Height effect on heat transfer characteristics of sintered porous blocks with a confined slot jet

This work numerically investigates the heat transfer of a sintered porous block under a confined slot air jet. The width of the jet nozzle (W) is 5 mm; the ratio of the porous block length to the jet nozzle width (L/W) is 12, and the Prandtl number (Pr) is 0.7. Variable parameters are the ratio of the porous block height to the jet nozzle width (H/W) and the Reynolds number (Re). The findings reveal that the cooling performance with the sintered porous block was better than that with an aluminum foam block, and was at least 5.8 times as large as that without it. The Nusselt number increased as the H/W fell. The effect of Reynolds number on the heat transfer was negligible at Re ≤ 1000 but considerable at Re > 1000.     

Exergy transfer in a porous rectangular channel

Present paper is performed to investigate the heat and exergy transfer characteristics of forced convection flow through a horizontal rectangular channel where open-cell metal foams of different pore densities such as 10, 20 and 30 PPI (per pore inches) were situated. All of the bounding walls of the channel are subjected to various uniform heat fluxes. The pressure drop and heat transfer characteristics are presented by two important parametric values, Nusselt number (Nu_H) and friction factor (f), as functions of Reynolds number (Re_H) and the wall heat flux (q). The Reynolds number (Re_H) based on the channel height of the rectangular channel is varied from 600 to 33?000, while the Grashof number (Gr_Dh) ranged from approximately 10⁵–10⁷ depending on q. Based on the experimental data, new empirical correlations are constructed to link the Nu_H. The results of all cases are compared to that of the empty channel and the literature. It is found that the results are in good agreement with those cited in the references. The mean exergy transfer Nusselt number (Nu_e) based on the Re_H, Nu_H, Pr and q for a rectangular channel with constant heat flux is presented and discussed.     

Convective mass transfer from a horizontal rotating large-diameter cylinder

The local and average convective mass transfer coefficients from the porous medium surface of a horizontal rotating large-diameter cylinder were measured at a constant mass transfer state. Characteristics of local and average Sherwood numbers varying with rotational Reynolds numbers were investigated. Further, the critical Reynolds number (Re_r,cri) was determined and analyzed. Based on the experimental results, equations correlating the average Sherwood number (Sh) and critical Reynolds number (Re_r,cri) with the rotational Reynolds number (Re_r), Schmidt number (Sc) and Grashof number (Gr) have been obtained, respectively.     

Variation of Nusselt number with flow regimes behind a circular cylinder for Reynolds numbers from 70 to 30 000

The dependence of the Nusselt number in the separated flow behind a circular cylinder to the cross-flow varies greatly with Reynolds number according to the flow regimes, i.e., laminar shedding, wake transition, and shear-layer transition regimes. The Nusselt number at the rear stagnation point, Nu_r/Re^0.5, increases with Reynolds number in the laminar shedding regime (Re < 150) and the shear-layer transition regime (3000 < Re < 15 000), corresponding to the shortening of the vortex formation region. On the contrary, the Nusselt number, Nu_r/Re^0.5, decreases with Reynolds number in the regime in which the wake develops to a complex three-dimensional flow (300 < Re < 1500), corresponding to the lengthening of the vortex formation region. This distinctive change affects the correlation of the overall Nusselt number with Reynolds number, i.e., the exponent of the Reynolds number has a lower value for 200 < Re < 2000 than that for 70 < Re < 200 and Re > 2000.     

A numerical study of the unsteady heat/mass transfer inside a circulating sphere

Numerical methods are used to investigate the transient, forced convection heat/mass transfer inside circulating spheres at low to moderate Reynolds numbers. The heat/mass balance equations were solved numerically in spherical coordinates system by a finite difference method. The values considered for the sphere interior Reynolds number are Re_int ? 1000. The computations were focused on the influence of the sphere Peclet number, Pe, and Re_int on heat/mass transfer rate for Pe/(1 + μ) ? 10⁴.     

