The objectives of the proposed research are to elucidate the two-phase flow instabilities in the massive arrays of parallel micro-channels (>10,000) in low temperature fuel cells using nonlinear analysis and to apply those approaches in devising new advanced diagnostics and control routines. Water management remains a significant technical challenge for low temperature fuel cells, such as polymer electrolyte fuel cells (PEFCs). Engineering measures to prevent severe liquid water accumulation, known as flooding, have resulted in several performance and cost compromises that include high air delivery parasitic loads and limitations on minimum plate thickness. Two-phase flow instabilities in the large arrays of parallel micro-channels are a key contributor to cell inefficiencies; however, these instabilities are not well understood or predictable. Furthermore, the influence of channel plurality (number of channels covering the electrode) on the stability has not been previously studied. The research will be performed in four phases. Phase 1: Investigate the dynamics of the two-phase instabilities using three ex situ experimental platforms: i) A single channel apparatus for flow visualization and pressure drop measurements serves as a control case. ii) A single channel with a parallel shunt channel that simulates parallel channels without inter-channel communication of two-phase flow disturbances. iii) A flow field with varying numbers of parallel channels. This three-tier approach distinguishes single-channel dynamics from those due to channel-to-channel communication of two-phase disturbances. Nonlinear time-series analysis techniques will be used to analyze the information-rich structure of the time-series. Phase 2: Establish a stability modeling framework for two-phase flow in large arrays of parallel micro-channels using a discretized, one-dimensional flow approach and advanced two-phase flow correlations. A broad parameter space will be simulated to elucidate relationships between the dynamics and the design and operating conditions. Spatio-temporal perturbations will identify marginal stability boundaries, allowing the construction of generalized flow stability maps. Phase 3: Investigate the effect of channel plurality on the in situ dynamics of operating PEFCs. Nonlinear analysis of voltage and pressure drop time-series will elucidate transitions in flow regime and onset of instability. From these data, a family of empirical stability maps will be constructed. Phase 4: Establish a diagnostic and control framework based on nonlinear techniques for chaotic systems. One component is devising an early warning detection technique, a key interest of industry. The work will establish chaos control methods to more efficiently prevent flooding. The proposed work has several avenues of new research. This work will be the first use of nonlinear analysis techniques in fuel-cell research, introducing these powerful techniques to the field. The PI will experimentally and theoretically investigate the influence of channel plurality on two-phase flow dynamics in large arrays of microchannels and its impact on fuel-cell performance. Fuel-cell developers have long known that channel plurality is an important factor in PEFC design, but it has not been fundamentally studied or quantified. To provide fundamental insight, a theoretical stability analysis will be used to construct a stability map from marginal stability boundaries. The PI will also investigate the novel approach of using nonlinear statistics for early warning detection and chaos control methods to mitigate flooding. In terms of the broader impacts, the PI aims to improve PEFC performance and robustness of operation. PEFCs are widely viewed as a key energy conversion device for a sustainable energy infrastructure. The work leverages relationships with industry for timely, practical impact. The research is also broadly applicable to other systems with two-phase flow in microchannels (e.g., electrolyzers). A targeted education, outreach, and mentoring plan is proposed. The PI will incorporate the research into his courses on fluid dynamics and energy systems. For outreach, the PI will maximize impact by leveraging a large number of established events to present engaging, hands-on activities on fuel cells and fluid dynamics. Furthermore, the Gelfand Center at Carnegie Mellon will support the efforts by helping to disseminate materials and information to Pittsburgh region schools, particularly underserved schools with at-risk student populations. In addition, the PI will integrate this research with his workshops on energy systems for teachers as part of the What is Engineering? workshop series at Carnegie Mellon.