Numerical Simulation of Heat Transfer Enhancement in Porous Pin Fins

International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 – 6340(Print),
ISSN 0976 – 6359(Online), Volume 5, Issue 7, July (2014), pp. 51-64 © IAEME
51
NUMERICAL SIMULATION OF FORCED CONVECTION HEAT TRANSFER
ENHANCEMENT BY POROUS PIN FINS IN RECTANGULAR CHANNELS
Manjunatha Reddy1
, Dr. G S. Shivanshankar M.E.,Ph.D
1
(M.Tech Student, Dept of Mechanical Engg, Siddaganga Institute of Technology, Tumkur)
2
(Professor and Head, Dept of Mechanical Engg, Siddaganga Institute of Technology, Tumkur)
ABSTRACT
Pin ﬁns have a variety of applications in industry due to their excellent heat transfer
performance, e.g., in cooling of electronic components, in cooling of gas turbine blades, and
recently, in hot water boilers of central heating systems. The forced convective heat transfer in three-
dimensional porous pin fin channels is numerically studied using ANSYS Fluent. Geometric
modelling is done using Design Modeller and CFD Meshing is carried out using ANSYS Meshing
Preprocessor. The effects of Reynolds number (Re), pore density (PPI) and pin fin form are studied
in detail.
The results show that, with proper selection of physical parameters, significant heat transfer
enhancements and pressure drop reductions can be achieved simultaneously with porous pin fins and
the overall heat transfer performances in porous pin fin channels are much better than those in
traditional solid pin fin channels. The effects of pore density are significant. As PPI increases, the
pressure drops and heat fluxes in porous pin fin channels increase while the overall heat transfer
efficiencies decrease and the maximal overall heat transfer efficiencies are obtained at PPI 20.
Furthermore, the effects of pin fin form are also remarkable. With the same physical parameters, the
overall heat transfer efficiencies in the long elliptic porous pin fin channels are the highest while they
are the lowest in the short elliptic porous pin fin channels.
Keywords: CFD, Heat Transfer, Pin Fin, Porous.
I. INTRODUCTION
Forced convection heat transfer in a channel or duct fully or partially packed with porous
material is of considerable technological interest. This is due to the wide range of applications such
as direct contact heat exchangers, electronic cooling, heat pipe etc. It has been demonstrated that
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ISSN 0976 – 6340 (Print)
ISSN 0976 – 6359 (Online)
Volume 5, Issue 7, July (2014), pp. 51-64
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52
insertion of a high-conductivity porous material in a cooling passage can have a positive effect on
convective cooling. An important class of problems directly related to porous matrix convection is
heat and fluid flow in composite systems, that is, systems consisting partly of a fluid-saturated
porous material and partly of a fluid. The convection phenomenon in these systems is usually
affected by the interaction of the temperature and flow fields in the porous spaces and the open
spaces. The importance of this class of problem is justified both in a fundamental and in a practical
sense. With reference to practical thermal engineering applications which stand to benefit if a better
understanding of heat and fluid flow processes in composite systems is acquired, the following
examples are cited: fibrous and granular insulation which occupies only part of the space between a
hot and a cold boundary, fault zones in geothermal systems, the cooling of stored grain, and heat
removal from nuclear debris beds in nuclear reactor safety.
The major challenges to the design of a heat exchanger are to make it compact, i.e., to
achieve a high heat transfer rate and, at the same time, to allow its operation with a small power loss.
These aims of research and development have not changed over the years but, most recently, high
energy and material costs have resulted in increased efforts to design and produce more and more
efficient heat exchanger equipment.
Fig.1: Pin-Fin Heat sink