Heat transfer study in a rotor–stator system air-gap with an axial inflow

Discoidal rotor–stator systems are nowadays sometimes used in electrical wind generator. The cooling of such a system is a major problem due to the fact that high electrical losses are dissipated for relatively low rotational speed, responsible of the cooling. A new cooling solution is then investigated in this paper. So, this paper presents an experimental study of the local heat transfers on the rotor surface in the air-gap of a discoidal rotor–stator system, in which an air jet comes through the stator and impinges the rotor. To determine the surface temperatures, measurements were taken on the rotor, using an experimental technique based on infrared thermography. A thermal balance equation was used to identify the local convective heat transfer coefficient. The influence of the axial Reynolds number Re_j and the rotational Reynolds number Re was measured and compared with the data available in the literature. Local convective heat transfer coefficients were obtained for an inter-disk dimensionless spacing interval G ranging from 0.01 to 0.16 for Re_j between 0 and 41,666 and for Re between 20,000 and 516,000. The rotating disk can thus be divided into zones: one dominated by the air jet near the center of the rotor and one affected by both the air jet and rotation. Even though these two zones are not located in the same place on the disk, the heat transfers with non-zero impinging jets appear to be continuously improved compared to those with no jets. Critical radii over the rotor surface are identified and correlations are given.     

Experimental study of forced and free convective heat transfer in the thermal entry region of horizontal concentric annuli

Forced and free convective heat transfer for thermally developing and thermally fully developed laminar air flow inside horizontal concentric annuli in the thermal entrance length has been experimentally investigated. The experimental setup consists of a stainless steel annulus having a radius ratio of 2 and an inner tube with a heated length of 900 mm subjected to a constant wall heat flux boundary condition and an adiabatic outer annulus. The investigation covers Reynolds number range from 200 to 1000, the Grashof number was ranged from 6.2 × 10⁵ to 1.2 × 10⁷. The entrance sections used were long tube with length of 2520 mm (L/D_h = 63) and short tube with length of 504 mm (L/D_h = 12.6). The surface temperature distribution along the inner tube surface, and the local Nusselt number distribution versus dimensionless axial distance Z_t were presented and discussed. It is inferred that the free convection effects tended to decrease the heat transfer at low Re number while to increase the heat transfer for high Re number. This investigation reveals that the Nusselt number values were considerably greater than the corresponding values for fully developed combined convection over a significant portion of the annulus. The average heat transfer results were correlated in terms of the relevant dimensionless variables with an empirical correlation. The local Nusselt number results were compared with available literature and show similar trend and satisfactory agreement.     

Mixed convection on jet impingement cooling of a constant heat flux horizontal porous layer

In the present study, numerical investigation of jet impingement cooling of a constant heat flux horizontal surface immersed in a confined porous channel is performed under mixed convection conditions, and the Darcian and non-Darcian effects are evaluated. The unsteady stream function-vorticity formulation is used to solve the governing equations. The results are presented in the mixed convection regime with wide ranges of the governing parameters: Reynolds number (1 ≤ Re ≤ 1000), modified Grashof number (10 ≤ Gr¹ ≤ 100), half jet width (0.1 ≤ D ≤ 1.0), Darcy number (1 × 10^?6 ≤ Da ≤ 1 × 10^?2), and the distance between the jet and the heated portion (0.1 ≤ H ≤ 1.0). It is found that the average Nusselt number (Nu_avg) increases with increase in either modified Grashof number or jet width for high values of Reynolds number. The average Nusselt number also increases with decrease in the distance between the jet and the heated portion. The average Nusselt number decreases with the increase in Da for the non-Darcy regime when Re is low whereas Nu_avg increases when Re is high. It is shown that mixed convection mode can cause minimum heat transfer unfavorably due to counteraction of jet flow against buoyancy driven flow. Minimum Nu_avg occurs more obviously at higher values of H. Hence the design of jet impingement cooling through porous medium should be carefully considered in the mixed convection regimes.     

