Energy-driven performance characteristics of two-phase natural circulation loops with steam drum for sustainable thermal systems

The rapid escalation of heat flux in modern data centres, AI accelerators, and power electronics has made thermal management a dominant contributor to overall energy consumption, with auxiliary pumping power and flow-control hardware increasingly limiting system-level efficiency. Pump-free two-phase natural circulation loops (TPNCLs) offer a compelling alternative by exploiting buoyancy-driven flow to dissipate large heat loads passively. However, the design and operating limits of mini-scale two-phase NCLs with steam separation, critical for compact, high-power applications, remain insufficiently understood. In this work, a comprehensive nonlinear homogeneous equilibrium model is developed for a mini two-phase natural circulation loop incorporating a steam drum, enabling systematic quantification of circulation strength and steam production under realistic operating conditions. Extensive parametric analysis is performed over heat input (5–45 kW), operating pressure (0.5–38 bar), inlet subcooling (4–20 K), and key geometric ratios, revealing distinct and decoupled sensitivities of mass flow rate and steam yields. The results reveal that mass flow rate exhibits strong nonlinear dependence on buoyancy friction competition and is highly sensitive to subcooling and geometric parameters, whereas the steam generation rate is predominantly energy-limited and governed mainly by heat input and pressure, showing negligible sensitivity to subcooling and geometric redistribution. The present study provides new physical insights into the coupled role of steam separation and two-phase flow dynamics and offers a framework for the design and optimization of compact passive natural circulation systems.