That's confusing. If the velocity at the exit is increased, that simply means kinetic energy is increased. But as the pressure is decreased and suppose the process is adiabatic (in case of compressible fluid), that means the temperature too is decreased. That simply means the enthalpy of the fluid is decreased. Where did the decreased enthalpy gone? It has converted into the kinetic energy. Thus the total amount of energy is conserved, so no violation of 1st law of thermodynamics.No (obviously). This is a direct result of Bernoulli’s principle, which is simply conservation of energy.
The motor power used by the blower is converted into kinetic energy of the fluid and both before and after the fluid entering and exiting the blower, it contains its own internal enthalpy. When the speed increases at the throat, its the enthalpy that has been converted into kinetic energy as both pressure and temperature decreases at the throat. So the Kinetic energy of the fluid at the throat is certainly higher than the energy supplied to it by the motor of the blower because inside the nozzle, the enthalpy has been converted into Kinetic Energy.
The nozzle will create backpressure because of the friction inside. A perfect frictionless nozzle wouldn't give any kind of backpressur. It's called the nozzle efficiency. The closer the efficiency of the nozzle to perfection i.e. 100%, lesser will the backpressure. And what the backpressure would do is to reduce the ratio. As for example, if the ratio is 2:1, then the velocity at the throat wouldn't 2 times at the inlet but 1.5 due to frictional loss i.e. backpressure.The nozzle will create back pressure on the fan - and that will slow the airflow and the fan.
Can you explain how a fan's performance can be affected by just putting a nozzle before it?The flaw is that you didn't do any real analysis of the fan's performance; you just multiplied the outlet velocity by 2!
Real fans have performance curves that must be used to analyze the fan's performance when attached to a given system