Jenn-Jiang Hwang

Measurement of Interstitial Convective Heat Transfer and Frictional Drag for Flow Across Metal Foams

Convective heat transfer and friction drag in a duct inserted with aluminum foams have been studied experimentally. The combined effects of foam porosity (e=0.7, 0.8, and 0.95) and flow Reynolds number (1900≤Re≤7800) are examined. Frictional drags for flow across the aluminum foam are measured by pressure taps, while interstitial heat transfer coefficients in the aluminum foam are determined using a transient single-blow technique with a thermal non-equilibrium two-equation model. Solid material temperature distribution is further measured for double check of the heat transfer results. To understand the frictional drag mechanisms, smoke-wire flow visualization is conducted in the aluminum-foam ducts. Results show that both the friction factor and the volumetric heat transfer coefficient increase with decreasing the foam porosity at a fixed Reynolds number. In addition, the aluminum foam of e=0.8 has the best thermal performance under the same pumping power constraint among the three aluminum foams investigated. Finally, empirical correlations for pore Nusselt number are developed in terms of pore Reynolds number under various foam porosities

Simulation and measurement of enhanced turbulent heat transfer in a channel with periodic ribs on one principal wall

This paper performs a numerical and experimental analysis to investigate the heat transfer and fluid flow behavior in a rectangular channel flow with streamwise-periodic ribs mounted on one of the principal walls. The k-e-A PDM turbulence model together with a smoothed hybrid central/skew upstream difference scheme (SCSUDS) and the PISO pressure-velocity coupling algorithm was applied to solving the accelerated, separated and recirculating flows. The real-time holographic interferometry technique was adopted to measure the time-dependent temperature field in the ribbed duct. The predicted fluid flow and temperature field were tested by previous LDV measurements and present holographic interferometry data, and reasonable agreement was achieved. By the examination of the local wall temperature distribution for the uniform wall heat flux (UHF) boundary condition the regions susceptible to the hot spots are identified. Moreover, the study provided the numerical solution to investigate the effect of geometry and flow parameters on the local as well as average heat transfer coefficients. The compact correlation of the average heat transfer coefficient was further developed and accounted for the rib height, rib spacing, and Reynolds number.

Effect of ridge shapes on turbulent heat transfer and friction in a rectangular channel

Abstract Turbulent heat transfer and friction in a channel with various-shaped ridges mounted on two opposite walls has been investigated experimentally. Experiments were conducted for three ridge shapes, namely triangular, semicircular, and square cross sections. The ridges are oriented transversely to the main stream in a periodic arrangement. The technique of real-time holographic interferometry is used to measure local and average heat transfer. The ridge pitch-to-height (radius) ratio varies from 8.0 to 20; the Reynolds number varies from 7800 to 50 000; and the ridge height(radius)-to-channel hydraulic diameter ratio is 0.08. The results show that the three types of ridged channels have comparable thermal performances. It is also noted that the semicircular and triangular ridge geometries are less likely to yield the hot-spots behind the ridges than the square one. Moreover, a comparison of the average heat transfer coefficient of the wall with thermally active ridges and that with thermally nonactive ridges is made. Compact correlations in terms of roughness parameters are developed for both the fully developed heat transfer coefficients and friction factors. The correlations can be used in the design of turbine blade cooling passages.

Turbulent Heat Transfer Augmentation and Friction in Periodic Fully Developed Channel Flows

Measurements are presented of the distribution of average friction factors (f) as well as local and average ({ovr Nu}) heat transfer coefficients for fully developed channel flows with two rib-roughened opposite walls. The temperature measurements were made by using both a laser holographic interferometer and thermocouples. In addition, the reattachment length was determined by flow visualization. The Reynolds number (Re) was varied from 5.0 {times} 10{sup 3} to 5.4 {times} 10{sup 4}; the rib pitch-to-height ratios (Pi/H) were 10, 15, 20; and the rib height-to-hydraulic diameter ratios (H/De) were 0.063, 0.081, and 0.106. The detailed results allowed the peaks of heat transfer augmentation and the regions susceptible to hot spots to be located and allowed the relative contribution of the rib surface and the channel wall to the heat transfer augmentation to be determined. Moreover, relative to a smooth duct, the enhancement of both {ovr Nu} and f at various Re, Pi/H, and H/De was documented in detail. Furthermore, compact correlations in terms of Re, Pi/H, and H/De were developed for both {ovr Nu} and f.

Thermal-Electrochemical Modeling of a Proton Exchange Membrane Fuel Cell

A thermal-electrochemical coupled model is presented to predict electrochemical and heat transfer behaviors in a proton exchange membrane (PEM) fuel cell. The Brinkman extension to Darcy flow describes the fluid flow characteristics in the porous electrodes. The Stefan-Maxwell correlations together with the Bruggemann modification illustrate the multispecies diffusion in the porous electrode. A two-equation approach is used to account for the local thermal nonequilibrium between the solid matrices and the fluids in the gas diffusion layers. In the catalyst layers, the heat dissipation due to irreversible-process heating is determined from the macroscopic electrochemical model. The present model is capable of simultaneously predicting the solid phase temperature and the fluid phase temperature inside the fuel cell, which enables a comprehensive understanding of the mechanisms responsible for thermal pathways. Most importantly, it has successfully assessed the possibility of hot spots within a PEM fuel cell. Increasing the interfacial heat-transfer coefficient between the solid phase and the fluid phase (hv) from 1.0 X 10 3 to 1.0 X 10 6 W/m 3 K has an advantage of alleviating the hot spot. Thermal effects on the active material degradation and hence fuel cell cycle life will be incorporated in the future work.

