A combustor assembly is generally provided. The combustor assembly includes a volute wall extended annularly around a combustor centerline. The volute wall is extended at least partially as a spiral curve from a circumferential reference line around the combustor centerline. The volute wall defines a combustion chamber therewithin. An annular inner wall is extended at least partially along a lengthwise direction from the volute wall. An annular outer wall is extended at least partially along the lengthwise direction from the volute wall. The inner wall and the outer wall are each separated along a radial direction from the combustor centerline. A primary flow passage is defined between the inner wall and the outer wall in fluid communication from the combustion chamber.