Heat transfer in a small gap between co-axial rotating cylinders

This paper investigates the local heat transfer of a co-axial rotating cylinder. In the inner flow field of the rotating cylinder, the dimensionless parameters include the rotational Reynolds number (Re_Ω) and buoyancy parameter (Gr). The test rig is designed to make the rotating in the inner cylinder and stationary in the outer cylinder. The local temperature distributions of the inner and outer cylinder on axial direction were measured. Under the experimental condition, whereas the ranges of the rotational Reynolds number are 2400 ≤ Re_Ω ≤ 45,000. Experimental results reveal that the rotational Reynolds number's increase is with the heat transfer coefficient distributions increase types. Finally, the local heat transfer rate on the wall are correlated and compared with that in the existing literature.     

Measurement of detailed heat transfer on a surface under arrays of impinging elliptic jets by a transient liquid crystal technique

Detailed heat transfer characteristics on a flat surface under arrays of impinging elliptic jets were measured by a transient liquid crystal technique. The elliptic jet holes of five different aspect ratios, AR = 4, 2, 1, 0.25, and 0.5, jet Reynolds numbers Re = 1500, 3000, and 4500, and three exit flow conditions are considered to investigate impingement heat transfer performance and the associated flow structure at various conditions. Results show that effects of the aspect ratio and crossflow have significant influences on the axial shift of the impingement/touchdown locations. The present thermographs with the high-Nu spots are very useful for understanding of the flow characteristics and jet structure deformation at various conditions. The axis-switchover phenomenon is found with the elliptic jets of aspect ratio AR > 1, but it does not occur in the cases of AR ? 1. Among the five aspect ratios considered, the mean heat transfer rates with the elliptic jets of AR = 0.5 are the highest at Re = 3000 and 4500; while, in the low-Re case of Re = 1500, the jet array with jet arrays with AR = 2 and 1 perform better than that with AR = 0.5. In addition, among the three, the two-way exit flow condition is most beneficial to the heat transfer characteristics of an impinging jet array.     

Heat transfer from a cylinder in axial turbulent flows

Local convective heat transfer coefficients were measured on a two-diameter long cylinder in axial flows of air at conditions unexplored so far, by using thermochromic liquid crystals (TLC) coated on an electrically heated strip-foil consisting bonded to the external surfaces. The Reynolds numbers (Re) based on the cylinder diameter were between 8.9 × 10⁴ and 6.17 × 10⁵, and the flow in front of the cylinder was modified in some cases by the use of a turbulence generating grid, or by circular disc inserts of two sizes placed upstream of the cylinder. These created a major change in the local convective heat transfer coefficient distribution on the cylinder. Increase of the turbulence intensity from Tu < 0.1% to Tu = 6.7% at the same Re increased the average calculated Nusselt number Nu over the cylinder by 25%, and decreased the Nu non-uniformity over the surface. One of the flow modification inserts also reduced significantly the Nu non-uniformity. The position of flow reattachment was measured using tufts. Our heat transfer data agree well with the small amount if data published of others, when extrapolated to their conditions. Correlations between the Nu and Re in the form Nu = CRe^e were established and presented for the average Nu on the front, middle and rear cylinder surfaces, and the variation of the local exponent e was shown along the cylinder. Introducing a new technique, a TLC-coated heated flat plate mounted in the flow above the cylinder in the meridional plane was demonstrated to help visualize the flow field above the cylinder. A track of maximum convective coefficients on this plate was found similar in position to the stream line dividing the forward and backward flows in a case measured for the separated flow in a past study.     