53
N. Sahiti et al.[1] demonstrated a considerable heat transfer enhancement by using small
cylindrical pins on surfaces of heat exchangers. It uses simple relationships for the conductive and
convective heat transfer to derive an equation that shows which parameters permit the achievement
of heat transfer enhancements.
N. Sahiti et al.[2] shown that the selection of elements for heat transfer enhancement in heat
exchangers requires a methodology to make a direct comparison of the performances of heat
exchanger surfaces with different elements.
Pei-Xue Jiang et al.[3] Experimentally investigated forced convection heat transfer of water and air
in sintered porous plate channels. The effects of fluid velocity, particle diameter, type of porous
media (sintered or non-sintered), and fluid properties on the convection heat transfer and heat
transfer enhancement were investigated.
Y. Wang and K. Vafai [4] conducted an experimental investigation of the convective heat
transfer and pressure loss in a rectangular channel with discrete flush-mounted and protru ding heat
sources. Six protruding obstacle heights, which represent the range of the dimensionless protrusion
of 0≤ h /H ≤ 0.805, are studied
Hyung Jin Sung et al.[5] did a numerical study of flow and heat transfer characteristics of forced
convection in a channel that is partially filled with a porous medium. The flow geometry models
convective cooling process in a printed circuit board system with a porous insert. The channel walls
are assumed to be adiabatic.
F. Benkafada et al.[6] carried a two dimensional numerical simulation of the laminar air
forced convection cooling of six blocks mounted on the lower wall of a plane horizontal channel
filled (or not filled) with a porous medium. Mounted in the channel filled with the porous matter.
Thus, the use of porous media when possible is recommended because it enhances the cooling of
heated blocks mounted in channels.
Habibollah sayehvand And Hossein Shokouhmand [7] did a numerical study of laminar fully
developed forced convection in a pipe partially filled with a porous medium.
Hadi Dehghan et al.[8] conducted a detailed numerical investigation of two-dimensional laminar
forced convection in a porous channel with inlet and outlet slot. A uniform heat flux is applied on
one wall of channel and an-other wall is isolated.
P.C.Huang, K.Vafai [9] presented a detailed investigation of forced convection enhancement
in a channel using multiple emplaced porous blocks. The brinkman-Forchheimer extended Darcy
model is used to characterize the flow field inside the porous regions in order to account for the
inertia effects as well as the viscous effects.
M.R.Asif et al. [10] carried out to investigate the mixed convective two dimensional flows in
a vertical enclosure with heated baffles on side walls. All walls are assumed to be adiabatic, but
baffles are considered as isothermally heated.
Somchai Sripattanapipat A et al.[11] Investigated Laminar periodic flow and heat transfer in
a two dimensional horizontal channel with isothermal walls and with staggered diamond-shaped
baffles numerically. The computations are based on the Finite volume method and the SIMPLE
algorithm has been implemented.
II. MATHEMATICAL MODELS OF FLUENT
All the fluids investigated in this research are Newtonian. This means that there exists a linear
relationship between the shear stress, σij , and the rate of shear (the velocity gradient). In CFX, this
is expressed as follows:
ߪ௜௝ ൌ െ‫ߜ݌‬௜௝ ൅ ߤ ቆ
߲‫ݑ‬௜
߲‫ݔ‬௝
൅
߲‫ݑ‬௝
߲‫ݔ‬௜
ቇ … … … … … … . .1

54
In FLUENT, these laws are expressed in the following form:
Law of Conservation of Mass: Fluid mass is always conserved.
ࣔ࣋
࢚ࣔ
൅
ࣔ
ࣔ࢞࢐
൫࢛࣋࢐൯ ൌ ૙ … … … … … . . . ૛
Newton’s 2nd
Law: The sum of the forces on a fluid particle is equal to the rate of change of
momentum.
ࣔ
࢚ࣔ
ሺ࢛࣋࢏ሻ ൅
ࣔ
ࣔ࢞࢐
൫࢛࣋࢏࢛࢐൯ ൌ
ࣔ
ࣔ࢞࢐
ቈെ࢖ࢾ࢏࢐ ൅ ቆ
࢛ࣔ࢏
ࣔ࢞࢐
൅
࢛ࣔ࢐
ࣔ࢞࢏
ቇ቉ ൅ ࡮࢏ … … … … … . . . ૜
First Law of Thermodynamics: The rate of head added to a system plus the rate of work done on a
fluid particle equals the total rate of change in energy.
ࣔ
࢚ࣔ
ሺ࣋ࡴࢋሻ ൅
ࣔ
ࣔ࢞࢐
൫࢛࣋࢐ࡴࢋ൯ െ
ࣔ
ࣔ࢞࢐
ቆࣅ
ࣔࢀ
ࣔ࢞࢐
ቇ ൌ
ࣔ࣋
࢚ࣔ
… … … … … . . . ૝
The fluid behaviour can be characterised in terms of the fluid properties velocity vector u
(with components u, v, and w in the x, y, and z directions), pressure p, density ρ, viscosity µ, thermal
conductivity λ, and temperature T. The changes in these fluid properties can occur over space and
time. H is the total enthalpy, given in terms of the static (thermodynamic) enthalpy, h:
After going through literature review certain gap findings have been determined. In the work
of Yang et al. [12] only air and water are investigated and the performances of other fluids are still
unknown. The performance of nano fluid in porous medium can have positive effect on heat transfer
augmentation is the important gap found during the literature review. The discrete heating of the
rectangular channel partially filled with porous medium is of considerable technological interest.
Removing the adiabatic walls of rectangular channel and maintaining them at constant temperature,
varying the cross sectional area of porous pin fin over the base wall area in single pin fin array unit
cell, changing the material properties of porous pin fin are some of the other gap findings that has
been determined.
III. GEOMETRIC MODEL
As shown in 2 the physical model is derived from traditional pin fin heat sink, which
generally consists of a bottom wall, two side walls, a top wall, and a pin fin array. The bottom wall is
hot and its temperature is kept at Th. The side and the top walls are kept adiabatic. The pin fin array
is made of high porosity metal foams aluminum and arranged in stagger; air and water are used as
the cold fluids. In order to obtain a basic understanding of flow and heat transfer performances in
porous pin fin heat exchangers, a simplified porous pin fin channel with appropriate boundary
conditions is adopted for the computations, which can be regarded as forced convection heat transfer
in a partially filled porous channel The computational domain is depicted in Fig. 4.1 b and 4.2 which
is composed of a developing inlet block L1=10 mm, two pin fin array unit cells L2=2×6.52 mm, and
a developing outlet block L3=70 mm.

