Relevant Heat Transfer Mechanisms Research Articles

Thermal characterization of the build chamber in electron beam melting

Abstract Electron beam powder bed fusion, commonly termed electron beam melting (EBM), offers great versatility in multiple-part processes and can produce high quality as-built components due to low residual stresses. The EBM process is complex and requires careful thermal management, including uniform and consistent preheating of the powder bed, in order to ensure quality and consistency of the product. However, most of the simulations in the literature focus on the selective melting stage of the process. As of today, optimal conditions for initial pre-heating temperatures are only available for specific materials. Thus, in order to extend the EBM technology to other desired build materials, a much better understanding of the pre-heating stages is required. In this study, numerical and experimental approaches are combined in order to investigate the effects and sensitivities of heat removal from the build plate during EBM pre-heating stages. For this purpose, a carefully reconstructed numerical model of the build chamber of an ARCAM Q20+ machine is developed. It includes all main parts of the chamber and all relevant heat transfer mechanisms, whereas special attention is paid to radiation heat exchange between various bodies. In order to validate the model, dedicated experiments are performed, in which a system of thermocouples is installed in the build chamber, allowing direct measurement of the local temperatures of the start-plate and heat shields. A good agreement between the simulation and experimental findings is achieved, leading to a better understanding of the thermal processes characteristic to the pre-heating stages. This basic analysis is followed by a representative pre-heating case, where a powder bed is also considered. The energy required to obtain the desired pre-heating temperatures is evaluated, and the role of the powder bed in heat transfer within the chamber is assessed. The pre-heating stage, simulated in the present work, is supposed to create proper conditions for sintering and consequent melting of the powder, which are highly dependent on the local temperatures and heat transfer features. Thus, the findings of the reported work present a step towards a better understanding of the thermal processes that characterize EBM. The reported model can be further used to provide realistic boundary condition inputs for other meso- or macro-scale models as a function of time and geometry. The model can serve also for verification of machine settings, i.e., jump-safe and melt-safe ones which actually provide desired preheat, and for development of settings for new powders.

Numerical Investigation of the Temperature Distribution in PCM-integrated Solar Modules

Both lifetime and efficiency of photovoltaic (PV) modules are negatively affected by high and/or strongly fluctuating temperatures. An approach to improve the thermal performance is to add a phase change material (PCM) to the layer composition of PV modules. In order to be able to determine design parameters of the PCM, e.g. layer thickness of the PCM, melting point or chemical composition, a mathematical 1D-model describing the transient temperature distribution across the module was developed. It considers all relevant heat transfer mechanisms – conduction, convection and radiation – as well as the melting-solidification processes of the PCM. The model is solved numerically by an iterative, implicit algorithm implemented in MATLAB. Input factors include geometrical parameters and material properties as well as time-dependent weather effects (irradiance, ambient temperature, wind speed and direction). To determine these factors, either locally measured data or values derived by empirical models from literature can be employed. The simulated temperature distribution was validated against measured temperatures with an average deviation of ±1.7 K and used as an input for a module lifetime model. The latter model showed that the integration of a PCM into the layer composition could increase the lifetime of a PV module by up to 10 y, yet at the cost of a lower energy generation which is reduced by about 12 % compared to PV modules without PCM.

Intense pulsed light processing for photovoltaic manufacturing

The solar manufacturing industry continues to expand and become a key source of energy as manufacturing costs have decreased. This has led to a rapid growth in revenue and a corresponding drop in margins that has suppliers implementing solutions to further reduce the costs associated with manufacturing solar panels. Despite these improvements, solar energy derived from photovoltaics still has an energy pay-back time of at least one year with some of this is associated with heating processes. To further reduce costs, a new technology called intense pulsed light is being implemented to eliminate expensive high energy thermal processes and encourage higher throughput. The technology also includes opportunities for devices that contain thermally limited materials. The technique was originally explored when the photovoltaic industry was quite small, and has only recently begun to gather interest as the printed electronics industry realized gains. This review discusses the use of intense pulsed light in photovoltaic manufacturing, detailing the process from a photonic physics perspective, the relevant heat transfer mechanisms, as well as its impact on the major photovoltaic sectors. Demonstrations of generation 1: crystalline silicon solar cells, generation 2: thin film solar cells, and emerging technologies: hybrid organic solar cell applications are discussed. Lastly, future outlooks for intense pulsed light in the photovoltaic manufacturing sector are offered.

