Environmental Engineering Reference
In-Depth Information
Internal combustion engines reject heat with higher
power densities because they produce much hotter com-
bustion gases. The firing temperatures of the most effi-
cient (@40%) gas turbines are in excess of 1300
C, and
the gases leave the machine usually at temperatures well
above 500
C. In stationary industrial turbines this heat
can be used to generate steam for steam turbines, achiev-
ing combined cycle efficiency of up to 60% and reducing
the heat flux through a stack to less than 20 W/m
2
.No
such combination is possible for gas turbines in flight,
and hence large turbofan jet engines like the GE90
(cruising thrust 70 kN, specific cruising fuel consumption
15.6 mg/Ns) directly reject about 25 MW/m
2
of hot
nozzle area (1 m
2
) at the design speed of 0.85 M (GE
Aviation 2006).
Even the most efficient automotive Otto cycle engine
loses about 30% of its initial fuel input through the radia-
tor and about 40% in exhaust gases. If all this gas-borne
heat were rejected through a tailpipe, even a compact ve-
hicle like the Honda Civic (engine rated at 85 kW, actual
driving fuel consumption@50 kW at 80-100 km) would
have thermal flux close to 1 kW/cm
2
(10 MW/m
2
)of
its exhaust pipe opening. In reality, most of that heat is
lost by radiation before it reaches the tailpipe. Exhaust
gases leave the engine at about 800
C, at a rate of 50
g/s and speed of 60 m/s, but exit the tailpipe at only
about 65
C, indicating rapid heat loss along the exhaust
piping. Because a large part of both exhaust and radiator
heat is absorbed by other car surfaces before it is finally
rejected into the atmosphere, and because the directly
rejected heat is commingled with heat generated by driv-
ing, it makes sense to divide a vehicle's power by its foot-
print (typically 7-9 m
2
). At full capacity this flux is about
11 kW
t
/m
2
Mercedes 600. In terms of heat rejection, cars are thus
equivalent to miniature movable power plant cooling
towers.
But cars are not the most ubiquitous high power den-
sity heat emitters; that primacy now belongs to micro-
processors. The first microprocessor, the Intel 4004,
released in 1971, had 2,300 transistors on a 135-mm
2
die and dissipated about 2.5 W/cm
2
. In 1978 the Intel
8086 had 29,000 transistors and dissipated 7.6 W/cm
2
,
a rate equal to that of a kitchen hot plate (1.2 kW in a
160-cm
2
circle). By 2001 ultralarge-scale integration of
Intel's Xeon Irwindale placed 50 million transistors on
a die of 130 mm
2
and consumed 115-130 W; its heat
rejection rate reached 100 W/cm
2
. The Itanium and
Apple's G4 have rates of about 110 W/cm
2
, or 1.1
MW/m
2
(fig. 11.5) (Azar 2000; Viswanath et al. 2000;
Joshi 2001). These power densities are of the same mag-
nitude as heat rising from large power plant stacks and
11.5 Heat rejection rates of Intel microprocessors, 1971-
2005. Plotted from data in Azar (2000), Viswanath et al.
(2000), and Joshi (2001).
for a Honda Civic and twice as much for a
