Thermal and hydraulic characteristics of brazed plate heat

Report
Engine heat transfer
Dr. Primal Fernando
[email protected]
Ph: (081) 2393608
1
Introduction
• Internal combustion engines
use heat to convert the energy
of fuel to power.
• Not all of the fuel energy is
converted to power.
• Excess heat must be removed
from the engine.
• In engines, heat is moved to
the atmosphere by fluids-water and air.
• If excess heat is not removed,
engine components fail due to
excessive temperature.
• Engine temperature is not
consistent throughout the
cycle.
• Heat moves from areas of
high temperature to areas of
low temperature.
2
• When fuel is oxidized (burned) heat is produced.
• Only approximately 30% of the energy released is
converted into useful work.
• The remaining (70%) must be removed from the engine to
prevent the parts from melting.
3
Additional heat is also generated by
friction between the moving parts.
•This heat must also be
removed.
4
Heat transfer
• Peak burned gas temperature ≈ 2500 K
• Maximum metal temperature for the inside of the combustion
chamber is much lower values due to
– Cracking on materials (cast iron - 400°C, aluminum alloys - 400°C
– Prevent deterioration of lubrication oil (keep below - 180°C)
– Spark plugs and valves must be kept cool to avoid knock and preignition problems
• Should maintain the combustion temperature: high heat transfer
reduce the engine efficiency
• Effects for emissions
• Heat transfer to inlet manifold reduces the air flow
5
Cooling System
• An automotive cooling system must perform several functions
– 1. Remove excess from the engine
– 2. Maintain a consist engine temperature
– 3. Help a cold engine warm-up quickly
– 4. Provide a means of warming the passenger compartment
6
Cooling system operation
Engine heat is transferred . . .
• through walls of the combustion chambers
• through the walls of cylinders
Coolant flows . . .
• to upper radiator hose
• through radiator
• to water pump
• through engine water jackets
• through thermostat
• back to radiator
7
Cooling system operation
Fans increase air flow through radiator
• Hydraulic fan clutches
• Hydraulic fans consume 6 to 8 HP
• Electric fans
Coolant (ethylene glycol)
• 50/50 mixture increases boiling point to 227°F (≈108°C)
• Automotive cooling systems operate around 180-212 degree F
(≈82 - 100°C)
• pressurizing system to 15 PSI increases to 265°F (≈ 1 bar, 130°C)
Coolant (propylene glycol)
• Less protection at the same temperatures
• Less toxic
8
Cooling Terms
• Thermal Conductivity
– Ability of a material to
conduct and transfer heat
• Thermal expansion
– Expansion of a material
when it is heated.
• Thermal growth
– Increase in size caused by
heating.
– When cooled does not
return to normal size.
• Thermal distortion
– Asymmetrical or nonlinear
thermal expansion.
Three means of heat transfer:
1. Conduction
2. Convection
3. Radiation
9
Heat Movement
• Conduction
–Movement of heat through materials ; Fourier’s Law: q  kT
Q
dT
one dimentional q x   k
A
dx
• Convection
–Movement of heat by fluids; Newton's Law of cooling
q  hc (T  Tw )
• Radiation
–Heat movement by transfer from one body to another. q   (T14  T24 )
Stefan-Boltzmann constant   5.67108W / m2 .K 4
10
Two Cooling Systems
• Small engines use two cooling systems;
– Air
– Liquid
• Both systems have two common features.
– Heat is transferred from the combustion
chamber to the crankcase by the oil.
– A large portion of the excess heat is removed
with the exhaust gases.
• The difference is in the medium used to move the
heat from the engine to the atmosphere.
11
Air Cooled Heat Movement
• In air cooled engines the excess
heat in the combustion chamber
moves through the cylinder
walls by conduction.
• The heat transfers from the
engine parts to the air at the
exterior surfaces and into the
atmosphere by convection.
• The air fins increase the surface
area between the engine and the
air--increasing heat transfer.
• The heart of the system is the fins on the flywheel which pumps
the air around the engine.
• The air flow is directed by the air shrouds.
12
Water Cooled Heat Movement
• Water cooled engines transfer the excess heat from the
combustion chamber through the cylinder walls by conduction.
• Water flowing past the exterior cylinder walls absorbs the heat
and transfers it to the radiator.
• Air flowing through the radiator absorbs the heat and transfers
it to the atmosphere.
• The system relies on a water pump to circulate the water
through the system and a fan to move air through the radiator.
13
Overall heat transfer from combustion chamber
T
Tg
Tg
q CV  q R
qCN
qCV
coolant
gas
Tw, g
Tw,c
tw
Tc
Tc
Distance, x
Schematic of temperature distribution and heat flow across the
combustion chamber
14
Overall heat transfer from combustion chamber
Gas side heat transfer
q cv  q R
T
Tg
q  hc, g (Tg  Tw, g )   (Tg4  Tw4, g )
Through the wall
q  k
q CN
(Tw, g  Tw,c )
q CV  q R
qCN
qCV
coolant
gas
Tw, g
tw
Coolant side
Tg
Tw,c
Tc
tw
q CV q  h (T  T )
c ,c
w,c
c
Tc
Distance, x
For force convection, convective heat transfer coefficient can be
calculated by Nusselt theory
Nu  C Re m Prn
 uL 
hL

