0
$\begingroup$

If I need to calculate the head coefficient using $c_h=\dfrac{gH}{N^2D^2}$ and the volume coefficient using $c_q= \dfrac{v}{ND^3}$ while keeping $N$ (impeller rotational speed in rpm), how can I make the units cancel out and have a dimensionless answer?

I am measuring $D$ (impeller diameter in meters), $g$ (gravity m/s^2), $H$ (total pressure head in meters), $V$ (volumetric flow rate in m/s).

I made it dimensionless by changing RPM to 1/s, however I have been asked to keep RPM and change the other units, can someone plz help.

$\endgroup$
0
$\begingroup$

Volumetric flow rate is not m/s, it is m^3/s...

But if you want rpm in minutes then have the volumtric flow rate in m^3/minute.

This is just balancing units.

$\endgroup$
  • $\begingroup$ Yes sorry my mistake, volumetric flow rate was measured in Liters/second. So if I convert l/s to m^3/min, do the units cancel out? $\endgroup$ – Shahad Feb 24 at 9:28
  • 1
    $\begingroup$ What is the difference between minutes and seconds? You should be able to cancel units... This is dimensional analysis 101... $\endgroup$ – Solar Mike Feb 24 at 9:33
  • $\begingroup$ I mean will 1/rpm cancel with minutes? Since I'm confused about the concept of rpm . I know its #of revolutions per minute. r/min. but do revolutions have units? $\endgroup$ – Shahad Feb 24 at 10:17
  • $\begingroup$ Think of rpm is (1/min) $\endgroup$ – Solar Mike Feb 24 at 10:18
  • $\begingroup$ Why not put all the terms into the equation, replace each with their units (ie Mass, Length or Time) and show they cancel out... $\endgroup$ – Solar Mike Feb 24 at 10:22

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.