Heat transfer enhancement in a tube using circular cross sectional rings separated from wall

A numerical study was undertaken for investigating the heat transfer enhancement in a tube with the circular cross sectional rings. The rings were inserted near the tube wall. Five different spacings between the rings were considered as p = d/2, p = d, p = 3d/2, p = 2d and p = 3d. Uniform heat flux was applied to the external surface of the tube and air was selected as working fluid. Numerical calculations were performed with FLUENT 6.1.22 code, in the range of Reynolds number 4475–43725. The results obtained from a smooth tube were compared with those from the studies in literature in order to validate the numerical method. Consequently, the variation of Nusselt number, friction factor and overall enhancement ratios for the tube with rings were presented and the best overall enhancement of 18% was achieved for Re = 15,600 for which the spacing between the rings is 3d.     

Mass-transfer to a channel wall downstream of a cylinder

The electrochemical method is used to measure the mass-transfer to a channel wall downstream of a cylinder. For Reynolds numbers based on cylinder diameter Re > 50, the flow is unsteady, and the mass-transfer rate is a function of time. When 50 < Re < 200, the mass-transfer rate is periodic with a frequency in the range of 1–3 Hz. When the ratio of cylinder diameter d to channel height h is 0.25, the Strouhal number is measured to be 0.27±0.02, and when d/h = 0.51, the Strouhal number is 0.49±0.01. The average mass-transfer rate at various positions downstream of the cylinder is reported. Experiments are compared to two-dimensional numerical simulations. The simulated and experimental variations of Nusselt number with position and Re contain similar features, but exact agreement is not found.     

Heat transfer in inclined rectangular receivers for concentrated solar radiation

We study heat transfer in inclined rectangular cavities, which may be used as receivers of concentrated solar radiation. One of the active walls is subject to concentrated solar radiation and the other is kept at constant temperature. Continuity, momentum and energy equations are solved by finite difference — control volume numerical method. The relevant governing parameters are: the Rayleigh numbers from 10³ to10¹², the cavity aspect ratio, A = L/H from 0.5 to 2, the inclination angle, from 30 to 90°.We found that the Nusselt number is an increasing function of the Rayleigh number, the aspect ratio and the inclination angle. Based on the computed data a correlation is derived in the form of Nu = f(Ra, A, ).     

Experimental study of mixed convection and pressure drop in helically dimpled tubes for laminar and transition flow

This paper presents the experimental results carried out in dimpled tubes for laminar and transition flows and completes a previous work of the authors focused on the turbulent region. It was observed that laminar flow heat transfer through horizontal dimpled tubes is produced in mixed convection, where Nusselt number depends on both the natural convection and the entry region. Employing water and ethylene glycol as test fluids, the following flow range was covered: x^*=10⁻⁴–10⁻² and Ra=10⁶–10⁸.

The experimental results of isothermal pressure drop for laminar flow showed dimpled tube friction factors between 10% and 30% higher than the smooth tube ones. Moreover, it was perceived that roughness accelerates transition to critical Reynolds numbers down to 1400. Correlations for the laminar friction factor f=f(Re,h/d) and for the critical Reynolds Re_crit=Re_crit(h/d) are proposed. The hydraulic behaviour of dimpled tubes was found to depend mainly on dimple height.

In mixed convection, high temperature differences in the cross section were measured and therefore heat transfer was evaluated by a circumferentially averaged Nusselt number. Experimenal correlations for the local and the fully developed Nusselt numbers and are given. Results showed that at low Rayleigh numbers, heat transfer is similar to the smooth tube one whereas at high Rayleigh, enhancement produced by dimpled tubes can be up to 30%.     

Effect of jet-jet spacing on convective heat transfer to confined, impinging arrays of axisymmetric air jets

The effects of jet-jet spacing (X_n/D), low nozzle-plate spacings (H/D = 0.25, 1.0 and 6.0) and spent air exits located between the jet orifices were studied on the magnitude and uniformity of the convective heat transfer coefficients for confined 3 × 3 square arrays of isothermal axisymmetric air jets impinging normally to a heated surface. Local and average Nusselt numbers are presented for Reynolds number range of 3500–20 400. The local Nusselt numbers illustrate the (non)uniformity of the heat transfer and aid in understanding the variations in the average Nusselt number. The jet-jet spacing affects the convective coefficient by varying the influence of the adjacent jet interference and fraction of the impingement surface covered by the wall jet. The addition of spent air exits increased the convective coefficient and influenced the location of the optimum separation distance. In addition, significant enhancement of the uniformity and the convective coefficients was observed at H/D = 0.25 and 1.0 when compared to H/D = 6.0.     

