You are testing a new jet engine in a test cell at sea level conditions. You measure the mass flow through the engine and find i

Question

2 years ago

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You are testing a new jet engine in a test cell at sea level conditions. You measure the mass flow through the engine and find i

t is 125 kg/sec of air plus 2.5 kg/sec of fuel. The velocity ahead of the engine may be assumed to be zero (it is a static test). The velocity of the exhaust is 340 meters/second. The load cell in the test stand (which measures engine thrust) reads 60,000 Newtons. Is the load cell reading correctly within 2%? If not, what should the load cell read?

Engineering

2 answers:

bulgar [2K] · Answer 1 · 2022-07-22T10:57:20+03:00

Answer:

43248 newtons.

Explanation:

Force = mass x accelerations and units of force are newtons which are given in the question.

here mass = 125 of air and 2.2 of fuel, total = 125+2.2=127.5kg/s and the velocity of the exhaust is 340m/s.

force = 340m/s * 127.5kg/s = 43248 newtons technically this is wrong (observe units) but i will expalin how i have taken acceleration as a velocity here and mass/unit time as simply mass.

see force is mass times acceleration or deceleration, here our velocity is not changing therefore it is constant 340m/s but if it were to change and become 0 in one second then there would be -340m/s^2 (note the units ) of deceleration and there would be force associated with it and that force is what i have calculated here. similarly there would be mass in flow rate of mass per second, which is also in that one second of time.

let's calculate error.

error = (actual-calculated)/actual. = (43248-60000)/43248= -38.734% less is ofcourse greater than 2%.

So the load cell is not reading correct to within 2% and it should read 43248newtons.