摘要:

A computer model is proposed for single droplet vaporization at high pressure and temperature. The model is one‐dimensional in space, with no gas or liquid motion. The model is used to calculate the transient heat‐up and subsequent vaporization of the droplet. The numerical results indicate a rapid heat up period during which the droplet temperature surpasses its thermodynamic critical point under some relatively high pressure and temperature conditions. The current model does rely on some provisional methods for describing near‐critical point transport properties. Thus, the model should be improved by experimental data. An experimental procedure is described to verify the model results. Microgravity drop tower studies using a compression device with a droplet placed at a position of compressive symmetry will be pursued. The photographic results obtained from experiment should provide vaporization rate data throughout the droplet lifetime.

ISSN:0094-243X    

DOI:10.1063/1.38970

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

Burning behavior of a fuel droplet in a quiescent high‐pressure atmosphere has been studied experimentally to explore the effects of the ambient pressure just around the critical pressure of fuel. Since the natural convection changes the transport field around the burning droplet, microgravity field made by a falling apparatus were used to suppress the natural convection generated by the droplet burning. Experiments showed that spherical flames were obtained for ambient pressures both below and above the critical pressure of fuel. As the ambient pressure is increased, the burning life time decreases and reaches the minimum at the critical pressure of fuel, beyond which the burning life time increase. Flame diameter increases during the burning time to a maximum and then decreases to burnout. This behavior is the same both below and above the critical pressure of the fuel. The maximum flame diameter attained during burning is a function of pressure and decreases with ambient pressure throughout the range investigated. There is no change in slope or minimum at the critical pressure.

ISSN:0094-243X    

DOI:10.1063/1.38971

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

A small‐scale (7.6m) drop tower was used for studying the combustion of unsupported fuel droplets (about 500 &mgr;m initial diameter) in a stagnant ambience under lower gravity. The experimental procedure consisted of generating a droplet in a near vertical trajectory and then releasing the chamber within which the droplet was introduced, as well as associated instrumentation, into free‐fall when the droplet reached the apex of its trajectory.Some results of the burning of n‐heptane, toluene, and heptane/hexadecane mixture droplets are reported. The range of the heptane data is discussed in terms of possibly varying ambient conditions around the droplet during burning due to droplet motion. Microexplosions were not observed for the mixture reported.

ISSN:0094-243X    

DOI:10.1063/1.38972

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

Isolated drops of many solutions or pure materials readily supercool below their equilibrium crystallization point. In some cases a glass forms; otherwise the crystallization process is usually initiaed at a specific point in the drop leading to a complex structure. Under moderate supercooling, in the first stage of crystallization dendrites grow throughout the drop, leading to latent heat release, and temperature rise. The dendrite growth velocity, tip radius and crystal orientation depend critically on the supercooling.The second stage of crystallization depends on the geometry of the heat loss from the periphery and results in freezing from one side of the drop for asymmetric heat loss or as a thickening shell for symmetrical heat loss. Most solute is rejected at this stage, to nucleate in a geometry determined by the original dendrite distribution and the growth interface, and the internal pressure as it responds to volume changes and cracking of the shell. New crystal orientation appear for nucleation at large supercooling and also in large drops, following reorientation of dendrite arms separated during Ostwald Ripening.Crystal growth is usually uninfluenced by the interface in a gas or liquid environment but changes habit to thin needle like crystals, and increases growth velocity for a solid interface. Drops of size of the tip radius will no longer be spherical on crystallization. For a solvent which evaporates, crystals grow faceted (as hydrates); surface tension forces may move solvent over the surface to give well formed faceted crystals.It is suggested that faceted defect free crystals can be grown from drops under low g using controlled nucleation in a controlled vapor and temperature environment in the absence of convective motion.

ISSN:0094-243X    

DOI:10.1063/1.38975

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

Interpretation of microwave scattering in rain depends on the shape of raindrops, normally considered to have an ‘‘equilibrium’’ axis ratio from a force balance at each point on the surface between surface tension, hydrostatic and aerodynamic pressure. Data from the Illinois State Water Survey raindrop cameras and also observations using a high resolution, dual‐polarization radar suggest a pronounced shift from equilibrium axis ratios for small raindrops. New laboratory measurements have been made for small water drops falling at terminal velocity. The experimental findings agree well with equilibrium axis ratios from perturbation theory ford≤1 mm, but scatter significantly above the theoretical curve ford≳1 mm. The scatter is in the same direction as the shifted axis ratios in the field observations. The one‐sided scatter is consistent with wake‐excited oscillations, but only for modes having a single nodal meridian through the poles (corresponding to spherical harmonics of degeneracy,m=1). These identified modes also best match the forcing pattern associated with eddy sheding.

