Heat Transfer In Turbulent Flow Research Articles

A three-dimensional fictitious domain method for direct numerical simulations of particle-laden flows with heat transfer

In the present work, a three-dimensional fictitious domain method for particulate flows with heat transfer is proposed. For the case of fixed particle temperature, an iterative scheme for the temperature Lagrange multiplier is proposed, in order to determine its initial value and overcome the spurious oscillation of the explicit scheme at the initial time stage for different initial fluid and particle temperatures. Both explicit and implicit schemes are proposed for the solution of coupled fluid and solid temperature equations in the case of freely evolving particle temperature. The implicit scheme is suited to the case of large density ratios, specific heat ratios, or thermal conductivity ratios. Our method for the case of fixed particle temperature is verified via the test problems of a stationary hot sphere heating the surrounding quiescent fluid, a fixed sphere, and spheroid, respectively, in uniform flow, and sedimentation of a sphere and spheroid, respectively, in a vertical channel. We propose a new correlation of particle Nusselt number for an isolated sphere in a relatively small domain. Our code for the case of varying particle temperatures is verified via the effective thermal conductivity of a motionless sphere and the rising of a catalyst particle in an enclosure. Our method is applied to the sedimentation of a sphere at different Grashof numbers, specific heat ratios, and conductivity ratios. In addition, some preliminary results on heat transfer in turbulent channel flows laden with neutrally buoyant spherical and spheroidal particles, respectively, from fully resolved simulations with our method are reported.

Heat Transfer and Pressure Loss of Turbulent Flow in a Wedge-Shaped Cooling Channel With Different Types of Triply Periodic Minimal Surfaces

Abstract Additive manufacturing enables highly efficient cooling fabrications such as triply periodic minimal surface (TPMS), which provides excellent heat transfer per unit volume. In a wedge-shaped channel representing trailing edge turbine blade cooling, conventional pin fins are replaced with different TPMS structures due to their topological features to enhance the flow mixing and heat transfer, strengthen the structural integrity, and reduce the manufacturing material. The turbulent flow and heat transfer characteristics of solid- and sheet-based TPMS models, including gyroid, diamond, and Schoen-I-graph and wrapped package (IWP), are numerically investigated. The heat transfer, pressure loss, and thermal performance are compared at Reynolds numbers of 10,000–30,000. Notably, among the studied TPMS structures, the diamond-sheet structure is selected as the optimal model. Compared to the baseline pin fin structure at an equal Reynolds number, it remarkably increases the overall heat transfer by up to 163.2%, the pressure loss by 181.8%, and the thermal performance by up to 77.3%. The numerical results indicate that the gyroid- and diamond-sheet structures effectively organize and interact with the cooling fluid, reducing low-velocity recirculation flow in the tip region of the trailing edge. The flow in the diamond-sheet network is distributed more evenly from the root to the tip region, improving the temperature uniformity throughout the channel. Overall, the diamond-sheet TPMS structure could effectively improve the heat transfer performance, temperature uniformity, and structural integrity in the turbine blades' trailing edge, thereby potentially extending the durability of the turbine blades.

Heat transfer characteristics in turbulent flow and flow patterns of PCM slurry using super-hydrophobic gel particles

An experimental study on the characteristics of heat transfer in turbulent flow and the flow patterns of an organic phase change marerial (PCM) slurry was conducted to investigate the latent heat transportation. As a PCM slurry, the particles of the super-hydrophobic marshmallow-like gel, adsorbing hexadecane on their surface, were dispersed in sodium carboxymethyl cellulose (CMC) solution. Hexadecane is a PCM with a melting point of 17.5 °C, and latent heat of 226.5 kJ·kg−1. For the heat transfer measurement, a double-tube heat exchanger with an inner tube with an inner diameter of 8 mm was used. The results demonstrated that the heat transfer of the marshmallow-like slurry with hexadecane was 1.5 times higher than that of Newtonian fluids. From the pressure drop in the test section, the heat transport efficiency was calculated, which was also higher than that of Newtonian fluids. Furthermore, the flow patterns of the marshmallow-like gel slurries were investigated by varying the CMC concentration and flow velocity. From the results, four patterns of particle dispersion in the flow were observed. The most dispersed flow pattern was observed with all CMC concentrations when the flow velocity was sufficiently high. Based on these results, it was concluded that heat transportation by the marshmallow-like gel slurry is promising.

Impact of human body shape on forced convection heat transfer

Predicting human thermal comfort and safety requires quantitative knowledge of the convective heat transfer between the body and its surrounding. So far, convective heat transfer coefficient correlations have been based only upon measurements or simulations of the average body shape of an adult. To address this knowledge gap, here we quantify the impact of adult human body shape on forced convection. To do this, we generated fifty three-dimensional human body meshes covering 1st to 99th percentile variation in height and body mass index (BMI) of the USA adult population. We developed a coupled turbulent flow and convective heat transfer simulation and benchmarked it in the 0.5 to 2.5m·s-1 air speed range against prior literature. We computed the overall heat transfer coefficients, hoverall, for the manikins for representative airflow with 2m·s-1 uniform speed and 5% turbulence intensity. We found that hoverall varied only between 19.9 and 23.2 W·m-2K-1. Within this small range, the height of the manikins had negligible impact while an increase in the BMI led to a nearly linear decrease of the hoverall. Evaluation of the local coefficients revealed that those also nearly linearly decreased with BMI, which correlated to an inversely proportional local area (i.e., cross-sectional dimension) increase. Since even the most considerable difference that exists between 1st and 99th percentile BMI manikins is less than 15% of hoverall of the average manikin, it can be concluded that the impact of the human body shape on the convective heat transfer is minor.

Heat transfer analysis of supercritical CO2 in a High-Speed turbine rotor shaft cooling passage

This study investigated the turbulent flow heat transfer in a high-speed turbine rotor shaft cooling passage using CFD. Supercritical CO2 is used as the coolant. The coolant temperature, heat transfer coefficient, convection heat transfer, and frictional heat transfer are investigated. The effect of clearance, mass flow rate, shaft rotational speed, and shaft temperature is also investigated. The result showed that the coolant temperature and heat transfer coefficient are independent of the radial clearance. The significant effect of mass flow rate on the coolant temperature is predicted. The convection heat transfer is substantially higher than the frictional heat transfer. This implies that the convection heat transfer is highly dominant over the frictional heat transfer by which the significant cooling provision could be achieved by the supercritical CO2 fluid through the annulus passage. A little agreement is found when the quantity of heat transfer is predicted using correlation and compared with the result obtained by CFD.

Dissimilar turbulent momentum and heat transfer associated with viscoelastic vibration in channel flow

This experimental study on the momentum and heat transfer in drag-reducing turbulent channel flow of surfactant solution, i.e., viscoelastic fluid, has revealed a viscoelastic vibration (VEV) as a novel type of ordered turbulent motion in the wall vicinity where the elastic energy is transformed into turbulent kinetic energy. The VEV is a wave-like motion of low-frequency modes in the streamwise direction and causes zero average Reynolds shear stress. However, thermal diffusion occurs continuously due to the movement of a fluid lump owing to the VEV via the additional action of pressure.

Структура течения и теплоперенос в газокапельном течении за внезапным сужением канала

The numerical study of the local flow structure and heat transfer in a gas-droplet turbulent flow behind a forward-facing step in the two-phase gas-droplet flow is carried out. Two-dimensional steady-state Reynolds-averaged Navier-Stokes (RANS) equations are used for the numerical solution. They were written taking into account the presence of a dispersed phase. The Eulerian two-fluid approach is used to describe the flow dynamics and heat and mass transfer in the gaseous and dispersed phases. The turbulence of the carrier phase was described using an elliptical model for the transport of Reynolds stress components, taking into account the presence of droplets. The effect of evaporating droplets flowing after the forward-facing step on the local flow structure, turbulence, distribution of the dispersed phase, and heat transfer inetensification is analyzed.

