An Experimental Study of Heat Transfer of a Porous Channel Subjected to Oscillating Flow

2000 ◽  
Vol 123 (1) ◽  
pp. 162-170 ◽  
Author(s):  
H. L. Fu ◽  
K. C. Leong ◽  
X. Y. Huang ◽  
C. Y. Liu
Keyword(s):  
Heat Transfer ◽  
Nusselt Number ◽  
Surface Temperature ◽  
Steady Flow ◽  
High Speed ◽  
Flow Direction ◽  
Porous Channel ◽  
Oscillating Flow ◽  
Oscillating Flows ◽  
Nusselt Numbers

Experiments have been conducted to study the heat transfer of a porous channel subjected to oscillating flow. The surface temperature distributions for both steady and oscillating flows were measured. The local and length-averaged Nusselt numbers were analyzed. The experimental results revealed that the surface temperature distribution for oscillating flow is more uniform than that for steady flow. Due to the reversing flow direction, there are two thermal entrance regions for oscillating flow. The length-averaged Nusselt number for oscillating flow is higher than that for steady flow. The length-averaged Nusselt number for both steady and oscillating flows increase linearly with a dimensionless grouping parameter k*/kfDe/L1/2Pe*1/2. The porous channel heat sink subjected to oscillating flow can be considered as an effective method for cooling high-speed electronic devices.

Heat Transfer Characteristics of Oscillating Flow Through Highly Porous Medium

2006 ◽  
Author(s):  
L. W. Jin ◽  
K. C. Leong
Keyword(s):  
Heat Transfer ◽  
Nusselt Number ◽  
Metal Foam ◽  
Metal Foams ◽  
Porous Channel ◽  
Oscillating Flow ◽  
Transient Temperature ◽  
Heat Transfer Characteristics ◽  
Flow Through ◽  
Highly Porous

Heat transfer in porous media has been investigated extensively with the motivation of enhancing heat removal in electronics cooling applications. Many investigations have been conducted on heat transfer in a channel filled with porous media. However, steady flow through a porous channel still yield a higher temperature difference along the flow direction. It is conceivable that oscillating flow through a porous channel will produce a more uniform temperature distribution due to the two thermal entrance regions of oscillating flow. As compared to a porous channel packed with metal particles, spheres or woven-screens, the highly porous open-cell metal foam possesses a different configuration. The polyhedral pore and reticulated ligament structures provide the extremely large fluid-to-solid contact surface area and tortuous coolant flow path inside the metal foam, which could increase dramatically the overall heat transfer rate. A survey of the literature shows that heat transfer in open-cell metal foam were mostly investigated under steady flow condition. Published literature on heat transfer in metal foams subjected to oscillating flow is scarce. This paper presents both experimental and numerical investigations on the heat transfer characteristics for oscillating flow through highly porous medium. Experiments were carried out to study the effect of the oscillatory frequency on the heat transfer in metal foams with various pore densities. The results show that the local Nusselt number increases with the kinetic Reynolds number. Higher total heat transfer rates for oscillating flow can be obtained by using high pore density metal foam. The numerical simulation is focused on the study of the variations of the transient temperature and Nusselt number at different locations in the porous channel during a complete cycle. The numerical results show that the profile of the transient temperature decreases with the increase of the distance along the vertical direction and the variation of the instantaneous Nusselt number at entrance region is more significant than that at the location close to the center of the porous channel. It is also found that the two-dimensional temperature distributions in the numerical domain are symmetric about the center of the channel at the cycle-steady state. The comparison shows that the results obtained by the simulation are in reasonably good agreement with the experimental data.

