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 Bob Haase February 15, 2001 11:14

Determining Pressure Drop via Reynolds Number

Hi,

I have just run a pressure loss simulation in CFD2000 for a very long duct. Now I want to validate my solution with an experiment. I have a blower to generate the volume flow, however, I don't have the means to heat the air to match the conditions in my simulation. The temperature of the air in my simulation was 773K. The temperature of the air for my experiment is more like 300K.

Here is my question. Can I use the Reynolds Number from my simullation to determine the air volume flow for the experiment? If I do this, can I expect that the measured pressure loss in the experiment will match the pressure loss under the same Reynolds Number flow at a higher temperature?

thanks, Bob

 John C. Chien February 15, 2001 14:20

Re: Determining Pressure Drop via Reynolds Number

(1). The viscous loss in a pipe is a function of the Reynolds number. (2). You can vary the temperature, the diameter, the velocity (or the mass flow rate), as long as the Reynolds number remains the same.

 Bob Haase February 16, 2001 10:57

Re: Determining Pressure Drop via Reynolds Number

O.K., so what if one of the flows is highly compressible? The high temperature flow has a Mach Number of 0.6, and the low temperature flow has a Mach Number of 0.2. Does the compressiblility contribute to the pressure loss in the duct?

thanks, Bob

 sylvain February 16, 2001 12:51

Re: Determining Pressure Drop via Reynolds Number

In that case, you should have to respect the Mach numbers also, to get the right compressible behavior.

This comes from the application of the Pi-theorem (or Bughingham-theorem).

 John C. Chien February 16, 2001 14:21

Re: Determining Pressure Drop via Reynolds Number

(1). The loss comes from the skin friction. And the skin friction coefficient is a function of Mach number. (2). You need to take a look at the book "Boundary Layer Theory", by Schlichting. It has a chapter on skin friction drag of a flat plate at sero incidence, which includes charts and formula to estimate the skin friction coefficient for compressible flows as a function of Mach number. (3). In your Mach number range, the effect is about 5% smaller based on the incompressible value. The higher the Mach number, the lower the skin friction coefficient. (3). You must try to get the book, it has all the information you need.(it is a popular standard reference book)

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