[coldncolder]

For fluid flow between two fixed plates when the ratio of width to distance between plates is around 200, how can the Reynolds number be calculated? Should the characteristic length be calculated by 4A/P or should the distance between the plates be directly used?

[FMDenaro]

Originally Posted by coldncolder

For fluid flow between two fixed plates when the ratio of width to distance between plates is around 200, how can the Reynolds number be calculated? Should the characteristic length be calculated by 4A/P or should the distance between the plates be directly used?

You can define the height, that is the distance

[coldncolder]

Originally Posted by FMDenaro

You can define the height, that is the distance

With air at around 2 deg C, flowing with a mean velocity of 1 m/s, and a geometry of cross-section of 2.000 m x 0.010 m, the Reynolds number becomes:
1. ~700 for 0.01 as the characteristic length
2. ~1400 for 4A/P as the characteristic length

Can you assure the validity of your answer?

[FMDenaro]

There is no one unique Reynolds value, in principle you could choice any lenght you want. But if you want to characterize the non dimensional parameters to assess the relevance of the ratio between convective and diffusive fluxes, the lenght has to be chosen properly.

[flotus1]

Originally Posted by coldncolder

With air at around 2 deg C, flowing with a mean velocity of 1 m/s, and a geometry of cross-section of 2.000 m x 0.010 m, the Reynolds number becomes:
1. ~700 for 0.01 as the characteristic length
2. ~1400 for 4A/P as the characteristic length

Can you assure the validity of your answer?

I can
Both are possible and valid definitions for a Reynolds number of your flow case.
However, defining the hydraulic diameter as the characteristic length of your flow problem for an aspect ratio of 200 is quite a stretch.
Think of it this way:
making the channel twice as wide (2m -> 4m) will not have a significant effect on the nature of the flow.
Increasing the height instead by a factor of 2 (0.01m -> 0.02m) can make the difference between a laminar and a turbulent flow.
So in your case the height would clearly be a better choice for the characteristic length.