Conjugate Heat Transfer Analysis Research Articles

Influence of secondary flow channels on heat transfer and fluid flow characteristics in curved microchannel

AbstractMicrochannel‐assisted cooling technologies have found extremely suitable for compact heat exchangers surpassing conventional fan‐assisted heat sink solutions. Though extensive research was done on straight channels, research on curved microchannel is scant. In the present study, hydraulic flow characteristics and cooling performance of curved microchannel with secondary flow (CMCSF) with four different oblique angles are investigated numerically and compared with conventional curved microchannel (CCMC). A 3D conjugate heat transfer analysis is performed for four CMCSFs and CCMC using commercial software ANSYS Fluent with Reynolds number (Re) ranging between 125 and 325. A part of fluid from main channel is diverted into the secondary channels, leading to re‐initialization of boundary layers. Also, this diverted flow that is fed into adjacent main channels consecutively results in better fluid mixing. Owing to these two phenomena, there is a huge enhancement in heat dissipation through the channel. The results revealed a Nusselt number increment of up to 80.12% for CMCSF in comparison with CCMC. Moreover, the average wall temperature was recorded lowest for CMCSF with oblique angle 30°, which is 8.83°C lower in comparison with CCMC for same Re. For the entire range of Re considered, the overall performance of all CMCSFs was found better than CCMC with maximum performance factor of 1.62 being achieved with secondary channel orientation of 24°.

Gas Flow and Ablation of 122 mm Supersonic Rocket Nozzle Investigated by Conjugate Heat Transfer Analysis

The propellant gas flow of a supersonic rocket in inappropriate operating conditions can cause excessive ablation inside a nozzle. In this research, conjugate heat transfer analysis (CHTA), consisting of computational fluid dynamics (CFD) and finite element analysis (FEA), was applied to investigate the gas flow and ablation of a 122 mm nozzle as a case study in the transient state, based on actual operating conditions. First, the nozzle was tested in a static experiment. Then, the experimental results were employed for CHTA settings and validation. Next, after completing the CFD calculation, the results revealed that the nozzle’s gas flow, temperature, pressure, Mach number, shock, etc. were consistent with theoretical results. Finally, using the CFD results as loads, the FEA results showed the equivalent von Mises stress (σv), which was consistent with the ablation results from the experiment, as expected. The more the σv, the greater the ablation. Both σv and ablation were high near the throat and decreased further away. In addition, increasing the insulators’ thickness reduced σv, leading to ablation reduction. The research findings contribute to an understanding of ablation and the methodology of employing CHTA to improve the design of 122 mm and other nozzles with reduced ablation for higher efficacy.

Numerical Analysis of Conjugate Heat Transfer of Supercritical CO2 and Pb‐Bi in Tube‐in‐Tube Channels

AbstractThis work presents a numerical analysis of conjugate heat transfer in an annular channel with turbulence models for supercritical CO2 (tube side) and Pb‐Bi (annular side) as working fluids to understand the impact of flow constraints and heat transfer. By using the Navier‐Stokes formulation and the energy equation clarifying concurrent flow buildup and conjugate heat transmittance in tubes, several turbulent Prandtl models were solved numerically. The turbulent Prandtl number Prt of Pb‐Bi was used to refine the consistency of the arithmetical simulation. By comparing Prt, temperature fluctuation, and heat flux density, it was concluded that the existing Prt model should be used with caution in the numerical calculation of liquid metals having low Pr. The heat transfer and flow phenomena at different Prt values are significantly different.

Airside Thermal Performance of Louvered Fin Flat-Tube Heat Exchangers with Different Redirection Louvers

The performance of heat exchangers is severely limited by airside thermal resistance. The effect of redirection louvers (RLs) on the airside thermal performance of a compact flat-tube louvered fin heat exchanger was investigated. A steady-state 3D numerical analysis was conducted for different fin configurations by varying the number of RLs (NRL = 1, 2, 3, and 5). Conjugate heat transfer analysis was performed at the low Re (50–450) for domestic and transport air-conditioning applications. Geometric parameters such as louver pitch, louver angle, fin pitch, and flow depth were set as 1.7 mm, 27°, 1.2 mm, and 20 mm, respectively. The effective heat transfer fin surface areas of different fin configurations were also kept identical for a comparative analysis. The influence of the RLs on the airside thermal–hydraulic performance was analysed by exploring the local and average Nusselt numbers, pressure drop, Colburn j factor, friction factor f, performance evaluation criteria (PEC), and flow efficiency of different fin configurations. The numerical results revealed that the asymmetric fin configuration with two RLs (NRL = 2) showed the best heat transfer performance for the entire Re range. It resulted in a 33% higher average Nusselt number, causing a 24% higher pressure drop compared to NRL=5. At low flow velocities (Re < 75), NRL = 3 showed better PEC; however, at high flow velocities (Re > 75), NRL = 1 outperformed other fin configurations. Finally, it was noted that increasing the number of RLs reduced the amplitude of the wavy-shaped flow formed between the neighbouring louvered fin, consequently deteriorating the flow efficiency.

