| Modeling Evaporation of Drops of Different Kerosenes |
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| NASA’s Jet Propulsion Laboratory, Pasadena, California | |
| May 01 2007 | |
One model applies to all three classes of hydrocarbon constituents.
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A mathematical model describes the evaporation of drops of a hydrocarbon liquid composed of as many as hundreds of chemical species. The model is intended especially for application to any of several types of kerosenes commonly used as fuels. Like evaporating- multicomponent- fuel-drop models described in several previous NASA Tech Briefs articles, the present model invokes the concept of continuous thermodynamics, according to which the chemical composition of the evaporating multicomponent liquid is described by use of a probability distribution function (PDF). However, as described below, the present model is more generally applicable than is its immediate predecessor.  The Asymptotic Evaporation Rate Constants of three kerosenes as a function of far-field pressure were computed for a far-field temperature of 1,000 K and zero far- field vapor fraction. The derivation leads to a new version of the double PDF, in which the square root of the molecular weight occupies the position previously occupied by the molecular weight and other parameters are modified accordingly. By design, this version of the double PDF applies to the three major homologous series in kerosene; hence, it is not necessary to use multiple double PDFs. An additional advantage of this formulation is that it is valid over the subcritical region in the pressure range from 1 to 15 bars (0.1 to 1.5 MPa). The model has been tested on three kerosenes used as aircraft and rocket fuels: Jet A, JP-7, and RP-1. The present version of the double PDF has been fitted to the discrete species distributions of these kerosenes and extensive calculations of evaporation of isolated drops performed. The results show that under the assumption of a quasi-steady gas phase, a relation known in the literature as the D2 law (pertaining to the rate of decrease of the square of the drop diameter) is recovered after an initial dropheating transient. A related quantity known in the literature as the asymptotic evaporation rate constant has been found to be an increasing function of the far-field temperature and pressure, a complex function of far-field composition, and a weak function of the difference between the drop-surface and farfield vapor molar fractions. In a comparison between results obtained (a) under the assumption that the interior of the drop is well mixed (the “wm” assumption) and (b) under the assumption that the drop can evaporate either in a wellmixed mode or at unchanging (frozen) composition (the “wm-fc” assumption), it was found that the differences between the asymptotic evaporation rate constants under the two assumptions is within the range of uncertainty in the transport properties (see figure). This work was done by Josette Bellan and Kenneth Harstad of Caltech for NASA’s Jet Propulsion Laboratory. For more information, download the Technical Support Package (free white paper) at www.techbriefs.com/tsp under the Information Sciences category. NPO-40437 This Brief includes a Technical Support Package (TSP).
Modeling Evaporation of Drops of Different Kerosenes (reference NPO-40437) is currently available for download from the TSP library. Login first to download.
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