Confinement Effects in Heat Transfer to a Miniature Compressible Impinging Air Jet

2007 ◽  
Author(s):  
Thomas L. Lupton ◽  
Darina B. Murray ◽  
Anthony J. Robinson
Keyword(s):  
Heat Transfer ◽  
Radial Distance ◽  
Local Heat Transfer ◽  
Local Heat ◽  
Heat Transfer Analysis ◽  
Transfer Coefficients ◽  
Adiabatic Wall ◽  
Air Jet ◽  
High Heat Transfer ◽  
Impingement Surface

The decreasing physical size of microchips accompanied by the increasing heat flux to be dissipated has led to the study of possible new innovative electronic cooling methods. Jet cooling has been successfully implemented in a variety of industrial applications and its capacity to maintain high heat transfer rates demonstrates its vast potential for incorporation into future cooling applications. The reduction in size of electronic components leads to the inevitability of jets used in this area being subjected to a degree of jet confinement. In this study the effects of confinement on the local heat transfer characteristics were experimentally investigated for a turbulent, fully developed, axisymmetric, compressible and submerged miniature air jet impinging normally onto an ohmically heated flat plate. The resulting surface temperature distribution was recorded via infrared thermography. Two 1mm diameter nozzles were examined, one confined and one unconfined. Tests were conducted for nozzle exit to impingement surface spacings of 1, 2, 4 and 6 jet diameters and for Reynolds numbers of 7,000 and 12,000 which corresponded to Mach numbers of 0.3 and 0.5. The heat transfer analysis accounts for compressibility effects by using the adiabatic wall temperature as the reference temperature in calculation of the heat transfer coefficients. Local heat transfer distributions are presented as a function of radial distance from the stagnation point. The results obtained indicate that confinement contributes to a flatter distribution of heat transfer over the impingement surface.

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.

Local Heat Transfer Coefficients Under an Axisymmetric, Single-Phase Liquid Jet

1991 ◽  
Vol 113 (1) ◽  
pp. 71-78 ◽  
Author(s):  
J. Stevens ◽  
B. W. Webb
Keyword(s):  
Heat Transfer ◽  
Nusselt Number ◽  
Local Nusselt Number ◽  
Single Phase ◽  
Radial Distance ◽  
Local Heat Transfer ◽  
Local Heat ◽  
Transfer Coefficients ◽  
Heat Transfer Coefficients ◽  
Plate Spacing

The purpose of this investigation was to characterize local heat transfer coefficients for round, single-phase free liquid jets impinging normally against a flat uniform heat flux surface. The problem parameters investigated were jet Reynolds number Re, nozzle-to-plate spacing z, and jet diameter d. A region of near-constant Nusselt number was observed for the region bounded by 0≤r/d≤0.75, where r is the radial distance from the impingement point. The local Nusselt number profiles exhibited a sharp drop for r/d > 0.75, followed by an inflection and a slower decrease there-after. Increasing the nozzle-to-plate spacing generally decreased the heat transfer slightly. The local Nusselt number characteristics were found to be dependent on nozzle diameter. This was explained by the influence of the free-stream velocity gradient on local heat transfer, as predicted in the classical analysis of infinite jet stagnation flow and heat transfer. Correlations for local and average Nusselt numbers reveal an approximate Nusselt number dependence on Re1/3.

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.

Effect of Acoustic Excitation on the Heat Transfer to an Impinging Air Jet

2007 ◽  
Author(s):  
Tadhg S. O’Donovan ◽  
Darina B. Murray
Keyword(s):  
Heat Transfer ◽  
Excitation Frequency ◽  
High Heat ◽  
Cooling Capacity ◽  
Mean Flow ◽  
Transfer Coefficients ◽  
Air Jets ◽  
Air Jet ◽  
High Heat Transfer ◽  
Impingement Surface

Impinging air jets are known as a method of achieving particularly high heat transfer coefficients and are employed in many applications including the cooling of electronics, manufacturing processes such as grinding, etc. The current investigation is concerned with acoustically exciting an impinging air jet to enhance its overall cooling capacity. Distributions of the heat transfer to an axially impinging air jet for a range of Reynolds numbers (Re) from 10000 to 30000, non-dimensional nozzle to impingement surface heights (H/D) from 0.5 to 2 and excitation frequencies (f) that range from 0.5 to 1 times the natural frequency of the jet are presented. For this low range of nozzle to impingement surface spacings it has been shown that the heat transfer distribution exhibits a peak at the stagnation point and secondary peaks at a radial location that is both excitation frequency and Reynolds number dependent. Distributions of the fluctuating component of the heat transfer coefficient are also presented for the range of parameters tested. These have been used, along with spectral analysis of the heat flux signal, to discern whether local variations in heat transfer are due to changes in the local vortex flow or to changes in the mean flow structure of the impinging jet.

