20th European Symposium on Computer Aided Process Engineering – ESCAPE20
S. Pierucci and G. Buzzi Ferraris (Editors)
© 2010 Elsevier B.V. All rights reserved.
Development of an Enhanced Heat Transfer
Surface
David J. Kukulkaa, Kevin G. Fullerb
a
State University of New York College at Buffalo,1300 Elmwood Avenue, Buffalo,New
York 14222 USA , kukulkdj@buffalostate.edu
b
Rigidized Metals, 658 Ohio Street, Buffalo, New York 14203 USA,
kevinfuller@rigidized.com
Abstract
Heat transfer enhancement has become popular recently in the development of high
performance thermal systems. Much work has been done to gain an understanding of
the fundamental flows that exist in arrays of smooth, parallel plates. A wide variety of
industrial processes involve the transfer of heat energy. These processes provide a
source for energy recovery and process fluid heating/cooling. Enhance heat transfer
surfaces can be designed through a combination of factors that include: increasing fluid
turbulence, generating secondary fluid flow patterns, reducing the thermal boundary
layer thickness and increasing the heat transfer surface area.
This study involves enhanced heat transfer surfaces that produce increased turbulence
and a better flow distribution. Heat transfer of several different surface configurations
have been evaluated over a wide range of conditions. Transient measurements of flow
rates and temperatures have been taken. In all cases, heat transfer increases as the
Reynolds number increases for all the enhanced heat transfer surfaces evaluated. The
enhanced heat transfer surfaces evaluated here show heat transfer performance gains in
excess of 40 percent and also produce a more evenly distributed flow.
Keywords: Enhanced heat transfer, extended surfaces, flow distribution
1. Introduction
Rapid heat removal from heated surfaces has become a major task in the design of heat
exchanger equipment. Development of high efficiency heat exchangers requires
efficient techniques to exchange large amounts of heat between extended surfaces and
the ambient fluid. Fin-and-tube heat exchangers are widely used in many industrial
applications. These heat exchangers are generally composed of continuous metal plate
fins pierced by an inline or staggered tube bank. Understanding the heat transfer and
flow mechanisms provides the necessary knowledge that allows the performance
maximization of high efficiency heat exchangers. In typical applications of air to fluid
heat exchangers, the dominant resistance is on the air-side. Therefore, improving the
heat transfer on the air-side heat transfer is required by the growing demand of high
efficiency heat transfer systems.
Enhanced heat transfer surfaces create a combination of: increased turbulence;
secondary flow generation; reduction in thermal boundary layer thickness and increased
heat transfer surface area. These factors will provide a design that increases heat
D. Kukulka and K. Fuller
transfer; that is more adaptive to flow blockages; and one that provides a more uniform
flow distribution.
A variety of enhanced surface studies have been previously performed that have
evaluated heat transfer and flow distribution of enhanced heat transfer surfaces. Webb
(1981) presents a performance evaluation for enhanced surfaces that relates heat transfer
and surface area. Sparrow and Hajiloo (1980) studied the heat transfer performance of
staggered plate arrays aligned parallel to the flow. They varied the Reynolds number
and the plate thickness in their experiment. Yu et al. (2005) performed experimental
and numerical studies to compare thermal performances of plate fin surfaces. Yakut et
al. (2006) experimentally analyzed thermal resistance and pressure drop and how they
were related to fin geometry; distances between fins; and flow velocity. Li and Chen
(2007) used infrared thermography to investigate the performance of plate-fin surfaces
under confined impinging jet conditions. Sahin and Demir (2008) discuss heat transfer
rates from fins and how they can be improved by employing slots or porosity. Molki
and Hashemi-Esfahanian (1992) discuss ways to enhance heat transfer in baffle plates
near boundaries. Sparrow and Carranco Ortiz (1982) experimentally determined heat
transfer coefficient on an upstream facing surface and how they are related to diameter
ratio and Reynolds number.
A number of studies have been conducted to evaluate the overall heat transfer
characteristics of louvered fin heat exchangers. There have been numerous studies
dealing with the local heat transfer of parallel-plate fins. Although louvered fins are
advantageous from a heat transfer efficiency standpoint, the formation of the louvers
increases the mechanical stresses on the fin sheets and cause deformation/failure of the
heat transfer surface. While a simple solid fin plate attached to the duct wall enhances
heat transfer, a perforated plate attached to the same duct wall poses less resistance to
the flow and produces better performance. Sara et al. (2000) determined thermal
performances of solid and perforated rectangular blocks that are attached on a flat
surface in a rectangular duct. Sahin et al. (2008) experimentally investigated overall
heat transfer, friction factor and the effect of various design parameters on heat transfer
and friction factor for the heat exchanger equipped with circular cross section perforated
pin fins in a rectangular channel and also obtained correlations for enhancement
efficiency. Dorignac et al. (2005) experimentally determined convective heat transfer on
a multi perforated plate.
The general objective of this study is to design an enhanced heat transfer surface to
minimize the material volume (weight) that is required to satisfy the required heat
dissipation. That includes:
1) Evaluation of the effect of material composition on the heat transfer surface
design (including heat transfer and flow distribution).
2) Evaluation of the effect of surface roughness on the heat transfer surface
design.
3) Evaluation of the effect of surface louver or surface openings on surface
design.
4) Determine the effect of Reynolds Number on the surface design.
The optimized surface can then be compared in design to an unenhanced surface. From
the results, an enhanced heat transfer surface can be designed that can be utilized for a
wide range of Reynolds numbers.
Development of an Enhanced Heat Transfer Surface
2. Experimental Methodology
2.1. Procedure
Sample surfaces were combined to create a heat transfer surface array that was placed in
a wind tunnel. This arrangement provided for both heat transfer and pressure-drop
measurements. The test section was constructed of clear acrylic that had movable side
walls, creating a sample area that could accommodate variable size arrays.
2.2. Materials evaluated
The following materials were evaluated in the enhanced heat transfer surface design:
• Copper
• Stainless Steel (304)
• Aluminum
• Copper Alloys
• Nickel Alloys
2.3. Surfaces evaluated
Rigidized heat transfer surfaces are deep-textured, three dimensional metals. Rigidized
metal is distributed above and below a neutral axis, thereby increasing strength in all
directions. Table 1 shows the heat transfer surface textures evaluated. Louvered and
solid surfaces were considered.
2.4. Surface Arrangement
The heat transfer surfaces evaluated included solid plain, solid textured, solid with
openings (louvered), and textured with openings. For any one test the same heat transfer
surface was the only surface considered.
3. Results and Conclusions
For Reynolds numbers around 350, little difference was observed in flow distribution or
heat transfer performance between the enhanced heat transfer surface and the plain,
solid heat transfer surface. At higher Reynolds numbers, heat transfer increased with
velocity, as the flow progressed through the array. Typical results are presented in Table
1. The difference in heat transfer efficiency is the result of the formation of undisturbed
boundary layers on the surface (in the case of the lower flow condition). The velocity
D. Kukulka and K. Fuller
Table 1
2RL
38 % Increase in
Heat Transfer with
Excellent Flow
Distribution
5WL
40 % Increase in
Heat Transfer with
Excellent Flow
Distribution
3SL
18 % Increase in
Heat Transfer with
Directional Flow
Distribution
4LP
25 % Increase in
Heat Transfer with
Average Flow
Distribution
RGM
30 % Increase in
Heat Transfer with
Excellent Flow
Distribution
Enhanced Heat Transfer Surface and the associated Enhancement in Heat Transfer and
Flow Distribution
Development of an Enhanced Heat Transfer Surface
is based on the measured inlet velocity incident to the test section. While measurements
have shown that the flow at the test section inlet was uniform, the flow diminished in
the undisturbed boundary layer.
At the lowest Reynolds numbers (for flow past the enhanced heat transfer surface) the
flow resembled flow past a flat plate. In the higher Reynolds number cases, more
complex flow conditions existed (comprising of turbulent and in some cases, impinging
jet flow). As a result of the interruption of the boundary layer at every surface, the flow
through the surface gap would be turbulent flow, with an increased heat transfer
coefficient.
Due to the enhanced surface (louvers, openings and surface variation) the flow patterns
near the surfaces are much different than those near a solid, smooth surface. Heat
transfer rate is higher due to flow separation and the resulting turbulence. In addition,
the friction coefficient and pressure drop will decrease (when compared to smooth, solid
surfaces) with increasing surface roughness and increasing louver (surface opening)
area. Heat transfer due to jets impinging on a surface has been studied by several
investigators [Li and Chen (2007)]. It has been found that the heat transfer rates
obtained with jets impinging on a surface are an order of magnitude higher than those
obtained in parallel flow and therefore the enhancement of heat transfer, in this case, is
considerable.
Heat transfer and flow distribution is dependent on the relative position of the
perforations in the surface and the shape of the opening. The dependence of the flow
and heat transfer on the different geometrical characteristics of the enhanced surfaces is
not well understood. It is therefore necessary to extend this study to evaluate the
transient heat transfer coefficient and the distribution of flow for a more developed set
of conditions. Additional work is also necessary in varying the opening area on the plate
and varying the combination of plates in the heat transfer module.
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