Document 6535637

Sample Questions from Dr Ahmed's ME
398 Finals from the last three years'"
A heat pump, working in a reversed Carnot cycle and driven by a
1.5 kW electric motor, provides heating for a building whose
interior is to be kept at 19 degrees Celsius.
(i) On a day whentheoutsidetemperature is -5.35 degrees
Celsius and energy is lost through the walls and the roof at a rate
of 63,000 kJ/h, would the heat pump suffice?
(ii) At 12 cents/kWh, estimate the electric bill for 8 hours of
operation of this heat pump (whether or not the heat pump has the
capacity to perform its function fully).
The electronic components ofa computer consume 370 W of
electrical power. The prevent overheating, a 20 W-fan blows in
air from the room at 20 degrees Celsius. The air exits the
computer enclosure at 35 degrees at nearly the same pressure.
Heat transfer through the walls of the computer is negligible. For
a steady state operation, determine:
(a) the flow rates of air through the computer, in kg/so
(b) the rate of entropy production in the computer, in kW/K.
A turbine operates with steam entering at 1.5 MPa at the rate of
200 kg/so The steam exits as saturated vapor at 0.1 MPa.
Assuming isentropic expansion in the turbine, estimate the power
output.
a) If a block of aluminum (p = 2700 kgim3, Cp= 0.903 kJ/kg. K)
and a block of steel (p = 8930 kgim3, cp = 0.385 kJ/kg.K) having
the same volume received the same amount of energy by heat
transfer (oQ), which one would experience the greater
temperature increase?
b) What are some of the principal irreversibilities present during
the operation of an automobile?
c) A system consists of two gases, initially at two different
temperatures, and separatedby a masslesspartition. The system
is isolated from its surroundings. The partition is removed and
the gasesmix with each other. Apply and simplify the
conservation of energy equation to the system as it goes through
the mixing process. Do the same with the equation describing
entropy changes for the system. Briefly explain the implications
of each of the final expressions.
One kg of air behaving like an ideal gas operates in a Carnot
cycle to produce power. The thermal efficiency of the cycle is
60%. The amount of heat added to the cycle (QH, at the high
constant temperature, TH) is 40 kJ. At the end of this heat
addition process the pressure and the specific volume of the air
are 560 kPa and 0.3 m3, respectively.
a) Show the process on the T-s plane.
A 30 0. resistor is used for heating a stream of air flowing through
an insulated duct. At steady state operation, the current flowing
through the resistor is measured at 15 A, and the temperature is
400 K. The air enters the duct at 290 K and 1 atm, and exits the
duct at 300 K, with negligible change in pressure.
(a) Determine the volumetric as well as mass flow rates of air
flowing through the duct.
(b) For the resistor as the system, find the rate of production of
entropy.
(c) For a CV enclosing the air in the duct and the resistor, find the
rate of production of entropy.
One kg of air behaving like an ideal gas operates in a Carnot
cycle to produce power. The thermal efficiency of cycle is 60%.
The amount of heat added to the cycle (QH. at the high constant
temperature, TH) is 40 kJ. At the end of this heat addition process
the pressure and the specific volume of the air are 560 kPa and
0.3 m3, respectively.
a) Show the process on the T-s plane.
b) Determine THand TLo
c) Determine the amount of heat (QJ that must be rejected at TLo
d) Determine the works done during the heat addition and heat
rejection processes.
e) Determine the works done during the isentropic processes
(assume constant specific heat).
f) Find the net work produced by the air during one cycle.
An engine working with 0.1 kg of air follows the Camot cycle.
The high temperature reservoir is at 940 K, and at the beginning
of isothermal expansion at this temperature the air pressure is 8.4
MPa. The heat added to the air in this expansion process is 8.4
kJ. The low temperature reservoir is at 300 K. Determine:
(a) the pressures at the 3 other "comers" of the Camot cycle;
(b) the net work developed per cycle, in joules;
(c) the thermal efficiency of the engine.
1 kg of water initially at 3 bar and 200 degrees Celsius undergoes
a process to its new state at 15 bar and 210 degrees Celsius while
being compressed in a piston-cylinder assembly. Heat transfer
occurs through a thin wall, with the surrounding at 22 degrees
Celsius. The work done is measured to be (-)175 kJ. Neglecting
the changes in KE and PE, determine whether the work
measurement may be in error.
The electronic components of a computer consume 300 kW of
electrical power. The prevent overheating, a 50 W-fan blows in
air from the room at 20 degrees Celsius. The air exits the
computer enclosure at 35 degrees at nearly the ambient pressure.
Heat transfer through the walls of the enclosure is negligible. For
a steady state operation, determine:
b) DetermineTHand TL.
c) Detennine the amount of heat (QL) that must be rejected at TL.
d) Detennine the works done during the heat addition and heat
rejection processes.
e) Detennine the works done during the isentropic processes
(assume constant specific heat).
t) Find the net work produced by the air during one cycle.
(a) the mass as well as volumetric flow rates of the air;
(b) the rate of entropy production in the enclosure.
. The abovedo not includecorrectionsmadeduring thc cxamination.The type of
questionsaswell asthe level of difficulty (along with the total numberof
problems)can vary from semesterto semester.Usually thereare 3-4 problems
that may requirethe full two hoursof the final exam.