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.

scholarly journals Heat Transfer Analysis and Modification of Thermal Probe for Gas-Solid Measurement

2016 ◽  
Vol 2016 ◽  
pp. 1-7
Author(s):  
Hong Zhang ◽  
Xiangying Qi
Keyword(s):  
Heat Transfer ◽  
Local Heat Transfer ◽  
Local Heat ◽  
Heat Transfer Analysis ◽  
Transfer Coefficients ◽  
Thermal Probe ◽  
Measuring Range ◽  
Two Phase ◽  
Heat Transfer Coefficients ◽  
Thermal Simulations

The presented work aims to measure the gas-solid two-phase mass flow-rate in pneumatic conveyor, and a novel modified thermal probe is applied. A new analysis of the local heat transfer coefficients of thermal probe is presented, while traditional investigations focus on global coefficients. Thermal simulations are performed in Fluent 6.2 and temperature distributions of the probe are presented. The results indicate that the probe has obviously stable and unstable heat transfer areas. Based on understanding of probe characteristics, a modified probe structure is designed, which makes the probe output signal more stable and widens the measuring range. The experiments are carried out in a special designed laboratory scale pneumatic conveyor, and the modified probe shows an unambiguous improvement of the performance compared with the traditional one.

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

2021 ◽  
pp. 1-22
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.

Microscale Heat Transfer of Confined Miniature Jets

2006 ◽  
Author(s):  
Colin Glynn ◽  
Anthony J. Robinson ◽  
Darina B. Murray ◽  
Thomas L. Lupton
Keyword(s):  
Heat Transfer ◽  
Heat Transfer Coefficient ◽  
Transfer Coefficient ◽  
Local Heat Transfer ◽  
Local Heat ◽  
Transfer Coefficients ◽  
Number Range ◽  
Microscale Heat Transfer ◽  
Air Jet ◽  
Experimental Apparatus

An experimental study of the microscale heat transfer characteristics of a 1.22 mm confined miniature jet is presented for both air and water. The experimental apparatus utilises infrared thermography to measure the temperature profiles on the underside of a heated thin-foil upon which the jet is impinging. The local heat transfer coefficient calculations account for lateral conduction within the foil, which, in contrast to conventional practice, has been shown to be non-negligible. The tests are carried out over a jet Reynolds number range of 5000–20000 and non-dimensional jet to target spacing (H/d) range of 2–5. Results are presented in terms of local heat transfer coefficients. It has been determined that (i) lateral heat conduction effects are significant for the air jet measurements but are negligible for the water jet results, and (ii) the two fluids display hugely different heat transfer coefficient profiles for ostensibly the same non-dimensional flow conditions.

Local Heat Transfer to Staggered Arrays of Impinging Circular Air Jets

1983 ◽  
Vol 105 (2) ◽  
pp. 354-360 ◽  
Author(s):  
A. I. Behbahani ◽  
R. J. Goldstein
Keyword(s):  
Heat Transfer ◽  
Reynolds Number ◽  
Flat Plate ◽  
Flow Direction ◽  
Local Heat Transfer ◽  
Local Heat ◽  
Transfer Coefficients ◽  
Heat Transfer Coefficients ◽  
Air Jets ◽  
Impingement Surface

Measurements are made of the local heat transfer from a flat plate to arrays of impinging circular air jets. Fluid from the spent jets is constrained to flow out of the system in one direction. Two different jet-to-jet spacings, 4 and 8 jet diameters, are employed. The parameters that are varied include jet-orifice-plate to impingement-surface spacing and jet Reynolds number. Local heat transfer coefficients vary periodically both in the flow direction and across the span with high values occurring in stagnation regions. Stagnation regions of individual jets as determined by local heat transfer coefficients move further in the downstream direction as the amount of crossflow due to upstream jet air increases. Local heat transfer coefficients are averaged numerically to obtain spanwise and streamwise-spanwise averaged heat transfer coefficients.

Entrainment Effects on Impingement Heat Transfer: Part II—Local Heat Transfer Measurements

1985 ◽  
Vol 107 (4) ◽  
pp. 910-915 ◽  
Author(s):  
B. R. Hollworth ◽  
L. R. Gero
Keyword(s):  
Heat Transfer ◽  
Local Heat Transfer ◽  
Target Surface ◽  
Local Heat ◽  
Impinging Jets ◽  
Transfer Coefficients ◽  
Air Jet ◽  
Impingement Heat Transfer ◽  
Local Recovery ◽  
The Difference

Convective heat transfer was measured for a heated axisymmetric air jet impinging on a flat surface. It was found that the local heat transfer coefficient does not depend explicitly upon the temperature mismatch between the jet fluid and the ambient fluid if the convection coefficient is defined in terms of the difference between the local recovery temperature and target surface temperature. In fact, profiles of local heat transfer coefficients defined in this manner were found to be identical to those measured for isothermal impinging jets.

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