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A small electric heating application
uses wire of 2 mm diameter with 0.8 mm thick insulation (k
= 0.12 W/m°C). The heat transfer coefficient (ho) on the insulated surface is 35 W/m2°C. Determine the critical thickness of insulation in this case and the percentage change in
the heat transfer rate if the critical thickness is used, assuming the temperature difference between the surface of the
wire and surrounding air remains unchanged.
In a steam engine cylinder the steam expands from 5.5 bar to 0.75 bar
according to a hyperbolic law, pv = constant. If the steam is initially dry and saturated, calculate
per kg of steam :
(i) Work done ;
(ii) Heat flow to or from the cylinder walls
Steam at 7 bar and dryness fraction 0.95 expands in a cylinder behind a
piston isothermally and reversibly to a pressure of 1.5 bar. The heat supplied during the process
is found to be 420 kJ/kg. Calculate per kg :
(i) The change of internal energy ;
(ii) The change of enthalpy ;
(iii) The work done.
A rigid cylinder of volume 0.028 m3 contains steam at 80 bar and 350°C.
The cylinder is cooled until the pressure is 50 bar. Calculate :
(i) The state of steam after cooling ;
(ii) The amount of heat rejected by the steam.
Two sides of a parallelogram are 120 degrees with each other. The lengths of the sides
are 10.0 m and 6.5 m. Find the length of the diagonal opposite the included angle.
3. The base of the triangle is 5 m and the base angles are 65 and 40. Find the other side of
the triangle.
9. During flight, the air speed of a turbojet engine is 250 m/s. Ambient air temperature is – 14°C. Gas temperature at outlet of nozzle is 610°C. Corresponding enthalpy
values for air and gas are respectively 250 and 900 kJ/kg. Fuel air ratio is 0.0180. Chemical energy of fuel is 45 MJ/kg. Owing to incomplete combustion 6% of chemical energy is not released in the reaction. Heat loss from the engine is 21 kJ/kg of air.Calculate the velocity of the exhaust jet.
. An air receiver of volume 5.5 m3 contains air at 16 bar and 42°C. A valve is
opened and some air is allowed to blow out to atmosphere. The pressure of the air in the receiver
drops rapidly to 12 bar when the valve is then closed.
Calculate the mass of air which has left the receiver
In a steam engine the steam at the beginning of the expansion process is
at 7 bar, dryness fraction 0.98 and expansion follows the law pv1.1 = constant, down to a pressure
of 0.34 bar. Calculate per kg of steam :
(i) The work done during expansion ;
(ii) The heat flow to or from the cylinder walls during the expansion.
verify experimentally that the ratio of the emissive and absorptive power is the same for all bodies
and is equal to the emissive power of a perfectly black body?
A mercury thermometer is at an initial temperature of 24 C and it is placed in a glass of water at 8 C. The thermometer has a spherical reservoir of diameter D = 1.4 cm3 , and a column of diameter d = 1.00 mm. At 4 C the mercury fills the reservoir but does not enter the column. The linear expansion coefficient of mercury = 6.00 × 104 K1 . The mercury column has initial height hi and it decreases to a final height hf = hi/2 during the process of arriving at equilibrium with the water. (a) Explain why the process is not a quasi-equilibrium process. (3) (b) Calculate the temperature of the water.
