Dynamic Thermal Performance of a Plate Heat Exchanger Under Viscosity–Velocity Combined: Implications for Seawater-Source Heat Pump Systems

ABSTRACT: To address the difficulty of predicting plate heat exchanger performance under variable-flow and fouling-prone coastal conditions, this study developed a novel combined framework for a BR50 plate heat exchanger by integrating a steady-state heat transfer model with a transfer-function-based dynamic wall-temperature model. The main innovation is that the framework simultaneously captures steady thermal performance and transient wall-temperature response, while explicitly quantifying the coupled effects of flow velocity and kinematic viscosity. The model was evaluated for sewage-side velocities of 0.8–1.5 m/s and viscosities up to ten times that of clean water. Results show that wall temperature increases slightly with velocity and can be described by a fourth-order polynomial. Its transient response follows first-order inertia, and the time constant decreases as velocity increases, indicating faster thermal response at higher flow rates. Both the sewage-side heat transfer coefficient and the overall heat transfer coefficient increase with velocity but decrease with viscosity; increasing velocity from 0.8 to 1.5 m/s raises the sewage-side coefficient by 49.2%. Sensitivity analysis identifies kinematic viscosity as the dominant factor affecting thermal performance, followed by flow velocity and wall temperature. The framework provides a practical basis for seawater-source heat pumps and coastal heat recovery systems under fouling-influenced conditions.