Value Of Average Nusselt Number Research Articles

A parametric study of the 2D model of solar air heater with elliptical rib roughness using CFD

The heat transfer can be improved by providing artificial roughness on absorber plate of the solar air heat. Many studies are available on circular, semi-circular, triangular and rectangular rib roughened solar air heater. But in present study heat transfer enhancement by providing elliptical ribs on absorber plate was analyzed by developing CFD code on non-commercial ANSYS (Fluent) 12.1 software. The simulations were performed on 2-D CFD model and analysis was carried out to study the effect of relative roughness width, relative roughness height and relative roughness pitch on heat transfer and friction factor. The Reynolds number range from 4000 to 15000 and turbulence phenomena is modeled by using Reynolds-average Navier-Stokes equations (RANS). The mathematical modeling is validated and compared with available results. The strong vortex formation takes place in the main stream flow because of elliptical roughness, which improved heat transfer augmentation in the solar air heater. The local turbulence kinetic energy strongly influenced by orientation of the elliptical ribs. The value of average Nusselt number increases by increasing relative roughness height but it decreases with the increase of relative roughness width and relative roughness pitch. The rib width has significant effects on heat transfer enhancement and maximum Nusselt number is observed for relatively small roughness width (i.e., 0.5) among the considered range of 0.5 mm to 2.0 mm. The maximum value of Nusselt number and friction factor is observed for relative roughness width of 0.5, relative roughness height of 0.045, and relative roughness pitch of 6.

Turbulent heat transfer and nanofluid flow in a protruded ribbed square passage

In this article, turbulent heat transfer of nanofluid flow in square passage with protruded rib shape is numerically and experimentally studied over Reynolds number ranges of 4000–18000. Different nanoparticles (Al2O3, CuO, and ZnO), with different concentration (φ) range of 1–4% and different nanoparticle diameter (dnp) range of 30–45nm are disperse in water (base fluid). Several parameters such as stream wise distance (Xs/dp) range of 1.4–2.6, span wise distance (Ys/dp) range of 1.4–2.6, ratio of protruded height to print diameter (ep/dp) range of 0.83–1.67 also studied to find the consequence on thermal and hydrodynamic characteristics. Simulations were carried out to obtain heat and fluid flow behaviour of smooth and ribbed square channel using commercial CFD software, ANSYS 15.0 (Fluent). Renormalization k-ε model was employed to assess the influence of protruded ribs on turbulent flow and velocity field. The outcome indicates that Al2O3 nanofluid has the highest value of average Nusselt number as compare to other nanofluids. The average Nusselt number increases as the concentration increases and it decreases as nanoparticle diameter increases. The thermal hydrodynamic performance parameter based on equal pumping power, average Nusselt number and average friction factor were found to be highest for Al2O3, φ=0.04, dnp=30nm, Xs/dp=1.8, Ys/dp=1.8 and ep/dp=1.0. The numerical data are compared with the corresponding experimental data. Comparison between CFD and experimental analysis results showed that good agreement as the data fell within ±7.0% error band.

Effect of geometrical and temperature-dependence parameters on forced convection of a nanofluid in a micro-channel heat sink

