Conjugate Heat Transfer Model Research Articles

Determination of heat transfer criterial equation when cooling aluminum ingots

A big problem when casting aluminum ingots is the uneven structure formation, which leads to an increased rejection of products. Nonequilibrium structure elimination is carried out by heat treatment. To obtain the required aluminum ingots’ physicochemical properties, it is necessary to know the conditions of heat transfer between the ingots and the cooling air, i.e. a mathematical model of conjugate heat transfer is needed. The mathematical model obtained by the authors makes it possible to analytically investigate the ingots temperature and cooling air during heat treatment. This mathematical model assumes the heat transfer coefficient calculation. The existing criterion equations for determining the heat transfer coefficient have a drawback - the heat transfer coefficient according to these equations is calculated in circular channels, while heat transfer between aluminum ingots and air occurs in rectangular channels. The article describes the criterion equation identification for heat transfer, used in the analytical study, by the data of the experimental study.

Conjugated Heat Exchange in Heat Treatment of Aluminum Ingots Simulation

Casting aluminum to obtain semi-finished products - round ingots, due to uneven cooling in the mold, leads to various defects that affect further machining. To eliminate such defects, heat treatment is carried out - homogenization annealing. One of the homogenization important stages is the cooling of the ingots after heating at a rate that does not lead to the ingot quenching. The cooling medium is air. Knowing the conditions of heat exchange between the cooling air and the high-temperature aluminum billet makes it possible to obtain the ingot’s necessary physical and mechanical properties. The article describes the developed mathematical model of conjugate heat transfer during homogenization annealing of aluminum ingot. It allows analytically calculating the temperature of the ingots depending on the cooling time. To verify the data obtained by the mathematical model, the conjugate heat transfer in the ANSYS program was simulated.

Comparison of lumped and 1D electrochemical models for prismatic 20Ah LiFePO4 battery sandwiched between minichannel cold-plates

In this paper, we present a detailed comparison study on the prediction accuracy by two different and simplified battery models, namely Lumped and Li-ion (1D electrochemical) model, for a prismatic 20Ah LiFePO4 battery sandwiched between minichannel cold-plates with distributed flow. A two-way coupling between the battery model (Li-ion/Lumped) and 3D conjugate heat transfer model is considered for heat generation and dissipation rates at different discharge rates (1-4C) and coolant inlet temperatures (15-35°C). The predicted battery voltage response at room temperature (22°C) and the performance of Battery Thermal Management System (BTMS) in terms of the battery surface temperatures (maximum temperature, Tmax and temperature difference, ΔT) are compared between the models. Further, to get the insights on the battery heating rate by these models, ten different locations are selected on the battery surface and temperature variation during the discharge process are compared. It is found that Lumped model predicted temperatures are in close agreement with that of Li-ion model with a difference of 2.1% at 1C discharge rate. However, the difference has increased to 16.4% at 4C discharge rate, and is attributed to the non-accountability of chemical reactions by the Lumped model in contrast to the Li-ion model (as a benchmark). Therefore, it is suggested to use Lumped model for preliminary studies on designing the BTMS because it requires only three measurable electrical parameters.

A comparison study on thermal performance enhancement of corrugated oil cooler with internal turbulators – Numerical and experimental approach

Oil coolers play an important role in dissipating excess heat from engine oil to surroundings. In this work, performance study of an oil cooler with internal turbulators of dimple and offset type is performed using numerical approach and compared with experimental results for validation. In numerical approach, oil cooler with turbulators are numerically modeled using Conjugate Heat Transfer (CHT) approach to solve for heat transfer characteristics. Turbulence physics modeling is done using Reynolds Averaged Navier Stokes equations (RANS) through realizable k-epsilon model with standard wall function. The air side Heat Transfer Coefficient (HTC) estimation is done with CHT model using one corrugation, as the air flow in fins is periodic. In experimental study, Required Inlet Temperature Difference (ITD), Oil Flow Rate, Air Flow Rate is maintained, measurements are recorded to create performance data matrix with combinations of various operating data points for air and oil sides respectively. Numerical results are compared with experimental results obtained; Deviation for heat rejection of oil-cooler with dimple turbulator fall in range of ±7%, offset turbulator fall in the range of −2.0 to 9.4%. Numerical data shows that the thermal performance of offset type is better than dimple type by 10–13.5%, while the test data depicts that performance of oil-cooler with offset turbulator is high by 12–19% than dimple type and thereby proving its performance superiority. In offset turbulator additional turbulent scales of the offset length scale range are generated, these additional scales are evident by increasing the Turbulent Kinetic Energy (TKE) of 46–51% than dimple turbulator from numerical study. Thus, for same plate and fin, oil-cooler with offset turbulator has better thermal performance than oil cooler with dimple turbulator.