Measurement of endwall heat transfer and pressure drop in a pin-fin wedge duct

Abstract This paper discusses the measurements of endwall heat transfer and pressure drop in a wedge-shaped duct inserted with an array of circular pin fins. The endwall surface is coated with a thin layer of thermochromic liquid crystals and a transient test is run to obtain detailed heat transfer distributions. Parametric studies include Reynolds number (10,000⩽ Re ⩽50,000), outlet flow orientation (straight and lateral) and pin configuration (staggered and in-line). The wedge duct has a convergent angle of 12.7°. The pin spacing-to-diameter ratios along the longitude and transverse directions are fixed at s x / d = s y / d =2.5. Pin-less wedge duct results are also obtained for comparison. Results indicated that the straight wedge duct with a staggered pin array is most recommended because of its significant endwall heat transfer and moderate pressure-drop penalty; while the turned wedge duct with a staggered pin array is least recommended since it yields the highest pressure drops and raises severe hot spots. A similarity of the pin Reynolds-number dependence of row-averaged Nusselt number is developed in the present wedge duct of accelerating flow.

Heat transfer and friction in a low-aspect-ratio rectangular channel with staggered perforated ribs on two opposite walls

Experiments are conducted to study the turbulent heat transfer and friction in a low-aspect-ratio rectangular channel in which two opposite walls are roughened by perforated ribs. The perforated ribs are arranged in a staggered manner. Effects of perforated rib open-area ratio (β = 10, 22, 38, 44, and 50 percent), rib pitch-to-height ratio (PR = 5, 10, 15, and 20), rib height-to-channel hydraulic diameter ratio (H/De = 0.063 and 0.081), rib alignment (staggered and symmetric), and Reynolds number (10,000 ≤ Re ≤ 50,000) are examined. It is found that approximately the same heat transfer enhancement and pressure drop penalty are obtained between symmetric and staggered rib arrangements. A permeability criterion is proposed by tracing heat transfer coefficient distributions, which compares well with previous flow-visualization results. Results also show that ribs with β = 44 percent give the best thermal performance under either the constant friction power or the constant flow rate constraint. Roughness functions for friction and heat transfer are further developed in terms of rib and flow parameters.

Heat Transfer-Friction Characteristic Comparison in Rectangular Ducts With Slit and Solid Ribs Mounted on One Wall

A comparison of fully developed heat transfer and friction characteristics has been made in rectangular ducts with one wall roughened by slit and solid ribs. The effects of rib void fraction and flow Reynolds number are examined. The rib height-to-duct hydraulic diameter and pitch-to-height ratios are fixed at H/D e = 0.167 and P i /H = 10, respectively. To understand the mechanisms of the heat transfer enhancement, smoke-wire flow visualization and measurements of mean velocity and turbulence intensity are conducted in the slit and solid-ribbed ducts. In addition, by separately measuring the floor and rib heat transfer, two contributive factors of heat transfer promotion, namely, the fin effect and the enhanced turbulence effect, have been isolated. Because of the greater turbulence-mixing effects the slit-ribbed geometry displays higher floor heat transfer than the solid-ribbed geometry. In addition, the fin effects for the slit rib are greater than that for the solid rib. The pressure drop across the slit ribs is lower than that across the solid ribs due to less duct blockage. Furthermore, slit ribs with larger void fractions in a lower flow Reynolds number range provide better thermal performance under a constant friction power constraint

Heat transfer in a rectangular channel with perforated turbulence promoters using holographic interferometry measurement

An experimental study is conducted to examine the effect of perforated ribs on turbulent heat transfer and friction in a rectangular channel. Perforated ribs with an open-area ratio of 50% are staggered on two opposite walls of the channel. Four rib pitch-to-height ratios (Pi/H = 5, 10, 15 and 20) and two rib height-to-channel hydraulic diameter ratios (H/De = 0.081 and 0.162) are examined. The Reynolds number ranges from 10 000 to 50 000. To facilitate comparison, results of conventional solid-type ribs are also compiled. Laser holographic interferometry is employed to measure the heat transfer coefficients of the ribbed wall. The results indicate that the perforated ribs have the advantages of eliminating the hot-spots and providing a superior heat transfer performance. Roughness friction and heat transfer correlations are also developed in terms of flow and rib parameters.

Augmented Heat Transfer in a Rectangular Channel With Permeable Ribs Mounted on the Wall

Turbulent heat transfer and friction in a rectangular channel with perforated ribs arranged on one of the principal walls are investigated experimentally. The effects of rib open-area ratio, rib pitch-to-height ratio, rib height-to-channel hydraulic diameter ratio, and flow Reynolds number are examined. To facilitate comparison, measurements for conventional solid-type ribs are also conducted. Laser holographic interferometry is employed to determine the rib permeability and measure the heat transfer coefficients of the ribbed wall. Results show that ribs with appropriately high open-area ratio at high Reynolds number range are permeable, and the critical Reynolds number of initiation of flow permeability decreases with increasing rib open-area ratio. By examining the local heat transfer coefficient distributions, it is found that permeable ribbed geometry has an advantage of obviating the possibility of hot spots. In addition, the permeable ribbed geometry provides a higher thermal performance than the solid-type ribbed one, and the best thermal performance occurs when the rib open-area ratio is 0.44. Compact heat transfer and friction correlations are also developed for channels with permeable ribs.