Simulation of mixed convection heat transfer in a horizontal channel with an open cavity containing a heated hollow cylinder

The objective of this paper is to numerically investigate the mixed convective flow and heat transfer controlled by a heated hollow cylinder inside an open cavity attached with a horizontal channel. All the boundaries of the channel and cavity are perfectly insulated while the inner surface of the cylinder is heated uniformly by heat flux q. The equations of conservation of mass, momentum, and energy were solved using adequate boundary conditions by Galarkin's weighted residual finite element technique. The solution has been performed in the computational domain as a whole with proper treatment at the solid/fluid interface. Computations have been conducted for Ra = 10³–10⁵, Prandtl number Pr varying from 0.7 to 7 and ratio of solid to fluid thermal conductivities from 0.2 to 50. Results are presented in terms of streamlines, isotherms, heat transfer rate in terms of the average Nusselt number (Nu_av), drag force (D), and maximum bulk temperature (θ_max). © 2012 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley Online Library ( wileyonlinelibrary.com/journal/htj ). DOI 10.1002/htj.21002     

An experimental investigation on the effect of a perforated fin on free convection heat transfer from a confined horizontal cylinder

Natural convection heat transfer from an isothermal horizontal fin attached to a cylinder, confined between two adiabatic walls of constant height is investigated by the Mach–Zehnder interferometry technique. This study is focused on the effect of a perforated fin attached to the bottom of a cylinder while the vertical position of the cylinder (Y ) changes between two walls with a constant distance of W measuring 1.5 times the cylinder diameter. The cylinder's average Nusselt numbers are determined for three ratios of vertical position to its diameter, Y /D = 0.5, 1.5, 2, and 3. The Rayleigh number ranges from 4.5 × 10³ to 1.2 × 10⁴. The distance between the walls is chosen to be 1.5 D, that is, an optimum distance at which the Nusselt number is maximum. The effect of the perforated fin on free convection heat transfer is investigated and compared with other works. Results show outstanding enhancement in heat transfer, with a minimum result of 40% and maximum of 90%. © 2013 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley Online Library (wileyonlinelibrary.com/journal/htj). DOI 10.1002/htj.21041     

Effect of viscosity variation on natural convection flow of water–alumina nanofluid in an annulus with internal heat generation

Heat transfer enhancement in a horizontal annulus using the variable viscosity property of an Al₂O₃–water nanofluid is investigated. Two different viscosity models are used to evaluate heat transfer enhancement in the annulus. The base case uses the Pak and Cho model and the Brinkman model for viscosity which take into account the dependence of this property on temperature and nanoparticle volume fraction. The inner surface of the annulus is heated uniformly by a constant heat flux q_w and the outer boundary is kept at a constant temperature T_c. The nanofluid generates heat internally. The governing equations are solved numerically subject to appropriate boundary conditions by a penalty finite‐element method. It is observed that for a fixed Prandtl number Pr = 6.2, Rayleigh number Ra = 10⁴ and solid volume fraction ? = 10%, the average Nusselt number is enhanced by diminishing the heat generation parameter, mean diameter of nanoparticles, and diameter of the inner circle. The mean temperature for the fluids (nanofluid and base fluid) corresponding to the above mentioned parameters is plotted as well. © 2012 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley Online Library ( wileyonlinelibrary.com/journal/htj ). DOI 10.1002/htj.21016     

Numerical investigation of turbulent heat transfer in channels with detached rib‐arrays

A numerical investigation is conducted to analyze the flow‐field and heat transfer characteristics in a rectangular passage of width‐to‐height ratio of 6:1 with detached ribs on one wall, where constant wall temperature condition is applied. The effect of detached‐rib geometry on heat transfer coefficient, friction factor, and thermal enhancement factor is investigated covering the range of the detached‐clearance ratios (c/a) of 0.1, 0.2, 0.3, and 0.4, the Reynolds number based on the channel hydraulic diameter ranges from 8000 to 24,000. The numerical results show that the flow‐field, temperature pattern, local Nusselt number distribution, average Nusselt number, and friction factor are strongly dependent on the detached‐clearance ratios. The thermal enhancement factor (TEF) under the same pumping power constraint is calculated in order to examine the overall effect of the detached‐clearance ratio. For the present range investigated, the maximum TEF of 1.22 is achieved by the use of the ribs with c/a of 0.1 at Reynolds number of 8000. © 2011 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley Online Library ( wileyonlinelibrary.com/journal/htj ). DOI 10.1002/htj.20357     

