Journal of Multidisciplinary Engineering Science and Technology (JMEST) ISSN: 3159-0040 Vol. 2 Issue 3, March - 2015 www.jmest.org JMESTN42350546 465 Recognition of Net Vapor Generation in Subcooled Flow Boiling Rouhollah Ahmadi School of New Technologies, Iran University of Science and Technology Narmak, Tehran, Iran, 1684613114 ahmadi@iust.ac.ir Tomio Okawa Department of Mechanical Engineering and Intelligent Systems, The University of Electro- Communications, Tokyo 182-8585, Japan okawa.tomio@uec.ac.ir Abstract— Point of net vapor generation (NVG) or onset of significant void flow (OSV) is investigated in a vertical rectangular channel. It is important point in subcooled flow boiling due to the dramatic increase in the amount of vapor. Experimental evidence shows that the onset of significant void (OSV) generally signals of flow instability in a system with pressure driven boundary condition and also influences on the reactivity of the liquid cooled nuclear reactors. Bubble detachment from nucleation site or heated surface was introduced frequently as a triggering mechanism of net vapor generation. However, bubble detachment was observed before point of NVG, in this work. Experimental results are explored in this work and then simple measurable way is proposed to recognize point of NVG in subcooled flow boiling. In this way, net vapor generation is a point that subcooling temperature is equal to excess wall temperature. This is a point that condensation rate is in balance with vaporization rate in subcooled flow boiling, and afterward vaporization exceeds from condensation, significantly. Keywords—subcooled flow boiling; void fraction; net vapor generation (NVG); onset of significant void (OSV) I. INTRODUCTION In describing the process of subcooled boiling in forced convective flow of liquid in a confined channel it has been noted that vapor in various amounts is generated at the heated surfaces in the channel. In the particular case of the design of liquid cooled nuclear reactors information on the void fraction under subcooled conditions is often required because of its influence upon the reactivity of the system. It is frequently postulated that in the representation of subcooled boiling, at first, for high degrees of subcooling the vapor generated remains as discrete bubbles attached to the surface whilst growing and collapsing; voidage in this region is essentially a wall effect. At somewhat lower subcoolings, bubbles detach from the surface, condensing only slowly as they move through the slightly subcooled liquid; voidage in this region is a bulk fluid effect. This point is known as point of net vapor generation (NVG). The void fraction rises sharply with length from the transition point of NVG. Bowring [1] proposed a model for the estimation of void fraction in this region. In addition, simple empirical methods of calculating the void fraction in the subcooled boiling of water have been suggested by several researchers. A simple expression for the void fraction at point of NVG has been suggested by Levy [2] from a consideration of the forces exerted on a vapor bubble attached to the wall and the temperature distribution in the single-phase liquid away from the heated surface. Bowring [1] applied the following criterion to establish this point. , w sub NVG f q T G     (1) Where ΔT sub,NVG is the subcooling at which bubble detachment occurs, η is an empirical factor derived from experimental data for water and found to depend only on the system pressure. He proposed the relationship for water over the pressure range 11 to 138 bar. Saha and Zuber [3] have proposed a simple method to calculate the point of NVG which can be assumed to be coincident with the point of bubble detachment. At low flow-rates the bubble detachment is assumed to be thermally controlled, occurring at a fixed value of Nusselt number [q w D/k f ΔT sub,NVG ]. At high flow-rates bubble departure is hydrodynamically induced and occurs at a fixed Stanton number [q w /Gc p ΔT sub,NVG ].   , 0 70 .0022 , 000 w sub NVG f qD k I T f Pe    (2)   sub, > 70, 000 153.8 w NVG p If Pe q T Gc   (3) Above correlation is simple to use and a recent critical review of models by Lee et at. [4] has shown that it remains the most accurate. However, many observational investigations show the contradictory with the base of NVG models. Before the point of NVG, at the condition close to onset of nucleate boiling (ONB), bubbles tended to be lifted off the wall and collapsed in subcooled bulk liquid, at atmospheric pressure [5-7, 9]. Whilst, under moderate pressure conditions, it was observed that bubbles mostly slid on the heated surface at the incipient boiling point [18]. In the experiments using FC-87 as a working fluid, bubbles departed from nucleation sites and slid on the surface for a long distance [8]. In this paper, experimental investigation was performed in a vertical upward subcooled flow boiling