How do you determine when mixed convection (combined natural and forced) must be considered?

Mixed convection occurs when both buoyancy and forced flow significantly affect heat transfer. The criterion uses Richardson number: Ri = Gr/Re² (or Gr/Re^2.5 for turbulent flow). When Ri << 1, forced convection dominates; Ri >> 1, natural convection dominates; Ri ≈ 1, mixed convection must be analyzed. For assisting flow (buoyancy aids forced flow): Nu^n ≈ Nu_forced^n + Nu_natural^n (n≈3 for laminar). For opposing flow: effects partially cancel. Mixed convection is common in vertical tubes at low flow rates, electronic equipment with fans, and building ventilation. Accurate analysis often requires CFD due to complex interactions.