Facility Effects

heat loss only occurs through a thin boundary layer in contact with the reactor walls, while the central core gas is unaffected, with a uniform temperature field. Calculation of the EOC conditions with relevance to the adiabatic core hypothesis are contained in section 3.3.2. IDTs are typically measured relative to the EOC temperatures, pressures, and equivalence ratio, allowing for the production of comprehensive IDT profiles (like that seen in figure 2.8). These profiles provide important information about a fuels autoignitive properties and knock resistance [85], with longer IDTs indicating less propensity for autoignition. They also provide validation targets for the development of chemical kinetic mechanisms in a thermodynamic region of high importance.

While recent shock tube efforts have achieved measurements of IDT up to 50 ms [89], shock tubes are typically used to investigate IDTs of <2 ms [86]. The RCM is capable of experimental durations typically much longer than those seen in shock tubes, with engine applicable fuel loading [95]. This experimental duration for RCMs is typically 2-150 ms, but is largely dependent on the RCM facility used and the ability of this RCM to create and maintain well specified thermochemical conditions within the reaction chamber for a relatively long period of time [86,96]. While the RCM is in theory an ideal, homogeneous reactor, several facility dependent effects and complex fluid mechanics are present in reality. These must be accounted for or minimised to facilitate the production of accurate IDT results and comparisons between RCM facilities.

2.3.1 Facility Effects

Despite the highly repeatable nature of RCM experiments and similarities in operational technique, it can be observed through literature sources that experiments under similar conditions, but produced in different facilities, are significantly different [97–101]. This makes the interpretation of RCM data found in the literature difficult, as it is often not accompanied by the characteristic errors and uncertainties. Heat losses from the combustion chamber post-compression and radical production during post-compression provide a degree of uncertainty in the final IDT measurement, but these are typically accounted for by the production of “non-reactive”

pressure traces [48,86], as described in section 3.3.3. A further large source of uncertainty in IDT measurements is the degree of homogeneity of the combustion chamber environment, which may vary considerably between different RCM designs and is largely affected by complex fluid dynamics initiated by the piston compression event [48,100,102]. The influence of fluid dynamic effects on the measured IDT is more difficult to account for and detect than the presence of heat losses and compression radical formation, as it does not always produce a

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clearly observable effect on the RCM pressure trace (a history of transient pressure measurements within the reaction chamber). However, these effects may cause the ultimate failure of the adiabatic core hypothesis due to mixing of the hot core and cool boundary layer gases, making the accurate determination of temperature difficult.

Figure 2.10. An illustration of the formation of roll-up vortices during the compression of a flat piston and containment of the boundary layer by a creviced piston [48].

Inhomogeneities in the reaction chamber, due to fluid motion during the compression stage, have been observed as far back as the 1950’s, when Schlieren and direct imaging techniques observed non-uniformities within the reaction chamber due to the interaction of the piston and the boundary layer gas [103,104]. This interaction results in the creation of a roll-up vortex, which mixes cooler gas from the boundary layer with hot gases in the adiabatic core region, as shown in figure 2.10 [105]. In the study of Clarkson et al. [106] it was observed, by the application of Rayleigh scattering and acetone laser induced fluorescence to image the temperature field within the RCM reactor, that by the EOC, roll-up vortices had penetrated the centre of the core gas. The resultant temperature difference between the hot gases and the cool roll-up vortices gases was estimated to be 50 K [106]. It is clear from these results that the influence of these vortices on the homogeneity of the temperature environment is significant.

Griffiths et al. [107] showed that, for di-tert-butyl peroxide, reactions proceeded faster in these localised regions of higher temperature. These observations have been reinforced by further experimental measurements [108–110] and multidimensional simulation results [102,111–113], which display the presence of similarly induced inhomogeneities.

