Airfoil blade having a serpentine cooling circuit and impingement cooling

An airfoil blade comprising:

• (a) a first and a second outer wall together defining an airfoil shape including a leading edge, a trailing edge, a pressure side along said first outer wall, a suction side along said second outer wall, a blade tip, and a blade root;
• (b) a longitudinal axis extending outward toward said blade tip and inward toward said blade root;
• (c) an internal serpentine coolant circuit having series-connected generally longitudinally extending coolant passageways including a downstream passageway bounded by four monolithic inner walls, wherein:
• (1) a first and a second inner wall are spaced apart and extend between, and are monolithic with at least a portion of, said first and second outer walls;
• and
• (2) a third inner wall extends between said first and second inner walls and is spaced apart from, and generally aligned with, one of said first and second outer walls;
• and
• (d) a first impingement chamber in downstream fluid communication with said downstream passageway and bounded by said first, second, and third inner walls and by said one outer wall, wherein said third inner wall includes a multiplicity of first impingement chamber air inlet orifices extending between said downstream passageway and said first impingement chamber.