Experimental Study of Steady-State Film Boiling Heat Transfer of Subcooled Water Flowing Upwards in a Vertical Tube Mohamed Mosaad* Research Assistant Klaus Johannsen Professor Institut far Energietechnik, Technische UniversitiitBerlin, Federal Republic of Germany IIA series of steady-state, post-DNB experiments have been performed on a vertically mounted Inconel 600 tubular test section of 0.28 m length (9 mm bore, 12 mm O.D.) with forced convective upflow of water at atmospheric pressure to investigate heat transfer characteristics at subcooled inverted annu- lar flow film boiling (IAFFB) conditions. Using directly heated hot patches, empirical results have been obtained for mass flux ranging from 50 to 500 kg/ (m 2 s) and inlet subcooling to 0-30"C. The accuracy of the heat transfer coefficient evaluated from the measure- merits was estimated at about +_ 7 %, except for the first few diameters from the point of initiation of film boiling. The experimental results have been correlated using a previously developed extended Bromley model that properly incorporates the effects of all relevant independent parameters on IAFFB heat transfer. The heat transfer coefficients predicted by this model are in good agreement with the experimental measure- merits, with an rms error of 12.2% and a mean deviation of 10.2%. This ac- curacy is significantly higher than that of other previously suggested heat transfer correlations. Keywords: film boiling, annular flow, subcooled boiling, heat transfer coefficient, water INTRODUCTION Background Current interest in subcooled and low-quality fdm boiling is primarily directed toward an improved understanding and prediction of coolant behavior during the reflooding phase of postulated loss-of-coolant accidents of light-water-cooled nu- clear reactors. During bottom reflooding, inverted annular flow film boiling (IAFFB) may be encountered downstream of the quench front when the liquid is subcooled and the void fraction is below about 50%. Presently available correlations as well as analytic models for IAFFB cannot be applied with sufficient confidence to predict heat transfer rates, especially at low flows and pressures [1]. This can be attributed primarily to the lack of a reliable empirical database for the overall heat transfer and, moreover, the complete absence of any data for interfacial transfer of heat, mass, and momentum. Correct modeling of the various heat transfer processes at the vapor- liquid interface has been identified as the controlling factor in developing heat transfer correlations as well as analytic * Currently Assistant Professor, Mansoura University, Mansoura, Egypt. calculational procedures that appropriately account for the effects of mass flux, inlet subcooling, wall temperature, and pressure [2]. However, no established (constitutive) relations for the individual interfacial heat flux components are cur- rently available [3]. Thus, in the last two or three years, the development of new or modified physical models of IAFFB heat transfer related to bottom flooding conditions seems to have somewhat leveled off considering the rather limited number of related publica- tions [2, 4-11]. On the other hand, there seems to be some hope that the large gaps in the database, as recently noted by Donevski et al [3], will be filled in the foreseeable future by the steadily increasing amount of experimental investigations that are carried out all over the world using steady-state tech- niques-that is, different versions of the so-called hot patch technique--to arrest the quench front, which was pioneered by Groeneveld and Gardiner in 1978 [12]. Previous Steady-State Experiments Most researchers have applied indirectly heated hot patches that usually consist of hollow copper cylinders, brazed on both ends of the test tube, that contain cartridge heaters. In the case of water as the test fluid, data at high flow and high pressure Address correspondence to Professor Klaus Johannsen, Institut fiir Energietechnik, Technische Universit~it Berlin, Marchstr. 18, D-1000 Berlin 10, Federal Republic of Germany. Experimental Thermaland Fluid Science 1989; 2:477-493 01989 by Elsevier Science Publishing Co., Inc., 655 Avenue of the Americas, New York, NY 10010 0894-1777/89/$3.50 477