Non‐Darcy assisted flow along a channel with an open cavity filled with water–TiO2 nanofluid

Numerical investigation on forced (assisted) convection heat transfer in a two‐dimensional horizontal porous channel with an open cavity is studied in this article. A non‐uniform heat flux is considered to be located on the bottom surface of the cavity. The rest of the surfaces are taken to be perfectly insulated. The physical domain is filled with a water‐based nanofluid containing TiO₂ nanoparticles. The fluid enters from the left and exits from the right with initial velocity U_i and temperature T_i. Governing equations are discretized using the penalty finite element method. The simulation is carried out for a wide range of Reynolds number Re (= 10–500) and Darcy number Da (= 10^?5–∞). Results are presented in the form of streamlines, isothermal lines, local and average Nusselt numbers, average temperatures of the fluid, horizontal and vertical velocities at mid‐height of the channel and mean velocity fields for various Re and Da. The enhancement of heat transfer rate is caused by the increasing Re and falling Da. © 2013 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley Online Library (wileyonlinelibrary.com/journal/htj). DOI 10.1002/htj.21046     

Power‐law flow and heat transfer over an inclined square bluff body: effect of blockage ratio

Flow and heat transfer of non‐Newtonian power‐law fluids over an inclined square cylinder placed inside a channel are studied numerically at low Reynolds numbers. In particular, calculations are carried out for Reynolds number (Re) = 1–40; power‐law index (n) = 0.4–1 and blockage ratio (β) = 12.5–50% at a Prandtl number (Pr) = 50. An increase in blockage ratio results in an increase in the total drag coefficient and decrease in the wake length. The Strouhal number and the root mean square value of the lift coefficient increase with the increasing Reynolds number for the fixed values of blockage ratio and power‐law index. The average Nusselt number increases with power‐law index and/or blockage ratio. The maximum enhancement in heat transfer is approximately 49, 41, and 35% for the values of blockages of 50, 25, and 12.5%, respectively, as compared to the corresponding Newtonian value. The average Nusselt number for the inclined square cylinder (at α = 45^°) is always greater than the average Nusselt number for the regular square cylinder (at α = 0). Finally, simple expressions of drag and Nusselt number have been established for the above range of settings. © 2013 Wiley Periodicals, Inc. Heat Trans Asian Res 43(2): 167‐196, 2014; Published online 20 June 2013 in Wiley Online Library ( wileyonlinelibrary.com/journal/htj ). DOI 10.1002/htj.21071     

Inner cylinder diameter effects on free convection heat transfer in a stable stratified fluid

An experimental study was performed to identify the effects of the inner cylinder diameter on free convection heat transfer in a stable stratified fluid between coaxial cylinders. The stratified layer was formed using a sucrose aqueous solution, and was heated from the outer cylinder (70 mm I.D.) at a constant heat flux and cooled from the inner cylinder at a constant temperature. The experiment was conducted for an inner cylinder of 50.8 mm O.D. and the results were compared with those for an inner cylinder of 30 mm O.D., which were previously reported by the authors. The results show that the inner cylinder size affects the heat transfer in the early stage of heating: increasing the diameter of the inner cylinder decreased the distance between the heating and cooling surfaces. It consequently reduced both the duration of heat conduction prevailing and the resulting temperature increase of the heating surface during that period. Increasing the diameter of the inner cylinder also made the heat transfer characteristics similar to those between vertical plates. © 2007 Wiley Periodicals, Inc. Heat Trans Asian Res, 36(3): 172– 186, 2007; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20148     

Experimental study of free convection heat transfer and condensation of vapor of humid air over an inclined cold tube

In this study condensation heat transfer on a cold inclined circular cylinder due to natural convection for various conditions is investigated experimentally. The cylinder is placed in an isolated test room to permit pure natural circulation of ambient air. Ambient temperature and humidity of the test room are controlled by a refrigeration cycle and humidifying. The ambient relative air humidity changed in the range of 30 to 50% and temperature from 25 to 35 °C. The ethylene‐glycol/water solution is used as a refrigerant to control and keep the temperature of the test section at a constant value. The cold surface temperature is varied from 2 to 6 °C. The condensation rate and heat flux are found to depend mainly on time, temperature difference between ambient air and cold surface, ambient relative humidity, and tube inclination. Results are plotted for various conditions with respect to time. The experimental results are used to propose a correlation to predict the condensate mass flow rate for free convection heat transfer. © 2012 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley Online Library ( wileyonlinelibrary.com/journal/htj ). DOI 10.1002/htj.21015     

