On the linear heat transfer correlation of a heated circular cylinder in laminar crossflow using a new representative temperature concept

Determination of the reference temperature for the kinematic viscosity of Reynolds number is the key problem for describing the forced convection of a heated bluff-body. The heat transfer of a heated circular cylinder in laminar cross airflow is experimentally investigated in this paper. New correlation equations based on the effective temperature have been firstly derived. All experimental data collapsed excellently on the correlation curves. Moreover, the derived Nusselt and Reynolds number relations are independent of the vortex shedding patterns. The correlation equations have further been transformed to an explicit form without temperature ratio. By introducing a newly proposed representative Reynolds number concept, linear correlation equations have been successfully derived.     

Laminar convective heat transfer from a circular cylinder exposed to a low frequency zero-mean velocity oscillating flow

Heat transfer characteristics of a circular cylinder exposed to a slowly oscillating flow with zero-mean velocity were investigated. The flow oscillation amplitude and frequency were changed in the range where the flow remains laminar and fluid particle travels back and forth over much larger distance compared to the cylinder diameter. The time- and space-averaged Nusselt number was measured by transient method, while two-dimensional numerical simulation was conducted to discuss the instantaneous flow and thermal fields around the cylinder. It was found that the time- and space-averaged Nusselt number can be correlated with the oscillating Reynolds number and Richardson number. Unique heat transfer characteristics under oscillating flow condition can be seen at the phases when the cross-sectional mean velocity is small or increasing from small value. During such period, heat transfer can be enhanced due to the local fluid motion induced by the vortices around the cylinder, which once moved away but returned back by the reversed flow. This heat transfer enhancement, however, is countered by the local warming effect of the hot vortices clinging around the cylinder at such phases.     

Experimental and numerical study on heat transfer enhancement in a channel with Z-shaped baffles

The influence of baffle turbulators on heat transfer augmentation in a rectangular channel has been investigated experimentally and numerically. In the experiment, the baffles are placed in a zigzag shape (Z-shaped baffle) aligned in series on the isothermal-fluxed top wall, similar to the absorber plate of a solar air heater channel. The aim at using the Z-baffles is to create co-rotating vortex flows having a significant influence on the flow turbulence intensity leading to higher heat transfer enhancement in the tested channel. Effects of the Z-baffle height and pitch spacing length are examined to find the optimum thermal performance for the Reynolds number from 4400 to 20,400. The Z-baffles inclined to 45° relative to the main flow direction are characterized at three baffle- to channel-height ratios (e/H = 0.1, 0.2 and 0.3) and baffle pitch ratios (P/H = 1.5, 2 and 3). The experimental results show a significant effect of the presence of the Z-baffle on the heat transfer rate and friction loss over the smooth channel with no baffle. The Nusselt number, friction factor and thermal performance enhancement factor for the in-phase 45° Z-baffles are found to be considerably higher than those for the out-phase 45° Z-baffle at a similar operating condition. The in-phase 45° Z-baffle with larger e/H provides higher heat transfer and friction loss than the one with smaller e/H while the shorter pitch length yields the higher Nu, f and TEF than the larger one. The numerical work is also conducted to investigate the flow friction and heat transfer behaviors in the channel mounted with the 45° Z-baffles, and the numerical results are found in good agreement with experimental data.     

Effect of channel-confinement and aiding/opposing buoyancy on the two-dimensional laminar flow and heat transfer across a square cylinder

The effect of channel-confinement of various degree (blockage ratio of 10%, 30% and 50%) on the upward flow and heat transfer characteristics around a heated/cooled square cylinder is studied by considering the effect of aiding/opposing buoyancy at −1 Ri 1, for Re = 100 and Pr = 0.7. With increasing blockage ratio, the minimum heating (critical Ri) required for the suppression of vortex shedding decreases up to a certain blockage ratio (=30%), but thereafter increases. The influence of buoyancy and channel-confinement on the recirculation length, drag and lift coefficient, pumping power, Strouhal number and heat transfer from the cylinder, is also investigated.     

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.     

