Jet impingement onto a tapered hole: Influence of jet velocity and hole wall velocities on heat transfer and skin friction

2009 ◽  
Vol 60 (9) ◽  
pp. 972-991 ◽  
Author(s):  
S. Z. Shuja ◽  
B. S. Yilbas ◽  
S. M. A. Khan
Keyword(s):  
Heat Transfer ◽  
Skin Friction ◽  
Jet Impingement ◽  
Hole Wall ◽  
Jet Velocity ◽  
Tapered Hole

Flow into a hole in relation to laser drilling: influence of coating thickness

2014 ◽  
Vol 24 (5) ◽  
pp. 988-1004
Author(s):  
Shahzada Zaman Shuja ◽  
Bekir Yilbas
Keyword(s):  
Heat Transfer ◽  
Nusselt Number ◽  
Flow Field ◽  
Skin Friction ◽  
Wall Temperature ◽  
Base Material ◽  
Jet Impingement ◽  
Content Type ◽  
Transfer Rates ◽  
Hole Wall

Purpose – In laser drilling applications, hole wall remains almost the melting temperature of the substrate material and the thermodynamic pressure developed at high temperature molten surface vicinity influences the heat transfer rates and the skin friction at the surface of the hole wall. This effect becomes complicated for the holes drilled in coated substrates. In this case, melting temperatures of the coating and base materials are different, which in turn modifies the flow field in the hole due to jet impingement. Consequently, investigation of the heat transfer rates from the hole wall surfaces and the skin friction at the hole surface becomes essential. The paper aims to discuss these issues. Design/methodology/approach – Numerical solution for jet impingement onto a hole with high wall temperature is introduced. Heat transfer rates and skin friction from the hole wall is predicted. The numerical model is validated with the experimental data reported in the open literature. Findings – The Nusselt number attains high values across the coating thickness and it drops sharply at the interface between the coating and the base material in the hole. Since fluid temperature in the vicinity of the substrate surface is higher than that of the wall temperature, heat transfer occurs from the fluid to the substrate material while modifying the Nusselt number along the hole wall. This results in discontinuity in the Nusselt variation across the coating-base material interface. The Raighly line effect enhances the flow acceleration toward the hole exit while increasing the rate of fluid strain. Consequently, skin friction increases toward the hole exit. The influence of average jet velocity on the Nusselt number and the skin friction is significant. Research limitations/implications – The findings are very useful to analyze the flow field in the hole at different wall temperature. In the simulations hole diameter is fixed in line with the practical applications. However, it may be changed to examine the influence of hole diameter on the flow field and heat transfer. However, this extension be more toward academic study than the practical significance. Practical implications – The complete modeling of turbulent flow jet flow impinging onto a hole is introduced and boundary conditions are well defined for the numerical solutions. The method of handing the physical problem will be useful for those working in the area of heat transfer and fluid flow. In addition, the importance of heat transfer rates and skin friction at the hole wall is established, which will benefit the practical engineers and the academicians working in the specific area of laser machining. Social implications – The findings are useful for those working to improve the laser technology in the machining area. Originality/value – The work presented is original and never being published anywhere else. The findings are reported in detail such that academicians and engineers are expected to benefit from this original contribution.

Novel Jet Impingement Cooling Geometry for Combustor Liner Backside Cooling

2009 ◽  
Vol 1 (2) ◽  
Author(s):  
E. I. Esposito ◽  
S. V. Ekkad ◽  
Yong Kim ◽  
Partha Dutta
Keyword(s):  
Heat Transfer ◽  
Velocity Profile ◽  
Jet Impingement ◽  
Local Heat Transfer ◽  
Reynolds Numbers ◽  
Impinging Jets ◽  
Transfer Coefficients ◽  
Jet Velocity ◽  
Corrugated Wall ◽  
Combustor Liner

