MIXED CONVECTION IN AN INCLINED CHANNEL WITH LOCALIZED HEAT SOURCES

Fully developed opposing mixed convection is numerically studied in an inclined channel that has discrete heating on the bottom and is insulated on the top. The numerical approach is based on the hypothesis that the solution is periodic according to the imposed wavelength of the heating elements. Considering that Ike heat produced by the heating elements is totally carried downstream, the temperature increment from one heating element to the other is defined on the basis of an energy balance. To verify the accuracy of the computational code, an analytical study of the extreme case with an entirely heated wall is investigated. Also, to validate that the solution of the problem is periodic with a wavelength corresponding to the imposed perturbation, a channel with entrance and exit sections containing four to six heating elements is simulated numerically. In the present study, the relative strength of the forced flow and buoyancy effects is examined for a broad range of Rayleigh numbers, Reynolds numbers, and inclination angles. Both overall and local recirculating flows are observed that are caused by buoyancy effects on the forced flow.     

EFFECT OF INCLINED VORTEX GENERATORS ON HEAT TRANSFER ENHANCEMENT IN A THREE-DIMENSIONAL CHANNEL

Three-dimensional unsteady flow and heat transfer in a channel with inclined block shape vortex generators mounted on one side of a channel flow are investigated for different Reynolds numbers, Re _H= 400 - 1500 , and Pr = 0.71. This study was informed to gain an understanding of the flow phenomena and calculate the heat transfer and pressure drop for different Reynolds numbers. The effect of computational domain, angle of incidence, size of the vortex generator, and the discretization schemes on the results are also investigated. Simulations use an incompressible finite volume code, based on a fractional step technique with a multigrid pressure Poisson solver and a nonstaggered grid arrangement.     

EFFECTS OF BINARY MIXTURE CONDENSATION ON TURBULENT MIXED CONVECTION HEAT AND MASS TRANSFER IN A VERTICAL CHANNEL

Abstract

A numerical analysis was carried out to study the detailed heat and mass transfer processes between a condensation liquid film and mixed turbulent moist airflow. Results show that the condensation latent heat transfer is more important for a system with higher inlet relative humidity or lower inlet Reynolds number of a moist airstream. The heat and mass transfer coefficients are higher for a system with higher inlet relative humidity and inlet Reynolds number of moist air. In addition, the aiding-buoyancy forces cause diminution in heat and mass transfer results compared with the corresponding results of forced convection.     

NAVIER-STOKES STUDY OF NATURAL CONVECTION AND HEAT TRANSFER IN VERTICAL SYMMETRICALLY HEATED PLATE-FIN HEAT SINKS

A numerical study was performed on buoyant flow and heat transfer in vertical plate-fin heat sinks. As a result of the problem's importance, a number of semiempirical correlations have been proposed to determine the Nusselt number along the wetted surfaces and establish the optimum fin spacing. This article aims to assess the accuracy and range of applicability of such correlations. A CFD program Coolit^TM specifically designed for modeling electronic equipment, was used to predict heat transfer over a wide range of Raleigh numbers and geometrical parameters. The computed results are presented and compared with semiempirical correlations. Ranges of validity of the correlations and three-dimensional flow mechanisms responsible for the observed discrepancies are discussed.     

NUMERICAL INVESTIGATION OF CONVECTION HEAT TRANSFER IN A HEATED ROOM

We describe numerical investigation of airflow and temperature field in a room with a convective heat source. The simulation involves using computational fluid dynamics (CFD) to validate different turbulence models, i.e., the standard k- k model and the low Reynolds number k- k model. The comparisons between computations and experiments show good and acceptable agreement. It can be concluded that the CFD simulations can capture the main flow features and provide satisfactory results. It can be seen that the thermal wall jet created by the heat source greatly influences airflow pattern and temperature field in the room. It can also be seen that the advanced turbulence model may produce better results than the standard one under a suitable mesh scheme.     

