Introduction
Exhaust
system has vital role in performance of IC engine which can significantly
affect the fuel consumption, exhaust smoke emission and temperature. The back
pressure in exhaust system should be limited to specific value in order to
contain emissions. The allowable back
pressure should be less than half of permissible value. The design
consideration for exhaust system designs is pipe size, silencer type and
configuration of system.
Figure
1: Fourcylinder engine exhaust manifold [15]
The
exhaust gases from engine cylinder head are carried away to exhaust system
using exhaust manifold. The emission efficiency and fuel consumption are also
dependent upon design and type of exhaust manifold.
The
exhaust gases from engine cylinder head are carried away to exhaust system
using exhaust manifold. The emission efficiency and fuel consumption are also
dependent upon design and type of exhaust manifold. The exhaust gases from
engine cylinder head are carried away to exhaust system using exhaust manifold.
The emission efficiency and fuel consumption are also dependent upon design and
type of exhaust manifold. Exhaust Manifolds are affected by â€śthermal stresses
and deformations due the temperature distribution, heat accumulation or
dissipation and other related thermal characteristicsâ€ť [14]. The exhaust stroke
piston work, gas exchange process is also influenced by exhaust manifold. The
backpressure is one of the important parameter which requires consideration in
the design of exhaust system. The backpressure has undesirable effect on engine
performance due to increase in residuals in the engine head. The temperature
increases at the beginning of the compression due to increased residuals and
deteriorate the thermal efficiency of engine.
II. Literature review
Jafar M Hassan et.al. [1] have conducted thermal
analysis on exhaust manifold design using FEA. The effect of longitudinal
tapered design on performance of exhaust manifold are also evaluated and the
results have shown enhanced performance of exhaust manifold with tapered
design. M. Usan et.al. [2] have conducted design optimization of catalytic
converter and exhaust manifold using integrated software. The optimized design
with varying shapes and dimensions of exhaust manifold is presented. Hessamed in Naemi et.al. [3] have conducted
CFD analysis on exhaust manifold to determine thermal and fluid flow
characteristics. The flow loss coefficient of manifold is evaluated and the
variables affecting the flow are presented. Masahiro Kanazaki et. al. [4] have
used multi objective genetic algorithm in optimization of exhaust manifold. Hong HanChi et.al. [5] In their research
paper they used GTPower, 1dimensional software, for estimating the engine
performance of a single cylinder IC engine. Taner Gocmez et.al. [6] have
conducted experimental investigation on exhaust manifold to improve its
strength using design optimization technique. The stresses and deformation are
evaluated from the analysis and critical failure regions are identified.
MartinezMartinez et.al. [7] have evaluated performance of exhaust manifold
using techniques of Computational Fluid Dynamics. In the analysis, the exhaust
manifold is placed nearer to catalytic converter. Benny Paul et.al. [8] have
conducted CFD simulation on exhaust system of FI engine using RANS solver. The
turbulence model used in the analysis was RNG kÎµ turbulence mode and pressure,
temperature is evaluated. MohdSajid Ahmed et.al. [9] have worked on
optimization of exhaust manifold of 4cylinder IC engine. The result shave
shown exhaust tube bending radius and thickness has significant effect on
temperature and pressure. Kandylas et.al. [10] have conducted steady state
thermal analysis on exhaust manifold. The temperature and radiation boundary
conditions are applied on the exhaust system. The heat dissipation plot (heat
flux) and temperature plots are evaluated. Bin Zou et al. [11] have conducted
CFD investigation on exhaust manifold to determine the effect of temperature on
engine performance. The free vibration analysis of exhaust manifold is also
conducted to determine natural frequency and mode shapes. P.L.S. Muthaiah
et.al. [12] have worked on improving performance of exhaust manifold using
techniques of Computational Fluid Dynamics. The back pressure is evaluated and
essential design changes to reduce backpressure are presented.
III.
Objective
The objective of current research is
to investigate the heat transfer characteristics of exhaust manifold
incorporated with fins. The CAD design of exhaust manifold is developed using
Creo design software and its thermal analysis was conducted using ANSYS v21
simulation package. The CFD analysis is also conducted to determine the effect
of fins on heat transfer characteristics of exhaust manifold.
IV.
Methodology
The
FEA simulation of exhaust manifold involves 3 stages i.e. preprocessing,
solution and post processing. The CAD model of exhaust system is developed by
using the dimensions from literature [13].
Figure 2: Dimensions of exhaust
manifold [13]
Figure 3: Exhaust manifold design
The
CAD design of exhaust manifold is developed using sketch, extrude and fillet
tool. The sketch profile is generated and sweep tool is used to generate tube
profile. The shell tool is used create hollow structure inside. The developed
CAD model is shown in figure 3 above.
Figure 4: Imported exhaust design
The
exhaust design is imported in ANSYS 21 design modeler. The inner cavity of
exhaust system is filled using tools. The other geometric errors like edge
defects, curvature defects are rectified. The exhaust manifold is discretized
using tetrahedral elements. The transition ratio is set to 0.77 and growth rate
is set to 1.2. The number of elements generated is 372548 and number of nodes
generated is 85527. The discretized model of exhaust design is shown in figure
5 below.
Figure 5: Discretized model
Figure 6: Copper domain definition
(without fins)
The
domains are defined for the simulation, i.e., solid domain and fluid domain.
The solid domain is defined using copper material as shown in figure 6 above.
The energy model is defined as thermal energy. The fluid domain is also defined
in the analysis with reference pressure set to 1atm and turbulence model is set
to komega. The material defined in fluid domain is exhaust gas. The domain
definition of fluid is shown in figure 7 below.
Figure 7: Fluid domain definition
(without fins)
The
similar boundary conditions are also applied on exhaust manifold with fins as
shown in figure 8 below. The fins are placed at small distance from the inlet
of exhaust gas tubes. In single tube, 3 fins are placed and multiple copies are
presented using pattern tool.
Figure 8: Fluid domain definition
(with fins)
The
simulation is run by defining RMS residual target values of .0001 and number of
iterations are set to 200. The convergence plot of mass, momentum and energy
are generated at the solution stage. .
V.
Results and discussion
The
simulation results are obtained for exhaust manifold with and without fins. The
results obtained from the analysis include total pressure, temperature, eddy
dissipation and turbulence kinetic energy.
Figure
9: Wall heat flux (without fins design)
The
wall heat flux plot is obtained from the analysis which represented heat
dissipation. The heat flux is observed to be maximum at the intersection region
of longitudinal member and 4 inlet tubes. The zones of maximum heat flux are
shown in blue color and magnitude of heat flux at this region is 9283W/m^{2}.
Figure 10: Temperature plot (without
fins design)
The
temperature plot is obtained from the analysis of exhaust manifold without fin
design as shown in figure 10 above. The temperature is maximum at the exhaust
gas inlet region. As the gas move towards the exit, the temperature increases
at the bottom region of exit tube which is shown in blue color with magnitude
of 530.7K. The pressure plot is obtained from the CFD analysis and is shown in
figure 11 below. The plot shows higher magnitude at the inlet section of tube
with magnitude of 3105Pa which is shown in red color. The exhaust gas pressure
reduces as it moves towards the exit tube where the pressure is nearly 86.96Pa.
Figure
11: Pressure plot (without fins design)
The
turbulence kinetic energy and eddy dissipation plot is generated which is shown
in figure 11 and figure 12 below. The turbulence kinetic energy is maximum at
the intersection of exit tube and longitudinal member with magnitude of 51.85m^{2}/s^{2}.
The change of fluid flow behavior at the corner region causes an increase in
turbulence which is represented by high magnitude of turbulence kinetic energy
at this zone.
Figure 12 (a): Turbulence kinetic
energy (without fins design)
Figure 12 (b): Turbulence eddy
dissipation (without fins design)
Similar
analysis is also conducted on exhaust system with fins. The heat flux plot
obtained from the analysis of finned exhaust manifold is shown in figure 13
below.
Figure
13: Wall heat flux (with fins design)
The
heat flux is observed to be maximum at the intersection region of longitudinal
member and 4 inlet tubes. The zones of maximum heat flux are shown in blue
color and magnitude of heat flux at this region is 9572W/m^{2}.
Figure 14: Temperature plot (with
fins design)
The
temperature is maximum at the exhaust gas inlet region and then reduces
suddenly due to enhanced heat dissipation by incorporated fins. The temperature
at the regions near the fins is low. As the gas move towards the exit, the
temperature increases at the bottom region of exit tube which is shown in blue
color with magnitude of 522.3K.
Figure
15: Turbulence kinetic energy (with fins design)
The
turbulence kinetic energy is maximum at the intersection of exit tube and
longitudinal member with magnitude of 53.78m^{2}/s^{2}. The
change of fluid flow behavior at the corner region causes an increase in
turbulence which is represented by high magnitude of turbulence kinetic energy
at this zone.
Table 1: Comparison of output
Design
Type

