Why Does Blowing on a Small Flame Put It Out But on a Big Flame Makes It Bigger?

Introduction to Combustion and Its Types

Understanding the behavior of flames when blown upon requires delving into the types of combustion and the mechanisms involved. Combustion is a chemical process that involves the rapid oxidation of a fuel with the release of heat and light. Depending on the type of combustion, the response to blowing can be quite different. This article explores why a small flame often extinguishes when blown on, while a larger flame can become more intense or even be extinguished using a strategic method.

TNT Detonation: Unaffected by Air Flow

TNT Detonation: TNT (Trinitrotoluene) detonation is a rapid and explosive process. It travels at an astounding speed of 7 kilometers per second. When you blow on a TNT-based flame, the speed of the air flow is far too slow to impact the detonation process effectively. The initial heat from the explosion is so rapid and concentrated that any attempt to blow it out is futile. In fact, the presence of oxygen from the atmosphere can help sustain and increase the combustion rate of TNT, rather than extinguish it. Explosion of TNT does not require atmospheric oxygen; it is an exothermic reaction that produces heat and brisance without a need for external oxygen.

Charcoal Burning: The Heat Capacity and Speed of Energy Transference

Charcoal Burning: Charcoal burning is a different affair. It has a significant heat capacity of around 7500 kcal/kg. Assuming 1 kg of charcoal, the required energy to extinguish it in one go is an astonishing 31 Megajoules. Even with rapid airflow, the energy transfer rate is insufficient to remove enough heat within a short period (e.g., 0.1 seconds). At 310 megawatts, the required airflow is so high that the friction from the air would reheat the charcoal. Internal temperatures persist, ensuring the flame can't be extinguished swiftly, if at all, with just a rapid breeze.

Candle Flames: A Common Example of Oxidation

Candle Flames: A candle flame is a more relatable example. It involves the gaseous paraffin from the wick combining with oxygen, producing water and heat. This is an oxidation process where a specific minimum temperature is required for the process to sustain itself. Blowing on a candle can either cool it down enough to disrupt the self-sustaining process or displace the air around it, hindering the necessary chemical reaction. Conversely, blowing gently can actually enhance the oxygen flow, leading to a more vigorous flame rather than extinguishing it.

Bigger Flames and Large-Scale Combustion

Bigger Flames and Large-Scale Combustion: When dealing with larger flames, the principles remain similar. However, to extinguish them, the scale of the effort required must also be larger. Large-scale fires, such as those at oil wells, are sometimes extinguished by detonating a large explosion near the flames. This rapid combustion can effectively blow out the flames, disrupting their self-sustaining process.

Blowing to Make a Flame Bigger

Blowing to Make a Flame Bigger: For a flame to grow larger when blown upon, the oxygen influx must be high enough to accelerate the oxidation process without disrupting the entire self-sustaining cycle. This is why a gentle breeze can actually intensify a candle flame, while a powerful blast can extinguish it.

Understanding these fundamental principles can help demystify the behavior of flames when exposed to air flow. By appreciating the differences in combustion types, we can better control and predict flame behavior, enhancing both safety and practical combustion applications.