Impinging jets are commonly used to enhance heat transfer in modern gas turbine engines. Impinging jets used in turbine blade cooling typically operate at lower Reynolds numbers in the range of 10,000–20,000. In combustor liner cooling, the Reynolds numbers of the jets can be as high as 60,000. The present study is aimed at experimentally testing two different styles of jet impingement geometries to be used in backside combustor cooling. The higher jet Reynolds numbers lead to increased overall heat transfer characteristics, but also an increase in crossflow caused by spent air. The crossflow air has the effect of rapidly degrading the downstream jets at high jet Reynolds numbers. In an effort to increase the efficiency of the coolant air, configurations designed to reduce the harmful effects of crossflow are studied. Two main designs, a corrugated wall and extended port, are tested. Local heat transfer coefficients are obtained for each test section through a transient liquid crystal technique. Results show that both geometries reduce the crossflow induced degradation on downstream jets, but different geometries perform better at different Reynolds numbers. The extended port and corrugated wall configurations show similar benefits at the high Reynolds numbers, but at low Reynolds numbers, the extended port design increases the overall level of heat transfer. This is attributed to the developed jet velocity profile at the tube exit. The best possible explanation is that the benefit of the developed jet velocity profile diminishes as jet velocities rise and the air has lesser time to develop prior to exiting.

Comparing Extended Port and Corrugated Wall Jet Impingement Geometry for Combustor Liner Backside Cooling

2007 ◽  
Author(s):  
E. I. Esposito ◽  
S. V. Ekkad ◽  
Yong Kim ◽  
Partha Dutta
Keyword(s):  
Heat Transfer ◽  
Velocity Profile ◽  
Jet Impingement ◽  
Local Heat Transfer ◽  
Reynolds Numbers ◽  
Impinging Jets ◽  
Transfer Coefficients ◽  
Jet Velocity ◽  
Corrugated Wall ◽  
Combustor Liner

Impinging jets are commonly used to enhance heat transfer in modern gas turbine engines. Impinging jets used in turbine blade cooling typically operate at lower Reynolds numbers in the range of 10,000 to 20,000. In combustor liner cooling, the Reynolds numbers of the jets can be as high as 60,000. The present study is aimed at experimentally testing two different styles of jet impingement geometries to be used in backside combustor cooling. The higher jet Reynolds numbers lead to increased overall heat transfer characteristics, but also an increase in crossflow caused by spent air. The crossflow air has the effect of rapidly degrading the downstream jets at high jet Reynolds numbers. In an effort to increase the efficiency of the coolant air, configurations designed to reduce the harmful effects of crossflow are studied. Two main designs, a corrugated wall and extended ports, are tested. Local heat transfer coefficients are obtained for each test section through a transient liquid crystal technique. Results show that both geometries reduce the crossflow induced degradation on downstream jets, but the individual geometries perform better at different Reynolds numbers. The extended port and corrugated wall configurations show similar benefits at the high Reynolds numbers, but at low Reynolds numbers, the extended port design increases the overall level of heat transfer. This is attributed to the developed jet velocity profile at the tube exit. The benefit of the developed jet velocity profile diminishes as jet velocities rise and the air has less time to develop prior to exiting.

Heat Transfer Enhancement of Impinging Row Jets in Cross-Flow with Mounting Baffles on Surface

2014 ◽  
Vol 931-932 ◽  
pp. 1218-1222
Author(s):  
Rattanakorn Pansang ◽  
Makatar Wae-Hayee ◽  
Passakorn Vessakosol ◽  
Chayut Nuntadusit
Keyword(s):  
Heat Transfer ◽  
Cross Flow ◽  
Jet Impingement ◽  
Transfer Characteristic ◽  
Impinging Jets ◽  
Upstream Region ◽  
Orifice Diameter ◽  
Thermochromic Liquid Crystal ◽  
Cross Flow Velocity ◽  
Jet Velocity

The aim of this research is to enhance heat transfer on a surface of row of impinging jets in cross-flow by mounting some baffles on the surface. A row of 4 jets with inline arrangement discharging from round orifices impinged normally on inner surface of wind tunnel with simulated cross-flow. The orifice diameter (D) was 13.2 mm. The jet-to-surface distance and jet-to-jet distance were fixed at H=2D and S=3D, respectively. Four couples of baffles with V-shaped arrangement at attack angle, θ=30o, were mounted on surface in upstream or downstream of impinging jets and the location of baffles attachment is L=1.5D apart from the jet impingement region. The velocity ratios (Jet velocity/cross-flow velocity) were varied from VR=3, 5 and 7 while the jet velocity was kept constant corresponding to Re=13,400. The experimental investigation was carried out for heat transfer characteristic by using Thermochromic Liquid Crystal sheet, and heat transfer coefficient distributions were evaluated using an image processing method. The results show that the impinging jets with mounting the baffles in the upstream region of jet impingement region can enhance the heat transfer rate throughout VR.

