# Experimental and Numerical Investigation of Turbulent Heat Transfer due to Rectangular Impinging Jets

Abstract (Summary)

Due to their efficient heat and mass transfer potential, impinging jets have received
attention in various applications. Heat transfer and flow characteristics of rectangular
turbulent impinging jets issued from a 24:1 aspect ratio and 24:1 contraction
ratio nozzle were investigated experimentally and numerically. In the heat transfer
measurements; a thin stainless-steel foil was utilized to obtain iso-flux boundary
conditions on the impingement surface. The target plate was free to translate in the
lateral direction and the heat transfer distributions were determined at 0 ? x/W ? 20
with the micro-thermocouples placed underneath the foil. The measurements were
conducted for Rej = 8900 ? 48600 at nozzle-to-target spacing of 0.5 ? H/W ? 12.0.
Both semi and fully confined jets were investigated. Heat transfer coefficients at
Rej = 28100, 36800, 45600 and H/W = 4.0 were determined by using adiabatic-wall
temperatures and the distributions were compared with those of the wall shear stress.
Off-center peaks were observed at high Rej and low H/W. Since the wall distributions
are susceptible to nozzle-exit conditions, velocity and turbulence profiles at the
nozzle-exit were measured for the velocity range of interest. Additionally, near-wall
mean velocity and turbulence profiles were determined at Rej = 21500 and 36800
at H/W = 4.0 to have a better understanding of the secondary peaks in the wall
distributions.
Numerical computations were performed by using several different turbulence
models (k ? ?, k ? ?, V 2F and Reynolds stress models). In wall-bounded turbulent
flows, near-wall modeling is crucial. Therefore, the turbulence models eliminating
wall functions such as the k ? ? and V 2F models may be superior for modeling
impingement flows. The numerical results showed reasonable agreement with the ex-
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perimental data for local heat transfer and skin friction coefficient distributions. The
occurrence of the secondary peaks was predicted by the k ? ? and V 2F models, and
for a few cases with the low-Re-k ? ? models. Near-wall measurements along with
the computed profiles were used to describe the “secondary peak” phenomena. It
was shown that the increase in turbulence production in the wall-streamwise direction
enhances turbulent momentum and heat transport in the wall-normal direction
which lead to secondary peaks in the wall distributions. The possibility of improving
surface heat transfer with fully-developed jets was also explored numerically as a case
study.
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Nomenclature
cf
cp
e
skin friction factor
specific heat (J/kg·K)
total energy per unit mass (J/kg)
fi damping functions (i = 1, 2, µ)
f22
h
H
I
elliptic relaxation function
heat transfer coefficient (W/m2·K)
nozzle-to-plate spacing (m)
current (A)
k turbulence kinetic energy per unit mass (m2/s2)
kf
?
Nu
P
Ps
P ?
Pr
thermal conductivity (W/m·K)
mixing length or turbulence length scale (m)
local Nusselt number
pressure (Pa)
stagnation pressure (Pa)
non-dimensional pressure or pressure coefficient
Prandtl number
Prt turbulent Prandtl number, Eq. 2.23
q
r
R
Re
Sij
heat transfer rate (W)
radial coordinate (m)
resistance (?)
Reynolds number
mean strain tensor (1/s)
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t
T
u
Um
x
X
v
V
w
W
y
time (s)
temperature (K)
velocity, velocity in the x-direction (m/s)
maximum velocity at a cross-section in the x direction (m/s)
lateral coordinate (m)
non-dimensional x-coordinate (x/W)
velocity in the y-direction (m/s)
voltage (V)
velocity in the z-direction (m/s)
nozzle width (m)
perpendicular distance from the wall, wall-normal coordinate (m)
y1/2 y coordinate where u = 0.5Um (m)
ym
yne
y+
y?
Y
z
Z
y coordinate where u = Um (m)
axial distance from the nozzle exit (H ? y) (m)
turbulence wall coordinate
turbulence wall coordinate
non-dimensional y-coordinate (y/W)
longitudinal coordinate (m)
non-dimensional z-coordinate (z/W)
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Greek Symbols:
?
thermal diffusivity (m2/s)
?
uncertainty
? turbulence dissipation rate (m2/ s3)
?
?
?
blending function
von Karman constant
blending function
µ absolute viscosity (Pa·s)
?
?t
?
?
momentum diffusivity (or kinematic viscosity)(m2/s)
turbulent (eddy) viscosity (m2/s)
specific turbulence dissipation rate (1/s)
vorticity (1/s)
? density (kg/m3)
?
?
?
??
shear stress (Pa)
stream function per unit depth (kg/s·m)
mean turbulence parameter
fluctuating turbulence parameter
? visocus dissipation (W/m2)
?
?
power (W)
non-dimensional stream function
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Subscripts and Superscripts:
a ambient
c conduction
D nozzle diameter
e effective
i index-i = 1, 2, 3
j nozzle exit or jet, index-j = 1, 2, 3
k index-k = 1, 2, 3
l loss
m maximum
P point
r radiation
st stagnation
t turbulent
w wall
W nozzle width
2W nozzle hydraulic diameter
?? flux
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Bibliographical Information:

Advisor:

School:The University of Arizona

School Location:USA - Arizona

Source Type:Master's Thesis

Keywords:

ISBN:

Date of Publication: