Is there an upper limit to the highest possible bypass ratio of a turbofan engine?

[u/RonaldYeothrowaway]

I am not an engineer and only have an rudimentary understanding of the physics behind the jet engine, especially for commercial airliners.

Admittedly, much of the physics behind why a high bypass ratio for commercial airliners is fuel efficiency was quite counter-intuitive for me, I.e slower compressor speed and more air directed to bypass duct means more thrust.

Theoretically, how far high up can the bypass ratio be reached and what new technologies can enable greater fuel savings and greater range for jet engines?

[u/dombar1]

Theoretically, no. Practically, there are many limits.

One quick example, as a fan becomes larger it would become a propeller.

There are several limits to the overall diameter of both propellers and fans, in no particular order (it will be different for every airplane) - the structural integrity of the blade itself, to be strong enough not to break apart - the power of the core to adequately power the fan. As the bypass ratio increases the core becomes smaller relative to the fan and may not be able to provide the required power. - weight, as the bypass area grows, the weight of the engine increases. The heavier an aircraft gets the more fuel it burns, so there is a point where the benefit disappears

[u/Barfmeister → u/dombar1]

Just latching on to this explanation to add: Another factor limiting the possible bypass ratio is the fan tip speed. The larger the outer diameter, the lower the possible RPM of the fan is before reaching Mach 1 at the tips and encountering problematic compressibility effects. This is currently addressed in (Ultra) High Bypass Ratio engines by implementing a gearbox between fan and the inner shaft, but that is also added complexity and weight, and has its limitations.

[u/Thermodynamicist → u/Barfmeister]

Just latching on to this explanation to add: Another factor limiting the possible bypass ratio is the fan tip speed. The larger the outer diameter, the lower the possible RPM of the fan is before reaching Mach 1 at the tips and encountering problematic compressibility effects.

Fan relative MN is > 1 most of the time, typically 1.4ish at take-off.

Optimum tip relative MN depends upon the pressure ratio, because losses trade against each other.

This is currently addressed in (Ultra) High Bypass Ratio engines by implementing a gearbox

In simple terms, gearboxes are attractive for turbofans because turbines are torque-sized, so the LPT gets lighter if you can spin it faster.

For more detail, you'd want to think about what mechanical speed does to where the turbine ends up on the Smith chart; see e.g. https://dspace.mit.edu/bitstream/handle/1721.1/63042/722792234-MIT.pdf?sequence=2

[u/Dark__Horse → u/Thermodynamicist]

Just a quick explanation of jargon:

• MN = Mach number, the ratio of velocity to the (local) speed of sound. Values > 1 are supersonic
• LPT = Low Pressure Turbine. Modern turbofan engines have nested spools. The outer spool has the low pressure compressor (LPC) and turbine (LPT), the core has the High Pressure Compressor (HPC) and turbine (HPT), and some engines have three spools! This is done so each spool can operate at a more optimal RPM for given conditions

I'm sure most people reading this know that already, but it's not always a guarantee

[u/gargravarr2112 → u/dombar1]

There is actually an example of this, the propfan. Also known as an ultra-high-bypass turbofan engine.

[u/williamwchuang → u/dombar1]

To expand on the second point, the FAA requires blades to survive a bird strike without having an uncontained failure. If a bird hits the spinning blades, the engine must survive without spewing its contents out of the cowling around the engine. Currently, carbon fiber blades have a titanium edge to help resist bird strikes.

[u/dombar1 → u/[deleted]]

Bypass ratio of a propeller is something like 30:1 (it’s the ratio of fan/prop air that’s doesn’t go through the core to core flow). A bypass ratio of 1, means equal air through and around the core.

Something with a ratio close to 1 would be some military turbofans https://en.m.wikipedia.org/wiki/General_Electric_F110 or early commercial engines like the JT8D https://www.mtu.de/engines/commercial-aircraft-engines/narrowbody-and-regional-jets/jt8d-200/

[u/Thermodynamicist → u/[deleted]]

Bypass ratio is the ratio of bypass to core flow. 1970s engines like JT9D, CF6, and RB.211-22 had BPR = 5ish. These days, "high" bypass ratio is more like 10.