Modeling a Burning Candle, How Would You Do It?

Fanny Littmarck December 25, 2012
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This holiday season most of us will have burned our fair share of candles. The flickering light of candles can really enhance the ambiance and put you in festive spirits. This reminds me of an analysis of a burning candle developed by AltaSim Technologies back in 2010. So, in tune with the holiday cheer, here are some candle physics.

Burning candle

Physics of Candles

The major components of a candle, as we all know, include some type of wax and a wick. The wax can be of stearin, beeswax, or a plant wax like palm or soy, to name a few. Alternatively, the candle could be made of a non-wax base such as gel made from resin and mineral oil. Did you know there are also several different types of wicks? I had never thought too deeply about that aspect of a candle, until now. Apparently, there is a point to trimming candle wicks; shorter wicks lead to slower and steadier burning, while also preventing smoke. Back in the day, wicks needed to be cut often but now they are created to curl as they burn, thus forcing the end of the wick into the hot area of the flame. When that happens, the end of the wick is burned off and becomes self-trimmed. That self-trimming is intentional was news to me, but it’s a rather fascinating, not to mention convenient, concept.

So what happens when you light a candle? First, the flame melts the wax closest to the wick. This leads to a phase change, allowing for mass transport via capillary flow. In other words, the wax (now in liquid state) is pulled up the wick. Next, the heat of the flame turns the liquid wax, thanks to combustion, into a hot gas. This process of combustion is maintained as the combustion heat melts more wax, until you either blow out the candle or it runs out of wax.

Modeling a Burning Candle in a Ceramic Vessel

Although not all candles come in a jar or a vessel of sorts, the one AltaSim Technologies chose to model did. Their paper shows how to use COMSOL Multiphysics for predicting the behavior of a burning three-wick candle with one-sixth symmetry. Four different scenarios showing the velocity field within the candle plume are portrayed in the analysis: a full, half-full, near-empty with a center flame, and near-empty with a flame licking the side of the ceramic vessel. These are shown in the images below, but you can also see them in a larger format in the original paper “Analysis of a Burning Candle”.

Modeling a burning candle in COMSOL Multiphysics
Velocity field within the plume of a burning candle flame at different stages.

This paper provides us with great insights about the modeling strategies necessary to predict such a complex phenomenon that requires a true multiphysics approach and all the power of the Heat Transfer Module.

Have you ever modeled a burning candle?

We’re curious to know on which modeling strategies you relied and which capabilities you have found to be the most useful.

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