Thermo Flashcards
(40 cards)
Self ignition would occur in the engine using certain brand of petrol if the temperature due to compression reached 350°C.
Calculate the highest ratio of compression that may be used to avoid pre-ignition if the law of compression is
PV 1 . 3 = c
PV 1 . 4 = c
Calculate the final pressure in each case. Assume inlet condition of 27°C and 1 bar.
a 23.5 bar
b 12.8 bar
Self-ignition would occur in an engine using certain brand of petrol if the temperature due to compression reaches 350o C; when the inlet condition is 1 bar, 27o C.
Calculate the highest compression ratio possible in order to avoif self-ignition, if the compression is according to
a) adiabatic, with index of 1.4; and
b) polytropic, with index of 1.3
a 6.2
b 11.4
The model ‘6SE-TCA3 Perkins’ diesel engine have a stroke of 190 mm and a bore of 160 mm. If its clearance volume is 5% of the swept volume, determine the pressure and temperature at the end of compression when the inlet condition is 1 bar, 27o C.
P 67.2 bar
T 956 K
The model ‘6SE-TCA3 Perkins’ diesel engine have a stroke of 190 mm and a bore of 160 mm. If its clearance volume is 5% of the swept volume, determine the pressure and temperature at the end of compression when the inlet condition is 1 bar, 27o C.
P 67.2 bar
T 956 K
A closed rigid container has a volume of 1 m 3 and holds air at 345 kPa and 20°C. Heat is added until the temperature is 327°C. Determine the change in Internal Energy:-
a) Using an average value of the specific heat.
b) Taking into account the variation of specific heat with temperature.
a 932 kJ
b 1018.7 kJ
An adiabatic steam turbine expands steam from a pressure of 6 MPa and a temperature of 500°C to a pressure of 10 KPa The isentropic efficiency of the turbine is 0.82 and changes in kinetic and potential energy may be neglected. Determine the state of the steam at exit from the turbine and the specific work transfer.
849 kJ/kg
A steam turbine receives steam at 2 MPa and 250o C, and exhausts at 0.1 MPa, 0.85 dry.
a) Neglecting heat losses and changes in ke and Pe, estimate the work output per kg steam.
b) If, when allowance is made for friction, radiation, and leakage losses, the actual work obtained is 80% of that estimated in (a), calculate the power output of the turbine when consuming 600 kg of steam per minute.
a 539 kJ/kg
b 4.31 MW
A boiler receives feed water at 40o C and delivers steam at 2 MPa and 500o C. If the furnace is oil fired, the calorific value of oil being 42000 kJ/kg and 4000 kg oil are burned while 45000 kg of steam are produced, determine :
a) the heat supplied in the boiler.
b) the efficiency of the boiler.
a 1.484 x 10^8 kJ
b 88%
An air compressor receives air at 27o C and delivers it to a receiver at the rate of 0.5 kg/s. It is driven by an electric motor which absorbs 10 kW and the efficiency of the drive is 80%.
Water jacket cooling is used at the rate of 6 kg/min while its temperature rises from 10o C to 20o C. Estimate the temperature of the air delivered.
Data : Cp w = 4.186, and Cp a = 1.005 kJ/kgK
43.5 C
An air compressor receives air at 27o C and delivers it to a receiver at the rate of 0.5 kg/s. It is driven by an electric motor which absorbs 10 kW and the efficiency of the drive is 80%.
Water jacket cooling is used at the rate of 6 kg/min while its temperature rises from 10o C to 20o C. Estimate the temperature of the air delivered.
Data : Cp w = 4.186, and Cp a = 1.005 kJ/kgK
43.5 C
Air at 27o C receives heat at constant volume until its temperature reaches 927o C. Determine the heat added per kilogram? Assume for air C V = 0.718 kJ/kgK.
646.2 kJ/kg
An insulated, constant-volume system containing 1.36 kg of air receives 53 kJ of paddle work. The initial temperature is 27o C. Determine
Q = 0 ( insulated system ) W = -53 kJ ( externally inputted work ) The change in internal energy ∆U is
a) the change of internal energy.
b) the final temperature.
Assume a mean value C v = 0.718 kJ/kgK.
a 53
B 81.3
An ideal gas occupies a volume of 0.5 m 3 at a temperature of 340 K and a given pressure. The gas undergoes a constant pressure process until the temperature decreases to 290 K. Determine
a) the final volume,
b) the work if the pressure is 120 kPa An ideal gas occupies a volume of 0.5 m 3 at a temperature of 340 K and a given pressure. The gas undergoes a constant pressure process until the temperature decreases to 290 K. Determine
a) the final volume,
b) the work if the pressure is 120 kPa
A 0.426 m3
B -8.88 kJ
0 kg/s steam at 3 MPa, 300oC expands isentropically in a turbine to a pressure of 100 kPa. If the heat transfer from the casing to surrounding air represents 1 per cent of the overall change of enthalpy of the steam, calculate the power output of the turbine. Assume exit is 2 m above entry and that initial velocity of steam is 10 m/s whereas exit velocity is 1 m/s.
