Is it practical to separate the fan shaft from the turbine shaft on a jet turbofan engine?

Question

The benefits of this would be a faster exhaust velocity from the fan shaft (since it's not losing energy to the turbine) that would increase thrust, the possibility of using more exotic fuel mixtures, and the ability to use a pulley ratio to increase the torque of the fan and allow for a larger fan blade radius.

It seems to me like having the turbine on the same shaft as the fan increase the losses in energy. Would it still work if the fuel mixture was burned separately powering a turbine that would then spin the fan on a separate shaft connected by a pulley or sprocket system? Then you could either store a separate oxidizer for increased burning efficiency or divert some air from the fan to the turbine for the oxidizer. Not sure yet if the increased weight from the oxidizer storage would diminish the benefits from the additional thrust.

The CO2 and H2O from the separate shaft with the turbine could be combined with the fan exhaust downstream.

To be honest, I know a bit about rocket propulsion but not a whole lot at all about jet propulsion. When I realized that the thrust from the air could be essentially broken down into the part of the oxygen being consumed by the fuel and the part that was composed of nitrogen and unused oxygen, my chem 101 knowledge said that the burned fuel/air mixture, released as CO2 and H2O, would interact very slowly with the N2 and O2 in the other part of the exhaust. Intermolecular forces aren't factored into ideal gases and these are far from ideal gases, but I'm working with what I've got here.

I've tried to research how heat flows from a hot gas, the CO2 and H2O from the fuel-air mixture burning, to a cold gas, the N2 and unused O2 in the same chamber, but I've hit a dead end that my Google research and limited college engineering classes have yet to solve. My chem knowledge would say that the heat transfer between cold Gas A and hot Gas B would be very slow, absent a conductor, but I don't know if that holds up to reality.

Answer 1

To move an airplane through the air, thrust is generated by some kind of propulsion system. Most modern airliners use turbofan engines because of their high thrust and good fuel efficiency.

How does a turbofan engine work? The incoming air is captured by the engine inlet. Some of the incoming air passes through the fan and continues on into the core compressor and then the burner, where it is mixed with fuel and combustion occurs. The hot exhaust passes through the core and fan turbines and then out the nozzle, as in a basic turbojet. The rest of the incoming air passes through the fan and bypasses, or goes around the engine, just like the air through a propeller. The air that goes through the fan has a velocity that is slightly increased from free stream. So a turbofan gets some of its thrust from the core and some of its thrust from the fan. The ratio of the air that goes around the engine to the air that goes through the core is called the bypass ratio.

Because the fuel flow rate for the core is changed only a small amount by the addition of the fan, a turbofan generates more thrust for nearly the same amount of fuel used by the core. This means that a turbofan is very fuel efficient. In fact, high bypass ratio turbofans are nearly as fuel efficient as turboprops. Because the fan is enclosed by the inlet and is composed of many blades, it can operate efficiently at higher speeds than a simple propeller. That is why turbofans are found on high speed transports and propellers are used on low speed transports. Low bypass ratio turbofans are still more fuel efficient than basic turbojets. Many modern fighter planes actually use low bypass ratio turbofans equipped with afterburners. They can then cruise efficiently but still have high thrust when dogfighting. Even though the fighter plane can fly much faster than the speed of sound, the air going into the engine must travel less than the speed of sound for high efficiency. Therefore, the airplane inlet slows the air down from supersonic speeds.

The above is excerpted and copied from here.

Answer 2

There are many ways to build a jet engine but the processes are well established

Modern turbofans will have compression, combustion, thrust from gases, and thrust from the bypass fan. Military engines add afterburners. How all of this gets done is up for debate and competing designs, but it seems fairly clear that the most efficient engines use the expanding gas to spin both the compression and the bypass fan on the same shaft. Because each component can be designed to extract on put in the exact amount of work needed, there is very little practical reason to add a new shaft into the mix.

Answer 3

Most are 2 shafts; some are three shafts. Look up Rolls Royce Trent engine in Wikipedia. I expect there are others but the Rolls RB 211 is the only one I encountered.