Originally Posted by Frog4
working out increased efficiency on a prop, and wondering why the losses at higher speed vs lower speed in rice nozzles? Looking at jet engines on aircraft, are they losing the same because of drag?
It isn't quite the same.
In a turbofan engine the fan diameter is substantially reduced (compared to a similar power open propeller or propfan) and the blade count is increased. If the blades didn't have a shroud the tip losses would kill the efficiency. The inlet of the fan is designed to match the airflow at high speed and is smaller than the fan tip. This allows the air to slow down as it enter the fan when you are at cruise. As the aircraft goes to high speed, the required fan airflow comes into line with the airflow passed by the inlet. At low speed the fan efficiency is better than a turbojet, but it isn't as good as a prop.
Propulsive efficiency is based on the differece between the speed of the vehicle and the speed of the acclerating flow that is creating the thrust. Moving more air at a lower velocity difference gives you high efficiency. At high speed (and altitude) the fan becomes efficient. This happens at higher velocities where the compressibility effects of air come into play (the fan blade tips are in the transonic range and flow velocity out the back end of a turbofan nearly sonic).
These "nozzles" reduce tip losses and provide more thrust at low speed than if the nozzle wasn't there. But the tip shroud ring results in a large drag increment at high speeds that is unacceptable.
You could do the same thing as these nozzles by putting a ring around a typical small airplane propeller and the results would be much the same, it would provide more low speed thrust and have higher drag at higher speed. There was an airplane that used a shrouded prop, it was called the Fantrainer, and it had a much smaller higher speed fan in a shrouded ring in the aft fuselage. It worked fine, but wasn't as efficient as an open prop.