Numerical simulations using computational
fluid dynamics (CFD) are performed to investigate the
flame length characteristics in rotary kilns using probability
density function (PDF) approach. A commercial CFD
package (ANSYS-Fluent) is employed for this objective. A
2-D axisymmetric model is applied to study the effect of
both operating and geometric parameters of rotary kiln on
the characteristics of the flame length. Three types of gaseous
fuel are used in the present work; methane (CH4), carbon
monoxide (CO) and biogas (50 % CH4 + 50 % CO2).
Preliminary comparison study of 2-D modeling outputs of
free jet flames with available experimental data is carried
out to choose and validate the proper turbulence model for
the present numerical simulations. The results showed that
the excess air number, diameter of kiln air entrance, radiation
modeling consideration and fuel type have remarkable
effects on the flame length characteristics. Numerical correlations
for the rotary kiln flame length are presented in terms of the studied kiln operating and geometric parameters within acceptable error.