3D Numerical study on the flow topology and heat transfer characteristics of turbulent forced convection in spirally corrugated tube

ABSTRACT

This paper presents a numerical analysis on flow configurations and heat transfer characteristics of turbulent forced convection in spirally corrugated tubes. The influences of corrugation depth (DR = 0.02–0.16), pitch ratio (PR = 0.10–1.00), and Reynolds number (Re = 5,000–20,000) on flow structure and heat transfer characteristics are described. Comparisons between the full length and periodic domains are also reported. The results show that spirally corrugated tubes induced vortex flows which helped to increase heat transfer due to enhanced fluid mixing. The maximum thermal enhancement factor of 1.16 was obtained by using the spirally corrugated tube with DR = 0.06, PR = 0.25 at Re = 5,000.     

Computations of Newtonian fluid flow around a square cylinder near an adiabatic wall at low and intermediate Reynolds numbers: Effects of cross-buoyancy mixed convection

ABSTRACT

The effects of cross-buoyancy mixed convection from a square cylinder in the proximity of a plane wall are studied for Reynolds number (Re) = 1–100, Richardson number (Ri) = 0–2, and gap ratio (G) = 0.25–1 at Prandtl number (Pr) = 0.7. The flow observed is steady for G = 0.25 and 0.5. The transition from a steady to a time-periodic system is observed for G = 1, and it is found at Re = 56, 60, and 74 for Ri = 0, 1, and 2, respectively. With increasing G and/or Ri, the drag coefficient and average Nusselt number increase for all Re values studied and the lift coefficient decreases with increasing Ri except at Re = 1. Maximum heat transfer augmentation is found about 89% at G = 0.5 (Re = 20, Pr = 0.7, Ri = 0) with respect to the corresponding value at G = 0.25 (Re = 20, Pr = 0.7, Ri = 0). Lastly, the correlations of drag coefficient and heat transfer have been obtained.     

Unsteady Mixed Convection in a Vented Enclosure Partially Filled with Two Non-Darcian Porous Layers

Mixed convection in a ventilated rectangular cavity with a horizontal strip occupied by two media of different permeability is studied by using the finite-volume method. The effect of the parameters that govern the problem, the Reynolds number (200–500), the Richardson number (0.1–10), the Darcy number (1.E–6–1.E–2), permeability ratio (0.01–100), aspect ratio of the cavity (1–1.5), and direction of flow with or against gravity, is investigated. From the numerical results, the influence of Darcy and Reynolds numbers on the fluid mechanics can be inferred. When the cold fluid flow is descending, the fluid–porous medium heat transfer increases because the hotter fluid is located in the upper zones of the cavity. As the aspect ratio (height/width) increases from 0.67 to 1.5, both the Nusselt number (7.5 < Nu < 27) and the friction coefficient increase. The friction coefficient on the walls is greater when Re = 500 and Ri = 10 in the two directions of flow. As the Darcy number increases, the velocity gradients increase near the walls, causing an increase of the friction coefficient (0 < Cf Re < 47).     

Turbulent heat transfer enhancement in a heat exchanger using helically corrugated tube

The augmentation of convective heat transfer in a single-phase turbulent flow by using helically corrugated tubes has been experimentally investigated. Effects of pitch-to-diameter ratio (P/D_H = 0.18, 0.22 and 0.27) and rib-height to diameter ratio (e/D_H = 0.02, 0.04 and 0.06) of helically corrugated tubes on the heat transfer enhancement, isothermal friction and thermal performance factor in a concentric tube heat exchanger are examined. The experiments were conducted over a wide range of turbulent fluid flow of Reynolds number from 5500 to 60,000 by employing water as the test fluid. Experimental results show that the heat transfer and thermal performance of the corrugated tube are considerably increased compared to those of the smooth tube. The mean increase in heat transfer rate is between 123% and 232% at the test range, depending on the rib height/pitch ratios and Reynolds number while the maximum thermal performance is found to be about 2.3 for using the corrugated tube with P/D_H = 0.27 and e/D_H = 0.06 at low Reynolds number. Also, the pressure loss result reveals that the average friction factor of the corrugated tube is in a range between 1.46 and 1.93 times over the smooth tube. In addition, correlations of the Nusselt number, friction factor and thermal performance factor in terms of pitch ratio (P/D_H), rib-height ratio (e/D_H), Reynolds number (Re), and Prandtl number (Pr) for the corrugated tube are determined, based on the curve fitting of the experimental data.     

