On the modeling of droplet transport, dispersion and evaporation in turbulent flows

Jorge Barata

doi:10.19206/ce-117399

Abstract

The present paper presents a numerical study on evaporating droplets injected through a turbulent cross-stream. Several models have been used with more or less success to describe similar phenomena, but much of the reported work deals only with sprays in stagnant surroundings. The ultimate goal of this study is to develop an Eulerian/Lagragian approach to account for turbulent transport, dispersion, evaporation and coupling between both processes in practical spray injection systems, which usually include air flows in the combustion chamber like swirl, tumble and squish in I.C. engines or crossflow in gas turbines. In this work a method developed to study isothermal turbulent dispersion is extended to the case of an array of evaporating droplets through a crossflow, and the performance of two different evaporation models widely used is investigated. The convection terms were evaluated using the hybrid or the higher order QUICK scheme. The dispersed phase was treated using a Lagrangian reference frame. The differences between the two evaporation models and its applicability to the present flow are analysed in detail. During the preheating period of the Chen and Pereira [1] model the droplets are transported far away from the injector by the crossflow, while with the Sommerfeld [2] formulation for evaporation the droplet has a continuous variation of the diameter. This result has profound implications on the results because the subsequent heat transfer and turbulent dispersion is extremely affected by the size of the particles (or droplets). As a consequence, droplet diameter, temperature and mass fraction distributions were found to be strongly dependent on the evaporation model used. So, a new formulation that takes into account also the transport of the evaporating droplets needs to be developed if practical injection systems are to be simulated. Also, in order to better evaluate and to improve the vaporization models more detailed measurements of three-dimensional configurations are required.

Highlights

The efficient use of sprays leading to proper evaporation/combustion reduces pollutants by decreasing NOx and CO2 emissions
The effect of the convective transport caused by the droplet motion relative to the gas was a accounted for by the so called “film theory”, which results in modified correlations for the Nusselt and Sherwood numbers
A different expression for the transit time is recommended in the literature [25, 26, 27], and was used in the τ FL

Summary

Introduction

The efficient use of sprays leading to proper evaporation/combustion reduces pollutants by decreasing NOx and CO2 emissions. Modelowanie ruchu kropel paliwa, rozpylenia i parowania. W szczególnoœci jakoœæ spalania oraz poziom emisji s1 zdeterminowane przez aerodynamiczn sprawnoœæ rozpylenia paliwa i mieszania go z powietrzem w komorze spalania, przez oczekiwany poziom temperatury i przez odpowiedni przebieg jej zmian. Jeszcze w pe3ni nie ustalono warunków rozpylania paliwa najkorzystniejszych do optymalizacji kszta3tu komory spalania, przez co zaprojektowanie optymalnej konstrukcji urz1dzeñ przemys3owych, wykorzystuj1cych w swym dzia3aniu proces spalania, nastrêcza trudnoœci od samego pocz1tku; wszystkie dane empiryczne uzyskane w toku badañ pracy tych urz1dzeñ okaza3y siê trudne do uogólnienia i ekstrapolacji. Dlatego te¿ w ubieg3ym roku zdecydowanie wzros3o zainteresowanie matematycznym modelowaniem rozpylenia paliwa i procesami spalania. Jeœli chodzi o w3asnoœci kropel, to wœród parametrów wp3ywaj1cych na spalanie mo¿na wymieniæ rozk3ad kropel œredniej wielkoœci w ka¿dym punkcie strugi paliwa, rozk3ad prêdkoœci kropel, zale¿noœæ prêdkoœci kropel paliwa od prêdkoœci powietrza, przestrzenne i czasowe zagêszczenie kropel w strudze paliwa oraz ich temperatura [4]

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Results

Conclusion