Convective Heat Transfer Enhancement Due to Intermittency in an Impinging Jet

1993 ◽  
Vol 115 (1) ◽  
pp. 91-98 ◽  
Author(s):  
D. A. Zumbrunnen ◽  
M. Aziz
Keyword(s):  
Heat Transfer ◽  
Convective Heat Transfer ◽  
Convective Heat ◽  
Local Heat Transfer ◽  
Local Heat ◽  
Impinging Jets ◽  
Transfer Coefficients ◽  
Stagnation Line ◽  
High Heat Transfer ◽  
Short Period

An experimental investigation has been performed to study the effect of flow intermittency on convective heat transfer to a planar water jet impinging on a constant heat flux surface. Enhanced heat transfer was achieved by periodically restarting an impinging flow and thereby forcing renewal of the hydrodynamic and thermal boundary layers. Although convective heat transfer was less effective during a short period when flow was interrupted, high heat transfer rates, which immediately follow initial wetting, prevailed above a threshold frequency, and a net enhancement occurred. Experiments with intermittent flows yielded enhancements in convective heat transfer coefficients of nearly a factor of two, and theoretical considerations suggest that higher enhancements can be achieved by increasing the frequency of the intermittency. Enhancements need not result in an increased pressure drop within a flow system, since flow interruptions can be induced beyond a nozzle exit. Experimental results are presented for both the steady and intermittent impinging jets at distances up to seven jet widths from the stagnation line. A theoretical model of the transient boundary layer response is used to reveal parameters that govern the measured enhancements. A useful correlation is also provided of local heat transfer results for steadily impinging jets.

Convective Heat Transfer of Alumina Nanofluids in a Microchannel

2010 ◽  
Author(s):  
Saeid Vafaei ◽  
Dongsheng Wen
Keyword(s):  
Heat Transfer ◽  
Heat Transfer Coefficient ◽  
Transfer Coefficient ◽  
Convective Heat Transfer ◽  
Convective Heat ◽  
Local Heat Transfer ◽  
Local Heat ◽  
Axial Distance ◽  
Transfer Coefficients ◽  
Heat Transfer Coefficients

This work reports an experimental study of convective heat transfer of aqueous alumina nanofluids in a horizontal microchannel under laminar flow condition. The variation of local heat transfer coefficients, in both entrance and developed flow regime, is obtained as a function of axial distance. The heat transfer coefficient of nanofluids is found to be dependent upon not only nanoparticle concentration but also mass flow rate. Different to the behavior in conventional-sized channels, the major heat transfer coefficient enhancement is observed in fully developed region in microchannels. Discussions of the results suggest that the heterogeneous nature of nanoparticle flow should be considered.

scholarly journals Empirical mapping of the convective heat transfer coefficients with local hot spots on highly conductive surfaces

2017 ◽  
Vol 21 (3) ◽  
pp. 1231-1240
Author(s):  
Murat Tekelioğlu
Keyword(s):  
Heat Transfer ◽  
Convective Heat Transfer ◽  
Convective Heat ◽  
Hot Spots ◽  
Local Heat Transfer ◽  
Local Heat ◽  
Transfer Coefficients ◽  
Transfer Rates ◽  
Heat Transfer Coefficients ◽  
Convective Heat Transfer Coefficients

An experimental method was proposed to assess the natural and forced convective heat transfer coefficients on highly conductive bodies. Experiments were performed at air velocities of 0m/s, 4.0m/s, and 5.4m/s, and comparisons were made between the current results and available literature. These experiments were extended to arbitrary-shape bodies. External flow conditions were maintained throughout. In the proposed method, in determination of the surface convective heat transfer coefficients, flow condition is immaterial, i.e., either laminar or turbulent. With the present method, it was aimed to acquire the local heat transfer coefficients on any arbitrary conductive shape. This method was intended to be implemented by the heat transfer engineer to identify the local heat transfer rates with local hot spots. Finally, after analyzing the proposed experimental results, appropriate decisions can be made to control the amount of the convective heat transfer off the surface. Limited mass transport was quantified on the cooled plate.

