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Abstract

The present study is the result of 4 years experimental research study aimed at understanding the hydrodynamic and heat transfer phenomena of a Diesel fuel droplet during the impact process with a heated flat and spherical surface. Such a phenomena are of a direct relevance to many engineering problems such as IC engines and fluid catalytic cracking (FCC). Due to the fact that the spray systems in the aforementioned applications may be comprised of millions of interacting droplets that prohibit detailed identification of the flow conditions during the impact of individual droplets, the current study focus on the characterisation of the impact dynamics of single droplets under well-controlled conditions. Several parameters, such as droplet velocity and diameter, liquid physical properties, surface conditions and geometry, wall surface temperature and ambient pressure are of key importance for the deformation of droplets upon impact and thus, define the impact outcome.

An experimental investigation of micrometric Diesel droplets impacting on a heated aluminium and a millimetric brass particle surface was carried out. Dual view high-speed imaging has been employed to visualise the evolution of the impact process at various conditions. The parameters investigated include wall surface temperature ranging from room temperature to above Leidenfrost temperature (~420°C), impact Weber, and Reynolds numbers and ambient pressure of 1 and 2 bar. The observed post-impact outcome regimes are defined by means of hydrodynamic regimes and droplet morphology (stick, splash, break-up and rebound); then for each surface geometry, the identified impact outcomes were illustrated on regimes maps as a function of surface temperature and impact Weber number. Comparisons with the available experimental data for the single component fluids clearly shows significant differences, especially in terms of transition to Leidenfrost and breakup regimes; differences in liquid composition and non-homogeneity of the Diesel fuel droplet at the temperature above any of its component’s boiling temperature, results in different flow process and evaporating behaviour during the impact, and consequently the final outcome.

Moreover, the temporal variation of the apparent dynamic contact angle and spreading factor has been determined as a function of the impact Weber number and surface temperature. The experimental results were compared against available numerical simulations, performed using a two-phase flow model with interface capturing, phase-change and variable physical properties in order to fully understand the physical mechanism behind the observed results; Increased surface temperature resulted to different spreading dynamic, in particular induced quicker and stronger recoiling behaviour, mostly attributed to the change of liquid viscosity. It has been also shown that the extension of the lamella spreading diameter on a spherical surface is larger than on a flat surface, which is due to the presence of the gravitational and centrifugal forces; yet the centrifugal force is the dominant effect.

In addition, a series of experimental results focusing on: (i) the effect of physical properties and additives on isothermal impact of fuel droplets onto the flat and inclined substrates and (ii) oblique droplet-particle impact, are reported. These parts of the work are included in the appendices as such results were known already from the literature (Appendix A and B), or a pilot study and thus not conclusive (Appendix C) to be presented in the main body of the thesis.

Publication Type:	Thesis (Doctoral)
Subjects:	T Technology > TJ Mechanical engineering and machinery
Departments:	Doctoral Theses School of Science & Technology > Engineering School of Science & Technology > School of Science & Technology Doctoral Theses

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