Enhancement of heat exchange in a wavy channel linked to a porous domain; a possible duct geometry for fuel cells

In this paper heat transfer and flow field analysis in a wavy channel linked to a porous Gas Diffusion Layer (GDL) is numerically studied. The domain is very similar to our earlier computations of proton exchange membrane fuel cells (see Khakbaz-Baboli and Kermani (2008)). The fluid temperature at the channel inlet (T_in) is taken less than that of the walls (T_w). The governing equations are numerically solved in the domain by the control volume approach based on the SIMPLE technique (1972). A wide spectrum of numerical studies is performed over a range of Reynolds number Re_H: 100 ≤ Re_H ≤ 1000, wave number β: 0 ≤ β ≤ 10, the wave amplitude α: 0 ≤ α ≤ 0.3 and Darcy number Da: 0.1 ≤ Da ≤ 0.001. Simulations show that heat transfer in channels can enhance up to 100%, depending on the duct α, β and flow Re_H. Computations show excellent agreement with the literature. The present work can provide helpful guidelines to the manufactures of the compact heat exchangers.     

Heat transfer from a spinning disk during semi-confined axial impingement from a rotating nozzle

Conjugate heat transfer from a uniformly heated spinning solid disk of finite thickness and radius during a semi-confined liquid jet impingement from a rotating nozzle is studied. The model covers the entire fluid region including the impinging jet on a flat circular disk and flow spreading out downstream under the spinning confinement plate and free surface flow after exposure to the ambient gaseous medium. The model examines how the heat transfer is affected by adding a secondary rotational flow under semi-confined jet impingement. The solution is made under steady state and laminar conditions. The study considered various plate materials such as aluminum, copper, silver, constantan and silicon. Ammonia, water, flouroinert FC-77 and MIL-7808 oil were used as working fluids. The range of parameters covered included Reynolds number (220–900), Ekman number (7.08 × 10^?5–∞), nozzle-to-target spacing (β = 0.25–1.0), disk thicknesses to nozzle diameter ratio (b/d_n = 0.25–1.67), Prandtl number (1.29–124.44) and solid to fluid thermal conductivity ratio (36.91–2222). It was found that a higher Reynolds number increased local heat transfer coefficient reducing the interface temperature difference over the entire disk surface. The rotational rate also increased local heat transfer coefficient under most conditions. An engineering correlation relating the Nusselt number with other dimensionless parameters was developed for the prediction of the system performance.     

Analysis of heat transfer and pressure drop characteristics in an offset strip fin heat exchanger

A steady-state three-dimensional numerical model was used to study the heat transfer and pressure drop characteristics of an offset strip fin heat exchanger. Water was the heat transfer medium, and the Reynolds number Re_dh ranged from 10 to 3500. Variations in the Fanning friction factor f and the Colburn heat transfer factor j relative to Re_dh were observed. General correlations for the f and j factors were derived, and these could be used to analyze fluid flow and heat transfer characteristics of offset strip fins in the laminar, transition, and turbulent regions. Finally, three performance criteria (j/f, j/f^1/3, and JF) were adopted, and the best performance criteria for the cases Pr = 7 and Pr = 50 were chosen to be JF and j/f^1/3, respectively.     

Turbulent heat transfer on the stationary disk in a rotor-stator system

A combined numerical and experimental study was performed to determine the turbulent heat transfer on a stationary disk, which is situated in a close distance from a rotating disk. The RNG k-ε model and the steady-state liquid crystal technique were employed respectively in the numerical simulation and the experiment. In the range of the rotational Reynolds number from 1.42×10⁵ to 3.33×10⁵, the heat transfer rate on the stator and the flow characteristics in the gap between the disks are presented. The results revealed that there exists an optimum rotor-stator distance for a given Reynolds number, at which the average heat transfer on the stator reaches maximum. When the Reynolds number increases, the maximum shifts towards smaller disk-distances.     

