A combustor can include: a combustion chamber; a cap covering the combustion chamber; a first swirler inlet cup passing through the cap and having a first length and a first bell mouth; a second swirler inlet cup passing through the cap and having a second length and a second bell mouth; and a third swirler inlet cup passing through the cap and having a third length and a third bell mouth, wherein the first length is different from the third length, and wherein the first bell mouth is different from the third bell mouth. The first, second, and third swirler inlet cups can have different diameters, respectively.