Experimental study on pool boiling heat transfer with reentrant cavity structures on tube external surfaces in the low heat flux regime

Abstract

This study aims to enhance the pool boiling heat transfer performance of heat exchange tubes used in industrial heat pump steam generators. Micro-structured surfaces were fabricated on the outer tube wall to improve boiling performance, and experiments were conducted on commercial low-fin and Turbo-E tubes as well as metal 3D-printed tubes that replicate reentrant cavity structures. All experiments were performed under atmospheric pressure using degassed deionized water at a saturated pool temperature of 100 °C. A low heat flux range of 0–70 kW/m², corresponding to ≤10 % of the known critical heat flux (CHF) for copper surfaces in pool boiling, was investigated. First, the pool boiling performance of smooth bare tubes was evaluated for different materials, including copper (Cu), stainless steel (SUS), carbon steel (CS), and carbon steel with removed mill scale (CS_r). Although SUS has a lower thermal conductivity, it initially exhibited better performance than CS and CS_r due to the incomplete removal of mill scale on the carbon steel surface, which degraded boiling performance. When the mill scale was subsequently removed via precise machining prior to boiling tests, the heat transfer performance followed a trend proportional to thermal conductivity, as expected. Subsequently, structured tubes with surface features were tested, including Cu low-fin, Cu Turbo- E, and SUS Turbo-E tubes. Compared to their corresponding bare tubes, the Cu low-fin and Cu Turbo- E tubes exhibited enhancements in the average heat transfer coefficient (HTC) by 1.40× and 3.17×, respectively, demonstrating significant performance improvement. The SUS Turbo-E tube showed a 1.92× increase in average HTC relative to the SUS bare tube; however, its enhancement was less pronounced than that of the Cu Turbo-E tube due to its significantly larger pore diameter, which limited the effectiveness of the reentrant cavity mechanism. Guided by the reentrant geometry of the Turbo-E surface, metal 3D-printed tubes incorporating reentrant cavities were fabricated. Axially oriented reentrant structures were first produced using a copper alloy; however, vapor entrapment within the axial tunnel led to severe thermal resistance, resulting in performance inferior to that of the bare tube. To overcome this limitation, reentrant cavities were instead oriented circumferentially, aligned with the primary bubble departure direction, and fabricated using SUS. All circumferential reentrant tubes outperformed the SUS bare tube. Among them, the optimized geometry (R1 tube) achieved a 4.9× increase in average HTC compared to the smooth 3D-printed reference tube (R0), demonstrating that circumferentially oriented reentrant cavities provide substantial enhancement in pool boiling heat transfer performance.