To limit the influence of detrimental fluid motion on the development of a uniform reactor environment, Park and Keck [105] made a series of recommendations for RCM design. For the RCM design applied in their study, it was shown through scaling analysis that the piston velocity should be limited to 10-20 m/s to maintain a laminar boundary layer during compression and

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avoid any heating of the gas mixture by sound waves, potentially generated by high piston velocities. To reduce the mixing of the boundary layer with the hot core gas, Park and Keck [105] proposed the addition of a crevice around the circumference of the piston head. This crevice provides a volume in which the boundary layer can be captured during the piston motion.

An example of this creviced design can be seen, compared to a flat piston head, in figure 2.10.

This original crevice design was further developed, for the same RCM configuration, by Lee and Hochgreb [102]. Several design recommendations were given with respect to the geometry of the creviced piston, stating that: the crevice volume should be large enough to contain the boundary layer gas during compression; the crevice should be shaped such that boundary layer gases are quickly cooled upon entering the crevice (preventing reactivity in these gases); and the clearance between the piston bore and reaction chamber wall should be sufficient to capture the full boundary, but small enough to limit the backflow of the boundary into the chamber after compression [102]. Similar recommendations have been validated by the use of computational fluid dynamic (CFD) modelling [100,114]. A novel sabot shaped floating piston head has also been applied to effectively capture the boundary layer gases, utilising similar principles [115].

Crevice geometry was further optimised to improve reactor homogeneity in several studies, including the work of Mittal and Sung [116] and Würmel and Simmie [100], where CFD simulations of several crevice geometries were performed, utilising laminar flow and k-ε models, respectively. From these studies, the optimal crevice volume was found to be between 9-14 % of the reaction chamber volume and was dependent on the diluent gasses used. It was also shown that, increasing the length of the crevice inlet channel does not affect the fluid mechanics of boundary layer capture, but does produce a cooler captured gas. Würmel and Simmie [100]

showed that the optimal geometry for this channel (in terms of boundary layer capture) was rectangular, with an optimum depth of 1.0 mm and length of 4 mm. Channel depths greater than 1.5 mm captured more boundary layer gas due to a more unrestricted flow, but the gas captured become hotter with increasing channel depth and a backflow of this gas into the reaction chamber was observed. While an angled design was seen to help somewhat with cooling of the crevice gas, it provided no obvious improvements in the homogeneity of the resultant temperature field [100].

The effectiveness of the creviced piston design was further investigated experimentally.

Mittal and Sung [114] applied the recommendations of their CFD modelling study [116] to produce an optimised creviced piston, the influence of which on the temperature environments was investigated. Acetone planar laser induced fluorescence (PLIF) measurements for RCM

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compression with a flat and creviced piston head showed that the flat piston head produced significant mixing of the cool boundary layer gas with the hot core gas, as shown in figure 2.11.

These PLIF measurements were taken at a compressed pressure of 39.5 bar and a compressed temperature of 770 K. The creviced piston displayed none of this temperature mixing up to a time of 200 ms after the EOC [114], producing a uniform temperature field.

However, at timescales longer than 200 ms the uniformity of the temperature field decayed significantly for the creviced piston also, due to mass transfer between the crevice and reaction chamber gases. Furthermore, the introduction of a creviced piston may cause additional multidimensional effects, which impact the integrity of zero dimensional modelling approaches in these cases [48,117–119]. This includes mass flow from the main reaction chamber to the crevice volume during cases of multi-stage ignition, which causes a reduction in the measured pressure rise and may affect the determination of IDT in these cases [117,120,121]. Several studies have applied crevice containment techniques in an effort to eliminate mass transfer between the reactor and crevice post-compression, physically separating the crevice volume from the reaction chamber with a seal [120]. This technique has been shown to produce a significant reduction in the post-compression pressure drop [120,121].

Figure 2.11. Acetone PLIF intensities and the corresponding derived temperature distributions for (a)-a flat piston head and (b)-a creviced piston head, at 2 and 25 ms after the EOC. Solid

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grey lines show the measured fluorescence intensities. Dashed grey lines show the fluorescence intensity for a fully homogeneous temperature environment. Red lines show the temperature distribution. Measurements made at Tc=770 K and Pc=39.5 bar. Adapted from [114].