17-04-2012, 11:17 AM
Jet Engine Design
jet engine design.ppt (Size: 118.5 KB / Downloads: 488)
Problem Statement
A jet aircraft moves with a velocity of 200 m/s where the air temperature is 20°C and the pressure is 101 kPa. The inlet and exit areas of the turbojet engine of the aircraft are 1 m2 and 0.6 m2, respectively. It is known that the exit jet nozzle velocity is 1522 m/s (from lab calculation) if the exhaust gases expand to 101 kPa at a temperature of 1,000°C. The mass flow rates of the inlet and exhaust flow are 240 kg/s and 252 kg/s, respectively. As a thermal engineer, your task is to (a) determine if the temperature of the exhaust gases is too high for the turbine blades as they exit from the combustion chamber.
Propulsion Efficiency
Define propulsion efficiency as the ratio of the thrust power (PT) to the rate of production of propellant kinetic energy (PT+PL). Where the total kinetic energy is the sum of the thrust power and the power that is lost to the exhaust jet (PL).
Propulsion Efficiency & Turbofan
The propulsion efficiency increases as the velocity ratio is increased.
It reaches a maximum at v=1 and h=1. No lost kinetic energy but also no thrust since V1=V6.
It also suggests that in order to increase the propulsion efficiency one would like to operate at relatively low jet nozzle velocity.
In order to avoid the loss of thrust, the mass flow rate has to be increased.