Turbulent Heat Transfer Research Articles

Multiphysics Analysis of Convergent-Divergent nozzle using FGM

Abstract: The study focuses on the Convergent-Divergent nozzle for the different types of flow regions and Mach numbers such as subsonic region, sonic region and supersonic region. The Mach number of the nozzle changes across the nozzle, the Mach number varies from the subsonic region at the inlet and varies to the supersonic region at the exhaust of the nozzle. The fluid domain is made across the nozzle to capture the effects of the pressure and temperature and Multi physics analysis is also done to find the deformation depending on the material. The multi-physics analysis of the nozzle is done in COMSOL Multiphysics software by using the system coupling of the turbulent model, solid mechanics and heat transfer models. The results show the deformation of the nozzle under the action of chamber pressure and chamber temperature as the nozzle is the crucial part of any rocket or jet engine of an aircraft or rocket or launch vehicle. The Convergent-Divergent nozzle first converges the flow from the combustion chamber and then diverges the flow to get the maximum thrust at the convergent section the pressure of the fuel increases and then at the exhaust or convergent section suddenly increases enhances the thrust and velocity of the coming the exhaust fuel.

Performance comparison of battery cold plates designed using topology optimization across laminar and turbulent flow regime

Liquid cooling with cold plates offers an efficient solution for battery thermal management. However, conventional cold plates in turbulent regime often result in inadequate temperature uniformity within battery modules and generate significant pressure drops. In this study, we employ the turbulent conjugate heat transfer topology optimization method based on the k-ε turbulent model for cold plate design. Then we derive two novel cold plate designs based on the laminar and turbulent topology optimization method, respectively, and the effectiveness of the two design methods are compared. A multi-objective function which minimizes pressure drop and average temperature is established to balance the thermal and hydraulic performance of the cold plates. The electrochemical-thermal coupled model is utilized to simulate heat production of the battery pack. The fluid flow and heat transfer performance of turbulent topology-optimized cold plate (TTCP) during constant rate discharging is analyzed and compared with that of laminar topology-optimized cold plate (LTCP), serpentine cold plate (SCP), and rectangular cold plate (RCP). Numerical simulation results show that TTCP gives the best overall cooling results among all the designs despite a slight disadvantage against LTCP at very low flow rates. At high-speed turbulent flow (8.488 × 10−2 kg/s), the performance evaluation criterion (PEC) number improvements for TTCP, LTCP, and SCP compared with RCP are 86.7 %, 78.5 %, and 85.4 %, respectively. Hence, cold plate topology optimization using turbulent conditions and methods is recommended for power battery systems, especially those with fast charging/discharging requirements.

Neural network analysis of bioconvection effects on heat and mass transfer in Non-Newtonian chemically reactive nanofluids

Using artificial neural networks, this study sought to investigate the magneto Williamson two-phase nanofluid, taking into account chemical reactions and the motion of gyrotactic motile microorganisms. Fluid flow behavior is influenced by chemical reactions, magnetic effects, Brownian motion, and thermophoresis, according to the study. Thermal transmission is enhanced in non-Newtonian fluids as a result of their propensity to thin under shear, increased turbulence, and superior convective heat transfer. As a result of the fluid's increased thermal conductivity, the incorporation of nanoparticles enhances heat conduction. Additionally, epidermis friction, Nusselt and Sherwood numbers, and the quantity of motile microorganisms were assessed in the study. The overall Absolute Errors lies in the range of 10−2to10−10.The mean squared error generated by Neural Networks lies in the range of 10−02−10−10, and 10−02−10−09 respectively. Suction or injection parameter and Prandtl number have an inverse relation with fluid temperature, while Thermophoretic parameter have a direct relation. Thermophoretic parameter, Schmidt number and suction or injection parameter have an inverse relation with the concentration of nanofluid and gyrotactic microorganisms' density, while micro-organisms density have a direct relation with the microorganisms. Engineering and medicine have utilized bioconvection, a process involving heat transfer and microorganism motion, in the development of nanomedicine, pharmacokinetics, drug delivery, and biosensors, among others. Solvers utilizing stochastic numerical computing include nonlinear networks, atomistic physics, thermodynamics, astrometry, fluid mechanics, nanobiology. As a result, variant scenarios are then tested, trained, and validated, in order to prove its accuracy.

