Using CFD for Cleaner Fireplace Design
Michael Alba posted on April 16, 2019 |
Wood-burning fireplaces, enjoyed by man and man’s best friend alike, must be designed to minimize harmful emissions. (Image courtesy of Stove Builder International.)
Wood-burning fireplaces, enjoyed by man and man’s best friend alike, must be designed to minimize harmful emissions. (Image courtesy of Stove Builder International.)

Nothing beats the pleasant crackling and gentle warmth of a dancing wood-fueled flame. As you settle into your hearthside armchair at the end of a long day, the last thing you want to worry about is the amount of particulate matter being emitted by your wood-burning fireplace. Is the smoke rising from your chimney as clean as it should be?

Worry not, because wood-fueled appliance manufacturers like Stove Builder International (SBI) worry for you. SBI manufactures wood and pellet fueled fireplaces, stoves and furnaces for the international market. We spoke with Guillaume Thibodeau-Fortin, a mechanical engineer and designer for SBI, to talk about how the company uses computational fluid dynamics (CFD) software to ensure its products are as clean as possible.

Minimizing Particulate Matter

SBI’s range of solid-fuel burning appliances produce heat by burning either wood or pellets made of compressed organic matter. However, heat isn’t the only product of this reaction. The smoke from wood- and pellet-burning stoves and fireplaces contains gases like carbon dioxide and monoxide, as well as solid or liquid particulate matter, tiny particles just 2.5 to 10 microns in size.

Particulate matter (PM) ranges from 2.5 to 10 microns in size, much smaller than a human hair or grain of sand. (Image courtesy of EPA.)
Particulate matter (PM) ranges from 2.5 to 10 microns in size, much smaller than a human hair or grain of sand. (Image courtesy of EPA.)

Particulate matter is not particularly healthy to have in one’s lungs, so it won’t come as much of a surprise that there are regulations surrounding its emission. In the United States, particulate matter emissions are currently limited to no more than 4.5 grams per hour. However, the Environmental Protection Agency (EPA) is tightening those restrictions to just 2.5 grams per hour in 2020.

To cope with the regulatory changes, SBI needed to revamp its considerable product line, and the company only had five years to do so. Though that sounds like plenty of time, as Thibodeau-Fortin explained, it could have been a close call.

“In the past, in order to do five fireboxes [the part of the fireplace or stove where the fire burns], it took probably five years. It was maybe three to four prototypes before the certification. Now we’re able to do one or two prototypes before being able to run a certification test,” he said.

So what changed? SBI turned to simulation to accelerate the design process. The company used Siemens’ FloEFD for Solid Edge, a CFD analysis tool integrated within its design software, to better analyze and optimize their designs.

Refining Design with FloEFD

One option to help reduce harmful emission in wood-burning products is to include a device called a catalytic combustor, which helps reduce the temperature necessary for gases to burn. However, according to Thibodeau-Fortin, catalytic combustors are fragile and difficult for end users to maintain, so SBI doesn’t use them. In order to reduce the particulate matter emissions of its appliances, SBI had no choice but to tweak its designs into compliance.

This tweaking was accomplished through the use of FloEFD. Though the CFD software doesn’t offer a direct method of determining the particulate matter emissions of a design, it can offer insight into emission quantity. Thibodeau-Fortin used data for wood gas flow to run a combustion simulation on SBI’s firebox designs. Though the simulations were intensive—10 hours of simulation for five minutes of real-time combustion—the analysis revealed the amount of residuals left in the chimney. The amount of residuals is an indicator of the amount of particulate matter emissions, with more residuals indicating more emissions.

SBI used FloEFD to simulate and optimize their stove and fireplace designs. (Image courtesy of SBI.)
SBI used FloEFD to simulate and optimize their stove and fireplace designs. (Image courtesy of SBI.)

With this feedback, Thibodeau-Fortin was able to tune the design in the direction of less residuals and, hence, less emissions.

“The combustion alters depending on how I tune my primary and secondary air intakes, the pilot and all those type of air intakes,” he said. “I looked at plenty of parameters in the flue pipe, and that's how I compared a wood stove that was quite dirty to one that was pretty clean. I got closer and closer to what a clean stove should be. At the end, I could do maybe 20 or 30 modifications without having to weld anything. So it helped a lot on the prototyping.”

Since starting to use FloEFD to modify designs, SBI has certified five new fireboxes in line with the 2020 requirements, which encompass three models of wood stoves, two models of fireplaces and two models of pellet stoves. Only a few fireboxes remain to be certified to complete SBI’s product portfolio.

Integrated CFD Analysis

Siemens acquired FloEFD in 2016 with the company’s acquisition of Mentor Graphics, allowing for even tighter integration with Siemens’ CAD applications Solid Edge and NX. FloEFD is embedded directly within the application, meaning there’s no need to transfer CAD models into a standalone CFD solver. That said, FloEFD is available in a standalone version to those who need it.

The tight integration of FloEFD within CAD programs encourages engineers to frontload their CFD analysis. FloEFD wants users to take advantage of CFD early on in the design process, rather than sending a nearly finished model to a CFD analyst late in the game. This means CFD can take a guiding role in the design, as it did for SBI. To the same end, FloEFD is designed to be user friendly, accessible even to users lacking a PhD in turbulence modeling.

“It’s quite user-friendly software,” Thibodeau-Fortin said. “It’s made for a non-expert of CFD, and that helps a lot.”

For his own training, Thibodeau-Fortin went through the tutorials that come packaged with FloEFD. He had also taken a few CFD and finite element analysis (FEA) classes during his undergraduate degree, which gave him some familiarity with the process. However, he found little information on simulating combustion, which is what he needed to do for SBI. He was fortunate that a former professor of his happened to be working on combustion simulations and was able to guide Thibodeau-Fortin.

Naturally, there’s always more to learn. Thibodeau-Fortin plans to complete further training with a company specializing in combustion and heat transfer simulations. The additional training will help him tackle even more wood-burning fireplace designs, as SBI, pleased with the benefits of FloEFD, will continue to simulate its appliances for a more detailed understanding of their performance.

In the meantime, you can rest easy knowing that there’s a little less particulate matter floating around out there, thanks to FloEFD and Thibodeau-Fortin’s design efforts.

You can learn more about Solid Edge FloEFD on Siemens’ website.


Siemens has sponsored this post. They have had no editorial input to this post. All opinions are mine. --Michael Alba

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