Techniques to enforce nearly uniform flow velocity in a rectangular duct

[MNS93]

I have a rectangular duct about 1 meter wide, 1 meter high and 6 meters long, the duct is filled with columns of trays arranged horizontally, air flow enters the duct through an 8" pipe and exit through another pipe of the same size, I'd like to know the best technique to achieve almost uniform flow velocity everywhere around the trays, I've tried using diffusers of different sizes, but the velocity was always higher in the middle portion of the duct than at the trays near the bottom and top, I've also tried using perforated plates, straightener packs, but those resulted in higher velocities near the walls than at the center, they also induced vortices immediately downstream at the middle portion, when I used both techniques simultaneously, I got better results, but still had considerably lower velocities at the trays located midway between the center and the top and bottom, I used Ansys Fluent to test these techniques.

 

[Andrew ONeill]

Here's a random paper that describes it fairly well. https://www.flowconditioner.com/wp-content/uploads/2020/07/FFC-Whitepaper-3_CPA.pdf

In practice there is a boundary layer, you will never get an equal velocity profile across the full cross-sectional area, however you can improve it /flatten it out somewhat
Ultimately some combination of flow straighteners / conditioners, on both the inlet and outlet, and an understanding of the details of the "trays" (they will have their own unique boundary layer / pressure drop characteristics) is all you can really do.

In my experience, when this stuff becomes critical, CFD modelling to generate ideas, followed up by proper scale modelling is what is used to come up with solutions that are then trialed full scale.

the only other alternative is to redesign the whole thing and have a dedicated fan for each tray, this can the guarantee the same flow rate of air over each tray, but I'm guessing likely to be way overkill for your scenario.

 

[MNS93]

Thanks for your reply, the idea of a fan for each tray or may be a fan for each column is a great one, the case is critical enough for this not to be overkill. Also, the paper is very useful, thank you.

 

[Compositepro]

You have basically built a tray drying oven using cross flow air. These are common, and they all have similar design. Your design, though, is unusual in that it is relatively long in the direction of airflow. The problem with that is that the trays at the inlet side will see different drying conditions than those at the exit side. As a result, the inlet side trays will dry long before those on the exit side. This seems a much more significant issue than velocity distribution.

Here is a common baffle design for Blue M ovens. https://www.lre.com/lre-inv/images/BLUEMOVEDC336CINTERIOR.jpg
The blower and heaters are below the oven chamber and blow hot air from a plenum on one side of the chamber to a plenum on the other side. The baffle plates on each side have a uniform pattern of punched holes, where the punched metal is left attached to the baffle plate by a bendable tab so that each hole opening can be adjusted to balance the air flow.

 

[LittleInch]

The sudden expansion area is one issue which you really need to think about. Going from an 8" pipe into a 1m x 1m duct is going to get a jet effect for some distance away from the entrance. I think you need to look at a much more gradual increase in area over say 1-2m gradually enlarging the inlet area and maybe put in some deflectors to spread out that unknown inlet velocity (what is the velocity here?).

Then I was going to suggest exactly what the picture shows - basically a bulkhead with a set of holes which can be blocked or adjusted as required.

After all that's exactly what a shell and tube HX does.

 

[MNS93]

You have basically built a tray drying oven using cross flow air. These are common, and they all have similar design. Your design, though, is unusual in that it is relatively long in the direction of airflow. The problem with that is that the trays at the inlet side will see different drying conditions than those at the exit side. As a result, the inlet side trays will dry long before those on the exit side. This seems a much more significant issue than velocity distribution.

Here is a common baffle design for Blue M ovens. https://www.lre.com/lre-inv/images/BLUEMOVEDC336CINTERIOR.jpg
The blower and heaters are below the oven chamber and blow hot air from a plenum on one side of the chamber to a plenum on the other side. The baffle plates on each side have a uniform pattern of punched holes, where the punched metal is left attached to the baffle plate by a bendable tab so that each hole opening can be adjusted to balance the air flow.

Yes, it's an air dryer, the tunnel being that long is what making velocity uniformity over the trays a critical issue so as not to increase the already significant drying rate difference between the first and last tray columns, the dryer uses significant area of flat plate solar collectors as its heating source, around 20 m2, I've used Ansys fluent along with some drying numerical model to calculate the drying rate over all trays and blowing air at a uniform speed of around 0.4 m/s over all trays ensures things will work fine but that's yet to be achieved, thanks for the suggestion but the reason why my design is long is because its drying capacity is high, it should be able to dry around 300 kg in about 30 hours.

 

[MNS93]

The sudden expansion area is one issue which you really need to think about. Going from an 8" pipe into a 1m x 1m duct is going to get a jet effect for some distance away from the entrance. I think you need to look at a much more gradual increase in area over say 1-2m gradually enlarging the inlet area and maybe put in some deflectors to spread out that unknown inlet velocity (what is the velocity here?).

Then I was going to suggest exactly what the picture shows - basically a bulkhead with a set of holes which can be blocked or adjusted as required.

After all that's exactly what a shell and tube HX does.

I really thought a punched plate will help take care of that jet effect but as I mentioned, it caused some serious circulation near the middle part of the tunnel right downstream of the plate, anyway the paper suggested by Mr. Andrew highlighted the flow phenomena I encountered in my simulations and pointed out the fact that it's going to take some long distance 5 to 8 hydraulic diameters of the duct for the flow to attain a fully developed velocity profile even after using the most advanced flow conditioning technique, so I'm redesigning to get rid of the piping completely, sorry for the late reply.
the velocity in the pipe is 13 m/s