06-10-2016, 04:40 PM
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Abstract:
The advancement in the field of aerodynamics and the
engine Management system has made the modern
vehicles lot more Faster as compare to one from the
previous generations. The engine size has not increased
much for reaching this much speed and torque
requirement.
Modern engine which produce efficient results in form
of fuel efficiency and power to weight ratio runs at
higher rpm and produce more power every cycle, this
is a great boon but the amount of heat to be released
from the engine to the surrounding increases with the
increase in maximum power and speed requirement.
That is why we need a more advance engine cooling
system that can achieve proper cooling of the engine
without any compromise with the aerodynamics shape
of vehicle.
Key words: engine cooling, convection, carbon foam,
radiator, Nano fluid.
I.INTRODUCTION
The transportation of goods and passengers using the
modern highways where the speed requirement is lot
higher with heavy load on the vehicle combined with
problems in hot summers surely require an advance
engine cooling system.
II. CONVENTIONAL ENGINE
COOLING SYSTEM
Combustion of air and fuel takes place inside the engine
cylinder and hot gases are generated inside an internal
combustion engine. The temperature of gases may rise
up to around 2300-2500 ℃ , which is a very high
temperature and may result into burning of oil film
between the moving parts, pre combustion and may
result into seizing or welding of the moving parts. So,
this temperature must be reduced to lower values at
which the engine will work properly and much more
efficiently. Too much cooling is also not desirable since
it reduces the thermal efficiency and reduces the vaporization of fuel thereby showing improper burning
in form of black smoke in exhaust.
Though the conventional engine cooling system which
are either air cooled or water cooled are designed to
remove about 30-35% of total heat that the engine
dissipates, for now these system somehow fulfilling the
existing requirements but with the advancement in the
engine technology, increasing relative brake horse
power (BHP), aerodynamic design requirements,
emissions standards and energy crisis, it need
advancement.
The radiators fitted in current engine cooling
system are limited by air side resistance and require
a large frontal area to meet cooling needs.
Current engine cooling system has limited heat
dissipation and does not meet the requirement at
high engine output.
Heat dissipation to volume ratio of the system is
less.
At high speeds it is difficult to maintain the
temperature of engine components.
Heat rejected by the system (about 35%of heat
generated) is wasted to the atmosphere.
III. NEW RADIATOR DESIGN
After developing several ideas those are based on
changing the radiator construction or assisting some
other components and process with the system. Such as
changing the type of fin material used, their
construction, use of turbocharger etc.
The pros/cons of these design ideas were considered
based upon the rate of heat dissipation and other
limitations as per individual idea. From which these two
design changes came to be much more promising than
the rest of others.
First one is the use of carbon foam fins instead of
aluminum fins on current radiator design, more heat
dissipation can be obtained. Because carbon foam
increases the surface area exposed to the air. This is
mainly due to the fact that the carbon foam is porous
and allows the air to flow through it in addition to
allowing the air to flow around it.
Apart from this the convective resistance between the
coolant and the tubing can be reduced by Nano-fluids, a
two-phase mixtures composed of very fine particles in
suspension in a continuous and saturated liquids (water,
ethylene glycol, engine oil), it may provide a very
important enhancement in heat transfer as compared to
conventional radiator coolant.
IV. DESIGN OF
CONVENTIONAL RADIATOR
Considering a conventional car radiator having Height
of 365 mm and Width is 610 mm and depth of radiator
(Lout) is 22mm. The fin dimensions are: thickness of
fin(Thfin) is 0.35, pitch of fin is 1.52 mm, and separation
between two adjacent tubes is 8mm and the tube wall
thickness of 0.4 mm.
The corresponding value of TH avg (hot water average
temperature) is 90OCwithQd tubesas discharge rate fluid
which is 1.30 x 10-3 m3/sec, Qreq= 35596 W at
maximum brake horse power of 37969 W, the wind
velocity which is mainly due to forward motion of
vehicle is (Vair) 150 km/hr.
Rate of heat transfer over the surface of radiator is
given by:
Q= U x A x ϴm
And ϴm = TH avg- TC avg = 90– 40 = 50OC
UA= 1 / Rtotal,(total thermal resistance between water
and air)
Where U is overall heat transfer coefficient between
two fluids and ϴm is AMTD (arithmetic mean
temperature difference)
Rtotal = Rin + Rf, in + Rcond + Rout + Rf, out(1)
Rin = 1/ (ℎ
in x A total,in)
A total,in= .979416
Defining ? ? as nusselt number, Kair as thermal
conductivity of air, Re as Reynolds number and Pr as
Prandtl number
Re = (ρ x v x Lc) / μ
= .9937x104
Pr = ( μ x cp) / k
= 1.8936
? ? = 3.66 + [ (0.668 (D/W) x Re x Pr) / (1 + 0.04 (
(D/W) x Re x Pr)2/3]
? ? = 6.3366
? ? = (ℎ
in x Lc) / k,Where Lc is characteristic length.
ℎ
in= 2122.3 W/ m2 K
Assuming Rin as convective resistance between the
water & the inner surface of the tube, Rf,in as fouling
resistance that occurs on the internal surface of the tube,
Rcond as resistance to conduction through tube wall, Rout
as resistance between the air and the surface of the fins
and the outer tube surface (it is due to both convection
and conduction resistance to the fin) and Rf out as
fouling resistance that occurs on the outer surface of the
tubes.
Rin= 4.81 x 10-4 0C W-1
Rf, in = R”f, in / Atotal in
CONCLUSION
The application of carbon foam as the radiator material
in the design shows an increased rate of heat transfer
(Q= 53414 W) which is much greater than the required
value (Qreq= 35596 W).
Hence, the existing radiator can be made smaller in size
by 203 mm (33 %) in width. That’s means there is an
extra availability of 1630 cm3
space in engine
compartment and decrease in the frontal surface area.
Nanofluid, shows an increase rate of heat transfer
(Q=38078 W) that is also a great way to enhanced the
rate of convection between the inner walls of the tubing
and the fluid, this rate of heat transfer is greater than the
required value (Qreq= 35596 W) and hence the radiator
can work efficiently even at higher load and speed
requirement in hot climatic conditions.