20-06-2013, 03:16 PM
CFD analysis of fin tube heat exchanger with a pair of delta winglet
vortex generators
CFD analysis.pdf (Size: 1.35 MB / Downloads: 100)
Abstract
Among tubular heat exchangers, fin-tube types are the most widely used in refrigeration and air-conditioning equipment. Efforts to enhance
the performance of these heat exchangers included variations in the fin shape from a plain fin to a slit and louver type. In the context
of heat transfer augmentation, the performance of vortex generators has also been investigated. Delta winglet vortex generators have
recently attracted research interest, partly due to experimental data showing that their addition to fin-tube heat exchangers considerably
reduces pressure loss at heat transfer capacity of nearly the same level. The efficiency of the delta winglet vortex generators widely varies
depending on their size and shape, as well as the locations where they are implemented. In this paper, the flow field around delta winglet
vortex generators in a common flow up arrangement was analyzed in terms of flow characteristics and heat transfer using computational
fluid dynamics methods. Flow mixing due to vortices and delayed separation due to acceleration influence the overall fin performance.
The fin with delta winglet vortex generators exhibited a pressure loss lower than that of a plain fin, and the heat transfer performance was
enhanced at high air velocity or Reynolds number.
Introduction
Generally used to exchange heat between a gas and liquid,
fin-tube heat exchangers are widely used in chemical processing
plants, power plants, and home appliances, including small
air conditioners and refrigerators [1]. While early fin-tube heat
exchangers primarily adopted plain fins, extensive research on
fin shape caused substantial progress in the development of
high performance fins to produce compact heat exchangers.
Current compact fin-tube heat exchangers adopt a slit or louver
fin if the working fluid is not significantly corrosive, such
as in the case of a fume gas.
Numerical result
Fig. 5 shows the air flow velocity vector at the mid-plane
between two fins when the frontal air velocity is 5 m/s. The air
enters from the left-hand side and flows around the heat transfer
tube. The air flow then separates from the tube wall and
develops a wake region behind the tube, and enters the fintube
heat exchanger where the flow slows down and circulates
[14]. For a plain fin heat exchanger, as seen in Fig. 5(a), the
wake is wide and extends to the tailing edge of the fin. For a
DWVG fin, as seen in Fig. 5(b), the flow velocity increases
between the DWVG and the heat transfer tube. Then the flow
separation point moves to the rear side of the tube, where in
this case, a narrower and shorter wake region still develops.
The wake region appears to be extended lengthwise to the
middle of the second row of tubes. Fig. 5(b) shows that the air
flow accelerates between the DWVG and tube wall, consequently
forming a delayed, adverse pressure gradient. Furthermore,
the flow separation occurs further downstream. This
delayed flow separation diminishes the wake region and increases
the uniformity of the overall velocity profile.
Concluding remarks
In the past decade, numerous studies investigated the effect
of vortex generators on the heat transfer of fins. While the
vortex generators were consistently reported to enhance the
heat transfer performance, the pressure loss measurements did
not yield agreement. Despite significant research in this area, a
lack of understanding remains on the mechanisms contributing
to heat transfer augmentation and a limited increase in
pressure loss. In the present work, the air flow and heat transfer
in a fin-tube heat exchanger were analyzed using the CFD
to obtain a better phenomenological understanding of the effects
of the delta winglet vortex generator on fin performance.