Theoretical and numerical analysis of conjugate heat transfer for supercritical CO2 flowing in printed circuit heat exchanger

The printed circuit heat exchanger (PCHE) is often utilized for the supercritical CO2 (SCO2) Brayton cycle. This study proposed a assessed method with the dual effects of temperature and pressure to investigate the heat transfer performance (HTP) for PCHE derived from theoretical analysis and numerical validation. The influence of various operating conditions on total and hot-side heat transfer coefficient (HTC) was analyzed. The results showed that the PCHE may achieve better HTP when the critical index was near 1. The assessed way had been verified by utilizing the existing experimental data. The total and hot-side HTC for SCO2-Water PCHE were higher than that of SCO2-SCO2 PCHE, which may increase as cold-side Reynolds number increases. The effect of entropy generation rate for heat transfer on the irreversibility was larger than that of entropy generation rate for viscosity. This study may offer theoretical guidance for optimization of heat exchangers.

Conjugate heat transfer analysis of laser-irradiated cylindrical-shaped biological tissue embedded with the optical inhomogeneity

The current study is associated with the numerical investigation of the thermal behavior of cylindrical-shaped biological tissue during laser-based photo-thermal therapy. The light-tissue interaction has been modeled using the transient radiative transfer equation (TRTE). The TRTE in a cylindrical coordinate system is solved using the modified discrete ordinate method to obtain the intensity distribution inside the biological tissue. The solution of TRTE is coupled with Pennes bio-heat transfer equation (BHTE) to understand the thermic response of biological tissue. The Pennes BHTE is solved using the finite volume method (FVM). The two different types of optical inhomogeneity (absorption and scattering inhomogeneity) are considered in the current study. The inhomogeneity's optical and thermophysical properties may be the same/different from the homogeneous biological tissue. This conjugate heat transfer problem (CHTP) is solved using the harmonic mean technique. First, the present result is verified with the data available in the literature. Subsequently, the effect of inhomogeneity's optical properties on the temperature distribution is investigated. A comparative study between the same and different thermophysical properties of the biological tissue and inhomogeneity is performed. This study will help to accurately model the thermal characteristics of the laser-irradiated biological tissue having different thermophysical properties than the inhomogeneity.

Thermal hydraulic analysis of the AFIP-7 irradiation test in the Advanced test Reactor – Model correlation and performance evaluation

As a system level code, RELAP5 is widely used in the nuclear field for the reactor hydraulic analysis. In the component/experimental level, it is frequently employed as well. This paper demonstrates that the RELAP5 flow simulation of the AFIP-7 irradiation experiment which has a complicated 3D geometry/flow path deviates by ∼28% from the flow evaluation through a computational fluid dynamics (CFD) simulation, which has been validated against a flow test performed at Oregon State University. Thermal safety compliance analysis of an experiment planned to be irradiated in the Advanced Test Reactor is usually performed using the finite element analysis code, Abaqus. This research reveals the disadvantages of the Abaqus simulation in the flow instability and departure from nucleate boiling evaluations via a conjugate heat transfer analysis of the AFIP-7 irradiation experiment. As a result, a detailed CFD simulation is suggested for irradiation experiments with complicated flow paths, rather than a simplified RELAP5 simulation or hand calculations. The validated simulation approach will be integrated with the Boehmite correlations to investigate the oxide growth prediction on the fuel cladding in Part II of this research.

Analysis of conjugate heat transfer for forced convective flow through wavy minichannel

PurposeThe purpose of this paper is to analyze numerically forced convective conjugate heat transfer characteristics for laminar flow through a wavy minichannel.Design/methodology/approachThe mass and momentum conservation equations for the flow of water in the fluidic domain and the coupled energy conservation equations in both the fluid and solid domain are solved numerically using the finite element method. The exteriors of both the walls are subjected to a uniform heat flux.FindingsThe results reveal that the theoretical model without consideration of the effect of wall thickness always predicts a lower value of average Nusselt number () as compared to the case of conjugate analysis, although it varies with the thickness as well as material of the wall. For the low amplitude of the wall (α = 0.2), the performance factor (PF) becomes very high for Re in the regime of 5 (⩽) Re (⩽) 15. For any geometrical configurations, conjugate heat transfer analysis predicts higher PF as compared to that of nonconjugate analysis.Practical implicationsThe present study finds relevance in several applications, such as solar collectors and heat exchangers used in chemical industries and heating-ventilation and air-conditioning, etc.Originality/valueTo the best of the authors’ knowledge, the analysis of combined influences of the thickness and the material of the wall of the channel together with the geometrical parameters of the channel, namely, amplitude and wavelength on the heat transfer and fluid flow characteristics for flow through wavy minichannel in the laminar regime is reported first time in the literature.