 C 
k
  
m
 c p 


 k 
n
15
Heat transfer and engine energy balance –
conservation of energy
Energy in
Energy in by fuel+ air
engine
Energy out
Energy out by Power (or call brake
power)+ coolant + (oil + convection
+ radiation ) + exhaust
 f hf  m
 a ha  Pb  Qcoolant  Q misc  (m
 f m
 a )he
m
16
Heat transfer and engine energy balance –
conservation of energy
 f hf  m
 a ha  Pb  Qcoolant  Q misc  (m
 f m
 a )he
m
Brake power
Enthalpy of burned and
unburned gas mixture
Heat rejected to oil (if separately cooled)
convection + radiation engine’s external
surface.
17
Heat transfer and engine energy balance –
conservation of energy
 f hf  m
 a ha  Pb  Qcoolant  Q misc  (m
 f m
 a )he
m
Enthalpy of burned and
unburned gas mixture
For the studies it is convenient to divide exhaust enthalpy into sensible
part + reference enthalpy
he (T )  he,s  he (298K )
Enthalpy relative to reference
18
Heat transfer and engine energy balance –
conservation of energy
he (T )  he,s  he (298K )
 f hf  m
 a ha  Pb  Qcoolant  Q misc  (m
 f m
 a )he
m
This equation can be rearrange to
 f QLHV  Pb  Q coolant  Q misc  H e,ic  m
 he,s
m
Exhaust enthalpy loss due to
incomplete combustion
Note: LHV uses when exhaust has water vapor (HHV=LHV+ hfg)
19
Energy balance for automotive engines at maximum power
Q cool
Pb
SI- engines
CI-engines
25-28
34-38
Q misc
H e,ic
Percentage of fuel heating value
17-26
3-10
2-5
16-35
2-6
1-2
m he,s
34-45
22-35
20
Working fluid constituents
Process
SI engine
CI-Engine
Intake
Air
Fuel (liquid + vapor)
Recycled exhaust-used to control Nox
Residual gas –within the cylinder
Air
Recycled exhaust-used to control Nox
Residual gas –within the cylinder
Compression
Air
Fuel (liquid + vapor)
Recycled exhaust-used to control Nox
Residual gas –within the cylinder
Air
Recycled exhaust-used to control Nox
Residual gas –within the cylinder
Expansion
Combustion products (Mixture of N2, H2O,
CO2, CO, H2, O2, NO, OH, O, H)
Combustion products (Mixture of N2,
H2O, CO2, CO, H2, O2, NO, OH, O, H)
Exhaust
Combustion products (Mainly N2, H2O, CO2,
and either O2 ( Ф < 1) or CO and (Ф >1)
Combustion products (Mainly N2,
H2O, CO2, and O2)
Ф – fuel/air equivalence ratio
21
Heat transfer analysis
• Overall time averaged
– Adequate for some analysis
• Instantaneous
– Necessary for realistic cycle calculations
Average values of temperatures, heat transfer coefficients are calculated at
each point in the cycle and using following equations heat transfer per
cycle is obtained, q().
Gas side heat transfer
Through the wall
Coolant side
q  hc, g (Tg  Tw, g )   (Tg4  Tw4, g )
q  k
(Tw, g  Tw,c )
tw
q  hc,c (Tw,c  Tc )
22
Convective heat transfer coefficients -I
• For force convection, Nusselt correlation
Nu  C Re m Prn
 uL 
hL