55
The dimensions of the computational domain are Length (L) 93.04 mm, Width (W) 3.26 mm,
Height (H) 10 mm. The total area of pin fin cross-sections over the base wall area in single pin in
array unit cell is 15%, which is reasonable for industry applications.
Fig.2: Physical model: a) porous pin fin heat sink and
b) representative computational domain
Fig.3: Porous pin fin cross-section Circular form

56
Fig.4. Porous pin fin cross-section long elliptic form
Fig.5: Porous pin fin cross-section short elliptic form
IV. CFD MESHING AND BOUNDARY CONDITIONS
CFD meshing is done by using ANSYS Meshing software. Total no of elements used in this
simulation is approximately for all cases is 35000.

57
Fig.6: CFD Meshing of Pin-fin
Fig.7: Boundary conditions
The temperature and velocity of inlet are kept at Tin and uin, respectively. The bottom wall
of pin fin array unit cells is the hot wall and the temperature is kept at Th. Two other bottom walls
and all top walls are kept adiabatic. The symmetry boundary conditions are adopted for two side
walls and the flow and heat transfer of outlet are considered to be fully developed. Furthermore,
three different kinds of porous pin fins with circular, long elliptic, and short elliptic cross-section
forms are employed to investigate the pin fin configuration effects and the cross-section areas of
different pin fins are identical with each other Apin =3.14 mm2
. The physical dimensions and cross-
section forms of different porous pin fins are presented in Fig. 4.3, 4.4 and 4.5.

58
Parameters studied in project
In this Project Air is employed as the cold fluids and the effects of Reynolds number (Re),
pore density (PPI) and pin fin form are studied.
Table 1: Parameters studied for the simulation
Solid Pin fin Pin fin-PPI 30 Pin fin-PPI 40
Fin Type Inlet Velocity
m/s
Inlet Velocity
m/s
Inlet Velocity
m/s
Circular 0.5 0.5 0.5
1 1 1
1.5 1.5 1.5
2 2 2
Long
Elliptical
0.5 0.5 0.5
1 1 1
1.5 1.5 1.5
2 2 2
Short
Elliptical
0.5 0.5 0.5
1 1 1
1.5 1.5 1.5
2 2 2
V. RESULTS AND DISCUSSIONS
The pressure distributions in solid pin fin channels are shown in Fig. 8. It shows the Pressure
drop of 0.7 Pa fro inlet velocity of 0.5 m/s. The temperature distributions in solid pin fin channels are
shown in Fig. 9. It shows that the internal temperatures of solid pin fins are quite uniform and the
average temperatures are high, which are 342.2 K. The temperature rise in the channel inlet to outlet
is 21.3K. The velocity vector distributions in solid pin fin channels are presented in Fig. 4.10. It
shows that large vortices are formed behind solid pin fins. In solid pin fin channels, the solid pin fins
are totally impermeable. Similar Trend is shown in 4.13.
Table 2: Comparison of Pressure drop and Temperature rise in Circular Pin-Fin
Fin Type Inlet
Velocity
m/s
Temperature
Rise K
Pressure
Drop Pa
Circular 0.5 21.3288 0.709819
1 14.92 2.35157
1.5 12.4976 4.9089
2 11.0747 8.32347

59
A. Circular Results
Fig.8: Pressure contours solid and circular pin fin channels-Inlet velocity 0.5m/s
Fig.9: Temperature contours solid and circular pin fin channels- Inlet velocity 0.5m/s