Numerical Simulations of a Postulated Methanol Pool Fire Scenario in a Ventilated Enclosure Using a Coupled FVM-FEM Approach

Numerical investigations have been carried out for a postulated enclosure fire scenario instigated due to methanol pool ignition in a chemical cleaning facility. The pool fire under consideration is radiation-dominated and poses a risk to the nearby objects if appropriate safety requirements are not met. The objective of the current study was to numerically evaluate the postulated fire scenario and provide safety recommendations to prevent/minimize the hazard. To do this, the fire scenario was first modeled using the finite volume method (FVM) based solver to predict the fire characteristics and the resulting changes inside the enclosure. The FDS predicted temperatures were then used as input boundary conditions to conduct a three-dimensional heat transfer analysis using the finite element method (FEM). The coupled FVM–FEM simulation approach enabled detailed three-dimensional conjugate heat transfer analysis. The proposed FVM–FEM coupled approach to analyze the fire dynamics and heat transfer will be helpful to safety engineers in carrying out a more robust and reliable fire risk assessment.

A combined Combustion-Conjugate heat transfer analysis for Design of partially insulated pistons

Insulating engine combustion chamber surfaces with thermal barrier coatings (TBCs) provides engine thermal efficiency improvement by reducing the heat transfer to the coolant when done appropriately. However, reducing heat losses using traditional ceramic insulation increases combustion chamber wall temperatures during the whole engine cycle, including intake stroke resulting in a decreased engine volumetric efficiency due to excessive intake air heating. This paper presents the concept of partial insulation coverage on a selective area of combustion chamber wall by simulations to improve the engine's thermal efficiency. A combined combustion–conjugate heat transfer analysis methodology was developed to determine the partial-coverage insulation area by analyzing the heat flux and surface temperature distribution across the combustion chamber wall. Engine cycle simulations were run to compare the engine performance with various partial insulation designs, and an optimal insulation area was proposed. To validate the simulation results, the full-coverage coated piston technology with 0.5 mm thick Yttria Stabilized Zrconia (YSZ) coating has been experimentally evaluated on a prototype engine and compared to the non-insulated baseline aluminum piston. It was found that the partial coverage insulation showed a thermal efficiency improvement of 1.98% relative to a full-coverage insulated piston efficiency of 2.06%.

Conjugate Heat Transfer Analysis of the Aero-Thermal Impact of Different Feeding Geometries for Internal Cooling in Lifetime Extension Processes for Heavy-Duty Gas Turbines

Regulations from the European Union move towards a constant reduction of pollutant emissions to match the single-digit goal by 2050. Original equipment manufacturers propose newly designed components for the lifetime extension ofgGas turbines that both reduce emissions and allow for increasing thermodynamic performance by redesigning turbine cooling geometries and optimizing secondary air systems. The optimal design of internal cooling geometries allows for reducing both blade metal temperature and coolant mass-flow rates. In the present study, four different geometries of the region upstream from the blade’s internal cooling channels are investigated by using computational fluid dynamics with a conjugate heat transfer approach. The baseline configuration is compared to solutions that include turbulators, vanes, and a diffuser-like shapes. The impact of each solution on the blade metal temperature is thoroughly analysed. The diffuser-like solution allows for a more uniform distribution of the coolant and may reduce the metal temperature by 30% in the central part of the blade. There are also regions where the metal temperature increases up to 15%, thus requiring a specific thermal fatigue analysis. Eventually, the non-negligible impact of the coolant flow purged in the tip clearance region on the generation of the tip leakage vortex is described.

플러쉬형 대기 자료 프로브 주위의 다상유동과 복합 열전달 전산해석

An air data probe is an important apparatus for measuring flight speed and altitude during flight. The tip of the probe could be blocked by ice, thus leading to malfunction and dangerous situations. In this study, multiphase flow and conjugate heat transfer around a flush-type air data problem were analyzed using COMSOL multiphysics software. For the validation, the numerical results of the L-type pitot probe were compared to the experimental data. The comparison between the results obtained by the single-phase and multiphase flow simulations showed that the surface temperature at the inlet region of the flush-type probe decreased in the multiphase flow due to the increased loss of heat. Moreover, the pressure fields around the flush-type probe were found to be slightly different in the single-phase and multiphase flow simulations. This indicates that the conjugate heat transfer analysis of the multiphase flow is required to evaluate the pressure, surface temperature, and power output of the flush-type probe.

