ISBN 9783843902069

978-3-8439-0206-9, Reihe Thermodynamik

Ingo Stotz
Shock Tube Study on the Disintegration of Fuel Jets at Elevated Pressures and Temperatures

222 Seiten, Dissertation Universität Stuttgart (2011), Softcover, A5

Zusammenfassung / Abstract

A new double diaphragm shock tube was developed for the investigation of fluid injection, disintegration, and mixing at elevated ambient and fuel pressures and temperatures. Its capabilities allow for the systematic study of fuel injection over a wide range of chamber pressure and temperatures. In addition, the state of the fuel can be varied as to perform sub-, near-, and supercritical jet disintegration experiments.

The performance of the shock tube has been evaluated numerically and experimentally over a wide range of operating conditions, using mixtures of helium and argon as the driver gas and pure argon as the driven gas. Emphasis was placed on the reproducibility and uniformity of the test conditions. In order to compensate for shock attenuation, a variable-area driver insert has been designed and tested ensuring conditions uniform in space and time.

The fuel was injected in the quiescent region behind the reflected shock wave using a temperature-controlled, fast-response fuel injector. As the fuel, n-hexane was used as a representative for hydrocarbon fuels. The fuel jets were investigated by means of high-speed schlieren, shadowgraph, and planar laser light scattering imaging. Schlieren and shadowgraph images were used to visualise the overall penetration and spreading of the jets, while in the elastically scattered light images only the liquid phase was revealed. Based on the shock tube experiments, the main features of each disintegration regime have been analysed and discussed and the findings compared to literature data.

A survey of the high-speed images revealed strong differences between injection modes. For subcritical fuel temperatures, the general trends of standard liquid atomisation were observed. For supercritical fuel temperature completely different behaviour was found. Jet expansion and the formation of shock structures similar to those found for underexpanded ideal gas jets is observed. Additionally, fuel condensation in the jet plume occurred. Analysis of the thermodynamic injection paths confirmed that for supercritical fuel temperatures the fuel jets were choked at the nozzle exit plane and the jets subsequently accelerate to supersonic velocity during expansion.

The study provided new results towards the understanding of supercritical fuel injection and breakup processes. Fuel-preheating to superheated and particularly to supercritical temperatures shows enhanced jet disintegration, fuel vaporisation, and mixing.