An Investigation of Microscale Explosive Vaporization of Water on an Ultrathin PT-Wire

Abstract

Abstract The present study investigates at the microscale level the explosive vaporization of water close to its superheat limit. In the course of the research, it was possible to obtain simultaneously unique results of visualization, temperature and pressure of the vapor microregion, thus accomplishing a marked step forward in understanding the complex behavior of explosive vapor nucleation growth and subsequent collapse, despite the very small time and length scales of the phenomena of interest. To this end, a short (1 mm in length) and ultrathin (10 μm in diameter) Pt-wire is very rapidly heated by imposing a short and intense current pulse. Deionized and degassed water in contact with the wire is, as a consequence, also rapidly heated during the time of application of the current (in the range of 6 to 50 μs) and vaporizes explosively. For the visualization of the very short boiling process, a strobe microscopy technique was developed and employed. The dynamic interaction between the exploding vapor microlayer and the surrounding liquid can be quantified and further characterized by measuring the acoustic pressure wave in the liquid during the boiling process. A fast pressure transducer allows us to capture the acoustic emission from the expanding vapor volume. From these data, the pressure inside the growing vapor layer and the mechanical energy released from its rapid expansion can be estimated. An important result of the present experiments is the determination of the vapor nucleation temperature. The method for measuring the maximum superheat temperature of the water involves measuring the change in resistance of the Pt-wire during a heating pulse and relating the change in resistance to the change in temperature. The slope of the temperature curve changes shape when the fluid in contact with the wire changes phase, because when the vapor phase covers the wire, it reduces the rate of heat transfer to the fluid. This produces an inflection point in the temperature curve. A curve-fitting method was developed, which enabled us to define the vapor transition point and thus to determine the temperature of the heater surface, when explosive boiling is initiated. In the present study we were able to operate with the shortest heating pulses (<4.5 μs) for an electrically heated body reported (to the best of our knowledge) and thus apply more electric power to the Pt-wire. The shorter the heating pulse, the more unlikely it is that nucleate boiling is promoted by gases trapped on the surface of the wire. It was shown that with a shorter pulse and higher power, the maximum attained superheat temperature increases. To the best of our knowledge, the superheat temperatures obtained for water (∼305 °C) in this study are higher than those reported in previous experiments in the open literature.