Влияние внезапного сужения плоского канала на вынужденную конвекцию в турбулентном газокапельном течении

Numerical modeling of the flow structure and heat transfer in a gas-droplet turbulent flow in a duct with forward-facing step is carried out. The two-dimensional RANS equations are used in the numerical solution. The Eulerian two-fluid approach is used for describing the flow dynamics and heat transfer in the gaseous and dispersed phases. The turbulence of the carrier phase is described using an elliptical Reynolds stress model with taking the presence of dispersed phase. It is shown that finely-dispersed droplets are involved in the separation recirculation motion of the gas phase. The addition of evaporating droplets to a single-phase turbulent flow in the forward-facing step leads to a significant intensification of heat transfer (more than 2 times) compared to a single-phase air flow, all other things being equal. This effect is enhanced with an increase in the initial mass fraction of the water droplets.

Effect of sudden constriction of a flat duct on forced convection in a turbulent droplet-laden mist flow

Numerical modeling of the flow structure and heat transfer in a gas-droplet turbulent flow in a duct with forward-facing step is carried out. The two-dimensional RANS equations are used in the numerical solution. The Eulerian two-fluid approach is used for describing the flow dynamics and heat transfer in the gaseous and dispersed phases. The turbulence of the carrier phase is described using an elliptical Reynolds stress model with taking the presence of dispersed phase. It is shown that finely-dispersed droplets are involved in the separation recirculation motion of the gas phase. The addition of evaporating droplets to a single-phase turbulent flow in the forward-facing step leads to a significant intensification of heat transfer (more than 2 times) compared to a single-phase air flow, all other parameters being equal. This effect is enhanced with an increase in the initial mass fraction of the water droplets. Keywords: Numerical simulation, Reynolds stress transport model, forward-facing step, droplet evaporation, turbulence, heat transfer enhancement.

The Impact of Collisions on Heat Transfer in a Particle-Laden Shearless Turbulent Flow

The Impact of Collisions on Heat Transfer in a Particle-Laden Shearless Turbulent Flow

Design of single and two phase modeled nanofluid CPU cooler and their numerical analysis

AbstractThe heating problems in electronic devices have become even more significant with the development of technology and the rapid increase in areas of use. In cases where passive cooling and active cooling processes are not sufficient to improve overheating in electronic devices, high temperature generation is prevented by liquid cooling if convenient. The effect of nanofluid use on heat transfer to improve the thermal performance of liquid cooling systems was examined with computational fluid dynamics from numerical methods in this research. For different cooler designs, both single‐phase and multiphase modeling methods were utilized in fluid modeling in the research involving forced convection and heat transfer in laminar and turbulent flow. The mathematical models utilized were validated on different experimental and numerical researches. The single‐phase modeling was conducted for pure H2O, 1% Al2O3, 5% Al2O3 nanofluids at different Reynolds numbers. The analyzes were conducted at different Reynolds values and the increase in Reynolds value decreases the maximum temperature value at the output. In the multiphase modeling method, the nanofluid Al2O3 is defined and modelled as a separate phase suspended in different sizes into the base fluid H2O. The use of nanofluid increased the thermal conductivity in both methods compared to the use of pure H2O. At the same time, the use of nanofluids at 5% concentration gave better results than 1% concentration. As the temperature distributions are examined, the Mixture model, one of the single‐phase and multiphase modeling methods, presents very similar distributions. The effect of nanofluids on thermal performance varies depending on many parameters. A significant one is the size of the suspended nanoparticle. Different particle sizes, 20, 30, and 40 nm, were investigated in both Mixture and Eulerian modeling approaches in this research. Two different design coolers were utilized in the researches. In these designs, the flow is distributed in parallel and serpentine. The flow distribution and pressure drop also had an effect on thermal performance. The temperatures were read lower in the serpentine block, which provides a more regular flow distribution.

