Dynamics of Intermittent Sprays

Sampling Technique ◽

Size Distributions ◽

Ambient Conditions ◽

Spray Formation ◽

Fuel Droplets ◽

Liquid Immersion ◽

Particle Sizer ◽

Abstract It is considered that droplet size distribution changes with time and space, since diesel fuel sprays are found to be transient and intermittent due to variations in ambient pressures. Therefore the obscuration signal (extinction of light due to particle field) obtained from a particle sizer for a single injection of fuel over the whole region of spray is necessary to determine the spray characteristics. Previous studies dealing with sprays have observed fuel droplets by use of the sedimentation tower method or liquid immersion sampling technique. However, in these technique droplets are usually sampled after spray formation is complete. The completion time of spray formation appears to vary with ambient conditions, thus making spray measurements under transient conditions during injection difficult. It is the objective of this paper to shine some light on the dynamics of spray motion, leading to a better understanding of the droplet size distributions.

Progress on Predicting the Breadth of Droplet Size Distributions Observed in Small Cumuli

Journal of the Atmospheric Sciences ◽

10.1175/jas-d-11-0153.1 ◽

2011 ◽

Vol 68 (12) ◽

pp. 2921-2929 ◽

Cited By ~ 8

Author(s):

Jennifer L. Bewley ◽

Sonia Lasher-Trapp

Keyword(s):

Droplet Size ◽

Cloud Condensation Nuclei ◽

Droplet Diameter ◽

Trade Wind ◽

Droplet Growth ◽

Size Distributions ◽

Model Resolution ◽

Modeling Framework ◽

Abstract A modeling framework representing variations in droplet growth by condensation, resulting from different saturation histories experienced as a result of entrainment and mixing, is used to predict the breadth of droplet size distributions observed at different altitudes within trade wind cumuli observed on 10 December 2004 during the Rain in Cumulus over the Ocean (RICO) field campaign. The predicted droplet size distributions are as broad as those observed, contain similar numbers of droplets, and are generally in better agreement with the observations when some degree of inhomogeneous droplet evaporation is considered, allowing activation of newly entrained cloud condensation nuclei. The variability of the droplet growth histories, resulting primarily from entrainment, appears to explain the magnitude of the observed droplet size distribution widths, without representation of other broadening mechanisms. Additional work is needed, however, as the predicted mean droplet diameter is too large relative to the observations and likely results from the model resolution limiting dilution of the simulated cloud.

Spatial distributions of cloud droplet size distributions from cloudbow observations measured with specMACS

10.5194/egusphere-egu21-8862 ◽

2021 ◽

Author(s):

Veronika Pörtge ◽

Tobias Kölling ◽

Tobias Zinner ◽

Linda Forster ◽

Claudia Emde ◽

...

Keyword(s):

Size Distribution ◽

Droplet Size ◽

Cloud Droplet ◽

Wide Field ◽

Size Distributions ◽

Vertical Profiles ◽

Camera System ◽

Cloud Droplet Size Distribution ◽

The evolution of clouds and their impact on weather and climate is closely related to the cloud droplet size distribution, which is often represented by two parameters: the cloud droplet effective radius (reff) and the effective variance (veff). The droplet radius (reff) determines the radiative effect of clouds on climate. The effective variance is a measure of the width of the size distribution which is, for instance, important to understand the formation of precipitation or entrainment and mixing processes. We present an airborne remote-sensing technique to determine reff and veff from high-resolution polarimetric imaging observations of the LMU cloud camera system specMACS.Recently the spectral camera system has been upgraded by a wide-field polarization resolving RGB camera which was operated for the first time on the HALO aircraft during the EUREC4A campaign. The new polarimeter is ideally suited for observing the cloudbow - an optical phenomenon which forms by scattering of sunlight by liquid water cloud droplets at cloud top. The cloudbow is dominated by single scattering which has two implications: Its visibility is significantly enhanced in polarized measurements and its structure is sensitive to the cloud droplet size distribution at cloud top. This allows the retrieval of reff and veff by fitting the observed polarized cloudbow reflectances against a look-up table of pre-computed scattering phase functions.The characteristics of the polarimeter are optimized for the measurement of the cloudbow. The wide field-of-view is key for observing the cloudbow (scattering angle 135&#176; -165&#176;) for a wide range of solar positions. Another advantage is the high spatial and temporal resolution which allows the study of small-scale variability of cloud microphysics at cloud top with a horizontal resolution of up to 20 m. Combining the polarimetric cloudbow technique with an existing stereographic retrieval of cloud geometry allows to derive vertical profiles of the droplet size distribution at cloud top. Observations of different EUREC4A cloud fields are used to demonstrate the retrieval technique and to present first spatial distributions and vertical profiles of cloud droplet size distributions.