Heat Transfer and Fluid Flow in Metal Foam Subjected to Oscillating Flow

2005 ◽  
Author(s):  
K. C. Leong ◽  
L. W. Jin
Keyword(s):  
Heat Transfer ◽  
Fluid Flow ◽  
High Speed ◽  
Metal Foam ◽  
Substrate Surface ◽  
Volume Ratio ◽  
Porous Channel ◽  
Oscillating Flow ◽  
Open Cell ◽  
Flow Through

The need for higher performance and an increased level of functional integration as well as die size optimization on the microprocessor leads to preferential clustering of higher power units on the processor. Conventional natural or forced convection cooling methods are not capable of removing such a high heat flux for maintaining a proper operational temperature. It is imperative to look for new methods of cooling the modern high-speed electronic components. The porous medium has emerged as an effective method of heat transfer enhancement due to its large surface area to volume ratio and intense mixing of fluid flow. Many researchers have studied heat transfer and fluid flow in a channel filled with metal particles or woven-screens. However, uni-directional flow through the porous channel yields a relatively high temperature difference along the flow direction on the substrate surface. For modern high-speed microprocessors, the reliability of transistors and operating speed are not only influenced by the average temperature but also by temperature uniformity on the substrate surface. Therefore, maintaining the uniformity of on-die temperature distribution below certain limits is imperative in thermal design. It is conceivable that oscillating flow through a porous channel will produce a more uniform temperature distribution, due to the presence of two thermal entrance regions for oscillating flow. In the present investigation, a novel porous material of open-cell metal foam was employed to study heat transfer and fluid flow of oscillating flow through a porous channel. The metal foam with fully inter-connected structure, large surface area to volume ratio and high permeability lends itself to applications in electronics cooling. This paper describes an experimental study on heat transfer and pressure drop behavior of oscillating flow through a channel filled with open-cell aluminum foam. Both cycle-averaged and length-averaged local Nusselt numbers were calculated to evaluate heat transfer rate of oscillating flow in metal foam channel. The effects of the dimensionless flow amplitude and frequency of oscillating flow on heat transfer were analyzed. A correlation equation of maximum friction factor of oscillating flow in metal foam was obtained and compared with the results for wire-screens obtained by other investigators under the oscillating flow condition. The results revealed that heat transfer performance can be enhanced substantially by oscillating flow through metal foam with moderate pressure drop.

scholarly journals Operating temperatures of oil-lubricated medium-speed gears: Numerical models and experimental results

2003 ◽  
Vol 217 (2) ◽  
pp. 87-106 ◽  
Author(s):  
H Long ◽  
A A Lord ◽  
D T Gethin ◽  
B J Roylance
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Surface Temperature ◽  
Rotational Speed ◽  
High Speed ◽  
Applied Load ◽  
Frictional Heat ◽  
Transfer Coefficients ◽  
Gear Teeth ◽  
Heat Transfer Coefficients

This paper investigates the effects of gear geometry, rotational speed and applied load, as well as lubrication conditions on surface temperature of high-speed gear teeth. The analytical approach and procedure for estimating frictional heat flux and heat transfer coefficients of gear teeth in high-speed operational conditions was developed and accounts for the effect of oil mist as a cooling medium. Numerical simulations of tooth temperature based on finite element analysis were established to investigate temperature distributions and variations over a range of applied load and rotational speed, which compared well with experimental measurements. A sensitivity analysis of surface temperature to gear configuration, frictional heat flux, heat transfer coefficients, and oil and ambient temperatures was conducted and the major parameters influencing surface temperature were evaluated.

Spatially Resolved Heat Transfer and Friction Factors in a Rectangular Channel With 45-Deg Angled Crossed-Rib Turbulators

2003 ◽  
Vol 125 (3) ◽  
pp. 575-584 ◽  
Author(s):  
P. M. Ligrani ◽  
G. I. Mahmood
Keyword(s):  
Heat Transfer ◽  
Reynolds Number ◽  
Nusselt Number ◽  
Rectangular Channel ◽  
Test Surface ◽  
Spatially Resolved ◽  
Nusselt Numbers ◽  
Friction Factors ◽  
Smooth Channel ◽  
Rib Turbulators

