# Pressure Drop vs Velocity in a Rectangular Pipe

1. Sep 27, 2009

### murasame

Hi guys

I am currently using Comsol to model a single rectangular channel, a typical one found in a fuel cell (with symmetry boundary). Dimension of the inlet and outlet area is of the order 10e-3 and the length of the pipe is 0.1 m.

I am having difficulty getting the expected result. The pressure drop vs velocity of fluid flow should be a curve that can be mapped by a second order quadratic equation (as the velocity increases, the pressure increases with increasing rate) But no matter how I try I keep getting a relationship that is close to linear, and only very slightly curved.

Here's what I've done, tried different solvers, played around with the meshing, changed the viscosity, but the result is the same, an almost linear graph, which does not fit the expectation.

Can anyone who is familiar with Comsol offer any advice regards to this problem? Thanks in advance.

2. Sep 28, 2009

### redargon

Are you taking frictional losses into account? are you taking heat transfer into account? Are you modelling it as a compressible or incompressible substance? Where did you get the expected result from?

3. Sep 28, 2009

### minger

With such a small diameter, you're going to see a lot of viscous effects, as long as your grid can resolve them.

Before anything can be determined, you'll need to provide some more information such as exact boundary conditions, information on the grid, etc, etc.

4. Sep 28, 2009

### Bob S

Try comparing to the pressure drop in a straight tube with a 1-mm diameter using an online calculator such as
http://www.pipeflowcalculations.com/pressuredrop/index.htm [Broken]
(click on calculator in lower left corner.

Last edited by a moderator: May 4, 2017