# How can I create uniform laminar flow in a short duct?

I have a small vessel with a rectangular cross-section which needs to be ventilated at a relatively slow rate, ~$3\ \tfrac{\text{L}}{\text{hr}}$. The connections to the vessel will be tubes with an approximately 0.5" diameter, but the cross-section of the vessel is 4" x 1". The diagram below gives a rough idea of the cross-section (viewed from above) with the height of the chamber being about 1". The part circled in orange is the topic of this question. The flow will be forced in from the left and will exit passively from the right (or vice versa).

How can I couple the 0.5" tube to the chamber such that the flow in the chamber is uniform across the entire cross-section for the entire length of the duct? In addition, how can I couple the two in the smallest amount of space possible?

• I'm curious if this is possible. Intuitively this much of a change in size and shape in the flow path seems like it should be a source of turbulence but with this slow of a flow rate maybe not. Commented Oct 22, 2015 at 15:28
• Can you pass the air through a packed granular bed? Air will come out turbulent but will diffuse laterally quite a bit. Commented Oct 22, 2015 at 15:34
• Based on the dimensions, it looks like you've got Reynolds numbers of about 5.5 and 13.5 in the tube and duct, respectively. Those are both well below the transition to turbulence. So the flow should be laminar everywhere. Uniformity near the ends might be a problem...
– Dan
Commented Oct 22, 2015 at 15:47
• Similar to the packed bed suggestion, I've seen some people suggest that flow straighteners can make a flow more uniform in addition to reducing the turbulence (an advantage over the packed bed). I'm not sure how true this is. You'd also expect the velocity profile to transition to the parabolic laminar one after the flow straightener. How quickly that occurs is not clear to me. Commented Oct 22, 2015 at 17:08
• According to this paper flow straighteners tend to preserve the velocity profile. So this suggests they wouldn't work for this. Otherwise, I know that contractions can be used to create more uniform velocity profiles, but there's an expansion here. Commented Oct 22, 2015 at 19:22

Quick and dirty CFD simulation of your problem using ANSYS Fluent 14.5: I used a 2D duct, 8" x 4" with a 45-degree angle going from the inlet pipes to the main chamber. Assuming 3 liters per hour flow through a half-inch diameter pipe gave me an inlet velocity of 6.6 mm/s. Air enters from the left and exits through the right. Inlet and outlet were set to a constant pressure of 101 kPa. I used the realizable $k-\epsilon$ turbulence model with standard wall functions. Computed velocity contours are below: