The conventional fuels that are used in the field of transportation are primarily composed of two or more components. Each component evaporates, mixes with hot oxidant gases, ignites, and burns. Since evaporation is the precursor of the sequence of events leading to combustion, the evaporation studies on the multi-component drops are essential for determining the governing parameters of spray evaporation. While single-component drop studies have been carried out extensively in the past, very limited literature exists on the multicomponent array evaporation. The present paper deals with the evaporation of multicomponent fuel droplets in an array using the recently developed point source method (PSM). First, the quasi-steady (QS) evaporation of an isolated, multicomponent droplet is briefly analyzed. The resultant governing equations, along with Raoult’s law and the Cox-Antoine relation, constitute the set of equations needed to arrive at the solutions for: (1) the droplet surface temperature, (2) the evaporation rate of each species, and (3) the vapor mass fraction of each species at the droplet surface. The PSM, which treats the droplet as a point mass source and heat sink, is then adopted to obtain an analytic expression for the evaporation rate of a multicomponent droplet in an array of liquid droplets. Defining the correction factor (η) as a ratio of the evaporation of a drop in an array to the evaporation rate of a similar isolated multi-component drop, an expression for the correction factor is obtained. The results of the point source method (PSM) are then compared with those obtained elsewhere for a three-drop array that uses the method of images (MOI). Excellent agreement is obtained. The treatment is then extended to a binary drop array to study the effect of interdrop spacing on vaporization. When the drops are close to each other, the evaporation rate of the droplet in the array containing the larger percentage of volatiles is higher than the rate under isolated conditions (η>1). The results qualitatively confirm the experimental data reported elsewhere. Parametric results were obtained for the effect of changing the composition on the correction factor and finally critical drop compositions in the binary array are given for which η>1. Even though the results for the average correction factor of the whole array of 2 to 9 drops obtained using PSM are almost the same as the results from MOI, the correction factor of the center drop under severe interaction may deviate from those results obtained with MOI.

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Shrouded aerosol sampling probes utilize an aerodynamic decelerator (shroud) placed about an inner probe. A model has been developed for predicting the transmission ratio (T) of aerosol from a free stream to the exit plane of the inner probe. This expression, T ) FA s A pr (1 -WL), is based on use of an existing empirical model to characterize the aspiration ratios of the shroud (A s ) and inner probe (A pr ) and based upon new models to characterize the wall loss ratio in the inner probe (WL) and to relate the concentration in the core region of the shroud to the mean concentration predicted by the existing aspiration model through a correlation function, F. Extensive computational results provide a data base for specification of the correlation function. The need for the correlation function results from the phenomenon that particle enrichment in a subisokinetic shroud is non-uniform, with the concentration higher near the wall than in the center region. However, the concentration in the core region of the shroud, which is the aerosol that is ultimately sampled, is quite uniform, albeit at a level that is somewhat higher than the concentration in the free stream. This correlation function depends on particle Stokes number and the velocity ratio between free stream and shroud inlet. The predictive equation was verified by comparing its results with data from physical experiments conducted in aerosol wind tunnels with several sizes of shrouded probes. The standard error of experimental data of aerosol transmission about the predictive equation was 7.7%. The model was also evaluated in-depth by examining its ability to predict the overall aspiration of aerosol from the free stream to the inlet plane of the inner probe, wall loss ratio, and transmission of aerosols from the free stream to the exit plane of the inner probe. The results show that the model underestimates the aspiration by approximately 2%. The model for wall loss ratio underpredicts the experimental values by 8% (which influences the transmission ratio by about 2%), and transmission ratio prediction is within 1% of average experimental data. Applications of shrouded probes involve sampling air from turbulent flows, and the model is based on conditions that simulate those encountered by shrouded probes in typical stack flows. The model takes into account turbulent, inertial, and gravitational effects. It is assumed that the shrouded probe is oriented parallel to the direction of flow and the inner probe is sufficiently small such that it only samples from the core region of the shroud.

The U.S. EPA has approved Alternate Reference Methodologies for sampling radionuclide aerosol particles from stacks and ducts of U.S. DOE facilities. The approach allows use of single point sampling with shrouded probes from locations where both fluid momentum and contaminant concentration are well mixed across the flow cross section. For existing stacks and ducts that do not have locations where there is adequate mixing, we have developed a generic mixing system that will generate conditions suitable for single point sampling. The coefficients of variation of the velocity, tracer gas, and 10 microm aerodynamic diameter aerosol particles profiles are all less than 10%, which are well within the EPA limit of 20%. Mixing is affected neither by size of the system nor by flow rate, provided the flow is turbulent.

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