Multiple scattering of thermal waves from double subsurface cylinders in semi‐infinite slab

In this paper, combined with the non‐Fourier equation of heat conduction and expansion method of wave functions, the multiple scattering of thermal waves from a subsurface cylinder in a semi‐infinite body is investigated. A general solution of scattered fields based on hyperbolic equations of heat conduction is presented for the first time. The effects of physical and geometric parameters on the temperature are analyzed. The thermal waves are excited at the front surface of opaque material by modulated optical beams. The circular cylinder is taken as a cavity with thermal insulation conditions. © 2007 Wiley Periodicals, Inc. Heat Trans Asian Res, 36(7): 398– 407, 2007; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20174     

MHD Flow and Heat Transfer of a Dusty Fluid Over a Stretching Hollow Cylinder with a Convective Boundary Condition

In this study, we deal with the problem of a steady two‐dimensional magnetohydrodynamic (MHD) flow of a dusty fluid over a stretching hollow cylinder. Unlike the commonly employed thermal conditions of constant temperature or constant heat flux, the present study uses a convective heating boundary condition. The multi‐step differential transform method (multi‐step DTM), one of the most effective methods, is employed to find an approximate solution of the system of highly nonlinear differential equations governing the problem. Comparisons are made between the results of the proposed method and the numerical method in solving this problem and excellent agreement has been observed. The influence of important parameters on the flow field and heat transfer characteristics are presented and discussed in detail. The results show that both the thermal boundary layer thickness and the heat transfer rate at the wall increases with increasing Biot number Bi, while it has no effect on the skin friction coefficient. © 2013 Wiley Periodicals, Inc. Heat Trans Asian Res, 43(3): 221–232, 2014; Published online 30 August 2013 in Wiley Online Library ( wileyonlinelibrary.com/journal/htj ). DOI 10.1002/htj.21073     

Free convection of composite porous medium in a vertical channel

A fully developed free convection flow of immiscible fluids in a vertical channel filled with a porous medium is analyzed in the presence of source/sink. The flow is modeled using the Darcy–Brinkman–Forchheimer equation model. The viscous and Darcy dissipation terms are included in the energy equation. The channel walls are maintained at two different constant temperatures. The transport properties of both fluids are assumed to be constant. Continuous conditions for velocity, temperature, shear stress, and heat flux of both fluids at the interface are employed. The resulting coupled nonlinear equations are solved analytically using regular perturbation method and numerically using finite difference method. The velocity and temperature profiles are obtained in terms of porous parameter, Grashof number, viscosity ratio, width ratio, conductivity ratio, and heat generation or heat absorption coefficient. It is found that the presence of porous matrix and heat absorption reduces the flow field. © 2011 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley Online Library ( wileyonlinelibrary.com/journal/htj ). DOI 10.1002/htj.20340     

Heat transfer enhancement for laminar flow of ethylene glycol through a square duct fitted with increasing and decreasing orders of twisted tape

The isothermal friction factor and heat transfer enhancement through a square duct fitted with increasing and decreasing order of twist ratio sets have been studied under nearly uniform wall temperature conditions. The ethylene glycol flows under laminar flow (Re = 30–1200) through a square duct and hot water flows through an annular channel formed between a square duct and circular tube, in a counter current fashion. The hot water at a very high flow rate is circulated though the annular channel to ensure a nearly uniform wall temperature condition. There is not much change in the magnitude of the heat transfer coefficient enhancement with the increasing twist ratio and with the decreasing twist ratio set, as the intensity of the swirl generated at the inlet or at the outlet in the order of increasing twist ratio or decreasing twist ratio, is the same in both the cases. Performance evaluation analysis on constant pumping power was made and a maximum performance ratio was obtained for each twist insert corresponding to a Reynolds number of 680. © 2012 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley Online Library ( wileyonlinelibrary.com/journal/htj ). DOI 10.1002/htj.20410     

