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|Document Type: ||Thesis or dissertation|
|Title: ||Heat transfer from swirling impinging jets|
|Authors: ||Mahmood, M.|
|Supervisors: ||Ward, J.|
|Issue Date: ||Apr-1980|
|Abstract: ||This thesis is concerned with the determination of the
local and average mass/heat transfer--from swirling jets
impinging orthogonally onto flat surfaces. ApplicatioCn>
of swirl alters the flow field of the' jet considerably
and eventually the maximum velocity in the jet is displaced
from the axis resulting in a typical 'double-peak'
profile. Further increase of swirl
can even result in a
recirculation in the inner core of the flow. The turbulence
characteristics of the jet are also affected. it
is expected that these changes will, in addition, modify
the heat transfer behaviour.
Consequently, a study of both single free jets and a
square array (of 3x 3) of jets was undertaken. The
range of swirls examined in this study was from zero
through to weak and then medium swirl, i. e. the swirl
number S was varied from 0 to 0.48. The other parameters
varied in the experiments were the nozzle-to-target
spacing z/D from 2 to 12, the nozzle pitches x/D (for
arrays of jets) which were 3.2,4.8 and 6.4, and the jGt
Reynolds number. In the single frce jet tests, two flow
rates corresponding to ReD-":2 32pOOO and 60,000 were studied
whilst, for the 'multiple' jets, the measurements
were confined. to the lower of these Reynolds numbers.
(ReD is based on the mean exit velocity in the non-swirling
case and the diameter of the nozzle. )
Limited velocity and turbulence measurements were also
undertaken on the single jet to characterise the flow
and also to compare the behaviour of the present jets
with those in previous studies. The flows were similar
to those observed previously for swirling jets so that
the heat transfer results should be generally applicable.
A 'thin-film' naphthalene sublimation technique was used
to measure mass transfers over the target surface. The
heat transfer coefficients were then derived using the
Chilton-Colburn analogy. A rig was developed to spray
a uniform coating of naphthalene on the target surface.
This 'thin-film' technique was found to provide repeatable
results and the validity of the experiments was further assessed by comparing the results with previously
published data for the no swirl case. These were in
For the single free jets, the application of swirl was
found to continuously reduce the heat transfers. The
heat transfers, however, became more uniform.
correlation has also been suggested for the average
Nusselt number associated with these single swirling '
jets and is valid for S= 0.12-to 0.48. In some circumstances
in the 'multiple' jet tests (e. g. at close nozzle-to-
target spacings) the average heat transfers increased
to a maximum of IS =0.24 (approximately). Further increases
in the degree of swirl brought about a subsequent
reduction in average heat transfer coefficients until
eventually the performance of the swirling jets was poorer
than that of the non-swirling flows. 'This discrepancy
in behaviour can be explained in terms of changes in the
local heat transfer distributions.|
|Appears in Collections:||PhD and Masters by research theses (School of Engineering)|
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