Fisher, Martin (2000) An experimental and numerical investigation into compact heat exchangers. PhD thesis, University of Warwick.

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Abstract

Experimental and numerical experiments were carried out on different heat exchanger
section types to determine the performance of vortex generators. The heat
exchanger sections investigated were plain channel fin types with delta and rectangular
winglet pairs, rectangular wings and embossed vortex generators. Fin-tube
heat exchangers were also investigated in both in-line and staggered arrangements
with and without vortex generators over the Reynolds number range 65 - 653 and
angle of attack of vortex generators 15° - 60°.
With the need for improved heat exchanger performance fin modifications are normally
used to enhance the gas side heat transfer coefficient. However, with many of
these fin modifications a significant pressure drop penalty can result. Vortex generators
enhance heat transfer with a minimal increase in pressure drop. The effect
of vortex generators in heat exchanger sections at low Reynolds numbers need to
be assessed so that optimal positioning can be determined. To do this local heat
transfer coefficients need to be measured.
Steady steady state physical tests with a constant heat flux boundary condition
were used to measure local and average heat transfer values and thereby measure
the effect of vortex generators on heat transfer and pressure drop. Numerical modelling
allows a detailed picture of the flow field and local heat transfer to be seen.
Such numerical modelling by Computational Fluid Dynamics (CFD) is still in its
infancy and comparisons against detailed experimental data are still needed before
simulations can be used without physical testing. The advantage of CFD is that a
large number of simulations can be completed in a short time span when compared
to full size physical tests.
At low Reynolds numbers, it was found that the inclusion of vortex generators in all
heat exchanger types had the effect of increasing the average heat transfer coefficient
and pressure drop. In all but one of the cases investigated, the in increase average
heat transfer coefficient was larger than the increase in pressure drop. For the case of
a staggered tube arrangement, with vortex generators at 60°, there was a reduction
in pressure drop when compared to the case without vortex generators. This was
due to delayed separation and reduced wake region behind the tube. This specific
vortex generator position is considered to be near optimal.
It is shown that CFD can successfully reproduce data from physical tests on heat
exchanger sections with and without vortex generators. The procedures given can
be generalised to optimise further geometries. The benefits of including vortex
generators in heat exchangers will enable smaller heat exchangers to be utilised or
an increase in effectiveness for the user.

Item Type:	Thesis or Dissertation (PhD)
Subjects:	T Technology > TJ Mechanical engineering and machinery
Library of Congress Subject Headings (LCSH):	Heat exchangers, Vortex generators
Official Date:	May 2000
Dates:	Date Event May 2000 Submitted
Institution:	University of Warwick
Theses Department:	School of Engineering
Thesis Type:	PhD
Publication Status:	Unpublished
Supervisor(s)/Advisor:	Critoph, Robert E. ; Shaw, Christopher T. (Christopher Thomas), 1956-
Sponsors:	Engineering and Physical Sciences Research Council (EPSRC) ; European Union (EU) (JOE3-CT97-0056)
Extent:	xxi, 330 p.
Language:	eng

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