Purpose The purpose of this study is the numerical prediction of the thermal and hydraulic characteristics (Nusselt number and shear stress) of a forced convection laminar flow through a rectangular micro-channel heat sink, using constant and temperature-dependent thermo-physical properties. The effects of the solids volume fraction and the size of the micro-channel on heat transfer enhancement have also been investigated. Design/methodology/approach The authors use the flow of a water-Al2O3 nanofluid and a single-phase approach. The equations are solved using the commercial code Fluent Version 6.3. This code uses the finite volume approach to solve the equations subject to the boundary conditions, which govern three-dimensional conjugate convection-conduction heat transfer model. The physical domain was meshed using the code GAMBIT. The mesh used is non-uniform and was obtained by sweeping in the Z direction an X-Y surface meshed with QUAD/pave type cells. Findings The results clearly show that the inclusion of nanoparticles produces a considerable increase in the heat transfer. Also, the temperature-dependent models present higher values of local and average Nusselt number than in the case of constant thermo-physical properties, and an increase in the channel dimensions leads to an important increase in heat transfer. Consequently, we ensure a better cooling of the base of the micro-channel heat sink. Research limitations/implications Because of the settling of nanoparticles, the research results may not be generalized to high values of solids volume fraction. Therefore, researchers are encouraged to find other techniques of cooling when the heat loads exceed values that cannot be dissipated using nanonofluids. Practical implications The paper includes implications for the miniaturization of electronic devices such as in microprocessors or those used in robotics and automotive industries, where continually increasing power densities are requiring more innovative techniques of heat dissipation from a small area and small coolant requirements. Originality/value This paper shows the implementation of variable property nanofluid models in CFD commercial codes.

Laminar natural convection characteristics in an enclosure with heated hexagonal block for non-Newtonian power law fluids

This work illustrates the steady state, two dimensional natural convective flow and heat transfer features in square enclosure containing heated hexagonal block maintained either at constant wall temperature (CWT) or uniform heat flux (UHF) thermal conditions. Governing equations (mass, momentum and energy) are solved by using finite volume method (FVM) with 3rd order accurate QUICK discretization scheme and SIMPLE algorithm for range of field pertinent parameters such as, Grashof number (103≤Gr≤106), Prandtl number (1≤Pr≤100) and power law index (0.5≤n≤1.5). The analysis of momentum and heat transfer characteristics are delineated by evolution of streamlines, isotherms, variation of average Nusselt number value and Colburn factor for natural convection (jnH). A remarkable change is observed on fluid flow and thermal distribution pattern in cavity for both thermal conditions. Nusselt number shows linear variation with Grashof and Prandtl numbers; while rate of heat transfer by convection decreases for power law index value. Higher heat transfer rate can be achieved by using uniform heat flux condition. A Nusselt number correlation is developed for possible utilization in engineering/scientific design purpose.

Numerical Simulation of Natural Convection in a Triangle Cavity Filled with Nanofluids Using Tiwari and Das' Model: Effects of Heat Flux

AbstractIn this article, we present a fully higher‐order compact (FHOC) finite difference method to investigate the effects of heat flux on natural convection of nanofluids in a right‐angle triangle cavity, where the left vertical side is heated with constant heat flux both partially and throughout the entire wall, the inclined wall is cooled, and the rest of walls are kept adiabatic. The Darcy flow and the Tiwari and Das’ nanofluid models are considered. Investigations with four types of nanofluids were made for different values of Rayleigh numbers with the range of 100 ≤ Ra ≤ 50,000, size of heat flux as 0.1 ≤ ε ≤ 1.0, enclosure aspect ratio as 0.5 ≤ AR ≤ 2.0, and solid volume fraction parameter of nanofluids with the range of 0% ≤ ϕ ≤ 20%. Results show that the average heat transfer rate increases significantly as particle volume fraction and Rayleigh numbers increase, and the maximum value of average Nusselt number is obtained by decreasing the enclosure aspect ratio. The results also show that the average heat transfer decreases with an increase in the length of the heater. Furthermore, multiple correlations in terms of the Rayleigh numbers and the solid volume fraction of four types of nanoparticles have been established in a general form.

Aspect ratio effect on natural convection in a square enclosure with a sinusoidal active thermal wall using a thermal non-equilibrium model

ABSTRACTA numerical investigation of the aspect ratio effect on natural convection in a square enclosure is carried out by adopting the local thermal non-equilibrium model. The top and bottom walls of the enclosure are adiabatic, the left vertical wall is partially heated and cooled by the sinusoidal thermal boundary condition, and the right vertical wall is maintained at uniform thermal boundary condition. The results show the value of periodicity parameter increasing. The streamlines vary in different patterns, rotating clockwise and counterclockwise simultaneously when N > 1, and the number of clockwise and counterclockwise rotating cells increases with the increase of N and equals the value of N. The sinusoidal local Nusselt number profiles are observed and the wave amplitude of local Nusselt number decreases with the increase of aspect ratio, and the absolute values of average Nusselt number at left wall of porous cavity reach maximum when Ar = 1. The absolute value of solid-to-fluid temperature differences decreases as the inter-phase heat transfer coefficient (H) increases and it increases as the value of aspect ratio increases. The total heat transfer of porous cavity can be enhanced by increasing the aspect ratio and the thermal conductivity ratio.

