Modeling heat and mass transfer in osmotic evaporation process

A Computational Fluid Dynamics simulation of near-field cough and sneeze droplet dispersion and heat and mass transfer is developed. In this study various sources of variability in cough and sneeze processes are considered. These are variations in injection volume (0.5l, 2.5l, and 5.0l) and ambient relative humidity (20%, 40% and 60%). There are a total of 9 simulations for coughs and sneezes in a quiescent background. A large ensemble (5000) of droplets are tracked with diameters in the range 1–500micron. Evaporation and dispersion are predicted as a function of droplet size. Generally, fine droplets evaporate faster than large droplets. Higher relative humidities slow the evaporation process. Larger droplets have greater axial penetration. They also exhibit greater vertical drop due to the effect of gravity. Sideway penetration is increased by higher injection volumes. The buoyancy effect due to thermal energy of the injection is very weak, at least for the 10-second computation duration.

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Modelling Droplet Heat and Mass Transfer in Aero-Engine Bearing Chambers

Volume 2C: Turbomachinery ◽

10.1115/gt2019-91657 ◽

2019 ◽

Author(s):

Dafne Gaviria Arcila ◽

Hervé Morvan ◽

Kathy Simmons ◽

Stephen Ambrose ◽

Michael Walsh ◽

...

Keyword(s):

Mass Transfer ◽

Heat And Mass Transfer ◽

Evaporation Rate ◽

Numerical Study ◽

Droplet Diameter ◽

Air Flow ◽

Core Region ◽

Evaporation Process ◽

Data Set ◽

The Core

Abstract The oil inside aeroengine bearing chambers can be found in many forms, including droplets which interact with the core airflow. The ability to model such bearing chambers computationally is desirable and thus a better understanding of the evaporation process of oil droplets is of great interest. Previous studies have analyzed the flow of isothermal droplets in bearing chambers. However, further investigation is needed into the heating of droplets in the highly rotating core region. This will enable designers to evaluate the behavior of droplets in a chamber and the likelihood that they will evaporate. The aim of this research is to analyze the oil droplet evaporation process under aeroengine bearing chamber representative conditions. An ultimate goal is the ability to predict the oil-air heat and mass transfer in the core flow region, as well as to develop an understanding of the flow inside a droplet, and how this affects evaporation. This latter is important as it has not been studied before. This paper presents the results of a numerical study of the evaporation process of a single droplet under bearing chamber temperature and air flow conditions. The two-phase flow is simulated using ANSYS Fluent with the volume of fluid approach and the evaporation process with the “D−square law”. First, the modelling approach is validated against previous experimental and numerical analysis of fuel droplets in an air flow with heat transfer. The simulation results were in excellent agreement with a benchmarking data set. The validated approach is then applied for investigation to smaller, bearing chamber representative droplets of an oil base stock used in jet engines. The oil evaporation rate was quantified as well as the evolution of droplet diameter, which revealed the effect of different air velocities and temperatures on the droplet. The extent to which evaporation rate increased with air velocity and temperature is quantified. It is concluded that droplets of initial diameters less than 200μm that remain in the chamber core region for more than 0.3s are likely to evaporate completely. This study allows us to estimate droplet heat and mass transfer and the associated phase change in a bearing chamber. It also provides best practice to predict the performance of small droplets under the effects of high temperature and velocity convective air flows. In future work this methodology will be applied in simulations in a representative bearing chamber to predict how the cooling process is affected by oil evaporation.

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