Four configurations of frustums (unrifled, inner-rifled, outer-rifled and two-faced rifled) were used to modulate the flow momentum. The rifle configuration transformed the axial flow velocity to radial direction for increasing the turbulence intensity (T.I.). The increased T.I. improved the mixing between the air and fuel flow. Direct photography, Schlieren photography, hot-wire anemometry, thermocouple, and gas analyzer were utilized to measure the flame length, turbulence intensity, flame temperature, heat release, and exhaust-gas concentrations. The Schlieren flame structures show that the lifted flame is dismissed behind the unrifled, inner-rifled, outer-rifled, and two-faced rifled frustums; and therefore, the flame can be further stabilized. Behind the two-faced rifled frustum, the T.I. increased from 0.15% to 4.6% and the non-dimensional flame length (H/D) decreased from 24 to 14 (i.e., a flame shortening of around 42%). The low flame temperature approximately corresponded to the low concentration of nitride oxide (CNO). The high air-fuel mixing (caused by the high T.I.) corresponded to a high total combustion intensity and a low NO. (C) 2015 Elsevier Ltd. All rights reserved.