CFD Candle Project - Femto Engineering - Femto Engineering

CFD Candle Project

Recently we investigated an everyday flow typical for the cold winter weather, a candle on a table. While it is easy to light and providing a nice ambiance to the room, the physics behind a candle are complex, more than you would initially guess.

Describing every part of a burning candle would require knowledge of melting wax and transport of said wax through the wick, before entering a complex chemical reaction where the wax reacts with oxygen, resulting in a flame, CO2 and H20 (and some soot).

This simulation shows the convection of heat from a candle. The flame is modelled as a volumetric heat source, providing sufficient energy to raise the maximum temperature in the flame to approximately 1400 degrees. For this we modelled the flame above the wick as a heat source releasing 11 million Watt/m^3. This resulted in a maximum temperature that is in line with an actual candle.

The heat from the candle causes the air to heat up and rise, in a process called convection. There are multiple forms of convection, namely free and forced convection. In this case we deal with free convection, with only the density difference caused by the temperature as the driving force.

Free convection is always an interesting phenomenon as it is dependent on chance. This is not something that Solvers handle well; they get the same results for the same inputs. For this convection, it meant that we got a much more steady flame than we would see when lighting a real candle. To create an instability, a draft is introduced into the domain, one of the walls is a velocity inlet with a small sinusoidal velocity, only 0.1 meter/second. The other wall is a pressure outlet.

IdealPressureFunction = 101325*(exp(-9.81*$$Centroid[2]/(287.058*300))-1)+9.81*1.17659*$$Centroid[2]


Corrected pressure function


Uncorrected pressure function

With forced convection and the height difference that we are looking at, the effect of this field function on the velocities tends to be small. However, since we only have the free convection, we’ll be looking at small velocities anyhow, so these will have a significant impact.

 

Without the corrected field function, the pressure difference over the height of the domain will cause velocities on the boundary of the domain to dominate the simulation, as displayed by an example from the Siemens websites, showing the flow without pressure function and with the pressure function.
With the pressure field and velocity in place, the simulation can be started with Simcenter STAR-CCM+, resulting in a flame that is unstably burning in a room.

 

We hope you enjoy the animation as much as we enjoyed making it.

January 26, 2021
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