An Experimental Study of the Forcing Effect on the Flow and Heat Transfer in a Turbulent Wall Jet

2015 ◽  
Author(s):  
Johnny Issa ◽  
Alfonso Ortega
Keyword(s):  
Heat Transfer ◽  
Nusselt Number ◽  
Stream Flow ◽  
Free Stream ◽  
Maximum Velocity ◽  
Local Maximum ◽  
Jet Flow ◽  
Wall Jet ◽  
Wall Jet Flow ◽  
Turbulent Wall

The effect of the exit wall jet flow excitation on the flow and thermal behaviors of the turbulent wall jet is experimentally investigated. Various forcing amplitudes and frequencies are used in the presence and absence of a free stream flow. Forcing the flow showed to have a major impact on the fluid mechanics of the turbulent wall jet which was clearly shown in the velocity fields and the associated time-averaged quantities such as the wall jet spread and the maximum velocity decay. The normal direction at which the local maximum velocity occurs, also known as the wall jet spreading, is shown to move further away from the wall and is increased by more than 20% under some forcing conditions. The local maximum velocity decay with the downstream direction is reduced by more than 2.5% at further downstream locations. At a given location, the increase in the wall jet spreading together with the reduction in the mean velocity results in a decrease in the wall skin friction calculated using the slope of the mean velocity in the viscous sublayer, a behavior consistent with the literature. Due to its importance in enhancing heat transfer phenomena, the effect of the forcing on the streamwise velocity fluctuations is also investigated under the various forcing conditions. The profiles of the fluctuating component of the velocity, u’, are measured at various downstream locations since they are essential in understanding the growth of the disturbances. Forcing the wall jet increased u’ in the inner and outer regions and revealed the two peaks corresponding to the inner and outer shear layers respectively. This phenomenon is attributed to the added disturbance at the jet exit in addition to the disturbance growth with the downstream direction. The introduction of wall jet flow forcing at various amplitudes and frequencies showed a significant effect on the thermal behavior of the wall jet and was more pronounced in the absence of a free stream flow, a fact related to the evolution of the mixing layer with the downstream direction. In the absence of a free stream flow, Nusselt number decreases with increasing forcing amplitude and frequency in the region close to the jet exit. The decay of Nusselt number in the downstream direction showed an inflection point at further downstream locations which leads to a larger Nusselt number value than the one observed in the unforced case. This behavior is related to the enhanced mixing between the wall jet flow and the free stream due to forcing, which results in a reduction in the wall skin friction and consequently a decrease in the heat transfer rate from the wall.

Experimental Investigation of the Effect of Free Stream Flow on the Thermal Behavior of a Turbulent Wall Jet

2014 ◽  
Author(s):  
Johnny Issa ◽  
Alfonso Ortega
Keyword(s):  
Experimental Investigation ◽  
Reynolds Number ◽  
Nusselt Number ◽  
Stream Flow ◽  
Free Stream ◽  
Flow Characteristics ◽  
Thermal Characteristics ◽  
Wall Jet ◽  
Reynolds Analogy ◽  
Turbulent Wall

A systematic experimental investigation is conducted to understand of the effect of the free stream flow on the thermal characteristics of the turbulent wall jet. The jet Reynolds number varies between 6000 and 10000. The effect of the free stream flow on heat transfer and flow characteristics of the wall jet is investigated for blowing ratio varying between 2.4 to infinity. In the absence of free stream flow, Nusselt number data showed a very good agreement with published correlations. The free stream flow reduced Nusselt number in the region close to the jet exit and increased it in the region far downstream, a behavior explained using Reynolds analogy. The local Nusselt number dependence on Reynolds number and on the downstream location is identified and the obtained experimental results are correlated for the various considered blowing ratios.

The asymptotic downstream flow of plane turbulent wall jets without external stream

2015 ◽  
Vol 779 ◽  
pp. 351-370 ◽  
Author(s):  
Klaus Gersten
Keyword(s):  
Experimental Data ◽  
Momentum Flux ◽  
Maximum Velocity ◽  
Jet Flow ◽  
Plane Wall ◽  
Wall Jet ◽  
Free Jet ◽  
Wall Distance ◽  
Wall Jet Flow ◽  
Turbulent Wall

The plane turbulent wall-jet flow without externally imposed stream is considered. It is assumed that the wall jet does not emerge from a second wall perpendicular to the velocity vector of the initial wall jet. The (kinematic) momentum flux $K(x)$ of the wall jet decreases downstream owing to the shear stress at the wall. This investigation is based on the hypothesis that the total friction force on the wall is smaller than the total inflow momentum flux. In other words, the turbulent wall jet tends to a turbulent ‘half-free jet’ with a non-zero momentum flux $K_{\infty }\;(\text{m}^{3}~\text{s}^{-2})$ far downstream. The fact that the turbulent half-free jet is the asymptotic form of a turbulent wall jet is the basis for a singular perturbation method by which the wall-jet flow is determined. It turns out that the ratio between the wall distance $y_{m}$ of the maximum velocity and the wall distance $y_{0.5}$ of half the maximum velocity decreases downstream to zero. Dimensional analysis leads immediately to a universal function of the dimensionless momentum flux $K(\mathit{Re}_{x})/K_{\infty }$ that depends asymptotically only on the local Reynolds number $\mathit{Re}_{x}=\sqrt{(x-x_{0})K_{\infty }}/{\it\nu}$, where $x_{0}$ denotes the coordinate of the virtual origin. When the values $K$ and ${\it\nu}$ at the position $x-x_{0}$ are known, the asymptotic momentum flux $K_{\infty }$ can be determined. Experimental data on all turbulent plane wall jets (except those emerging from a second plane wall) collapse to a single universal curve. Comparisons between available experimental data and the analysis make the hypothesis $K_{\infty }\neq 0$ plausible. A convincing verification, however, will be possible in the future, preferably by direct numerical simulations.

