cubic enclosure and a hot inner cylinder. Simulations have been carried out for Rayleigh numbers ranging from 10 3 to 10 7 and a titled angle of the enclosure varying from 0° to 90°. The developed mathematical model is governed by the coupled equations of continuity, momentum and energy and is solved by finite volume method. The effects of cylinder inclination and Rayleigh number on fluid flow and heat transfer are presented. The distribution of isocontours of temperature, components of velocity and streamtraces eventually reaches a steady state for Rayleigh numbers ranging from 10 3 to 10 7 for titled inclination of 90°. However, for the remaining inclinations, Rayleigh numbers must be in the range 10 3 –10 6 to avoid unsteady state which is manifested by the subdivision of the area, containing the maximum local heat transfer rate, into three parts for a Rayleigh number equal to 10 7 and an inclination of 90°. We mention that instability study is not included in the present paper which is solely devoted to three dimensional calculations. Results indicate also that optimal average heat transfer rate is obtained for Rayleigh number set to 10 6 and an inclination of 90° for both cases of the inner cylinder and lateral walls of cubical enclosure. [ABSTRACT FROM AUTHOR]

In this paper, a numerical investigation is presented to study Buongiorno's mathematical model for hydromagnetic free convection flow in an isosceles triangular cavity filled with nanofluid. The inclined walls of the cavity are maintained at constant cold temperature whereas various combinations of the thermal boundary conditions at the bottom heated wall are considered. The cavity is permeated by an inclined uniform magnetic field and the effects of Brownian motion and thermophoresis are incorporated into the nanofluid model. The Galerkin weighted residual finite element method has been employed to solve the governing partial differential equations after converting them into a non-dimensional form using a suitable transformation of variables. In the numerical simulations, alumina-water based nanofluid has been taken into account. Comparisons with previously published work are performed and excellent agreement is obtained. The effects of various parameters such as Hartmann number, Rayleigh number and inclined magnetic field angle on streamlines, isotherms and isoconcentrations have been displayed graphically. The heat transfer augmentation for various combination of model parameters as well as various thermal boundary conditions have been done in light of the average Nusselt number from the bottom heated wall. The results show that the heat transfer rate can be decreased with the increasing values of the Hartmann number and inclination angle of the magnetic field but it can be increased by increasing the Rayleigh number and by reducing the diameter of the nanoparticles. The obtained numerical results also indicate that the variable thermal boundary conditions have significant effects on the flow and thermal fields. [ABSTRACT FROM AUTHOR]

This paper presents a theoretical study on the optimization of key material parameters of multilayer clothing assemblies for thermal comfort in cold climate. A mathematical model of heat and moisture transfer through a multilayer clothing assembly is established. The existence and uniqueness of the solution of the model are verified by using the fixed point theorem. The model predicts the optimum thicknesses and porosities of the two internal layers of the clothing assembly, with which the thermal comfort index will fall within the required intervals with the greatest probability. This study is significant for the optimum design of multilayer clothing assembly for thermal comfort. [ABSTRACT FROM AUTHOR]

In this paper, a distributed parameter model for tubular reactor in the ethylene cracking furnace based on 3-D temperature reconstruction is developed. The mathematical model was formulated to predict the distributions of the heat flux and the tube temperature while considering a non-uniform distribution of the surface heat transfer and the ethylene pyrolysis process. The results show that the heat flux distribution on the tubular reactor has four high heat flux zones, and the tube temperature reaches a maximum of 1088 °C. The value of the maximum tube temperature decreases compared with the value of the maximum heat flux due to the heat exchange coefficient of pyrolysis gas increasing slowly in the bend region. The tube temperature distribution was validated by using a spectrometer system and the errors between three major product yields at the tube outlet obtained by the proposed model and in-situ measured values were within 5%. This distributed parameter model can be used as a guide for ethylene cracking furnace operators and as a tool for design. [ABSTRACT FROM AUTHOR]

Abstract: The paper presents a new proposal of a discrete event simulation (DEVS) model for heat transfer in an enclosure. To our knowledge this is the first proposal in this direction, therefore the model is not comprehensive (has no windows, no ground coupling, etcetera). It is a first step of DEVS into the heat transfer in buildings. The model presented is just one possible implementation for a single thermal zone, although it has been designed to be used in multi-zone models. Common methods like CTF (conduction transfer functions) or RTF (response transfer functions) cannot be used. Instead it employs the successive transition state formulation for the 1D conduction heat transfer through multi-layered walls. An example test room has been calculated and the results compared with software that uses well known methods (CTF, RTF). Finally the DEVS model has been tested with a random convective internal-load signal. [Copyright &y& Elsevier]