NUMERICAL SIMULATION OF THREE-DIMENSIONAL SEPARATED FLOW AND HEAT TRANSFER AROUND STAGGERED SURFACE-MOUNTED RECTANGULAR BLOCKS IN A CHANNEL

ABSTRACT

Numerical results simulating a three-dimensional laminar separated flow and heat transfer around staggered surface-mounted rectangular blocks in a plane channel are presented. Treated in the present study is a case of staggered three-row blocks. The finite-difference method is employed to solve the Navier-Stokes and energy Equations directly, and the resulting finite-difference Equations are solved with the SMAC method for Re = 100–500 and Pr = 0.7. The present numerical results are found to simulate well the visualization results such as horseshoe vortices and recirculating flow. The heat transfer coefficient greatly varies on the different side surfaces of blocks and also with Reynolds number.     

CONJUGATE COMPUTATION OF TRANSIENT FLOW AND HEAT AND MASS TRANSFER BETWEEN HUMID AIR AND DESICCANT PLATES AND CHANNELS

This is a numerical study of dehumidification of humid air in laminar and turbulent flows (333 ≤ Re ≤ 6,000) over desiccant (silica gel)-lined finite flat plates and in channels. The problem is treated as conjugate flow, heat, and mass transfer, and solved by using a finite control-volume method. The effects of the plate thickness (3 ≤ b ≤ 7 mm), the Reynolds number (333 ≤ Re ≤ 3,333), and the turbulence intensity (1 ≤ TI ≤ 10%) on the dehumidification process are investigated. The results show that increasing the desiccant plate thickness decreases the heat and mass transfer coefficients by 25% and 22% at t = 10 s and x = 0.11 m, respectively, in comparison to a thin plate. Mass transport rates increase with Re, e.g., at t = 20 s, W_ave increases by 50% as Re is increased 10-fold from 333 to 3,333. Turbulent flow in channel desiccants increases the rate of dehumidification, e.g., an increase in Re from 600 (laminar) to 6,000 (turbulent) results in an increase in W_ave by 22% at t = 20 s. Also, increasing the turbulence intensity from 1% to 10% increase W_ave by 7%.     

Opposing buoyancy characteristics of Newtonian fluid flow around a confined square cylinder at low and moderate Reynolds numbers

ABSTRACT

The influence of opposing-buoyancy mixed convection from a square cylinder in a vertical channel has been studied at Reynolds numbers (Re) = 1–100, Richardson numbers (Ri) = 0 to ?1, and blockage ratios (β) = 10–50% for air as a working fluid. The onset of a steady to a time-periodic regime is found for Ri = 0 (at Re = 35, 65, 74, and 62), Ri = ?0.5 (at Re = 12, 39, 48, and 54), and Ri = ?1 (at Re = 9, 30, 39, and 50) for β = 10%, 25%, 30%, and 50%, respectively. The initiation of flow separation is also determined. Finally, the correlations of Strouhal number, drag coefficient, and the Colburn heat transfer factor were obtained.     

Heat Transfer and Flow Characteristics in Rib-/Deflector-Roughened Cooling Channels with Various Configuration Parameters

The present paper deals with a detailed numerical investigation of the turbulent flow inside a stationary rib-/deflector–roughened cooling channel. Various downstream-shaped deflectors including sloping-board deflectors [Cases A1, A2], guide-shaped deflectors [Cases B1, B2], and drop-shaped deflectors [Cases C1, C2] and configuration parameters such as channel aspect ratio (AR = 0.5, 1 and 2), and rib-pitch-to-rib-height ratio (P/e = 5, 8, and 10) are investigated. The main objective is to design an appropriate deflector to improve the flow characteristics in the wake of the deflectors and guide the flow between two neighboring rib turbulators to enhance the heat transfer performance. A quasi-three-dimensional flow structure, supported by the stream tracer field in some planes, is established to improve and deepen the understanding as well as the analysis of the complex flow field in the rib-/deflector–roughened channels. In addition, the thermal performance corresponding to various rib-pitch-to-rib-height ratios and aspect ratios emphasizes the role of the configuration parameters in the heat transfer and flow resistance performance. The results demonstrate that the deflectors trip the boundary layer and blend the fluid flow, and that the sloping board deflectors contribute to enlarging the turbulence level of the whole cooling channel, more than in the wake region. It is found that Cases A1 and A2 provide the best heat transfer performance, while Case C1 presents the largest thermal enhancement factor Nu/Nu₀/(f/f₀)^1/3 at high Reynolds number. The wide-aspect-ratio channel with deflectors and large pitch-to–height ratio ribs exhibits much better heat transfer performance.     

