Turbulent Convective Heat Research Articles

Energy and exergy analysis of conjugate 3D unsteady heat conduction with solidification of water by turbulent buoyancy-driven freezing of solid food in air

In this work, the influence of the position of salmon in a cooling cavity on heat transfer, freezing rate and local exergy destruction was investigated using mathematical modeling and numerical simulation. The predicted temperature evolution in the food during freezing by turbulent heat convection in air inside a cooling chamber was validated with experimental results. The conjugate turbulent model in three dimensions included the natural heat convection and the unsteady heat diffusion with the phase change of water to ice in the food meat. Three positions of the block of salmon in the freezer were studied: bottom-left, bottom-center, and center of the freezer. The local exergy destruction was calculated to quantify the contribution of viscous dissipation and heat transfer during the salmon freezing process. The unsteady description of air velocity, temperature, and freezing rate were analyzed. The highest cooling rate calculated was for the food at the bottom left, and the lowest when located at the center of the cooling cavity. The exergy destruction analysis determined the useful energy lost during the freezing process.

DNS Study of turbulent heat transfer in an elliptical pipe flow subjected to system rotation about the major axis

Turbulent heat transfer in an elliptical pipe flow subjected to spanwise system rotation about the major axis has been studied through direct numerical simulations (DNS). The influence of the Coriolis force on the turbulent heat convection has been analyzed through analyses of the first- and second-order statistical moments of the temperature field in both physical and spectral spaces. The mean temperature, turbulent heat fluxes, pre-multiplied spectra of temperature fluctuations, and coherent thermal structures of the rotating flows are compared against those of the non-rotating flow. It is observed that the secondary flow pattern is sensitive to the rotation number and has a significant impact on the transport process of thermal energy and on the value of the Nusselt number. Through a budget analysis of the vertical turbulent heat flux, it is revealed that the profiles of the budget terms are symmetrical about the minor axis and the turbulent production is the primary source term in a non-rotating flow. However, as soon as the system rotation is imposed, the profiles become asymmetrical and the Coriolis term starts to dominate the turbulent production process as the primary source for turbulent heat flux at a moderate rotation number. Examination of the coherent thermal structures within the flow further reveals that the Coriolis force acts to reduce the vertical turbulent heat flux on the pressure side of the pipe.

Two-phase modeling of low-Reynolds turbulent heat convection of Al2O3-water nanofluid in a 2-D helically corrugated channel

Low-Re-number turbulent flow and convective heat transfer of Al2O3-water nanofluid inside a helically corrugated channel (HCC) were examined numerically. The helically corrugated channel (HCC) was studied for the first time by analogy to a helically corrugated tube. The Reynolds number (based on the nanofluid properties) varies between 5000 and 10000. The nanoparticles volume fraction ranges from 0.1% to 5%. Also, the nanoparticle sizes of 20 nm, 40 nm, and 60 nm were considered. The two-phase mixture model was utilized in the simulations. The numerical results were compared with the experimental data. It was illustrated that a previously established correlation for the effective viscosity could be erroneous in certain circumstances. Hence, that model was amended carefully, and a correlation for nanoparticles with the mean diameter of 20 nm was presented. It was argued that the 20 nm nanoparticles slightly decrease the thickness of the velocity boundary layer. Besides, it was shown that there is a critical particle size above that the heat transfer decreases. Based on the present study, the recirculation region in the corrugation cavity has a significant impact on the turbulent mixing for Re ≥ 10000. Among the tested cases, the maximum ratio of Nusselt number was 1.413 for the particle size of 20 nm and the volume fraction of 5%, while the corresponding pressure drop ratio was 7.122.

Theoretical analysis of convective heat flux structure in the incompressible turbulent boundary layer on a porous plate with uniform injection and suction

Although many experimental and numerical studies on thermal convection exist for turbulent transpiration boundary layer flows on a porous structure, the general convective heat flux formula is urgently needed to clarify the influence of injection or suction on the energy transfer mechanism of turbulent heat convection. The two-dimensional (2D) theoretical convective heat flux structure obtained previously is therefore applied to the incompressible turbulent thermal boundary layer on a porous flat plate with uniform injection and suction. The energy balance relationship between the streamwise and wall-normal heat flux components is established and the effects of injection and suction on turbulent convective heat fluxes are analyzed. The explicit analytical expressions of turbulent skin friction factor, convective heat transfer coefficient and Stanton number are obtained based on the heat flux theory and verified by the previously experimental measurements. The turbulence characteristics of convective heat transfer with uniform injection and suction are revealed by the proposed key parameters. A hybrid control volume analysis for energy transfer inside the porous media plate is performed and applied to the cases of transpiration cooling and enhancing heat transfer, respectively, and a novel calculation formula for convective heat transfer is proposed in the form of heat flux instead of the heat transfer coefficient. It is also found that the direction of the resultant convective heat flux combining the conduction with advection is not always to remain that of the negative temperature gradient, and it depends not only on the temperature difference direction as well as velocity direction, but also on the magnitude contrast of the conductive and advective heat fluxes.

Turbulent mixed convective heat transfer in a ventilated enclosure with a cylindrical/cubical heat source: a 3D analysis

ABSTRACT A numerical study of three-dimensional turbulent mixed convection in a ventilated, thermally insulated cavity containing a hot body (cylindrical or cubical) is conducted. The cavity has an inlet and an outlet in the bottom of the left wall and at the top of the right wall, respectively. The governing equations are solved using the finite volume method, and realizable k-ε turbulence model is adopted to predict turbulence. The influence of hot body on the ventilation efficiency is analyzed in terms of pathlines, temperature field, average Nusselt number (Nuavg ), turbulence characteristics and shape of the hot body by considering different Richardson numbers ranging from Ri = 0.01 to 30. A strong dependence between the heat transfer characteristic and Richardson number is observed, and the heat body geometry plays a significant role in ventilation efficiency. The average Nusselt number is significantly affected when Ri < 1; however, it remained nearly constant for values of Ri > 10. The Nuavg values on the cylindrical heat source are lower compared to those of the cubical heat source; however, the ventilation efficiency of the cavity including the cylindrical heat source is better.

Numerical study on turbulent natural convection and radiation heat transfer of nanofluids in a differentially heated square enclosure

Due to the low performance observed during natural convection with fluids such as air and water, nanofluid research was carried out to analyse them for convective heat transfer applications. Heat transfer comparison between water and different water-based nanofluids comprising of MWCNT, alumina and silver has been established for Rayleigh Number 1.5 × 109. Numerical analysis of 2-D differentially heated square cavity has been performed under turbulent natural convection. The top and bottom walls are considered to be at constant temperatures, while the side walls are adiabatic in nature. Based on previous research, nanofluid volume concentration of 0.1 vol% has been considered for this study, and the nature of flow physics and natural convection heat transfer has been explored. It is observed that multiple vortices (almost similar size) exist and evenly speared within the computational domain for air. The viscosity of the fluid was observed to be a deciding factor for the degree of turbulence, thickness and velocity variations within the boundary layer. The average fluid velocity and vortex circulation strength are highest for alumina nanofluid and least for water. The other two nanofluids lie in between with the velocity and vorticity variations of 12.8 and 16.5%, respectively. In terms of heat transfer, alumina nanofluid performs admirably well, followed by MWCNT nanofluid with a variation in radiation heat transfer of 60.8 and 53.19% when compared to air, while for convective heat transfer the variation observed was 98.4 and 97.8%, respectively. Due to the high densities of these nanofluids as compared to air, achieving thermal stratification is difficult. Alumina nanofluid performs admirably well in enhancing convection and radiation heat transfer.

Heat and Moisture Transfer in a Rectangular Cavity Partially Filled with Hygroscopic Porous Media

This work deals with turbulent natural convection heat and moisture transfer with thermal radiation in a rectangular cavity partially filled with hygroscopic porous medium. The governing equations for the momentum and heat transfer in both free fluid and hygroscopic porous media and moisture content transfer in hygroscopic porous medium were solved by the finite element method. Comparisons with experimental and numerical results in the literature have been carried out. Effects of thermal radiation, Rayleigh number on natural convection and heat transfer in both free fluid and porous medium and moisture content transfer in porous medium were analyzed. It was found that surface thermal radiation can significantly change the temperature and moisture content fields in the regions of free flow and hygroscopic porous medium. With the increase in Rayleigh number, the temperature of porous medium at the interface increased slightly, and the magnitude of moisture change becomes smaller.

