Brent A. Rankin

Imaging of OH* Chemiluminescence in an Optically Accessible Nonpremixed Rotating Detonation Engine

The detonation propagating through the annular channel of an optically accessible nonpremixed rotating detonation engine (RDE) is visualized in this work using OH* chemiluminescence imaging. The fuel and air are injected from separate streams and partially premix in the channel in front of the detonation wave. The OH* chemiluminescence images allow observation of the size and shape of the detonation structure, trailing edge oblique shock wave, and possible presence of deflagration between the fuel fill region and expansion region containing detonated products. The OH* chemiluminescence images are useful for evaluating the effects of the air mass flow rate, equivalence ratio, air injection area, and fuel injection scheme on the detonation structure and its corresponding impact on RDE operation and performance. The detonation increases in height as the air mass flow rate is increased for low flow rates, experiences subtle changes in size and shape for intermediate flow rates, and transitions from one-wave to two-waves as the flow rate is further increased. For fuel lean conditions, the high OH* emissions from the detonation are distributed more broadly in space. For stoichiometric and fuel rich conditions, the high OH* emissions typically are confined to a more narrow region near the detonation wave front. The wave front is more concave with respect to the fuel fill region in front of the detonation as the air injection slot is increased from low to intermediate values. The angle between the wave front and fuel injection surface in front of the detonation becomes more acute as the air injection slot is further increased. Reducing the number of fuel injection holes has significant effects on the detonation structure including transition from one-wave to two-waves. The waves typically co-rotate with the detonations propagating in the same azimuthal direction for most conditions in which two-waves are established in the channel. Counter-rotating waves with the detonations propagating in the opposite azimuthal direction are observed for some conditions. The observation of two counter-rotating detonation waves demonstrates one occasional effect of non-ideal mixing between the fuel and air in a nonpremixed RDE. The OH* chemiluminescence images are useful for evaluating RDE models and simulations, improving fundamental understanding of the detonation structure in nonpremixed RDEs, and identifying critical design parameters that influence RDE operation and performance.

Experimental and Numerical Evaluation of Pressure Gain Combustion in a Rotating Detonation Engine

The detonation structure, pressure gain, and thrust production in a rotating detonation engine (RDE) are studied using a combination of experimental and numerical approaches. High frequency time-dependent and low frequency time-averaged static pressure and thrust measurements are acquired for a range of operating conditions and geometry configurations. Acoustic coupling between the detonation channel and air plenum is important for low air mass flow rates and large air injection slots based on analyses of the pressure measurements in the time and frequency domains. The static pressure increases across the air inlet by up to approximately 15% when utilizing a large air injection slot. The pressure increase across the air inlet demonstrates encouraging progress towards realizing pressure gain combustion in RDEs with corresponding challenges associated with isolating the inlet plenums. The time-dependent pressure measurements acquired using a semi-infinite tube arrangement and time-averaged pressure measurements acquired using a capillary tube attenuated arrangement agree to within 30% depending upon location. Quantification of the similarities and differences between the two techniques represents important progress towards acquiring quantitative time-dependent pressure measurements in the challenging environment presented by RDEs. Twodimensional simulations of the RDE capture the essential features of the flow field such as the detonation wave height and angle, trailing edge oblique shock wave, shear layer between the freshly and previously detonated products, and deflagration between the fuel fill region and expansion region containing detonated products. The presence of air purging from the plenum to the channel behind the detonation wave is suggested by the comparison of measured and simulated channel pressure distributions. The pressure, thrust, and wave speed measurements provide benchmark data that are useful for evaluating low and high fidelity simulations of RDEs and improving fundamental understanding of the critical design parameters that influence RDE operation and performance.