Computational analysis on solar air heater with combination of alternate dimple protrusions and intrusions on absorber plate with one rounded corner triangular duct.

This study focuses on improving heat transfer by converting one of the corners of the duct to a rounded structure. To study the effect of dimpled shaped protrusions and intrusions on the rounded corner triangular duct with a constant radius of curvature by varying relative streamwise distance (z/e) with a constant transverse distance x'/e = 10,14 and 18. Steady-state, turbulent flow heat transfer under thermal boundary conditions is to be analyzed by varying different Reynolds numbers (5600 to 21000). The duct with dimple-shaped protrusions and intrusions is compared with a simple triangular duct. Optimization of relative horizontal distance (z'/e) by keeping constant protrusion to protrusion distance as z/e = 28 and relative transverse distance as x/e = 10, 14, and 18. It was noted that there was a significant loss in friction and a rise in heat transfer. The relationship between friction factor and Nusselt number was formulated using operating and roughness parameters, using the data collected from the numerical investigation. The friction factor increases significantly with roughness elements, and it is maximum for x'/e = 20 at a low Reynolds number. Nusselt number increases with roughness elements, and it is maximum for x'/e = 14 for all Reynolds numbers and all the models. Enhancement of Nusselt number is due to increase of local heat transfer because of local vortex neat heat transfer zone. The maximum outlet temperature is obtained at a low Reynolds number. The maximum temperature of the heated surface is obtained for Rc = 0.67h and the minimum for Rc = 0.33h.

Large-eddy-simulation of turbulent buoyant flow and conjugate heat transfer in a cubic cavity with fin ribbed radiators

Turbulent convective flow in a cubic cavity is a fundamental model observed in various processes that appeared in environmental and industrial applications. The turbulent fluctuations in the flow field, fluid-solid interaction/coupling, and the transient nature of the problem make it challenging to establish numerical modeling. The turbulent flow and convective heat transfer in a cubic cavity fixed with three fins are studied here. The numerical solutions are obtained through large-eddy simulation with Smagorinsky subgrid-scale model combined with conjugate heat transfer to incorporate temperature distribution in solid fins. The upper and lower walls of the enclosure are differently heated, while the lateral walls are adiabatic, and equally spaced conductive fins are located at the bottom surface. The solutions are initially compared with the numerical and experimental results for validation purposes. Successively, the model is applied to explore the impact of a material (copper) that makes up the horizontal wall on the flow field. We found that the heat transfer essentially alters the turbulent field, and the flow field becomes less homogeneous along the vertical direction. A number of streaky and coherent turbulent structures are found with varying magnitudes. The Q-criterion, second-invariant of the velocity-gradient tensor, predicted that strong vortices occur near the fins and in the surrounding regions of the cavity. Moreover, the energy (entropy) spectral, which plays a crucial role in engineering applications and turbulent theory, is also presented, showing the contribution of each frequency component to the velocity (temperature) variance at a given point. The velocity and temperature fields are found to be anti-symmetric, except close to the front and back walls. The major cause for this is the conducting bottom and fins, which produces thermal stratification in the cavity. The conducting bottom induces the locally unstable thermal stratification in the vicinity of the wall, which intensifies the turbulence as the flow advances toward the temperature-controlled boundaries. Further, the turbulent exchange in the central region is more responsible for the heat transfer than convection that occurs due to the differentially heated walls.

Drag and heat transfer in turbulent channel flow over shallow circular dimples: The shift of the deepest point of dimples

In this study, the flow characteristics and enhanced heat transfer performance of circular dimples in a channel flow were numerically analyzed using DNS and compared with a flat channel. The effect of shifting the deepest point of the dimples in a turbulent channel flow on their drag and heat transfer performance are discussed. The strength and extent of the induced recirculating flow is suppressed significantly when the deepest point is shifted downstream, enhancing the heat transfer performance of the dimpled wall. At the same time, the flow structure above the dimpled wall is also manipulated by the geometry changes. The flow impingement on the dimpled wall increases the drag; consequently, the power required to drive the flow is increased. A study is conducted to investigate the effect of the shift in the deepest point of circular dimples on its heat transfer performance while minimizing the drag increase.

