Ultra-high critical heat flux (CHF) for subcooled water flow boiling—I: CHF data and parametric effects for small diameter tubes

Abstract

Ultra-high critical heat flux (CHF) data, with many values exceeding 100 MW m −2, were obtained using high mass velocity, subcooled water flow through short, small diameter tubes. These tests produced the highest CHF of 276 MW m −2 reported in the literature for a uniformly heated tube which surpassed the prior record of 228 MW m −2. The data include broad ranges of tube diameter (0.406–2.54 mm) , heated length-to-diameter ratio (2.4–34.1) , mass velocity (5000–134 000 kg m −2 s −1) , inlet temperature (18–70°C) , and outlet pressure (2.5–172.4 bars) . The parametric trends of CHF were ascertained relative to all important flow and geometrical parameters. CHF increased with increasing mass velocity, increasing subcooling, decreasing tube diameter, and decreasing heated length-to-diameter ratio. For a constant inlet temperature, CHF increased with increasing pressure for pressures up to 30 bars, remained fairly constant between 30 and 150 bars, and decreased afterwards as the critical pressure was approached. CHF was accompanied by physical burnout of the tube wall near the exit and tube material had little effect on the magnitude of CHF. The pressure drop for most conditions was fairly constant, albeit as high as 153.4 bars, over the entire range of heat fluxes, from the single-phase flow condition corresponding to zero heat flux up to CHF, proving that CHF was triggered even with negligible net vapor production. These high pressure drops indicate special attention should be exercised when employing the high mass velocity flows necessary to attaining ultra-high CHF. These high pressure drops also render the practice of referencing CHF data reSlative to a single measured pressure value very misleading.