Section A
Q1. 1.0 m3 of air is heated reversibly at a constant pressure from 25°C to 500°C, and is then cooled reversibly at constant volume back to initial temperature. If the initial pressure is 1 bar, find the (i) net heat flow, and (ii) overall change in entropy. Represent the process on T-s plot. An air compressor takes in air at 1 atm and 25°C. The compressed air is discharged through a pipeline having an internal diameter of 2 cm. The average velocity of the discharged air is 10 m/s and the discharge pressure is 5 bar. Assuming that the compression occurs quasistatically and adiabatically, calculate the work input to the compressor. Consider the inlet velocity to the compressor is negligible. A four-cylinder, four-stroke SI engine is operating at a speed of 4000 rpm. The following particulars refer to a furnace wall:
(a) Take Cp = 1.005 kJ/kg K and R = 0.287 kJ/kg K for air.
(b) Take Cp = 1.005 kJ/kg K and γ = 1.4 for air.
(c) Determine: (i) How many times the spark will trigger in one minute per cylinder. (ii) Number of thermodynamic cycles per cylinder per second. (iii) If the combustion phenomenon takes place over 72° of crank rotation, find out duration of combustion in seconds.
(d) Hot gas temperature = 2000°C Room air temperature = 45°C Heat flow by radiation from gases to inside surface of the wall = 23.26 kW/m2 Convective heat transfer coefficient at the interior surface = 11.63 W/m2 K Thermal conductance of the wall = 58 W/m2 K Heat flow by radiation from external surface to surroundings = 9.3 kW/m2 Interior wall surface temperature = 1000°C Determine for the external surface of the wall (i) surface temperature, and (ii) convective heat transfer coefficient.