Investigation of fires in a mechanically ventilated compartment using the CFD code FireFOAM

The outbreak of fires in nuclear power plants is a major risk due to the potential leak of radioactive materials. The use of traditional prescriptive fire safety regulations has shown its limitations and there is now a shift towards performance-based fire safety engineering, which requires well-validated fire models. Nuclear power plants require the use of mechanical ventilation, which provides dynamic confinement for nuclear materials by maintaining the required pressure. The occurrence of fires could potentially result in pressure variations within power plants. Although dynamic confinement along with other safety measures are in place to prevent fires, there is a continuous need to assess fire safety measures and reduce the risk of fire propagation with the use of fire simulation codes.The current study aims to build on existing research by making use of an emerging open-source computational fluid dynamics (CFD) fire simulation code known as FireFOAM, to predict fire behaviour in a mechanically ventilated nuclear compartment. An existing in-house modified version of FireFOAM developed by the authors’ research group, is further modified in the present work to include a Conjugate Heat Transfer (CHT) model to account for the heat transfer between combustion gases and solid boundaries. The CHT is validated using the experimental wall temperatures and heat fluxes. Furthermore, a mechanical ventilation model has been developed and implemented into FireFOAM. This newly modified version of FireFOAM is employed to predict the pressure variations in a nuclear compartment and the flow rates in the ventilation network. The predictions are compared to some experimental data from the open literature. Overall, it is shown that the mechanical ventilation model and the modified FireFOAM with CHT can predict the pressure variations and flow rates with a relatively good level of accuracy.

Conjugate Heat Transfer Analysis of Flat Plate Subjected to Impingement and Film Cooling

Abstract: The present computational study involves a flat plate subjected to combined effect of jet impingement and film cooling. A conjugate heat transfer model in conjunction with k-ω SST turbulence model is employed to study the turbulence effects. The effect of Reynolds number varying from 389 to 2140 on static temperature, Nusselt number and film cooling effectiveness has be discussed for the blowing ratios of 0.6, 0.8, 1.0. The variation in the size of vortices formed on the impinging surface with Reynolds number is studied. It has been observed that the local Nusselt number shows a rising trend with the increase in Reynolds number, while the static temperatures follow the downfall in its values. As a result, an enhancement in the effectiveness is observed, which is credited to the capabilities of combined impingement and film cooling. At Reynolds number of 972, the coolant jet is found to be attached to the surface, for this condition the heat transfer phenomena for blowing ratios of 0.6, 0.8, 1.0, 1.2, 1.6, 2.0, 2.4, 2.6 are studied to understand the flow distribution on the plate surface. Keywords: Jet impingement, film cooling, effectiveness, conjugate heat transfer

Design and optimization study of a densely packed ultrahigh concentration photovoltaic thermal array for desalination usability

The cogenerated energy from the densely packed ultrahigh concentration photovoltaic thermal systems (UHCPV/T) can not only promote higher exergetic efficiency but also enable the usability of the collected thermal energy for potential desalination purposes. In the present study, three finned heat sink designs of UHCPV/T have been proposed toward maximum collected thermal power and minimum required pumping power. A conjugate heat transfer model has been developed and validated to simulate the different designs under various operating conditions. The cooling setups are based on copper millimeter-scale finned heat sinks immersed in fully developed laminar flow in a rectangular microchannel. It was found that the configuration from the more efficient heat sink design to the poorest thermal one is the rectangular fins (RFs), elliptical fins (EFs), diamond pin-fins (DPFs), and then traditional round pin-fins (TRPFs) heat sink. The RF heat sink has demonstrated the best thermal performance with a maximum multi-junction solar cell (MJSC) temperature of 81.4°C and water temperature increase of 14.21°C under ×1200 and water inlet velocity of 0.02 m/s. Despite the RF heat sink has recorded the highest values of pressure drop and required pumping power equal to 78.74 Pa and 570.7 mW, respectively, it was selected as the optimal heat sink design, ensuring the highest net electrical output power and thermal collected power.