Abstract: The present paper describes a mathematical study on peristaltic flow of viscoelastic fluids (with the robust Jeffrey model) through a finite length channel under the influence of heat transfer. The study is motivated by the need to further elucidate the mechanisms inherent in swallowing of diverse food bolus types (bread, fruit jam and almost all edible semi-solids) through the oesophagus, by taking account of the viscous and elastic effects. The expressions for temperature field, axial velocity, transverse velocity, volume flow rate, pressure gradient, local wall shear stress, mechanical efficiency, stream function, and reflux limit are obtained, when the Reynolds number is small and the wavelength is large, by using appropriate analytical and numerical methods. The computational results are presented in graphical form. The influence of thermophysical (heat transfer), relaxation time and retardation time parameters on pressure distribution, local wall shear stress profiles, temperature profiles and velocity profiles are studied in detail. Furthermore we investigate the effects of these parameters on two inherent phenomena (reflux and trapping) characterizing peristaltic flow using streamline plots. The present study emphasizes an important observation, namely that pressure along the entire length of the channel reduces when the magnitude of relaxation time (retardation time is fixed) or Grashof number or indeed thermal conductivity increase, whereas pressure is enhanced by increasing the magnitude of retardation time (relaxation time is fixed). [Copyright &y& Elsevier]

Abstract: Air is a cheap and safe fluid, widely used in electronic, aerospace and air conditioning applications. Because of its poor heat transfer properties, it usually flows through extended surfaces, such as finned surfaces, to enhance the convective heat transfer. In this paper, experimental results are reviewed and numerical studies during turbulent air forced convection through extended surfaces are presented. The thermal and hydraulic behavior of a reference trapezoidal finned surface, experimentally evaluated by present authors in an open-circuit wind tunnel, has been compared with numerical simulations carried out by using the commercial CFD software COMSOL Multiphysics. Once the model has been validated, numerical simulations have been extended to other rectangular finned configurations, in order to study the effects of the fin thickness, fin pitch and fin height on the thermo-hydraulic behavior of the extended surfaces. Moreover, several pin fin surfaces have been simulated in the same range of operating conditions previously analyzed. Numerical results about heat transfer and pressure drop, both for plain finned surfaces and for pin fin surfaces, have been compared with empirical correlations from the open literature, and more accurate equations have been developed and proposed. As shown with an optimization example, these new equations can be used as an easy-to-implement calculation approach for heat sink design in electronic thermal management. [Copyright &y& Elsevier]

Abstract: There are several methods adopted by the scientists in modeling the heat transfer phenomenon during solid–liquid phase change. However, the validity of the various models developed and the complexity required for a given problem is not well reported in the literature. This paper presents a comparative study of three different mathematical models for the packed bed latent heat storage system, comprised of a cylindrical storage tank filled with paraffin encapsulated spherical containers. The enthalpy formulation technique is used in the models to accommodate the phase change behavior of the paraffin over a range of temperatures. The results obtained by solving the models using the fully explicit finite difference method are initially validated with experimental results. Further numerical analysis is performed using the models for two different heat transfer fluids of air and water at different mass flow rates and ball sizes and the validity of the models are compared and reported. It is found that one model is sufficient when air is the heat transfer fluid and another model is recommended when water is the heat transfer fluid. [Copyright &y& Elsevier]

