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Fuel injection from subcritical to supercritical P-T conditions: a unified methodology for coupled in-nozzle flow, atomisation and air-fuel mixing processes

Periodic Reporting for period 1 - UNIFIED (Fuel injection from subcritical to supercritical P-T conditions: a unified methodology for coupled in-nozzle flow, atomisation and air-fuel mixing processes)

Reporting period: 2018-10-08 to 2020-10-07

Fossil fuel consumption is expected to increase by over 20% over the next 3 decades in order to meet the increasing demand for infrastructure, trade and transportation. Development of engines complying with the forthcoming 2020 emission legislations, relies on the effective design of advanced high-pressure fuel injection systems and represents a key industrial priority.

Such advanced injection systems can improve internal combustion engine performance in the following ways:
- Formation of high velocity fuel jets that lead to finer atomization and better air/fuel mixing.
- Increase of engine efficiency.
- Reduction of soot and CO2 emissions.

The objectives of the present project involve the study of high pressure fuel systems injecting fuel at high temperature and pressure conditions, well above the fuel’s critical point; the prevailing supercritical fluid conditions result to disappearance of the liquid-gas interface, which in turn, reduces vaporisation time and enhances significantly air-fuel mixing. The project will involve experiments (outgoing phase at Sandia National Laboratories) and simulations (return phase) to study the phenomena occurring during fuel injection and how they can be exploited to improve engine design.
The work performed so far involves numerical predictions / simulation methodologies and experimental procedures/results that refer to fuel sprays.

Concerning numerical predictions/simulations: an advanced numerical methodology for handling (Diesel) fuel sprays at elevated temperatures and pressures has been developed. This methodology involves two aspects:

(1) thermodynamic property modelling, including non-ideal effects that occur at the extreme conditions of modern fuel injection systems. This modelling involves high order Equations of State (e.g. PC-SAFT, NIST Refprop) to accurately predict property variation and consequent thermodynamic effects.
(2) diesel fuel spray simulations at modern engine conditions, which exceed the fuel's critical point (T>700K, p>20bar).

Concerning experiments for fuel studies, several aspects have been pursued:
(1) one refers to the use of various diagnostics (e.g. Schlieren for vapor penetration, Diffuse Backlight Illumination for liquid penetration) for examining various gasoline-like fuels at standardised conditions, as established by the Engine Combustion Network (ECN).
(2) the second refers to the further development of a 3d tomographic reconstruction technique, that is capable of producing 3D volumetric distribution of fuel sprays for quantitative comparisons of the liquid penetration.
(3) the third refers to the visualisation of internal flow in fuel injector orifices. The orifices are transparent, made of acrylic for refractive index matching with the diesel fuel and are based on standardised geometries publicly available through the Engine Combustion Network (ECN). These orifices have been examined with optical methods (Diffuse Back Illumination and Schlieren / Shadowgraphy) to determine fuel penetration characteristics and/or correlation to turbulence and cavitation.

The aforementioned aspects have been investigated and important findings (validated methodology and predictions) have been published in the relevant journals (see also publication section).
"The present program advanced the state of the art both through numerical simulation tools and experimental techniques.

The advancements in simulation tools offer a better understanding of spray characteristics and related effects, by including accurately thermodynamic property variations and induced effects such as viscous heating and Joule-Thomson cooling. The inputs needed for thermodynamic modelling are based on the molecular characteristics of the fuel, providing a versatile tool for examining ""what-if"" scenarios, which can be exploited in design studies in fuel and automotive industries.

The fuel studies on gasoline-based blends is closely associated with the standardisation of fuels, investigation of fuel surrogates and composition on spray characteristics, relevant to different engine operating conditions (e.g. cold start, early injection, high temperature etc), which subsequently affect emissions and performance.

Hence this work is a direct advancement in the quest of efficient, less polluting engines, either diesel or gasoline. However, on the more fundamental level, the diagnostics developed and used are relevant to the understanding of sprays (volumetric distribution of liquid, probability density functions of droplet sizes), relevant even to biomedical applications."
Dodecane mass fraction distribution of high temperature/high pressure planar dodecane spray.
Vortical structures during dodecane injection in nitrogen, ECN Spray-A conditions.
Vortical structures during dodecane injection in nitrogen, ECN Spray-A conditions.
Temperature distribution of high temperature/high pressure planar dodecane spray.