Heat transfer from hollow cylinder using optimal homotopy asymptotic method

Optimal homotopy asymptotic method (OHAM) is employed to investigate steady‐state heat conduction with temperature dependent thermal conductivity and uniform heat generation in a hollow cylinder. Analytical models are developed for dimensionless temperature distribution and heat transfer for two cases using mixed boundary conditions (Dirichlet, Neumann, and Robin). The inner cylinder is assumed to be insulated in both cases. In the first case, the outer cylinder is assumed to be isothermal whereas in the second case, the outer cylinder is convectively cooled by a fluid of temperature T₂ through a uniform heat transfer coefficient h. The effects of Biot number, dimensionless heat generation, and thermal conductivity parameters on the temperature distribution and heat transfer are determined analytically and validated numerically using MAPLE 14. In both cases, the results obtained by OHAM are found to be in good agreement with the numerical results. It is found that as the Biot number increases, the results approach that of the isothermal case. © 2012 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley Online Library ( wileyonlinelibrary.com/journal/htj ). DOI 10.1002/htj.20407     

Lattice Boltzmann Method Based Simulation of Natural Convection in Partially Open Enclosure

A thermal lattice Boltzmann method‐based analysis was performed to numerically investigate the heat transfer by natural convection from an enclosure with a large vertical side opening. The height of the opening was less than the enclosure height and the vertical wall opposite to the opening was maintained at constant temperature. A parametric study was carried out for different values of Rayleigh number (Ra) ranging from 10³ to 10⁵ with air as the working fluid for three opening sizes and three opening locations. The Prandtl number was fixed at 0.71 and the enclosure aspect ratio was also fixed at 2 in all calculations. With Boussinesq approximation, the temperature distribution and stream functions in the enclosure were predicted. The profile of the normal velocity component at the opening location was determined. The opening size affects the stratification and recirculation pattern within the enclosure. The average Nusselt number at the heated wall was determined for all cases. © 2013 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley Online Library ( wileyonlinelibrary.com/journal/htj ). DOI 10.1002/htj.21110     

Heat transfer enhancement for combined convection flow of nanofluids in a vertical rectangular duct considering radiation effects

In this paper, combined convective heat transfer and nanofluids flow characteristics in a vertical rectangular duct are numerically investigated. This investigation covers Rayleigh numbers in the range of 2 × 10⁶ ≤ Ra ≤ 2 × 10⁷ and Reynolds numbers in the range of 200 ≤ Re ≤ 1000. Pure water and five different types of nanofluids such as Ag, Au, CuO, diamond, and SiO₂ with a volume fraction range of 0.5% ≤ φ ≤ 3% are used. The three‐dimensional steady, laminar flow, and heat transfer governing equations are solved using finite volume method (FVM). The effects of Rayleigh number, Reynolds number, nanofluids type, nanoparticle volume fraction of nano‐ fluids, and effect of radiation on the thermal and flow fields are examined. It is found that the heat transfer is enhanced using nanofluids by 47% when compared with water. The Nusselt number increases as the Reynolds number and Rayleigh number increase and aspect ratio decreases. A SiO₂ nanofluid has the highest Nusselt number and highest wall shear stress while the Au nanofluid has the lowest Nusselt number and lowest wall shear stress. The results also revealed that the wall shear stress increases as Reynolds number increases, aspect ratio decreases, and nanoparticle volume fraction increases. © 2011 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley Online Library ( wileyonlinelibrary.com/journal/htj ). DOI 10.1002/htj.20354     

Natural convection heat transfer of high temperature gas in an annulus between two vertical concentric cylinders

Water cooling panels have been adopted as the vessel cooling system of the High Temperature Engineering Test Reactor (HTTR) to cool the reactor core indirectly by natural convection and thermal radiation. In order to investigate the heat transfer characteristics of high temperature gas in a vertical annular space between the reactor pressure vessel and cooling panels of the HTTR, we carried out experiments and numerical analyses on natural convection heat transfer coupled with thermal radiation heat transfer in an annulus between two vertical concentric cylinders with the inner cylinder heated and the outer cylinder cooled. In the present experiments, Rayleigh number based on the height of the annulus ranged from 2.0 × 10⁷ to 5.4 × 10⁷ for helium gas and from 1.2 × 10⁹ to 3.5 × 10⁹ for nitrogen gas. The numerical results were in good agreement with the experimental ones regarding the surface temperatures of the heating and cooling walls. As a result of the experiments and the numerical analyses, the heat transfer coefficient of natural convection coupled with thermal radiation was obtained as functions of Rayleigh number, radius ratio, and the temperatures and emissivities of the heating and cooling wall surfaces. © 2005 Wiley Periodicals, Inc. Heat Trans Asian Res, 34(5): 293–308, 2005; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20070     