Effect of a thermal sleeve on the temperature non-uniformity of an annuli under solar irradiation

Heat transfer of an annuli under solar irradiation or forced convection is numerically studied. A conjugate heat transfer model, which includes conduction, natural convection and surface radiation, is developed and solved by finite volume method. The effects of the thickness of the inner cylinder on the temperature contours are analyzed. Results show that the temperature in the domain is non-uniform when it is exposed directly to the sun radiation, and is almost uniform when a thermal sleeve is used. Neglecting the surface radiation between the barrel wall and sleeve wall will underestimate the temperature of the annuli. During continuous firing conditions, natural convection in the air gap shows chaotic behavior.

Thermal and electrical performances of actively cooled concentrator photovoltaic system

• Reliable thermal management for CPV is mandatory for efficient/safe operation. • 3D heat transfer/thermal model for DL-MCHS coupled with CPV layers are developed. • Including and ignoring the header of DL-MCHS is considered in the simulation work. • Without headers, CPV/T system attained a lower average and local cell temperature. • With headers, electrical efficiency is improved while pumping power is augmented. The double-layer microchannel heat sink (DL-MCHS) is commonly applied for thermal management of high heat flux surfaces, including electronic chips and concentrator photovoltaic (CPV). However, most of the recent studies neglected the header effect in the simulation of the DL-MCHS. The current study compressively investigated the impact of considering the inlet and outlet headers on the CPV systems' performance. A three-dimensional conjugate heat transfer model of the fluid flow in the DL-MCHS coupled with the CPV layers' thermal model was employed. The thermal performances of DL-MCHS with and without considering the headers were evaluated. Parallel flow (PF) and counter flow (CF) conditions at a solar concentration ratio ( CR ) of 20 suns were studied. The results showed that the header section should be considered in the numerical model for obtaining a more realistic average and local silicon layer temperature profile, especially in the CF operation. For instance, the difference in solar cell temperature between including and ignoring the header section is about 13.86 K for CF operation at CR of 20 and 200 ml/hr coolant flowrate. The predicted pumping power and net gained electric power were also significantly affected by neglecting the headers in the numerical modelling. The electrical efficiency deviation reached about 23.5% at CF, while PF reported almost 4.0%.

Numerical investigation of turbocharger turbine temperature field based on conjugate heat transfer

Aiming at the issues of low accuracy and poor feasibility of the analytical results of the turbocharger turbine temperature field under operating conditions, a full-domain conjugate heat transfer numerical model was established by the conjugate heat transfer and finite volume method. The temperature field characteristics of each component of the turbocharger turbine were analyzed. The numerical and experimental test results were compared and analyzed. The global conjugate heat transfer model avoids the input of a large number of hypothetical data on the interface between fluid and solid in the traditional model, and makes the calculation process closer to the actual situation. Through the comparison with the experimental results, the accuracy of the turbine temperature field obtained by the global conjugate heat transfer model is more reasonable and more accurate than that of the traditional model, which verifies the reliability and accuracy of the global conjugate heat transfer model.

Study on flow and heat and mass transfer characteristics of cross-flow poly propylene hollow fiber membrane module

ABSTRACT Hollow fiber membrane solution dehumidification technology is a new type of dehumidification technology combining solution dehumidification and membrane separation. It is currently widely used in the field of air conditioning dehumidification. In this study, membrane dehumidification technology was applied to low-temperature flue gas dehumidification, and a model of regularly arranged hollow fiber membrane modules was established on the basis of a conjugate heat and mass transfer model. The accuracy of the model was verified by comparing it with literature and experimental data, applying a random arrangement of components. Numerical simulation is carried out by COMSOL software. The flue gas velocity, temperature and water vapor concentration field in the module are coupled together, and the flue gas flow and heat and mass transfer characteristics are affected by the distance between the membrane filament tubes. The dehumidification and cooling efficiency of the randomly arranged hollow fiber membrane module can reach 68.3% and 39.1%,respectively, which Close to 72.1% and 48.7% of regularly arranged components. And the components can be optimized based on the model.