Numerical simulation of turbulent flow and heat transfer though sinusoidal ducts

Turbulent forced convection heat transfer in corrugated plate surfaces was studied by means of CFD. Flow through corrugated plates, which are sets of sinusoidal ducts, was analyzed for different inlet flow angles (0° to 50°), aspect ratios (0.1 to 10), Reynolds numbers (2000 to 40,000) and Prantdel numbers (0.7 to 5). Heat transfer is affected significantly by variation of aspect ratio. A maximum heat transfer coefficient is observed at a particular aspect ratio although the aspect ratio has a minor effect on friction factor. Enlarging inlet flow angle also leads to a higher heat transfer coefficient and pressure loss in aspect ratios close to unity. Dependency of Nusselt and friction factor on the angle and aspect ratio was interpreted by means of appearance of secondary motions and coexistence of laminar and turbulent flow in a cross section. Comparing the results with experimental data shows a maximum 12.8% difference. By evaluating the results, some correlations were proposed to calculate Nusselt number and friction factor for entrance and fully developed regions. A corrugated plate with an aspect ratio equal to 1.125 and an inlet flow angle equal to 50° gives the best heat transfer and pressure drop characteristics.

Numerical study on the influence of turbulence on the pyrolysis of hydrocarbon fuel in mini-channel

The turbulent heat convection with pyrolysis is very complicated and occurs in cooling system of hypersonic aircraft with hydrocarbon fuel as the coolant. Since the mixing controlled by turbulence directly affects the reaction and leads to variations in both conversion and selectivity, a 2D numerical model was established to study the influences of turbulence on the pyrolysis of hydrocarbon fuel in mini-channel. The numerical results indicated that the turbulence enhanced the heat and mass transfer in core flow. And turbulence became weak as closing to the wall, which leads to a layer distribution of reaction rate with great gradient forming near the wall. So the distribution of the reaction rate in core flow is more uniform compared with that near the wall. According to analyze the radial distribution of relative rate of that between reaction rate and mixing, the result showed that relative rate presented multi-scale in radial direction under the influence of turbulence. In addition, the increase in turbulent intensity was beneficial to the increase in uniformity of radial distribution of conversion, which was helpful to enhance the utilization level of chemical absorption capacity of fuel.

An Experimental Study of Chilton–Colburn Analogy Between Turbulent Flow and Convective Heat Transfer of Supercritical Kerosene

In this paper, characteristics of turbulent flow and convective heat transfer of supercritical China RP-3 kerosene in a horizontal straight circular tube are studied experimentally, and the validity of Chilton–Colburn analogy is examined. Using a three-stage heating system, experiments are conducted at a fuel temperature range of 650–800 K, a pressure range of 3–4 MPa, and a Reynolds number range of 1 × 105–3.5 × 105. The Nusselt number and skin friction coefficient are calculated through control volume analysis proposed in this paper. Heat transfer enhancement and deterioration were observed in the experiments as well as the similar change of skin friction coefficient. The present results show that Chilton–Colburn analogy is also valid for turbulent flow and heat transfer of supercritical kerosene in horizontal straight circular tubes.

Turbulent natural convection heat transfer with thermal radiation in a rectangular enclosure partially filled with porous medium

ABSTRACTIn this paper, the effects of surface radiation on turbulent natural convection and heat transfer in an enclosure partially filled with a porous medium have been studied numerically. The governing equations for the momentum and thermal energy in both free fluid and porous medium were solved by the finite element method. Effects of thermal radiation on natural convection and heat transfer in both free fluid and porous media were analyzed. It was found that with the increase of Rayleigh number, the convective Nusselt number at the interface between the free fluid and the porous medium increases, and the mean temperature at the interface decreases. Surface thermal radiation can significantly change the temperature fields of free flow. With the increase of surface emissivity, the mean convective Nusselt number at the interface decreases slightly, and the radiative and total Nusselt numbers at the interface increase. The thickness of the porous medium also has a significant effect on heat transfer in the region of free flow, and the convective Nusselt number at the interface decreases with the increase of the ratio of thickness between the porous media and free flow until the ratio is 0.25. Comparing the enclosure with the adiabatic upper wall, the mean temperature at the interface is higher, and the convective Nusselt number at the interface is lower when the upper wall is nonadiabatic.