Heat Transfer Characteristics of Fractionalized Hydromagnetic Fluid with Chemical Reaction in Permeable Media

This manuscript optimizes the conjugate heat transfer and thermal-stress analysis for hydromagnetic Brinkman fluid with chemical reaction in permeable media. The governing equations of non-Newtonian Brinkman fluid have been traced out and then fractional derivative approach, namely, Caputo–Fabrizio, is invoked, subject to the exponential boundary conditions. The Fourier Sine and Laplace transforms are applied on governing partial differential equations for generating the analytical results of temperature, concentration and velocity. A comparative study of velocity field is investigated for the sake of long memory and hereditary properties. The analytical investigation of temperature, concentration and velocity field have strong effects on chemical reaction. The graphical depiction of vibrant characteristics of hydromagnetic Brinkman fluid with chemical reaction in permeable media is exhibited for disclosing the sensitivities of different embedded rheological parameters of fluid flow. The results suggested that temperature distribution for smaller and larger Prandtl number has disclosed quick and thicker heat diffusivity.

Conjugate Heat Transfer and Flow Features of Single-Hole and Combined-Hole Film Cooling With Rib-Roughened Internal Passages

Abstract Internal cooling and film cooling, as two main cooling methods in modern gas turbines, work together to protect the high-temperature components of gas turbines. This paper presents the results of a computational study on cooling performance for a flat plate with both film cooling and internal cooling using a conjugate heat transfer analysis. Three internal delivery channel geometries, smooth channel, channel roughened by square ribs (SR), and channel roughened by crescent ribs (CR), are studied with two film cooling geometries, cylindrical hole, and sister holes (SS). The respective conjugate cooling performances are compared. Detailed flow and heat transfer characteristics are presented and discussed. Results show that both film cooling effectiveness and internal cooling performances are influenced by the delivery channel geometry near the hole inlets. The sink flow effects of film cooling enhance the heat transfer coefficient near the film cooling hole inlet. At the same time, film cooling performance is affected by the internal channel as the flow inside the film cooling hole is influenced by the ribs near the hole inlets. When using sister holes, ribs in the internal channels make the anti-kidney vortex structure created by sister holes more effective by changing the mass flow distribution among the three holes.

Effects of Thin Film Heat Spreader on Hot Spots Mitigation in Heat Sinks

Abstract The effects of graphene platelets and diamond-based thin film heat spreaders on maximum temperature of integrated electronic circuits were investigated. A fully three-dimensional conjugate heat transfer analysis was performed to investigate the effects of thin film material and thickness on the temperature of a hot spot and temperature uniformity on the heated surface of the integrated circuit when subjected to forced convective cooling. Two different materials, diamond and graphene, were simulated as materials for thin films. The thin film heat spreaders were applied to the top wall of an array of micro pin fins having circular cross sections. The integrated circuit with a 4 × 3 mm footprint featured a 0.5 × 0.5 mm hot spot located on the top wall, which was also exposed to a uniform background heat flux of 500 W cm−2. A hot spot uniform heat flux of magnitude 2000 W cm−2 was centrally situated on the top surface over a small area of 0.5 × 0.5 mm. Both isotropic and anisotropic properties of the thin film heat spreaders made of graphene platelets and diamond were computationally analyzed. The conjugate heat transfer analysis also incorporated thermal contact resistance between the thin film and the silicon substrate. It was found that isotropic thin film heat spreaders significantly reduce the hot spot temperature and increase temperature uniformity, allowing for increased thermal loads. Furthermore, it was found that thickness of the thin film heat spreader does not have to be greater than a few tens of microns.

Conjugate heat transfer analysis of molten salt in annular heater with rectangular wire coil

In this study, the detailed flow and heat transfer characteristics of molten salt for conjugate annular channel heater with rectangular wire coil were numerically investigated. Three-dimensional computational models are established based on the CFD software FLUENT, and are validated against the established empirical correlations. Good agreement is achieved within the considered range of Reynolds (Re = 673–1318). The effect of inner tube is evaluated and discussed based on numerical results. The simulated results show that radial velocity and tangential velocity are generated, and meanwhile two vortices are formed, as compared to the smooth annular channel heater. And The heat transfer is strengthened due to vortices. The distribution of local Nusselt number is not uniform, and hot spot zones are generated in the windward and leeward sides of rectangular wire coil, which are less than that of non-conjugate annular channel heater. This result revealed that the conjugate model must be established when considering the hot spot zone. Nu and f correlations are proposed to predict heat transfer and pressure drop for conjugate annular channel heater. Finally, the overall performance is evaluated based on the principle of entropy production, the value of which is in the range of 0.212–0.245.

