questions about heat transfer in rectangular duct

[xuehua]

I want to design a rectangular duct(height=2cm,width=5cm) through liquid. Can I use the circular duct empirical correlations to get the heat transfer coefficient if hydraulic diameter replaces the diameter? I want to get hydraudynamically and thermaly fully developed flow in the duct,so what is the minimum length of the duct? Another question is what the critical Reynolds number if fully turbulent flow in the duct?

Thank you very much!

 

[chicopee]

Check out one of the mechanical engineering handbooks, there is an equation that will give equivalent round duct diameters to that of rectangular ducts. I know that my Kent "Power" ME handbook has this formula. Be sure to read the basis of the formula. This equivalent diameter is not the hydraulic diameter.

 

[vpl]

This sounds like an experiment of some sort. Generally the minimum distance is based on moving the fluid from Point A to Point B, not on trying to get 'hydraulically and thermally fully developed flow'. Where you get 'hydraulicaly and thermally fully developed flow' depends a lot on parameters you've not mentioned and can be highly dependent on the situation, such as the flowrate and whether different streams of fluid are joining together.

You also asked about what is the critical Reynolds number for fully turbulent flow. In general, turbulent flow is presumed if Re> 4000. However, in real life, IT DEPENDS.
My Crane Handbooks notes that laminer flow can terminate at a Reynolds number as low as 1200 and can exist at a Reynolds number as high as 40,000.

Speaking of which, you might also want to get your hands on a Crane Handbook 210 "Flow of Fluids Through Valves, Fittings and Pipes." Since your handle indicates you're an electrical, you might want to borrow it from a mechanical engineer coworker. It's also relatively inexpensive to buy and well worth having if you're going to be working with fluids.

Patricia Lougheed

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