CFD study of the Effect (1)

2019

One of the conditions for controlling the aerodynamics in the reaction chamber is designing a crevice volume on the surface of the piston head. The importance of the crevice volume is to contain the cool boundary layers generated as a resulting of the moving reactor piston. However, this crevice volume consequently drops the end gas pressure and temperature at the end of the stroke. The CFD study of the aerodynamic effect of a piston movement in a reaction chamber was modelled using the commercial code of Ansys Fluent and assuming a 2-Dimensional computational moving mesh. A starting optimal crevice volume of 282 mm3 was used for further optimisation. This resulted in five crevice lengths of 3 mm, 5 mm, 7 mm, 9 mm and 12 mm, respectively. The crevice height of 5 mm was found to improve the compressed gas pressure at the end of the stroke to about 2 bar and temperature about 17.7 K and also maintained a uniform temperature field, while that of 12 mm had the least peak compressed gas ...

Combustion and Flame, 2006

The aerodynamics inside a rapid compression machine after the end of compression is investigated using planar laser-induced fluorescence (PLIF) of acetone. To study the effect of reaction chamber configuration on the resulting aerodynamics and temperature field, experiments are conducted and compared using a creviced piston and a flat piston under varying conditions. Results show that the flat piston design leads to significant mixing of the cold vortex with the hot core region, which causes alternate hot and cold regions inside the combustion chamber. At higher pressures, the effect of the vortex is reduced. The creviced piston head configuration is demonstrated to result in drastic reduction of the effect of the vortex. Experimental conditions are also simulated using the Star-CD computational fluid dynamics package. Computed results closely match with experimental observation. Numerical results indicate that with a flat piston design, gas velocity after compression is very high and the core region shrinks quickly due to rapid entrainment of cold gases. Whereas, for a creviced piston head design, gas velocity after compression is significantly lower and the core region remains unaffected for a long duration. As a consequence, for the flat piston, adiabatic core assumption can significantly overpredict the maximum temperature after the end of compression. For the creviced piston, the adiabatic core assumption is found to be valid even up to 100 ms after compression. This work therefore experimentally and numerically substantiates the importance of piston head design for achieving a homogeneous core region inside a rapid compression machine. (C.-J. Sung). chamber at the end of compression. Although in principle RCM simulates a single compression event, complex aerodynamic features can affect the state of the reacting core in the reaction chamber. Previous studies (e.g., ) have shown that the motion of the piston creates a roll-up vortex, which results in mixing of the cold gas pockets from the boundary layer with the hot gases in the core region. Such undesired mixing leads to difficulties in accurately characterizing the state of the reacting mixture.

Combustion and Flame, 2005

A transient 2-dimensional moving mesh CFD computer model was created, validated against experimental data, and used to investigate the flow and resulting temperature fields in a rapid compression machine. The sensitivity of the horizontally opposed twin-piston RCM to nonsynchronized and non-uniform piston strokes was determined and the effect of non-uniform heating on resulting pressure profiles was investigated. Predictions of the ignition temperature in a rapid compression machine are made very difficult due to the existence of a highly non-uniform temperature field at the end of the compression stroke. An optimally designed piston head crevice, determined by a number of criteria, can largely overcome this problem by eliminating the mixing of the cool boundary layer gas with the hot compressed core gas. We used the CFD model to optimize the piston head crevices for our RCM and determined some new factors that are important when optimizing the piston head crevice design. Our best crevice design was then applied to a range of test gases and recommendations regarding the use of these as bath gases were made.

Fuel, 2012

The performance of a rapid compression machine (RCM) with a creviced piston is assessed over a range of operating conditions through computational fluid dynamics simulations with systematic demonstration of the effects of compressed gas pressure, temperature, stroke length, and clearance on altering vortex formation and temperature homogeneity inside the reaction chamber. Simulated results show that as compressed gas pressure is reduced, the temperature homogeneity deteriorates due to the combined effect of thicker boundary layer and increased flow velocities. A further optimization of the creviced piston geometry is then required to completely suppress the roll-up vortex. Stroke length and clearance volume are also noted to significantly affect vortex formation. A basis for quantifying the extent of the rollup vortex is suggested and the operating regime of an RCM with a creviced piston, that is free from the roll-up vortex, is delineated. This work emphasizes the importance of assessing the performance of an RCM over the associated range of operating conditions in order to obtain reliable chemical kinetics data.

