Combustor heat shield with carbon avoidance feature

A gas turbine engine combustor comprising:

• a combustor shell defining a combustion chamber;
• a heat shield mounted to an inner surface of the combustor shell, the heat shield having a back face facing the inner surface of the combustor shell and being spaced therefrom to define an air gap, the heat shield having localized areas on an opposed front face thereof where carbon residues tend to build-up during engine operation;
• and
• an airflow path extending along a generally straight line across the combustor shell, the air gap and the heat shield at each of said localized areas;
• each airflow path comprising an impingement hole defined in the combustor shell and disposed directly on top of a group of carbon purge air holes defined in the heat shield so that an axial projection of the impingement hole onto the back face of the heat shield captures the group of carbon purge air holes, wherein the axial projection is taken along a longitudinal axis of the combustor, the carbon purge air holes having respective axes extending in a direction generally parallel to an axis of the impingement hole and generally normal to the front face of the heat shield, the impingement holes and the carbon purge air holes being sized to preserve enough air momentum at the exit of the purge air holes to deflect fuel droplets away from the front face of the heat shield to impede carbon build-up on said front face,
• wherein rails extend from the back face of the heat shield in sealing engagement with the inner surface of the combustor shell to seal the air gap, and wherein effusion film cooling holes are defined in the heat shield, the cross-sectional area of the carbon purge air holes being greater than that of the of the effusion film cooling holes.