Experimental Investigation of Local Heat Transfer Distribution on Smooth and Roughened Surfaces Under an Array of Angled Impinging Jets

2001 ◽  
Author(s):  
Lamyaa A. El-Gabry ◽  
Deborah A. Kaminski
Keyword(s):  
Heat Transfer ◽  
Reynolds Number ◽  
Cross Flow ◽  
Local Heat Transfer ◽  
Reynolds Numbers ◽  
Local Heat ◽  
Impinging Jets ◽  
Transfer Coefficients ◽  
Heat Transfer Coefficients ◽  
Roughened Surface

Abstract Measurements of the local heat transfer distribution on smooth and roughened surfaces under an array of angled impinging jets are presented. The test rig is designed to simulate impingement with cross-flow in one direction which is a common method for cooling gas turbine components such as the combustion liner. Jet angle is varied between 30, 60, and 90 degrees as measured from the impingement surface, which is either smooth or randomly roughened. Liquid crystal video thermography is used to capture surface temperature data at five different jet Reynolds numbers ranging between 15,000 and 35,000. The effect of jet angle, Reynolds number, gap, and surface roughness on heat transfer efficiency and pressure loss is determined along with the various interactions among these parameters. Peak heat transfer coefficients for the range of Reynolds number from 15,000 to 35,000 are highest for orthogonal jets impinging on roughened surface; peak Nu values for this configuration ranged from 88 to 165 depending on Reynolds number. The ratio of peak to average Nu is lowest for 30-degree jets impinging on roughened surfaces. It is often desirable to minimize this ratio in order to decrease thermal gradients, which could lead to thermal fatigue. High thermal stress can significantly reduce the useful life of engineering components and machinery. Peak heat transfer coefficients decay in the cross-flow direction by close to 24% over a dimensionless length of 20. The decrease of spanwise average Nu in the crossflow direction is lowest for the case of 30-degree jets impinging on a roughened surface where the decrease was less than 3%. The decrease is greatest for 30-degree jet impingement on a smooth surface where the stagnation point Nu decreased by more than 23% for some Reynolds numbers.

Heat Transfer Enhancement by Turbulent Impinging Jets

2000 ◽  
Author(s):  
M. Kumagai ◽  
R. S. Amano ◽  
M. K. Jensen
Keyword(s):  
Heat Transfer ◽  
Heat Transfer Coefficient ◽  
Transfer Coefficient ◽  
Stokes Equations ◽  
Local Heat Transfer ◽  
Local Heat ◽  
Impinging Jets ◽  
Transfer Model ◽  
Transfer Coefficients ◽  
Heat Transfer Coefficients

Abstract A numerical and experimental investigation on cooling of a solid surface was performed by studying the behavior of an impinging jet onto a fixed flat target. The local heat transfer coefficient distributions on a plate with a constant heat flux were computationally investigated with a normally impinging axisymmetric jet for nozzle diameter of 4.6mm at H/d = 4 and 10, with the Reynolds numbers of 10,000 and 40,000. The two-dimensional cylindrical Navier-Stokes equations were solved using a two-equation k-ε turbulence model. The finite-volume differencing scheme was used to solve the thermal and flow fields. The predicted heat transfer coefficients were compared with experimental measurements. A universal function based on the wave equation was developed and applied to the heat transfer model to improve calculated local heat transfer coefficients for short nozzle-to-plate distance (H/d = 4). The differences between H/d = 4 and 10 due to the correlation among heat transfer coefficient, kinetic energy and pressure were investigated for the impingement region. Predictions by the present model show good agreement with the experimental data.

Heat Transfer Measurements Using Multiple Thermochromic Liquid Crystals in Symmetric Cooling Channels

2020 ◽  
Author(s):  
Tobias Krille ◽  
Stefan Retzko ◽  
Rico Poser ◽  
Jens von Wolfersdorf
Keyword(s):  
Heat Transfer ◽  
Local Heat Transfer ◽  
Local Heat ◽  
Single Channels ◽  
Ambient Conditions ◽  
Transfer Coefficients ◽  
Experimental Conditions ◽  
Cooling Channels ◽  
Heat Transfer Coefficients ◽  
High Heat Transfer