Numerical Simulation of Double Diffusive Mixed Convection in a Lid-Driven Square Cavity Using Velocity-Vorticity Formulation

In this article, convection driven by combined thermal and solutal concentration buoyancy effects in a lid-driven square cavity is examined using velocity-vorticity form of Navier-Stokes equations. The governing equations consist of vorticity transport equation, velocity Poisson equations, energy equation, and concentration equation. Validation results are discussed for convection due to heat and mass transfer in a lid-driven square cavity at Re = 500, Le = 2, and G_RT  = G_RS  = 100. These results indicate that the present velocity-vorticity formulation could predict the characteristic parameters of flow, temperature, and solutal concentration fields using a much coarser mesh compared to the mesh used in a stream function-vorticity formulation. The capability of the proposed algorithm to handle complex geometry is demonstrated by application to mixed convection in a lid-driven square cavity with a square blockage. The effect of buoyancy ratio on the convection phenomenon is discussed for buoyancy ratio varying from ? 100 to 100 at Re = 100. Under opposing temperature and concentration gradients along the vertical direction, the negative buoyancy ratios give rise to aiding flows.     

Effect of Channel-Confinement and Rotation on the Two-Dimensional Laminar Flow and Heat Transfer across a Cylinder

Effect of channel-confinement and rotation on the flow and heat-transfer across a cylinder is studied for various blockage ratio (β = 0–50%), nondimensional rotational velocity (α = 0–2), and Reynolds number (Re = 35–170). The cylinder is maintained at a constant wall temperature with air as the working fluid. Criss-cross motion of the shed-vortices is noticed for the channel-confined flow across a rotating cylinder at intermediate blockage ratio. The effect of channel-confinement (rotation) is an enhancement (reduction) in the drag force and heat transfer. A downward lift force is generated under the influence of counterclockwise rotation, which increases with increasing blockage ratio. Rotation and channel-confinement have a stabilizing effect and can be used for flow control.     

The Effect of a Blockage on Forced Convection Heat Transfer From a Heated Square Cylinder to Power-Law Fluids

Forced convection heat transfer characteristics of a long, heated square cylinder blocking the flow of a power-law fluid in a channel is numerically investigated in this study. In particular, the role of the power-law index n, Reynolds number Re, Prandtl number Pr, and blockage ratio β(=B/H) on the rate of heat transfer from a square cylinder in a channel has been studied over the following ranges of conditions: 0.5 ≤ n ≤ 1.8, 60 ≤ Re ≤ 160, β = 1/4, 1/2, and 0.7 ≤ Pr ≤ 50. A semi-explicit finite-volume method is used on a nonuniform collocated grid arrangement. The third-order QUICK and the second-order central difference schemes are used to discretize the convective and diffusive terms, respectively, in the momentum and energy equations. Irrespective of the type of behavior of fluid (different values of n), the average Nusselt number increases as the blockage ratio increases. Similar to the unconfined flow configuration, the average Nusselt number increases monotonically with Reynolds and Prandtl numbers for both values of the blockage ratio and for all values of power-law index considered here. Further insights into the heat transfer phenomenon are provided by presenting isotherm contours in the vicinity of the cylinder for a range of values of the Reynolds number, Prandtl number, and power-law index for the two values of β considered in this work.     

Thermal behavior in solar air heater channel fitted with combined rib and delta-winglet

Effects of combined ribs and delta-winglet type vortex generators (DWs) on forced convection heat transfer and friction loss behaviors for turbulent airflow through a solar air heater channel are experimentally investigated in the present work. Measurements are carried out in the rectangular channel of aspect ratio, AR = 10 and height, H = 30 mm. The flow rate is presented in the form of Reynolds numbers based on the inlet hydraulic diameter of the channel ranging from 5000 to 22,000. The cross-section shape of the rib placed on the absorber plate to create a reverse-flow is an isosceles triangle with a single rib height, e/H = 0.2 and rib pitch, P_l/H = 1.33. Ten pairs of the DW with its height, b/H = 0.4; transverse pitch, P_t/H = 1 and three attack angles (α) of 60°, 45° and 30° are introduced and mounted on the lower plate entrance of the tested channel to generate longitudinal vortex flows. The experimental results show that the Nusselt number and friction factor values for combined rib and DW are found to be much higher than those for the rib/DW alone. The larger attack angle of the DW leads to higher heat transfer and friction loss than the lower one. In common with the rib, the DW pointing upstream (PU-DW) is found to give higher heat transfer rate and friction loss than the DW pointing downstream (PD-DW) at a similar operating condition. In comparison, the largest attack angle (α = 60°) of the PU-DW yields the highest increase in both the Nusselt number and friction factor while the lowest attack angle of the PD-DW provides the best thermal performance.     