Analysis of mixed convection in a vertical porous layer using non-equilibrium model

The problem of two-dimensional steady mixed convection in a vertical porous layer is investigated numerically in the present paper using the thermally non-equilibrium model. The vertical porous layer is assumed to have a finite isothermally heated segment on one vertical wall which is otherwise adiabatic and the other vertical wall is cooled to a constant temperature. Non-dimensionalization of the governing equations results in four parameters for both aiding and opposing flows: (1) Ra, Rayleigh number (2) Pe, Péclet number (3) K_r, thermal conductivity ratio parameter, and (4) H, heat transfer coefficient parameter. The numerical results are presented for 0.01  H  100, 0.01  K_r  100, 0.01  Pe  100 and Ra = 10, 50 and 100. The results show that, the thermal equilibrium model cannot predict the average Nusselt number correctly for small values of H × K_r. In both the aiding and opposing flows, the total average Nusselt number is decreasing with increasing the heat transfer coefficient parameter at low values of Pe, while for high values of Pe, higher H will enhance the total heat transfer rate. Increasing the thermal conductivity ratio leads to increase in the total average Nusselt number. It is found also that the total average Nusselt number depends strongly on the thermal conductivity ratio parameter and depends slightly on the heat transfer coefficient parameter.     

Entropy generation due to conjugate natural convection in enclosures bounded by vertical solid walls with different thicknesses

Entropy generation due to conjugate natural convection heat transfer and fluid flow has been studied inside an enclosure with bounded by two solid massive walls from vertical sides at different thicknesses. Enclosure is differentially heated from vertical walls and horizontal walls are adiabatic. Governing equations which are written in streamfunction-vorticity form solved by finite difference technique for the governing parameters as Rayleigh number, 10³ ≤ Ra ≤ 10⁶, length ratio of solid walls as ₁ (for left vertical wall) and ₂ (for right vertical wall) and thermal conductivity ratio of solid to fluid (k), 1 ≤ k ≤ 10. Entropy generation contours due to fluid friction and heat transfer irreversibility, isotherms, streamlines, Nusselt numbers and velocity profiles were obtained. It is found that entropy generation increases with increasing of thermal conductivity ratio and thicknesses of the walls. Entropy generation due to heat transfer is more significant than that of fluid flow irreversibility for all values of thickness of the solid vertical walls.     

Three-dimensional driven-cavity flows with a vertical temperature gradient

Numerical studies of three-dimensional flows in a cubical container with a stable vertical temperature stratification are carried out. Flows are driven by the top lid, which slides in its own plane at a constant speed. The top wall is maintained at a higher temperature than the bottom wall. The end walls and the side walls are thermally insulated. Numerical solutions are obtained over a wide range of physical parameters, i.e. 10² ≤ Re ≤ 2 × 10³, 0 ≤ Ri ≤ 10.0 and Pr = 0.71, where the mixed-convection parameter Ri Gr. Re⁻². Systematical comparison of the three-dimensional numerical solutions with the previously reported two-dimensional results illuminates the impact of thermal stratification on the three-dimensional flow characteristics. When Ri < O(1), the effect of the vertical temperature gradient is minor, and the flow structures are similar to those of the non-stratified fluid flows in a conventional lid-driven cavity flow. Fluids in the primary vortex are well mixed, and the temperature is fairly uniform in the main circulating region. When Ri ≥ O(1), the stable temperature distribution tends to suppress the vertical fluid motion. Much of the fluid motion takes place in the vicinity of the top sliding lid and the bulk of the cavity region is nearly stagnant. When Ri > O(1), the fluid motion exhibits vertically layered vortex structures. The Nusselt number is computed at the top and bottom wall, and this also illustrates the varying flow characteristics as Ri encompasses a broad range. Extensive numerical flow visualizations are conducted. Plots demonstrating the primary flows in the (x−y) plane and the secondary flows in the (y−z) plane are presented. These display the influences of Re and Ri on the basic character of the flow and the three-dimensional effects.     

Cooling strategy by mixed convection of a discrete heater at its optimum position in a square cavity with ventilation ports

We studied cooling strategy in a square enclosure with ventilation ports and a discrete heat source at its optimum position. We searched the optimum heater position by maximizing the global conductance at different Rayleigh and Reynolds numbers and considered three different ventilation ports arrangements. We solved the conservation equations of mass, momentum and energy for mixed convection. We found that the heater position is at off center in all cases, its optimum position is insensitive to the variation of Ra and Re; it solely depends on the ventilation ports arrangement. The Nusselt number is dependent on Ri = Ra/Re²: at its low values, Nu is a decreasing function of Ri and at its high values, it is an increasing function of it.     

