(178f) Modeling Evaporation and Microexplosion of Water-in-Alkane Emulsion Droplets

Water is sometimes added to fossil-fuel-based combustion processes by separate injection (liquid or vapor) inside the combustion chamber, or directly to the reacting fuel by means of emulsifying processes. Furthermore, the development of coal-to-liquid and liquid synfuel technologies as well as the salutary growing use of biomass pyrolysis oils as renewable energy, have also led to a substantial increase in the water fraction with respect to other components. For example, biomass pyrolysis oils have up to 25wt% water in their final composition. According to Dryer(1) (1976) the potential benefits of adding water to the evaporation/combustion process can be classified as those arising from the chemical reactions kinetics and those arising from the so-called secondary-atomization effect or micro-explosion. Concerning chemical reactions kinetics, the resulting effects of adding water range from the passive heat sink effect to more active contributions where water vapor acts as a catalyzer on some intermediate reactions or affects the flame velocity. Moreover, independently from the type of fuel (fossil, synthetic or liquid-coal) the presence of water in such multicomponent-oil mixtures leads to a higher water vapor concentration and a temperature reduction in the gas fuel-rich region1. This can significantly reduce the gas phase soot formation process1 and diminish the chemical activities at the flame, which should reduce the production of NOx1. Also, the reduced temperature can be interpreted in terms of lost of heat supply for the cracking reactions that lead to the formation of carbonaceous residue. Besides thermo-chemical effects, the addition of water, which is characterized by a substantially lower boiling temperature with respect to most practical fuel components, leads also to a mechanical effect, namely micro-explosion, which improves the combustion efficiency. Microexplosion occurs when the water mixed in the fuel droplet under various degrees of miscibility reaches its superheat limit(2). Then, the onset of homogeneous nucleation in a water-fuel mixture or the onset of heterogeneous nucleation at the interface of a water micro-droplet embedded in the fuel parent droplet both lead to disruptive boiling followed by secondary atomization of the parent droplet with various intensities. The benefits are a substantial increase in the surface of evaporation and an enhanced mixing of fuel vapor with air. Recent studies concerning these emerging fuels have proven that micro-explosion events are not marginal and therefore can substantially contribute to the overall atomization process. In order to account for all the aforementioned beneficial effects one has to capture accurately their respective kinetics. In particular, some of those beneficial effects depend on the onset of micro-explosion, thus on the superheat limit of water in the fuel droplet. Therefore, the proposed article aims at coupling the thermodynamic limit of superheat and the theory of Blander and Katz(2) (1975) for the kinetic limit of superheat to an all-pressure droplet evaporation model. The Peng-Robinson cubic equation of state is used for the n-alkane and water VLE at the droplet surface and for the determination of the mixture spinodals (thermodynamic limit). The single droplet evaporation model is validated against experimental data at atmospheric as well as high pressure conditions(3). Then, the derived superheat limits for water-in-alkane-fuel droplets is combined to a secondary-atomization model(4) for emulsified droplets and results from the computation of the evaporation of a single water-in-alkane emulsion droplet will be compared to the experimental measurements of Wang and Law(5) (1985) who studied experimentally the influence of pressure on the onset of micro-explosion.

REFERENCES

(1) Dryer, F. L., Water addition to practical combustion systems: concepts and applications, Proceedings of the Sixteenth (International) Symposium on Combustion, 279-295, 1976.

(2) Blander, M. and Katz J. L., Bubble nucleation in liquids, AIChE J. 21, 833-848, 1975.

(3) Nomura, H., Rath, H., Sato, J. and Kono, M., Experimental study on hogh-pressure droplet evaporation using microgravity conditions, Proceedings of the Twenty-Sixth (International) Symposium on Combustion, 1267-1273, 1996.

(4) Le Clercq, P. C., Noll, B. and Aigner, M., Modeling evaporation and secondary atomization of water-in-multicomponent-oil emulsion droplet, SPRAY-05, International Symposium on Heat and Mass Transfer in Spray Systems, Antalya, Turkey, June 5-10 2005.

(5) Wang, C. H. and Law, C. K., Micro-explosion of fuel droplets under high pressure Combustion and Flame, 59, 53-62, 1985.