Heat transfer and pressure drop in a channel with multiple 60° V-baffles

The research work has been conducted to assess turbulent forced convection heat transfer and friction loss behaviors for airflow through a channel fitted with a multiple 60° V-baffle turbulator. Measurements have been carried out for the channel of aspect ratio, AR = 10 and height, H = 30 mm with three different baffle blockage ratios, (e/H = 0.10, 0.20 and 0.30) and three baffle pitch spacing ratios, (PR = P / H = 1, 2 and 3) while the transverse pitch of the V-baffle is set to 2H and kept constant. The air flow rate is in terms of Reynolds numbers based on the inlet hydraulic diameter of the test channel ranging from 5000 to 25,000. The experimental results show that the V-baffle provides the drastic increase in Nusselt number, friction factor and thermal enhancement factor values over the smooth wall channel due to better flow mixing from the formation of secondary flows induced by vortex flows generated by the V-baffle. In addition, substantial increases in Nusselt number and friction factor values are found for the rise in blockage ratio and/or for the decrease in pitch ratio values. Assessing thermal performance of the V-baffled channel, the use of the V-baffle with PR = 1 and e/H = 0.10 leads to maximum thermal enhancement factor of about 1.87 at lower Reynolds number.     

Momentum and Heat Transfer Phenomena for Power–Law Liquids in Assemblages of Solid Spheres of Moderate to Large Void Fractions

This work extends our previously reported results for the flow of and heat transfer from expanded beds of solid spheres to power–law fluids by using a modified and more accurate numerical solution procedure. Extensive results have been obtained to elucidate the effects of the Reynolds number (Re), the Prandtl number (Pr), the power–law index (n), and the bed voidage (ε) on the flow and heat transfer behavior of assemblages of solid spheres in the range of parameters: 1 ≤ Re ≤ 200, 1 ≤ Pr ≤ 1000, 0.6 ≤n ≤ 1.6, and 0.7 ≤ε ≤ 0.999999. The large values of bed voidage are included here to examine the behavior in the limit of an isolated sphere. As compared to Newtonian fluids, for fixed values of the Reynolds number and the voidage, the total drag coefficient decreases and the average Nusselt number increases for shear thinning fluids (n < 1); whereas, for shear thickening fluids (n > 1), the opposite behavior is observed. The drag results corresponding to bed voidage, ε = 0.99999, are very close to that of a single sphere; whereas, the heat transfer results approach this limit at ε = 0.999. Based on the present numerical results, simple correlations for drag coefficient and average Nusselt number are proposed which can be used to calculate the pressure drop for the flow of a power–law fluid through a bed of particles, or rate of sedimentation in hindered settling and the rate of heat transfer in assemblages of solid spheres in a new application. Broadly speaking, all else being equal, shear-thinning behavior promotes heat transfer, whereas shear-thickening behavior impedes it.     

Numerical Study on Mixed Convection Heat Transfer in Inclined Triangular Cavities

This article presents a parametric study on flow behavior and heat transfer in an inclined triangular cavity subjected to a moving lid and temperature differential. The systematic study considers three physical parameters (inclination angle, Reynolds number, and Grashof number) and explores the influence of these parameters on flow pattern and heat transfer characteristics. A series of computations were performed for the inclination angle (θ) ranging from 0° to 360° (in increments of 45°), Reynolds number (Re) from 100 to 1,500, and Grashof number (Gr) from 10⁵ to 10⁷. The numerical results show that there are three kinds of flow regime in a triangular cavity inclined from 0° to 360°: buoyancy-dominant, inertia-dominant, and intermediate transition (mixed convection flow). It is interesting that the case with Re = 100, Gr = 10⁷, and θ = 0° exhibits five circulation cells and induces excellent thermal performance, corresponding to wavy profiles in local Nusselt number and local friction factor. The study also reveals that the good thermal performance within a local region can generate higher friction force on the neighboring boundary and this friction force may reduce the strength of the vortex.     