Numerical analysis of a developing turbulent flow and conjugate heat transfer for molten salt and liquid sodium in a solar receiver

• Temperature uniformity within a solar receiver using guide vanes was investigated. • Friction coefficient and Nu number were reported for liquid sodium and molten salt. • The effect of wall heat flux on fluid flow and heat transfer was studied. A numerical analysis of the developing flow and conjugate heat transfer in a solar receiver tube with a three-blade guide vane at the tube inlet was performed to investigate the effect of guide vane and working fluid on the flow and temperature distribution in a solar receiver tube subjected to the boundary condition of the non-uniform wall heat flux. Three dimensional Navier-Stokes, energy, and Laplace equations were solved numerically using Fluent code for the molten solar salt and liquid sodium as the working fluids. The SST k-ω turbulence model was used to predict the flow behavior inside the tube. The values of the friction coefficient and Nu number predicted by the numerical analysis were compared to and validated against experimental correlations from the literature. The numerical results show that a significantly lower pressure drop, and more uniform temperature distribution in the receiver tube are achievable by using the liquid sodium, compared to the molten solar salt. Application of a three-blade guide vane improves temperature uniformity across the receiver tube cross-section, but increases pressure drop in the developing flow region and, thus the overall pressure drop in the tube. The increase in the pressure drop is attributed to the swirling flow and higher friction coefficient compared to the fully developed flow for both working fluids studied in this work.

Experimental investigation of the heat transfer and friction loss of turbulent flow in circular pipe under low-frequency ultrasound propagation along the mainstream flow

The characteristics of the heat transfer and friction loss of turbulent water flow in a circular pipe were investigated experimentally at a constant surface temperature of 45 ℃ for 28 kHz ultrasound propagation along the mainstream flow. Transducers were installed in five rows and three columns in the upstream section of the test pipe, and the number of active transducers was varied (1, 3, and 15) for a Reynolds number range of 10,000–25,000. The results indicated that the ultrasonic effects yielded positive results for both the heat transfer and pressure loss of the pipe flow. Under the influence of 15 ultrasonic transducers, the maximum Nusselt number ratio was 1.57 and the greatest reduction in the friction factor was 21.6 % for a Reynolds number of 10,000. The corresponding maximum thermal performance factor was approximately 1.7. However, the thermal efficiency tended to decrease with an increase in the number of transducers. The maximum thermal efficiency values under ultrasonic waves with 1, 3, and 15 transducers were 5.43, 3.37, and 1.95, respectively. When the change in the friction factor per ultrasonic input power was considered, the most suitable number of ultrasonic transducers was three. Finally, predictive formulas were proposed for the Nusselt number ratio and friction factor ratio under low-frequency ultrasound, with deviations from −5.5 % to 5.4 % and −7.4 % to 7.4 %, respectively.

Stationary Gas Dynamics and Heat Transfer of Turbulent Flows in Straight Pipes at Different Turbulence Intensity

The gas-dynamic and heat-exchange behaviours of air flows in gas-dynamic systems have a significant impact on the efficiency and environmental performance of most technical equipment (heat engines, power plants, heat exchangers, etc.). Therefore, it is a relevant task to obtain reliable experimental data and physical laws on the influence of cross-sectional shape and initial turbulence intensity on gas dynamics and the level of heat transfer. In this study, data were experimentally obtained on the instantaneous values of the local velocity and local heat transfer coefficients of stationary air flows in straight pipes with circular, square, and triangular cross-sections at different initial values of the turbulence intensity. The measurements were carried out with a constant temperature hot-wire anemometer, thermocouples, and pressure sensors. Based on the research results, data on the turbulence intensity and averaged local heat transfer along the length of pipes with different cross-sections were summarised. It has been established that turbulence intensity in a square pipe is up to 40% higher than in a round channel; in a triangular channel, on the contrary, it is up to 28% lower. After the air flow’s initial turbulence, the relaxation of the flow in square and triangular pipes occurs faster than in a round channel. It is found that the initial intensity of turbulence leads to an increase in the averaged local heat transfer, which is typical of all investigated pipe configurations and initial conditions.

