Closed-loop pulsating heat pipes (CLPHPs) have emerged as versatile, passive two-phase thermal management devices for electric-vehicle batteries, electronics cooling, and aerospace systems. Traditional performance assessments rely on average thermal resistance (TR) over the entire operating range, obscuring dynamic transitions among start-up, effective pulsation, and dry-out regimes. The objectives of this work are to identify and quantify ineffective and effective heat-transfer regimes in a vertical CLPHP charged with deionized water at 60% filling ratio, and to determine critical thermal thresholds for evaporator temperature (T e ), condenser temperature (T c ) and internal pressure. To achieve these objectives, experiments were conducted at heat inputs of 80 W, 100 W, and 120 W. An in-house Python library, PyPulseHeatPipe , was used to fit smooth polynomial regressions by optimal degree to high-resolution TR data and compute its first, second and third derivatives with respect to T e . Four characteristic points (A–D) were extracted from derivative extrema to delineate three operational regimes: A → B (ineffective heat transfer), B → C (effective slug–plug pulsation) and C → D (transition toward dry-out). The novelty of this study lies in applying derivative-informed critical thermal analysis to explicitly identify regime boundaries and corresponding TR ranges—0.29–0.59 KW −1 at 80 W, 0.22–0.45 KW −1 at 100 W and 0.20–0.39 KW −1 at 120 W—in place of conventional single-value metrics. Results demonstrate that the most vigorous oscillations and highest heat-removal rates occur between points B and C, while rapid T c rises beyond point C signal imminent dry-out. This methodology furnishes thermal engineers with a powerful tool to optimize CLPHP design parameters and enhance reliability and safeguard against thermal instability in real-world applications.
Parmar et al. (Sat,) studied this question.