Improving wood stove combustion using Simcenter FLOEFD

Reducing time to market

A typical wood stove of the old type (pre 1998) including open fireplaces, utilizes the wood quite poorly. A lot of the flammable gases that evaporate from the wood continue up the chimney without catching fire. This results in poor efficiency and a lot of pollution.

Simulation made easy

Old stoves without secondary combustion can emit 40 grams or more particles per kilo of dry wood. The very best stoves, without catalyst or other measures, emit around 2-4 grams. To achieve this, most stove producers simply relay on experience and numerous experiments when they develop a new model. A cumbersome and expensive exercise.

In a Norwegian innovation project (2017-2019) granted by the Research Council of Norway, the CFD-tool Simcenter FLOEFD was selected as part of the research to determine if this could be used to improve the combustion in selected stoves. The industry partners were Jøtul AS, Dovre AS and Norsk Kleber AS.

A selection of CFD software solutions exists on the market, some quite complex, more suited for research and then others, less costly and more user-friendly, more targeted for industrial use. Of the latter, only Simcenter FLOEFD included combustion modelling, which was the main reason for choosing this software. Fast gridding, quick convergence and ease-of-use were also important aspects.

Frontloading CFD Simulation

This picture shows the result from a wood stove simulation of a modern clean-burning Jøtul F163 with Simcenter FLOEFD by Morten Seljeskog at SINTEF Energy research and shows how the air in a furnace is distributed in the original geometry.

It is the amount of primary air at a given air valve position that determines the wood consumption (kg/hour) and thus the release of energy in the combustion chamber. The primary air keeps the windows clean and supplies air to keep the temperature in the primary zone (around the logs) sufficiently high to evaporate the logs (around 75% of the logs will evaporate and burn as gases in the secondary combustion zone, the remains being charcoal). These wood gas will then burn out below the vault where preheated secondary air is introduced. Radiation from the secondary combustion zone helps to keep the evaporation going. Besides radiation through the stoves windows, heat exchange to the room happens mainly in the section above the vault, before the chimney exit. Ignition air from the bottom is only used during the ignition phase, the first 5-10 minutes.
Simcenter FLOEFD can be used to understand the combustion in such a stove, either as steady-state or its transient behavior.

Simcenter FLOEFD was successfully used to optimize the geometry of the secondary air nozzles and the shape of the primary air outlet to obtain better window flushing and better mixing in the secondary combustion zone. Further Simcenter FLOEFD was used to determine optimized geometry for minimum pressure drop in the air supply channels in several other stoves.

More challenges

Simcenter FLOEFD is the only fully CAD-embedded CFD software on the market. FLOEFD helps design engineers conduct up-front, concurrent CFD analysis using the familiar MCAD interface. This reduces design times by orders of magnitude when compared to traditional methods and products. Concurrent CFD can reduce simulation time by as much as 65 to 75 percent over traditional CFD tools. It allows optimization of product performance and reliability while also reducing physical prototyping and development costs, without time or material penalties.

Discover how Sintef successfully made use of Simcenter FLOEFD

"With Simcenter FLOEFD the development time can be reduced up to 29%”

“With Simcenter FLOEFD I can see the result from a wood stove simulation in a section that make it easy to understand how the air moves through the combustion process“

Morten Seljeskog SINTEF Energi

Simcenter FLOEFD was used for

Only Simcenter FLOEFD includes combustion modelling, fast gridding, quick convergence and ease-of-use. These are important aspects.

  • Optimize air distribution through the supply channels regarding pressure drop, velocity and pre-heating of the air
  • Optimize nozzle geometries in the secondary combustion zone for improved mixing
  • Determine residence time and temperatures in the combustion zones
  • Heat distribution and rediation
  • Demonstrate for the industry, the necessity of using such software to be able to cope with much more stringent emission limits in the near future

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