TUTORIAL QUESTIONS ON THERMODYNAMICS (GEC 221).

Q1. 0.05m3 of a gas at 6.9 bar expands reversibly in a cylinder behind a piston according to the law

pv1.2 = Constant until the volume is 0.083. Compute the work done by the gas and sketch the process

on a p-v diagram (15300NM).

Q2. 1Kg of a fluid is compressed reversibly to a law pv = 0.25 where p is in bar and v in m3/kg. The

fluid volume is 25% of the initial volume. Calculate the work done on the fluid and sketch the process

on a p-v diagram. (34660NM).

Q3. 1kg of a fluid expands reversibly according to a linear law from 4.2bar to 1.4bar. The initial and

final volumes are 0.004m3 and 0.02m3 respectively. The fluid is then cooled reversibly and constant

pressure and finally compressed reversibly according to a law pv = constant back to the initial

condition of 4.2bar and 0.004m3 .calculate the work done in each process stating whether it is done on

or by the fluid and calculate the net work of the cycle. Sketch the cycle on a p-v diagram (4480Nm, -

1120Nm, -1845Nm, 1515Nm).

Q4. A fluid expands adiabatically from a volume of 1m3 to a volume of 2m3 by flowing into an

evacuated chamber. The initial pressure of the fluid is 10bar. What is the change in internal energy of

the fluid? (0)

Q5. A steam turbine receives a steam flow of 1.35kg/s and delivers 500kW. The heat loss from the

casing is negligible.

a. Find the change of enthalpy across the turbine when the velocities at the entrance and exit and

the difference in elevation at the entrance and exit are negligible.

b. Find the change of enthalpy across the turbine when the velocity at entrance is 60m/s, the

velocity at exit is 360m/s and the inlet pipe is 3m above the exhaust pipe. (370kJ/kg,

433kJ/kg).

Q6. The mass flow rate of air flowing steadily through a compressor is 0.5kg/s. The velocity,

Pressure and Specific volume of air are 6m/s, 1bar and 0.85m3/kg respectively at entry and

4.5m/s, 7.0bar and 0.2m3/kg respectively at exit from the compressor. The internal energy of the

air leaving is 80kJ/kg greater than that of air entering cooling water in a jacket surrounding the

cylinder absorbs heat from the air at the rate of 59kJ/s. Calculate the power required to drive the

compressor at the inlet and outlet pipe Cross-Sectional areas. (-68.105kJ/s, 0.071m2, 0.022m2).

Q7. A mass of 2kg of ammonia existing as a saturated vapor at temperature 500C is taken by a

reversible process to become saturated vapour at temperature 200C. What is the maximum useful

work that can be obtained from this process in an environment at 1bar and 150C? The following

properties apply to saturated ammonia vapour.

Note: Subscript, s, indicates saturation values and subscript, g indicates gaseous phase.

(95.186KJ/Kg).

Q8. A system consists of a pure dissipative element and a pure thermal element with a heat

capacity of 50kJ/K. it experiences an irreversible work transfer interaction which takes the system

from state 1 at a temperature of 300K to state 2 at a temperature of 310K. If no heat is

transferred during this irreversible process, calculate the change in entropy of the system.

(1.6395kJ/kgK).

t(s) 0C P(s) bar Ug m3/kg hg, kJ/kg Sg kJ/kg

50 20.33 0.0635 1474.7 4.773 20 8.57 0.1494 1462.6 5.095

Q9. When a certain perfect gas is heated at constant Pressure from150C to 950C, the heat

required is 1136kJ/kg. When the same gas is heated at constant volume between the same

temperatures the heat required is 808kJ/kg. Calculate Cp, Cv, 𝛾, R and the molecular weight of

the gas. (14.2kJ/kg, 10.1kJ/kg, 1.406, 4.1kJ/kgK, 2.028).

Q10. 1kg of the steam at a pressure of bar and having a volume of 0.312m3 is confined inside a

cylinder equipped with a frictionless piston. If 1500kJ of heat is allowed into the steam its pressure

is kept constant, how much work does the steam do? (286kJ).

Q11. A compressor takes in 500kg/min of air at 0.98bar and 180C and delivers it at 5.5bar and

680C. The diameter of the intake and delivery pipes are respectively 450mm and 200mm and the

Compressor is driven by a motor 1000kW. Determine the air velocities in the intake and delivery

pipes and assuming that all the energy from the motor is transmitted to the air, estimate the rate at

which heat is transferred to or from the air during compression. (U1=44.7m/s, U2=47.2m/s, Q=-

580W).