Heat transfer behavior in a rotating aluminum foam heat sink with a circular impinging jet

This work experimentally studied heat transfer associated with an impinging jet onto a rotating heat sink. Air was used as the impinging coolant, and a square Al-foam heat sink was adopted. The variable parameters were the jet Reynolds number (Re), the relative nozzle-to-foam tip distance (C/d), the rotational Reynolds number (Re_r) and the relative side length of the square heat sink (L/d). The effects of Re, C/d, Re_r and L/d on the dimensionless temperature distributions and the average Nusselt number were considered. For a stationary system, the results reveal that the average Nusselt number (Nu₀) with Al-foam was two to three times that without Al-foam. Nu₀ increased with Re. A larger L/d responded to a larger Nu₀ based on the same jet flow rate. The effect of C/d on Nu₀ was negligible herein. For a rotating system, when Re and L/d were small and C/d was large, the average Nusselt number (Nu_Ω) increased considerably with Re_r. Additionally, for Nu_Ω/Nu₀ ? 1.1, the results suggest that rotation was substantial at Re_r/Re ? 1.13 when L/d = 4.615 with C/d = 0–5 and at Re_r/Re ? 1.07 when L/d = 3.0 with C/d = 0–5. For L/d = 2.222, rotation was substantial at Re_r/Re ? 1.44 when C/d = 0 and was always substantial when C/d ? 1.     

Thermal characteristics of turbulent rib-grooved channel flows

Experimental work has been performed to examine the combined effects of rib-grooved turbulators on the turbulent forced convection heat transfer and friction characteristics in a rectangular duct under a uniform heat flux boundary condition. In the experiments, three types of rib-groove arrangements: rectangular-rib and triangular-groove (RR–TG), triangular-rib and rectangular-groove (TR–RG) and triangular-rib with triangular-groove (TR–TG), were examined. Measurements were carried out for the duct of one aspect ratio, AR = W/H = 20 and duct height, H = 9 mm with rib height, e = 3 mm at three pitch ratios, PR = P/e = 6.6, 10 and 13.3. Experiments were conducted for the Reynolds number range of 3000 to 10,000. Influences of rib-groove arrangements on the Nusselt number and friction factor have been discussed and compared with smooth duct results under similar test conditions. Isothermal friction factors were also taken and presented. The obtained results of the smooth duct are in good agreement with the previous studies found in the literature. Experimental results also show that the duct with RR–TG arrangement provides maximum heat transfer rate and friction factor than others. On the other hand, the thermal enhancement index obtained at constant pumping power reveals that the TR–TG provides the highest values for all pitch ratios studied. Finally, correlations for the heat transfer (Nu), friction factor (f) and the enhancement index (η) have been developed as a function of pitch ratio (PR) and Reynolds number (Re).     

Heat transfer on a radially rotating heated cylinder

Experiments are conducted to investigate the convective heat transfer on a radially rotating heated cylinder. In the experiment, one uses cold air-hot cylinder instead of hot air-cold blade in a real engine. The hollow bakelite test rotating cylinder is pasted with a heater made of 0.03 mm thin film of stainless steel. The maximum air stream velocity is 20 m/s with the corresponding Reynolds number of 1.2 × 10⁵ that is high enough to simulate the real turbine blade of Re ≈ 10⁵. The rotation-induced cross stream flow affect the heat transfer coefficient on the cylinder surface. The effect is more prominent for the cases with higher rotational speeds and lower Reynolds numbers. Due to rotation, the heat transfer enhancement at lower Reynolds number is greater than those at a higher one.     

Experimental investigation of cooling of heated circular disc using inclined circular jet

An experimental study is performed to examine the heat transfer characteristics of impinging circular jet onto a heated circular disc. The disc is heated under constant heat flux and it has an inclination angle with impinging jet in the range of 90° ≤ φ ≤ 150°. The air is supplied using a radial fan. The fluid flows through a designed tunnel. Experiments were performed under different Reynolds number, 2800, 9000, and 36,000, and different values of inclination angle of the disk and jet-to-plate distance to jet diameter ratio H/D_h as 5, 10, and 15. The results of experiments showed that the most effective parameter is the inclination angle between jet and heater. Both locations of stagnation point and heat transfer are affected from this parameter.