Why will a fuel not burn if it is above the Upper Explosive Limit (UEL)?

Why will a fuel not burn if it is above the Upper Explosive Limit (UEL)?

• A. There is too much fuel compared to the amount of oxygen.
• B. There is too much oxygen compared to the amount of fuel.
• C. Vaporization is occurring below the rate required for ignition.
• D. Combustion requires an outside ignition source

Answer: (A)

The main idea is the flammable range of a fuel–air mixture. A flame will only sustain if the mixture sits between the lower and upper explosive limits. When the mix is above the upper limit, there is too much fuel relative to oxygen. In other words, the environment becomes fuel-rich and there aren’t enough oxygen molecules to oxidize all the fuel present. Without enough oxygen to support the oxidation reaction, the flame cannot propagate, so the mixture won’t burn even with an ignition source. As for other ideas, a lean (oxygen-rich) mixture can still burn as long as it’s within the flammable range, so having too much oxygen isn’t the limiting factor here. The rate of vaporization isn’t what determines ignition in this context—the crucial factor is the fuel-to-oxygen ratio. And while an ignition source is needed to start combustion, the question is about why the mixture can’t burn once it’s above the upper limit, which relates to being too fuel-rich rather than the presence or absence of an ignition source.

The main idea is the flammable range of a fuel–air mixture. A flame will only sustain if the mixture sits between the lower and upper explosive limits. When the mix is above the upper limit, there is too much fuel relative to oxygen. In other words, the environment becomes fuel-rich and there aren’t enough oxygen molecules to oxidize all the fuel present. Without enough oxygen to support the oxidation reaction, the flame cannot propagate, so the mixture won’t burn even with an ignition source.

As for other ideas, a lean (oxygen-rich) mixture can still burn as long as it’s within the flammable range, so having too much oxygen isn’t the limiting factor here. The rate of vaporization isn’t what determines ignition in this context—the crucial factor is the fuel-to-oxygen ratio. And while an ignition source is needed to start combustion, the question is about why the mixture can’t burn once it’s above the upper limit, which relates to being too fuel-rich rather than the presence or absence of an ignition source.