Abstract: Propagation of uncertainty through the physical model has been investigated in the present paper by solving two specific simple stochastic problems using the Non-Intrusive Spectral Projection method. The uncertain parameters are described by either a Gaussian or a LogNormal probability distribution function. For each of the problems, the stochastic and the deterministic mean solutions have been compared and the respective confidence intervals have been obtained. For the deterministic problems, the confidence intervals have been estimated using both one-dimensional and multi-dimensional bound methods. From the results it has been observed that the differences between the stochastic and the deterministic mean solutions are apparent only when large uncertainties are introduced in the random variables. For both the specific problems, considered in the present study, the confidence intervals for the stochastic problems have been exactly predicted by the deterministic limits when uncertainty is introduced only in one of the parameters. For more than one uncertain parameters, the multi-dimensional bound method produces better agreement with the stochastic confidence intervals than the one-dimensional bound method. The findings are expected to be applicable to problems in heat and mass transfer with similar characteristics or input–output relations. [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: The flash evaporation of superheated water in direct contact with its own vapor involves very high heat transfer rates and the existing solutions for the problems of heat diffusion in the superheated water drops underestimate the actual heat transfer rates, even when the gas-side heat transfer coefficient is assumed to be extremely high. This happens due to the poor thermal conductivity of the water which causes the heat transfer to be dominated by the heat flow within the liquid. Accommodation between the analytical results of the heat diffusion in superheated water and those obtained from the experimental measurements requires considering the effective thermal conductivity of the liquid owing to the violent nature of the flow, which is thought to be much larger than the mere molecular thermal conductivity. This paper presents initial attempts on modeling the liquid-side heat transfer process accompanying the surface evaporation of superheated water drops resulting from the flashing of superheated water jets. [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: The design of technological devices and systems, with reduced size but unchanged or increased performances, has been one of the most important scientific challenges faced in the last decade by many fields of the world industry. The capability to remove and exchange heat in more compact components has allowed great advantages in the optimization of all final products as well as in industrial processes. However the never-ending race to miniaturization of appliances has been taking into account problems concerning computational models and design criteria developed in the past for macro-systems. Nowadays most of these methods are no longer reliable for emerging micro-systems because of the substantial differences in transport phenomena and heat exchange, mainly in two-phase flow (scaling effects). This paper aims to provide its contribution to the burgeoning research activity on boiling heat transfer under forced convection in microchannels. The BO.E.MI.A. experimental facility was designed and specifically developed to perform different regimes heat transfer tests on mini and microchannels: a total of 150 tests were performed using a fluorinert fluid FC-72 as coolant. The test section consists of a horizontal 1mm inner diameter stainless steel tube having a heated length (Joule effect) of approximately 60mm. The mass flux range is between 1000 and 2000kgm−2 s−1 while the applied heat flux is between 10 and 150kWm−2. By means of a preheater we are able to change coolant inlet temperature and by setting the operating pressure in a range between 3 and 7bar, a broad spectrum of subcooling degree at inlet test section is achieved. The local heat transfer coefficients were evaluated for both subcooled and saturated flow boiling regimes. The experimental data show an increase of local heat transfer coefficient for increasing values of heat flux, with a weak dependence on the vapour quality. The experimental values obtained were compared with those from the adoption of one of the main heat transfer correlations in the literature. [Copyright &y& Elsevier]

Abstract: In this paper, a distributed parameter model for the evaporation system of a subcritical once-through, coal-fired, twin-furnace boiler based on 3-D temperature field reconstruction is developed. The imaginary wall surface was put forward to simplify the twin-furnace problem. The mathematical model was formulated for predicting the transient distributions of parameters, such as the heat flux, the metal-surface temperature and the steam quality; while considering the non-uniform distributions of the surface heat transfer coefficient and frictional resistance coefficient. The model was based on the 3-D temperature distribution got by a flame image processing technique. The model was validated by some measurable parameters of the evaporation system at three typical loads. The results show that the heat flux and the temperature which are located at the overlapping region of the two tangential flames are higher than those in other corners. This distributed parameter model on evaporation system reflects the in-situ operating status of the power plant boiler, which may lead to the subsequent research on a supercritical boiler. [Copyright &y& Elsevier]

Abstract: The work presented in this paper investigates the degradation effect of flow maldistribution on the thermal and hydraulic performance of a heat exchanger. A new mathematical model is derived based on Taylor series expansion to describe the contribution of each of the four statistical moments of distribution on the degradation problem. It is found that the first two moments, i.e. mean and standard deviation, have the highest effect on the performance degradation. Subsequent higher moments will give declining degradation effects until the fourth moment, kurtosis, which has no significant effect. Maldistribution with low standard deviation and high positive skew will give low thermal deteriorations, though a distribution with negative skew is preferred for low hydraulic performance losses. The design of the heat exchanger could be modified to give these favourable moments which would minimize the degradation effects of the maldistribution. Consequently, any effort to modify the flow distribution profile should be focused on optimizing the first three moments. [ABSTRACT FROM AUTHOR]

Abstract: A computational model is developed to describe the thermo-fluid-dynamic behaviour of a helically coiled steam generator device working with water and widely adopted in the nuclear industry. The discretized governing equations are coupled using an implicit step by step method. The mathematical model includes: a subcooled liquid region, a two-phase flow region, and a superheated vapour region (according to the once-through nature of the heat exchanger). All the flow variables (enthalpies, temperatures, pressures, vapour qualities, velocities, heat fluxes, etc.), together with the thermo-physical properties, are evaluated at each point of the grid in which the domain is discretized. A full-scale experimental investigation carried out at SIET thermal-hydraulics labs in Piacenza (Italy), and aimed at characterizing the fluid-dynamic behaviour of two-phase flows in helically coiled tubes, is referenced in the present paper. Two-phase pressure drops data reduction allowed optimizing a suitable form of the friction factor multiplier required by momentum balance equation. Comparisons of the numerical simulations with a wide range of two-phase pressure drops measurements (experiments conducted both in diabatic and adiabatic conditions) are shown in order to validate the proposed model. [ABSTRACT FROM AUTHOR]

