How do you select the appropriate convection correlation for a given heat transfer problem?

Selection depends on: Flow geometry (internal/external, flat plate/cylinder/sphere), Flow regime (laminar/turbulent based on Re), Convection type (forced/natural/mixed based on Gr/Re² ratio), and Boundary conditions (constant wall temperature or constant heat flux). For external flow over flat plate: use Blasius (laminar, Re<5×10⁵) or Dittus-Boelter type. For internal pipe flow: use Sieder-Tate or Gnielinski correlation. For natural convection: use Churchill-Chu correlation. Always check validity range (Re, Pr, L/D limits). For non-standard geometries, CFD or experimental correlations are needed.