Flow and heat transfer of couple‐stress permeable fluid sandwiched between viscous fluid layers

The fully developed flow and heat transfer in a horizontal channel consisting of couple‐stress permeable fluid sandwiched between viscous fluid layers is investigated analytically. The channel walls are maintained at two different constant temperatures. The transport properties of the fluids in all regions are assumed to be constant. The governing equations are linear ordinary differential equations and hence closed form solutions are obtained. Effects of physical parameters such as viscosity ratio, thermal conductivity ratio, Eckert number, and Prandtl number on the flow are reported. An interesting and new approach is incorporated to analyze the flow for strong, weak, and comparable porosity with couple‐stress parameter. The variation of rate of heat transfer for different values of couple stress parameter and porosity is also discussed. © 2012 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley Online Library ( wileyonlinelibrary.com/journal/htj ). DOI 10.1002/htj.21007     

Simulating magnetohydrodynamic natural convection flow in a horizontal cylindrical annulus using the lattice Boltzmann method

A numerical study is presented about the effect of a uniform magnetic field on free convection in a horizontal cylindrical annulus using the lattice Boltzmann method. The inner and outer cylinders are maintained at uniform temperatures and it is assumed the walls are insulating with a magnetic field. Detailed numerical results of heat transfer rate, temperature, and velocity fields have been presented for Pr=0.7, Ra=10³ to 5 × 10⁴, and Ha=0 to 100. The computational results show that in a horizontal cylindrical annulus the flow and heat transfer are suppressed more effectively by a radial magnetic field. It is also found that the flow oscillations can be suppressed effectively by imposing an external radial magnetic field. The average Nusselt number increases by increasing the radius ratio while it decreases by increasing the Hartmann number. © 2012 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley Online Library ( wileyonlinelibrary.com/journal/htj ). DOI 10.1002/htj.21008     

CFD Simulation of Heat Transfer Augmentation in a Circular Tube Fitted with Alternative Axis Twisted Tape in Laminar Flow under a Constant Heat Flux

CFD simulation of heat transfer and friction factor characteristics in a circular tube fitted with twisted tape inserts in laminar flow and constant heat flux is conducted using FLUENT version 6.3.26. Plain and alternate axis twisted tape inserts with twist ratios (y = 2.93, 3.91, 4.89) and alternative angles (β = 30^°, 60^°, 90^°) have been used for the simulation. The simulated results are matched with the literature correlations for a plain tube with a discrepancy of less than ±8% for Nusselt number and ±6.25% for friction factor. The results also revealed that the heat transfer in terms of the Nusselt number increased with increases of Reynolds number, alternative angles, and decreases of twist ratio. Among the various twist ratios and alternative angles, the twist ratio of y = 2.93 and alternative angle β = 90^° offered a maximum heat transfer enhancement. © 2013 Wiley Periodicals, Inc. Heat Trans Asian Res, 43(4): 384–396, 2014; Published online 3 October 2013 in Wiley Online Library ( wileyonlinelibrary.com/journal/htj ). DOI 10.1002/htj.21089     

Mixed Convection Over a Vertical Plate in a Doubly Stratified Fluid‐Saturated Non‐Darcy Porous Medium with Cross‐Diffusion Effects

The present article investigates the influence of Dufour and Soret effects on mixed convection heat and mass transfer over a vertical plate in a doubly stratified fluid‐saturated porous medium. The plate is maintained at a uniform and constant wall heat and mass fluxes. The Darcy–Forchheimer model is employed to describe the flow in porous medium. The nonlinear governing equations and their associated boundary conditions are initially transformed into dimensionless forms. The resulting system of nonlinear partial differential equations is then solved numerically by the Keller‐box method. The variation of the dimensionless velocity, temperature, concentration, heat, and mass transfer rates for different values of governing parameters involved in the problem are analyzed and presented graphically. © 2013 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley Online Library ( wileyonlinelibrary.com/journal/htj ). DOI 10.1002/htj.21114