Abstract: Microjet impingement cooling devices promise high heat removal rates for the development of advanced thermal management solutions. However, understanding of microjet hydrodynamics is needed to optimize cooling performance. In this paper, we combined experiments and modeling to obtain three-dimensional (3-D) microjet flows. We fabricated single-jet and multi-jet arrays with 50μm diameter orifices and used micron-resolution particle image velocimetry (μPIV) to capture two-dimensional (2-D) images of the flow field at different imaging planes. The data was subsequently used to obtain the out-of-plane (z-component) velocities, which play an important role in enhancing heat transfer at the impingement surface. The results from the reconstruction of the 3-D flow field offers new insights into the impact region of a single jet and optimized design of microjet cooling devices. [ABSTRACT FROM AUTHOR]

Abstract: In the present paper, a numerical model of spot pulsed current GTA welding for partially and fully penetrated weld pools is presented. Heat transfer and fluid flow in the weld pool driven by the combination of electromagnetic force, buoyancy force, surface tension gradient and latent heat are included in our model. A new formulation of the electromagnetic problem is introduced to take into account eddy current in the weld pool. The shape of the free deformable surface under the action of pulsed arc force is also handled after the magneto-hydrodynamic calculation. The numerical model was applied to 304 stainless steel welding. We compare the influence of various pulsed welding parameters such as pulse frequency and current ratio on the weld quality. Experimental study is conducted to compare our numerical prediction with welding macrographies. It shows a good agreement of the model. [Copyright &y& Elsevier]

Abstract: This paper presents a novel strategy for optimizing the design of air curtains for open vertical refrigerated display cases which is based on an air curtain two-fluid of cooling loss (CLTF) model and a support vector machine (SVM) algorithm. A model for air curtain cooling loss, one important performance factor of display cases, is proposed. To verify the air curtain cooling loss and determine which design parameters significantly influence the performance of air curtains in open vertical display cases, the CLTF model was built to study the flow and transfer of heat through air curtains used in such display cases. After the object function for cooling loss is constructed, it is solved using an SVM algorithm with different input design parameter combinations. As a result, the predicted cooling loss is reduced by 19.6%. After being validated using experimental data, the TEC/TDA of optimum display case is found to be reduced by 17.1%. The experimental results show this strategy to be an effective method of optimizing the air curtain design. [Copyright &y& Elsevier]

Abstract: Analytical solutions are proposed in this paper to calculate the thermal impedance and transient temperature in a semi-infinite body subjected to a heat spot. Two cases of uniform and non-uniform heat flux are considered. These solutions are developed using integral transforms and are given in exact expression forms, without any restrictive hypothesis. They include special functions such as Bessel, Struve and hypergeometric functions. Many softwares to treat these functions are available (eg. Maple, Mathematica and others). The solutions are validated through comparisons with available models treating particular cases. The transient temperatures and impedances are presented for different spatial distributions of the heat flux dissipated by the spot. [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]

Abstract: Determining laminar burning velocities from the pressure trace in constant volume combustion requires knowledge of the burnt fraction as a function of pressure, . In recent literature is either determined via numerical modeling or via the oversimplified assumption that is equal to the fractional pressure rise. Recently, we have shown that the latter violates energy conservation, and derived alternative analytical relations based on zone modeling which are more simple to apply than numerical models. However we had to assume perfect gas behavior, neglecting dissociation. In this paper we systematically compare our analytical models with a numerical two-zone model and with a 1D unsteady simulation (1DUS) of a spherical stoichiometric methane–air flame in a constant volume. Results indicate that our analytical models reasonably describe the burnt fraction as a function of fractional pressure rise. However the relation also involves the (theoretical) end pressure . Its value significantly affects , with a relative sensitivity close to minus one, and is influenced by dissociation. Evaluating from an equilibrium code, in combination with the analytical model, provides results within 3% accuracy. This approach removes the need for numerical modeling of intermediate stages of combustion. Still, highest accuracy for is achieved using numerical models that account for dissociation also for intermediate stages. Comparing results of the 1DUS with the two-zone equilibrium model shows that the combined effect of detailed chemistry, flame stretch, heat transfer between zones, and the temperature gradient in the burnt mixture is limited to about 1% for the example case. [Copyright &y& Elsevier]

