Brazed-plate heat exchangers (BPHEs) are widely used as recuperators in organic Rankine cycle (ORC) systems, but their corrugated passages make full-scale Computational Fluid Dynamics (CFD) simulations prohibitively expensive, limiting the use of CFD for performance prediction and for deriving design-oriented heat-transfer correlations. This work presents a validated domain-reduced CFD methodology for a BPHE recuperator operating with R134a. The computational domain is reduced to a single exchange unit (one vapor and one liquid corrugated channel separated by one heat-transfer plate), further downscaled by truncating the plate length to the fully developed corrugated region, exploiting longitudinal symmetry, and approximating the corrugation profile with a trapezoidal shape; the resulting mesh (≈4.5 × 10⁶ cells) is approximately 44.5 times smaller than the estimated 2 × 10⁸-cell full-stack mesh. Five steady operating points from a micro-scale ORC test rig (mass-flow rate 0.08–0.22 kg/s, Re ≈ 500–18,500) are used for validation by mapping experimental data to the reduced domain through an ε–NTU formulation. CFD predictions of hot- and cold-side temperature changes agree within 1–10% of the reference values. Building on this validated model, single-phase Nusselt-number correlations are proposed, valid over 500 < Re < 1400 for the liquid phase and 6000 < Re < 18,500 for the vapor phase. The proposed method and results support preliminary, geometry- and model-specific assessment of BPHE recuperators; broader transferability requires additional geometries, operating points and model-form validation.