Electroluminescent cooling mechanism in InGaN/GaN light-emitting diodes

GaN-based light-emitting diodes (LEDs) are able to emit photons of higher energy than the injected electrons, resulting in an above-unity electrical efficiency. This phenomenon is generally attributed to heat extraction from the crystal lattice. In good agreement with measurements, we investigate the microscopic mechanism and the magnitude of such electroluminescent cooling by advanced numerical simulation including all relevant heat transfer mechanisms. Peltier cooling near the InGaN light-emitting layer is found to reduce the internal LED temperature rise significantly.

Comparison of modelled heat transfer and fluid dynamics of a flat plate solar air heating collector towards experimental data

Research concerning the field of Liquid Heating Collectors (SLHCs) cannot be directly transferred to the large area of applications for Solar Air Heating Collectors (SAHC). Larger cross sections for transporting the air are necessary. This results in rather complicated fluid distributions within the products. In this paper we combine simulation and experimental techniques providing a high level of detail. Highly resolved three dimensional fluid dynamic simulations contain all the relevant heat transfer mechanisms in a plate SAHC: Heat conduction, convection patterns and radiation are modelled in the two air gaps. In all solid materials of the collector only heat conduction has to be accounted for. For the exchange of heat between the two air zones the heat transfer through the absorber is modelled. Nevertheless careful simplification is needed to be able to concentrate on the important details. Furthermore we shed light on numerical instabilities observed in the air gap between absorber and glass in the simulated example.A consistent numerical description is given, concluding, that Computational Fluid Dynamics (CFD) is an appropriate tool for design and optimisation of SAHC concepts. The effects of collector tilting, insulation level and heat transfer surface increase have been assessed as a prove of applicability. The performance of these models will finally be compared to the experimental data obtained using a high-precision SAHC testing facility. For the experimental data base refined local measurement techniques, which meet highest accuracy requirements, are developed and employed.

Parametric analysis of a high temperature packed bed thermal storage design for a solar gas turbine

The development of a high temperature Thermal Energy Storage (TES) system will allow for high solar shares in Solar Gas Turbine (SGT) plants. In this research a pressurised storage solution is proposed that utilises a packed bed of alumina spheres as the storage medium and air from the gas turbine cycle as the heat transfer fluid. A detailed model of the storage system is developed that accounts for transient heat transfer between discrete fluid and solid phases. The model includes all relevant convective, conductive and radiative heat transfer mechanisms and is validated against high temperature experimental data from a laboratory scale test facility. The validated model is further utilised to conduct a parametric design study of a nominal six hour TES (1.55MWhth) for a gas micro-turbine. The concepts of utilisation factor and storage efficiency are introduced to determine the optimal storage design. The results of the study indicate that a storage efficiency of 88% and utilisation factor of 85% can be achieved when combining thermal storage and hybridisation with fossil fuels.

Dynamic simulation of integrated rock-bed thermocline storage for concentrated solar power

In contrast to wind and photovoltaic, concentrated solar power plants can be equipped with thermal energy storage in order to decouple intermittent energy supply and grid feed-in. The focus of this study is the technical evaluation of a cost-efficient storage concept for solar tower power plants. Consisting of a quartzite-rock bed that is charged with a hot air flow and discharged by cold air counter-flow, the storage essentially operates like a regenerator. For such systems, the discharge temperature typically declines with time. Furthermore, the use of a randomly packed bed results in considerable pressure loss. In order to describe the relevant flow and heat transfer mechanisms in rock beds used for thermal storage, a mathematical model written in the modelling language Modelica is developed and validated. Good agreement with experimental data from literature is obtained. With the aid of the validated model, a rock-bed thermal storage for application in a semi-industrial scale solar power plant (1.5MWel) is designed and optimised with respect to electrical efficiency of the plant during the charge and discharge cycle. The storage capacity is equivalent to four hours of full-load operation. Results show that compressor work should be considered directly in the selection of packed-bed geometry in order to minimise the efficiency penalty of storage integration in the solar plant.

Experimental and numerical investigation of a propane-fueled, catalytic mesoscale combustor

Small-scale thermochemical devices driven by hydrocarbon-fueled combustors have attracted increased attention for portable power generation applications due to their superior power density compared to state of the art Li-ion batteries. A combined experimental and numerical investigation is presented for the performance characteristics of a mesoscale, propane-fueled, catalytic combustor, to be used in an integrated, gas-turbine-based, mesoscale (ca. 100 W el) power generation system. Experiments in an optically accessible reactor led to a global reaction rate for the total oxidation of propane on platinum, valid over the pressure range 1 bar ≤ p ≤ 7 bar. Parametric numerical studies were subsequently carried out in a single catalytic channel, using a 2D elliptic code with the aforementioned validated kinetic scheme and all relevant heat transfer mechanisms. The predictions identified favorable materials and operating conditions for the burner under consideration. A subscale monolithic honeycomb reactor was finally constructed, based on the findings of the parametric study. The reactor met the set goals for power output as a function of mass throughput. However, heat losses to the environment were responsible for measuring reduced combustor efficiency at certain operating conditions. A continuum model for the entire monolithic structure complemented the experiments and provided the 2D temperature field. Computed exhaust gas temperatures of the monolith were in good agreement with the measurements.

