Turbojets: Thermodynamics for Mechanical Engineers

Turbojets allow us to create the thrust an airplane needs to fly. A Brayton cycle engine lies at the heart of a turbojet, but it’s sandwiched between a diffuser and a nozzle. These new components allow us to convert kinetic energy to enthalpy (or vice versa). This video will show you a systematic process you can use to solve any turbojet problem!
In this example, you’ll learn how to solve a turbojet problem using variable and specific heat analysis. I think the trickiest parts of this analysis are (i) converting between moving and stationary frames of reference, (ii) units on the kinetic energy terms in metric and imperial questions, and (iii) remembering that the back work ratio of the Brayton cycle here is limited to 1 so that we can maximize the velocity at the exit of the nozzle.

[@rickybasutu1066]

faith thrust and pixie dust

[@harvirr.7110]

This video really helped me.

[@JoseApolinarCirilo]

question what numbers did you plug in for (V_0 - V_in)V_jet to find thermal efficiency

[@hakan2499]

how did we find p3

[@mikeschertzer941]

Pressure stays approximately constant as you go through the combustor.

[@harvirr.7110]

If there was no diffuser what would the specific enthalpy be at the inlet of the compressor? Would it just be Cp * temperature?

[@mikeschertzer941]

Just whatever the ambient value is. Your want to look it up on a table. Cp x delta T works for changes, but not for the single value. 1st law only needs changes anyway though.

[@harvirr.7110]

@@mikeschertzer941 I see, I don't have the table for air in the booklet given by my professor. So I calculated each of the enthalpies with Cp*Temperature. I needed the change in enthalpies to calculate the net power output and didn't know how to calculate the enthalpy. I don't know if I've done it correctly I will have to wait to see what he says when he marks my paper.

[@ericklamilla]

Nice explanation, we are building a turbo jet engine (basic) for educational purposes... Does it exist the possibility to contact you via email or zoom? Regards

[@mikeschertzer941]

Thanks Erick. I'm glad that this helped you. I'm not sure how much I'd be able to help with the design and build of a TJ engine though. My guess is that you'd get better, more valuable help from someone you could speak to IRL.

[@ericklamilla]

@@mikeschertzer941 maybe could you help me with a e-mail for contact you? Regards

[@mikeschertzer941]

@@ericklamilla I'd prefer not to write it in the comments here.