Turbulent forced convection heat transfer in triangular cross sectioned helically coiled tube

Turbulent forced convection heat transfer in triangular cross sectioned helically coiled tube

Numerical Analysis of Experimental Turbulent Forced Convection Heat Transfer for Nanofluid Flow in a Tube

A numerical model for determining the characteristics of flow and heat has been presented by modifying the eddy diffusivity equation of Sarma et al. The experimental data of thermo-physical properties determined using spherical particles in a wide range of concentration, particle size, materials and operating temperatures are available in the literature. The numerical analysis employed equations, which were developed using the experimental data of thermo-physical properties, friction factor and Nusselt number. Based on the agreement of the numerical results with the experimental data, the influence of concentration and temperature on the turbulent characteristics is presented. It is observed that SiO2 nanofluid attained higher velocity and lower eddy diffusivity compared to Cu nanofluid at a concentration. The temperature gradient increases with concentration and decreases with temperature.

TURBULENT-FORCED CONVECTIVE HEAT TRANSFER AND PRESSURE DROP ANALYSIS OF FE3O4 MAGNETIC NANOFLUID IN A CIRCULAR MICROCHANNEL

A numerical simulation was accomplished in this study that investigated the turbulent force convective heat transfer and pressure drop in straight circular copper pipe with a hydraulic diameter of 0.0005m and 0.1m in length, as given by Lee and Mudawar [11]. The enhancement of heat transfer for water and nanofluids (Fe3O4) under 100 [W/m2] constant heat flux was applied around the wall of the pipe. In this study, standard k-ɛ turbulence model was employed and was performed at a steady state flow, incompressible turbulent flow, and three-dimensional structure. Various volume concentrations of nanoparticles were conducted in the range of 1% to 15% at constant nanoparticle diameter size, which was 32 nm. The heat transfer enhancement was obtained in the range of Reynolds number from 3000 to 10,000. The results displayed an increase in Reynolds number and volume concentrations, as well as an increase in the Nusselt number. The optimum Nusselt number gained was about 5% to 6% of volume concentration at each Reynolds number tested. Besides, with the increase of Reynolds number, the variation pressure saw a dropped for inlet, whereas an increase in the outlet section. Moreover, the increase in volume concentration also caused a small increment in the pressure drop compared to pure water.

Experimental Study on the Heat transfer of MPCM Slurry as a Heat-transfer Fluid

The experiment of turbulent heat convection of microencapsulated PCM slurry which flows in a circular tube was based on the MPCM slurry. The slurry used isopropanol/water mixed solution as the base fluid and MPCM28 as the phase change microcapsule particles. The results showed that the pressure drop of heating pipe increased with the increase of turbulent intensity, and decreased with the increasing temperature. However, Nux of the MPCMS increased with the increasing Re as well as the concentration of slurry. But increased first and decreased then with the increase of heat flux.

Numerical simulation of conjugate forced turbulent heat convection with induced natural laminar convection in a 2D inner cavity

Unsteady natural convection process of a fluid inside an inner walled vessel caused by an external forced turbulent convective flow of air in a rectangular duct has been analyzed numerically for Re = 5000; Re = 22,000. Fluid mechanics and conjugate convective heat transfer, for both inner fluid (water) and outer fluid (air), described in terms of continuity, linear momentum, k–ε turbulence model and energy equations were predicted by using the finite-volume method (FVM). Results for the streamlines, isotherms, local Nusselt numbers along the cavity walls and the average friction factor are presented. A comparison between obtained results for both Reynolds number is made. In general, it was found that the cooling process is improved up to 79% when Re number is increased from 5000 to 22,000; and the fluid mechanics characteristics inside the vessel is not considerable affected by the velocity inlet of the cooling air.

Numerical study of turbulent flow and heat transfer of Al2O3–water mixture in a square duct with uniform heat flux

Turbulent flow and convective heat transfer of a nanofluid made of Al2O3 (1–4 vol.%) and water through a square duct is numerically studied. Single-phase model, volumetric concentration, temperature-dependent physical properties, uniform wall heat flux boundary condition and Renormalization Group Theory k-e turbulent model are used in the computational analysis. A comparison of the results with the previous experimental and numerical data revealed 8.3 and 10.2 % mean deviations, respectively. Numerical results illustrated that Nu number is directly proportional with Re number and volumetric concentration. For a given Re number, increasing the volumetric concentration of nanoparticles does not have significant effect on the dimensionless velocity contours. At a constant dimensionless temperature, increasing the particle volume concentration increases the size of the temperature profile. Maximum value of dimensionless temperature increases with increasing x/Dh value for a given Re number and volumetric concentration.