60
Fig.10: Velocity contours solid and circular pin fin channels- Inlet velocity 0.5m/s
The flow and heat transfer performances in circular porous pin fin channels are carried out.
The circular porous pin fin form see Fig. 5.1 is selected for present study. Air Pr=0.7 are used as cold
fluids and the Reynolds number Re varies from 1000 to 2291 with φ=0.9 PPI =30. Tin=293 K, and
Th=343 K.
The pressure distributions in Porous circular pin fin channels are shown in Fig. 11. It shows
the Pressure drop of 0.67 Pa for inlet velocity of 0.5 m/s. It shows the lower pressure drop compared
to circular solid channel due to Porosity in the solid region. The temperature distributions in circular
porous pin fin channels are shown in Fig. 12. The internal temperatures of porous pin fins are not so
uniform and the average temperatures are much lower, which are 330 K. Also, the fluid temperatures
in porous pin fin channels are higher than those in solid pin fin channels. The average exit
temperature rise in porous pin fin channels is 26.64 K. However in the solid circular pin fin the
internal temperatures of solid pin fins are quite uniform and the average temperatures are high, which
are 343K. Also the solid the temperature rise in the channel inlet to outlet is 21K. These results
indicate that more heats can be transported away by using porous pin fins and their heat transfer
performances would be better. This is because the porous pin fins can greatly enlarge the contact
surface areas and mix the fluid flow inside, which may lead to significant heat transfer
enhancements.
Table 3: Comparison of Pressure drop and Temperature rise in Circular pin-fin(PPI=30)
Fin Type Inlet
Velocity
m/s
Temperature
Rise K
Pressure
Drop Pa
Circular 0.5 26.6488 0.6774
1 21.6226 1.95834
1.5 18.7446 3.44732
2 16.7798 5.07131

61
B. Porous (PPI=30) Circular Pin-Fin results
Fig.11: Pressure contours porous (PPI=30) circular pin fin channels-Inlet velocity 0.5m/s
The velocity vector distributions in solid pin fin channels are presented in Fig. 13. It shows
that with the same Reynolds number, the fluid velocities in solid pin fin channels are much higher
than those in porous pin fin channels. Large vortices are formed behind solid pin fins while no such
vortices are found in porous pin fin channels. In solid pin fin channels, the solid pin fins are totally
impermeable and this would narrow the flow passages and enhance the flow tortuosities inside.
While in porous pin fin channels, the porous pin fins are permeable and the fluid can flow through
them directly. This would widen the flow passages and lower the flow tortuosities inside. Therefore,
the flow resistances and pressure drops in porous pin fin channels would be lower. Similar Trend is
shown in Fig.5.6 for inlet velocity of 1 m/s.
Fig.12: Temperature contours porous (PPI=30) circular pin fin channels- Inlet velocity 0.5m/s

62
Fig.13: Velocity contours porous (PPI=30) circular pin fin channels- Inlet velocity 0.5m/s
Table 4: Comparison of Pressure drop and Temperature rise in Circular, Long Elliptical and
short Elliptical (PPI=30)
Fin Type Inlet
Velocity
m/s
Temperature
Rise K
Pressure
Drop Pa
Circular 0.5 26.6488 0.6774
1 21.6226 1.95834
1.5 18.7446 3.44732
2 16.7798 5.07131
Long
Elliptical
0.5 21.2587 0.362434
1 16.0238 1.10911
1.5 13.6143 2.0637
2 12.2815 3.17423
Short
Elliptical
0.5 32.7796 1.15565
1 27.4522 2.8647
1.5 23.5242 4.64382
2 20.6503 6.46394
The velocity vector distributions in solid pin fin channels are presented in Fig. 5.15.It shows
that with the same Reynolds number, the fluid velocities in solid pin fin channels are much higher
than those in porous pin fin channels. Large vortices are formed behind solid pin fins while no such

63
vortices are found in porous pin fin channels. In solid pin fin channels, the solid pin fins are totally
impermeable and this would narrow the flow passages and enhance the flow tortuosities inside.
While in porous pin fin channels, the porous pin fins are permeable and the fluid can flow through
them directly. This would widen the flow passages and lower the flow tortuosities inside. Therefore,
the flow resistances and pressure drops in porous pin fin channels would be lower. Similar Trend is
shown in Fig.5.18 for inlet velocity of 1 m/s.
VI. CONCLUSION
The forced convective heat transfer in three-dimensional porous pin fin channels is
numerically studied in this paper. Air is used as the cold fluids and the effects of Reynolds number,
pore density, and pin fin form are performed using ANSYS CFD Fluent software. Geometric
modeling is carried out using ANSYS Design Modeler and CFD meshing is done by ANSYS
meshing platform.
The flow and heat transfer performances in porous pin fin channels are also compared with
those in traditional solid pin fin channels in detail. The major observations are as follows.
• With proper selection of metal foams, such as PPI=30, significant heat transfer enhancements
and pressure drop reductions can be achieved simultaneously by using porous pin fins and the
overall heat transfer efficiencies in porous pin fin channels are much higher than those in
solid pin fin channels, which are 50%.
• The effects of pin fin form are also remarkable. With same physical parameters, the pressure
drops and heat fluxes are the highest in short elliptic porous pin fin channels and lowest in
long elliptic porous pin fin channels.
• With the same physical parameters, the overall heat transfer efficiencies in the long elliptic
porous pin fin channels are the highest while they are the lowest in the short elliptic porous
pin fin channels.
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