Heat release analysis for rapid compression machines: Challenges and opportunities
Introduction
Heat release analysis (HRA) has a long history in application to both internal combustion (IC) engines [1], [2], [3] and fundamental apparatuses [4], [5], [6], providing a means to derive insight into governing chemical and physical processes in situations where direct measurement is challenging, or not practical. Applications exist also for numerical simulations [7].
Rapid compression machines (RCMs) are typically used to characterize the autoignition of gaseous and liquid fuels, covering a wide range of thermodynamic and fuel-loading conditions [8]. Some configurations are also capable of generating stratification and micro-mixing representative of combustion chambers, as well as integrated ignition sources to explore additional phenomena. While a number of facilities incorporate sophisticated diagnostics, such as laser-based velocimetry and sampling-based speciation, these are challenging to implement, expensive and can limit the operating range (e.g., density) of the devices [8]. As such, pressure is the primary diagnostic, where this is used to, predominantly, quantify ignition delay times via the sharp rises in pressure that accompany first-stage and main ignition.
A few groups have previously quantified the exothermic characteristics of fuels in RCM tests. Tanaka et al. [9] used pressure-time histories to define parameters such as pressure rise from first-stage ignition (ΔP1), maximum pressure rise relative to the compressed condition (ΔPmax = Pmax–Pc), and burn rate, based on the time taken for pressure to rise from 20% of ΔPmax to 80% of ΔPmax. Griffiths and Hasko [10], Tanaka et al. [11], and Shiga et al. [12] more formally derived heat release rates (HRRs) for various fuels based on energy conservation principles, incorporating reduced-order models for heat loss and an assumption of mixture uniformity. Some results were presented to illustrate how fuel structure, compressed condition, and fuel loading influence autoignition phenomenology, rates of reaction, and the development of pressure waves.
Past works have discussed challenges with HRA, and how to achieve rigorous, quantitative results. These include properly representing physical phenomena in the reaction chamber (e.g., heat loss to the walls, growth of the boundary layer, gas flow to the crevice, and condensation of heavy fuels near cold surfaces); describing the thermophysical properties of the reacting mixture, including changes in composition; and adequately recording the time-varying conditions in the reaction chamber, with minimal perturbation by the data acquisition (DAQ) system. However, there is significant potential to extract a wealth of information from pressure-time histories via HRA, including (a) quantifying the evolution and trends of preliminary exothermicity, i.e., low- and intermediate-temperature heat release (LTHR and ITHR, respectively), across a range of thermodynamic conditions; (b) detecting the existence of non-uniform ignition phenomena during a test; and (c) providing additional targets for the evaluation and improvement of chemical kinetic models.
The objectives of this work are to identify and highlight benefits of conducting HRA with RCM data, and discuss challenges that must be addressed to derive quantitative representations, at low uncertainty, of the chemical exothermicity for a range of fuels.
Section snippets
Formalism
HRA begins by first applying the energy conservation equation to the gas in the reaction chamber, as expressed inwith Us representing the total sensible internal energy, the rate of chemical enthalpy, or heat released, the rate of heat exchange with the walls, the rate of piston work on the gas, and , the rates of enthalpy flow to/from the reaction chamber, respectively. When the energy flows are adequately tracked, the excess
Experimental
Autoignition measurements are acquired in Argonne's twin-piston RCM (tpRCM). A detailed overview, as well as experimental uncertainties can be found elsewhere [21]; it is briefly described here. The tpRCM is pneumatically-driven and hydraulically controlled. A ring-groove arrangement in the hydraulic chambers is used at the end of the stroke to facilitate deceleration, while the hydraulic chambers are pressurized during the test period to minimize piston rebound at ignition. The pistons in the
Results
In this section it is demonstrated how a variety of combustion behaviors can be extracted from pressure-time histories via HRA.
Conclusions
HRA is not often, or consistently used in RCM studies to derive understandings of exothermic behavior from the measurements. There is significant potential that can be realized through its application however, including quantifying preliminary exothermicity, detecting non-uniform ignition events and improving chemical kinetic models. A number of physical modeling and experimental challenges, like adequate representation of exothermicity-induced crevice flows and DAQ capabilities, need to be
Acknowledgments
This manuscript has been created by UChicago Argonne, LLC, Operator of Argonne National Laboratory, a U.S. Department of Energy Office of Science laboratory, under Contract No. DE-AC02-06CH11357. The U.S. Government retains for itself, and others acting on its behalf, a paid-up nonexclusive, irrevocable worldwide license in said article to reproduce, prepare derivative works, distribute copies to the public, and perform publicly and display publicly, by or on behalf of the Government. The DOE
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