Lattice Boltzmann analysis of conjugate heat transfer in the presence of electrohydrodynamic flow

The externally imposed direct current (DC) electric field mightily impacts on the conjugate heat transfer existing somewhere between the solid and liquid phases, which is numerically investigated via Lattice Boltzmann method (LBM) in this paper. The model is a square cavity filled with dielectric liquid, in which has a centred heat-generating solid body, and the spatially inhomogeneous physical properties such as thermal conductivity, heat capacity, permittivity and mobility are considered distinctively. The lattice Boltzmann method is used to solve the entirely coupled equations and the developed code is first verified by three cases for the hydrostatic solution, conjugate heat transfer with and without heat source. The LBM results show the highly agree well with both the derived analytical solution and the results obtained by other papers. Then, enhancement effect of the flow and heat transfer when electrohydrodynamic (EHD) forces in existence are studied under the different governing parameters and the internal heat source intensity. It is found that the solid-liquid two phases conjugate heat transfer can be significantly enhanced when applying a DC electric field across the domain.

A Study on Computational Analysis for Natural Convection in Tall Building – A Macroscopic Approach

The development of convective heat transfer in buildings are found to be due to the movement of air with respect to the turbulence in air to the temperature. The heat transferred is observed in 3 levels ‐ the roof exposed floor, the seventh floor and the lower floor of the 14 storey naturally ventilated tall building. This paper discusses the eulerian approach which is a macroscopic approach to understand the temperature in the building since the density stratification occurs in the indoor volume of the building. The volume averaged data is considered to be in eularian approach. The CFD analysis is used to artificially create the environment with the different temperatures and velocities. Heat transfer happens due to convection, hence the boundary conditions ‐ initial outdoor temperature has been kept constant at 30°C and 23°C. The change in the outdoor temperature at the air velocities 10 m/sec at 12 noon were observed as 29.37°C, 29.49°C, 29.89°C at constant temperature 30 with respect to 1st floor, 7th floor and roof. Finally, it is noted that, as the air velocity increases, the outdoor temperature also increases. Conjugate heat transfer analysis is followed in CFD analysis to include the resistance of the exterior concrete wall to transfer the temperature to the control volume. K‐omega model is used to resolve the turbulence and to analyse the interior domain with Boussinesq hypothesis in the momentum equation to include the effect of the density difference in heat transfer. It is observed the velocity helps to take the temperature from the exterior domain to the interior part of the building. Hence the results clearly indicate that when the outdoor temperature is higher and the outdoor velocity is also higher, the temperature which enters the indoor environment is also high.

Multi-objective optimization of multi-stage heat sink of electric aircraft using three-dimensional thermal network analysis

The efficiency, reliability, and safety of aircraft have been improved by substituting hydraulic and mechanical systems with electric systems. Furthermore, in future electric aircraft, where fan-driving engines are partially replaced by electric motors, solving the thermal management of heat generation from the motor-controlling power-devices will become a crucial problem. In this study, three-dimensional steady thermal network analysis (TNA) was carried out to analyze the temperature field in a heat sink, for motor-controller air cooling, which is expected to be used for the future electric aircraft. The numerical procedure used was verified by comparing the results of TNA with those of three-dimensional fluid-solid conjugate heat transfer analysis using finite volume method. After the verification, TNA was carried out for an aircraft flight scenario, where the optimum geometry of the heat sink was investigated, minimizing the following three objective functions: the pressure loss of air flow through the heat sink, the maximum local wall temperature of the heat sink at the surface of motor controller, and the weight of the heat sink. In this multi-objective optimization, the design of experiments technique was used to decide the space-filling points for the six design parameters. The three objective functions at each sampling point were calculated using TNA. A response surface was created for each objective function. A multi-objective optimization on the response surfaces, using a genetic algorithm, was iteratively carried out to find the Pareto optimal solutions for the air-cooled multi-stage heat sink. From the results of the Pareto optimal solutions, the effects of the design parameters on the objective functions were discussed. In addition, multiple regression analysis was performed for quantitative evaluation of the results of the Pareto optimal solutions, and the dominant design parameters for each objective function were identified.

Conjugate Heat Transfer Analysis of a Diamond Finned Double Pipe Heat Exchanger

Conjugate Heat Transfer Analysis of a Diamond Finned Double Pipe Heat Exchanger