Direct numerical simulation of turbulent heat transfer over surfaces with hemisphere protrusions

Direct numerical simulations (DNSs) of turbulent heat transfer over walls with regularly distributed hemispheric protrusions were conducted to explore how the arrangement and number density of hemispheres affect the turbulent heat transfer. For the rough walls, we considered eight rough surfaces, in which the number density, i.e., the number of hemispheres per unit area, and the distances between two neighboring hemispheres in the streamwise and spanwise directions were systematically varied. The friction Reynolds number was fixed at 660, and we considered an incompressible airflow with a Prandtl number of 0.71, neglecting the buoyancy effects. The results showed that the velocity roughness function strongly depends on the hemisphere arrangement and number density. The spanwise-aligned hemisphere array yields the larger velocity roughness function than its streamwise-aligned hemisphere counterpart, whereas the temperature roughness function depends only on number density. The Reynolds analogy factor decreases with increasing inner-scaled equivalent sand grain roughness, and the decreasing trend is weakly affected by the number density. We analyzed the momentum and heat transfer budgets and found that the pressure drag, which dominates the momentum transfer near the wall, is strongly affected by the hemisphere arrangement and number density. In contrast, the roughness-induced wall heat transfer term, which is the dominant contributor to near-wall heat transfer, depends only on the number density. This difference is the driving mechanism that causes dissimilar trends in momentum and heat transfer over walls with hemispheric protrusions.

Direct numerical simulation of turbulent flow and heat transfer in a particle-laden turbulent channel flow

Direct numerical simulation (DNS) is conducted to investigate the impact of solid particles on heat transfer in a plane channel flow. For comparison, two corresponding DNSs of unladen channel flow are also performed. The numerical simulation of the particle-laden case employs a two-way coupled Eulerian–Lagrangian computational model, which allows for the consideration of momentum transfer between the two phases: discrete particles and the continuous fluid phase. The presence of particles results in an increase in the mean temperature, the root mean square (rms) of temperature fluctuations, and the streamwise turbulent heat flux within the core region. Furthermore, particles exert a more significant influence on these characteristics at higher solid volume fractions. Additionally, the correlation between velocity and temperature is affected by the presence of particles. When comparing heat transfer in particle-laden channel flow to that in the corresponding unladen channel flow, it is observed that the difference in heat flux arises from changes in the rms of the wall-normal velocity and temperature fluctuations. The impact of particles on turbulent heat flux transport is significant in the vicinity of the wall, while their influence within the core region is relatively small.

Direct numerical simulation of sodium in vertical channel flow: From forced convection to natural convection at friction Reynolds number 180

The buoyancy-aided sodium flow in a vertical channel is investigated using direct numerical simulation (DNS) to study turbulent flow and heat transfer at six different Richardson numbers (Ri = 0, Ri = 0.025, Ri = 0.25, Ri = 2.5, Ri = 7.5, and Ri = 15) with a fixed friction Reynolds number (Reτ = 180). The results reveal that the velocity profile shows an “M” shape under buoyancy effect and reverses at the center under strong buoyancy. Additionally, the temperature profile exhibits a thicker boundary layer compared to the velocity profile. Global coefficients, such as the skin friction coefficient and the Nusselt number, are analyzed using Fukagata, Iwamoto, and Kasai (FIK) decomposition to elucidate their respective contributions. Furthermore, anisotropy analysis indicates that buoyancy makes the turbulence more isotropic, and the buoyancy also makes the turbulent Prandtl number (Prt) unpredictable; however, a comparison among the molecular heat flux, the definition of turbulent heat flux, and the calculation of the standard gradient diffusion hypothesis (SGDH) model suggests that the turbulent heat flux can be neglected without significant influence in this study. Finally, the turbulent structures in the viscous layer are presented, and the quadrant analysis is performed to quantitatively analyze the influence of buoyancy on the turbulent structure.

Construction of a mathematical model of turbulent heat and mass transfer processes for the case of electron beam melting of titanium alloy casts

This paper describes a mathematical model built for turbulent heat and mass transfer processes in the case of electron beam melting of titanium alloy ingots. The object of research is the conditions that ensure the quality of ingots. The model makes it possible to calculate the distribution of hydrodynamic flows in the liquid metal and temperature fields in the ingot, to determine the profile of the metal crystallization front, taking into account the interphase transition zones. The model solves the problem of finding the necessary melting regimes of ingots by calculation, in contrast to high-cost natural experiments. The thermal and hydrodynamic processes during the melting of a cylindrical ingot with a diameter of 110 mm of the newest titanium alloy Ti-6Al-7Nb for medical use were calculated and its melting parameters were determined. The small diameter of the ingot significantly facilitates its further machining. The geometry of the two-phase zone of the liquidus-solidus transition, which determines the crystallization front of the metal, was calculated. The position and geometry of this front greatly affects the quality of ingot formation and the concentration of the distribution of alloying elements and the homogeneity of the metal across its volume. A sufficiently flat crystallization front has been obtained, under which the given conditions are ensured. It was found that heat transfer in the liquid phase of the metal is mainly caused by heat and mass transfer due to its movement, and heat and mass transfer significantly depends on the power of the electron beam and its distribution on the surface of the bath. According to the calculated regimes, at the Institute of Electric Welding named after E. O. Paton, the National Academy of Sciences of Ukraine, high-quality ingots for the needs of the medical industry were smelted. The castings are used for the manufacture of light and ultra-strong endoprostheses and implants, which are chemically neutral and biologically and biomechanically compatible with the human body and do not cause rejection.

Particle resolved DNS and URANS simulations of turbulent heat transfer in packed structures

An accurate prediction of turbulent heat transfer in packed-bed reactors is fundamental for the design and performance of such reactors. Various classes of Reynolds-Averaged Navier-Stokes models are available in the literature for predicting turbulent heat transfer via closing the Reynolds stress and Turbulent Heat Flux terms. In this work, we investigate the accuracy of representative unsteady RANS turbulence models in modeling heat transfer characteristics in packed-bed reactors. For this accuracy assessment, the performance of the examined unsteady RANS models is compared with particle-resolved Direct Numerical Simulations including heat transfer. The simulations are conducted in a three dimensional periodic domain consisting of randomly arranged cylindrical particles for Reynolds number Rep=1000. The extracted mean quantities (e.g. velocity and temperature), turbulent heat flux components and turbulent kinetic energy are used as a reference for the RANS models' accuracy assessment. The SST k−ω and Spalart-Allmaras models performed well in predicting mean fields. However, all models tended to mispredicted the turbulent quantities including the turbulent viscosity and heat flux vector. The thermal field calculations rely on the turbulent viscosity based Simple Gradient Diffusion Hypothesis for modeling turbulent heat flux, which introduce challenges in capturing the coupling between thermal and turbulent fields.

Enhancement of turbulent heat transfer by using CuO nano-particle and twisted tape

AbstractA computational model is developed to investigate the convective heat transfer properties and the fluid flow characteristics of cupric oxide - water nano-fluid in a horizontal circular pipe aiming to provide insights into optimizing heat transfer in such systems. A twisted tape with varied twist ratios is inserted. This quantitative investigation used five Reynolds number from 4,000 to 12,000 under a uniform heat flux scenario of 25,000 W m−2. All experiments were performed as a single-phase fluid with cupric oxide values of 0, 0.4, 1, and 2% by volume. By reducing the twist ratio and increasing volume concentration, the average heat transfer coefficient of cupric oxide-water nano-fluid was improved. For a twist ratio of 4D, the maximum heat transfer improvement was 228% greater than the plain pipe. The presence of twisted tape with modest step ratios causes the friction factor to grow.

