**Streamline Diffusers and that pesky Third Dimension**

Table 12-1 gives a table of coordinates for a streamline diffuser of different expansion ratios (0.25, 0.30, 0.40, 0.50). The coordinate system here is a bit odd -- I'm sure it's explained somewhere else in the book, but at a flip through I can't find it. The main dimension Y_B is the height of the heat exchanger; but it looks like this coordinate runs from -1 to 1. (Logic: For the 0.50 expansion ratio, the final y coordinate is 0.5/Y_B; no other context I can think of makes sense here.)

This table is for a two dimensional diffuser, and the coordinates are modified from true streamlines to give a total recommended length of 3*Y_B. So, if I'm reading this right, for a 12" high diffuser, max(Y_B) = 6", so the diffuser length would be 18". I'd appreciate a check that I'm interpreting this correctly.

Where things get interesting is applying this to a three dimensional heat exchanger, with height and width. Suppose a moderate aspect ratio -- so 24" width, 8" height, irrelevant depth, as an example. With a 2D diffuser you can diffuse the air in two directions -- if the inlet is 24" x 4", you get an expansion ratio of 0.5 with a diffuser length, by the table, of 12"; if the inlet is 12" x 8" you have a diffuser length of 36".

If makes moderate intuitive sense to me that, for a 2D streamlined diffuser, you can build it either way, and will get different results, but I'd love a sanity check. The obvious follow-up question is which of these would be preferred.

It also seems obvious that, in these days of CFD, it would make sense to drop that 24" x 8" heat exchanger into CFD, modeled as a thin plate perpendicular to the flow with the appropriate pressure drop across it (no geometry modeling), and selecting 3D streamlines. There's still a lot of approximations here (no boundary layer), but it would give a 3D inlet shape with the streamline property as much as the 2D examples in the book have, and probably also give some nice visualization of how to to deal with e.g. corners. However, this will still give infinitely long streamlines, which have to be truncated somewhere. Is the appropriate length to truncate to here based on the maximum wall angle, or based on equivalent length (so 3/2*sqrt(24*8) = ~21" length)? Too long means excess wall friction losses; too short means separation losses. Ideas?