Heat transfer characteristics in a two‐dimensional channel with an oscillating wall

Numerical calculations have been carried out for the laminar heat transfer in a two‐dimensional channel bounded by a fixed wall and an oscillating wall. In this calculation, the moving boundary problem was transformed into a fixed boundary problem using the coordinate transformation method, and the fully implicit finite difference method was used to solve the mass, momentum, and energy conservation equations. The calculated results are summarized as follows: (i) The wall oscillation has an effect of enhancing the heat transfer and an effect of increasing the additional pressure loss. (ii) An optimum Strouhal number for the enhancement of heat transfer exists, and this optimum value is strongly affected by the amplitude of wall oscillation. © 2001 Scripta Technica, Heat Trans Asian Res, 30(4): 280–292, 2001     

Heat and mass transfer in a reforming catalyst bed: Analytical prediction of distributions in the catalyst bed

Heat and mass transfer characteristics within a reforming catalyst bed have been analytically investigated. A numerical analysis was carried out in a two‐dimensional steady‐state model of a reforming catalyst bed. The reforming tube was filled with catalyst and the tube wall was uniformly heated; a mixture of steam and methane was reformed through the catalyst bed. The predicted distributions of temperature, formed gas composition, methane conversion rate, and heat transfer coefficient in the catalyst bed are in good agreement with the experimental data. The effects of space velocity, steam carbon molar ratio, and wall temperature on the heat transfer coefficient were analytically presented. From temperature and composition distributions simulated by the two‐dimensional analysis, the effects of the above‐mentioned factors and diffusion on both heat and mass transport phenomena were qualitatively predicted. © 2003 Wiley Periodicals, Inc. Heat Trans Asian Res, 32(4): 367–380, 2003; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.10101     

Wall‐Confined Flow and Heat Transfer Around a Square Cylinder at Low Reynolds and Hartmann Numbers

This article discusses the results obtained through a two‐dimensional numerical simulation following a finite volume approach on the forced convection heat transfer for the hydromagnetic flow around a square cylinder at low Reynolds and Hartmann numbers. The magnetohydrodynamic (MHD) flow of a viscous incompressible and electrically conducting fluid is assumed to take place in a rectangular channel subjected to externally imposed magnetic fields and the cylinder is fixed within the channel. The magnetic fields may be applied either along the streamwise or transverse directions. Simulations are performed for the range of kinetic Reynolds number 10 ≤ Re ≤ 60 with Hartmann number 0 ≤ Ha ≤ 15 and for different thermal Prandtl numbers, Pr = 0.02 (liquid metal), 0.71 (air), and 7 (water) for a blockage ratio β = 0.25. A steady flow can be expected for the above range of conditions. Besides the channel wall, the magnetic field imparts additional stability to the flow as a consequence of which the recirculation region behind the obstacle reduces with increasing magnetic field strength for a particular Re. The critical Hartmann numbers for the complete suppression of flow separation in the case of a transversely applied magnetic field are computed. The rate of heat transfer is found almost invariant at low Re whereas it increases moderately for higher Re with the applied magnetic field. The heat transfer increases in general with the Reynolds number for all Hartmann numbers. Finally, the influence of obstacle shape on the thermohydrodynamic quantities is noted. © 2013 Wiley Periodicals, Inc. Heat Trans Asian Res, 43(5): 459–475, 2014; Published online 3 October 2013 in Wiley Online Library ( wileyonlinelibrary.com/journal/htj ). DOI 10.1002/htj.21091     

Free Convective Flow in an Open‐Ended Vertical Porous Wavy Channel with a Perfectly Conductive Thin Baffle

The problem of steady two‐dimensional free convective flow of a Walters fluid (model B ′) in a porous medium between a long vertical wavy wall and parallel flat wall in the presence of a heat source is discussed. The channel is divided into two passages by means of a thin, perfectly conductive plane baffle and each stream will have its own pressure gradient and hence the velocity will be individual in each stream. The governing equations of the fluid and the heat transfer have been solved subject to the relevant boundary conditions by assuming that the solution consists of two parts: a mean part and disturbance or perturbed part. Exact solutions are obtained for the mean part and the perturbed part is solved using long wave approximation. Results are presented graphically for the distribution of velocity and temperature fields for varying physical parameters such as Grashof number, wall temperature ratio, porous parameter, heat source/sink parameter, product of non‐dimensional wave number, and space‐coordinate and viscoelastic parameter at different positions of the baffle. The relevant flow and heat transfer characteristics, namely, skin friction and the rate of heat transfer at both walls, has been discussed in detail. © 2013 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley Online Library ( wileyonlinelibrary.com/journal/htj ). DOI 10.1002/htj.21118     