Heat Transfer in a Lid-Driven Enclosure Filled with Water-Saturated Aluminum Foams

Mixed convection in a lid-driven square enclosure filled with water-saturated aluminum foams is investigated numerically. The driving forces of fluid flow in such a system include the buoyancy force due to temperature gradient and the shear force due to lid movement, while the interaction of these forces results in various heat transfer modes. This work uses the Brinkman-Forchheimer model for fluid flow and the two-equation model for heat transfer. The top moving wall and the bottom heated wall are maintained at different constant temperatures, while the other walls are thermally insulated. The relevant parameters are the porosity of aluminum foams (ε = 0.91, 0.97), the Grashof number (Gr = 10⁴–3 × 10⁶) and the Reynolds number (Re = 10^?2–10⁴). The fluid flow and heat transfer characteristics of the present porous system are identified. Parametric study indicates that the average Nusselt number (Nu) generally increases with Gr and Re. The higher porosity promotes much more enhancement of convective heat transfer, but the lower porosity is desired for higher total heat transfer due to the higher value of effective thermal conductivity. Finally, the Nu correlation is established based on the numerical results.     

Numerical investigation on heat transfer of supercritical water in a roughened tube

ABSTRACT

The turbulent mixed convection heat transfer of supercritical water flowing in a vertical tube roughened by V-shaped grooves has been numerically investigated in this paper. The turbulent supercritical water flow characteristics within different grooves are obtained using a validated low-Reynolds number κ-ε turbulence model. The effects of groove angle, groove depth, groove pitch-to-depth ratio, and thermophysical properties on turbulent flow and heat transfer of supercritical water are discussed. The results show that a groove angle γ = 120° presents the best heat transfer performance among the three groove angles. The lower groove depth and higher groove pitch-to-depth ratio suppress the enhancement of heat transfer. Heat transfer performance is significantly decreased due to the strong buoyancy force at T_b = 650.6 K, and heat transfer deterioration occurs in the roughened tube with γ = 120°, e = 0.5 mm, and p/e = 8 in the present simulation. The results also show that the rapid variation in the supercritical water property in the region near the pseudo-critical temperature results in a significant enhancement of heat transfer performance.     

Influence of Tube Arrangement on the Thermal Hydraulic Performance of a Membrane Helical-Coil Heat Exchanger

Thermalhydraulic performances of membrane helical-coil heat exchangers in in-line and staggered arrangements, are numerically investigated. The influences of Reynolds number, dimensionless pitch, and arrangement on heat transfer and flow are discussed. Moreover, the axial and tangential velocity distributions were presented for various pitches and arrangements. The results show that the membrane helical-coil arrangement has a profound effect on the thermalhydraulic performance. For the smaller radial pitch (s ₁/d < 1.8), the heat transfer coefficient in in-line arrangement is higher than that in staggered arrangement. Meanwhile, the in-line arrangement gives a significantly higher friction factor than the staggered arrangement. However, for the larger radial pitch (s ₁/d ≥ 1.8), the coil arrangement appears to have no effect on the heat transfer and friction factor. In addition, the average Nusselt number Nu and the friction factor f for in-line and staggered arrangements were calculated and correlated against the Reynolds number and structural parameters. According to the thermalhydraulic performance evaluation criterion, the staggered arrangement is recommended as the optimal coil arrangement.     

A Numerical Study of Fluid Flow and Heat Transfer around a Square Cylinder at Incidence using Unstructured Grids

A numerical investigation of flow and heat transfer around a square cylinder at incidence (α = 0° ? 45°) is presented for a range of Reynolds numbers ( Re = 60 ? 150). A finite-volume code suitable for unstructured grids has been developed to simulate the flow. The unstructured grid has been generated using the Delaunay triangulation algorithm. A modified pressure-velocity correction scheme with semi-explicit time-stepping is implemented to solve the Navier-Stokes equations. Collocated grid arrangement has been used for the dependent variables. Convective terms have been discretized using a second order upwind least squares scheme. The formation of Karman vortex street has been captured and the Strouhal number associated with the wake has been determined. The dependence of Strouhal number, force coefficients (drag and lift), moment coefficient and average Nusselt number on Reynolds number, and angle of incidence for a fixed blockage ratio has been reported and analyzed.     

Free Liquid Jet Impingement from a Slot Nozzle to a Curved Plate

This article explores the heat transfer characteristics of a free liquid jet discharging from a slot nozzle and impinging vertically on a curved cylindrical shaped plate of finite thickness. Computations were done for Re = 500–1800, β = 0.75–3, R _i /d _n  = 4.16–16.66, b/d _n  = 0.08–1.5, and d _n  = 0.3–2.4 mm. Results are presented for dimensionless solid–fluid interface temperature, dimensionless maximum temperature in the solid, and local and average Nusselt numbers. The local Nusselt number increases with Reynolds number. Decreasing the nozzle width increases the local heat transfer coefficient. Decreasing the nozzle to target spacing or plate thickness or plate inner radius of curvature all enhances the Nusselt number.