A combustor for use in a gas turbine engine, the combustor enclosing a combustion chamber having a combustion area, the combustor comprising: a heat shield panel having a first surface and a second surface opposite the first surface, the first surface being oriented towards the combustion area; a combustion liner having an inner surface oriented towards the second surface of the heat shield panel, an outer surface opposite the inner surface of the combustion liner, and a primary aperture extending from the outer surface of the combustion liner to the inner surface of the combustion liner through the combustion liner, wherein the heat shield panel and the combustion liner are in a facing spaced relationship wherein the second surface of the heat shield panel and the inner surface of the combustion liner define an impingement cavity therebetween in fluid communication with the primary aperture for cooling the second surface of the heat shield panel, wherein the heat shield panel is removably mounted to the combustion liner by one or more attachment mechanisms; wherein the combustion liner has a variable thickness along a first length of the combustion liner, characterised in that the impingement cavity includes an exit defined between the second surface of the heat shield panel and the inner surface of the combustion liner, and the variable thickness being configured to adjust a distance (D1, D2) between the inner surface of the combustion liner and the second surface of the heat shield panel throughout the first length of the combustion liner such that a Mach number of a cross airflow within the impingement cavity is adjusted to promote movement of particulate through the impingement cavity and towards the exit of the impingement cavity. a heat shield panel having a first surface and a second surface opposite the first surface, the first surface being oriented towards the combustion area; a combustion liner having an inner surface oriented towards the second surface of the heat shield panel, an outer surface opposite the inner surface of the combustion liner, and a primary aperture extending from the outer surface of the combustion liner to the inner surface of the combustion liner through the combustion liner, wherein the heat shield panel and the combustion liner are in a facing spaced relationship wherein the second surface of the heat shield panel and the inner surface of the combustion liner define an impingement cavity therebetween in fluid communication with the primary aperture for cooling the second surface of the heat shield panel, wherein the heat shield panel is removably mounted to the combustion liner by one or more attachment mechanisms; wherein the combustion liner has a variable thickness along a first length of the combustion liner, characterised in that the impingement cavity includes an exit defined between the second surface of the heat shield panel and the inner surface of the combustion liner, and the variable thickness being configured to adjust a distance (D1, D2) between the inner surface of the combustion liner and the second surface of the heat shield panel throughout the first length of the combustion liner such that a Mach number of a cross airflow within the impingement cavity is adjusted to promote movement of particulate through the impingement cavity and towards the exit of the impingement cavity.