Expert: Paul Soderman - 4/19/2010

Question
QUESTION: How to determine the amount of combustion energy to provide the thrust in turbojet and turbofan engine?

ANSWER: The energy in a static jet is equal to rho*A*V^3/2 where
rho = jet gas density
A = jet diameter
V = average velocity of the jet

Thus, the combustion energy must equal the jet energy plus any losses in the combustor or in the exhaust nozzle. What those losses are depend on the specific engine and nozzle.  In flight, the equations are slightly different.
Paul

---------- FOLLOW-UP ----------

QUESTION: Sir the combustion energy must be equal to jet energy  means to turbine power plus jet thrust and in some loss

A jet aircraft moves with a velocity of 200 m/s where the air tem perature 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. (b) Determine the amount of combustion energy necessary to provide the thrust. The maximum tolerable temperature of the blades is 3,000 K. It is known that the pressure ratio of the multi-stage compressor is 8 to 1.

in the above problem The total thermal energy supplied into the engine has determined as the difference of the energy in and out of the combustion chamber

Q = m4h4-mah3  = 537494 (kj)

 Wturbine = Wcompressor = 61037 (Kj)

Jet Thrust = Q - (Wturbine = Wcompressor)

         = 537494 (kj) - 61037 (Kj)

         = 476457 (kj) is this the energy available as the thrust?

so the total combustion energy should be
    = turbine + jet thrust + some loss

is this correct sir?

Answer
Well, I am not sure how you got your numbers, and I would be careful not to give jet thrust units of energy.  But your final sentence seems correct - the total energy added to the combustor must equal the energy used to compress the air and provide thrust along with losses.