Periodic Exhaust Flow through a Converging-Diverging Nozzle Downstream of a Rotating Detonation Engine

Periodic exhaust flow around a conical centerbody and through a converging-diverging nozzle downstream of a rotating detonation engine (RDE) is studied in this work using experimental and computational methods. Time-dependent and time-averaged static pressure measurements are acquired along the nozzle to provide insights into the unsteadiness of the flow for a range of RDE operating conditions. Unsteady flow computations are performed on a three-dimensional domain using an unstructured finitevolume compressible flow solver and prescribing a time-dependent boundary condition at the inlet of the nozzle. The time-dependent pressure measurements and computations indicate that the periodic flow at the nozzle inlet remains periodic up to the nozzle throat. The flow is non-periodic with no characteristic frequency near the nozzle exit. The conclusions regarding the transition from periodic to non-periodic flow are supported quantitatively by statistical analysis of the time-dependent pressure measurements including temporal autocorrelation coefficients and power spectral density functions. The measurements and computations reported in this work demonstrate that the combination of a conical centerbody and converging-diverging nozzle provides a useful passive flow control technique for eliminating the periodic nature of the flow downstream of rotating detonation engines.

Imaging Fourier-transform spectrometer measurements of a turbulent nonpremixed jet flame

This work presents recent measurements of a CH4/H2/N2 turbulent nonpremixed jet flame using an imaging Fourier-transform spectrometer (IFTS). Spatially resolved (128×192 pixels, 0.72  mm/pixel) mean radiance spectra were collected between 1800  cm(-1)≤ν˜≤4500  cm(-1) (2.22  μm≤λ≤5.55  μm) at moderate spectral resolution (δν=16  cm(-1), δλ=20  nm) spanning the visible flame. Higher spectral-resolution measurements (δν=0.25  cm(-1), δλ=0.3  nm) were also captured on a smaller window (8×192) at 20, 40, and 60 diameters above the jet exit and reveal the rotational fine structure associated with various vibrational transitions in CH4, CO2, CO, and H2O. These new imaging measurements compare favorably with existing spectra acquired at select flame locations, demonstrating the capability of IFTS for turbulent combustion studies.

Overview of Performance, Application, and Analysis of Rotating Detonation Engine Technologies

Recent accomplishments related to the performance, application, and analysis of rotating detonation engine technologies are discussed. The pioneering development of optically accessible rotating detonation engines coupled with the application of established diagnostic techniques is enabling a new research direction. In particular, OH* chemiluminescence images of detonations propagating through the annular channel of a rotating detonation engine are reported and appear remarkably similar to computational fluid dynamic results of rotating detonation engines published in the literature. Specific impulse measurements of rotating detonation engines and pulsed detonation engines are shown to be quantitatively similar for engines operating on hydrogen/air and ethylene/air mixtures. The encouraging results indicate that rotating detonation engines are capable of producing thrust with fuel efficiencies that are similar to those associated with pulsed detonation engines while operating on gaseous hydrocarbon fuels....

Evaluation of Mixing Processes in a Non-Premixed Rotating Detonation Engine Using Acetone PLIF

The fuel and air mixing processes in an optically accessible non-premixed rotating detonation engine (RDE) are visualized using acetone planar laser induced fluorescence (PLIF) imaging. The acetone PLIF images are used to observe the transient fuel injection processes and evaluate the extent of partial premixing upstream of the detonation wave. The acetone PLIF images complement past OH* chemiluminescence images which showed the instantaneous size and shape of the detonation structure, oblique shock wave, and possible presence of deflagration between the fuel-fill zone and expansion region containing detonation products. The acetone PLIF data presented in this work represents a recent and ongoing experimental investigation that provides insightful information on the transient processes in the RDE. The acetone PLIF images of the non-reacting flow show an impinging jet in crossflow consistent with the fuel injection scheme of the current RDE design. A recirculation zone with minimal fuel concentration is observed in the outer corner near the fuel injection surface of the annular detonation channel. The acetone PLIF images of the reacting flow indicate that there is a purging period (60 – 75 μs corresponding to 18 – 22 % of the cycle) in which fuel is not being injected into the channel after the detonation wave travels past a particular fuel jet. This observation suggests that the high-pressure detonation wave inhibits the inflow of fuel during the purging period. The application of established experimental techniques such as acetone PLIF and OH* chemiluminescence imaging is providing new insights into RDEs. The results provide benchmark measurements that are useful for evaluating RDE models and simulations, improving fundamental understanding of the detonation structure in RDEs, and identifying critical design parameters that influence RDE operation and performance.

