large aspect ratio hydraulic diameter & pressure drop

[tope]

I have a cooling application where I want to flow air thru a rectangular channel with an extreme aspect ratio. The channel would be 0.75 in. x 148 in. Typically books say to use (cross sectional area)/(wetted perimeter) for the hydraulic radius. For this sort of configuration the hydraulic radius is then equal to one half the width of the passage. Using this value for hydraulic radius gives what I consider a rediculous pressure drop for turbulent flow. I think its more like flow between parallel plate. Does anyone have experience with turbulent flow pressure drop in such a situation?

 

[johnp]

Have a look at the American Society of Heating, Refrigeration and Airconditioning Engineers (ASHRAE) Design Handbook. This type of thing often crops up in the Refrigeration and Air Conditioning field - e.g flow trough plate heat exchangers.

Most flow charts, and formulae, are based on circular sections, with a conversion factor for surface area. In your case you have a lot of surface area and therefore a lot of drag will occur due to the shearing of boundary layers.

 

[tope]

Thanks, I will look there. I was thinking that if I divided the flow area into a number of smaller channels and then divided the flow accordingly this would be a conservative upper bound pressure drop estimate. I thought this because that would be increasing the surface area.

 

[KenRad]

tope,

The hydraulic radius formula should be 2*A/WP, not A/WP.