High-Speed Internal Nozzle Flow Visualization of Flashing Jets

Flashing or thermodynamic breakup of a liquid jets occurs when a pressurized, subcooled or saturated liquid is released to a lower pressure, resulting in violent vapor nucleation, expansion, and breakup of the liquid phase. Flashing is known to produce very fine droplet atomization, often not possible by other means. Despite its usefulness as an atomization method, the fundamental processes involved in flashing remain poorly understood. This has limited its applicability due to a lack of control of spray characteristics. In a previous study, several new flashing modes emanating from long tube nozzles were discovered through high-speed imaging and depended on the level of superheat. Breakup mode and frequency appeared to be highly dependent on the state of two-phase internal flow within the nozzle. In this study, a review of the state of knowledge of flashing sprays is presented along with motivation for continuing research. Ongoing work is also described in which internal flashing phenomena were observed using transparent glass tube nozzles. Water was used as the working fluid and was preheated and pressurized within a sample cylinder prior to release to the atmosphere. These nozzles allowed for imaging of the developing internal two-phase flow with a high-speed video camera set at 10000 fps. Internal flow phenomena were then related to observed external jet breakup. Results revealed the bubble nucleation, migration, and coalescing processes occurring during flashing. Bubble nucleation near the nozzle wall could be observed near the tube entrance. At higher superheats, an annular flow pattern develops near the nozzle exit and corroborates previous conjectures to the internal flow pattern during flare flashing. Introduction Flashing occurs when a pressurized supercritical, subcooled or saturated fluid is released to a lower pressure, resulting in expansion, violent vapor nucleation, and break up of the liquid phase due to thermodynamic instability. Flashing of liquid jets has been studied since the early 1960’s [1]. Early works were primarily qualitative visualization studies documenting the phenomenology of the flashing process [2-4]. Later, empirical and semi-theoretical correlations were developed to predict spray properties based on initial conditions, though applicable conditions for these relations were limited [5]. Modeling work of jet breakup and droplet dispersion has also been performed for limited situations [6-8]. Recently, due to advances in spray diagnostics, some quantitative spray characteristic measurements have been performed [9-11] though currently a lack of comprehensive measurements exists and more are needed to facilitate modeling. Interest in thermodynamic atomization persists due to applications in a variety of areas. Because of the low temperatures possible from a flashing jet, it is being actively studied in the area of cryogenic spray cooling [12-14]. The use of low boiling point liquids such as refrigerants or cryogens in spray cooling is ideally suited in applications requiring very intense cooling or low temperatures, namely in dermatologic laser therapies and high power electronics. Advancement in this area requires better control of spray characteristics, and in turn, of cooling characteristics. Fine droplet atomization is another attractive feature of flashing sprays. In fuel injection, flashing is being explored to improve fuel atomization in internal combustion engines [15, 16], especially for diesel or direct injection applications where atomization and, thus, combustion efficiency is poor. The fine droplet atomization also has potential in other process applications such as emulsification of immiscible liquids [17] and nanoparticle production by flame spray pyrolysis [18], though the application of flashing to the latter has not yet been explored. Also, of great public concern is the risk of release of hazardous pressure liquefied gases (PLG’s) during transportation or storage [19, 20]. In recent decades, a variety of highly destructive and deadly release scenarios have occurred (Seveso, Italy, 1976; Bhopal, India, 1984; Mexico City, 1984; Milwaukee, 2006) exposing the need for further risk assessment and safety measures. Past dispersion studies have avoided flashing atomization issues by ICLASS 2009 High-Speed Internal Nozzle Flow Visualization of Flashing Jets 2 assuming arbitrary initial release characteristics, making results questionable and of limited usefulness [20]. Scaleup of existing flashing research to the dimensions relevant to industrial releases is currently not possible due to a lack of fundamental understanding of the atomization and dispersions processes. More detailed study of the initial release processes would greatly improve the accuracy of subsequent dispersion predictions. In the present study, a review of the current state of knowledge of flashing liquid jets is presented along with motivation for continuing research. The study will present a review of the topic on a fundamental level, as well as describe the ongoing work of the authors on high-speed imaging of internal flashing flow processes. Preliminary results and findings are discussed as well as ideas for future work. Figure 1. Phase diagram of the flashing process. Thermodynamic Breakup Mechanisms The basic mechanism behind thermodynamic or flashing breakup involves the transfer of energy from the expansion of vapor bubbles nucleated within the bulk liquid to the surface energy of droplets. This occurs when a pressurized supercritical, subcooled or saturated fluid is suddenly released to a lower, thermodynamically unstable pressure by way of a throttling process. Figure 1 illustrates this process with a P vs. V phase diagram. Obviously, if the pressure drop does not cross the liquid/vapor saturation line, flashing will not occur. Also, the intensity of flashing is dependent upon the superheat, ∆Tsh, with a certain minimum threshold superheat required for flashing to occur. Peter et al. [2] identified four breakup regimes, depending on the level of superheat. These are illustrated in Figure 2 and represent changes in flow characteristics with increasing superheat. Type 4 or “flare flashing,” represents a transition from external flashing to internal flashing in which most or all of the vaporization and liquid phase breakup takes place within the ejection orifice or nozzle. Flashing has also been found to be influenced by the nozzle diameter with larger diameters promoting more violent liquid breakup. This finding has been corroborated by many authors [2, 5, 21]. The energy exchange taking place was first conceptualized by Brown and York [1] by considering single bubble nucleation. Under adiabatic conditions, the latent heat of vaporization for bubble formation comes from the sensible heat of the bulk liquid. Equilibrium is reached when the residual liquid has cooled to the saturation temperature. A nucleated bubble is subject to three forces: the liquid pressure pl, the vapor pressure inside the bubble pg, and the pressure exerted by surface tension 2σ/r. Bubble growth can only occur when the pressure acting outward exceeds the pressure acting inward: