Fluid world: Question on Centrifugal impellers: I am blowing air through a single one sided impeller that has a defined rpm and mass flow rate. if I want to accelerate the flow withe the least change, What should I change first? Restrict the outlest? Change the number of vanes in the impeller? What is the practical way of doing this? fco.

Hi, Francisco.

Your question is not perfectly clear. What do you mean by 'accelerate the flow' if the mass flow rate is fixed? What is an outlest?

But anyway, to increase the energy added to fluid, with rpm and mass flow fixed, you can increase the length of the vanes.

Sergei

(1). The continuity equation says: Mass Flow Rate= Density*velocity*Area. where velocity is the average velocity, and the Area is the cross-sectional area. (2). Since you are blowing air, the density can be considered as constant. (3). So, the only way to increase the velocity (accelerate the flow) is to decrease the area. (4). As a matter of fact, I used to have a hair dryer which has a sliding door at the exit (a rectangular exit duct instead of a round exit duct.), so that I can increase the hot jet velocity by reducing the exit area. ( you can also try to attach a tapered cone to a hair dryer and make your observation.)

What do you mean by blowing? Is the impeller used as a compressor or as a turbine? If the latter is the case, does the flow go from the tip of the impeller to the the eye? Is your question referring to changing the operating condition of a given impeller or the design change to an impeller?

Rainer: Thanks for your answer. My question was on changing the design of an impeller. I cann't change the diameter. It is a compressor. The flow comes axially at the eye and leaves from the tip of the impeller into an involute. Should I change the angle exit of the vane? or should I put pre-whirl? I do not have too much room at the eye. I want effectively increase the exit velocity.

I don't want yo sound difficult. However, why would you want to increase the exit velocity (are we talking relative velocity?) from the impeller. Usually, the attempt is to decccelate the relative velocity in the impeller.

Rainer: The application is a little air blower for garden applications. I want to increase the speed because the current one is weak and does not blow enough air after a travel in a tube of about 30 in long. This is not the conventional use for the impeller.

Ok, your basic problem is, that you need to increase the flow through your fan, which requires that you generate a higher pressure ratio. Because you run at constant rpm (ac electric motor?), there are two things you can do: 1- reduce the inlet and exhaust pressure losses by widening/smoothing/optimizing the ducts 2-increase the impeller diameter or impeller vane diameter (this may cause structural problems). If the vanes don't end radially, you may want to try to bend them more towards radial direction(although it depends on a bunch of circumstances whether that will help or makes things worse) .

(1). Ken Elms pump site is a good place to look for design information. (2). Actually, there is a good undergraduate Fluid Mechanics book with a good chapter on the pump and turbomachinery.( Book by Prof. White, Rhode Island Univ.?) These are analytical theories easy to understand. (3). If a 30 in. tube is creating large pressure drop (loss), you can reduce the pressure loss in the tube by replacing it with larger diameter one.