Turbulent Radiation Statistics of Exhaust Plumes Exiting from a Subsonic Axisymmetric Nozzle

near the tip of the potential core and downstream. Axial and radial variation in radiation intensity fluctuations is similar to those reported for flames. Autocorrelation coefficients of the radiation intensity are approximated reasonably well by exponential curves. Integral time and length scales increase monotonically downstream of the core region and are consistent with Taylor’s hypothesis. The break frequency and slope of the normalized power spectral density function are comparable to those reported for turbulent jet flames. These findings suggest that reacting flows can be used to predict trends in turbulent radiation properties of exhaust plumes.

Modeling and simulation of bluff body stabilized turbulent premixed flames

Bluff body stabilized turbulent premixed flames represent a canonical configuration with relevance to reacting flows in gas turbine engines. In addition to its relevance, the geometric simplicity of this flame has prompted the collection of significant amounts of experimental data that have been used by many researchers for model validation. A review of the literature shows that many of these efforts focus on stable combustion as opposed to the more challenging phenomena of lean blowout and thermoacoustic oscillations. Using the stable combustion condition as a starting point, Detached Eddy Simulations (DES) are performed to identify the mesh resolution requirements for this flow and the selected solver. Stable combustion results show reasonable agreement with mean and second moment statistics of velocity, as well as mean temperature profiles. Simulations of lean blowout and thermoacoustic instability are also presented in order to assess and understand the level of fidelity needed to capture more complex bluff body flame phenomena.

Temperature Estimations in the Near-Flame Field Resulting from Hypergolic Ignition Using Thin Filament Pyrometry

Time-dependent infrared images, temperature profiles, and oxidizer-to-fuel (O/F) mixture ratios are reported for unsteady flames resulting from the hypergolic ignition of a droplet of monomethyl hydrazine (MMH) contacting a small pool of red fuming nitric acid (RFNA). The infrared images of the hypergolic combustion process provide insights into the size, shape, and intensity of the flame ignition, growth, and extinguishment progression. Thin filament pyrometry is used to estimate time-dependent and time-averaged flame temperature profiles across three heights above the propellant contact interface. The flame temperature linearly decreases with distance above the contact interface for the ignition heights (9–19 mm) studied in this work. The flame temperature is largest near the droplet centerline and nonlinearly decreases in the radial direction as the hypergolic combustion products mix with the surrounding nitrogen. A parametric sensitivity analysis is used to show that mixture ratio, gas velocity, and the convective heat transfer coefficient correlation have minimal effect on the flame temperature estimates. A chemical equilibrium optimization analysis indicates that the O/F mixture ratios are smallest near the flame centerline, nonlinearly increase in the radial direction, and asymptotically approach infinity near the edge of the flame. The results provide important data for quantitative comparison with hypergolic ignition models and future studies of alternative hypergolic propellants of practical interest.

Radiation Measurements and Temperature Estimates of Unsteady Exhaust Plumes Exiting from a Turbine Driven by Pulsed Detonation Combustion

Unsteady exhaust plumes exiting from a turbine driven by pulsed detonation combustion are studied using radiation intensity measurements acquired with a high speed infrared camera. The gas temperature near the turbine exit is estimated using inverse analysis of the radiation measurements. Phase-averaged and time-averaged radiation intensity and temperature values are reported for a range of operating frequencies (10 – 20 Hz), equivalence ratios (1.0 – 1.4), fuel/air fill fractions (0.6 – 0.9), and air purge fractions (0.50 – 0.75). An increase in the operating frequency results in an increase in the time-average temperature. An increase in the equivalence ratio from stoichiometric to fuel rich conditions results in an increase in the peak temperature and smaller effects on the time-averaged temperature. An increase in the fill fraction or decrease in the purge fraction causes the peak and time-averaged temperature to increase for all operating conditions considered in this work. Cycle-to-cycle variation in the peak radiation intensity and temperature values is found to be minimal. The temperature values are useful for improving unsteady efficiency calculations of turbines driven by pressure gain combustion. This work demonstrates that imaging in the mid-infrared spectrum coupled with inverse radiation analysis is an effective non-intrusive technique for estimating gas temperatures in low luminosity, unsteady, high speed flows of practical interest.