 C 
k
  
m
 c p 


k


n
23
Convective heat transfer coefficients -II
• For time averaged heat flux,
Taylor and Toong
– Correlated heat transfer data
for 19 different engines
– They defined average effective
gas temperature, Tg,a over the
engine cycle, which is the
temperature of the wall that
stabilize if no heat is removed
from the out side (obtained by
extrapolating plotted data).
– Nu plotted against Re
– Suggested power law of 0.75
 Ah
c
(T  Tg ,a )d  0
Q B
4Q
Nu 

(B 2 / 4)(Tg ,a  Tc )k g Bk g (Tg ,a  Tc )
Re 
m B
4m

 g (B 2 / 4)  g B
  chargemass flow rate
m
B  Cylinder bore
24
Convective heat transfer coefficients -III
• For instantaneous spatial average
b
 S p B 
 hc B 
coefficients - Annad


  a

– a varies with intensity of
 k 
  
charge motion and engine
For normalcombustion, 0.35  a  0.8,
design
– Gas properties are evaluated
pVM
Tg 
at the cylinder average charge
~
m
R
temperature, Tg
b  0.7
25
Convective heat transfer coefficients -IV
• For instantaneous spatial average
coefficients - Woschni
– Assumed average gas
velocity equal to piston speed
Nu  0.035Re
m
assuming k  T 0.75 ,   T 0.62 and p  RT
hc  CB
m 1
m
m
p w T
0.75 1.62 m


VT
w  C1 S p  C 2 d r ( p  p m )
p r Vr


displaced volume Vd
instantaneous cylinderpressure p
mean pistomspeed S p  2 LN
engine speed, stroke N , L
motoredcylinderpressure at thesame carnkangleas p, pm
workingfluid pressure, volume,tempat
some referencestate(ex : start of combustion) pr , Vr , Tr
For thegas exchangeperiod C1  6.18, C2  0
For thecompression period C1  2.28, C2  0
For thecombustionand expansionperiod C1  6.18, C2  3.24  103
For engine with swirl
For t hegas exchangeperiod C1  6.18  0.417
with m  0.8
hc (W / m 2 .K )  3.26B(m) 0.2 p(kPa) 0.8 T ( K ) 0.55 w(m / s) 0.8
For t herest of cycle C1  2.28  0.308
vs
Sp
vs
Sp
vs  B p / 2,  p is theroatationof thepaddle wheel
used to measre theswirl velocit y
26
Convective heat transfer coefficients -V
• For instantaneous local
coefficients – LeFeuvre et al. and
Dent Sulaiman
– For direct injection diesel
engines with swirl
 vl   c p 
 hc l 

 

  0.036
 k 
    k 
0.8
0.333
; l  2r and v  r
Heat flux with any radius with Pr = 0.73
k  r
q (r )  0.023 
r v
2



0.8
T
g
(r )  Tw (r )

27
Radiative heat transfer – Diesel engines are about 20-35%
of total heat transfer, SI engines small compared to convective part
• Two sources radiative heat transfer
within the cylinder
– Gases
– Soot particles (about 5 times
compared to gases)
Annand
q R   (Tg4  Tw4 );  was proposedas  0.6 and later
proposedas 1.6
Flynn et al. for instantaneous heat transfer
 (   s ) a 1 
  s 
q R  2q R b(a  1)
 exp

360
360




correlations for q R , a, b and s were presented
a
28
Example
If radiation in the combustion chamber is negligible and time-averaged
overall heat transfer of the engine can be approximated as
q  hc,o (Tg  Tc )
Give an expression for hc,o
29
Solution
If radiation in the combustion chamber is negligible and time-averaged
overall heat transfer of the engine can be approximated as
q  hc,o (Tg  Tc ) Give an expression for hc,o
Gas side heat transfer
q  hc, g (Tg  Tw, g )   (Tg4  Tw4, g )
Through the wall
q  k
(Tw, g  Tw,c )
tw
Coolant side
q  hc,c (Tw,c  Tc )
T
Tg
Tg
q CV  q R
qCN
qCV
coolant
gas
Tw, g
Tw,c
tw
Tc
Tc
Distance, x
30
Solution
q  hc,o (Tg  Tc )
Gas side heat transfer
q  hc, g (Tg  Tw, g )
q  k
(Tw, g  Tw,c )
tw
q  hc,c (Tw,c  Tc )
q
 Tg  Tw, g
hc , g
qt w
 Tw, g  Tw,c
k
qt
q
q
 w
 Tg  Tc
hc, g
k
hc,c
q
 Tw,c  Tc
hc,c
t
1
1
1
 w

hc, g k hc,c hc,o
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