Jet Engine Questions and Answers | Turbofan | Turbojet | Engine Exhaust Temperature

These questions and answers are helpful for mechanical engineering students seeking jet engine engineering jobs.

Question 1: Will turbofans still make more thrust today if modern turbojets with similar metallurgy were available?

Answer: Yes. With a higher mass flow flowing through the combustor, more fuel can be burned and hereby higher thrust can be produced. However, even though the cores operate at pretty the same efficiency, the turbofan will still have the better overall efficiency for a given SFC. To really capitalize on the increased mass flow, the fuel drain will significantly drop the range of the aircraft. This is the reason why General Electric in its YF120 engine opted to go for variable bypass to achieve greater flexibility. The new technology engine NPO Saturn AL-41F developed by Saturn also has variable bypass for supercruise flight.

Large jet engine exhaust
Large jet engine exhaust | Source

Question 2: Does the modern fighter turbofan engine’s bypass air simply pass through a fan (accelerated at low pressure) or it bypasses air from LP compressor? Most text I read on the Internet doesn’t specify but illustrations make it look like only transports have real fans, fighter engines use the LP compressor as the source of bypass air.

Answer: It depends on the engine application. If you are talking about an engine used in an airline, for example the new GE90-115B, the bypassed air is driven only by one gigantic fan producing most of the thrust whereas if you look at the F119, it is bypassed after going through LP fans.

Question 3: I always thought a hotter exhaust would be a good thing, propulsion wise but some say that the exhaust should exit at the same temperature, the heat energy converted to kinetic energy. Besides diluting the exhaust with more cool air to absorb the heat before exiting the nozzle, is there any other way to more efficiently reduce heat dissipation into the atmosphere?

Answer: The temperature of the exhaust does not contribute to thrust in any way. The temperature of the exhaust serves as a sign of the thrust that can be expected (similar to inflation being a symptom of high growth rates in an economy). The amount of energy rejected to the atmosphere in form of heat represents the amount of energy just wasted.

Well, to cool down the exhaust, you have to take the heat out and dump it into something else (conservation of energy principle). So the question is where do you transfer the heat to? The plain and simple answer is the atmosphere. You can do that by bypassing extra air or cooling down the exhausts as in the F-117.

Gas Turbine Theory by Top Mechanical Engineering Authors Saravanamuttoo, Rogers, and Cohen

Question 4: Are there general figures available on exhaust temperature, comparing turbojet and turbofan exhaust temperatures at turbine entry and at nozzle exit?

Answer: Performance figures of aero-engines are available in the public domain. Just not of the engines flying on aircraft in service. Look on the internet and you can find plenty or even better, invest in good books. (You can start with Gas Turbine Theory by Herbert I. H. Saravanamuttoo, Rogers and Cohen).

Lengthening the exhaust section can help with proper mixing of the bypassed air with the hot exhaust. Also, the exhaust section is that long because afterburner design is such that you need that space.

Afterburner design isn’t as easy as just dumping the fuel into the hot exhaust where the fuel burns automatically. It is a lot more complicated than that.

Under subsonic flow conditions to accelerate the flow, one needs a nozzle with a converging cross-section. However, as the velocity of the compressible fluid goes sonic and flow gets choked, the nozzle thereafter needs to expand in order to continue accelerating the flow. That’s what a convergent-divergent nozzle is used for. It is just another nozzle used to convert pressure and thermal energy into kinetic energy, only here the flow velocity exceeds sonic velocity at the nozzle exit.

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