Combustion and Flame, 2011

Rapid compression machines (RCMs) typically incorporate creviced pistons to suppress the formation of the roll-up vortex. The use of a creviced piston, however, can enhance other multi-dimensional effects inside the RCM due to the crevice zone being at lower temperature than the main reaction chamber. In this work, such undesirable effects of a creviced piston are highlighted through computational fluid dynamics simulations of n-heptane ignition in RCM. Specifically, the results show that in an RCM with a creviced piston, additional flow of mass takes place from the main combustion chamber to the crevice zone during the first-stage of the two-stage ignition. This phenomenon is not captured by the zerodimensional modeling approaches that are currently adopted. Consequently, a novel approach of 'crevice containment' is introduced and computationally evaluated in this paper. In order to avoid the undesirable effects of creviced piston, the crevice zone is separated from the main reaction chamber at the end of compression. The results with 'crevice containment' show significant improvement in the fidelity of zero-dimensional modeling in terms of predicting the overall ignition delay and pressure rise in the first-stage of ignition. Although the implementation of 'crevice containment' requires a modification in RCM design, in practice there are significant advantages to be gained through a reduction in the rate of pressure drop in the RCM combustion chamber and a quantitative improvement in the data obtained from the species sampling experiments.

Journal of KONES, 2018

The main aim of this study to reproduce methane combustion experiment conducted in a rapid compressionexpansion machine using AVL FIRE TM software in order to shed more light on the in-cylinder processes. The piston movement profile, initial and boundary conditions as well as the geometry of the combustion chamber with a prechamber were the same as in the experiment. Authors by means of numerical simulations attempted to reproduce pressure profile from the experiment. As the first step, dead volume was tuned to match pressures for a non-combustion (air-only) case. Obtained pressure profile in air compression simulations was slightly wider (prolonged occurrence of high pressure) than in the experiment, what at this stage was assumed to have negligible significance. The next step after adjusting dead volume included combustion simulations. In the real test facility, the process of filling the combustion chamber with air-fuel mixture takes 15 s. In order to shorten computational time first combustion simulations were started after the chamber is already filled assuming uniform mixture. These simulations resulted in more than two times higher maximum pressure than recorded in experiments. It was concluded that turbulence decays quickly after filling process, what was also confirmed by next combustion simulations preceded by the filling process. Then the maximum pressure was significantly decreased but still it was higher than in the experiments. Based on the obtained results it was assumed that the discrepancy noticed in air cases is further increased when combustion is included. Moreover, the obtained results indicated that pre-combustion turbulence level is very low and suggested that either piston profile movement is not correct or there is high-pressure leak in the test facility.

Combustion and Flame, 2016

The behavior of transient, compressible and combusting pre-mixed methane-air jets was experimentally studied with high speed imaging in a rapid compression machine. The jets were generated with a turbulent jet ignition system, which is a prechamber initiated combustion system. The absence of physical analyses of the characteristics of premixed turbulent jets was the motivation for the present study. Experiments were completed for turbulent jet ignition system orifice diameters of 2.0, 2.5 and 3.0 mm each at lean-to-stoichiometric equivalence ratios of φ=0.67, 0.8 and 1.0. The hot jet velocity at the orifice exit was calculated using mathematical correlations. The Mach number and Reynolds number were also computed. The high speed imaging shows the influence of orifice diameter on the flame propagation and the shape and structure of vortices resulting from the turbulent jet. Results revealed a direct relationship between orifice exit area reduction and a decrease in hot jet penetration speed. There was a reduction in hot jet penetration speed with an increase in the equivalence ratio. For the orifice diameters and equivalence ratios tested here, results showed that the jet evolved downstream of the orifice exit in partial agreement with existing correlations. Moreover, the jet was turbulent with calculated Reynolds numbers of around 20,000 or greater.