Natural convection of silica nanofluids in square and triangular enclosures: Theoretical and experimental study

In this paper, the values of average Nusselt number and heat transfer coefficient ratio for natural convection of silica/water nanofluids in square and triangular cavities are estimated using theoretical correlations. The correlations that are used to calculate the heat transfer characteristics of enclosures are a function of thermophysical properties of nanofluids. Many studies demonstrated that using classic thermophysical models to determine the properties such as thermal conductivity, viscosity, and density sometimes may lead to substantial errors. To avoid such errors, it is helpful to determine the thermophysical properties through experiments. For this purpose, first SiO2 nanoparticles with a diameter of 7nm are suspended into water to prepare stable nanofluids. Next, thermal conductivity, viscosity, and density of nanofluids with volume fractions of 0.5%, 1%, and 2% are measured at a temperature range of 25–60°C. Measured data are compared with the outcomes of theoretical models. The comparison of experimental data and classic models shows a considerable difference between the outputs particularly at high volume fractions of nanoparticles. Moreover, theoretical models predict a descending trend for variations of thermal conductivity ratio with temperature, but, on the contrary, experimental data unveil an ascending tendency. Finally, Nusselt number and heat transfer coefficient ratio for various cavities including square cavity in horizontal and inclined (angle of 45°) positions and the right triangular enclosure are predicted at Rayleigh numbers of 105 and 106. The findings indicate that even without having measured data of thermophysical properties, the average Nusselt number could be estimated with the same trend and maximum difference of 4.5%. However, the results of heat transfer coefficient ratio reveal the significant of using experimental data for properties to find the optimum working fluid for application in cavities since the theoretical models and experiment-based model may give different trends for heat transfer coefficient in enclosures.

Hydromagnetic double-diffusive mixed convection in trapezoidal enclosure due to uniform and nonuniform heating at the bottom side: Effect of Lewis number

This paper presents unsteady numerical results of double diffusive mixed convection flow in a trapezoidal enclosure with the uniform magnetic field effect applied in negative horizontal direction. At the bottom wall, the uniform and non-uniform heat and mass are applied while the heat and mass absorbed uniformly at the top wall. Other side walls are impermeable and adiabatic. The top wall moves along x-axis direction with a constant velocity. The transport phenomenon of this problem can be expressed by the coupled governing equation derived from the conservation of mass and momentum along with the energy equation for temperature and concentration. The finite element method (FEM) based on Galerkin weighted residual technique is used to compute the numerical result from these governing equations. The numerical computation is carried out for Lewis number (Le=0.1–50) and Richardson’s number (Ri=0.1–100). Computed numerical results of mass, temperature and velocity distribution are expressed graphically as iso-concentration lines, isotherm lines and streamlines respectively. Average Sherwood and Nusselt number values are used to show the mass and heat transfer rate from the heated and concentrated surface of the enclosure. It is found from the analysis that mass transfer strongly depends on Lewis number. Heat and mass transfer for uniformly heated and concentrated bottom wall is larger than the non-uniformly heated and concentrated bottom wall. Finally, a correlation has been done for average Nusselt and Sherwood numbers for both of the cases.