Numerical Investigation of Turbulent Heat Convection from Solid and Longitudinally Perforated Rectangular Fins

Micro heat sinks are adopted in electronics cooling together with different technologies to enhance the heat transfer process. To improve the cooling performance of heat sink, perforations such as small channels of square and circular cross sections are arranged along with stream wise fin's length. A numerical investigation is conducted in this study for three-dimensional fluid flow and convective heat transfer from an array of solid and perforated fins that are mounted on a flat plate. Incompressible air as working fluid is modeled using the Navier–Stokes equations and RNG based k- turbulent model is used to predict turbulent flow parameters. Temperature field inside the fins is obtained by solving Fourier law of heat conduction equation. Flow and heat transfer characteristics are presented for Reynolds numbers from 2×104 to 3.9 ×104 based on the fin length and Prandtl number is taken as Pr = 0.71. Numerical simulation is validated with the published experimental results of the previous investigators and good agreement is observed. Results show that the fins of circular perforations have remarkable heat transfer enhancement and reduced pressure drop. The results of this study can help designing micro heat sinks for heat removal from electronic devices.

Forced turbulent heat convection in a square duct with non-uniform wall temperature

In the present work, the numerical simulation to calculate the problem of the turbulent convection with non-uniform wall temperature in a square cross-section duct was adopted. To solve this problem some assumptions for the flow, such as: the condition of fully developed turbulence and incompressible flow have been assumed. The methodology of the dimensionless energy equation was used to calculate the fluid temperature field in the square cross-section in function of the non-uniform wall temperatures prescribed. Numerical simulations were done using two different turbulent models to resolve the momentum equations and two more models to resolve the energy equation. The models of turbulence k–ε Nonlinear Eddy Viscosity Model (NLEVM) and the Reynolds Stress Model (RSM) were used to determine the turbulent intensities as well as the profiles of axial and secondary mean velocities. The turbulence model RSM was simulated using a commercial software. The thermal field was determined from other two models: Simple Eddy Diffusivity (SED), based in the hypothesis of the constant turbulent Prandtl number; and Generalized Gradient Diffusion Hypothesis (GGDH). In this last model, as the turbulent heat transfer depends on the shear tensions, the anisotropy is considered. These two last equation models of the energy equation of the fluid have been implemented in FORTRAN, a code of programming. The performances of the models were evaluated by validating them based in the experimental and numerical results published in the literature. Two important parameters of great interest in engineering are presented: the friction factor and the Nusselt number. The results of this investigation allow the evaluation of the behavior of the turbulent flow and convective heat fluxes for different square cross-sectional sections throughout the direction of the main flow, which is mainly influenced by the temperature distribution in the wall.

Large-eddy simulation of turbulent heat convection in a spanwise rotating channel flow

In this paper, we investigate the effects of the Coriolis force in a heated plane channel flow subjected to spanwise rotation using the method of large-eddy simulation. We present both the general and simplified transport equations for the resolved turbulent stresses, which are essential for understanding the unique pattern of turbulent kinetic energy production in a rotating system. Numerical simulations are performed using primarily two dynamic subgrid-scale stress models and one dynamic subgrid-scale heat flux model; namely, the conventional dynamic model (DM) and a novel dynamic nonlinear model (DNM) for closure of the filtered momentum equation, and an advanced dynamic full linear tensor thermal diffusivity model (DFLTDM) for closure of the filtered thermal energy equation. The turbulent flow field studied herein is characterized by a Reynolds number Re τ = 150 and various rotation numbers Ro τ ranging from 0 to 7.5. In order to validate the LES approach, turbulent statistics obtained from the simulations are thoroughly compared with the available experimental results and direct numerical simulation (DNS) data. A detailed comparative study has been conducted in order to evaluate the performance of the DM and DNM in terms of their prediction of characteristic features of the velocity and temperature fields and their capability of reflecting both forward and backward scatter of kinetic energy between the filtered and subgrid scales.