Idle descent on piston & turbine engines

[shumway76]

It is not advisable to carry out prolonged idle descends on piston engine aircraft due to "shock cooling" effect.
But for turbine engines this is not so.
Why is that?

[Tu.114]

I am not sure if I understood Your question correctly - do You want to know why piston engines are prone to shock cooling or why turbines are not?

Anyway, let me offer the following thought: A turbine engine has no heat sink surfaces exposed to free flowing air. Cooling takes place only where the operating temperatures are beyond what the material can handle - in the combustion chamber and the turbine section. (Depending on the engine type, the oil system may or may not have an air-cooled oil cooler as well) The cooling air in a turbine engine is taken from the high pressure compressor stages and has a temperature of a few 100°C - this is sufficient for cooling in areas like the combustion chamber or forward compressor stages that would otherwise face temperatures beyond 2.000°C.

Now in an idle descent, the engine is still running, combustion still takes place and the compressor still delivers hot high pressure air for the cooling systems; both mass flows are lower than in other phases of flight and the temperatures are a bit cooler of course. On the Dash 8-400, normal ITTs are around 350°C in descent compared to about 650-750°C in climb.

In a piston-powered aircraft by contrast, the heat generated in an idle descent is strongly reduced, but the airflow across the constant area of the cooling fins is not (assuming no cowl flaps are installed) due to the typically high speeds in descent.

So the shock cooling is caused by the strong airflow that removes more heat than the piston engine provides in this situation. On a turbine by contrast, the engine produces less heat as well, but as the cooling air flow from the compressor is still much warmer than the outside air and provided at a lower mass flow rate, the ratio between produced and removed heat is still rather well balanced.

[shumway76]

Thanks for the explanation, that's exactly what my question was about.

I guess piston engines "exterior" surfaces are cooled by air, therefore shock cooling at low power & aircraft high speed is a great possibility. Internally the engine is still cooled by oil.

As for turbine engines, it doesn't actually rely on air cooling, but oil. But can I say that turbine engine doesn't need "exterior" surface cooling like a piston engine cylinder does?

[blackhand]

As for turbine engines, it doesn't actually rely on air cooling, but oil. But can I say that turbine engine doesn't need "exterior" surface cooling like a piston engine cylinder does?

Thats fairly correct, although oil cooling isn't a factor in cooling the exterior of the combustion chamber(s). As already discussed is "cooled" by compressor discharge air.
In fact approx 75% of air flow is for cooling, the remaining 25% for combustion.

BH

[Tu.114]

Actually, the parts of a turbine engine that bear the strongest thermal load are air cooled indeed. Oil is used to remove heat from the shaft bearings and some accessory systems (engine type dependent), but as mentioned by Blackhand, the combustion chamber and most importantly the turbine section is impossible to cool by oil.

Take a look at the following picture:
http://ec.europa.eu/research/transport/images/projects/49_1.jpg

The holes in the blade have compressor bleed air flowing out of them that forms a protection layer between the metal and the hot gasses coming from the combustion chamber. If this layer was not there, the exhaust gas would be way too hot for the material to handle. Other, older engines may not use film cooling but simply route cooling air through the blade from foot to tip, but that method is less effective.

[Saint Jack]

It is also worth noting that compressor discharge air is sometimes used in the turbine section to oppose (balance) the axial thrust of the gas stream impinging on the turbine wheels, this relieves side loads on the bearings supporting the rotating assemblies. This is generally referred to as 'cooling and balance air'.