A combustor assembly having a fuel pre-mixer comprising: a duct for mixing an airflow and a fuel therein; a center body coaxially aligned within the duct for receiving the fuel from a fuel source and configured to distribute the fuel to at least one axial location within the duct; a planar vane section in communication with the airflow and the fuel source to provide a first injection of fuel and a flow conditioning effect on the airflow, the planar vane section comprising a plurality of relatively planar vanes circumferentially spaced from each other, each planar vane having a leading edge axially spaced from a trailing edge, wherein the leading edge and the trailing edge of each planar vane are axially aligned, wherein the plurality of planar vanes is circumferentially spaced around the centerbody, each of the plurality of relatively planar vanes having a planar portion disposed in a longitudinal direction of the duct to straighten the airflow; and a swirler vane section disposed downstream of the planar vane section, the swirler vane section comprising a plurality of circumferentially spaced swirler vanes, each of the plurality of swirler vanes having a leading edge positioned downstream from corresponding trailing edges of two circumferentially adjacent planar vanes of the plurality of planar vanes, wherein the leading edge of each swirler vane is positioned circumferentially offset from an in-line plane of the plurality of planar vanes, thereby forming a staggered formation between the planar vane section and the swirler vane section, characterized in that the swirler vane section is configured to provide a second injection of fuel and a mixing of the fuel and the airflow. a duct for mixing an airflow and a fuel therein; a center body coaxially aligned within the duct for receiving the fuel from a fuel source and configured to distribute the fuel to at least one axial location within the duct; a planar vane section in communication with the airflow and the fuel source to provide a first injection of fuel and a flow conditioning effect on the airflow, the planar vane section comprising a plurality of relatively planar vanes circumferentially spaced from each other, each planar vane having a leading edge axially spaced from a trailing edge, wherein the leading edge and the trailing edge of each planar vane are axially aligned, wherein the plurality of planar vanes is circumferentially spaced around the centerbody, each of the plurality of relatively planar vanes having a planar portion disposed in a longitudinal direction of the duct to straighten the airflow; and a swirler vane section disposed downstream of the planar vane section, the swirler vane section comprising a plurality of circumferentially spaced swirler vanes, each of the plurality of swirler vanes having a leading edge positioned downstream from corresponding trailing edges of two circumferentially adjacent planar vanes of the plurality of planar vanes, wherein the leading edge of each swirler vane is positioned circumferentially offset from an in-line plane of the plurality of planar vanes, thereby forming a staggered formation between the planar vane section and the swirler vane section, characterized in that the swirler vane section is configured to provide a second injection of fuel and a mixing of the fuel and the airflow.