EFFECT OF HEATED WALL POSITION ON MIXED CONVECTION IN A CHANNEL WITH AN OPEN CAVITY

Mixed convection in an open cavity with a heated wall bounded by a horizontally insulated plate is studied numerically. Three basic heating modes are considered: (a) the heated wall is on the inflow side (assisting flow); (b) the heated wall is on the outflow side (opposing flow); and (c) the heated wall is the horizontal surface of the cavity (heating from below). Mixed convection fluid flow and heat transfer within the cavity is governed by the buoyancy parameter, Richardson number (Ri), and Reynolds number (Re). The results are reported in terms of streamlines, isotherms, wall temperature, and the velocity profiles in the cavity for Ri=0.1 and 100, Re=100 and 1000, and the ratio between the channel and cavity heights (H/D) is in the range 0.1-1.5. The present results show that the maximum temperature values decrease as the Reynolds and the Richardson numbers increase. The effect of the H/D ratio is found to play a significant role on streamline and isotherm patterns for differentheating configurations. The present investigation shows that the opposing forced flow configuration has the highest thermal performance in terms of both maximum temperature and average Nusselt number.     

MAGNETOHYDRODYNAMIC MIXED CONVECTION FROM A VERTICAL PLATE EMBEDDED IN A POROUS MEDIUM

Magnetohydrodynamic mixed convection flaw about a vertical flat plate embedded in a porous medium is considered. The effect of the magnetic field strength on the local Nusselt number and local wait shear stress is presented. The non-Darcian model including both the inertia and boundary effects is used. A particular transformation for the governing equations is adopted to cover the whole mixed convection regime within two finite limits. Appreciable effects of the magnetic field strength on the local Nusselt number as well as on the local wall shear stress in the mixed convection regime are found.     

NUMERICAL INVESTIGATION OF MIXED CONVECTION HEAT TRANSFER IN A HORIZONTAL CHANNEL WITH A BUILT-IN SQUARE CYLINDER

Structures of laminar wakes and heat transfer in the presence of thermal buoyancy art investigated from the numerical solution of complete Navier-Stokes and energy equations in a two-dimensional horizontal channel with a built-in square cylinder. Results show that mixed convection can initiate periodicity and asymmetry in the wake at lower Reynolds numbers than forced convection alone. For a given Reynolds number, the heating of the fluid in the channel is improved by mixed convection up to a certain Grashof number and deteriorates if the Grashof number is further increased.     

FULLY DEVELOPED MIXED CONVECTION IN A HORIZONTAL CHANNEL HEATED UNIFORMLY FROM ABOVE AND BELOW

Abstract

Calculations were performed for fully developed, laminar, mixed convection flow in a horizontal, parallel plate channel heated uniformly at the top and bottom plates. Spanwise conduction within the plates was considered, and calculations were performed for Pr = 0.7, 0 < Ra? < 2.5 × 10⁴, and values of a nondimensional conductance ratio in the range 10^?5 < γ < 10³. It is shown that mixed convection heat transfer enhancement is restricted to the lower surface and that the attendant secondary flow induces large spanwise surface temperature variations for which the maximum temperature exceeds values associated with the upper surface. Increased conduction within the bottom plate weakens the secondary flow and decreases spanwise temperature variations. Results of the calculations have important implications in situations for which there is interest in maintaining reduced temperatures, as well as large heat transfer enhancement, at the bottom plate.     

INTERNAL HEAT TRANSFER AUGMENTATION IN A CHANNEL USING AN ALTERNATE SET OF POROUS CAVITY-BLOCK OBSTACLES

A numerical investigation for forced convection in a constant-temperature parallel plate channel with porous cavity and block alternately emplaced on the bottom plate is presented in this work. The Brinkman-Forchheimer-extended Darcy model, which accounts for the effects of impermeable boundary and inertia, is used to characterize the flow field inside the porous region. Solutions of the coupled governing equations are tarried out through the stream function-voracity analysis. The characteristics of fluid flow and forced convection heat transfer have been obtained by the examinations of various governing parameters, such as the Reynolds number, Darcy number, inertial parameter, Prandtl number, and two geometric parameters. Several interesting phenomena such as the heat transfer augmentation in the channel were presented and discussed. The results of this investigation indicate that the size of recirculation caused by porous block will have a profound effect on the flow and     

