Customer Brief: Enfield

Defying assumptions
Enfield Technologies specializes in designing proportional pneumatic controls for automation, medical equipment, animatronics and specialty instrumentation. They ran into a flow problem when designing a valve as the primary control element for inhaling and exhaling within a lightweight and portable respirator that is about the size of a toaster.

Fortunately, the design team had two things working in its favor: an engineer’s nuclear reactor operator experience, and CFdesign software that gave them a good picture of flow characteristics within the control valve in the early design stages.

The respirator valve needed to operate with a non-linear flow profile in relation to the poppet position. A very shallow and well-behaved flow control was needed for the first 50 percent of the valve stroke, followed by a rather steep flow profile for the remaining 50 percent.

Fast design/visualization iterations
Enfield uses Autodesk Inventor and the models were read directly into CFdesign. The tight integration between Inventor and CFdesign makes it quick and easy to change geometry and then see the effect of those changes on streamlines, mass flows, and flow velocities.

The valves were tested at various positions and in different upstream and downstream conditions to develop preliminary flow profiles. With CFdesign, each design idea can be explored relatively quickly without the added expense of physical prototypes. Capabilities such as being able to cut a plane through visual representations of velocity and pressure to see flow characteristics at exact points, displaying bulk parameter mass flow, and viewing particle trajectory using cylinders and spheres were priceless for better understanding the effects of design changes.

The engineers expected results from the physical prototype testing to be within 10 to 15 percent of CFD results, so they were amazed when the data agreed to within two to three percent at nearly every position.

The nuclear reactor effect
CFdesign was used extensively to study flow effects at the valve position of interest. The CFdesign simulations revealed a discrepancy: The convergence monitor window in CFdesign showed that the solver took longer than normal to converge and was less stable than expected around the suspect valve position.

They initially assumed that part tolerances might be allowing a small shift in the mechanical elements, but the anomaly was still present when all valve elements were rigidly mechanically constrained. Further mechanical testing that separated the valve from the electronics/sensors revealed that the problem was isolated to the valve assembly.

The problem supported a theory that was proposed based on nuclear reactor physics: they were witnessing a bi-stable flow effect – a flow variation caused by voids or mechanical obstructions – that is seen on rare occasions in water reactor physics.

Conquering bi-stable flow
While they could not prove the bi-stable phenomena with CFdesign, it was instrumental in isolating the unbalanced flow, messy vortices and crossing streamlines that indicated a need for geometry changes to the poppet. Further iterations between Inventor and CFdesign helped them hone in on a design that resulted in the smooth streamlines and balanced flow velocities we were seeking.

Providing an intrinsic understanding
With CFdesign, each design idea can be explored relatively quickly without the added expense or associated schedule delays of additional physical prototypes. A workable design for the respirator valve with the same specs might not have been possible without CFdesign, resulting in loss of a valuable client or in a final product with some undesirable features, neither of which is an acceptable outcome at Enfield Technologies.