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PROB SOLVING PBE > HEATRA PROB > Flashcards

HEATRA PROB Flashcards

1
Q

An uninsulated steam pipe passes through a room in which the air and walls are at 25C

A

998

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2
Q

A hydrogen-air Proton Exchange Membrane (PEM) fuel

A

9.4 m/s

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3
Q

Humans are able to control their heat production rate and heat loss rate to maintain a nearly constant temperature

A

Still air - 34C, 146W
In water - 300K, 1320 W

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4
Q

A thin silicon chip and an 8mm thick aluminum substrate

A

Tc = 75.3C, chip will be operate below its max allowable temperature

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5
Q

A thermal conductivity of a D = 14nm diameter carbon nanotube is measured with an instrument that is fabricated
Estimate the thermal conductivity of carbon nanotube

A

3113 W/mK

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6
Q

A spherical, thin walled metallic container is used to store liquid nitrogen at 77K. The container has a diameter of 0.5 m and is covered with an evacuated

What is the rate of heat transfer to the liquid nitrogen?
What is the rate of liquid boil-off?

A

13.06W
7L/day

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7
Q

A plane wall is a composite of two materials, A and B. The wall of materials A has a uniform heat generation
Determine the temperature To of the insulated surface and temperature at2 of the cooled surface

A

105C
To = 140C

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8
Q

Find the specific volume for H2O at 10 MPa and 500C

A

0.032 m^3/kg

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9
Q

300 kg/min of steam at 3 MPa and 400C
Can you deduce the specific heat of the steam under this condition?

A

2133.5 kW

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10
Q

The model ‘6SE-TCA3 Perkins’ diesel engine have a stroke of 190mm and a bore of 160 mm.

A

956 K

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11
Q

An adiabatic steam turbine expands steam from a pressure of 6 Mpa and a temperature of 500C 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.

A

State: mixture
W = 850 kJ/kg

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12
Q

Steam at 1 MPa, 0.95 dry is throttled to 10 kPa. What is the quality of steam after throttling?

A

Superheated

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13
Q

A boiler receives feed water at 40C and delivers steam at 2 MPa and 500C. If the furnace is oil fired, the calorific value of oil being 42000 kJ/kg and 4000 kh oil are burned while 45000 kg if steam is produced, determine:
Heat supplied in the boiler
Efficient of the boiler

A

1.484x108
88%

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14
Q

An air compressor receives ait at 27C 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 its temperature rises from 10C to 20C.
Estimate the temperature of the air delivered.

A

43.5C

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15
Q

30 kg/s stea at 3 MPa, 300C 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 1m/s.

A

+19 kW

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16
Q

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 = contant. Calculate: the work transfer and heat transfer in the expansion process

A

527 J and 137 J

17
Q

A reciprocating steam engine cylinder contains 2 kg 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. Final state of the steam
B. The work transfer
C. The heat transfer in the process

A

0.875
884
-63.4 kJ/kg

18
Q

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

m = 1.216 kg/s
m = 1.187 kg/s or an error of 2.4%

19
Q

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.

A

V = 782m/ s 2

20
Q

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 30C. The condenser is cooled by water which enters at a temperature of 5C and leaves at a temperature of 25C. 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.

A

m kg s w ∴ = 75 /

21
Q

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

0.788 kg
x3 = 0.715

22
Q

Steam at a pressure of 2 MPa and a temperature of 240ºC enters a nozzle with a velocity of 15m/s.
The steam expands reversibly and adiabatically in the nozzle to a pressure of 100 kPa and a dryness
fraction of 0.9. Calculate the velocity of the steam at exit from the nozzle,

A

715m/ s

23
Q

1kg of gas occupies a volume of 0.4m3 at a pressure of 100 kN/m2. The gas is compressed
isothermally to a pressure of 450 kN/m2. Determine the work of compression, and the change in
entropy of the gas during the compression. Assume for the gas R = 300 J/kgK

A
  • 60 kJ
    -0.451
23
Q

A one pass steam generator receives saturated water at 20 bar, and converting it into steam at
400ºC. The mass flow rate of steam is 1200 kg/h, calculate the heat transfer in the generator.

A

150 kW

24
Q

The cylinder of an engine has a stroke of 300mm and a bore of 250mm. The volume ratio of
compression is 14:1. Air in the cylinder at the beginning of compression has a pressure of 96
kN/m2 and a temperature of 93 ºC. The air is compressed for the full stroke according to the law
PV1.3 = C. Determine the work transfer per unit mass of air. Assume air R = 287 J/kgK.

A

-424 kJ/kg

24
Q

A nuclear reactor generates 3000 MW of heat. The heat is transferred in a heat exchanger of
energy transfer efficiency 75% into steam which is expanded in a turbine in order to produce a
power output. The steam is condensed in a condenser, releasing 1800 MW of heat, and pumped
back through the heat exchanger by a feed pump which requires 3% of the power output from the
turbine. Determine:
a) The net power output from the plant.
b) The power output from the turbine.
c) The overall thermal efficiency of the plant.

A

463.9 MW
450 MW
15%

25
Q

Steam at a pressure of 10 bar and dryness fraction of 0.96 expands adiabatically to a pressure of 2
bar according to PV1.12 = constant. Determine the work done during expansion per unit mass of
steam.

A

246kJ kg

26
Q
A
26
Q

You have a 200 gram cup of coffee at 100C, too hot to drink
A. How much will you cool it by adding 50 gms of water to 0C
B. Ice

A

A. 80
B. 64

26
Q

Calculate the density of ethane at 171 bar and 458 K. Assume for ethane

A

117 kg/m^3
146 kg/m^3

27
Q

A motorist equips his automobile tyres with a reflief-type valve so that the pressure inside the tyre will never exceedds 220 kPa (gauge). He starts the trip with a pressure of 200 kPa (gauge) and a temperature of 23C in the tyres. During the long drive the temperature of the air in the tyres reaches 83C. Each tyre contains 0.11 kg of air. Determine:
a. The mass of air escaping each tyre
b. The pressure od the air inside the tyre when the temperature returns to 23C

A

mass - 0.0094
Pressure - 183 kPa

28
Q

A closed rigid container has a volume of 1 m^3 and holds air at 345 kPa and 20C. Heat is added until the temperature is 327C. Determine the change in internal energy.
A. Using ave value of specific heat
B. Taking into account the variation of specific heat with temperature

A

A. 932 kJ
B. 1018 kJ

29
Q

The diagram shows a conical section fabricated from pyroceram.

It is of corcular cross section with the Diameter = ax

A

-2.12 W

30
Q

The possible existence of sn optimum insulation thickness for radial system is suggested

A thin-walled copper tube of radius r,

A

0.006 m

31
Q

A spherical thin-walled metallic container is used to stored liquid nitrogen at 77K.

A

5.64 kg/d

32
Q

A very long rod 5 mm in dismeter has one end maintained at 100C. The surface of the rod is exposed to ambient air

A

Q = 8.3 W
L = 0.19 m

PROB SOLVING PBE (11 decks)

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