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FMFP2018 paper 563

Proceedings of the 7th International and 45th National Conference on Fluid Mechanics and Fluid Power (FMFP)
December 10-12, 2018, IIT Bombay, Mumbai, India
FMFP2018–PAPER NO. 563
Heat Transfer and Fluid Flow behaviours of a Square Twisted Micro-Channel
Sampad Gobinda Das
Department of Mechanical
Jadavpur University, Kolkata,
West Bengal, India.
[email protected]
Suvanjan Bhattacharyya
Department of Mechanical and
Aeronautical Engineering,
University of Pretoria,
South Africa.
[email protected]
Arnab Banerjee
Department of Mechanical
MCKV Institute of Engineering,
Howrah. West Bengal. India.
[email protected]
Himadri Chattopadhyay
Department of Mechanical Engineering,
Jadavpur University,
Kolkata, West Bengal, India.
Email: [email protected]
efficient cooling systems. Since the extensive
investigation by the previous researchers illustrated that
the utilization of micro-channels are allowed for raised
heat flux level to be sustained, micro-channels have
become a concerning topic of investigation. Successive
assessments by many researchers followed, and a variety
of contradictory results were published in terms of friction
factor and Nusselt number characteristics in comparison
to the conservative macro range ducts behaviour.
The heat transfer and pressure drop analysis of twisted
square micro-channel is studied numerically for Reynolds
number (Re) ranging from 10 to 250. The laminar flow
model is used for formulation in Ansys fluent 15.0, since
the flow is laminar. The governing equations are solved
with a finite-volume-based numerical method. A threedimensional non uniform grid was generated, in order to
critically examine the flow and heat transfer. The effects
of aspect ratio and twist ratio on the Nusselt number and
friction factor are investigated. It is found that the twisted
geometry of the channel evidently enhance heat transfer
by generating longitudinal vortices, which strengthen
flow mixing. Such small vortices also upsurge the
turbulent kinetic energy (TKE) and decrease thickness of
the boundary layer, which lowers local temperature
nearby the target surface. Computational results show that
the simulation with twisted micro-channel geometry
considerably better than the case with straight microchannel with respect to heat transfer performance.
Kandilkar et al. [1] reported in details about heat
exchange characteristics and the flow behaviour for
micro-channels and mini-channels. The device properties
were also listed. Kheirabadi et al. [2] presented a
comparison of different cooling technologies. The work
examined liquid cooling; spray cooling, air-cooling, pool
boiling, heat pipes and jet impingement. It was predicted
that by the year 2020, liquid cooling would be the norm
for high performance computing, whereas air-cooling will
be viable for general purpose computing. Leng et al. [3]
numerically experimented and proposed design
improvement in micro-channel heat sink having double
layers. In integrated circuits, forced liquid cooling having
high-performance was experimentally investigated by
Tuckerman et al. [4]. Micro-channel heat sinks with liquid
flow in single phase was studied by Steinke et al. [5], to
improve upon the high heat removal characteristics. The
influence of geometrical parameters in cooling efficiency
Keywords: Micro-channel, Twisted channel, Heat
transfer enhancement, Fluid flow, Thermal performance.
The pressure drop and heat transfer phenomena in microchannels are significant in thermal design engineering,
Understanding of the effectiveness of micro-channels in
the laminar through transitional to turbulent regime flow
is crucial to ensure thermally effective and energy
momentum equations for a three dimensional, laminar
flow forced convection of heat transfer are solved.
was analysed by Ghani et al. [6] with varying channel
shapes. Dirker et al. [7] studied laminar and transitional
regime in micro-channels and evaluated corresponding
heat exchange coefficient and friction factor. For
sinusoidal micro-channel having rectangular crosssections, the heat exchange and flow characteristics were
experimentally investigated by Sui et al. [8]. Flow and
thermal characteristics, were numerically investigated by
Xia et al. [9] in micro-channel heat sinks having Y-shaped
bifurcations internally. Secondary flow's effect on
augmentation of heat exchange was numerically
investigated by Lee et al. [10] for oblique fin microchannel. For micro-channel heat sink, the heat exchange
and laminar flow characteristics were investigated by Li
et al. [11], for modifications with rectangular ribs and
triangular cavities.
Grid sizes of a variety of dimensions were tested as a part
of study for grid independence. Performing a rigorous
grid independency test, a mesh containing 98,775 nodes
and 1,99,333 elements were utilized for this current
numerical analysis. Theoretical equations of laminar
model were solved for 3D model consisting diameter of
0.5 mm and length of 25 mm, under the heating condition
of the test fluid, air Having Prandtl Number (Pr = 0.707).
The 3D geometry model was constructed using ANSYS
Design Modeler15.0.
The equations for discretization of control volume were
deduced from these basic equations by utilizing the hybrid
scheme. The numerical estimations of the flow field are
executed utilizing the Semi-Implicit Method for PressureLinked Equations (SIMPLE) algorithm.
