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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 217 records · Page 12
All sourcesNASA NTRSDOE OSTI
DOE OSTI2015

Large eddy simulation of a reacting spray flame with multiple realizations under compression ignition engine conditions

An n-dodecane spray flame (Spray A from Engine Combustion Network) was simulated using a detailed combustion model along with a dynamic structure LES model to evaluate its performance at engine-relevant conditions and understand the transient behavior of this turbulent flame. The liquid spray was treated with a traditional Lagrangian method and the gas-phase reaction was modeled using a detailed combustion model. A 103-species skeletal mechanism was used for the n-dodecane chemical kinetic model. Significantly different flame structures and ignition processes are observed for the LES compared to those of RANS predictions. The LES data suggests that the first ignition initiates in lean mixture and propagates to rich mixture, and the main ignition happens in rich mixture, preferable less than 0.14 in mixture fraction space. LES was observed to have multiple ignition spots in the mixing layer simultaneously while the main ignition initiates in a clearly asymmetric fashion. The temporal flame development also indicates the flame stabilization mechanism is auto-ignition controlled and modulated by flame propagation. Soot predictions by LES present much better agreement with experiments compared to RANS both qualitatively and quantitatively. Multiple realizations for LES were performed to understand the realization to realization variation and to establish best practices for ensemble-averaging diesel spray flames. The relevance index analysis suggests that an average of 2 and 5 realizations can reach 99\% of similarity to the target average of 16 realizations on the temperature and mixture fraction fields, respectively. However, more realizations are necessary for OH and soot mass fraction due to their high fluctuations.

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DOE OSTI2018

A review of control methodologies for vapor compression and absorption heat pumps; Examen des méthodes de régulation des pompes à chaleur à compression et à absorption de vapeur

Here this paper presents an overview of control systems for heat pumps, and more specifically, a critical review of control methodologies for thermally activated absorption heat pumps. The objectives of a control system are identified and discussed, including optimal system operation, ability for continuous turndown, and fast response to changes in the operating conditions. Key details of different aspects of research and development such as steady-state and transient thermodynamic models, control algorithm development, and implementation are presented. Advanced multivariable feedback controls are able to perform better than individual feedback control loops and conventional on/off controls. The knowledge of the transient response of heat pump systems through models or experimental data is crucial for advanced control system design and implementation. Recently, data-based methods have also gained interest, but they are difficult to generalize. A general outlook with limitations and the potential of different approaches, and recommendations for future work are provided.

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