Describe the challenges in predicting heat transfer in two-phase flow and approaches used.

Challenges: flow regime transitions (bubbly, slug, annular, mist) dramatically change heat transfer mechanisms; non-equilibrium effects (vapor superheat, subcooled boiling); interfacial phenomena (bubble dynamics, entrainment); and strong coupling between hydrodynamics and heat transfer. Approaches: Homogeneous model (simplest, assumes phases at same velocity, uses mixture properties); Separated flow models (Lockhart-Martinelli for pressure drop, Chen correlation for boiling combining nucleate and convective contributions); Drift-flux models (relative velocity via drift velocity); Two-fluid models (separate conservation equations for each phase, closure via interfacial transfer terms). For heat exchangers with phase change: zone analysis dividing into subcooled, two-phase, superheated regions. CFD approaches: VOF/Level-set for resolved interfaces, Eulerian-Eulerian for dispersed flows. Correlations remain industry standard due to modeling complexity; Shah, Kandlikar correlations widely validated for in-tube flow boiling.