Heat transfer enhancement in a channel with a built-in square cylinder

A two dimensional numerical investigation of the unsteady laminar flow pattern and forced convective heat transfer in a channel with a built-in square cylinder is presented. The channel in the entrance region has a length to plate spacing of ten. The computations were made for several Reynolds number and two square cylinder sizes. Hydrodynamic behavior and heat transfer results are obtained by solution of the complete Navier-Stokes and energy equation. The results show that these flow exhibits laminar self-sustained oscillations for Reynolds numbers above the critical one. This study shows that oscillatory separated flows result in a significant heat transfer enhancement but also in a significant pressure drop increase.     

Heat transfer in channel flow around a rectangular cylinder

Heat transfer and flow visualization experiments have been made in a channel with a rectangular cylindrical section having various width-to-height ratios. Vortices were observed to shed periodically from the cylinder and then reattach to the channel wall. This reattachment of the vortices induces a periodic fluctuation in heat flux at the wall and enhances the heat transfer in the downstream region of the cylinder. The streamwise position of the maximum Nusselt number moves downstream with decreasing width-to-height ratio, b/h, of the cylinder. When b/h = 2.0, however, the heat flux periodicity disappears because the wake narrows intermittently owing to reattachment of the separated flows to the upper and lower surfaces of the cylinder. © 1998 Scripta Technica. Heat Trans Jpn Res, 27(1): 84–97, 1998     

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.     

Numerical predictions of a PEM fuel cell performance enhancement by a rectangular cylinder installed transversely in the flow channel

This paper numerically investigates the installation of the transverse rectangular cylinder along the gas diffusion layer (GDL) in the flow channel for the cell performance enhancement of a proton exchange membrane fuel cell (PEMFC). The effects of the blockage at various gap sizes and the width of the cylinder on the cell performance enhancement have been studied with changing the gap ratios λ = 0.05–0.3, for the same cylinder) and the width-to-height ratios (WR = 0.66–1.66, for the same cylinder height and gap ratio). The results show that the transverse installation of a rectangular cylinder in the fuel flow channel effectively enhances the cell performance of a PEMFC. In addition, the influence of the width of the cylinder on the cell performance is obvious, and the best cell performance enhancement occurs at the gap ratio 0.2 among the gap ratios of 0.05, 0.1, 0.2, and 0.3.     

Heat transfer around a circular cylinder in separated air flow

An experimental study has been conducted to determine the heat transfer characteristics around a circular cylinder attached to the separated flow of air shed from a fence. The fence was located vertically to the flow with a height of H = 40 mm. d/H was constant at 0.638, where d is the cylinder diameter of 25.5 mm. X/H were 0.50 and 0.775 and Y/H ranged from 0.525 to 1.50, where X and Y are, respectively, the distances between the axis of the cylinder and the front face of the fence, and the bottom wall of the test section. The Reynolds number based on the cylinder diameter and the velocity of the undisturbed flow ranged from 1.9 × 10⁴ to 6.0 × 10⁴. It was found that the maximum local Nusselt number changes drastically in the vicinity of Y/H = 1.0–1.11 and that the maximum mean Nusselt number occurs in the neighborhood of Y/H = 1.24–1.43 for X/H = 0.50 and 1.3–1.4 for X/H = 0.775. © 1999 Scripta Technica, Heat Trans Asian Res, 28(3): 211–226, 1999     

Investigation of heat transfer and exergy loss in oscillating circular pipes

In this study, the effect on heat transfer rates, friction factor and exergy loss of oscillating pipe were investigated. Air was used as working fluid. To obtain oscillating motion of the test pipe, the experimental setup was designed. Thermocouple, the temperature control system and the other measurement systems were installed on the oscillating section. For both steady and oscillating flows, the bulk and local wall temperature distribution, pressure drop, inlet and outlet temperatures and frequency were measured. The parameters for this study were chosen as Reynolds number from 5000 to 20,000 at oscillating frequencies with 10 and 20 Hz. The variations of Nusselt number and exergy loss with these parameters were determined and presented graphically. In oscillatory flow, a heat transfer enhancement of up to at constant pumping power was achieved. The exergy loss was decreased with the increase of Reynolds number. The exergy loss affected slightly from the increase of oscillating frequencies.     