The Local and Average Heat Transfer Characteristic of Confined Jet Array Impingement Boiling of Aqueous Ethylene Glycol Solutions

2013 ◽  
Author(s):  
F. J. Hong ◽  
C. Y. Zhang ◽  
W. He ◽  
P. Cheng ◽  
G. Chen
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Ethylene Glycol ◽  
Critical Heat Flux ◽  
Wall Temperature ◽  
Heating Surface ◽  
Jet Impingement ◽  
Jet Velocity ◽  
Aqueous Ethylene Glycol ◽  
Jet Array

Liquid Jet impingement cooling is deemed as one of the most promising high heat flux cooling technologies. Compared with single phase cooling, two-phase cooling has advantages of more uniform heating surface temperature, lower pressure drop and less mass flow rate. In this paper, a closed-loop experimental setup is built to study confined jet array impingement boiling of 43% mass concentration aqueous ethylene glycol solution. The rectangular heating surface made of thin metal film is 20 mm × 40 mm and with the thickness of 0.03 mm. The in-line jet array has the jet orifice diameter d = 1 mm, the dimensionless jet-to-target spacing H/d = 1, and the dimensionless jet-to-jet spacing S/d = 5. The experiments are performed at atmospheric pressure to explore the effects of jet impingement velocity and liquid subcooling. The tested jet velocity is 0.2, 0.31 and 0.5 m/s respectively, while the inlet subcooling is ranged from 36°C to 96°C. The results showed that wall temperature and even heat transfer mode at different locations of the heating surface are quite different, with the lowest temperature on the heating surface directly under the jets and the highest temperature on the heating surface under the center of four jets where the nucleation boiling incepts earliest and the critical heat flux (CHF) occurs. Increasing subcooling and jet velocity can delay the onset of nucleate boiling and enhance the critical heat flux dramatically. Wall temperature overshooting phenomenon can only be found on the heating surface under the center of four jets when the jet velocity is low and sub-cooling is high.

scholarly journals Turbulent Heat Transfer From a Slot Jet Impinging on a Flat Plate

2013 ◽  
Vol 135 (10) ◽  
Author(s):  
Dahbia Benmouhoub ◽  
Amina Mataoui
Keyword(s):  
Heat Transfer ◽  
Reynolds Number ◽  
Nusselt Number ◽  
Flow Field ◽  
Skin Friction ◽  
Velocity Ratio ◽  
Surface Velocity ◽  
Turbulent Heat Transfer ◽  
Turbulent Heat ◽  
Jet Velocity

The flow field and heat transfer of a plane impinging jet on a hot moving wall were investigated using one point closure turbulence model. Computations were carried out by means of a finite volume method. The evolutions of mean velocity components, vorticity, skin friction coefficient, Nusselt number and pressure coefficient are examined in this paper. Two parameters of this type of interaction are considered for a given impinging distance of 8 times the nozzle thickness (H/e = 8): the jet-surface velocity ratio and the jet exit Reynolds number. The flow field structure at a given surface-to-jet velocity ratio is practically independent to the jet exit Reynolds number. A slight modification of the flow field is observed for weak surface-to-jet velocity ratios while the jet is strongly driven for higher velocity ratio. The present results satisfactorily compare to the experimental data available in the literature for Rsj ≤ 1.The purpose of this paper is to investigate this phenomenon for higher Rsj values (0 ≤ Rsj ≤ 4). It follows that the variation of the mean skin friction and the Nusselt number can be correlated according to the surface-to-jet velocity ratios and the Reynolds numbers.

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