To slow down large prime movers like locomotives, a process termed dynamic electric braking is used to switch the traction motor to a generator mode in which mechanical power from the drive wheels is absorbed and used to generate electrical current. As shown in the schematic, the electric power is passed through a resistor grid (a), which consists of an array of metallic blades electrically connected in series (b). The blade material is a high-temperature, high electrical resistivity alloy, and the electrical power is dissipaled as heat by internal volumetric generation. To cool the blades, a motor-fan moves high-velocity air through the grid. (a) Treating the space between the blades as a rectangular channel of 22 0 − mm × 4 − mm cross section and 7 0 − mm length. estimate the heat removal rate per blade if the airstream has an inlet temperature and velocity of 25 ° C and 5 0 m / s . respectively. while the blade has an operating temperature of 6 00 ° C . (b) On a locomotive pulling a 10-car train, there may be 2000 of these blades. Based on your result from part (a), how long will it take to slow a train whose total mass is 1 0 6 kg from a speed of 12 0 km / h to 5 0 km / h using dynamic electric braking?
To slow down large prime movers like locomotives, a process termed dynamic electric braking is used to switch the traction motor to a generator mode in which mechanical power from the drive wheels is absorbed and used to generate electrical current. As shown in the schematic, the electric power is passed through a resistor grid (a), which consists of an array of metallic blades electrically connected in series (b). The blade material is a high-temperature, high electrical resistivity alloy, and the electrical power is dissipaled as heat by internal volumetric generation. To cool the blades, a motor-fan moves high-velocity air through the grid. (a) Treating the space between the blades as a rectangular channel of 22 0 − mm × 4 − mm cross section and 7 0 − mm length. estimate the heat removal rate per blade if the airstream has an inlet temperature and velocity of 25 ° C and 5 0 m / s . respectively. while the blade has an operating temperature of 6 00 ° C . (b) On a locomotive pulling a 10-car train, there may be 2000 of these blades. Based on your result from part (a), how long will it take to slow a train whose total mass is 1 0 6 kg from a speed of 12 0 km / h to 5 0 km / h using dynamic electric braking?
Solution Summary: The author calculates the cross-sectional area of a channel as follows: Dimension of the channel, Inlet temperature, Velocity, Specific Heat, Thermal Conductivity, and Prandtl number.
To slow down large prime movers like locomotives, a process termed dynamic electric braking is used to switch the traction motor to a generator mode in which mechanical power from the drive wheels is absorbed and used to generate electrical current. As shown in the schematic, the electric power is passed through a resistor grid (a), which consists of an array of metallic blades electrically connected in series (b). The blade material is a high-temperature, high electrical resistivity alloy, and the electrical power is dissipaled as heat by internal volumetric generation. To cool the blades, a motor-fan moves high-velocity air through the grid.
(a) Treating the space between the blades as a rectangular channel of
22
0
−
mm
×
4
−
mm
cross section and
7
0
−
mm
length. estimate the heat removal rate per blade if the airstream has an inlet temperature and velocity of
25
°
C
and
5
0
m
/
s
. respectively. while the blade has an operating temperature of
6
00
°
C
.
(b) On a locomotive pulling a 10-car train, there may be 2000 of these blades. Based on your result from part (a), how long will it take to slow a train whose total mass is
1
0
6 kg
from a speed of
12
0
km
/
h
to
5
0
km
/
h
using dynamic electric braking?
Branch of science that deals with the stationary and moving bodies under the influence of forces.
An electric resistance heater is embedded in a long cylinder ofdiameter 30mm. When water with a temperature of 25 C and velocityof 1m/s flows crosswise over the cylinder, the power per unit lengthrequired to maintain the surface at a uniform temperature of 90 C is28kW/m. When air, also at 25 C, but with a velocity of 10m/s isflowing, the power per unit length required to maintain the samesurface temperature is 400W/m. Calculate and compare the convectioncoefficients for the flows of water and air.
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