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Digital Debunking: Is the Tip of a Flame Hotter than the Base?

Most of you will remember the first time you sat in a science class, safety goggles fastened, eyes focused on a Bunsen burner sitting on the lab table. The teacher had just introduced you to this novel piece of equipment and was telling you how to avoid scorching yourself (or someone else, or more). Your teacher may have also mentioned that the tip of the flame is hotter than the base – but is this true? Most classroom thermometers can’t handle fire, and you probably wouldn’t want to use your finger to measure the temperature (it wouldn’t give you much quantitative data, anyway). Thankfully, Altair solutions can help us find out if your teacher was right or full of hot air.  

To investigate, we used LOGEresearch, the ultimate simulation tool to investigate reactive flows using complex chemical kinetics. LOGEresearch can predict emission levels of soot, NOx, CO, and unburned hydrocarbons. The software offers faster computation times than those of traditional CFD analysis, and has additional benefits, like the ability to show users detailed chemistry.

Setting Up

LOGEresearch has a graphical user interface (GUI) that lets the user build models by connecting blocks, similar to 1D simulation tools.

The LOGEresearch interface
The LOGEresearch interface

You can see the basic data required for the calculation (definition of chemical reaction formulas related to gas combustion, etc.) in the table below:

Data required for the calculation
Data required for the calculation

To investigate accurately, we needed to recreate the chemical reaction process of a burning flame. Typically, a Bunsen burner uses methane gas, known chemically as CH4; when combined with air and an ignition source, it can oxidize. LOGEresearch decided the ratio of fuel to oxidizer based on a complete fuel burn. In this instance, the chemical equation to solve is:

CH 4 + 2O 2 + 8N 2- > CO 2 + 2H 2 O + 8N 2.

The fuel and oxidizer parameters set in LOGEresearch
The fuel and oxidizer parameters set in LOGEresearch

The conditions are shown in the figure below:

Conditions for the simulation
Conditions for the simulation

With this process set, we then needed to identify a steady state. The distribution of fuel during this process almost disappeared within 1 millimeter from the ignition point, as you can see in the graph below:

Establishing a steady state
Establishing a steady state

With a steady state established, a temperature distribution was produced recording the hottest temperature at 1850°C. The temperature of a gas burner is typically around 1800°C, so we felt confident with the results so far. Based on these results, the conclusion is that the hottest point was 5 millimeters from the ignition source.

Maximum temperature recorded
Maximum temperature recorded

Conclusion

So, was your teacher right? What we found is that the base of the flame has the highest amount of oxygen reacting with the fuel source during combustion. Thanks to LOGEresearch’s ability to simulate flows as well as chemical reactions, it represents the heat rising and therefore registers a hotter temperature at the tip of the flame. To us, it looks like your teacher was right – at least in Bunsen burners, the tip is the flame’s hottest region. Maybe teachers do know a thing or two after all, contrary to what our high school selves often thought. A quick disclaimer: The flame we simulated for this experiment was small. A regular flame from a Bunsen burner would be larger, but the same principles would likely apply.

LOGEresearch is available through the Altair Partner Alliance (APA) program, designed to increase your return on investment from your existing units pool. Users benefit from unmatched flexibility and access to 65+ additional products, resulting in maximum software utilization and productivity.