Global Journal of Engineering Sciences (GJES)
Heat
Transfer Characteristics of Submerged and Free Surface Air-assistant Water Jet
Impingement
Authored by Kyosung Choo
Abstract
Heat
transfer and fluid flow characteristics of air-assistant submerged and free
surface water impinging jets are experimentally investigated. Results showed
that the Nusselt number and pressure decrease exponentially in both free
surface and submerged jets for all volumetric qualities in jet deflection
region. In transition region, the Nusselt number and pressure of the submerged
jet and free surface jet decrease since the jet deflection effect was
disappeared. In buoyance effect region, the Nusselt number and pressure
increase in the free surface jet since the potential energy increases, while
the Nusselt number and pressure decrease in the submerged jet since the bubbles
are not impinged the plate due to the buoyancy effect.
Keywords: Two-phase; Impinging jet; Heat
transfer; Circular jet
Introduction
Impinging
jets have been studied in great depth due to their high rates of heat transfer
and wide range of application [1-4]. Several researchers have observed heat
transfer and fluid flow characteristics for single phase and two-phase free
surface jet impingement. For the single-phase free surface jet impingement,
Kuraan et al. [5] studied the nozzle-to-plate spacings effect on heat transfer
and found correlations for hydraulic jump diameter and local Nusselt number.
Friedrich et al. [6] investigated the effects of volumetric quality on thermal
performance of two-phase impinging jets and showed the optimum design of the
jet impingement.
In a
submerged jet, the jet ejects into a fluid of the same state before impinging
on a surface, where the entrainment of the surrounding fluid is significant.
Choo et al. [7] experimentally investigated thermal characteristics on a flat
plate and showed that the governing parameter of the heat transfer is
stagnation pressure. Bieber et al. [8] numerically investigated the submerged
laminar slot jet impingement. They suggested correlations for submerged slot
jets as functions of nozzle-to-plate distance, heat flux, and wall temperature.
The purpose of this study is to determine the heat transfer and fluid flow
characteristics of two-phase submerged and free surface impinging jets and
compare the two impinging jets.
Material and Methods
Figure
1 shows a schematic diagram of the test section [9- 12]. The water flow is
regulated by a flowmeter valve (Dwyer® RMB-84-SSV). The range of the flowmeter
valve is 0- 2.52 liter per minute. The air was also regulated by two mass flow
controllers. The flow controller used depended on the volumetric quality of the
mixture. For lower volumetric qualities (β = 0.1,0.3,0.5), the Omega® FMA5514A
was used, which has a full-scale range of 0-1000 mL/min. For higher volumetric
qualities (β = 0.7), the Omega® FMA5520A was used with an accuracy level of ±1%
and a repeatability of ±0.15%, and a full-scale range of 0-10 L/min. To create
the two-phase impinging jet, the two fluids are regulated in separate tubes and
then combined in a single nozzle. The volumetric quality was controlled by
keeping the flow rate of the water constant at 1.47 x 10-5 m3/s throughout the
experiment and varying the air flow rate in order to change the volumetric
quality. The nozzle in which the fluids are combined is 470 mm long and is made
of extruded acrylic. The nozzle has a diameter of 5.86 mm, and is circular,
producing an axisymmetric velocity profile. The nozzle is mounted on a 3-axis
stage (Thorlabs, Inc, PT3A/M) that has a resolution of 10 μm. Two digital
manometers were used to measure the pressure at the stagnation zone of the
impinging jet: Meriam® M200-DI0001 manometer with a range of 0 – 6.9 kPa and an
accuracy of ±0.05% FS; and Meriam® M200-DI0005 manometer with range of 0 – 34.5
kPa and similar accuracy.
The circular
impingement surface is inside the test section, but elevated so that water can
fall off of it and fall into the pool below. This is useful in the case of the
free surface jet, because the water exiting the nozzle can flow off of the
plate after it passes over the stagnation point without effecting the flow at
the stagnation point. The focus of this study is to compare the free surface
jet and the submerged jet. For the submerged portion, the jet is submerged in a
cylindrical tank that is 141.7 mm in diameter and 43.2 mm in height as shown in
Figure 2. The impingement surface was made of PTFE Teflon and is a disk that is
20 mm thick and 297 mm in diameter.
Results and Discussion
The variation of the
normalized Nusselt number, Nu0* (Nu0 / Nu0, H/d = 1) and the normalized
pressure P0* (P0 / P0, H/d = 1) of each nozzle-to-plate spacing can be seen in
Figures 2 and 3. Analyses of the results are divided into three regions: Jet
deflection region (Region I) ranging from 0.03
Comparing the values of
the free surface and submerged jet, the free surface jet has a slightly higher
Nusselt number for low nozzle-to-plate spacings as shown in Figure 5(a). As the
nozzle-toplate spacing increases, the Nusselt number of the submerged jet
decreases while the Nusselt number of the free surface jet increases. At some
point, they intersect and the free surface jet Nusselt number becomes larger
than that of the submerged jet. The point of intersection varies for each
volumetric quality, which occurs at H/d = 2 - 4. In Figure 5(a), the value of
the submerged jet is only larger by a small amount until H/d=0.51 at β=0.1.
From 2.05
Conclusion
In this work, an experimental
study was conducted for a comparison of two-phase submerged jets and free
surface jets. The Nusselt number and pressure decreased exponentially in both
free surface and submerged jets for all volumetric qualities in Region I due to
the jet deflection. The Nusselt number and pressure of the submerged jets and
free surface jets continue to decrease since the jet deflection effect
disappears in Region II. The Nusselt number and stagnation pressure increased
in the free surface jet since the potential energy increases, while the Nusselt
number and pressure decreased in the submerged jet since the bubbles are not
impinged the plate due to the buoyancy effect in Region III. The submerged jet
showed higher heat transfer performance than the free surface jet up to the
intersection points of H/d = 2 – 4 due to the entrainment of surrounding water,
while the free surface jet had higher thermal performance than the submerged
jet beyond the intersection points due to the diminishment of the bubble impact
by buoyancy effect.
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