Numerical Simulation of Two Phase Turbulent Flow of Nanofluids in Confined Slot Impinging Jet

In this article, a numerical investigation is performed on flow and heat transfer of confined impinging slot jet, with a mixture of water and Al2O3 nanoparticles as the working fluid. Two-dimensional turbulent flow is considered and a constant temperature is applied on the impingement surface. The k − ω turbulence model is used for the turbulence computations. Two-phase mixture model is implemented to study such a flow field. The governing equations are solved using the finite volume method. In order to consider the effect of obstacle angle on temperature fields in the channel, the numerical simulations were performed for different obstacle angles of 0° − 60°. Also different geometrical parameters, volume fractions and Reynolds numbers have been considered to study the behavior of the system in terms of stagnation point, average and local Nusselt number and stream function contours. The results showed that the intensity and size of the vortex structures depend on jet- impingement surface distance ratio (H/W) and volume fraction. The maximum Nusselt number occurs at the stagnation point with the highest values at about H/W = 1. Increasing obstacle angle, from 15° to 60°, enhances the heat transfer rate. It was also revealed that the minimum value of average Nusselt number occurs in higher H/W ratios with decreasing the channel length.

Experimental Analysis of Laminar Flow and Heat Transfer in Multi-Port Finned Mini-Channel

Due to their high heat transfer rate, small scale channels have been a popular area of study for the past three decades, especially for heat exchangers. In this study, fluid flow and heat transfer in a multi-port finned minichannel with a rectangular cross section was investigated experimentally under the constant heat flux boundary condition. The minichannel, which has a length of 638 mm, consists of 16 ports, 14 of which are identical finned rectangular channels with a width of 2.10 mm and a height of 5.85 mm while the remaining two ports at the outer edges of the channel were considered to be identical with the other ports. Deionized water was used as the working fluid with Reynolds number ranging between 75 and 190 in a single port. In order to correctly evaluate local heat transfer and friction coefficient values, thermal entrance effects and varying thermo-physical properties of the working fluid were taken into consideration throughout the study. Local Nusselt number varying with dimensionless axial thermal length, friction factor and average Nusselt number values varying with Reynolds number, and temperature distribution along the wall were evaluated to study fluid flow and heat transfer of the finned minichannel. The results were compared to theoretical values and presented graphically. Local Nusselt number values indicated fairly good agreement with theory while friction factor was overestimated. This was considered to be due to the effect of fins. In addition, three correlations were suggested in order to evaluate friction factor, local Nusselt number and average Nusselt number for the minichannel in given ranges of parameters. Keywords: Fin; Convective Heat Transfer; Laminar; Multi-Port; Nusselt Number DOI: 10.17350/HJSE19030000032 Full Text:

Effect of Buoyancy Ratio on Double-Diffusive Natural Convection in a Porous Rhombic Annulus

This paper reports on the effect of buoyancy ratio due to both heat and mass transfer on natural convection in a porous enclosure between two isothermal concentric cylinders of rhombic cross sections. For negative values of the buoyancy ratio, buoyancy forces due to heat and mass transfer are in opposite directions (opposing mode), while for positive values they are in the same direction (aiding mode). Numerical results demonstrate that the flow strength increases as the absolute value of the buoyancy ratio increases. In the opposing mode, the eye of the vortex flow is located in the lower half of the enclosure, while in the aiding mode it is positioned in the upper part of the annulus. The average Nusselt and Sherwood number values increase as the absolute value of the buoyancy ratio moves away from 1, with values obtained in the aiding mode being higher than corresponding values achieved in the opposing mode. A comparison is also made between the computed average Nusselt and Sherwood number values and similar ones obtained in a circular annulus having the same inner and outer perimeters as the rhombic enclosure. Predictions indicate large percent difference in values, demonstrating that circular geometries cannot be exploited to accurately predict heat and mass transfer in complex geometries.