Without
Fins

With
Fins

Turbulence
Kinetic Energy (m^{2}/s^{2})

51.85

53.78

Max.
Temperature (K)

535

522

Heat
Flux (W/m2)

9283

9572

Figure 16: Turbulence kinetic energy
comparison
The
turbulence kinetic energy comparison shows higher magnitude for exhaust
manifold design with fins as compared to exhaust system design without fins.
Similarly, heat flux plot comparison shows higher magnitude for exhaust system
with fins which represents higher heat dissipation rate. The exhaust manifold
with fins has 289 W/m^{2} higher heat flux as compared to exhaust
system without fins.
Figure 17: Heat flux comparison
Figure 18: Maximum temperature
comparison
The temperature
comparison plot is obtained for both designs as shown in figure 18 above. The
plot shows higher maximum temperature for exhaust system design without fins
with magnitude of 535K and maximum temperature for design with fins has
522K.


VI. Conclusion
The
heat dissipation from exhaust system design can be significantly improved by
incorporating fins attached to tubes of exhaust manifold. The thermal
characteristics of exhaust manifold system using CFD has shown an enhancement
of heat dissipation capability by 3.01%. The incorporation of fins augmented
turbulent flow which enhanced heat dissipation capability of exhaust manifold
with 2.42% reduction in maximum temperature. Further research can be done in
changing material of exhaust system and also by changing turbulence models.