ISSN:0094-243X    

DOI:10.1063/1.38941

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

The model of Beard and Chuang (1987), using the complete form of Laplace’s formula and adjustments to the aerodynamic pressure distribution for the effect of drop distortion, has been extended to raindrop shapes under the influence of vertical electric fields and drop charges. A finite volume method with numerically generated transformation to a boundary‐fitted coordinate system was used to calculate the shape‐dependent electric field. Sufficient constraints (viz, drop volume, overall force balance, and shape‐dependent surface distributions of aerodynamic and electrostatic stresses) allow the calcualtion of a unique shape by integration from the upper to lower pole using a multiple iteration scheme. The model has been verified against solutions for a stationary drop in a uniform electric field (Taylor, 1964; Brazier‐Smith 1971; Zrnicet al. 1984). Numerical shapes of drops falling in electric fields show a pronounced extension of the upper pole. The increased fall speed of electrostatically stretched drops enhances the aerodynamic flattening of the base. The resultant triangular drop profiles are similar to wind tunnel observations (Richards & Dawson 1973; Rasmussenet al. 1985).

ISSN:0094-243X    

DOI:10.1063/1.38942

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

The light scattered by a transparent sphere can be made unusually bright in the forward or backward directions by proper choice of the sphere’s refractive indexM. An axial focusing effect in the forward and backward directions gives rise to the strong scattering enhancement known as the glory. For certainMvalues, rainbow scattering coincides with the glory, giving an additional enhancement. Conditions for the existence of rainbow‐glory scattering have been explored using ray optics. Mie‐theory computations of scattered irradiance show prominent rainbow‐glory effects in the predictedMranges. Rainbow‐glory backscattering was observed from a glass fiber‐coupling sphere in a liquid whereMwas varied by adjusting the liquid temperature.

ISSN:0094-243X    

DOI:10.1063/1.38943

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

The series of sea surface aerosol generation models beginning with that of Monahan,et al., which relate the flux of spray droplets up from the interface to the fractinal whitecap coverage, have been used successfully by, for example, Burk and Stramska, to predict the aerosol population of the MABL. Combining these models with the insights into parent bubble‐daughter jet droplet relationships found in Blanchard, it has been possible to infer, again in terms of fractional whitecap coverage, the associated bubble flux up to the sea surface. This bubble flux model, when taken together with the information on effective bubble rise velocities provided by Thorpe, leads to estimates of mixed layer buble populations, estimates which are in first order agreement with the limited collection of bubble populations reported for comparable sea states.

ISSN:0094-243X    

DOI:10.1063/1.38944

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

Experiments and theory pertinent to light scattered by bubbles in water are examined. Topics includes: the transition to total reflection at the critical angle, complex angular momentum theory of this transition, colors seen in sunlit bubble clouds, optical effects of surface films on bubbles, Brewster and scattering, and the backscattering patterns and caustics of spherical and oblate bubbles. Responses of bubbles to optical radiation pressure include: levitation and optically stimulated acoustic emissions.

ISSN:0094-243X    

DOI:10.1063/1.38945

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

Upon the introdution of gas bubble into a liquid possessing a uniform thermal gradient, an unsteady thermocapillary flow begins. Ultimately, the bubble atains a constant velocity. This theoretial analysis focuses upon the transient period for a bubble in a microgravity environment and is restricted to situations wherein the flow is sufficiently slow such that inertial terms in the Navier‐Stokes equation and convective terms in the energy equation may be safely neglected (i.e., both Reynolds and Marangoni numbers are small). The resulting linear equation were solved analytically in the Laplace domain with the Prandtl number of the liquid as a parameter; inversion was accomplished numerically using a standard IMSL routine. In the asymptotic long‐time limit, our theory agrees with the steady‐state theory of Young, Goldstein, and Block. The theory predicts that more than 90% of the terminal steady velocity is achieved when the smallest dimensionless time, i.e., the one based upon the largest time scale‐viscous or thermal‐equals unity.

ISSN:0094-243X    

DOI:10.1063/1.38946

出版商:AIP    

年代:1990

数据来源: AIP