Engine flow rate with varying power settings

[stevero390]

Most engine specifications I have found only provide the engine mass flow rate for take-off setting. For example, the V2500 has a mass flow rate of 354.25 kg/s.

How does the flow rate vary through different power settings? For example, if the engine is on idle, will the mass flow rate be 0 kg/s? And if the engine power setting is at 50%, will the mass flow rate be half of 354.25 kg/s?

Thanks for your time

 

[jerromyjon]

How does the flow rate vary through different power settings?

That I don't know specifically, and can't find any charts which show it.

For example, if the engine is on idle, will the mass flow rate be 0 kg/s?

Common sense tells me that 0 kg/s would only be while the engine is off, it takes mass flow to maintain an idle.

And if the engine power setting is at 50%, will the mass flow rate be half of 354.25 kg/s?

I'm sure it is more complicated than that, velocity would have an effect for sure.

 

[stevero390]

Hmm what if I phrased it this way: If the engine needs a mass flow 354.25 kg/s (which is maximum power setting), would the engine need half of 354.25 when the engine power setting is at 50%?

 

[Asymptotic]

Hmm what if I phrased it this way: If the engine needs a mass flow 354.25 kg/s (which is maximum power setting), would the engine need half of 354.25 when the engine power setting is at 50%?

Caveat: I know nothing about jet engines, and am totally winging it. Regardless, take heed to this.

Common sense tells me that 0 kg/s would only be while the engine is off, it takes mass flow to maintain an idle.

What you need to find out is mass flow rate at minimum engine power setting. Consider a gasoline auto engine with zero mass flow at 0 RPM (that is, when it isn't running), but pulling in a mixture of air and fuel whenever it is. The engine has a minimum idle speed rating, and although you can adjust it somewhat below specification there comes a point where an engine is simply too slow to remain running.

If a jet engine is similar in this respect, and assuming the relationship between engine power setting and mass flow is linear (it probably isn't), one way to look at it is

(maximum mass flow - minimum mass flow)/(maximum power setting - minimum power setting).

For example, if minimum mass flow is 54.25 kg/s at a minimum power setting of 5%, then (354.25-54.25)/(100%-5%) = 300/95%, a 3.16 kg/s increase in mass flow for every percent increase of the power setting from a minimum mass flow of 54.25 kg/s.

 

[jack action]

A turbine is designed to work (efficiently) at a specific flow rate. Therefore, it shouldn't vary much. This is a plot defining turbine characteristics:

In green, you have the mass flow plotted against the pressure ratio for different corrected values of rpm (ranging from 80 000 to 180 000 rpm). In orange, you have the turbine efficiency plotted against the pressure ratio for the same rpm values.

Note that at high rpm, the mass flow is pretty constant across the pressure ratio range. That is because the flow is choked. Note also that maximum efficiency is when the flow is at high rpm and high pressure ratio (i.e. when the flow is choked).

One would only consider the points where the efficiency is maximum for each rpm. If you plot only the mass flow values corresponding to their respective maximum efficiency for their rpm, you will get something looking like this:

Here, N_F is the maximum allowable rpm. So near a pressure ratio of 1.4, the rpm stay constant (for example, like using solely the 160 000 rpm line from the previous plot for all points above that pressure ratio threshold). You can see that for take-off, cruise or climb, the mass flow is relatively identical, i.e. the flow is choked. The only exception is idling where a lower rpm is used to reduce friction losses for less fuel consumption (and probably lower noise level too).

The lower power settings are set by using less fuel, which will reduce the pressure ratio and you can find the corresponding mass flow on the plot. But, because of the lower efficiency, a turbine is not usually used that way in its normal use.

 

[Nidum]

The V2500 engine is a fan jet with about 5:1 bypass ratio . At ground idle the fan is turning relatively slowly and mass flow of air through it is quite low .

This engine is two shaft and for several reasons this means that the fan does have a critical minimum turning speed to allow the engine to self sustain .

Three shaft but otherwise similar engines will self sustain with the fan almost at idle .

In both cases though the fans only move high mass flow rates of air when the engines are running at higher power levels during take off and cruise .