Optimizing Home Heating with CFD Analysis

Fireplaces are a lousy way to heat a home, says Rand Simulation CFD specialist.

How does a computation fluid dynamics (CFD) specialist spend their holiday? If you were Troy Baumgardner, CFD specialist for Rand Simulation, you would have been watching your fireplace very intently—and wondering why the wood burning so hot was not making you warmer.

Troy Baumgardner, simulation specialist at Rand Simulation (Picture courtesy of LinkedIn).

“I remember seeing the Christmas movie characters and a scene where they are in bed, a fireplace burning in the bedroom, they in their blankets and hats,” recalled Baumgardner. “The fire was not helping heat them either.”

We catch up with Troy to find out more about his fireplace research.

“Why are we so interested in analyzing hypersonic flight craft when something [like] the common fireplace is not understood?” he wondered and then commenced to use simulation to answer his own questions.

Indeed, fireplaces do more to warm the heart than the body. The primary mode of usable heat transfer for a fireplace is radiation—not convection as most people think. Most of the heat from convection goes right up the chimney.

“You don’t want the hot air from the fire in your room,” said Baumgardner. “You will have a room full of smoke.”

“For thousands of years, everyone from kings to paupers counted on fireplaces for warmth,” noted Baumgardner. Yet, the fireplace seems to have not been sufficiently studied.

It was thousands of years ago that someone got the bright idea of moving a wood burning fire inside to heat a home. What could go wrong? An open fireplace is not only dangerous; it is also unhealthy (polluting both indoor and outdoor )—not to mention grossly inefficient (most of the heat from combustion goes up through the chimney).

The Philadelphia Fireplace was the original Franklin Stove, which had its hot exhaust circulating in the stove so that more heat would be transferred to the room. Unfortunately, this system requires the smoke to go down, exiting from the bottom of the stove, when it naturally wants to rise. (Picture courtesy of Wikipedia.)

Much better are Franklin stoves. Named after Ben Franklin, the Franklin stove, aka the Philadelphia fireplace, was invented in 1741.

“The Franklin stove does a much better job of getting the heat into the room than a fireplace,” said Baumgardner.

But the original Franklin stove didn’t work. The “inverted siphon” required the exhaust to be on the bottom of the stove—despite smoke’s tendency to rise. While the Philadelphia Fireplace cannot be counted as one of Franklin’s successful inventions, the stoves that evolved from his design—with better exhaust paths—became popular in America.  Another inventor and scientist, David Rittenhouse, kept the concept of using baffles to make the exhaust snake through chambers but redesigned the passage for better natural convection. He called the redesigned stove the Rittenhouse Stove, but this name didn’t stick. The stoves continue to be called by the name of his more famous contemporary.

“The late 17th century saw significant advancements with fireplaces, improving ventilation and efficiency. Franklin added baffling that kept hot exhaust gasses in contact with cooler room air, causing heat transfer. Wood-burning heaters continue to improve, especially with modern pellet-type heaters,” noted Baumgardner.

A modern wood-burning stove uses the same principles as a Franklin stove but can employ a catalytic converter and a secondary combustion chamber. A pellet-burning stove (not shown) has the added advantage of having fuel conveniently flow into the fire from a hopper. (Picture courtesy of Pinterest.)

The Simulation

Fire modeled as a half-cylinder (red) inside a wood-burning stove inside a bedroom. (Picture courtesy of Rand Simulation.)

Reducing flow into the chimney, as well as a poorly designed or blocked chimney, could allow smoke to escape into the room, making an unpleasant indoor environment. (Picture courtesy of Rand Simulation.)

Velocity gradients inside a Franklin stove. Arrows show direction only, with magnitude shown by color. Half-cylinder shape is a model of fire. (Picture courtesy of Rand Simulation.)

Home heating is an old concept, but its simulation is far from simple. “Even without modeling the chemical energy changes during combustion, we still have three modes of heat transfer to consider,” explained Baumgardner. “Convection, conduction and radiation. Architects may do hand calculations to size a stove, but a 3D CFD makes it easier to see the picture.

“The ability to make the invisible visible—that’s what CFD does,” said Baumgardner.

Baumgardner found that the results of analysis gave him some answers—but also generated questions. What would happen if that chimney were larger? Would it be better if the woodstove was toward the front of the fireplace? Should the fire grate be higher? What if the back of the fireplace was more reflective?

It certainly seems that simulation like this would help not only product designers (a further improved Franklin stove) but also architects, who can provide a more efficient heating option to clients who must have a romantic or nostalgic fire or the quaint retrospective look of a wood- or pellet-burning stove.

The Making of a CFD Specialist 

Baumgardner is part a six-person team that does CFD simulation for Rand Simulation clients. He has taken graduate courses in mechanical engineering at Case Western Reserve University and has earned a BSME from Ohio State.

Was that helpful in becoming a CFD specialist? Not so much, says Baumgardner. Although he took the obligatory courses in heat transfer, thermodynamics and fluid flow, he did not use CFD while in college.

“My first CFD program used was Autodesk’s while working in industry,” said Baumgardner. “Using a CFD program in university meant taking so many theory courses that you practically had to make a minor out of it.” But three out of six of his team members have PhDs. “We give them the harder problems,” he said

The mesh is the hardest thing to master, Baumgardner has learned. Getting the mesh right is the key to getting the right results, he says. Baumgardner refines the mesh to see if the CFD is converging to a solution, which serves him to validate the results.

“If we change the mesh but the results don’t change, we know we are converging to the right solution,” said Baumgardner.

Rand Simulation, a division of Rand Worldwide, is an Ansys reseller for North America that also provides FEA, CFD and electromagnetic simulation services across multiple industries. When Autodesk acquired Blue Ridge Numerics in 2011, several employees left to do consulting at IDP, which was acquired in 2012 by IMAGINiT, an Autodesk reseller. In 2018, the group started doing business as Rand Simulation and replacing Autodesk simulation with simulation from Ansys.

Find out more about Rand Simulation at www.randsim.com.