Slug flow heat transfer in a semi‐infinite microtube with temperature jump wall condition

This paper presents an analytical solution of steady‐state heat transfer for laminar, two‐dimensional, and rarefied gas flow in a semi‐infinite microtube. To account for the slip‐flow characteristics of microscale heat transfer, temperature jump condition at the wall has been included in the model while the fluid velocity is assumed to be constant (slug flow). The solution yields closed form expressions for fully‐developed Nusselt numbers in terms of Knudsen number and Prandtl number under both isothermal and isoflux wall conditions. © 2009 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20263     

A spatially advancing turbulent flow and heat transfer in a curved channel

Direct numerical simulation was performed for a spatially advancing turbulent flow and heat transfer in a two‐dimensional curved channel, where one wall was heated to a constant temperature and the other wall was cooled to a different constant temperature. In the simulation, fully developed flow and temperature from the straight‐channel driver was passed through the inlet of the curved‐channel domain. The frictional Reynolds number was assigned 150, and the Prandtl number was given 0.71. Since the flow field was examined in the previous paper, the thermal features are mainly targeted in this paper. The turbulent heat flux showed trends consistent with a growing process of large‐scale vortices. In the curved part, the wall‐normal component of the turbulent heat flux was twice as large as the counterpart in the straight part, suggesting active heat transport of large‐scale vortices. In the inner side of the same section, temperature fluctuation was abnormally large compared with the modest fluctuation of the wall‐normal velocity. This was caused by the combined effect of the large‐scale motion of the vortices and the wide variation of the mean temperature; in such a temperature distribution, large‐scale ejection of the hot fluid near the outer wall, which is transported into the near inner‐wall region, should have a large impact on the thermal boundary layer near the inner wall. Wave number decomposition was conducted for various statistics, which showed that the contribution of the large‐scale vortex to the total turbulent heat flux normal to the wall reached roughly 80% inside the channel 135^° downstream from the curved‐channel inlet. © 2009 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20275     

An experimental and theoretical investigation into the heat transfer of a finned water wall tube in a circulating fluidized bed boiler

Heat transfer improvement in a water wall tube with fins was investigated in a circulating fluidized bed (CFB) boiler. Experiments were first conducted in a 6 MWth CFB boiler then a model was developed to analyse and interpolate the results. Temperatures at some discrete points within the wall cross‐section of the tube were measured by burying 0.8 mm thermocouples within a tube. Experimental data showed an increase in heat absorption up to 45 per cent. A good agreement between measured and predicted values was noted. The distribution of temperature in the metal wall and of heat flux around the outer wall of a tube with longitudinal and lateral fins was analysed by numerical solution of a two‐dimensional heat conduction equation. Effects of bed‐to‐wall heat transfer coefficient, water‐to‐tube inside heat transfer coefficient, bed temperature, water temperature and thermal conductivity of the tube material on the heat flux around the water tube are discussed. The present work also examines the influence of the length of the longitudinal fin and the water tube thickness. Heat flux was highest at the tip of the longitudinal fin. It dropped, but increased again near the root of the lateral fin. Copyright © 2000 John Wiley & Sons, Ltd.     

Numerical study of heat transfer enhancement and fluid flow with inline common‐flow‐down vortex generators in a plate‐fin heat exchanger

Three‐dimensional numerical simulations are performed on a plate‐fin heat exchanger (with triangular fins as inserts between the plates) to evaluate the laminar heat transfer and fluid flow characteristics with longitudinal vortex generators (LVGs). The effect with an inline rectangular winglet pair (RWP) with a common‐flow‐down (CFD) configuration is studied. The numerical results indicate that the application of inline LVGs effectively enhances the heat transfer of the channel. The heat transfer further increases with the increase in the Reynolds number from 200 to 500 and angle of attack from β = 15° to 22.5°. The computations are also performed to find the best location for the second RWP. © 2012 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley Online Library ( wileyonlinelibrary.com/journal/htj ). DOI 10.1002/htj.20414     

A theoretical study of evaporative heat transfer in high‐velocity two‐phase flow of air–water in a small vertical tube

A theoretical study was performed to investigate the evaporative heat transfer of high‐velocity two‐phase flow of air–water in a small vertical tube under both heating conditions of constant wall temperature and constant heat flux. A simplified two‐phase flow boundary layer model was used to evaluate the evaporative heat transfer characteristics of the annular two‐phase flow. The analytical results show that the gravitational force, the gas–liquid surface tension force, and the inertial force are much smaller than the frictional force and hence can be neglected for a small tube. The evaporative heat transfer characteristics of the small tube with constant wall temperature are quite close to those of the small tube with constant heat flux. The mechanism of the heat transfer enhancement is the forced convective evaporation on the surface of the thin liquid film. © 2003 Wiley Periodicals, Inc. Heat Trans Asian Res, 32(5): 430–444, 2003; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.10110     