Fog Classification by Their Droplet Size Distributions: Application to the Characterization of Cerema’s Platform

Atmosphere ◽

10.3390/atmos11060596 ◽

2020 ◽

Vol 11 (6) ◽

pp. 596 ◽

Cited By ~ 1

Author(s):

Pierre Duthon ◽

Michèle Colomb ◽

Frédéric Bernardin

Keyword(s):

Size Distribution ◽

Droplet Size ◽

Transportation Systems ◽

Distribution Range ◽

Size Distributions ◽

Atmospheric Conditions ◽

Different Types ◽

Radiation Fogs ◽

Fog is one of major challenges for transportation systems. The automation of the latter is based on perception sensors that can be disrupted by atmospheric conditions. As fog conditions are random and non-reproducible in nature, Cerema has designed a platform to generate fog and rain on demand. Two types of artificial fog with different droplet size distributions are generated: they correspond to radiation fogs with small and medium droplets. This study presents an original method for classifying these different types of fog in a descriptive and quantitative way. It uses a new fog classification coefficient based on a principal component analysis, which measures the ability of a pair of droplet size distribution descriptors to differentiate between the two different types of fog. This method is applied to a database containing more than 12,000 droplet size distributions collected within the platform. It makes it possible to show: (1) that the two types of fog proposed by Cerema have significantly different droplet size distributions, for meteorological visibility values from 10 m to 1000 m; (2) that the proposed droplet size distribution range is included in the natural droplet size distribution range; (3) that the proposed droplet size distribution range should be extended in particular with larger droplets. Finally, the proposed method makes it possible to compare the different fog droplet size distribution descriptors proposed in the literature.

Time Evolution of the Droplet Size Distribution in Dropwise Condensation

Journal of Heat Transfer ◽

10.1115/1.4047488 ◽

2020 ◽

Author(s):

Maofei Mei ◽

Feng Hu ◽

Chong Han ◽

Yan Sun ◽

Dongdong Liu

Keyword(s):

Size Distribution ◽

Droplet Size ◽

Power Law ◽

Droplet Growth ◽

Size Distributions ◽

Dropwise Condensation ◽

Dominant Feature ◽

Droplet Size Distributions ◽

Instantaneous Distribution

Abstract Droplet growth processes during dropwise condensation are simulated with a help of computer. We focus on instantaneous and time-averaged characteristics of droplet size distributions. Based on simulation results, shift of a single peak from small to large size is a significant characteristic for the instantaneous distribution before the first departure. Once condensing surface was refreshed time and again by shedding droplets, then coexistence, shift and combination of multiple peaks is the dominant feature. This indicates that the instantaneous droplet size distribution highly depends on growth time and target area. The findings can explain why different distribution characteristics were reported in experiments. Different from the instantaneous distribution, time-averaged size distributions for coalesced droplets follow a power-law style due to a collaboration of coalescence events and re-nucleation behaviors. However, the size range for the power-law distributions were affected by nucleation density. This requires an appropriate usage of the empirical or fractal model to predict theoretically heat transfer rate of dropwise condensation. The present work provides a comprehensive understanding of the instantaneous and time-averaged characteristics of droplet size distributions.

Effect of Inlet Tangential Port Area on Droplet Size Distribution of Small-Scale Simplex Atomizer

ASME 2013 Gas Turbine India Conference ◽

10.1115/gtindia2013-3549 ◽

2013 ◽

Author(s):

Muthu Selvan Govindaraj ◽

Muralidhara H. Suryanarayana Rao ◽

Vinod Kumar Vyas ◽

Karthy Shanmugasundaram ◽

Narendran Venugopal ◽

...

Keyword(s):

Size Distribution ◽

Droplet Size ◽

Flow Direction ◽

Injection Pressure ◽

Working Fluid ◽

Small Scale ◽

Size Distributions ◽

Port Area ◽

An experimental investigation was conducted to study the effects of increased area of inlet tangential ports on the droplet size distribution of small-scale simplex atomizer. The spray characteristics of four different simplex atomizers representing increasing area of inlet tangential ports (diameter range 0.6 mm to 0.9 mm) are examined using water as working fluid. Measurements of droplet size and droplet size distributions of the four different atomizer configurations were carried using Malvern droplet size instrument at various downstream locations from final orifice exit. These measurments has been taken for five different injection pressures of spray. Variation of droplet size and droplet size distribution along the flow direction of spray was examined. The effect of increase in injection pressure on droplet size distribution of the spray was examined. Increase in inlet tangential port area significantly affects droplet size and droplet size distributions of the spray and affects the length of primary breakup region.