59th Annual Report on Research 2014

      The main objectives of this project can be summarized as (1) experimentally-determining iso-octane ignition delay times and their susceptibility to heat loss effects and (2) using these data to recommend a heat loss modeling approach for rapid compression machine (RCM) experiments.

      Experimental measurements were proposed in a variable-speed compression-expansion machine (VSCEM) that rapidly compresses a reactive mixture to elevated temperatures and pressures to induce autoignition. The VSCEM is designed so that during the ignition delay period, the induced mixture will be slightly expanded to cool the mixture. Operating conditions of the VSCEM allow this rate of expansion to be altered, and provide the basis for experimentally emulating the influence of heat loss characteristics on ignition delay measurements. A schematic of the VSCEM appears in Figure 1, where it is noted that the shape of the cam can be altered to change the expansion (i.e., emulated heat loss) characteristics for an experiment. More mild changes to the expansion characteristics can be achieved by changing the operating conditions and working fluid in the hydraulic brake. Photographs of the VSCEM appear in Figures 2 and 3.

Figure 1. Top-down VSCEM schematic.

      The VSCEM was commissioned shortly after the start of the project, but one of the components failed during testing, damaging the machine and requiring a redesign of some components. During a test, tensile failure of the piston rod connecting the pneumatic cylinder rod to the linear cam occurred, causing the cam to become airborne. The cam and rod assembly was damaged in the process as it collided with the wall. The weak point in the design has been identified and the redesign completed. The new parts are presently being fabricated, and the PI estimates that the iso-octane ignition delay measurements will resume in one month.

Figure 2. Photograph of the VSCEM showing the test chamber.

Figure 3. Photograph of the VSCEM showing the linear cam.

      Given the delays in experimentation, we have been conducting computational fluid dynamics (CFD) simulations to explore the influence of non-uniform boundary conditions on ignition delay time measurements from RCMs. These simulations support this research project because both efforts are directed at gaining additional insight into the lack of agreement in experimental data from different RCMs. In heated RCM experiments, open loop temperature control is often used and can lead to non-uniformities in the boundary temperatures, especially on the piston crown. The non-uniform boundary temperatures lead to a non-homogeneous initial temperature field in the gas. The role of this thermal heterogeneity is disregarded when modeling RCM experiments, but no analysis has been done to determine whether that assumption is acceptable. In these simulations, an initial condition is obtained by performing a steady-state simulation of the temperature field using a set of prescribed boundary conditions. The case of fully-uniform boundary temperatures is compared to the case where the piston crown is 25 K cooler than the surrounding walls (at 398 K). The initial temperature fields for these cases appear in Figure 4. Qualitative analysis of the temperature fields that develop during the simulation suggest that a creviced piston is effective at suppressing the influence of the initially-cool boundary layer on the compressed temperature field (Figure 5). Simulations with a flat piston (not shown) indicate that the non-uniform boundary temperatures play a more influential role on the compressed temperature field. The temperature field analysis has been conducted using inert mixtures. Ongoing simulations are assessing how the non-uniformities impact the actual ignition delay times. Simulations of the compressed temperature fields were reported in January 2014 at the 52^nd AIAA Aerospace Sciences Meeting, which was sponsored by the American Institute of Aeronautics and Astronautics. The additional simulation work for ignition delay times will be used as the basis for a peer-reviewed journal publication.

Figure 4. Initial temperature fields for uniform boundary and “cool piston” cases.

Figure 5. Compressed temperature fields for uniform boundary temperatures and a cool piston case where the piston crown is 25 K cooler than the wall temperatures. Temperature fields are compared at various post-compression times.

      We have also used the CFD model to assess how the expansion stroke of the VSCEM will influence temperature field homogeneity when a creviced piston is used. Fluid dynamic effects are minimized when using a creviced piston as the boundary layer mass flows into the crevice. The reversal of this flow during expansion will influence the temperature field in the VSCEM. Simulation results shown in Figure 6 indicate that even at rapid expansion rates, the core region of the gas remains relatively unaffected by the crevice flow. These simulations will be an important justification for treating the core gas in the VSCEM experiments as being thermally-homogeneous.

Figure 6. VSCEM temperature fields during expansion. Impact of Research

      Preliminary simulation results from this project have been reported in a conference venue, and have provided insight into the influence of non-uniform boundary conditions on compressed temperature field development in heated RCM experiments. These results are significant because of the lack of agreement in RCM data in the literature. These simulations suggest that boundary conditions play a minor role in the discrepancy provided that a creviced piston is used for the experiments. The PI expects that the forthcoming iso-octane ignition delay data will be highly-valuable for the RCM testing community when determining how to compare their data with other measurements in the literature. By obtaining the grant award early in his career cycle, the funding has been very critical to jump-starting the PI’s career. It has provided recognition within the College of Engineering and has supported the PI’s successful application for additional internal funding. New sources of external funding have been extensively sought, but with no success to date. One graduate student (John Neuman) is supported by the grant, and working on the project has been critical for his development as a researcher. The project has provided Mr. Neuman the opportunity to learn about ignition chemistry, CFD modeling, numerical analysis, and reactor design.