Proceedings of the Combustion Institute, 2019

The study of auto-ignition under temperature stratification is of great interest. Indeed, further understanding of the thermo-kinetic interactions and its influence on the combustion propagation regime is needed. In a previous work [1], experiments in a flat piston Rapid Compression Machine (RCM) demonstrated that the apparent propagation of reaction fronts is highly influenced by the typical temperature stratification observed at inert conditions. Nevertheless, the influence of low temperature heat release (LTHR) on the internal aerodynamics and temperature of the RCM is not well understood. In the present study, we first address the LTHR-flow interaction then address the LTHR-temperature interaction. We performed 2D-PIV experiments at 10 kHz for inert and reactive lean isooctane mixtures. We averaged spatially the acceleration to present the time evolution during the cool flame period. We found that the normalized acceleration has a decreasing trend in both inert and reactive tests. No significant effect of the cool flame was observed on the trend. We performed temperature measurements using thin wire (7.6 µm) type K thermocouples at inert and reactive n-hexane mixtures (same test conditions of fig.7 in [1]). The temperature evolution of the hot (adiabatically compressed) and the colder gases were recorded when cool flame occurs. The corrected gas temperature showed good agreement with the theoretical adiabatic core temperature as well as previous measurements with toluene LIF. In the tested case, we found that the cool flame induces an equal temperature rise of approximately 110 K in both the adiabatically compressed and the colder vortex gases. These results confirm quantitatively that LTHR does not significantly affect the mixing of the

Combustion and Flame, 2008

In modeling a rapid compression machine (RCM) experiment, a zero-dimensional code is commonly used along with an associated heat loss model. However, the applicability of such a zero-dimensional modeling needs to be assessed over a range of accessible experimental conditions. It is expected that when there exists significant influence of the multidimensional effects, including boundary layer, vortex roll-up, and nonuniform heat release, the zero-dimensional modeling may not be adequate. In this work, we simulate ignition of hydrogen in an RCM by employing computational fluid dynamics (CFD) studies with detailed chemistry. Through the comparison of CFD simulations with zero-dimensional results, the validity of a zero-dimensional modeling for simulating RCM experiments is assessed. Results show that the zero-dimensional modeling based on the approach of "adiabatic volume expansion" generally performs very well in adequately predicting the ignition delay of hydrogen, especially when a well-defined homogeneous core is retained within an RCM. As expected, the performance of this zero-dimensional modeling deteriorates with increasing temperature nonuniformity within the reaction chamber. Implications for the species sampling experiments in an RCM are further discussed. Proper interpretation of the measured species concentrations is emphasized and the validity of simulating RCM species sampling results with a zero-dimensional model is assessed. (G. Mittal). have highlighted the effect of roll-up vortex caused by the piston movement during the compression stroke and the temperature nonhomogeneity induced by such fluid motion. On the basis of the suggestion of Park and Keck [6], Lee and Hochgreb [2] computationally showed that a properly designed piston crevice can suppress the vortex formation, leading to better definition of reacting core conditions by confining the cold gases to the wall regime. Recently, Mittal and Sung [7] conclusively demonstrated through experiments and simulations that the resulting temperature 0010-2180/$ -see front matter

Applied Thermal Engineering, 2016

The present study focuses on a design analysis of a shaped liquid piston compression chamber based on CFD. The liquid piston compression chamber is for application to Compressed Air Energy Storage (CAES), which can be used to even the mismatch between power generation and power demand, and, thus, the objective of the design exploration is to maximize the compression efficiency. Within the compression chamber is an open-cell metal foam medium for enhancement of heat transfer. Traditionally, the chamber has a cylindrical shape. The present study explores the effects on compression efficiency of varying the profile of cross-sectional diameter along the axis of the chamber. This leads to a compression chamber with curved walls that assumes a gourd-like shape. A set of exploratory design cases is completed using the orthogonal array concept based on the Taguchi method, hence reducing the number of realizations. CFD simulations provide insight into how the chamber shape affects the flow physics during compression. A quantitative design analysis shows that, in general, a large aspect ratio and a steep radius change of the chamber is preferred, which is in line with a visualization of the CFD flow fields. The relative importance of each different shape parameter is analyzed.