A gas turbine engine includes a combustion section, a turbine section, and a compressor section. The combustion section includes a combustor casing, a combustor, a cooling duct, and an outer duct. The combustor casing defines at least in part a diffuser cavity and a fluid inlet. The combustor disposed is in the diffuser cavity. The cooling duct is in fluid communication with the fluid inlet in the combustor casing and is configured to transport a flow of cooled air. The outer duct surrounds at least a portion of the cooling duct and extends along a portion of an entire length of the cooling duct. The outer duct defines a gap with the cooling duct and is configured to transport a flow of buffer air. The turbine section is disposed downstream from the combustion section. The cooling duct is in fluid communication with the turbine section.