Design and Optimization of a Compact U‐Shaped Internal‐Condenser Heat‐Pipe Heat Exchanger for High‐Ventilation Heating, Ventilation, and Air Conditioning in Hot–Dry Climates

ABSTRACT High outdoor‐air (OA) ventilation improves infection control but can sharply increase HVAC energy use in hot–dry climates. This study designs and optimizes a sealed, U‐shaped internal‐condenser heat‐pipe heat exchanger (HPHE) that thermally couples exhaust and supply ducts without air mixing. A hybrid porous/ ε –NTU screen is used to identify a practical operating band of 2.0–2.5 m·s⁻¹ (Δ T ≈ 20 K) for penalty‐aware operation. Transient two‐phase VOF simulations map loop stability versus condenser length and working‐fluid fill ratio, rejecting 300 mm legs due to flooding and selecting a 150 mm U‐leg at FR ≈ 50% in a laminar, capillary‐dominated regime. Conjugate CFD at the selected operating conditions predicts a ≈ 15.6 K reduction in hot‐duct outlet temperature and an ≈8.4 K increase in supply‐duct outlet temperature, with only watts–tens of watts of added fan power; the loop pumping requirement is O (10⁻² W). Verification and validation follow AHRI Guideline V conventions, including three‐level mesh/time refinement (GCI₉₅% < 5%), two‐phase energy‐balance checks, and cross‐study comparisons ( ε–v and ) against recent HPHE literature (2021–2024). At the nominal point, the recovered sensible heat is = 1.621 kW with ±1 σ = 0.064 kW (4.0%) and a 95% confidence interval of 1.497, 1.748 kW (±7.8%). The results establish a compact, stable HPHE module that delivers strong, net positive heat recovery for high‐ventilation, infection‐control HVAC operation in extreme hot–dry climates.