Computation of flow and heat transfer in rotating two-pass rectangular channels (AR = 1:1, 1:2, and 1:4) with smooth walls by a Reynolds stress turbulence model

Numerical predictions of three-dimensional flow and heat transfer are presented for rotating two-pass smooth channels with three aspect ratios (AR = 1:1; 1:2; 1:4). Detailed predictions of mean velocity, mean temperature and Nusselt number for two Reynolds numbers (Re = 10,000 and 100,000) were carried out. A total of fifteen calculations have been performed with various combinations of rotation number, Reynolds number, and coolant-to-wall density ratio. The rotation number and inlet coolant-to-wall density ratio varied from 0.0 to 0.28 and from 0.13 to 0.40, respectively. The focus of this study is to investigate the effect of the channel aspect ratio, the Reynolds number, and the coolant-to-wall density ratio on the nature of the flow and heat transfer. A multi-block Reynolds-averaged Navier–Stokes (RANS) method was employed in conjunction with a near-wall second-moment turbulence closure. In the present method, the convective transport equations for momentum, energy, and turbulence quantities are solved in curvilinear, body-fitted coordinates using the finite-analytic method.     

Local heat transfer and recovery factor with impinging free-surface circular jets of transformer oil

Measurements were made to investigate the local behavior of the recovery factor and the heat transfer coefficient with free-surface circular jets. The experiments were performed with transformer oil jets impinging on a vertical constant-heat-flux surface from small pipe and orifice nozzles of 1 mm diameter in the ranges of Re = 183–2600 and Pr = 82–337. Large values of recovery factor over 20 were recorded with medium jet velocity about 20 m s⁻¹. Radial distribution of the recovery factor was determined and expressed in empirical equations. The heat transfer coefficient at stagnation point was found to be nearly independent of nozzle-to-plate spacing, but proportional to the square root of the jet Reynolds number. Profiles of local heat transfer coefficients were obtained and correlated. Based on the local measurements, integral average heat transfer coefficients were obtained and correlated.     

The Numerical Analysis of Oscillating Rectangular Impinging Jets

In this study, the flow and heat transfer characteristics of oscillating air jets impinging on a flat surface were numerically analyzed. The jet velocity oscillated sinusoidally in time. A computer program, based on the control volume method and SIMPLE algorithm, was developed to numerically analyze the problem. Numerical simulations were performed to investigate the effects of the Reynolds number, amplitude, and frequency of the jet oscillation on the flow and heat transfer. It was observed that when the jet is oscillated, the Nusselt number moderately increases compared to the Nusselt number of steady jets.     

Technical Note Transient behavior of vertical buoyancy layer in a stratified fluid

horizontal length scale of the vertical channelg acceleration of gravityRa Rayleigh number [ ≡ gβT_oD⁴ν]T temperaturew velocity component in the z-directionx horizontal coordinatez vertical coordinate.Greek symbols coefficient of thermometric expansionδ thermal perturbation thermal diffusivityν kinematic viscosityσ Prandtl number [ ≡ νχ].     

Numerical and experimental analysis of unsteady heat transfer with periodic variation of inlet temperature in circular ducts

This work focusses on a numerical and experimental analysis of unsteady forced convection in hydrodynamically developed and thermally developing laminar air flow in a circular duct, subjected to a periodic variation of the inlet temperature. The experiments were conducted over a wide range of Reynolds number (281.2 ≤ Re ≤ 1024.3) and inlet frequency (0.01 ≤ β ≤ 0.20 Hz) of the periodic heat input. In the numerical study, the non-uniform inlet temperature amplitude profile derived from the experiments, was included in the numerical model. A fully explicit, second-order accurate finite difference scheme was developed and used for the solution of the unsteady energy equation. Numerical results are obtained with the fully developed parabolic velocity profile under the boundary condition of the first kind, which was verified by the experiments. Temperature variations along the centerline of the circular duct are observed to be thermal oscillations with the same frequency as the inlet periodic heat input and amplitudes that decayed exponentially with distance along the duct. Thermal response along the wall exhibits negligible amplitude variation with changes in Reynolds number and inlet frequency. The variation in the periods and amplitudes of the thermal oscillations are observed to be a function of spacial system variables only. Satisfactory agreement between the numerical and experimental results are obtained.     

A numerical study of the unsteady flow and heat transfer in a transitional confined slot jet impinging on an isothermal surface

A numerical finite-difference approach was used to compute the steady and unsteady flow and heat transfer due to a confined two-dimensional slot jet impinging on an isothermal plate. The jet Reynolds number was varied from Re=250 to 750 for a Prandtl number of 0.7 and a fixed jet-to-plate spacing of H/W=5. The flow was found to become unsteady at a Reynolds number between 585 and 610. In the steady regime, the stagnation Nusselt number increased monotonically with Reynolds number, and the distribution of heat transfer in the wall jet region was influenced by flow separation caused by re-entrainment of the spent flow back into the jet. At a supercritical Reynolds number of 750 the flow was unsteady and the net effect in the time mean was that the area-averaged heat transfer coefficient was higher compared to what it would have been in the absence of jet unsteady effects. The unsteady jet exhibited a dominant frequency that corresponded to the formation of shear layer vortices at the jet exit. Asymmetry in the formation of the vortex sheets caused deformation or buckling of the jet that induced a low-frequency lateral jet “flapping” instability. The heat transfer responds to both effects and leads to a broadening of the cooled area.     