Enhancing heat transfer with a hybrid vortex generator combining delta wing and winglet designs: A numerical study using the GEKO turbulence model

This paper presents a numerical study on the enhancement of heat transfer in a solar air heater (SAH) duct with Hybrid Vortex Generators (HVG) placed periodically along the flow direction on its heated wall (absorber plate) for fluid flow and convection heat transfer at Reynolds numbers between 4121 and 25,365. HVGs consist of a trapezoidal winglet (TWL) for HVG-winglet-base-angles θ = 45° and 30°, or a combination of a delta winglet (DWL) with a delta wing (DWG) for angles θ = 26°, 18° and 12°. The angle of attack for the winglet (TWL or DWL) in the HVG is set at 30°, while the merging angle with the DWG is 45°, and the relative height BR = e/H = 0.48 remains constant. Initially, the effects on heat transfer and friction factor of Trapezoidal Winglet Vortex Generator (TWVG) and HVG45 (θ = 45°) obtained by basic modification on TWVG are investigated under the same flow conditions for PR = 1.5 and Re = 11,205. Subsequently, parametric studies are performed for five different HVG-winglet-base-angles (θ = 45°, 30°, 26°, 18° and 12°) and three different longitudinal pitch ratios (PR = PL/H = 1.0, 1.5 and 2.0). The calculations are performed with ANSYS Fluent 2021R2 based on the finite volume method. In order to model turbulent flow, the GEKO (GEneralized K-Omega) turbulence model recently developed by ANSYS and based on the k-ω formulation is used. HVG45, obtained by basic modification on TWVG, produces slightly stronger and more effective longitudinal vortices compared to TWVG, showing a 3.87 % decrease in friction factor and a 4.11 % increase in Nusselt number and thus a 5.43 % increase in Thermal Enhancement Factor (TEF). Parametric investigations are conducted across a range of values for both θ and PR. The highest rise in friction factor is attained at the lowest Reynolds number, reaching a value of f/f0 = 54.52, while the most significant enhancement in heat transfer rate is noted at the highest Reynolds number, with Nu/Nu0 = 5.86 for θ = 45° and PR = 1.0. The TEF reaches 2.42 at the lowest Reynolds number for the HVG18 with a winglet-base-angle of 18° and a longitudinal pitch ratio of 1.5. The resulting findings not only offers a new approach to improve the thermal performance of solar air heaters but also hints at the potential success of the GEKO model in predicting turbulent flow and heat transfer analysis.

Optimization of Turbulent Flow Heat Transfer in a 3D Cubic Shell Heat Exchanger using Non-Mixture Multiphase Nanofluids

The present work investigates the turbulent flow heat transfer in a three-dimensional cubic shell and tube heat exchanger employing non-mixture multiphase nanofluid, specifically SiO₂-H₂O. Computational Fluid Dynamics (CFD) simulations were carried out to study the variation of both shell and tube inlet velocities and optimize the thermal performance of the exchanger. The Reynolds-averaged Navier-Stokes (RANS) solver and the k-epsilon turbulent model were employed, and the results were validated against experimental data and numerical results from the available literature. Key results showed that reducing the tube’s Reynolds number to 75% of the shell’s inlet velocity yields the highest total heat transfer rate of 26,692 Watt, marking an impressive 54% improvement over the baseline conditions. Lowering the tube inlet velocity improved the fluid residence time leading to enhanced heat absorption and higher performance. However, reductions in either fluid Reynolds number below 75% led to diminished heat transfer rates, attributed to reduced turbulent kinetic energy and less effective thermal mixing. These findings provide valuable insights into the role of nanofluids in boosting heat transfer efficiency, offering practical implications for industries seeking to enhance energy conservation and optimize thermal systems such as air conditioning and heating systems, thermal power plants, automotive and aerospace industries, as well as solar thermal power plants.

Validation of a Central-Difference Finite Volume Method for Analysing Wall Turbulent Heat Transfer Using Transport Equations for Turbulent Heat Fluxes

Abstract The aim of this study is to validate the finite volume method approach using the transport equations for turbulent heat fluxes in turbulent heat transfer at the wall surface. The finite volume method technique is a numerical analysis based on conservation laws of physical quantities in the flow field, with particular emphasis on the conservation of velocity fluctuation intensity in the flow field. The behaviour of turbulent heat flows is analysed in detail using this method and its validity is tested. In the study, heat transfer in channel turbulence is analysed under a constant heat flux heating condition. The individual terms of the temperature variance equation are analysed and the results are shown to be in agreement with a previous study. The residual is small, confirming the high accuracy of the results. The present study analyses the transport equation for the dissipation of the temperature variance. The transport equations for turbulent heat fluxes in the flow direction and in the vertical wall direction are analysed and show good agreement with those of the previous study. The results show that in the vertical wall direction, non-zero terms such as the pressure diffusion term appear to contribute and reduce the accuracy of the transport equations.

Impacts of building modifications on the turbulent flow and heat transfer in urban surface boundary layers

This study examines turbulent airflow and upward heat transport in real urban environments using a building-resolving large-eddy simulation model to understand the characteristics of turbulent airflow and upward heat transport when geometrical distributions of buildings are modified. The target areas were two real urban districts within Osaka City, Japan, having different morphological features. In the numerical experiments, the initial condition was set to a neutral condition in which temperature is uniformly distributed vertically, and buildings emitted heat at a constant rate. The results in the two districts indicated that the features of turbulence and heat transport distinctly differed with different building arrangement. Specifically, taller buildings significantly decelerated airflows and induced warming behind buildings. More high-rise buildings (which resulted in a larger building variability) in a district with a larger building density caused a large heat flux and warming at higher levels. The sensitivity experiments in which a density and height variability of buildings were modified showed that a building density at higher levels and a building height variability significantly influenced warming at upper levels. An increased building height variability weakened wind speed and disturbed horizontal heat advection, whereas a large building density caused numerous heat sources.

Simulations of Cryogenic Line Chilldown with Advanced Subgrid Wall Boiling Models

A mesoscale model for boiling processes in water was adapted for cryogens and demonstrated for chilldown of propellant transfer lines in both liquid nitrogen and hydrogen. The subgrid boiling model accurately captures the contributions to heat transfer from the generation of bubble nuclei, growth, and interaction of the bubbles in the microlayer as well as quenching of the boiling surface following bubble departure. It was adapted for cryogenic fluids using thermodynamic scaling concepts, considering nondimensional pressures and temperatures that are scaled by the corresponding critical values for the fluid. The boiling model was demonstrated for line chilldown in liquid nitrogen. The predicted wall temperature at which quenching occurs was close to the test data for liquid nitrogen, while the slope of the temperature curve after quenching is initiated shows a steeper variation than the test data. Simulations were also performed for liquid hydrogen. Chilldown times in liquid hydrogen are much more rapid due to higher heat transfer in the film boiling regime, and accurately modeling the impact of turbulent heat transfer was found to be important. Furthermore, due to the much colder fluid temperatures in liquid hydrogen, it is critical to model the variable thermal properties of the solid material.

Mechanism analysis of turbulent convective heat transfer of supercritical carbon dioxide in serpentine tubes

This study examines turbulent convective heat transfer in serpentine tubes, a critical topic given their widespread use in supercritical carbon dioxide (SCO2) cycle heat exchangers. Despite their importance, this area remains underexplored. The novelty of this work lies in analyzing various serpentine tube orientations to optimize heat exchanger design under different mass and heat fluxes. Using a validated numerical method, the study explores the impact of SCO2 thermophysical variations on gravitational and centrifugal forces. Furthermore, heat transfer effects are evaluated by analyzing Morton and Dean vortices as well as Richardson numbers under various conditions. The results show that changes in mass and heat fluxes have a greater impact on the gravitational Richardson number than on the centrifugal Richardson number. When both effects are considered together, the Richardson ratio increases with higher mass and heat fluxes, emphasizing the dominance of centrifugal force over gravitational force. The study also found that changes in SCO2 flow direction most significantly affect the thermal and hydraulic characteristics of serpentine tube in a vertical orientation. The improvement index reaches values of 1.13, 2.38, and 3.26 at heat fluxes of 20, 40, and 60 kW/m2, respectively. However, under conditions of low mass fluxes and high heat fluxes, the serpentine tube with a lateral orientation and downward SCO2 flow demonstrates optimal overall performance. In this configuration, the performance index for thisconfiguration increases by up to 15 % at a heat flux of 60 kW/m2.

