Search
A wire 1.5 mm in diameter and 150 mm long is submerged in water at
atmospheric pressure. An electric current is passed through the wire and is increased until the
water boils at 100°C. Under the condition if convective heat transfer coefficient is 4500 W/m2°C
find how much electric power must be supplied to the wire to maintain the wire surface at 120°C ?
When 0.5 kg of water per minute is passed through a tube of 20 mm
diameter, it is found to be heated from 20°C to 50°C. The heating is accomplished by condensing
steam on the surface of the tube and subsequently the surface temperature of the tube is maintained at 85°C. Determine the length of the tube required for developed flow.
Take the thermo-physical properties of water at 60°C as :
ρ = 983.2 kg/m2, cp = 4.178 kJ/kg K, k = 0.659 W/m°C, ν = 0.478 × 10–6 m2/s.
The volume of a liquid is 25.5ml. A student measures the volume and finds it to be 25.2ml, 25.1ml, 24.9ml and 25.3ml in the first, second, third, and fourth trial respectively. Which of the following statements is true for his measurements?
a. They have poor precision
b. They have poor accuracy
c. They are neither precise nor accurate
d. They have good precision
When 0.5 kg of water per minute is passed through a tube of 20 mm
diameter, it is found to be heated from 20°C to 50°C. The heating is accomplished by condensing
steam on the surface of the tube and subsequently the surface temperature of the tube is maintained at 85°C. Determine the length of the tube required for developed flow.
Take the thermo-physical properties of water at 60°C as :
ρ = 983.2 kg/m2, cp = 4.178 kJ/kg K, k = 0.659 W/m°C, ν = 0.478 × 10–6 m2/s
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.