Thermal radiation effects of a high‐temperature developing laminar flow in a tube

The thermal radiation effects of a high‐temperature developing laminar flow in a tube are investigated numerically. The two‐dimensional steady flow and heat transfer are considered for an absorbing‐emitting gray medium, whose density is dependent on the temperature. The governing equations of the coupled process are simultaneously solved by the discrete ordinate method combined with the control volume method. For a moderate optical thickness, the velocity distribution, the temperature distribution, and the radial heat flux distribution in the medium as well as the heat flux distribution on the tube wall are presented and discussed. The results show that the thermal radiation effects of a high‐temperature medium are significant under a moderate optical thickness. The flow and convective heat transfer are weakened, and the development of temperature distribution is accelerated noticeably. © 2004 Wiley Periodicals, Inc. Heat Trans Asian Res, 33(5): 299–306, 2004; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20018     

Longitudinal heat transfer enhanced by fluid oscillation in a circular pipe with conductive wall

The longitudinal heat transfer in capillary pipes is enhanced by fluid oscillation. The analytical solution to this phenomenon was obtained considering the wall thermal conductivity. Based on this solution, the effects of wall conductivity and thickness were investigated for the case of large amplitude of fluid motion. The longitudinal heat transfer through the fluid part was more enhanced in highly conductive thick pipes. This is because the region where the heat is transferred backwards is smaller in these pipe sections. The direction of longitudinal heat transfer depends on the phase difference of temporary change between the velocity and temperature; it depends on whether or not the difference exceeds π/2. From this point of view, the most effective wall regarding this problem is presented, where the wall temperature does not change preserving the mean temperature of the location during the oscillation. © 2004 Wiley Periodicals, Inc. Heat Trans Asian Res, 33(2): 129–139, 2004; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.10126     

Study on heat transfer characteristics of porous metallic heat sink with conductive pipe under bypass effect

The work investigated the forced convection heat transfer of the heat sink situated in a rectangular channel by considering the bypass effect. The fluid medium was air. The relevant parameters were the Reynolds number (Re), the relative top by‐pass gap (C/H), and the relative side by‐pass gap (S/L). The size of the heat sink was 60 mm (L)×60 mm(W)×24 mm(H). Two heat sinks were employed as test specimens: (A) the 0.9‐porosity aluminum foam heat sink and (B) the 0.9‐porosity aluminum foam heat sink with a 20 mm diameter copper cylinder. The copper cylinder was used as a conductive pipe of heat sink. The average Nusselt number was examined under various forced convection conditions. Experimental results demonstrate that increasing by‐pass space decreased the Nusselt number. Besides, the average Nusselt number of mode B heat sink was higher than that of mode A heat sink by 30% for the case without by‐pass flow. The heat transfer enhancement by the copper cylinder would decline as the by‐pass space grew. © 2009 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20247     

Effect of dynamic load on heat transfer characteristic of a two‐phase pipe boiling flow

An experimental investigation was performed to obtain the flow and heat transfer characteristics of a single‐phase water flow and a two‐phase pipe boiling water flow under dynamic load in the present work. By analyzing the fluid resistance, effective heat, flow pattern, and heat transfer coefficient of the experimental data, the effects of dynamic load on the flow and heat transfer characteristics of single‐phase water and two‐phase boiling water flow were investigated. The results show that the dynamic load significantly influences the flow characteristic and boiling heat transfer of the two‐phase pipe flow. It will enhance the fluid resistance and heat dissipation toward the ambient environment, and reduce the heat transferred to the two‐phase fluid. The impact mixing flow caused by the dynamic load breaks the uniform and varying principle of the wall temperatures. As a result of that, the greater the dynamic load, the lower the wall inner bottom temperature and the higher the wall inner top temperature in a certain extent. © 2011 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley Online Library ( wileyonlinelibrary.com/journal/htj ). DOI 10.1002/htj.20378     

Numerical simulation of separated flow transition and heat transfer around a two‐dimensional rib

Numerical simulations of separated flow transition and heat transfer around a two‐dimensional rib mounted in a laminar boundary layer were performed. The separated shear layer becomes unstable due to the Kelvin–Helmholtz instability and generates a two‐dimensional vortex. This vortex becomes three‐dimensional and collapses in the downstream part of the separation bubble. As a result, transition from laminar to turbulent flow occurs in the separated shear layer. Streamwise vortices exist downstream of the reattachment flow region. The low‐frequency flapping motion and transition of the separated shear layer are influenced by three‐dimensional dynamics upstream of the separation bubble. Large‐scale vortices around the reattachment flow region have substantial effects on heat transfer. Downstream of the reattachment point, the surface friction coefficient and Nusselt number are different from their profiles in the laminar boundary layer and approach the distributions seen in the turbulent boundary layer. © 2007 Wiley Periodicals, Inc. Heat Trans Asian Res, 36(8): 513–528, 2007; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20177     

