A combustor includes an end cap that extends radially across a portion of the combustor and includes an upstream surface axially separated from a downstream surface. A combustion chamber is downstream of the end cap. Premixer tubes extend from a premixer tube inlet proximate to the upstream surface through the downstream surface to provide fluid communication through the end cap and include means for conditioning flow through the plurality of premixer tubes. A method for conditioning flow through a combustor includes flowing a working fluid through a first and second set of premixer tubes that extend axially through an end cap, wherein the second set of premixer tubes includes means for conditioning flow through the second set of premixer tubes, and flowing a fuel through the first or second set of premixer tubes.