Air Quality Implications of Using Ammonia as a Renewable Fuel: How Low Can NOx Emissions Go?

In addition to their lifecycle carbon emissions, another important issue with decarbonized energy pathways is their air quality, water, or land use implications. This paper considers the air quality issue for ammonia combustion. When directly combusting ammonia, reactions of its N atom with atmospheric oxygen lead to NOx emissions that are O(103) ppm, 2 orders of magnitude higher than EPA limits or the amount emitted by current natural-gas-fired technologies. In order to provide guidance to policymakers and technologists on what is fundamentally possible, this Perspective analyzes the fundamental minimum NOx emissions that can be produced from ammonia combustion. The analysis shows that it is possible to achieve quite low NOx emission levels of O(10) ppm, but these designs differ markedly from those used in today's lean, premixed combustion systems.

Simultaneous multiple laser beam intensity profile correction and its application to a vitiated bluff body combustor field.

Simultaneous high-speed stereo-particle image velocimetry, OH planar-laser-induced fluorescence (PLIF), and CH2O PLIF measurements in a vitiated bluff body combustor are considered. An ex situ, simultaneous, time-resolved laser sheet intensity profile correction procedure is introduced. This procedure is easily implemented experimentally and is capable of correcting multiple sheets at the same time. As a proof of concept, the procedure is applied to perform correction of the CH2O PLIF images in vitiated and unvitiated conditions. The challenges associated with CH2O PLIF under these combustor operating conditions are also discussed.

Characterization of forced response of density stratified reacting wake.

The hydrodynamic stability of a reacting wake depends primarily on the density ratio [i.e., ratio of unburnt gas density (ρu) to burnt gas density (ρb)] of the flow across the wake. The variation of the density ratio from high to low value, keeping ρu/ρb>1, transitions dynamical characteristics of the reacting wake from a linearly globally stable (or convectively unstable) to a globally unstable mode. In this paper, we propose a framework to analyze the effect of harmonic forcing on the deterministic and synchronization characteristics of reacting wakes. Using the recurrence quantification analysis of the forced wake response, we show that the deterministic behaviour of the reacting wake increases as the amplitude of forcing is increased. Furthermore, for different density ratios, we found that the synchronization of the top and bottom branches of the wake with the forcing signal is dependent on whether the mean frequency of the natural oscillations of the wake (fn) is lesser or greater than the frequency of external forcing (ff). We notice that the response of both branches (top and bottom) of the reacting wake to the external forcing is asymmetric and symmetric for the low and high density ratios, respectively. Furthermore, we characterize the phase-locking behaviour between the top and bottom branches of the wake for different values of density ratios. We observe that an increase in the density ratio results in a gradual decrease in the relative phase angle between the top and bottom branches of the wake, which leads to a change in the vortex shedding pattern from a sinuous (anti-phase) to a varicose (in-phase) mode of the oscillations.

Fiber-coupled LWIR hyperspectral sensor suite for non-contact component surface temperature measurements.

We report on the development of a robust fiber-coupled long-wavelength infrared (LWIR) hyperspectral sensor suite for accurate and reliable non-contact surface temperature measurements in propulsion systems with limited optical access. We first experimentally investigate various state-of-the-art LWIR optical fibers and identify the ideal fiber for efficient coupling and transmission of LWIR signals. The effects of the fiber material, structure, bending, and thermal heating on LWIR fiber transmission are characterized. Subsequently, we discuss the development of a fiber-coupled LWIR hyperspectral sensor using a multi-mode polycrystalline fiber. The temperature measurement accuracy and precision of the sensor are determined using a well-calibrated blackbody radiation source and heated thermal barrier coating. The sensor is integrated into a homemade water-cooled probe housing and environmental protection box and subsequently used for reliable combustor liner temperature measurements in a high-pressure, liquid-fueled combustor rig with no built-in optical access. We also discuss the measurement challenges associated with flame interference and potential solutions. The LWIR sensor shows significant promise in its application to surface temperature measurements, and our findings can aid propulsion system engineers and researchers in system design and operation optimization.

Nonlinear dynamics and intermittency in a turbulent reacting wake with density ratio as bifurcation parameter.

The flame or flow behavior of a turbulent reacting wake is known to be fundamentally different at high and low values of flame density ratio (ρ_{u}/ρ_{b}), as the flow transitions from globally stable to unstable. This paper analyzes the nonlinear dynamics present in a bluff-body stabilized flame, and identifies the transition characteristics in the wake as ρ_{u}/ρ_{b} is varied over a Reynolds number (based on the bluff-body lip velocity) range of 1000-3300. Recurrence quantification analysis (RQA) of the experimentally obtained time series of the flame edge fluctuations reveals that the time series is highly aperiodic at high values of ρ_{u}/ρ_{b} and transitions to increasingly correlated or nearly periodic behavior at low values. From the RQA of the transverse velocity time series, we observe that periodicity in the flame oscillations are related to periodicity in the flow. Therefore, we hypothesize that this transition from aperiodic to nearly periodic behavior in the flame edge time series is a manifestation of the transition in the flow from globally stable, convective instability to global instability as ρ_{u}/ρ_{b} decreases. The recurrence analysis further reveals that the transition in periodicity is not a sudden shift; rather it occurs through an intermittent regime present at low and intermediate ρ_{u}/ρ_{b}. During intermittency, the flow behavior switches between aperiodic oscillations, reminiscent of a globally stable, convective instability, and periodic oscillations, reminiscent of a global instability. Analysis of the distribution of the lengths of the periodic regions in the intermittent time series and the first return map indicate the presence of type-II intermittency.