Heat transfer in a PEMFC flow channel

A numerical method was applied to the heat transfer performance in the flow channel for a proton exchange membrane fuel cell (PEMFC) using the finite element method (FEM). The heat transfer enhancement has been analyzed by transversely installing a baffle plate and a rectangular cylinder to manage flow pattern in the flow channel of the fuel cell. Case studies include baffle plates (gap ratios from 00.05 to 0.2) and the rectangular cylinder (width-to-height ratios from 0.66 to 1.66 with a constant gap ratio of 0.2; various gap ratios from 0.05 to 0.3 with a constant width-to-height ratio 1.0) at constant Reynolds number. The results show that the transverse installation of a baffle plate and a rectangular cylinder in the flow channel can effectively enhance the local heat transfer performance of a PEMFC. The installation of a rectangular cylinder has a better effective heat transfer performance than a baffle plate; the larger the width of the cylinder is the better effective heat transfer performance becomes.     

Heat transfer augmentation in a wedge-ribbed channel using winglet vortex generators

Experimental investigations have been carried out to study the effect of combined wedge ribs and winglet type vortex generators (WVGs) on heat transfer and friction loss behaviors for turbulent airflow through a constant heat flux channel. To create a reverse flow in the channel, two types of wedge (right-triangle) ribs are introduced: wedge ribs pointing downstream and pointing upstream. The arrangements of both rib types placed inside the opposite channel walls are in-line and staggered arrays. To generate longitudinal vortex flows through the tested section, two pairs of the WVGs with the attack angle of 60° are mounted on the test channel entrance. The test channel has an aspect ratio, AR = 10 and height, H = 30 mm with a rib height, e/H = 0.2 and rib pitch, P/H = 1.33. The flow rate in terms of Reynolds numbers is based on the inlet hydraulic diameter of the channel ranging from 5000 to 22,000. The presence of the combined ribs and the WVGs shows the significant increase in heat transfer rate and friction loss over the smooth channel. The Nusselt number and friction factor values obtained from combined the ribs and the WVGs are found to be much higher than those from the ribs/WVGs alone. In conjunction with the WVGs, the in-line wedge pointing downstream provides the highest increase in both the heat transfer rate and the friction factor while the staggered wedge pointing upstream yields the best thermal performance.     

Forced convection heat transfer from a circular cylinder in crossflow to air and liquids

Local and average heat transfer by forced convection from a circular cylinder is studied for Reynolds number from 2 × 10³ to 9 × 10⁴ and Prandtl number from 0.7 to 176. For subcritical flow, the local heat transfer measurement indicates three regions of flow around the cylinder: laminar boundary layer region, reattachment of shear layer region and periodic vortex flow region. The average heat transfer in each region is calculated and correlated with the Reynolds number and the Prandtl number. The Nusselt number in each region strongly depends on the Reynolds number and the Prandtl number with different power indices. An empirical correlation for predicting the overall heat transfer from the cylinder is developed from the contributions of heat transfer in these three regions.     

Effects of porosity and thickness of porous sheets on heat transfer enhancement in a cross flow over heated cylinder

An experimental study is carried out to investigate the thermal impact of wrapping an aluminum porous sheet over a circular tube in a heat convection configuration. The experimental apparatus consists of a heated horizontal cylinder with a constant heat flux. The cylinder is then covered with porous sheets of different thicknesses. The tube is exposed to a cross flow of air at different speeds which corresponds to different Reynolds numbers. The effect of the added porous layer on the pressure loss over the cylinder was also investigated. It is observed that heat transfer is greatly enhanced with the addition of the porous layer. Also, the addition of the porous layer doesn’t appear to increase the pressure loss.     

Heat transfer enhancement in a parallel plate duct by an oscillating thin plate insertion

Various arrangements were considered for two thin plates, oscillated by a flow in a parallel plate duct, with a view to enhancing the heat transfer along the duct. Heat transfer and pressure distributions were measured at varying the clearances from the wall and various plate separations. The maximum and mean Nusselt numbers have a Reynolds number dependence of Re^0.8, and were, respectively, 2.3 and 1.6 times as large as those in fully developed turbulent flow, for air with Reynolds number ranging from 9,000 to 37,500. Full-field infrared imaging, a relatively new technique, was used to obtain the temporal and spatial temperature profiles on the wall surface. Isotherm contours of the infrared images correspond well to the heat transfer characteristics and flow. © 1997 Scripta Technica, Inc. Heat Trans Jpn Res, 25 (8): 554–567, 1996     

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