Can the thrust of a turbofan be increased by increasing bypass ratio?

Question

I read that the CFM56 is based upon the F101. Id on't know how true that is, but it generated the following question: If a larger fan is fitted to a low bypass turbofan, would dry thrust be increased? Or is it the same amount of thrust produced with less fuel consumption? That is what I was told on another message board but it doesn't seem to make sense. Assuming that the proper size turbine is fitted, the same turbine airflow should be turned into more thrust with a larger fan. Which is correct?

Answer 1

GE used the core of the F101 engine as the core of the CFM56 (with some modifications, presumably). The core of a turbofan consists of the high pressure compressor, the combustion chamber and the high pressure turbine. In this case, we talk about a 9-stage HPC and a single-stage HPT. Everything else was newly developed for the high-bypass turbofan.

Unlike just putting a larger fan on the low-pressure shaft, this is possible becasue the high-pressure components and the low-pressure components are relatively independent in a turbofan, i.e. the core of the engine is not affected too much by the size of the fan, the bypass ratio or other changes in the low-pressure area.

The new low pressure turbine (4-stage instead of 2-stage) can operate at a lower rpm and transmit more of the exhaust gas energy from the engine core to the larger fan (the exhaust gas coming out of the new LPT is probably cooler and slower than that coming out of the F101 LPT). The new low-pressure compressor increases the overall pressure ratio and therefore increases the amount of energy that goes to the low pressure compressor and fan, thereby improving the overall efficiency. This is probably possible because the non-supersonic operational envelope of the CFM56 means that intake air total temperatures will be much lower so that the same EGT margin can be achieved with a higher pressure ratio. In other words, becasue the incoming air is cooler, it can be compressed further without damaging the high-pressure turbine because it is too hot.

The static thrust of the CFM56 is 19500 lbf, which is up about 15 % from the F101's 17000 lbf without the afterburner. While a small part of this is probably an aerodynamic loss due to the unused afterburner chamber, most of it has to come from the better use of the energy offered by the engine core exhaust in the larger fan. The difference might even be higher becasue the maximum thrust of the F101 might be adjusted for a much lower mean time between shop visits. I'm not sure about that though.

This shows that the main benefit of the high-bypass low pressure section of the engine is a significantly increased fuel efficiency, an increase in the static thrust and (much) lower noise levels. The F101, on the other hand, can operate at higher speeds, produce more thrust at high speeds, is lighter and has a much smaller cross-section.

(Edited per comment by fooot, thanks for finding the source.)

Answer 2

Fitting a larger fan does two things to the thrust of an engine:

1. The static thrust is increased. Static thrust is the thrust produced when the engine is not moving.
2. The thrust gradient over speed becomes more negative, which means that thrust will decline more with speed when the bypass ratio is increased.

But there is more: Exit speeds at the nozzle are reduced, and a bigger intake is required, since the bigger fan will need more air. Increasing the bypass ratio means taking some of the kinetic energy of the core flow and turning this into a higher fan mass flow.

If we look at the formula for the propulsive efficiency $\eta_p$ of an airbreathing engine: $$\eta_p = \frac{v_{\infty}}{v_{\infty} + \frac{\Delta v}{2}}$$ where $v_{\infty}$ is the speed of the engine and $\Delta v$ the speed increase of the gas flowing through the engine, the speed dependency becomes clear: When $v_{\infty}$ is low, a smaller $\Delta v$ acting on a higher mass flow makes the engine more efficient. When $v_{\infty}$ is high, however, this effect disappears, and now the smaller engine with a smaller and lighter intake becomes more attractive.

When the core of the engine stays the same, its fuel consumption will also stay the same, but the bigger fan will create more thrust, especially at low speed. The core mass flow will be the same, regardless of fan size, and the amount of fuel to heat this mass flow will also not change.

Since efficiency is defined as thrust per unit of fuel consumed, the bigger fan will also increase efficiency.

Answer 3

The jet engine propulses the aircraft based on moving air behind the aircraft at a higher speed than the airplane. Turbofan tries to move a higher quantity of air at lower speed that is more efficient. That is what is called propulsion efficiency.

This can be easily understood thinking about how can you propulse yourself when you are in a skateboard, will be more efficient if you use your hands over a wall or over something that can slip on floor? The heavier that "thing" the best for your propulsion reaching the building wall.

Ok, that's a good concept, but you need to do it!!! For being able to do that you need to connect a turbine at the exhaust of the previous turbine connected mechanically to the new fan of the engine.

Easy but no so good...

• You are adding a new compressor stage (the fan) in front of the previous stages. So you need to make sure that all your materials are prepared for it.
• You exhaust of the core might not be properly prepared to the new conditions.
• Probably external shape of the previous engine is not prepared for a secondary flow.
• Systems need to be reviewed.

In principle, with some modifications, that operation you suggest will work but just if we are talking about small modifications and not going to significant bypass ratio, where modifications will be of the same cost of a complete new engine design.

Also I see economical difficulties, in the current context there is significant competition for making the lowest comsuption. If you perform what you are proposing you will have an engine optimized for not having secondary flow adapted for secondary flow competing with engines optimized for working with secondary flow. Engine will be unefficient compared with competitors.

Finally, you will increase weight and thrust.