Viscous dissipation effect on mixed convection flow of a micropolar fluid over an exponentially stretching sheet

Micropolar boundary-layer flow and heat transfer characteristics associated with a heated exponential stretching continuous sheet being cooled by a mixed convection flow are examined. The relevant heat transfer mechanisms are of interest in a wide variety of practical applications such as hot rolling, continuous casting, extrusion, and drawing. The wall temperature and stretching velocity are assumed to vary according to specific exponential forms. The contributions of buoyancy along with viscous dissipation on the convective transport in the boundary-layer region is analyzed in the opposing and assisting flow situations. Local similarity solutions are obtained for the boundary-layer equations governing the problem. A parametric study of the mixed convection parameter ξ, the micropolar parameter Δ, the Eckert number Ec, the parameter of temperature distribution n, and Prandtl number Pr is conducted and a representative set of numerical results for the velocity, angular velocity, temperature profiles, local skin friction coefficient, wall couple stress parameter, and local Nusselt number are illustrated graphically to show typical trends of the solutions.

Ignition and Extinction in Catalytic Partial Oxidation of Methane-Oxygen Mixtures with Large H2O and CO2 Dilution

The ignition and extinction in catalytic partial oxidation (CPO) of CH4/O2 mixtures with large exhaust gas dilution (46.3% H2O and 23.1% CO2 vol.) has been investigated experimentally and numerically at 5 bar. Experiments were carried out in a short contact time Rh-coated honeycomb reactor and involved temperature measurements along the reactor and exhaust gas analysis. Numerical predictions were performed with a 2-D transient elliptic code that included detailed chemical reaction schemes and relevant heat transfer mechanisms in the solid. The employed heterogeneous reaction scheme reproduced the measured minimum inlet temperatures required for catalytic ignition (light-off), the elapsed times for the propagation of the reaction front, and the steady-state exhaust gas compositions at a fuel-to-air equivalence ratio of ϕ = 4.0. The chemical impact of the added H2O, although important already at the early light-off stages, was minimal on the ignition delay times because the latter were dominated by total oxidation and not by partial oxidation or reforming reactions. The key reaction controlling catalytic ignition was the surface oxidation of CO to CO2, which was the main exothermic heat release step in the induction zone. Measurements and predictions indicated that vigorous combustion could be sustained at inlet temperatures at least as low as 473 K and 298 K in CPO with and without exhaust gas dilution, respectively. The extended stability limits of CPO combustion were due to a shift from partial to total oxidation products, and hence to higher exothermicity, with decreasing inlet temperature. The key parameter controlling extinction was the CO(s) coverage, which led to catalyst poisoning. Finally, operation at non-optimal stoichiometries (ϕ = 2.5) was shown to be beneficial in CPO of power generation systems with large exhaust dilution, due to the moderating effect of dilution on the maximum reactor temperature.

Experimental and numerical investigation of supported rhodium catalysts for partial oxidation of methane in exhaust gas diluted reaction mixtures

The partial oxidation of methane/oxygen mixtures with large exhaust gas dilution (46.3 vol% H 2 O and 23.1 vol% CO 2 ) has been investigated experimentally and numerically over Rh / CeO 2 – ZrO 2 , Rh / ZrO 2 and Rh / α - Al 2 O 3 catalysts. Experiments were carried out in a short-contact-time ( ∼ 8 ms ) reactor at 5 bar and included exhaust gas analysis, temperature measurements along the reactor, and catalyst characterization. Additional experiments were performed in an optically accessible channel-flow reactor and involved in situ Raman measurements of major gas-phase species concentrations over the catalyst boundary layer and laser-induced fluorescence (LIF) of formaldehyde. A full elliptic two-dimensional numerical code that included elementary hetero-/homogeneous chemical reaction schemes and relevant heat transfer mechanisms in the solid was used in the simulations. The employed heterogeneous reaction mechanism, including only active Rh sites, reproduced the experiments with good accuracy. The ratio of active to geometrical surface area, deduced from hydrogen chemisorption measurements, was the single model parameter needed to account for the effect of different supports. This indicated that water activation occurring on support sites, resulting in inverse OH spillover from the support to the noble metal sites, could be neglected under the present conditions with high water dilution. An evident relationship between noble metal dispersion and catalytic behavior, in terms of methane conversion and synthesis gas yields, could be established. Both measurements and predictions indicated that an increasing Rh dispersion (in the order Rh / α - Al 2 O 3 , Rh / ZrO 2 , and Rh / CeO 2 – ZrO 2 ) resulted in higher methane conversions, lower surface temperatures, and higher synthesis gas yields.

