Experimental investigation of the optimum operating conditions of a two-phase closed thermosiphon using water at sub-atmospheric pressure

Abstract

Two-phase closed thermosiphon (TPCT) heat exchangers have been widely investigated due to their high efficiency in heat transfer and heat recovery applications. In this study, an experimental investigation was conducted to assess the feasibility of using water as a working fluid at a boiling temperature compatible with the operating temperature of central processing units (CPUs) in servers (55 °C recommended for long-term operation), while maintaining the cooling fluid temperature as high as possible for heat recovery. Four cooling temperatures (5 °C, 15 °C, 25 °C, and 30 °C) and six filling ratios (FRs) ($20\text{\%}\le \mathrm{FR}\le 150\text{\%}$) were tested under sub-atmospheric pressure. The heat input ranged from 40.9 W to 201.9 W. A copper tube (500 mm length, 20 mm inner diameter, 1 mm thickness) was used. Temperature distribution and vapor pressure were measured to determine evaporator and condenser heat transfer coefficients (HTCs) and thermal resistance. Stable and unstable regimes were identified depending on FR and heat input. The best operating conditions of the system were achieved at moderate to high FRs (80%–100%) and coolant temperatures of 25–30 °C, with evaporator temperatures below 40 °C at 201.9 W. Low FRs led to intermittent boiling and large fluctuations, while excessively high FRs slightly reduced performance coming from the increased hydrostatic pressure. In the stable regime, no superheating was observed, and the liquid temperature at the evaporator bottom matched the saturation temperature. Water demonstrates high heat transfer under conditions that promote continuous vapor generation and stable two-phase flow, with a heat transfer rate of 201.9 $W$ for an average $\Delta T\approx 6°C$ (temperature difference between the evaporator and the condenser), supporting its potential application in sustainable thermal management for data centers.