Spatially resolved Nusselt numbers, spatially averaged Nusselt numbers, and friction factors are presented for a stationary channel with an aspect ratio of 4 and angled rib turbulators inclined at 45 deg with perpendicular orientations on two opposite surfaces. Results are given at different Reynolds numbers based on channel height from 10,000 to 83,700. The ratio of rib height to hydraulic diameter is .078, the rib pitch-to-height ratio is 10, and the blockage provided by the ribs is 25% of the channel cross-sectional area. Nusselt numbers are given both with and without three-dimensional conduction considered within the acrylic test surface. In both cases, spatially resolved local Nusselt numbers are highest on tops of the rib turbulators, with lower magnitudes on flat surfaces between the ribs, where regions of flow separation and shear layer reattachment have pronounced influences on local surface heat transfer behavior. The augmented local and spatially averaged Nusselt number ratios (rib turbulator Nusselt numbers normalized by values measured in a smooth channel) vary locally on the rib tops as Reynolds number increases. Nusselt number ratios decrease on the flat regions away from the ribs, especially at locations just downstream of the ribs, as Reynolds number increases. When adjusted to account for conduction along and within the test surface, Nusselt number ratios show different quantitative variations (with location along the test surface), compared to variations when no conduction is included. Changes include: (i) decreased local Nusselt number ratios along the central part of each rib top surface as heat transfer from the sides of each rib becomes larger, and (ii) Nusselt number ratio decreases near corners, where each rib joins the flat part of the test surface, especially on the downstream side of each rib. With no conduction along and within the test surface (and variable heat flux assumed into the air stream), globally-averaged Nusselt number ratios vary from 2.92 to 1.64 as Reynolds number increases from 10,000 to 83,700. Corresponding thermal performance parameters also decrease as Reynolds number increases over this range, with values in approximate agreement with data measured by other investigators in a square channel also with 45 deg oriented ribs.

Local Heat Transfer and Flow Structure on and Above a Dimpled Surface in a Channel

2000 ◽  
Author(s):  
G. I. Mahmood ◽  
M. L. Hill ◽  
D. L. Nelson ◽  
P. M. Ligrani ◽  
H.-K. Moon ◽  
...  
Keyword(s):  
Heat Transfer ◽  
Surface Temperature ◽  
Local Heat Transfer ◽  
Streamwise Velocity ◽  
Test Surface ◽  
Channel Height ◽  
Stagnation Temperature ◽  
Vortex Pairs ◽  
Nusselt Numbers ◽  
Flow Visualizations

Experimental results, measured on and above a dimpled test surface placed on one wall of a channel, are given for Reynolds numbers from 1,250 to 61,500 and ratios of air inlet stagnation temperature to surface temperature ranging from 0.68 to 0.94. These include flow visualizations, surveys of time-averaged total pressure and streamwise velocity, and spatially-resolved local Nusselt numbers, which are measured using infrared thermography, used in conjunction with energy balances, thermocouples, and in situ calibration procedures. The ratio of channel height to dimple print diameter is 0.5. Flow visualizations show vortical fluid and vortex pairs shed from the dimples, including a large upwash region and packets of fluid emanating from the central regions of each dimple, as well as vortex pairs and vortical fluid which form near dimple diagonals. These vortex structures augment local Nusselt numbers near the downstream rims of each dimple, both slightly within each depression, and especially on the flat surface just downstream of each dimple. Such augmentations are spread over larger surface areas and become more pronounced as the ratio of inlet stagnation temperature to local surface temperature decreases. As a result, local and spatially-averaged heat transfer augmentations become larger as this temperature ratio decreases. This is due to the actions of vortical fluid in advecting cool fluid from the central parts of the channel to regions close to the hotter dimpled surface.

Forced Convection in a Porous Channel With Discrete Heat Sources

2000 ◽  
Vol 123 (2) ◽  
pp. 404-407 ◽  
Author(s):  
C. Cui ◽  
X. Y. Huang ◽  
C. Y. Liu
Keyword(s):  
Heat Transfer ◽  
Reynolds Numbers ◽  
Heat Fluxes ◽  
Experimental Results ◽  
Porous Channel ◽  
Heat Sources ◽  
Nusselt Numbers ◽  
Discrete Heat Sources ◽  
Flow Through ◽  
Good Agreement

An experimental study was conducted on the heat transfer characteristics of flow through a porous channel with discrete heat sources on the upper wall. The temperatures along the heated channel wall were measured with different heat fluxes and the local Nusselt numbers were calculated at the different Reynolds numbers. The temperature distribution of the fluid inside the channel was also measured at several points. The experimental results were compared with that predicted by an analytical model using the Green’s integral over the discrete sources, and a good agreement between the two was obtained. The experimental results confirmed that the heat transfer would be more significant at leading edges of the strip heaters and at higher Reynolds numbers.