Heat Transfer Characteristics from Single Group Slot Jet Impingement on the Strip Surface for the Hot Rolling Ultra-Fast Cooling

As for the impinging jets of single group slot nozzles, the heat transfer of the top and bottom strip surface is uneven which is mainly caused by the various jet distances of the top and bottom nozzle to the strip surface and the effect of gravity. In this study, the convective heat transfer process of the top and bottom strip surface due to single group slot nozzles in the ultra-fast cooling (UFC) system was studied by the fluid-structure interaction finite element method. The distributions of the flow field and heat transfer for the top and bottom strip surfaces were obtained under the various parameters. The results showed that, the difference value of the average Nusselt numbe for the top and bottom strip surface was decreased with the increase of the jet velocity, but when the jet velocity was up to 5m/s it remained almost the same. The uniformity of the top and bottom surface was improved by the jet impingement height (h) for the h<45mm. It was found that for h=25mm, the heat transfers of the top and down surfaces were more evenly, and the different value of average Nusselt number for the decreased by about 12.6%-28% as the jet impingement height increased from 25mm to 45mm when the slot width was 5mm.

Convection heat transfer of NaK-78 liquid metal in a circular tube and a tri-lobe channel

Computational Fluid Dynamics (CFD) analysis of turbulent convection heat transfer of NaK-78, liquid metal alloy in a uniformly heated circular tube are performed and the results are compared to reported experimental measurements. Also presented are the results for a tri-lobe channel, of the same heated length, equivalent hydraulic diameter, wall heat flux, liquid mass flux, setup and the inlet and exit bulk temperatures as in the circular tube experiments. The tri-lobe channel has 28% larger cross-sectional and surface areas than the circular tube. Examined are the effect of using the two-layer realizable k–ε, the Shear Stress Transport (SST) k–ω, and the SST Detached Eddy Simulation (DES) turbulence models on the computation time and the accuracy of the results. The calculated axial distribution of the wall temperature for the circular tube compares favorably to reported measurements at Pe=163 and 796. This agreement confirms the soundness of the implemented methodology and the numerical meshing grid. The calculated and experimental Nu values for fully developed flows in the uniformly heated circular tube, covering a wide range of Pe values, are correlated to within ±10%, as:Nu=5.6+0.013Pe0.863Although the calculated Nu values using the different turbulence models are well within the ±10% of the experimental data, the computation time using the DES turbulence model is 13 and 14 time that using the k–ω and the k–ε model, respectively. Unlike for the circular tube, the local Nu in the tri-lobe channel varies azimuthally with the values in the corners are much lower than in the concavities of the tri-lobe channel. The calculated average values of Nusselt number, N¯u for the fully developed flows of liquid NaK-78 in the tri-lobe channel are correlated, to within ±5% as:N¯u=6.6+0.007Pe0.94For Pe<100, the N¯u values for the tri-lobe channel are ∼15% higher than for the uniformly heated circular tube, but the difference gradually decreases with increasing Pe.

Mixed convection from a hemisphere in Bingham plastic fluids

In this work, mixed convection in a Bingham plastic fluid from a heated hemi-sphere has been investigated numerically in the so-called aiding buoyancy configuration. The results reported herein, spanning the following ranges of conditions: Reynolds number, 1⩽Re⩽100, Prandtl number, 1⩽Pr⩽100, Richardson number, 0⩽Ri⩽10 and Bingham number, 0⩽Bn⩽100, are used to describe the momentum and heat transfer characteristics of a hemi-sphere oriented with its curved surface towards the oncoming fluid stream. In particular, flow characteristics are analyzed in terms of the streamlines and the morphology of fluid-like/solid-like regions and the corresponding heat transfer characteristics are examined via isotherm contours, local Nusselt number and average Nusselt number values to delineate the role of each of these parameters. The flow separates itself from the hemi-sphere at a much lower Reynolds numbers as compared to that for a sphere due to the sudden loss of the rear surface. The average value of the Nusselt number is influenced by four non-dimensional parameters, namely, Reynolds number, Prandtl number, Bingham number and Richardson number. Finally, the present results have been consolidated using the Colburn j-factor for predicting the value of the average Nusselt number in a new application.