MIXED CONVECTION HEAT TRANSFER IN A TEE BRANCH

The heat transfer characteristics of a Newtonian fluid in a two-dimensional, planar, right-angled tee branch are studied over a range of inlet Reynolds numbers and Grashof numbers. The flow and heat transfer equations, subject to the Boussinesq approximation, are solved using a finite-element discretization. The effects of the branch length and the grid size on the interior flow field are examined to assess the accuracy of the solutions. Results are presented for two types of experimentally realizable boundary conditions—equal exit pressure at the outlet of each branch and specified flow split between the branches. The thermal boundary condition of uniform wall temperature is examined. The effect of increasing Reynolds number is to increase the size and strength of the recirculation zones in both the main and side branches, while that of increasing Grashof number is to decrease such an effect. For the case of equal exit pressures there is a significant flow reversal in the side branch and the exit flow rate from the main branch increases linearly with increasing Gr/Re². For the case of specified flow split, an increasing back pressure is required to be maintained at the exit of the main branch to regulate the flow split at the desired level.     

TRANSIENT CONJUGATE NATURAL CONVECTION HEAT TRANSFER ALONG A VERTICAL PLATE FIN IN A HIGH-POROSITY MEDIUM

The non-Darcian effects on transient conjugate natural convection-conduction heat transfer from a two-dimensional vertical plate fin embedded in a high-porosity medium are studied numerically. The coupled nonlinear partial differential equations for transient natural convection in the porous medium and transient heat conduction in the fin are solved numerically with a cubic spline collocation method. Numerical results for the heat transfer characteristics are presented for water and air at selected values of the convection-conduction parameter at different times. It is shown that the inertial effect on heat transfer characteristics is negligible at short times; its effect, however, becomes increasingly important at longer times. The time taken to reach steady state is longer for a fluid with a smaller Prandtl number and for a system with a smaller value of the convection-conduction parameter.     

NATURAL CONVECTION IN A VERTICAL,ASYMMETRICALLY HEATED,PERMEABLE WALLED CHANNEL

Developing natural convection in an asymmetrically heated, open-ended vertical channel was studied both experimentally and numerically. A tightly stretched, perforated, plastic radiation shield was suspended parallel to an electrically heated aluminum plate to form a vertical channel. In order to model the heat transfer and fluid flow in the vertical channel, the unsteady, two-dimensional Navier-Stokes equations were solved using a primitive variable, finite-difference formulation. Flow through the perforated boundary was modeled using a modified form of Darcy's law. Radiative exchange between the boundaries and between the boundaries and the environment was included. The predicted mass flow was within 3% of that measured experimentally. Both the average plate temperature and the bulk exit channel air temperature were within 1·2°C of the measured values. However, the predicted average temperature of the radiation shield was 8°C higher than that measured.     

UNSTEADY FREE CONVECTION IN A VERTICAL CHANNEL WITH A BUILT-IN HEATED SQUARE CYLINDER

The cause of high operating temperatures experienced by grate blocks in a waste-to-energy power plant is investigated. A three-dimensional numerical analysis is performed to assess convection from the grate to the underfire air supply and conduction within the grate. A number of geometric complexities, including variable solid thickness, extended surfaces, sharp curvatures, and narrow passages, are accounted for via a finite element approach. The grate temperature is found to be very sensitive to the flow characteristics beneath the grates. Analysis of a modified grate structure was performed, showing a 14% reduction in maximum grate temperature.     

倾斜平板热面朝上自然对流换热实验研究

对不同倾斜角度的平板在空气中的自然对流换热系数进行了实验测量.采用直接电加热方法对倾斜平板进行加热,在倾斜铜板背面嵌人式布置热电偶测量平板表面平均温度,同时测量电加热时的电压及电流.实验结果表明,随着倾斜表面与空气温差的增加,实验得到的平均努谢尔特数与经典的准则关联式得到的努谢尔特数呈现不同的变化趋势;随着倾斜角度的增加自然对流换热得到了强化,而且当倾斜平板趋于水平或者竖直时实验结果与经典准则关联式的计算结果偏差逐渐增大,最大偏差达到51%.     

TRANSIENT BUOYANCY-INDUCED FLOW THROUGH A HEATED,VERTICAL CHANNEL OF FINITE HEIGHT

A detailed numerical analysis is carried out to study the transient natural convection in a symmetrically heated vertical plate channel of finite height due to a step change in plate temperature. Both the inlet and exit effects are included in the analysis. In particular, a numerical scheme derived from the SIMPLER algorithm is employed to solve the governing differential equations. Emphasis is placed on the temporal evolution of the flow and thermal characteristics. Results are obtained for air with Ra varying from 10³ to 10⁶ and the aspect ratio from 5 to 10. The existence of multiple cells in the channel and in the region surrounding the top end during the transient stage is predicted. Over a certain period, temperature overshoot is noted, causing a dip in the time variation of the Nusselt number. Correlation equations are proposed for the average Nusselt number and the amount of air drawn into the channel at the steady state.     

