Influence of the liquid viscosity on air core size in the pressure swirl atomizer was investigated. And theoretical analysis model was established in a control volume with the conservation law of angular momentum in the particles system. Eventually

the semi-theoretical prediction formula was derived

and used to calculate air core size and spray cone angle. In addition

validity of the prediction formula was verified by experimental data under different operation conditions. Results show that the present prediction formula can availably and accurately calculate air core size and averaged spray cone angle

exhibiting a prediction uncertainty of ±13% for air core diameter calculation and from －20% to 6% for spray cone angle determination.