Experimental and Numerical Investigation of Combined Convection Heat Transfer and Fluid Flow around Circular Cylinder through Rectangular and Trapezoidal Open-Cell Aluminum Foams

Combined convection heat transfer and fluid flow around a circular cylinder surface placed in open-cell aluminum foams and subjected to constant heat flux inside a rectangular, water-filled horizontal channel was numerically and experimentally studied. Two models (rectangular and trapezoidal open-cell aluminum foam shapes) made of 6101-T6 alloy with pore densities of 10 and 40 pores per linear inch (PPI) and 7–9% relative density were employed as test sections. The aluminum foam dimensions were 35.7 × 35.7 × 36.85 mm, the Reynolds number range was 60–2000, and the modified Grashof number range was 2 × 102–2.6 × 107. Governing equations (continuity, momentum, and energy) were solved using the finite-volume method (FVM). Effects of the porous characteristics of aluminum foams and mixed convection heat transfer parameters on buoyancy force, Nusselt number, friction factor, and pumping power values of the two models were investigated. The results show that high mixed convection occurred with the trapezoidal model. A high average Nusselt number value was obtained at 40PPI in the rectangular model. In the trapezoidal model, average Nusselt number decreased with increased aluminum foam pore density. Friction factor increased slightly with increasing modified Grashof number and decreased with increasing Reynolds number. Pumping power increased with increased pore density of aluminum foam and mixed convection parameters. The comparison shows good agreement between the numerical and experimental work and that the average results produced have an 8.02% deviation in average Nusselt number.

MIXED CONVECTION JET IMPINGEMENT COOLING OF A RECTANGULAR SOLID HEAT SOURCE IMMERSED IN A POROUS LAYER

The jet impingement cooling of a solid rectangular block heated from below and immersed in a fluid-saturated porous medium is considered for investigation numerically. The jet direction is considered to be perpendicular from the top to the solid rectangular block. Therefore the jet flow and the buoyancy-driven flow are in opposite directions. For a fixed block length, the governing parameters in the present Darcy flow problem are: Rayleigh number (Ra), Peclet number (Pe), solid-to-porous thermal conductivity ratio (Kr), the dimensionless thickness (or height) of the solid wall (H), in addition to the dimensionless jet width (D). The results are presented in the mixed convection regime with wide ranges of the governing parameters. At low values of Pe (natural convection cases), it is found that the effect of Pe and the jet width are negligible and the average Nusselt number (Nu) is increasing with the increase of either Rayleigh number or the thermal conductivity ratio, or decreasing the thickness of the solid wall. At high values of Pe (forced convection cases), it is found that the effect of Ra is negligible and Nu increases with either increasing Pe values, the thermal conductivity ratio or jet width, or decreasing the thickness of the solid wall. At moderate values of Pe (opposing mixed convection cases), it is observed that the values of average Nusselt number show minimum values. The value of Pe at which minimum Nu occurs depends on Ra, thermal conductivity ratio, jet width, and the thickness of the solid wall. At low values of either Ra or thermal conductivity ratio, this case where Nu shows a minimum value is not obvious. It is found that the thinner solid walls have higher values of the average Nusselt number with other parameters fixed. Therefore, for cooling applications, the numerical results indicate that the solid wall should be as thin as possible, with high thermal conductivity. The present results show that the cooling rate with the opposing missed convection mode is lower than that with the natural convention mode. The results show that the effect of the jet width D can be incorporated by plotting the average Nusselt number against Pe × D, where the results form a single curve for different values of the jet width.

Enhancement of natural convection heat transfer using different nanoparticles in an inclined semi-annular enclosure partially heated from above

