Simplified Approach to the Prediction and Analysis of Temperature Inhomogeneity in Rapid Compression Machines

Abstract

1Substantial progress has been made in understanding and reducing temperature inhomogeneity in rapid compression machines (RCMs) with the help of computational modelling. To date, however, it has not been possible to investigate and map the full range of possible RCM designs, working gases and operating conditions. In this article, we present a framework which simplifies the task of comprehensive and general RCM performance prediction. A set of thermophysical and geometrical parameters have been defined to characterize the design and operating conditions of a general RCM. Dimensional analysis was applied to reduce the number of variables and a sensitivity analysis, based on computational simulations, was used to rank the dimensionless parameters and eliminate unimportant ones. The results of this analysis show that Reynolds number, Prandtl number, aspect ratio, and crevice volume ratio are the most important parameters determining temperature inhomogeneity.A further set of computational simulations was conducted to predict post-compression temperature inhomogeneity over the full range of RCM design and operating parameters. These results are well represented by a simple power law equation that correlates a dimensionless temperature inhomogeneity parameter (mass-averaged over the main chamber) as a function of post-compression time with just three parameters -Peclet number (the product of Reynolds and Prandtl numbers), aspect ratio, and crevice volume ratio. This equation can serve as a simple and general tool for RCM designers and users who wish to determine optimal configurations that minimise temperature inhomogeneity for combustion experiments.2

Advances in rapid compression machine studies of low- and intermediate-temperature autoignition phenomena

“…From these studies, it is clear that properly configuring the piston crevice for particular machine geometry and operating conditions is key to adequately suppressing the corner vortex formation, and that a single configuration may not be appropriate across a wide range of facilities, or a range of experimental conditions explored during an experimental campaign. Yousefian et al [150] used non-dimensional scaling to highlight that Peclet number, bore:stroke aspect ratio, and crevice volume : swept volume ratio are key overall parameters towards suppressing fluid motion and minimizing thermal non-uniformities.…”

Simulation of turbulent flow in a rapid compression machine: Large Eddy Simulation and computationally efficient alternatives for the design of ignition delay time experiments

“…Three simulations are conducted for different mesh sizes at the initial pressure and temperature of 50 kPa and 298 K for nitrogen, which is described in detail in the mesh methodology section. The results of previous study [9] showed that various initial pressures resulted in different velocity and temperature distributions inside the RCM main chamber. Therefore, another simulation is conducted for the selected mesh at initial pressure and temperature of 100 kPa and 298 K for nitrogen to study the effect of operating conditions.…”

“…In this study, seven LES simulations are conducted to study mesh sensitivity, the effects of operating conditions on flow regimes, and the assessment of temperature inhomogeneity correlation which was developed in previous study [9]. Three simulations are conducted for different mesh sizes at the initial pressure and temperature of 50 kPa and 298 K for nitrogen, which is described in detail in the mesh methodology section.…”

“…Mittal et al [8] experimentally studied autoignition of iso-octane with and without crevice containment using a laminar model and showed that post-compression induced mass transfer to the crevice can lead to considerably longer ignition delays. Yousefian et al [9] presented a simplified framework for the prediction of temperature inhomogeneity in RCMs using a laminar model. The approach was applicable to any RCM configuration and any gas mixture properties.…”

“…Moreover, there is a need to characterize the performance of an operating RCM for a range of operating condition and crevice configuration to assess the level of temperature inhomogeneity using more computationally-efficient tools. Therefore, the objectives of this work are: (1) to characterize the in-chamber fluid velocity and temperature fields in the NUI Galway twin-piston RCM using a three-dimensional (3D) large eddy simulation (LES) approach, (2) to compare the post-compression temperature field predictions of axisymmetric two-dimensional (2D) laminar and 3D LES approaches, (3) to compare post-compression temperature inhomogeneity predictions given by 2D laminar CFD, 3D LES CFD and a previously-developed correlation [9], and (4) to propose a computationally-efficient framework for RCM testing campaigns.…”