SIMULATION OF MIXED CONVECTION HEAT TRANSFER AT THE WALL OF A PACKED BED

Numerical investigations of the effect of mixed convection on the fluid flow pattern and heat transfer at the boundary layer of a packed bed are reported. A volume-averaged Navier-Stolces equation incorporating the Boussinesq approximation is used to predict the fluid flow, and a volume-averaged heat balance equation, the heat transfer. An exponential variation in the porosity of the packing is assumed in the region near the wall. Simulations are performed using a modified penalty Galerkin finite-element method. The Nusselt number is found to depend on the Grashof to Reynolds number ratio, Graetz number, and ratio of thermal conductivity of the solid and fluid phases. The heat transfer rate is enhanced by a positive Grashof number and hindered by a negative value. The velocity and temperature profiles in the bed are also found to be strong functions of the Gr/Re² ratio. At higher values of this ratio the flow becomes unsteady     

MIXED CONVECTION FROM A MULTIBLOCK HEATER IN A CHANNEL WITH IMPOSED THERMAL MODULATION

Enhancement of mixed-convection heat transfer in a multiblock heater arrangement in a channel is studied. At the most upstream heated block, a time-periodic heat generation is present, while the heat generation is constant in other heater blocks. The explicit effect of using thermal modulation (time-periodic heat generation) at the upstream heater is examined by acquiring comprehensive numerical solutions. The heat transfer enhancement is pronounced at the heaters at far downstream, and the augmentation is maximized when resonance is realized. The resonance frequency is close to the natural frequency of the system, which scales with the time for the main stream to travel through the interblock region. Plots are illustrated to demonstrate the formations of a pair of circulations downstream of the most upstream heater, which leads to identifying the natural frequency. The increase in the overall pressure drop is calculated. The benefit of heat transfer augmentation, as opposed to the increased pressure drop, is assessed to justify the use of thermal modulation in the upstream heater.     

CONJUGATE NATURAL CONVECTION HEAT TRANSFER IN A VERTICAL ANNULUS WITH INTERNAL CIRCUMFERENTIAL FINS

A numerical investigation is carried out to study natural convection heat transfer in a vertical annular enclosure with circumferential fins mounted on the inner cylinder. Heat is generated within the inner solid cylinder, while the top, the bottom, and the outer walls are exposed to convection. The results show that, even though the presence of fins does not alter the main features of the flow in the large-scale recirculation zone, it reduces the mean temperature of the inner cylinder by a maximum of 9.6% at low Rayleigh numbers. Also, the number and length of fins have a pronounced effect on the mean temperature.     

EFFECTS OF ASPECT RATIO ON VORTEX FLOW PATTERNS IN MIXED CONVECTION OF AIR THROUGH A BOTTOM-HEATED HORIZONTAL RECTANGULAR DUCT

Abstract

Buoyancy-induced vortex flow structures and the associated heat transfer were numerically investigated in a mixed convective airflow in a bottom-heated horizontal rectangular duct of different aspect ratios. The unsteady three-dimensional Navier-Stokes and energy equations were directly solved by a higher order upwind finite difference scheme. Results were presented in particular for Reynolds numbers ranging from 5 to 15, Rayleigh numbers up to 9000, and aspect ratios from 4 to 12. The predicted results clearly show significant differences in vortex structures induced in ducts with small and large aspect ratios. For an aspect ratio less than 6 the transverse vortex rolls are periodically generated in the duct entry and gradually transform into longitudinal rolls when moving downstream. The resulting vortex flow eventually evolves to a time periodic state with the upstream and downstream portions of the duct dominated by the transverse rolls and longitudinal rolls, respectively. For a large aspect ratio (A > 9) the transverse rolls prevail in the duct core, with two to three longitudinal rolls existing near each sidewall. The flow oscillation in the region dominated by the transverse rolls is much higher than that dominated by the longitudinal rolls. At high Ra the flow becomes chaotic in time, and the duct is filled with unstable irregular vortex rolls.