This study addresses a numerical analysis of heat transfer and fluid flow in an inclined half-annulus partially heated from above and filled with nanofluids. The conservation equations in cylindrical coordinates are solved using an in-house FORTRAN code based on the finite volume method coupled with multigrid acceleration. A localized heat source owing constant temperature is placed along the outer cylinder of the annular region. Water-based nanofluid containing various volume fractions of Au, Cu, CuO and Al2O3 nanoparticles is used to examine potential heat transfer enhancement in the annulus. The effective thermal conductivity and viscosity of nanofluid are calculated using the Maxwell-Garnetts (MG) and Brinkman models, respectively. Different configurations are considered and numerical simulations were performed for different governing parameters such as the Rayleigh number, the size γ and/or location θ p of the heat source and inclination angle of enclosure α. It is found that the average Nusselt number exhibits an increasing trend as dual functions of the Rayleigh number and the solid volume fraction of the nanoparticles. The highest values of enhancement are obtained when using Au nanoparticles. Moreover, it is observed that the size, γ, and location, θ p , of the heater source significantly affect the resulting convective flow. An optimum size of the heater source is manifested in which the average Nusselt number attains a minimum for a given Rayleigh number. Reliable correlations formulae expressing the average Nusselt number in terms of Ra, φ, γ and θ p are established. Also for γ = 180°, we note that the change of inclination angle has a significant impact on the thermal and hydrodynamic flow field. It is shown that the minimum values of average Nusselt numbers are found to be at αmin = 45°, 90°, 90° for Ra = 103, 104 and 105, respectively. However, its maximum occurs at αmax = 180° for all Ra number.

Numerical Study of Heat Transfer and Separated Flow over Rectangular Obstacle in Jet

This paper is about a separated reattaching flow over a hot rectangular obstacle. Two types of incoming flow are examined in order to show the influence of the external zone of the flow on the reattachment process. It comes about due to a wall jet and a boundary layer. The inner region of these two flows is similar, but their external regions are extremely different. The separating and reattaching flow phenomena are of particular interest in engineering fields such as for an aeronautical application. Wall jet flow over an obstacle occurs in many engineering applications such as environmental discharges, heat exchangers, fluid injection systems, cooling of combustion chamber wall in a gas turbine, automobile design, and others. In electronics cooling, the prediction of the Nusselt number distribution along the obstacles is necessary before the design of the apparatus. For a heated obstacle at a constant temperature, T = 350 K and an aspect ratio of 10 (L = 10 H), the problem parameters are: (a) jet exit Reynolds number (Re) ranged from 1000 to 50000, (b) incoming flow configuration (boundary layer and wall jet). The ratio between the thickness of the nozzle (b) to the obstacle height (H) are examined simultaneously. The formulation is based on the SST k–ω turbulence model. The results show that the increasing of nozzle thickness; enhances the heat transfer and considerably modifies the stagnation point location. The highest incoming flow momentum provides the greatest values of average Nusselt number. Such as the boundary layer case in comparison with the wall jet cases. The average Nusselt number is correlated according to problem parameters .

Cross-Buoyancy Mixed Convection Around a Confined Triangular Bluff Body

Two-dimensional numerical simulations are carried out to understand the effects of cross-buoyancy on the confined flow and heat transfer characteristics over an equilateral triangular bluff body at low and intermediate Reynolds numbers (Re). For a fixed Richardson number (Ri), an average cylinder Nusselt number increases with increasing Re. However, the average Nusselt number shows a soft response toward increasing Ri. The critical Re for the onset of periodic unsteady flow increases with increasing cross-buoyancy (Ri). A simple heat transfer correlation is also obtained to relate the values of average cylinder Nusselt number, Re, and Ri.

Natural convection heat transfer under constant heat flux wall in a nanofluid filled annulus enclosure

In this investigation, the Control Volume based Finite Element Method (CVFEM) is used to simulate the natural convection heat transfer of Cu–water nanofluid in an annulus enclosure. The Maxwell–Garnetts (MG) and Brinkman models are also employed to estimate the effect of thermal conductivity and viscosity of nanofluid. The governing parameters are the Rayleigh number, nanoparticle volume fraction and the aspect ratio (ratio of the outer radius to the inner one). Results are presented in the form of isotherms, streamlines, local and average Nusselt numbers. The results indicate that increment of the aspect ratio increases the value of average Nusselt number. Moreover, the angle of turn for the boundary condition of the inner cylinder significantly affects the values of local Nusselt number, average Nusselt number, streamlines and isotherms.