5 Ways to Improve Flow Design with Perforated Plates

The use of perforated plates in controlling fluid flow is widespread.  They are one of our favorite flow control devices for many situations.  Check out these 5 ways perforated plates can improve your flow designs.

#1:  Diffusers (aka duct expansions)

Diffusers come in all shapes and sizes.  Diffusers are most commonly used to reduce a fluid’s velocity when it's needed for downstream equipment performance.  Industrial and power generation equipment such as heat exchangers, electrostatic precipitators, spray dry absorbers and selective catalytic reduction (SCR) systems often rely on diffusers in this way.  Left to their own, diffusers will result in flow separation and poor flow distribution entering the downstream equipment which can inhibit performance.  Perforated plates are one type of flow distribution device that can be installed in a diffuser to improve this situation and provide flow uniformity as needed.

The following animations, created from Computational Fluid Dynamics (CFD) simulations, demonstrate the flow uniformity improvements in an asymmetric diffuser, such as would be seen upstream of a Heat Recovery Steam Generator, utilizing two perforated plates.  The geometry shown here is taken from the experimental study of Noui-Mehidi et al. [1].

#2:  Flow Conditioning

 
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Many hydraulic instruments, such as flow meters and orifice plates, perform best when incoming flow is as uniform as possible.  In this sense, it is an unfortunate fact that piping turns lead to swirling flow effects first identified by W.R. Dean in 1958 [2].  The resulting paired vortices are commonly referred as Dean vortices.  Perforated plates have long been used as flow conditioners able to eliminate such swirling conditions.  Using perforated plates in this way has been shown to reduce the required upstream flow diameters by more than 75% [3] while improving accuracy.

The following animations, the result of CFD simulations, demonstrate the elimination of swirl realized through the installation of a perforated plate downstream of two 90-degree turns in a curved pipe. 

#3:  Flow balancing in plenums

 
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In many instances across a range of industries, flow needs to be equally divided between a multitude of paths connected to a common plenum.  Often times these flow paths have widely differing resistances to fluid flow that would naturally cause flow rate imbalances among the paths.  Flow is lazy, we like to say, and gravitates toward the easiest route possible.  Such differences can be offset by adding resistance to the lowest resistance paths through the use of perforated plates.  This functionality is analogous to that provided by louvers but without the moving parts and negative impact on local flow uniformity.  A well-known example is found in many types of HVAC systems where a common inlet plenum feeds multiple outlet vents spread across a building. 

Another simple example is an engine air manifold.  The following animations demonstrate the difference in mass flows through six engine manifold outlets for designs with and without perforated plates.

#4:  Distributed Resistance on Suction Intakes

 
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In many cases, one would like to pull flow into a system uniformly over a large area while using fans or pumps located further downstream.  An example of this is the common computer or server casing where flow should be evenly drawn in across the whole frontal face of the intake to ensure proper cooling of electronic components within the casing.  We recently completed a study of Rackspace’s high-performance Open Compute / OpenPOWER server design, the Barreleye G2, in which modeling the impact of the perforated cover plate was especially important for making accurate heat dissipation predictions.  The following animation demonstrates one such result where streamline colors indicate the evolving temperature of fluid parcels moving through the rack server. 

 
 

#5:  Turbulence Generation

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One of the least known functionalities of perforated plates is their ability to enhance turbulence homogenously.  There are many examples in which such an increase in turbulence will improve system performance, a few of which are combustion systems and heat transfer devices.  Stay tuned for an example of a CFD simulation of the turbulence generation capabilities of perforated plates.

References

[1] W. J. S. I. G. C. Noui-Mehidi M.N., "Velocity distribution downstream of an asymmetric wide-angle diffuser," Experimental Thermal and Fluid Science, pp. 649-657, 2005.

[2] D. W.R., "Fluid Motion in a Curved Channel," Proceedings of the Royal Society A, vol. 121, no. 787, pp. 402-404, 1928.

[3] Southwest Research Institute, "Review of Ultrasonic Flow Meter Installation Effects," Pipeline Research Council International, 2015.