CFD validation of condensation heat transfer in scaled-down small modular reactor applications, Part 2: Steam and non-condensable gas

This paper presents the computational fluid dynamics (CFD) validation and scaling assessment of the condensation heat transfer (CHT) models in the presence of non-condensable gas for the passive containment cooling system (PCCS) of the small modular reactor (SMR). The STAR-CCM+ software with 3D scaled-down SMR containment geometries was used in CFD simulations with steam and non-condensable gas (NCG). The limitations and approximations of the previous studies were resolved to avoid scaling distortion and uncertainties. Air was used as the NCG gas with steam. The multi-component gas model was used to define the steam-NCG mixture, and the condensation-seed parameter was used as the source term for the fluid film model. Three different turbulence models were used to check the heat flux performances and temperature distributions on the coolant side. The heat flux was estimated from the axial coolant bulk temperature, which was identical to the test data reduction method. An implicit-unsteady numerical solver was applied to the conjugate heat transfer models between the gas, liquid, and solid regions. Detailed simulations were performed, and simulation results were validated with the measured parameters experimentally. The condensation heat transfer performance was quantified using non-dimensional numbers and compared for different scaled geometries to identify the scaling distortions.

Multi-objective topology optimization of pin-fin heat exchangers using spectral and finite-element methods

Forced convective pin-fin heat exchangers, due to the high wet surface area per volume and the hindered thermal boundary layers, feature low thermal resistances. However, the considerable coolant pressure drop, particularly for densely packed fin arrays, imposes operational costs for pumping power supply. This paper develops a multi-objective topology optimization approach to optimize sink geometries in order to minimize thermal resistance and pressure loss, concurrently. In accordance to the pin-fin geometrical characteristics, a dedicated pseudo-3D conjugate heat transfer model is utilized, by assuming periodic flow and fin design pattern, to reasonably reduce the high cost of full-3D model optimization. For the solution of flow part, a pseudo-spectral scheme is used, which is intrinsically periodic and features a high spectral accuracy, and the finite element method for the non-periodic conjugate heat transfer model. Exact partial derivatives of the discrete solutions are obtained analytically by hand-differentiation. This task is rather convenient for the flow part, due to the simplicity of the pseudo-spectral implementation; however, the MATLAB symbolic toolbox is selectively utilized for the finite element code to ensure an error-free implementation. Finally, the sensitivities are computed directly or via a discrete adjoint method, for the flow and heat models, respectively. To examine the present approach, two approaches are used for optimization of a practical cooling task: constrained and unconstrained multi-objective formulations, where in all cases more emphasis is placed on thermal resistance minimization. A series of optimized heat sink geometries via constrained or unconstrained multi-objective optimizations are obtained to examine practical utility of the present approach. The optimized topologies demonstrated superior cooling performances at lower costs of pressure losses compared to conventional (circular) in-line and staggered fins, and confirmed the supremacy of topology over pure sizing optimization.

Performance enhancement in minichannel heat sinks using supercritical carbon dioxide (sCO2) as a coolant

Minichannel heat sinks are widely adopted heat dissipation solutions from miniaturized heat flux generating components, such as microprocessors. This paper presents further enhancements in the thermal and hydraulic performance of a minichannel heat sink by employing supercritical carbon dioxide (sCO2) as a coolant. In this study, thermal and hydraulic performance of CO2-cooled minichannel heat sink at three inlet pressures (Pin,CO2) (i.e., 8.0, 10 and 12 MPa)and inlet temperatures (Tin,CO2) ranging between 30∘C−60∘C is compared with a water-cooled minichannel heat sink at inlet conditions (0.1 MPa,30 °C−38 °C). To investigate the performance of the minichannel heat sinks, a conjugate heat transfer model is solved in a commercial code ANSYS-CFX. Whereas the variations in thermo-physical properties of supercritical carbon dioxide (sCO2) are incorporated in ANSYS-CFX through a real gas property (RGP) table. The results reveal a maximum enhancement of 42.13% in average heat transfer coefficient (h¯) for minichannel heat sink using CO2 as a coolant at inlet conditions of 8.0 MPa and34∘C. Similarly, a pressure drop reduction of 55.79% is computed for CO2 as a coolant at corresponding inlet conditions as compared to water-cooled minichannel heat sink. Moreover, the results suggest that replacing water with CO2 as a coolant at higher inlet pressures (Pin,CO2) i.e., 10 MPa and 12 MPacan further reduce pressure drops in the minichannel heat sink by 60.65% and 62.41%, respectively. Apart from the lower base temperatures, performance evaluation criteria (PEC) suggests that the overall performance of the minichannel heat sink using CO2 at 8.0 MPa is roughly enhanced by 2 times as compared to water-cooled minichannel heat sink.

Numerical Investigation of a Radially Cooled Turbine Guide Vane Using Air and Steam as a Cooling Medium

Gas turbine performance is closely linked to the turbine inlet temperature, which is limited by the turbine guide vanes ability to withstand the massive thermal loads. Thus, steam cooling has been introduced as an advanced cooling technology to improve the efficiency of modern high-temperature gas turbines. This study compares the cooling performance of compressed air and steam in the renowned radially cooled NASA C3X turbine guide vane, using a numerical model. The conjugate heat transfer (CHT) model is based on the RANS-method, where the shear stress transport (SST) k−ω model is selected to predict the effects of turbulence. The numerical model is validated against experimental pressure and temperature distributions at the external surface of the vane. The results are in good agreement with the experimental data, with an average error of 1.39% and 3.78%, respectively. By comparing the two coolants, steam is confirmed as the superior cooling medium. The disparity between the coolants increases along the axial direction of the vane, and the total volume average temperature difference is 30 K. Further investigations are recommended to deal with the local hot-spots located near the leading- and trailing edge of the vane.

Retrofitting abandoned petroleum wells as doublet deep borehole heat exchangers for geothermal energy production—a numerical investigation

This study aims to develop a simulation model to investigate the feasibility of extracting geothermal energy from abandoned petroleum wells using doublet deep borehole heat exchangers (DBHEs) in the Western Canadian Sedimentary Basin near Hinton. A three-dimensional simulation model that simplifies heat convection to one dimension was built and verified with a conjugate heat transfer model. The simulation results show that doublet DBHEs perform better than single-well DBHEs in extracting geothermal energy from abandoned wells. The lateral branch contributed much greater to the performance than other sections in the doublet DBHE: the average temperature increase rates per unit length of different sections were 1.59, 3.53, and 1.17 °C/km after 25 years in the injection well, the lateral branch, and the production well, respectively. Employing multilateral branches can significantly improve the performance of doublet DBHEs. Besides, the outlet of the doublet DBHE had a higher static pressure than the inlet due to the thermosiphon effect. A higher production temperature led to a lower static pressure difference between the inlet and outlet of the doublet DBHE. Using doublet DBHEs, abandoned wells in the Western Canadian Sedimentary Basin near Hinton can work as a heat source either for direct use or power generation.

A quasi-transient coupling approach to the modeling of conjugate heat transfer in the autoclave

Structural aerospace composite parts are generally cured in an autoclave. To achieve a homogeneous curing, computational fluid dynamics simulations have been increasingly used in thermal optimization. However, a transient computational fluid dynamics simulation of autoclave processing is resource intensive. This article outlines the concept of a quasi-transient coupling strategy to deal with the conjugate heat transfer problem inside an autoclave. In this approach, a computational fluid dynamics model is coupled with a finite element method (FEM) model through incorporating an empirical-based analytic equation, which describes the dependence of the heat transfer coefficient on pressure and temperature, into the computational fluid dynamics computations. This approach bridges the temporal disparities between the fluid and the solid, thus minimizing the global computing time. To validate this method, two simulation cases have been studied. In both cases, two different coupling computations are compared, namely a full-transient simulation as the reference computation and the introduced quasi-transient simulation. First, the quasi-transient coupling approach is implemented by performing the transient heat transfer analysis on a flat plate. The results indicate that this approach can predict accurate transient temperature fields, and the computational effort is reduced by up to 87%. Subsequently, this method is used in a real autoclave and validated by known experimental data. The simulation results are in good agreement with the experimental results, with a mean temperature error lower than 1.9°C. This indicates the capability and efficiency of this approach in solving a conjugate heat transfer problem for autoclave processing.