19
A piston and cylinder mechanism contains 2 kg of a perfect gas. The gas expands reversibly and isothermally from a pressure of 10 bar and a temperature of 327 C to a pressure of 1.8 bar.
Calculate:
a) the work transfer,
b) the heat transfer; and
c) the specific change in enthalpy of the gas.
Take R=0.3 kJ/kg K and n=1.4
A 617 kJ
B 617 kJ
C 0
The gas expanding in the combustion space of a reciprocating engine has an initial pressure of 50 bar and an initial temperature of 1623 C. The initial volume is 50000 mm 3 and the gas expands through a volume ratio of 20 according to the law pV 1 . 25 = constant. Calculate
a) the work transfer and
b) heat transfer in the expansion process.
Take R = 270 J/Kg K and C v = 800 J/Kg K.
A 527 J
B 136 J
A reciprocating steam engine cylinder contains 2kg of steam at a pressure of 30 bar and a temperature of 300 C. The steam expands reversibly to a final pressure of 2 bar, according to the law pv 1. 2 = c Calculate
a) the final state of the stream,
b) the work transfer and
c) the heat transfer in the process.
A 0.875
B 884 kJ
C -63.4 kJ/kg
Steam at a pressure of 6 MPa and a temperature of 500 C enters an adiabatic turbine with a velocity of 20 m/s and expands to a pressure of 50 kPa, and a dryness fraction of 0.98. The steam leaves with a velocity of 200 m/s. The turbine is required to develop 1MW. Determine:
a) the mass flow rate of steam required, when KE is neglected, and
b) What is the effect of KE on the answer?
A 2599.8kJ / kg
B 1.187 kg/s
Air, which may be considered a perfect gas, enters an adiabatic nozzle with negligible velocity. The entry pressure is 6 bar and the exit pressure is 1 bar; the entry temperature is 760 K. The flow throughout the nozzle is reversible and the mass flow rate is 2 kg/s. Calculate the exit velocity.
Take Cp = 1004.5 J/kg K and n = 1.4
782m / s
3 kg/s of steam enters an adiabatic condenser at a pressure of 100 kPa with dryness fraction 0.80,
and the condensate leaves the condenser at a temperature of 30 C. The condenser is cooled by
water which enters at a temperature of 5 C and leaves at a temperature of 25 C. Calculate the mass flow rate of cooling water required if all changes in kinetic and potential energy may be neglected. Assume Cp = 4.2 kJ/kgK, and the enthalpy at 100 kPa & 30oC = 125 kJ/kg.
75 kg / s
A reciprocating compressor delivers 0.1 kg/s of air at a pressure of 12 bar. The air enters the compressor at a pressure of 1 bar and a temperature of 15 C. Calculate the delivery temperature of the air, the work transfer rate and the heat transfer rate in the compression process for:
i. reversible polytropic compression, PV 1 . 2 = constant;
ii. reversible adiabatic compression;
iii. reversible isothermal compression.
A −10.6kW
B 0
C 20.5kW
A reciprocating internal combustion engine has a clearance volume of 0.0001m 3 and a compression ratio (volume ratio) of 10. The pressure and temperature of the combustion gases when the piston is at top dead centre are 4000 kN/m 2 and 1800ºC respectively.
Assuming that the expansion process follows PV 1.3 = constant, calculate:
a) the work transfer in this process, and
b) the temperature of the gases at the end of the process.
A 66.7 J
B 1039 K
A reciprocating steam motor is supplied with dry saturated steam at a pressure of 1.6MPa ( specific volume = 0.1238 m3 /kg). The stroke of the motor is 0.8m and the bore is 0.3m. The clearance volume is negligible. The steam enters the cylinder, expands at constant pressure for ¼ of the stroke and then expands reversibly according to a law PV = constant, til the end of the stroke. Calculate
a) the mass of the steam,
b) the work transfer and the heat transfer in the process?
A 31.3 kJ
B 31.3 kJ
A piston and cylinder mechanism has its piston fixed so that the volume contained is 0.0025m3 . The mechanism is filled with wet steam at a pressure of 2 bar. The steam is heated until it reaches the critical point. The piston is released and the steam expands adiabatically to a pressure of 2 bar and a volume of 0.5m3 . Calculate:
a) the mass of steam in the mechanism,
b) the dryness fraction of the steam after expansion.
A 0.788 kg
B 0.715
