Effects of dissolved air on subcooled and saturated flow boiling of water in a small diameter tube at low pressure Andrea Cioncolini * ,1 , Lorenzo Santini 2 , Marco E. Ricotti 3 Department of Nuclear Engineering, Politecnico di Milano, via Ponzio 34/3, 20133 Milano, Italy Received 8 November 2006; received in revised form 24 January 2007; accepted 25 January 2007 Abstract Experimental results on flow boiling of air saturated water flowing through a tube of inner diameter d i = 4.03 mm are presented. Both subcooled and saturated flow boiling are investigated, covering pressures from 177 to 519 kPa, mass fluxes from 478 to 839 kg m 2 s 1 and heat fluxes from 210 to 736 kW m 2 . By comparing the measured results with existing prediction methods derived for degassed liq- uids the study concludes that the presence of dissolved air in the testing fluid can be regarded as a second order effect, and no special attention to dissolved air appears required. Ó 2007 Elsevier Inc. All rights reserved. Keywords: Two-phase flow; Flow boiling; Heat transfer; Dissolved gas; Noncondensable; Enhancement 1. Introduction Virtually all liquids contain some dissolved gas [1,2], depending on the solubility of the gas in the liquid and on exposure history. Since the presence of dissolved gases may affect the thermal-hydraulics of boiling systems, the subject has been investigated quite extensively. Dissolved gas effects on pool boiling, in particular, received considerable attention [3–12]. Prior to nucleate boiling inception, when the heater temperature is below the liquid saturation temperature and a gas free liquid would be single-phase, with gas saturated liquids a region of degassing may be observed. If the solubility of the gas decreases with temperature, as frequently happens, during the heating process the local solubility may be reduced below that actually present, thus causing the gradual release of the dissolved gas. Bubbles composed mostly of gas appear at the heater surface, giving rise to a two com- ponents, two-phase flow. The resulting apparent boiling enhances the heat transfer process, due to the disruption of the thermal boundary layer during bubble growth and sliding on the heater surface and due to the stirring action induced by departed bubbles. Due to the presence of dissolved gas, nucleate boiling inception may be antici- pated to lower wall superheats than those required to nucleate the corresponding gas free liquid. The combined presence of gas and vapor within the embryonic bubbles in the cavities on the heater surface reduces the vapor pres- sure and hence the wall superheat required to activate the cavity. The number of active nucleation sites is higher in a gassed liquid and gas microbubbles streaming in the flow can as well act as nucleation sites, should they come close to the heater surface, so that subcooled boiling heat trans- fer is enhanced with respect to gas free subcooled boiling. Besides, due to the increased active nucleation sites density, the hysteresis of nucleate boiling inception that is typically observed with highly wetting liquids is smoothed and 0894-1777/$ - see front matter Ó 2007 Elsevier Inc. All rights reserved. doi:10.1016/j.expthermflusci.2007.01.007 * Corresponding author. Present address: Westinghouse Electric Com- pany, Science and Technology Department, 1344 Beulah Road, Pitts- burgh, PA 15235, USA. Tel.: +39 02 2399 6327; fax: +39 02 2399 6309. E-mail addresses: andrea.cioncolini@polimi.it, cioncoa@westinghou- se.com (A. Cioncolini), lorenzo.santini@polimi.it (L. Santini), marco. ricotti@polimi.it (M.E. Ricotti). 1 Tel.: +1 412 256 1656; fax: +1 412 256 2444. 2 Tel.: +39 02 2399 6327; fax: +39 02 2399 6309. 3 Tel.: +39 02 2399 6325; fax: +39 02 2399 6309. www.elsevier.com/locate/etfs Experimental Thermal and Fluid Science 32 (2007) 38–51