Effects of Free-Stream Turbulence on the Instantaneous Heat Transfer in a Wall Jet Flow

1997 ◽  
Vol 119 (2) ◽  
pp. 359-363 ◽  
Author(s):  
S. Yavuzkurt
Keyword(s):  
Heat Transfer ◽  
Free Stream ◽  
Flow Direction ◽  
Vertical Direction ◽  
Jet Flow ◽  
Wall Jet ◽  
Axial Component ◽  
Free Stream Turbulence ◽  
Wall Jet Flow ◽  
Hot Film

This is a preliminary study in order to understand how free-stream turbulence increases heat transfer. Effects of free-stream turbulence on instantaneous heat transfer were investigated in a wall jet flow. Heat transfer traces obtained by a hot-film probe flush-mounted with the surface showed an intermittent structure with definite peaks at certain time intervals. The number of peaks per unit time increased with increasing turbulence intensity. A wall jet test rig was designed and built. The initial thickness and the velocity of the wall jet were 10 cm and 24.4 m/s, respectively. The hot-film probe, which was flush with the surfaces, was positioned at 10 cm intervals on the surface in the flow direction. The profiles of mean velocity and axial component of the Reynolds stress were measured with a horizontal hot-wire probe. Space correlation coefficients for u′ and q′ were obtained in the vertical direction to the wall. This paper concentrates on the effects of turbulence level on instantaneous heat transfer at the wall. It is speculated that the intermittent structures of the heat transfer traces are related to burst phenomena and increase in heat transfer is due to increased ejections (bursts) at the wall with increasing turbulence levels.

scholarly journals Effects of Free Stream Turbulence on the Instantaneous Heat Transfer in a Wall Jet Flow

1995 ◽  
Author(s):  
Savash Yavuzkurt
Keyword(s):  
Heat Transfer ◽  
Free Stream ◽  
Flow Direction ◽  
Vertical Direction ◽  
Jet Flow ◽  
Wall Jet ◽  
Axial Component ◽  
Free Stream Turbulence ◽  
Wall Jet Flow ◽  
Hot Film

This is a preliminary study in order to understand how free stream turbulence increases the heat transfer. Effects of free stream turbulence on the instantaneous heat transfer were investigated in a wall jet flow. Heat transfer traces obtained by a hot film probe flush-mounted with the surface showed an intermittent structure with definite peaks at certain time intervals. Number of peaks per unit time increased with increasing turbulence intensity. A wall jet test rig was designed and built. The initial thickness and the velocity of the wall jet were 10 cm and 24.4 m/s respectively. The hot film probe which was flush with the surfaces was positioned at 10 cm intervals on the surface in the flow direction. The profiles of mean velocity and axial component of the Reynolds stress were measured with a horizontal hot wire probe. Space correlation coefficients for u′ and q′ were obtained in the vertical direction to the wall. This paper concentrates on the effects of turbulence level on the instantaneous heat transfer at the wall. It is speculated that intermittent structure of the heat transfer traces are related to burst phenomena and increase in heat transfer is due to increased ejections (bursts) at the wall with increasing turbulence levels.

Measurements of Skin Friction in a Plane Turbulent Wall Jet

1958 ◽  
Vol 62 (576) ◽  
pp. 873-877 ◽  
Author(s):  
A. Sigalla
Keyword(s):  
Skin Friction ◽  
Free Stream ◽  
Maximum Velocity ◽  
Local Maximum ◽  
Power Laws ◽  
Skin Friction Coefficient ◽  
Rate Of Change ◽  
Turbulent Pipe Flow ◽  
Wall Jet ◽  
Turbulent Wall

SummaryResults of an experimental investigation of the distribution of skin friction along the wall of a plane turbulent wall jet are presented. The measurements show that it is possible to describe the variation of skin friction coefficient by a formula similar to the Blasius formula which is based on experimental results of turbulent pipe flow. This is simply achieved by considering the inner layer of fluid between the wall and the position where the velocity is a maximum as a boundary layer with an outer uniform free stream of velocity equal to the local maximum velocity.Other measurements of velocity distribution indicate that within the experimental range and accuracy, the velocity profiles in the jet are similar and that the rate of change of velocity and width of the jet can be expressed by simple power laws. These results are then partially compared with theory.

NUMERICAL INVESTIGATION OF CONJUGATE HEAT TRANSFER FROM LAMINAR WALL JET FLOW OVER A SHALLOW CAVITY

2018 ◽  
Vol 49 (12) ◽  
pp. 1151-1170 ◽  
Author(s):  
Maheandera Prabu Paulraj ◽  
Rajesh Kanna Parthasarathy ◽  
Jan Taler ◽  
Dawid Taler ◽  
Pawel Oclon ◽  
...  
Keyword(s):  
Heat Transfer ◽  
Numerical Investigation ◽  
Conjugate Heat Transfer ◽  
Jet Flow ◽  
Wall Jet ◽  
Shallow Cavity ◽  
Wall Jet Flow
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