DC motor advice for fan (tea leaf blower) - advice
It's normal for a centrifugal fan like you have to speed up as the exit is restricted. This means you've reduced the mass of air passing through the fan, the motor is doing less work on the airflow, so the load on the motor is less, and so the speed increases.
If the motor is intended for car use, then 14 V is a reasonable voltage to run it on long term, this is the voltage at which cars tend to charge their batteries. While it will run for a while at 18 V, that's not recommended if you want the motor to last.
The fan you have bought is a high-speed low pressure fan. You'll notice from the fan datasheet that the blades are canted forward. Any stall in the output airflow will radically reduce the output pressure. You can turn speed into pressure with a flare from the fan to the plenum.
The nozzles you are using, with their small cyclindrical section and their right-angle connection to the plenum will generate a high back-pressure. You need to design the airflow into the nozzles from the plenum more carefully. Amongst the many things that may help you increase velocity after the nozzles are

increase the bore of the nozzles
reduce the number of nozzles
add a diffuser flare to the fan outlet
flare the diameter from the plenum to each nozzle
use a larger radius of bend from the plenum to the nozzles
use a higher pressure fan, perhaps cascade two fans of the type you have
maybe replace the individual nozzles with a single slot in the plenum, the flare may be easier to achieve if you build the plenum with a single strip folded into a tear-drop cross-section