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For example, in a turbofan engine the air is taken from the ambient air outside the engine or is it taken from the compressed air from one of the compressor stages ?

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  • $\begingroup$ Can you give an example for a jet engine with an air cooled oil cooler? The jet engines I'm familiar with use a fuel-oil heat exchanger. I know that some use air cooling for IDG oil. Is that what you mean? $\endgroup$
    – Bianfable
    Commented Jan 11, 2023 at 8:08
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    $\begingroup$ Bleed air, is going to be hot compared to the outside air due to being compressed. That will make it less effective for cooling purposes. $\endgroup$
    – jwh20
    Commented Jan 11, 2023 at 13:12

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Kerosene burning high altitude airplanes generally use fuel as the oil cooling medium, because you need to heat the fuel anyway due to the low operating temperatures at altitude. It limits ice formation in the fuel controller, and improves atomization. Sometimes it's just called a fuel heater, not an oil cooler.

If not, then you would use an air/oil heat exchanger, but you still need to heat the fuel, in which case you would have to use bleed air or an electric heater to heat that.

So much simpler to use the hot oil in the engine to warm the fuel. In no case would you use engine bleed, which is hot, to cool engine oil. If it's not an oil/fuel heat exchanger, a stand alone oil cooler will take ambient air ducted to it.

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  • $\begingroup$ Yes, bleed air would be a bad practice due to its temperature. But if for example the oil temperature is such that the fuel/oil heat exchanger is not enough then and air cooled oil cooler might be used right? Moreover, fuel needs to be heated, as you said to prevent ice particle formation and to protect the filter. In a top of climb case of a civil transport turbofan engine, what would be a reasonable fuel temperature supplied to the fuel oil heat exchanger? Thank you in advance. $\endgroup$
    – qsct
    Commented Jan 11, 2023 at 18:29
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    $\begingroup$ A fuel/oil heat exchanger is WAY more efficient than a oil/air heat exchanger. The supplied fuel will be somewhere above ambient after a long climb, and will cool toward ambient. So if it's -30C, eventually the fuel entering the heater will be close to that. $\endgroup$
    – John K
    Commented Jan 11, 2023 at 18:40
  • $\begingroup$ Adding to that, the fuel after serving its purpose of obtaining the heat loads from the various engine subsystems ends up heated to the burner at an increased temperature. So my next question is: should the system's designer goal be a high burner entry fuel temperature with respect to an upper limit ? The upper temperature limit being the point where self ignition and fuel decomposition takes place. In the literature, I bumped into a temperature of around 120 C for Jet A, would that be reasonable? $\endgroup$
    – qsct
    Commented Jan 11, 2023 at 19:14
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    $\begingroup$ Not sure. You'd need to talk to a powerplant specialist with an engine OEM. $\endgroup$
    – John K
    Commented Jan 11, 2023 at 21:04
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    $\begingroup$ @qsct In automotive diesel applications (similar fuel as Jet A) the fuel temperature limit is set by the loss of lubricity and the impact on the high pressure fuel pump. As temperature increases, fuel density drops and power is lost. I suspect a similar factor in jet engines. $\endgroup$
    – user71659
    Commented Jan 12, 2023 at 1:23

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