Abstract: Understanding the heat and mass transfer phenomena in a coal gasifier is very useful for the assessment of gasifier performance and optimization of the design and operating parameters. In this paper, performance of an entrained flow air blown laboratory scale gasifier is numerically simulated with Fluent software. In the model, the continuous phase conservation equations are solved in an Eulerian frame, while those of particle phase are solved in a Lagrangian frame, with coupling between the two phases carried out through interactive source terms. The dispersion of the particles due to turbulence is predicted using a stochastic tracking model, in conjunction with the k–ε equations for the gas phase. The coal gasification model adopted includes devolatilization, combustion of volatiles, char combustion and gasification. The gasification performance inside the gasifier has been predicted for different air ratios as well as for different air and steam inlet temperatures. The overall temperature inside the gasifier is found to increase when the degree of air/steam pre-heating is increased, resulting in acceleration of the different reaction steps in the gasifier. The overall gasification performance indices such as carbon conversion, heating value of the exit gas and cold gas efficiency have been predicted. The predicted results show good agreement with available experimental data in literature. [Copyright &y& Elsevier]

Abstract: In present paper, a numerical model combined radiation and conduction heat transfer was developed to predict the effective thermal conductivity of fibrous insulation at various temperatures and pressures. Effective thermal conductivities of the fibrous insulation were measured over a wide range of temperature (300–973 K) and pressure () using a developed apparatus. The transmittance spectra in the wavelength range of 2.5–25 μm were also measured at temperatures up to 973 K using a Fourier Transform Infrared Spectrometer. From transmittance data, the spectral extinction coefficients and Rosseland mean extinction coefficients were obtained at various temperatures to investigate the radiative heat transfer in fibrous insulation. The numerical and experimental data were compared. It was found that the calculated values corresponded with the experimental values within an average of 14.7 percent. Numerical results were consistent with experimental results throughout the investigated environmental conditions. [Copyright &y& Elsevier]

Abstract: An improved understanding of the heat transfer in materials consisting of two layers (splat and substrate) is essential for many industrial applications. We are interested in the deposition, rapid cooling and solidification of metal droplets (known as splats) brought into contact with a cold substrate. We therefore need to understand the temperature history in both the splat and the substrate, including phase change phenomena. A new model of the thermal contact resistance based on a random distribution of contact points, rather than the uniform distribution commonly used, is presented in this paper. Phase change has been also considered, using the enthalpy-porosity formulation. Simulations have been conducted with the commercial package CFX-4. The computational results for the cooling rate of the splat obtained using the random contact distribution model are in good agreement with available experimental results. In addition, results obtained from the random model provide information on the inhomogeneity affecting the temperature at the interface between the splat and the substrate. [Copyright &y& Elsevier]

Abstract: This paper presents a mathematical model for determining optimal configuration of plate finned tube condensers widely used in air conditioning system. The condenser investigated in this work is a cross-flow air-refrigerant type with aluminium plain fin geometry. Refrigerant HFC 134a flows inside while air flows outside the copper tubes. Minimum entropy change of air and refrigerant is used as the criterion for analysis. This entropy change resulting from losses due to heat transfer and pressure drop can be represented in a form of entropy generation number, . The proposed mathematical model is divided into three regions: superheated, two-phase and sub-cooled regions. Effects of heat transfer and pressure drop of air side and refrigerant side on entropy generation rate are simultaneously analyzed through the implicit method. Results from the mathematical model show that entropy generation number decreases with increasing fin pitch and decreasing number of rows and tube diameter. In addition, this work also presents an optimal configuration of condenser. The optimal tube diameter and the number of fin per inch are 6.35–12.7 mm ( inch) and 11–12, respectively. [Copyright &y& Elsevier]

Abstract: This paper presents a detailed analysis of the heat and mass transfer processes during the absorption of ammonia into water in a co-current vertical tubular absorber. The absorber configuration is of the shell and tubes type. The absorption process progresses as the vapour and liquid contact inside the tubes. Water is used as the absorber cooling medium. A differential mathematical model has been developed on the basis of mass and energy balances and heat and mass transfer equations, in order to provide further understanding of the absorber behaviour. The model takes into account separately for the churn, slug and bubbly flow patterns experimentally forecasted in this type of absorption processes inside vertical tubes and considers the simultaneous heat and mass transfer processes in both liquid and vapour phases, as well as heat transfer to the cooling medium. The model equations have been solved using the finite-difference method. Results obtained for specific data are depicted to show local values of the most important variables all along the absorber length. Parametric analyses have been performed to show the influence of design parameters and operating conditions on the absorber performance. The effect of the heat and mass transfer coefficients has also been evaluated. [Copyright &y& Elsevier]