# What is physically happening when you use a bypass nozzle on a turbofan?

I am having trouble understanding how a fan affects the flow of air through the bypass in a turbofan engine. Solving for the flow of air using thermodyanic states requires using a bypass nozzle to "convert" the stagnation enthalpy to some amount of exit velocity. However, intuitively I see a fan that generates thrust on its own, much like a propeller on a turboshaft engine. So my question is, what exactly is physically happening here with the turbofan?

• The inlet slows the air, increasing it's static pressure. The fan accelerates this high-static-pressure, low-speed air without changing it's static pressure. The nozzle then accelerates the air even more, while decreasing it's static pressure. Does this help? – Abdullah Jul 14 at 16:07
• The fan accelerates this high-static-pressure, low-speed air without changing its static pressure?? Several folks here have attempted to propose combining a prop and nozzle to try to use pressure differential thrust (the least efficient). Notice "choking" the airstream makes the fan work harder and reduces mass flow. Mass flow is the second worst way to use a prop, but for a fan, it works. The outflow exit may be a "nozzle" in name only. Props generate lift. – Robert DiGiovanni Jul 16 at 8:48
• @RobertDiGiovanni WDYM "props generate lift"? If you mean the propeller blades work like wings, then yes, and the same is true of fan b;ades, axial compressors...... Also, I don't know why this diagram doesn't show it, but generally, turbofan inlets look like nozzles in reverse, and that's what they are. (Note: the diagram doesn't show anything that looks like a bypass "nozzle" either. The bypass duct shown is just a straight line.) – Abdullah Jul 16 at 10:28
• @Abdullah "nozzles in reverse" would make sense, especially at higher altitudes. "the inlet slows the air, increasing its pressure" helps the fan move more mass. And yes, the bypass duct does not appear to contract. – Robert DiGiovanni Jul 16 at 13:44
• @RobertDiGiovanni for a fan of a given diameter spinning at a given rpm, slower airflow certainly means that the fan can have a lower pitch, wasting less energy in swirl flow. Maybe that's part of the reason for the duct? – Abdullah Jul 16 at 15:49