A combustor assembly includes a volute wall extended annularly around a combustor centerline, extended at least partially as a spiral curve from a circumferential reference line around the combustor centerline, and defining a combustion chamber therewithin, an annular inner wall extended at least partially along a lengthwise direction from the volute wall, an annular outer wall extended at least partially along the lengthwise direction from the volute wall, the annular inner wall and the annular outer wall being separated along a radial direction from the combustor centerline, and a primary flow passage being defined between the annular inner wall and the annular outer wall in fluid communication with the combustion chamber, and a flow passage wall extending between a portion of the volute wall and a portion of the annular outer wall.