Non-thermal equilibrium melting of granular packed bed in horizontal forced convection. Part I: experiment

A series of experiments are conducted to investigate the non-thermal equilibrium characteristics of melting of a packed bed under horizontal forced and mixed convections. This configuration imposes a complex treatment in phase change heat transfer that involves not only the coupled heat, mass and momentum exchanges but also the local geometric change of the packed bed (packing effect). Using visualization observations and measurements, we determine experimentally the volumes and packing patterns of the melting granular packed beds and the time variation of average melting rate per unit bed volume and average heat transfer coefficient for Re=71–2291, Gr/Re²=1.48×10⁻⁵–17.32, and Ste=0.0444–0.385. The effects of water velocity and water temperature on the melting and heat transfer in the melting process are analyzed. The effects of packing patterns on Nusselt number correlations are presented. Using the definition of a terminal velocity, a Reynolds number ratio is developed as the criterion defining the floating, non-floating or transitional packing pattern.     

CFD investigation of the flow and heat transfer characteristics in a tangential inlet cyclone

This work presents a computational fluid dynamics (CFD) calculation to investigate the flow field and the heat transfer characteristics in a tangential inlet cyclone which is mainly used for the separation of the dens phase of a two phase flow. Governing equations for the steady turbulent 3D flow were solved numerically under certain boundary conditions covering an inlet velocity range of 3 to 30 m/s. Finite volume based Fluent software was used and the RNG k −  turbulence model was adopted for the modeling highly swirling turbulent flow. Good agreement was found between computed pressure drop and experimental data available in the literature. The structure of the vortices and variation of local heat transfer were studied under the effects of inlet velocity.     

Alterations to coherent flow structures and heat transfer due to pulsations in an impinging air-jet

An investigation was carried out to study the effect of flow pulsation on the characteristics of a planar air jet impinging normally on a heated surface. Such information was further utilized to determine the influence of flow characteristics in the plane of impingement on Nusselt number distribution. Time-resolved system properties were investigated with modern instrumentation that allowed instantaneous heat transfer and flow velocity measurements to be performed simultaneously. Based on good coherence function estimates between the signals, heat transfer measurements were used in return to infer flow dynamics near the impingement surface. Experiments were performed for steady and pulsating jets at jet Reynolds numbers of 1 000, 5 500, and 11 000, pulse frequencies up to 82 Hz (corresponding to Strouhal numbers below 0.13), and pulse amplitude at the nozzle exit up to 50 % of the mean flow velocity. Special techniques commonly used for periodically disturbed flow fields elucidated the dynamics of the pulse and associated coherent flow structures. Results indicated the parametric conditions for which alterations are expected in time-averaged heat transfer from the surface. Engineering applications include cooling of electronic packages and heat transfer to gas turbine blades.     

A study on the heat and mass transfer properties of multiple pulsating impinging jets

In order to explore the potential effect of unsteady intermittent pulsations on the heat and mass transfer rate of multiple impinging jets, a numerical study is performed on a two-dimensional pulsating impinging jet array under large temperature differences between jet flows and impingement wall when the thermo-physical properties can change significantly in the flow domain. Computational fluid dynamic approach is used to simulate the flow and thermal fields of multiple pulsating impinging jets. The numerical results indicate a significant heat transfer enhancement due to intermittent pulsation over a wide range of conditions. The oscillatory flow periodically alters the flow patterns in contrast to steady jets, which can eliminate the formation of a static stagnation point and enhance the local Nusselt number along the impingement wall between adjacent jets. Examination of the velocity field shows that the instantaneous heat transfer rate on the target surface is highly dependent on the hydrodynamic and thermal boundary layer development with time.     

Nonlinear flow and heat transfer dynamics of a slot jet impinging on a slightly curved concave surface

The nonlinear flow and heat transfer characteristics for a slot jet impinging on a slightly curved concave surface are experimentally studied here. The effects of jet Reynolds number on the jet velocity distribution and circumferential Nusselt numbers are examined. The nozzle geometry is a rectangular slot and the dimensionless nozzle-to-surface distance equals to L^⁎ = 8. The constant heat fluxes are accordingly applied to the surface to obtain an impingement cooling by the air jet at ambient temperature. The measurements are made for the jet Reynolds numbers of 8617, 13 350 and 15 415. New correlations for local, stagnation point, and average Nusselt numbers as a function of jet Reynolds number and dimensionless circumferential distance are reported.