Abstract The transient Thermochromic Liquid Crystal (TLC) method is applied to determine the distribution of the local heat transfer coefficients using a configuration with parallel cooling channels at an engine relevant Reynolds number. The rectangular channels with a moderate aspect ratio and a high length-to-diameter ratio are equipped with one-sided oblique ribs with high blockage, which is a promising configuration for turbine near wall cooling applications. In this arrangement, the three inner channels should experience same flow and thermal conditions. Numerical simulations are performed to substantiate this assumption. The symmetric single channels are sprayed with narrowband TLC with various indication temperatures. Multiple experiments were conducted. All start at ambient conditions before the fluid is heated up to several temperatures between 46°C and 73°C. The results show that the determined local heat transfer coefficients and therefore the Nusselt numbers vary significantly for the different experimental conditions especially at locations of high heat transfer coefficient behind the ribs. A simplified procedure with respect to measurement uncertainties is applied to enable an easy and fast valuation on the data quality. This might be used within the data reduction analysis for such experiments directly. The approach is illustrated using the obtained experimental data.

Convective Heat Transfer in a Triple-Cavity Structure Near Turbine Blade Trailing Edge

2002 ◽  
Author(s):  
Minking K. Chyu ◽  
Unal Uysal ◽  
Pei-Wen Lee
Keyword(s):  
Heat Transfer ◽  
Convective Heat Transfer ◽  
Turbine Blade ◽  
Convective Heat ◽  
Internal Heat ◽  
Local Heat Transfer ◽  
Local Heat ◽  
Trailing Edge ◽  
Cavity Structure ◽  
Exit Slot

The present study explores the internal heat transfer in a triple-cavity cooling structure with a ribbed lip for a turbine blade trailing edge. The design consists of two impingement cavities, two sets of crossover holes, a third cavity and an exit slot with eleven ribs attached to it. Local heat transfer in each subregion is determined. Results indicate that the highest heat transfer occurs in the second impingement cavity. The exit slot area between the ribs is identified as a region of low heat transfer in the overall design. A comparison with enhancement induced by arrays of pin fins and fins of other geometries reveals that the triple-cavity design represents a lesser quality cooling scheme in the range of Reynolds numbers tested. Further improvement of the convective heat transfer at the exit slot with either film cooling, or different rib geometries appears to be essential to make the triple-cavity strategy superior to those of the traditional approaches for cooling of blade trailing edge.

Forced Convective Heat Transfer in Cross-Corrugated Solar Air Heaters

1994 ◽  
Vol 116 (4) ◽  
pp. 212-214 ◽  
Author(s):  
Y. Piao ◽  
E. G. Hauptmann ◽  
M. Iqbal
Keyword(s):  
Heat Transfer ◽  
Convective Heat Transfer ◽  
Convective Heat ◽  
Fluid Velocity ◽  
Local Heat Transfer ◽  
Local Heat ◽  
Working Fluid ◽  
Solar Air Heater ◽  
Forced Convective Heat Transfer ◽  
Radiant Heater

Forced convective heat transfer in a cross-corrugated channel solar air heater has been studied experimentally using air as a working fluid. The channel was formed by two transversely positioned corrugated sheets and two flat thermally insulated side walls. One corrugated sheet was heated by a radiant heater, while the other was thermally insulated. The fluid velocity and temperature, and the wall temperature and the local heat flux across the heated corrugated sheet were measured for a variety of operating flow rates. Experimental results for the channel geometry have yielded the correlation Nu=0.0743(Re)0.76. This heat-transfer coefficient is about 2.8 times that of a smooth flat channel. The experiments showed that local heat transfer rate was smaller on the valley of the corrugation than that on the peak. The ratio of the local heat transfer rates on the two locations was related to the Reynolds number.

scholarly journals An Interferometric Study of Free Convective Heat Transfer in a Double Glazed Window With a Between-Panes Venetian Blind

2021 ◽  
Author(s):  
Bertha Lai
Keyword(s):  
Heat Transfer ◽  
Convective Heat Transfer ◽  
Convective Heat ◽  
Numerical Study ◽  
Local Heat Transfer ◽  
Local Heat ◽  
Test Fluid ◽  
Flat Plates ◽  
Set Up ◽  
Free Convective Heat Transfer