Heat Transfer Experiments in a Confined Jet Impingement Configuration Using Transient Techniques

2010 ◽  
Author(s):  
Florian Hoefler ◽  
Nils Dietrich ◽  
Jens von Wolfersdorf
Keyword(s):  
Heat Transfer ◽  
Heat Transfer Coefficient ◽  
Transfer Coefficient ◽  
Flow Direction ◽  
Jet Impingement ◽  
Thermochromic Liquid Crystal ◽  
Transient Techniques ◽  
Nusselt Numbers ◽  
The Heat Transfer Coefficient ◽  
Good Agreement

A confined jet impingement configuration has been investigated in which the matter of interest is the convective heat transfer from the airflow to the passage walls. The geometry is similar to gas turbine applications. The setup is distinct from usual cooling passages by the fact that no crossflow and no bulk flow direction are present. The flow exhausts through two staggered rows of holes opposing the impingement wall. Hence, a complex 3-D vortex system arises, which entails a complex heat transfer situation. The transient Thermochromic Liquid Crystal (TLC) method was used to measure the heat transfer on the passage walls. Due to the nature of the experiment, the fluid as well as the wall temperature vary with location and time. As a prerequisite of the transient TLC technique, the heat transfer coefficient is assumed to be constant over the transient experiment. Therefore, additional measures were taken to qualify this assumption. The linear relation between heat flux and temperature difference could be verified for all measurement sites. This validates the assumption of a constant heat transfer coefficient which was made for the transient TLC experiments. Nusselt number evaluations from all techniques show a good agreement, considering the respective uncertainty ranges. For all sites the Nusselt numbers range within ±9% of the values gained from the TLC measurement.

Study of a Cooling Feed Pipe With a Covering Plate on a Ribbed Turbine Case

2019 ◽  
Vol 141 (7) ◽  
Author(s):  
Fangyuan Liu ◽  
Junkui Mao ◽  
Chao Han ◽  
Yuanjian Liu ◽  
Xingsi Han ◽  
...  
Keyword(s):  
Heat Transfer ◽  
Reynolds Number ◽  
Nusselt Number ◽  
Experimental Tests ◽  
Impinging Jet ◽  
Nusselt Numbers ◽  
Spacing Ratio ◽  
Wall Flow ◽  
Complicated Geometry ◽  
Clearance Control

Considering the complicated geometry in an active clearance control (ACC) system, the design of an improved cooling feed pipe with a covering plate for a high pressure ribbed turbine case was investigated. Numerical calculations were analyzed to obtain the interactions between the impinging jet arrays fed by the pipe. Experimental tests were performed to explore the effect of the Reynolds number (2000–20,000) and the jet-to-surface spacing ratio (6–10) on the streamwise-averaged Nusselt numbers. Additionally, the effect of the crossflow produced by the configuration was investigated. Results showed a confined curved channel was formed by the pipe and ribbed case, which resulted in crossflow. The crossflow evolved into vortices and the streamwise-averaged Nusselt number on the high ribs was subsequently increased. Furthermore, the distribution of the heat transfer on the entire surface became more uniform compared with that of traditional impinging jet arrays. A higher Nusselt number was achieved by decreasing the jet-to-surface spacing and increasing the Reynolds number. This investigation has revealed a cooling configuration for controlling the wall flow and evening the heat transfer on the case surface, especially for the ribs.

