Dynamics of vapor explosions : rapid evaporation and instability of butane droplets exploding at the superheat limit

vapor explosions

Shepherd, Joseph Emmett ; Sturtevant, Bradford

University:California Institute of Technology

Department:Engineering and Applied Science

关键词: vapor explosions;   

Others  :  https://thesis.library.caltech.edu/4521/1/Shepherd_je_1981.pdf

美国|英语

来源: Caltech THESIS

【 摘 要 】

A preliminary experimental investigation of the vapor explosion of a single droplet (~ 1 mm diameter) of liquid butane at the superheat limit has been completed. These experiments provided the first detailed look at rapid evaporation taking place under conditions such that departures from equilibrium, evaporative fluxes and fluid accelerations are orders of magnitude larger than observed under ordinary circumstances. Single short-exposure photographs and fast-response pressure measurements were used to obtain a description of the complete explosion process within a superheated drop immersed in a bubble-column apparatus. Emphasis was placed on the early (microsecond-time-scale) evaporative stage. Despite the apparant simplicity of the vapor explosion of a single superheated droplet, the present experiments revealed a wide range of phenomina of varying complexity occurring at different stages of the explosion.

The explosion is initiated by the spontaneous formation within the drop of a single vapor bubble, which grows until the drop liquid is completely evaporated. The resulting vapor bubble undergoes volume oscillations and eventually breaks up via Taylor instability. Several new and unusual features of the early evaporative stage of the explosion have been observed, three of which are remarkably repeatable. First, photographs of the evaporative surface show a highly roughened and disturbed interface for most of the evaporative stage. At the earliest observed times (8 [mu]sec) the roughening appears to begin as a rather regular pattern on an otherwise spherical surface, suggestive of a fundamental instability due to evaporative mass flux. Second, due to the asymmetric location of the initial nucleus within the drop, a portion of the evaporating surface contacts the surrounding fluid first and becomes nonevaporating. As the bubble grows, a unique, axisymmetric structure of circumferential waves terminated by a spherical cap appears on this nonevaporating surface. Apparently, these waves are driven by the impinging jet of vapor coming from the opposing evaporating surface. Third, nucleation and initial development of the bubble in the first 10[mu]sec is accompanied by a characteristic two-step pulsating pressure signal, suggesting that a fundamental and repeatable unsteadiness, perhaps connected with the above mentioned instability, is taking place at this stage.

A preliminary estimation of the evaporative mass flux has been made from photographically-determined bubble volumes and pressure signals measured in the first 30 [mu]sec. As might have been expected in view of our observations of the highly roughened surface, the inferred mass flux (~ 400 gm/cm[superscript 2]-sec) is two orders of magnitude larger than that predicted by the classical, diffusion-limited theory of bubble growth. We propose that the interface roughening is due to an inertial instability of the evaporative surface. A preliminary calculation for the Landau mechanism of instability, supplemented by an ad hoc correction for sphericity indicates that, indeed, the classical mode of bubble growth would be unstable under the conditions found in the present experiment. An explanation of the present observations that is consistent with this theoretical prediction is that the actual instability does occur in the first 1-2 [mu]sec of bubble growth and the instability has developed well into the nonlinear stage by 8 [mu]sec, the earliest time at which bubbles have been observed in the present experiment.

The present observations are completely different than what might be predicted from previous experiments and analyses of near-equilibrium evaporation. The generality of the present results needs to be verified in detail, but they clearly indicate that evaporation at the superheat limit can be much more complex than previously expected.