Mechanism Analysis of Turbulent Heat Transfer in Porous Coking of Hydrocarbon Fuels under Supercritical Pressure

Mechanism Analysis of Turbulent Heat Transfer in Porous Coking of Hydrocarbon Fuels under Supercritical Pressure

Investigation of airflow thermal ice melting process under various flight conditions

The challenge of preventing and removing ice from exposed regions of aircraft is a significant engineering concern. Understanding the melting process of ice due to aerodynamic heating during flight is essential for developing UAV flight strategies in icy conditions. This paper analyzes the heat transfer mechanisms involved in airflow over ice and introduces a theoretical model to estimate the ice melting rate, using principles of turbulent heat transfer at the stagnation point. The study also discusses the impact of environmental conditions on ice melting rates. Findings indicate that the melting speed of the ice surface exhibits a negative linear correlation with the initial temperature of the ice, whereas it shows a nonlinear correlation with airflow velocity and total temperature of the incoming flow. Lower airflow velocity or total temperature of the incoming flow enhances the sensitivity of ice melting speed to changes. Additionally, lower ice density results in a higher melting speed, showing an exponential relationship with factors like average droplet diameter, airflow velocity, and airfoil leading edge diameter in the cloud field during icing. Experiments conducted using a small jet test bench and an icing wind tunnel confirmed the impact of varying conditions on ice melting rates. The deviation between experimental results and theoretical predictions was under 5 %. These conclusions offer valuable insights for flight safety planning of unprotected iced aircraft in challenging environments.

Machine learning algorithms on predicting the turbulent mixed convection flow in a driven-cavity with two horizontal cylinders

Turbulent mixed convection flow and heat transfer properties in a driven cavity with two circular cylinders arranged one above another are analyzed numerically with the incorporation of a finite element scheme, based on the Galerkin method of weighted residuals. A comparison of streamlines, isotherms, and local, average Nusselt number is provided to illustrate how the Richardson number Ri, Reynolds number Re and the eddy viscosity ε affect the transport phenomena inside the cavity. The local and average Nusselt number have been evaluated and it is found that strong convection at the top wall causes the average Nusselt number to progressively rise with respect to Ri and ε, while steady or slightly varying profiles are seen with regard to Re. In order to predict the thermal distribution inside the cavity due to these hot cylinders, Artificial Neural Network (ANN) and Gaussian Process Regression (GPR) have been developed with these model parameters as input and average Nusselt Number as target values. Several plots have been depicted to come to a conclusion that both models have been trained very effectively with the minimal observed Mean Square Error (MSE) values of 0.0026018 and 0.0026428 for ANN and GPR, respectively. In support to these findings, the coefficient of determination R2 suggests that ANN (R2 = 0.89) outperforms GPR (R2 = 0.87) in testing accuracy, hence it is recommended for prediction task.

Effects of oscillated wall on the turbulent structure and heat transfer of three-dimensional wall jet

Effects of oscillated wall on the turbulent structure and heat transfer of three-dimensional wall jet

Turbulent pipe flow and heat transfer of a binary mixture at supercritical pressure: Influences of cross-diffusion effects

Cross-diffusion effects, including Soret and Dufour effects, are enhanced around the pseudo-critical temperature (Tpc) of a binary mixture. Their influences on heat transfer at supercritical pressure have been scarcely studied. To bridge this gap, large-eddy simulations (LES) are conducted to investigate forced convective heat transfer of a CO2–ethane mixture at supercritical pressures in a circular pipe subject to a uniform heat flux. Both heating and cooling conditions, along with varying initial concentrations and thermodynamic pressures, are included in the simulations. The LES results reveal that the Soret effect causes concentration separation, resulting in a concentration boundary layer. The magnitudes of the thermodiffusion factor (kT) and the radial temperature gradient control the intensity of separation, which is more pronounced at near-critical pressure and high heat flux. Since kT is significant only around Tpc, downstream decay of the concentration separation is observed as the loci of T=Tpc migrate away from the wall so that the local radial temperature gradient diminishes. The primary factors affecting heat transfer are the variations in thermal conductivity and isobaric specific heat resulting from concentration separation. In contrast, the Dufour effect and the accompanying inter-diffusion play negligible roles. In deterioration scenarios, the bulk Nusselt number (Nub) shows a maximum relative drop of 8%, whereas in enhancement scenarios, Nub shows a maximum relative increase in 10%, with both deterioration and enhancement decaying downstream. Cross-diffusion effects have negligible impacts on density and streamwise velocity, but noticeably alter streamwise velocity fluctuation and turbulent kinetic energy.