A MATHEMATICAL-MODEL OF SOME ASPECTS OF JET NEBULIZER PERFORMANCE

A simple mathematical model describing the performance of an Acorn jet nebuliser system (Medic-Aid Ltd) has been developed in which nebuliser efficiency E, defined as the volume fraction of solution (water) released as an aerosol or lost by evaporation, and nebulisation time T, are given as functions of the the initial volume of solution. The model identifies an initial phase during which the nebuliser output is at a constant, continuous rate of 1.007 ml s-1. This is followed by an intermittent phase of operation during which output is estimated to occur for 0.25 of the total duration of the phase and results from solution deposited on the walls being recycled. The model indicates that increasing the flow of solution to the nebulisation region in the nebuliser or decreasing the fraction of aerosol intercepted by the baffle, will decrease T whilst leaving E unaffected. Two residence times tau-1, tau-2 which are a measure of the time that solution droplets adhere to the inner walls of the nebuliser are also identified; tau-1 (= 1.6 s) is the residence time associated with the rapid recycling of the majority (a fraction eta-1 = 0.992) of the aerosol while tau-2 (= 200 s) the residence time associated with the remaining fraction 1 - eta-1. Increasing eta-1 will increase E for constant tau-1, tau-2 while increasing tau-1, tau-2 will decrease E, T remaining essentially unchanged. It is proposed that the model may be applicable to other jet nebulisers.