Experimental study on the effect of wall roughness on heat transfer characteristics of supercritical carbon dioxide in vertical tubes

The heat transfer characteristics of supercritical carbon dioxide(S-CO2) are vital for S-CO2 power cycles. Previous studies primarily focused on the heat transfer of S-CO2 in smooth or ribbed tubes. Rough walls can affect the heat transfer in turbulent flows. Whereas roughness effects on S-CO2 heat transfer have rarely been studied. The heat transfer characteristics of S-CO2 in tubes (d=4.57 mm,16207≤Re≤91735) with various roughness (1.5 μm∼40 μm) are investigated experimentally in this study. The results reveal that rough wall helps alleviate heat transfer deterioration, which is embodied in: the critical heat load(q/G) for heat transfer deterioration increases from 0.11 kJ/kg to 0.17 kJ/kg as roughness increases. Heat transfer deteriorates in all tubes under high q/G, the temperature peak and deterioration length drop from 106.9 °C to 73.7 °C and 0.75 m to 0.25 m as roughness increases. No heat transfer deterioration in various tubes under low q/G, the heat transfer coefficient practically triples with increasing roughness. The effect of operation parameters on heat transfer performance of S-CO2 in various tubes is similar. A heat transfer correlation for rough tubes is proposed based on 5460 experimental data, which catches 90% of experimental data within ±20% error.

Prediction of turbulent flow characteristics and heat transfer in a dilute droplet-laden flow over a backward-facing step

We present numerical investigation of the influence of the thermophysical properties of droplets on turbulence modification, scattering of droplets, and heat transfer augmentation in a two-phase mist flow over a backward-facing step. Predictions are made for droplets of water, ethanol, acetone, and glycerol, with droplet diameters in the range d 1 =1–100 μm and mass fractions in the range ML 1=0.01–0.1 at the inlet. The RANS approach is used to simulate the gaseous phase, and the motion and heat transfer of the dispersed phase are computed using Eulerian model. Carrier phase turbulence is predicted using a second moment closure model. The effect of attenuation of the gas phase turbulent kinetic energy is shown to be minimal for acetone droplets (more than 7%), whereas the largest effects are seen for glycerol and water droplets (up to 15%). Heat transfer enhancement is shown to reach a maximal value with the use of ethanol (more than twice as high as for single-phase flow over the backward-facing step), while minimal intensification is obtained for the evaporation of acetone droplets (up to 25%).

Turbulent Fluid Flow and Heat Transfer Enhancement Using Novel Vortex Generator

One of several effective methods for improving heat transfer in heat exchangers is the use of vortex generators in channels. The flow of fluid through a channel with a vortex generator is disrupted by the development of recirculation zones on the step wall, which improves fluid mixing and heat transmission. The current work investigates the flow and heat transfer of turbulent fluid flow through channels with various vortex generator designs mathematically (triangular, half-circle, and quarter circle). The finite volume method (FVM) is used to discretize and solve the continuity, momentum, and energy equations. The SIMPLE algorithm approach is used to connect the pressure and velocity fields within the domain. The effect of geometrical factors such as step height (2, 3, 4, 5, and 6 mm) on the flow and heat fields is demonstrated and analyzed, as is the effect of Reynolds number. As the step height increases, so do the surface Nusselt number and skin friction coefficient, which peak at 4 mm. According to the findings, as Re increases, so does the average Nusselt number. The quarter circle vortex generator has the best thermal-hydraulic performance at a 4 mm amplitude height, followed by the triangle vortex generator. The simulation results are consistent with those found in the literature.