Determination of Ignition Delay Time

Autoignition behaviour is often characterised by the experimental measurement of the ignition delay time, τi. Another parameter of interest, within τi, is the excitation time, τe, this is the period of time in which the majority of heat release occurs. However, whereas τi is typically measured in the order of milliseconds, τe values are in the order of microseconds, making there accurate experimental measurement impossible due to insufficient response times achievable from dynamics pressure transducers, particularly under such high pressures and temperatures, where thermal shock becomes an issue.

There have been many measurements of τi for a variety of fuels and blends under different equivalence ratios, temperatures and pressures using a variety of techniques and apparatus, reference to which are given in the following subsections for three of the most common experimental techniques.

The complex flow fields within engines and turbines serve to obscure τi measurements, therefore a common theme amongst typical τi measuring apparatus is to negate such effects by simplification of the environment in which it is measured. Three common techniques are introduced below: Flow reactors, shock-tubes and that used in the present work, rapid compression machines. Figure 1.9 shows their typical operational boundaries.

Chapter 1 - Introduction

23 Figure 1.9: Typical operational boundaries of shock-tubes, rapid compression machines, and flow reactors. A comparison to a representative ignition delay curve of iso-octane is included; ignition delay of iso-octane is obtained from the reduced mechanism of Pepiot-Desjardins and Pitsch (2008) at an equivalence ratio of 0.6 and a pressure of 2.0MPa (Grogan et al., 2015).

1.5.1.1 Flow Reactors

Flow reactors have been successfully employed in the measurement of τi values for many years, with Mullins (1951), Lezberg (1957) Chang et al., (1958) and Miller (1958) acquiring some of the earliest data. Figure 1.10 shows a schematic of typical flow reactor. Essentially, they comprise of a cylinder duct in which a preheated turbulent oxidiser (usually air) flow mixes with fuel that is usually radially injected, slightly downstream, this creates a homogeneous premixed combustible mixture, and under the heat and pressure of the flow, ignition of the combustible mixture occurs sometime further downstream.

Figure 1.10: Schematic of a typical flow reactor (Beerer et al., 2009).

Chapter 1 - Introduction

24 The measurement of τi is defined between the point at which the fuel and oxidiser are sufficiently mixed and the point at which ignition occurs, which is usually detected by light emission or a pressure spike. Heating is usually limited to 1000K and most systems operate in the range of 0.1-3.0 MPa. Using optical access, Beerer et al., (2009) employ a laser/photo-detector system to accurately detect the premixed charge just after the fuel is injected and photodiodes to detect luminescence at the onset of combustion.

Fig. 1.11 shows a typical measurement of τi for a methane/air mixture using this setup.

Figure 1.11: Measurement of τi for methane/air mixture using laser detection for entering mixture and photodiode detection for luminous from onset of combustion (Beerer et al., 2009).

1.5.1.2 Shock Tubes

Shock tubes are generally used in the measurement of relatively short τi values under the high temperatures (Campbell et al., 2015; Davidson et al., 2005; Hawthorn and Nixon, 1966; Zhu et al., 2015). Figure 1.12 shows an operational schematic of a typical shock tube. They comprise of a length cylindrical duct in which an inert high pressure driving gas is separated from a relatively low pressure combustible test mixture via a thin diaphragm. Depending on the pressure ratio between the two gases, this is typically made from aluminium, copper or steel. Upon rupturing the diaphragm, usually by means of an electronically operated needle, a shock wave is generated by the high pressure driving gas as it enters the low pressure mixture zone. This shock wave, known as the incident shock wave, compresses the mixture up to a high autoignition pressure

Chapter 1 - Introduction

25 and temperature to allow ignition to take place, with a reflective shock wave returning down the duct.

Figure 1.12: Schematic of a typical shock (http://www.ucalgary.ca/johansen/node/7.)

As indicated by the number five in Fig. 1.13 measurements of τi values are limited by the time between the point at which the incident shock wave compresses the mixture to sufficient pressure and temperature within the test zone and the return of the reflective shockwave, which is a function of the contact surface between that of the inert driving gas and the test mixture. Thus, only measurements of τi values shorter than this duration are possible and are typically within the range 1-3ms.

Chapter 1 - Introduction

26 Figure 1.13: Wave system in the shock tube (Campbell et al., 2015).

However, recent developments have successfully increased this range by methods such as specifically tailored driving gases and extended driving gas sections, this has shown to increase the distance between the shock wave and the driving gas contact surface, resulting in increased times up to 55ms (Campbell et al., 2015).

1.5.1.3 Rapid Compression Machines

Many τi values for a variety of fuels over an array of conditions have been measured using rapid compression machines (Affleck and Thomas, 1968; Griffiths et al., 1993;

Grogan et al., 2015; Mittal et al., 2014). They have since undergone much refinement with advances in technology allowing for higher pressures and temperatures to be achieved, typically in the region of 2.0-4.0 MPa and 900-1500K, and are responsible for a vast array of τi measurements for a large variety of fuels. Regardless of different operating mechanisms, a common theme to all RCM’s is the simplistic nature of rapidly compressing a premixed combustible mixture within a cylinder via a piston to a constant end of compression (EOC) volume. The compression increases the mixture pressure and temperature to autoignition conditions at EOC, and a pressure spike indicates the onset of ignition.

Chapter 1 - Introduction

27 The value of τi is generally defined as the duration between the EOC and the point of the maximum rate of pressure rise (Griffiths and Nimmo, 1985; Mittal, 2006; Westbrook et al., 1998). Rapid compression is essential to reduce the likelihood of any pre-reactions during the pressure and temperature rise of compression. RCM’s are generally well suited to the measurement of longer τi values due to their ability to hold the required autoignition EOC pressures and temperatures for relatively long periods, typically over 100ms, which is typically two fold that of shock tubes (Mittal and Bhari, 2013).