Amongst incidental scenarios, natural gas releases have become particularly relevant within the process industry environment. With an overall composition of 94.7% of methane, it is important to consider how this gas can cause incremental damage when it is released. Methane gas is a non-toxic, flammable gas, that, at higher concentrations, can behave as an asphyxiant due to oxygen displacement from the lungs. The main concern with methane is related to its flammability and the potential damage that could be dealt to its surroundings if the high pressure jet were to ignite into a jet fire.
Three parameters that must be considered when dealing with flammable gasses are their flash point (FP), upper flammability limit (UFL) and lower flammability limit (LFL), as they are set values for any compound at a specific pressure and temperature. If we consider the fire triangle model, to induce a combustion reaction three components are needed: a fuel, an oxidizing agent and heat. 
When release happens in an ambient filled with air, the oxidizing agent will be oxygen (air has a constant concentration of 21% in standard conditions). At an almost pure concentration, a few centimeters from the exit plane, the concentration of natural gas is too high and oxygen too low to generate any kind of combustion reaction, but as the high pressure jet develops, the concentration of its components will dilute as air entrainment increases, allowing an enrichment of oxygen within the jet. Assuming a constant concentration for oxygen, the jet must dilute enough to enter within its flammability range; below its UFL. Within this range, a flammable mixture can be made and any source of heat can jump-start the reaction.
To properly judge the damage and potential risk that the jet fire can generate, several studies regarding the maximum distance that the cloud generated by the jet can reach have been made. As dilution of the jet continues due to air entrainment in the farfield, going below its UFL, the maximum distance that the flammable mixture can reach is at the point in which the concentration of the cloud is equals to the LFL of the gas, as it is the lowest concentration allowable that permits the formation of a flammable mixture between air and natural gas at standard conditions (the LFL for natural gas is 4% ).  
Considering a free jet at sub-critical pressure (beyond the nearfield zone), its mean volume fraction axial concentration decay of any gas released in air can be defined as follows: