The lowest concentration of fuel at which an air/fuel mixture will ignite is known as what?

The lowest concentration of fuel at which an air/fuel mixture can ignite is referred to as the Lower Explosive Limit (LEL). This is a critical concept in understanding combustion and safety in various applications, including gas furnace operation. At or below this concentration, the mixture cannot sustain combustion because there is not enough fuel to support a flame. When the concentration of fuel is below the LEL, the mixture is too "lean" to ignite.

In operational contexts, knowing the LEL is essential for evaluating the safety of gas systems, as it helps prevent situations where a flammable mixture could form. It is important to monitor and maintain concentrations above the LEL to ensure that combustion is controlled and efficient within gas appliances.

The other options represent different concepts related to combustion. The Upper Explosive Limit (UEL) marks the concentration at which a mixture becomes too rich to ignite. The flash point refers to the lowest temperature at which a volatile substance can vaporize to form an ignitable mixture in air, while the combustion point is a more general term and may not precisely describe the thresholds necessary for ignition in the context of air/fuel mixtures. Thus, understanding the LEL is vital for ensuring safe operation in gas systems.