A simple model of gas heating and acceleration in a plasma torch is proposed. With simple conditions, easy to get from the torch operating parameters, this model leads to apprehend the gas behavior interacting with the surrounding atmosphere outside the torch, in steady or unsteady state. To better apprehend the air introduction into the plasma flow, different values of the Schmidt number have been investigated according to a classic k - epsilon turbulence model. Comparisons between velocity, temperature and Nitrogen percentage measurements and calculations were made in steady state.