Modeling the physical processes relevant during hot pressing of wood-based composites. Part I. Heat and mass transfer

A three-dimensional model based on fundamental principles to simulate heat and mass transfer and mat densification during batch or continuous hot pressing of wood-based composites is presented in this pair of papers. The present paper gives a comprehensive description and mathematical formulation of the relevant heat and mass transfer mechanisms, which are water vapor and air transfer by gas convection and molecular gas diffusion, conductive and convective heat transfer, including sorptive effects. The heat and mass transfer model consists of a set of constitutive flux equations that are coupled by local energy and mass balances.

Mathematical modeling for the solidification heat-transfer phenomena during the reflow process of lead–tin alloy solder joint in electronics packaging

In electronic packaging lead–tin alloy is frequently used to joint electronic components. For the formation of solder joint, lead–tin alloy usually undergoes a reflow process which includes spreading, re-melting, and then solidification of the alloy. Therefore, the properties of lead–tin alloy solder joint and in turn the success of electronic packaging will be significantly affected by the reflow process. In this study, solidification heat-transfer analysis is conducted for a auto-rectifier to obtain the temperature distribution and variation of the lead–tin alloy as well as the whole electronic part during the reflow process. Explicit finite difference method is employed to conduct the solidification simulation for the reflow process. First a solid model for analysis is constructed based on the actual shape and size of the electronic component including the solder joints. Then the model is meshed for the subsequent numerical analysis. The next step is to investigate all possible heat-transfer mechanisms. All the heats to be extracted and relevant heat-transfer mechanisms are considered in the solidification model to calculate for the temperature distribution and variation of the lead–tin alloy solder joints during the reflow process. Temperature measurement at one particular location of the auto-rectifier is conducted during the whole reflow process in an actual production line. The calculated temperature for the same position is then compared to the measurement and satisfactory consistency is found. Then a shrinkage criterion is also used to quantitatively predict the extent of shrinkage formation in the solder joints.

Thermal analysis of a simulated CANDU 37-element spent fuel bundle with air backfill

Using a thermal mock-up of a simulated CANDU 37-element spent fuel bundle within a fuel basket, a series of experiments were performed to obtain the maximum fuel rod temperature along with the radial and axial temperature distributions within the fuel bundle. The main purpose of these experiments was to characterize the relevant heat transfer mechanisms in a dry, vertically oriented CANDU spent fuel bundle and to verify the MAXROT code developed earlier for the thermal analysis of a CANDU spent fuel bundle in a dry storage basket. A total of 48 runs were made with eight different power input to the 37-element heater rod bundle ranging from 5 to 40 W, while using six different band heater power input from 0 to 250 W to maintain the basket wall at a desired boundary condition temperature at the steady state. The experimental data were compared with the predictions of the MAXROT code to examine the code’s accuracy and validity of assumptions. The MAXROT code explicitly models each representative fuel rod in a CANDU fuel bundle and treats the conductive and radiative heat transfer amongst the rods. Comparisons between the measured and predicted maximum fuel rod temperatures of the simulated CANDU 37-element spent fuel bundle for all 48 tests show that the MAXROT code slightly overpredicts the measured data within 2.8% with a mean deviation of 3.2°C.

Mixed convection cooling of a heated, continuously stretching surface

An numerical study of the flow and heat transfer characteristics associated with a heated, continuously stretching surface being cooled by a mixed convection flow has been carried out. The relevant heat transfer mechanisms are of interest in a wide variety of practical applications, such as hot rolling, continuous casting, extrusion, and drawing. The surface velocity of the continuously stretching sheet was assumed to vary according to a power-law form, that is, u w (x)=Cx p . Two conditions of surface heating were considered, a variable wall temperature (VWT) in the form T w (x)−T ∞=Ax n and a variable surface heat flux (VHF) in the form q w (x)=Bx m . The governing differential equations are transformed by introducing proper nonsimilarity variables and solved numerically using a procedure based on finite difference approximations. Results for the local Nusselt number and the local friction coefficient are obtained for a wide range of governing parameters, such as the surface velocity parameter p, the wall temperature exponent n, the surface heat flux exponent m, the buoyancy force parameters (ξ for the VWT case and χ for the VHF case), and Prandtl number of the fluid. It is found that the local Nusselt number is increased with increasing the velocity exponent parameter p for the VWT case, while the opposite trend is observed for the VHF case. The local friction coefficient is increased for a decelerated stretching surface, while it is decreased for an accelerated stretching surface. Also, appreciable effects of the buoyancy force on the local Nusselt number and the local friction coefficient are observed for both VWT and VHF cases, as expected.

