Substance related investigation of the evaporation characteristics of free falling alkane-ethanol droplets using Raman spectroscopy
Thomas Hillenbrand  1, *@  , Dieter Brüggemann  1@  
1 : Chair of Engineering Thermodynamics and Transport Processes (LTTT), Bayreuth Engine Research Center (BERC), University of Bayreuth
Universitätsstraße 30 D-95447 -  Germany
* : Corresponding author

A detailed understanding of the evaporation process is a key issue in the design of internal combustion engines. The emerging of renewable fuels has led to recent challenges in the prediction of this process especially for blended fuel sprays. In this context, the blending of ethanol into gasoline in various substance ratios is an already widely spread approach. Even though this mixtures are already in use, the evaporation process of this fuel blends is still insufficiently characterized. The particular occurrence of component-by-component evaporation compromises the efficient and the low-emission operation of internal combustion engines due to inhomogeneous carburation. In order to meet these challenges a precise understanding of the mechanisms causing the component-by-component evaporation is inevitable. Initially, a precise and extensive experimental investigation is essential. Referring to this, to identify the fundamental mechanisms raising these challenges the evaporation of droplets under controlled conditions needs to be considered first. Available sources for experimental validation focus on lying or levitated droplets. Therefore, the emphasis of this work is the experimental evaluation of the component-by-component evaporation of free falling droplets consisting of various binary mixtures of n-hexane, iso-octane or n-decane blended with ethanol. A high-resolution Raman setup combined with a fast triggering system is established. The gaseous phase directly behind free falling droplets is resolved quantitatively and substance related. The Raman signals of the components overlap additively but differ considerably. Therefore, the relative concentration ratio in the gaseous phase behind the droplets is determined. To ensure engine-like conditions the droplet diameter is kept below 60 µm. Single droplets as well as droplet chains and an intermediate droplet character are investigated. A systematic variation of the droplet temperature, the falling distance, the droplet generating frequency and the relative ratios of the applied substances is accomplished. The evaluation reveals that the component-by-component transfer into the gaseous phase is strongly depending on the adjusted parameters and the applied substances. Furthermore, the results indicate a depletion of the more volatile component at the droplet surface with increasing droplet temperature at selected component ratios.

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