874 | ThermodynamicsStagnation Properties
17–1C A high-speed aircraft is cruising in still air. How
will the temperature of air at the nose of the aircraft differ
from the temperature of air at some distance from the
aircraft?
17–2C How and why is the stagnation enthalpy h 0 defined?
How does it differ from ordinary (static) enthalpy?
17–3C What is dynamic temperature?
17–4C In air-conditioning applications, the temperature of
air is measured by inserting a probe into the flow stream.
Thus, the probe actually measures the stagnation temperature.
Does this cause any significant error?
17–5 Determine the stagnation temperature and stagnation
pressure of air that is flowing at 44 kPa, 245.9 K, and 470
m/s. Answers:355.8 K, 160.3 kPa
17–6 Air at 300 K is flowing in a duct at a velocity of (a) 1,
(b) 10, (c) 100, and (d) 1000 m/s. Determine the temperature
that a stationary probe inserted into the duct will read for
each case.
17–7 Calculate the stagnation temperature and pressure for
the following substances flowing through a duct: (a) helium at
0.25 MPa, 50°C, and 240 m/s; (b) nitrogen at 0.15 MPa, 50°C,
and 300 m/s; and (c) steam at 0.1 MPa, 350°C, and 480 m/s.
17–8 Air enters a compressor with a stagnation pressure of
100 kPa and a stagnation temperature of 27°C, and it is com-
pressed to a stagnation pressure of 900 kPa. Assuming the
compression process to be isentropic, determine the power
input to the compressor for a mass flow rate of 0.02 kg/s.
Answer:5.27 kW
17–9E Steam flows through a device with a stagnation
pressure of 120 psia, a stagnation temperature of 700°F, and a
velocity of 900 ft/s. Assuming ideal-gas behavior, determine
the static pressure and temperature of the steam at this state.
17–10 Products of combustion enter a gas turbine with a
stagnation pressure of 1.0 MPa and a stagnation temperature
of 750°C, and they expand to a stagnation pressure of 100
kPa. Taking k1.33 and R0.287 kJ/kg ·K for the prod-
ucts of combustion, and assuming the expansion process toPROBLEMS*be isentropic, determine the power output of the turbine per
unit mass flow.
17–11 Air flows through a device such that the stagnation
pressure is 0.6 MPa, the stagnation temperature is 400°C, and
the velocity is 570 m/s. Determine the static pressure and tem-
perature of the air at this state. Answers:518.6 K, 0.23 MPaSpeed of Sound and Mach Number
17–12C What is sound? How is it generated? How does it
travel? Can sound waves travel in a vacuum?
17–13C Is it realistic to assume that the propagation of
sound waves is an isentropic process? Explain.
17–14C Is the sonic velocity in a specified medium a fixed
quantity, or does it change as the properties of the medium
change? Explain.
17–15C In which medium does a sound wave travel faster:
in cool air or in warm air?
17–16C In which medium will sound travel fastest for a
given temperature: air, helium, or argon?
17–17C In which medium does a sound wave travel faster:
in air at 20°C and 1 atm or in air at 20°C and 5 atm?
17–18C Does the Mach number of a gas flowing at a con-
stant velocity remain constant? Explain.
17–19 Determine the speed of sound in air at (a) 300 K and
(b) 1000 K. Also determine the Mach number of an aircraft
moving in air at a velocity of 280 m/s for both cases.
17–20 Carbon dioxide enters an adiabatic nozzle at 1200 K
with a velocity of 50 m/s and leaves at 400 K. Assuming con-
stant specific heats at room temperature, determine the Mach
number (a) at the inlet and (b) at the exit of the nozzle.
Assess the accuracy of the constant specific heat assumption.
Answers:(a) 0.0925, (b) 3.73
17–21 Nitrogen enters a steady-flow heat exchanger at 150
kPa, 10°C, and 100 m/s, and it receives heat in the amount of
120 kJ/kg as it flows through it. Nitrogen leaves the heat
exchanger at 100 kPa with a velocity of 200 m/s. Determine
the Mach number of the nitrogen at the inlet and the exit of
the heat exchanger.
17–22 Assuming ideal-gas behavior, determine the speed of
sound in refrigerant-134a at 0.1 MPa and 60°C.
17–23 The Airbus A-340 passenger plane has a maximum
takeoff weight of about 260,000 kg, a length of 64 m, a wing
span of 60 m, a maximum cruising speed of 945 km/h, a
seating capacity of 271 passengers, maximum cruising alti-
tude of 14,000 m, and a maximum range of 12,000 km. The
air temperature at the crusing altitude is about 60°C. Deter-
mine the Mach number of this plane for the stated limiting
conditions.*Problems designated by a “C” are concept questions, and students
are encouraged to answer them all. Problems designated by an “E”
are in English units, and the SI users can ignore them. Problems
with a CD-EES icon are solved using EES, and complete solutions
together with parametric studies are included on the enclosed DVD.
Problems with a computer-EES icon are comprehensive in nature,
and are intended to be solved with a computer, preferably using the
EES software that accompanies this text.cen84959_ch17.qxd 4/21/05 11:08 AM Page 874