Predicting effective thermal conductivities of unbonded and bonded silica sands

A theoretical model to predict the effective thermal conductivities of unbonded and bonded sands is developed by considering the relevant heat transfer mechanisms and their relationship to the model geometry. Comparison between model results and experimental measurements indicates that the assumptions leading to the introduction of two model parameters (φef and δ) were justified. As a result, the model was proved to predict the effective thermal conductivities of unbonded and bonded silica sands within reasonable accuracy and to estimate the effects of important parameters reasonably well.

Computational model for analysis of dynamic thermal performance of a hybrid slab-collector system with passive discharge

The paper presents a mathematical model developed to simply analyze a hybrid hollow-core-floor-slab-wall-air-collector system with passive discharge to the rooms above and below the slab. It is based on a one-dimensional (longitudinal) detailed thermal analysis of the basic module, that is comprised of two parts (one along the cores with incoming air, and the other along those with outgoing air). The model accounts, in addition to the exact heat exchange along the module, for transverse heat exchange between the two parts within the module, transverse heat exchange with the adjacent modules, and heat exchange with the surrounding air at every point along the system. It incorporates thus the relevant heat transfer mechanisms associated with all the physical factors of the system that require optimization, while avoiding the time consuming complete 3-D thermal analysis required for a thorough and totally accurate analysis. The field equations consist of four second order partial differential equations, with four unknowns, at every point along the system. The numerical solution was accomplished by means of a finite difference scheme, that yielded a banded coefficients matrix. The integration was performed for 1 h time steps by means of a computer program (DEKOLTA—Decke-Kollektor-Thermal-Analysis) developed specifically for the purpose. In order to illustrate the application of the procedure for thermal performance analysis of a hybrid system the paper presents results for three case studies of a system with two different rooms along the slab.

Thermal Effects During Infrared Solder Reflow—Part I: Heat Transfer Mechanisms

An experimental study has been conducted to reveal the relevant heat transfer mechanisms which exist within an infrared reflow oven. Simulated card assemblies are used and their transient thermal responses, induced by combined radiative and convective heating, are measured. A simple numerical model is developed with which relevant heat transfer mechanisms are identified and quantified. The study shows that radiative and mixed convective heat transfer processes induce a variety of system thermal responses. Model predictions, which incorporate measured forced convection heat transfer coefficients and accurate descriptions of surface-to-surface radiative exchange, are in excellent agreement with experimental data for cases where the thermally induced buoyancy forces within the oven air are relatively small. The results of the experimental and analytical study provide guidelines for the development of more sophisticated models of the infrared reflow process.

Thermal Transport From a Heated Moving Surface

A numerical and analytical study of the transport process arising due to the movement of a continuous heated body has been carried out. The relevant heat transfer mechanisms are of interest in a wide variety of practical applications, such as continuous casting, extrusion, hot rolling, and crystal growing. The conjugate problem, which involves a coupling between the heat transfer in the moving material and the transport in the fluid, is considered. The thermal fields in the material and in the fluid are computed. The temperature level is found to decay gradually with distance along the moving material, as expected. Results are obtained for a wide range of governing parameters, particularly the Peclet number Pe and the parameter R, which depends on the properties of the fluid and the material. The results obtained are compared with those for the idealized cases of an assumed surface heat transfer coefficient and of a moving isothermal surface. Of particular interest were the nature of the flow generated by the moving surface and the resulting thermal transport. The results obtained are also considered in terms of the underlying physical processes in the problem.

Thermal structure of the accreting Earth

The energy associated with the accretion of the earth and the segregation of the core is more than sufficient to melt the entire earth. In order to understand the thermal evolution of the early earth it is necessary to study the relevant heat transfer mechanisms. In this paper we postulate the existence of a global magma ocean and carry out calculations of the heat flux through it in order to determine its depth. In the solid mantle heat is transferred by the upward migration of magma. This magma supplies the magma ocean. The increase in the mantle liquidus with depth (pressure) is the dominant effect influencing heat transfer through the magma ocean. We find that a magma ocean with a depth of the order of 20 km would have existed as the earth accreted. We conclude that the core segregated and an atmosphere was formed during accretion.