Experimental study of pool boiling on tube bundles at 3 to 5 kPa

This paper presents an experimental investigation of pool boiling of water on isolated copper tubes and vertically aligned tandem tube bundles under subatmospheric conditions, with saturation pressures of 3 and 5 kPa. The experimental setup represents compact, staggered tube arrangements used in low-temperature evaporators. All tests were performed on 25.4 mm diameter tubes with a pitch-to-diameter ratio of 1.0 and at two liquid levels (0 cm and 20 cm), to explore the effects of the hydrostatic head and subcooling on boiling heat transfer. For these thermohydraulic and geometrical configurations, boiling curves were obtained under systematically controlled conditions.

High-speed imaging was used to correlate bubble dynamics and vapor structures with heat transfer performance. The results demonstrate the sensitivity of the boiling behavior to system pressure, subcooling, and vertical tube interaction. In particular, on the top tube under base level ($L_h=0$ cm) conditions, phenomena such as vapor blanketing, rupture, and droplet and film evaporation on partially exposed surfaces were observed. The onset of nucleate boiling occurred at wall superheats of approximately 4.5–5 K. The corresponding heat fluxes ranged from 12.7 to 21.2 kW/m${}^2$, depending on tube position, liquid level, and system pressure. In tandem tube configurations, vapor interactions from the bottom tube enhance nucleation and reduce wall superheat on the top tube by approximately 1–2 K, confirming the presence of a bundle effect.

Detailed boiling curve measurements and high-speed visualization for compact staggered tube bundles at saturation pressures as low as 3 kPa are shown in this paper. These findings provide new insights into low-pressure boiling and the performance of compact heat exchangers.