Effusion cooling represents the state of the art of liner cooling technology for modern combustors. This technique consists of an array of closely spaced discrete film cooling holes and contributes to lower the metal temperature by the combined protective effect of coolant film and heat removal through forced convection inside each hole. Despite many efforts reported in literature to characterize the cooling performance of these devices, detailed analyses of the mixing process between coolant and hot gas are difficult to perform, especially when superposition and density ratio effects as well as the interaction with complex gas side flow field become significant. Furthermore, recent investigations on the acoustic properties of these perforations pointed out the challenge to maintain optimal cooling performance also with orthogonal holes, which showed higher sound absorption. The objective of this paper is to investigate the impact of a realistic flow field on the adiabatic effectiveness performance of effusion cooling liners to verify the findings available in literature, which are mostly based on effusion flat plates with aligned cross flow, in case of swirled hot gas flow. The geometry consists of a tubular combustion chamber, equipped with a double swirler injection system and characterized by 22 rows of cooling holes on the liner. The liner cooling system employs slot cooling as well: its interactions with the cold gas injected through the effusion plate are investigated too. Taking advantage of the rotational periodicity of the effusion geometry and assuming axisymmetric conditions at the combustor inlet, steady state RANS calculations have been performed with the commercial code Ansys® CFX simulating a single circumferential pitch. Obtained results show how the effusion perforation angle deeply affects the flow-field around the corner of the combustor, in particular, with a strong reduction of slot effectiveness in case of 90 deg angle value.