Expert: Ray Wilkinson - 5/18/2006

Question
hi ray. im a mechanical engineer in the polymer sector by trade, but have been asked by by my nephew now studing engineering a turbofan question and need a little help

His question reads

An aircraft powered by turbofan engines cruises at a Mach No. of 0.82, at an altitude where Ta = 223.3K and Pa = 0.265 bar. Each engine has a by-pass ratio of 4.8 and a total mass flow rate of 210 kg/s at these conditions. The fan pressure ratio is 1.75 and the fan efficiency is 97%. The intake efficiency is 94% and the by-pass nozzle efficiency is 100%.
Calculate the thrust from one of the engines produced by the by-pass air alone.

now i understand that the thrust is going to be a combination of the bypass and the main core thrust.I can get the compressor work but the problem which i am having is figuring the turbine entry temperature, or am i going in the wrong direction?

help much appreciated

regards
emmett

can any of you pros out there assist

much appreciated  

Answer
Hi Emmett

I'm not a GT specialist and don't have the textbooks handy to look it up, but here goes:

You don't need to calculate the core thrust. Thrust from the bypass only is going to be a function of the mass flow and energy added or pressure difference. You can calculate the flow through the bypass duct because you have the total mass flow and bypass ratio (4.8 times as much air goes through the bypass as the core). The Mach number and altitude allow you to calculate the intake total pressure, with 94% recovery. The fan pressure ratio and its efficiency will give you the total pressure rise. I'm not sure whether this is enough, but you may also be able to get something from the work done.

Other random thought - total temperature is Ta + 0.7pM^2, where p is the static pressure.

Sorry I can't give more help, and if I had time I'd pursue it further, but hopefully this will give you some clue.

Regards

Ray