It has been observed from the literature review that heat
transfer by virtue of fluid flow in twisted square channel
has not been reported in the past. In this paper, therefore,
the laminar flow numerical heat transfer and pressure
drop results of twisted square channel of different twist
and aspect ratio are presented.
The 3D co-ordination of grid is fabricated utilizing
ANSYS Fluent 15.0. Considering the air flow through the
duct with heat exchanging, the applied mathematical
model is constructed of the conservation equations of
energy, momentum and mass for incompressible flow for
The full length square twisted micro-channel as shown in
Figure 1, of inlet diameter (D) with pitch (p) and length
(L) is employed. Air flow was introduced at inlet of the
channel and a pressure outlet condition was applied at
exit. Air inlet temperature of 298 K was set in the
direction of fluid flow. The temperature of wall was kept
constant at 350 K throughout the experiment. At mean
bulk temperature, the thermal and the physical properties
of air were taken to be invariant and no-slip condition was
implemented at wall.
Figures 3(a) show the important understanding of
dimensionless velocity ((u = ui/V)) at Re 250 for the
twisted channel with AR 0.8 and Y = 2.0 at 5% TI at
inlet. It can be witnessed that twisted geometry has solid
impact over the flow field. The dimensionless thermal
interaction [((T-TIN) / (TW-TIN))] is shown in Figure 3 (b).
Figure displays the contour plots of temperature field in
transverse planes Re = 250 for the twisted channel with
AR 0.8 (AR stands for channel Aspect Ratio which is
defined by height to width ration of the channel) and Y =
2.0. (‘Y’ denotes the twist ratio which is defined by the
ratio of inlet diameter to the pitch of the channel). One
can see from the figure that there is a change in the
temperature field throughout.
Figure 1: Computational Domain
Steady state, incompressible flow of a Newtonian fluid
with constant material properties and negligible buoyancy
are assumed. The basic forms of continuity, energy and
Figure 2: Surface Grid
Correlations established by Shah and London [16] were
utilized to evaluate and validate the results of the analysis.
The data found simulation of the plain micro-channel are
quite close with the predicted results from the proposed
correlations with little margin of error with data range of
+5.2% to +6.5% and +1.8% to +2.3% for the Nusselt
number and friction factor, respectively as revealed in
Figure 4.
As established by Shah and London [26] and also
presented in Figure 5, the Nusselt number (Nu) shows a
specific trend that Nu rising with the rise in Reynolds
number. The twisted nature in the channel aids to
efficiently increase the heat transfer by distracting the
boundary layer growth and also by creating local
turbulence and swirl flow and thus, increasing the value
of Nusselt number which can be clearly observed in
Figure 5.
The characteristic of pressure drop in the form of friction
factor (f) is presented in Figure 6. The figure represents
the association between the friction penalty and Reynolds
number for variety of aspect ratio and twist ratio of the
square twisted micro-channel. It could be well established
from Figure 6 that the friction penalty was found to
follow the analogous trend, both for the plain and twisted
micro-channel. The friction penalty of square twisted
micro-channel is inversely proportional to Reynolds
number. At a distinct Reynolds number, the microchannel with twisted nature phenomena results in greater
friction factor over those of the plain micro-channel. This
was due to the blockage of flow, greater contact area of
the surface, the phenomenon caused by the swirl flow as
well as the dynamic pressure dissipation of the fluid due
to the viscosity loss nearby the micro-channel wall.
Furthermore, the pressure penalty had a greater prospect
to occur by the interaction of the inertia forces with
pressure forces in the boundary layer.
Figure 3: Dimensionless contours (a) axial velocity, and (b) temperature
On Figure 7, the curve of thermo-hydraulic performance
parameter (PEC) with Reynolds number is shown. The
thermal performance is the ratio of the dimensionless
Nusselt number to dimensionless friction factor and
shows the amount of the energy saved. By doing all the
simulation on square twisted micro-channel and plain
micro-channel it was found somewhat efficient from the
energy point of view. fig. 7 shows that the channel with
A.R. 1.0, with different twisted ratio provided higher
thermal enhancement efficiency.
Figure 4: Plain channel: (a) Nu vs Re, (b) f vs Re
Thermo-hydraulic transport phenomena of square twisted
micro-channel of various aspect ratio and twist ratio were
assessed numerically. The utilization of square twisted
micro-channel produces extensive augmentation of heat
transfer with a substantial increase in friction factor. In
general assessments, it was ascertained that the heat
exchange, temperature at outlet and friction factor
extensively rise at AR = 0.8, Y = 2.0. It is also evident
that, the Nusselt number proportionally varies with the
Reynolds number while the contrary was found for the
instance of friction factor. The Nusselt number for the
micro-channel with twisted geometry has a reasonable
enhancement than those of the plain micro-channel
Figure 5: Variation of Nusselt number with Reynolds number at different
pitch and aspect ratio
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Figure 6: Variation of friction factor (f) with Reynolds number at different
pitch and aspect ratio
Figure 7: Variation of PEC with Reynolds number at different pitch and
aspect ratio