Hydrothermal Behavior of a Ferrofluid in a Corrugated Channel in the Presence of a Magnetic Field

In this paper, results of applying a non‐uniform magnetic field on a dilute ferrofluid (water and 3% vol. Fe₃O₄) flow in a corrugated channel under a constant heat flux boundary condition have been reported. The thermal behavior of the flow is investigated numerically using a two‐phase mixture model and control volume technique. It is concluded that using a magnetic field with a negative gradient on a nanofluid flow in corrugated channels can be proposed as a suitable method to achieve higher heat transfer performance and augment the heat transfer coefficient and also reduces the wall temperature. This method can lead to the design of more compact heat exchangers. © 2013 Wiley Periodicals, Inc. Heat Trans Asian Res, 43(1): 80–92, 2014; Published online in Wiley Online Library ( wileyonlinelibrary.com/journal/htj ). DOI 10.1002/htj.21060     

Wall–bounded flow and heat transfer around a circular cylinder at low Reynolds and Hartmann numbers

A two–dimensional numerical simulation is performed following a finite volume approach to analyze the forced convection heat transfer for the hydromagnetic flow around a circular cylinder at low Reynolds numbers. The cylinder is placed within a rectangular channel subjected to externally applied magnetic fields and acted upon by the magnetohydrodynamic (MHD) flow of a viscous incompressible and electrically conductive fluid. The magnetic field is applied either along the streamwise or transverse directions. The simulation is carried out for the range of Reynolds number 10 ≤ Re ≤ 80 with Hartmann number 0 ≤ Ha ≤ 10 and for different Prandtl numbers, Pr = 0.02 (liquid metal), 0.71 (air), and 7 (water) for a blockage ratio β = 0.25. The flow is steady for the above range of conditions. Apart from the channel wall, the magnetic field provides additional stability to the flow as a result of which the recirculation region behind the obstacle reduces with increasing magnetic field strength for a particular Reynolds number. The rate of heat transfer is found almost invariant at low Re whereas it increases slightly for higher Re with the applied magnetic field. The heat transfer increases as usual with the Reynolds number for all Hartmann numbers. © 2012 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley Online Library ( wileyonlinelibrary.com/journal/htj ). DOI 10.1002/htj.21025     

Augmenting Natural Convection in a Vertical Flow Path Through Transverse Vibrations of an Adiabatic Wall

Natural-convection enhancement methods may provide more efficient and effective, low-cost, reliable alternative cooling options for certain operating conditions. In this work, the introduction of locally applied oscillations is explored as a means of improving natural-convection cooling. A simple geometric system consisting of a vertically oriented flow path with one uniformly heated wall and one transversely oscillating, insulated wall is utilized. A finite-volume SIMPLER-based technique is developed to study the system in a transformed “fixed” domain. Analysis over a range of oscillation frequencies and displacements, flow channel length-to-width ratios, and heat rates allows for the examination of the characteristics of the resulting velocity and temperature fields under various operating conditions. The combination of the oscillations, the natural convection, the fluid inertia, and other flow drivers produced up to a 340% increase in the local heat transfer coefficient.     

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     

Prediction of turbulent flow and heat transfer within rotating U‐shaped passages

Numerical predictions of three‐dimensional flow and heat transfer are presented for rotating serpentine passages with and without rib turbulators. The coolant air is pressurized and its operating conditions are selected closely to match actual turbine operating parameters. Two different arrangements of rib turbulators were studied: (1) transverse ribs on the leading and trailing walls and (2) transverse ribs on all four walls. The rib height‐to‐hydraulic diameter ratio (e/D_h) is 0.143; the rib pitch‐to‐height ratio (s/e) is 7. Results for the rib‐roughened serpentine passages were compared with those of smooth ones calculated in the literature. It was shown that a significant enhancement is achieved by means of rib turbulators in a serpentine passage at a stationary state as well as in a rotating state. In the radially‐outward flow passages, the effect of rotation on heat transfer is relatively prominent. The secondary flows induced by the Coriolis forces are most intensive in the channel with four ribbed surfaces. The heat transfer after a 180° sharp turn in the smooth channel is influenced more by the sharp‐turn‐induced flow than the rib‐roughened ones. © 2006 Wiley Periodicals, Inc. Heat Trans Asian Res, 35(6): 410–420, 2006; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20125     

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