Experimental Investigation of Coil Curvature Effect on Heat Transfer and Pressure Drop Characteristics of Shell and Coil Heat Exchanger

2014 ◽  
Vol 7 (1) ◽  
Author(s):  
M. R. Salem ◽  
K. M. Elshazly ◽  
R. Y. Sakr ◽  
R. K. Ali
Keyword(s):  
Heat Transfer ◽  
Nusselt Number ◽  
Heat Exchanger ◽  
Friction Factor ◽  
Heat Exchangers ◽  
Average Nusselt Number ◽  
Transfer Coefficients ◽  
Nusselt Numbers ◽  
Fanning Friction Factor ◽  
Coil Heat

The present work experimentally investigates the characteristics of convective heat transfer in horizontal shell and coil heat exchangers in addition to friction factor for fully developed flow through the helically coiled tube (HCT). The majority of previous studies were performed on HCTs with isothermal and isoflux boundary conditions or shell and coil heat exchangers with small ranges of HCT configurations and fluid operating conditions. Here, five heat exchangers of counter-flow configuration were constructed with different HCT-curvature ratios (δ) and tested at different mass flow rates and inlet temperatures of the two sides of the heat exchangers. Totally, 295 test runs were performed from which the HCT-side and shell-side heat transfer coefficients were calculated. Results showed that the average Nusselt numbers of the two sides of the heat exchangers and the overall heat transfer coefficients increased by increasing coil curvature ratio. The average increase in the average Nusselt number is of 160.3–80.6% for the HCT side and of 224.3–92.6% for the shell side when δ increases from 0.0392 to 0.1194 within the investigated ranges of different parameters. Also, for the same flow rate in both heat exchanger sides, the effect of coil pitch and number of turns with the same coil torsion and tube length is remarkable on shell average Nusselt number while it is insignificant on HCT-average Nusselt number. In addition, a significant increase of 33.2–7.7% is obtained in the HCT-Fanning friction factor (fc) when δ increases from 0.0392 to 0.1194. Correlations for the average Nusselt numbers for both heat exchanger sides and the HCT Fanning friction factor as a function of the investigated parameters are obtained.

Experimental Analysis of the Heat Transfer Variations Within an Internal Passage of a Typical Gas Turbine Blade Using Varied Internal Geometries

2012 ◽  
Author(s):  
Todd Hahn ◽  
Bryant Deakins ◽  
Andrew Buechler ◽  
Sourabh Kumar ◽  
R. S. Amano
Keyword(s):  
Heat Transfer ◽  
Nusselt Number ◽  
Gas Turbine ◽  
Heat Transfer Rate ◽  
Turbine Blade ◽  
Experimental Analysis ◽  
Transfer Rate ◽  
Copper Foil ◽  
Nusselt Numbers ◽  
Turbulent Air Flow

This paper describes the experimental analysis of the heat transfer rate within an internal passage of a typical gas turbine blade using varied internal geometries. This method of alteration, using rib turbulator’s within the serpentine cooling passages of a hollow turbine blade, has proven to drastically cool turbine blades more significantly than a smooth channel alone. Our emphasis is to determine which rib geometry will yield the highest heat transfer rate, which was examined in the form of a comparison between theoretical to experimental Nusselt numbers. For testing purposes, an enclosed 2 in. × 2 in. square Plexiglas channel was constructed to model an internal cooling passage within a turbine blade. Silicon heat strips, wrapped in copper foil, were placed on the bottom surface of the channel to ensure even heat distribution throughout. To measure internal surface temperatures, thermocouples were placed on the surface of heat plate as well as in the opening of the channel throughout. The four different rib geometries which were individually wrapped in copper foil were then placed on top of the heating element. To compare the rib geometry results with a control, a test was run with no ribs. To simulate turbulent air flow through the channel, a blower supplied velocities of 23.88 m/s and 27.86 m/s. These velocities yielded a Reynolds number ranging between 70,000 and 90,000. Final results were found in the form of the experimental Nusselt number divided by the theoretical Nusselt number, a standard when comparing surface heat transfer rates. The 60 degree staggered arrow geometry pointing away from the inlet and outlet (geometry 4) proved to create the highest heat transfer rate through the way it produced turbulent air flow. The average Nusselt number of this design was found to be 718.2 and 868.3 for 23.88 and 27.86 m/s respectively. From the calculated data it was found that higher Nusselt numbers were more prone to occur in higher air velocities.

Sign in / Sign up

Export Citation Format

Share Document