Influence of conjugate heat transfer on the minimization of entropy generation for forced convective flow through parallel plate channel filled with porous material

AbstractA fluid–solid conjugate heat transfer model is developed to analyze the characteristics of entropy generation for forced convective steady hydrodynamically fully developed laminar flow of a Newtonian fluid through a parallel plate channel filled with porous material by modulating the following parameters: substrate thickness, the ratio of thermal conductivity of wall to fluid, Biot number, the axial temperature gradient in the fluid, and Peclet number. The exteriors of both the walls are subjected to the thermal boundary conditions of the third kind. The mass and Brinkman momentum conservation equations in the fluidic domain and the coupled energy conservation in both the solid and fluidic domain are solved analytically using the local thermodynamic equilibrium model, so as to derive closed‐form expressions for the velocity in the fluid and the temperature both in the fluid and solid walls in terms of relevant parameters. Suitable combinations of influencing factors, namely the geometric parameters of the system, fluid, flow, and substrate properties are identified for which global entropy generation rate is minimized. The findings may be helpful in the design of thermal systems frequently used in diverse engineering applications having heat transfer in the solid wall being a crucial parameter.

3D modeling framework and investigation of pollutant formation in a condensing gas boiler

Condensing gas boilers exhibit complex global flow-field patterns, which lead to noteworthy inhomogeneities of the burnt gas temperature distribution and levels of local pollutant formation in the combustion chamber. To enable numerical simulations of realistic heating systems, a modeling framework was developed. Challenges like high computational times and the combined multi-physics aspects that occur in quite complex geometries were addressed. The flamelet progress variable (FPV) model was applied, which tabulates detailed reaction kinetics as a function of progress variable and enthalpy. As the larger timescales of CO and NO formation inhibit a direct lookup of local CO and NO mass fractions from the chemistry table, the so called NOMANI model was utilized for NO and a rescaling model was applied for CO. All models were validated against resolved simulation results and experimental measurements. The multi-hole burner was modeled as a porous medium, because it was computationally unfeasible to resolve each burner hole. While local gas acceleration and preheating was considered, the exact flame structure as well as the recirculation zone behind the flame front could not be represented. To still capture the burnt gas temperature, the missing heat losses from the recirculation zone were modeled and coupled to the conjugate heat transfer (CHT) model. The modeling framework was applied to a commercial condensing gas boiler device at nominal load utilizing a lean methane-air mixture. The simulation shows reasonable agreement with experimentally measured emission levels and critical regions with CO and NOx emissions twice as high as in other regions were identified. Temperature variations of 200 K in the flue gas throughout the combustion chamber and 450 K in the solid region of the multi-hole burner were found. Based on these results, design aspects are correlated to CO and NO formation and suggestions on possible design modifications are discussed to further reduce pollutant emissions of such condensing gas boilers.

Numerically Based Reduced-Order Thermal Modeling of Traction Motors

This article presents an approach based on numerical reduced-order modeling to analyze the thermal behavior of electric traction motors. In this article, a single conjugate heat transfer analysis provides the possibility to accurately predict thermal performances by incorporating both computational fluid dynamic and heat transfer modules. Then, the developed model is used as the basis for deriving a fast reduced-order model of the traction motor enabling prediction of motor thermal behavior in duty cycles with a high number of operating points. All the results achieved are verified using flow and temperature measurements carried out on a traction motor designed and built for a traction application. A good agreement between the measured and estimated values of flows and temperatures is achieved while keeping the computation time within a reasonable range for both the full-order and reduced-order conjugate heat transfer models. The optimized full-order model can be run in minutes and the reduced-order model computation time is less than one second per operating point. The transient simulation based on the reduced-order model is conducted and both the learning phase and validation results are well illustrated. It is shown than the deviation of the reduced-order model in estimating the motor thermal performance is less than one celsius degree from the full-order model.