The free convective heat transfer in a double-glazed window with between-panes Venetian blinds was measured using a Mach-Zehnder interferometer. A vertical cavity with differentially heated/cooled flat plates was set up with an internal blind at slat angles of ø=0⁰, ø=45⁰, and ø=90⁰ from the horizontal and tip-to-plate spacings of s=2mm, s=4mm, and s=8mm. Heat transfer measurements were taken with air as the test fluid and at Rayleigh numbers of Ra~4.5x10(4), RA~6.7X10(4), and Ra~13.1x10(4), based on cavity widths of W=28.7mm, W=32.7mm, and W=40.7mm, respectively. Finite fringe interferograms were used to obtain local and average heat transfer data. Infinite fringe interferograms were taken to visualize the temperature field within the cavity. A preliminary numerical study of the experimental geometry was also conducted. The results show that there was substantial variation in local heat transfer rates caused by the presence of the between-panes blind inside the window cavity. In general, experimental average Nusselt numbers were found to be lower than those of a cavity without blinds.

The Influence of Flexible Strip Height on Convective Heat Transfer Enhancement

2019 ◽  
Author(s):  
Yang Yang ◽  
David S.-K. Ting ◽  
Steve Ray
Keyword(s):  
Heat Transfer ◽  
Convective Heat Transfer ◽  
Heat Transfer Enhancement ◽  
Convective Heat ◽  
Local Heat Transfer ◽  
Local Heat ◽  
Vortical Flow ◽  
Transfer Enhancement ◽  
Local Flow ◽  
Convective Heat Transfer Enhancement

Abstract A 12.7 mm wide flexible rectangular strip, made from 0.1 mm-thick aluminum sheet, is experimentally explored as a vortical flow generator for promoting heat convection from a flat plate in a wind tunnel. The strip is positioned normal to the freestream with an incoming velocity of 10 m/s, resulting in a Reynolds number, based on the strip width, of 8,500. The influence of the height of the flexible strip on the convective heat transfer enhancement is of interest. Three strip heights, 25.4 mm, 38.1 mm and 50.8 mm, were investigated. The heat transfer results are expressed in terms of Nusselt number, Nu, normalized by the unperturbed reference Nu0. The shortest, 25.4 mm high flexible strip resulted in the highest peak and overall heat transfer enhancement. The distribution of the local heat transfer enhancement is found to correlate well with the turbulent flow motion detailed using a triple-sensor hot-wire anemometer. Pointedly, the heat transfer rate is most elevated when the local flow is moving toward the hot plate, sweeping across a stretch of the surface before moving away from it. These effective convective motions are most effectively generated by the vortices produced by the shortest strip.

Flow Distribution and Heat Transfer Coefficients Inside Gas Holes Discharging Into an Orthogonal Crossflow

2011 ◽  
Author(s):  
S. Acharya ◽  
A. Eshtiaghi ◽  
R. Schilp
Keyword(s):  
Heat Transfer ◽  
Heat Transfer Coefficient ◽  
Transfer Coefficient ◽  
Flow Simulation ◽  
Local Heat Transfer ◽  
Local Heat ◽  
Transfer Coefficients ◽  
Ir Camera ◽  
Blowing Ratio ◽  
High Heat Transfer

Fluid flow and heat transfer coefficient associated with flow inside short holes (L/D = 1) discharging orthogonally into a crossflow was investigated experimentally and numerically for Re ranging from 0.5×105 to 2×105, and blowing ratio ranging from 1.3 to 3.2. The basic configuration studied consists of a feed tube with five orthogonally located gas holes. Four different hole configurations were studied. The transient heat transfer study employs an IR-camera to determine the local heat transfer coefficient inside each hole. Velocity measurements and numerical flow simulation were used to better understand the measured heat transfer distribution inside the hole. The Nusselt number distribution along the hole surface exhibits significant circumferential non-uniformity associated with impingement and separation, with localized high heat transfer regions caused by flow impingement. The heat transfer coefficient was observed to be a strong function of the Reynolds number, but a weak function of the blowing ratio.

Sign in / Sign up

Export Citation Format

Share Document