Modern Engineering Thermodynamics

Problems 313
heat is added to the box by heat transfer from an external
source. The surface temperature of the box is isothermal at
300.°F, and no work is done on or by the mixing box.
Determine
a. The quality (or temperature if superheated) of the exit flow.
b. The rate of entropy production inside the mixing box.
34. You are now a world famous energy researcher commissioned by
the National Entropy Foundation to determine the effect of vapor
generation on the entropy production rate in a two-phase
mixture. Your experiment consists of a steady state, closed loop
flow system in which a liquid-vapor mixture of Refrigerant-134a
flows through a test section consisting of a stainless steel tube
0.319 in. in diameter and 4.00 ft long. A constant wall heat flux
is imposed along the length of the tube of 450. Btu/(h · ft 2 )
The inlet quality is 0.00% and the outlet quality is 72.6%. The
wall surface temperature along the length of the tube is given
in °F by
R-134a
x in = 0
T w = 199:28 − 0:007217 × T + 0:39449 × T 2
for 0 ≤ x ≤ 4.00 ft. The mass flow rate of the R-134a is
1000. lbm/h, and the inlet and outlet temperatures are 10.0°F
and 0.00°F, respectively. Determine the total entropy
production rate in the test section of Figure 9.29.
FIGURE 9.29
Problem 34.
x
q wall= 450. Btu/h •ft 2 = constant
T W = f(x)
m R-134a = 1000. lbm/h
x out = 0.726
35. A steam turbine receives steam at 250. psia and 900.°F and
exhausts it at 20.0 psia. The turbine is adiabatic and does the
work of 190.4 Btu/lbm of steam flowing. Find the entropy
production per lbm of steam flowing.
36.* What mass flow rate is required to produce 75.0 kW from a
steam turbine with inlet conditions of 2.00 MPa, 900.°C and
exit conditions of 0.100 MPa, 200.°C if the turbine is reversible
but not adiabatic? The heat transfer from the turbine occurs at a
surface temperature of 50.0°C. Neglect any changes in kinetic
and potential energy.
37. Calculate the isentropic efficiency of a continuous flow adiabatic
compressor that compresses 20.0 lbm/min of a constant specific
heat ideal gas. The test data for this compressor are inlet state =
1.00 atm and 25.0°C; outlet state = 1.00 MPa and 350.°C;
c p = 1.00 KJ/(kg · K), R = 0.250 kJ/(kg · K). The isentropic
efficiency of a compressor is defined as
η s =
_ W isentropic compression
_W actual compression
38. A steady flow, steady state air compressor with a surface
temperature of 80.0°F handles 4000. ft 3 /min measured at the
intake state of 14.1 psia, 30.0°F and a velocity of 70.0 ft/s. The
discharge is at 45.0 psia and has a velocity of 280. ft/s. Both the
inlet and exit stations are located 4.00 ft above the floor.
Determine the discharge temperature and the power required to
drive the compressor for
a. A reversible adiabatic process.
b. An irreversible adiabatic process with a compressor work
transport efficiency of 80.0%.
39.* Determine the power required to compress 15.0 kg/min of
superheated steam in an uninsulated, reversible compressor
from 0.150 MPa, 600.°C to 1.50 MPa, 500.°C in a steady state,
steady flow process. Neglect any changes in kinetic and potential
energy. The boundary temperature is 20.0°C.
40. An adiabatic, steady flow compressor is designed to compress
superheated steam at a rate of 50.0 lbm/min. At the inlet to
the compressor, the state is 100. psia and 400.°F; and at the
compressor exit, the state is 200. psia and 600.°F. Neglecting
any kinetic or potential energy effects, calculate
a. The power required to drive the compressor.
b. The rate of entropy production of the compressor.
41.* A steady flow air compressor takes in 5.00 kg/min of
atmospheric air at 101.3 kPa and 20.0°C and delivers it at an
exit pressure of 1.00 MPa. The air can be considered an ideal
gas with constant specific heats. Potential and kinetic energy
effects are negligible. If the process is not reversible but is
adiabatic and polytropic with a polytropic exponent of n = 147,
calculate
a. The power required to drive the compressor.
b. The entropy production rate of the compressor.
42. An uninsulated, irreversible steam engine whose surface
temperature is 200.°F produces 50.0 hp with a steam mass
flow rate of 15.0 lbm/min. The inlet steam is at 400.°F,
100. psia; and it exits at 14.7 psia, 90.0% quality. Determine
(a) the rate of heat loss from the engine and (b) its entropy
production rate.
43.* A design for a turbine has been proposed involving the
adiabatic steady flow of steam through the turbine. Saturated
vapor at 300.°C enters the turbine and the steam leaves at
0.200 MPa with a quality of 95.0%. (a) Draw a T-s diagram
for the turbine, and (b) determine the work and entropy
production per kilogram of steam flowing through the turbine.
The turbine’s boundary temperature is 25.0°C.
44. An uninsulated, warp drive steam turbine on a Romulan battle
cruiser has a surface temperature of 200.°F. It produces 50.0 hp
with a steam mass flow rate of 150. lbm/min. The inlet steam is
at 400.°F, 100. psia, and it exits at 16.0 psia, 90.0% quality.
Determine
a. The heat transfer rate from the engine.
b. The entropy production rate of the engine.
c. Show whether the Romulans have discovered how to build
steam engines that violate the second law of
thermodynamics.
45.* Steam enters a turbine at 1.50 MPa and 700.°C and exits the
turbine at 0.200 MPa and 400.°C. The process is steady flow,
steady state, and adiabatic. The system boundary temperature is
35.0°C. Determine the following on the basis of a steam flow
rate of 6.30 kg/s:
a. The entropy production rate of the turbine.
b. The work transport energy efficiency of the turbine.
c. The turbine’s actual output power.