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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
NASA NTRS2005

Time-Accurate Computations of Isolated Circular Synthetic Jets in Crossflow

Results from unsteady Reynolds-averaged Navier-Stokes computations are described for two different synthetic jet flows issuing into a turbulent boundary layer crossflow through a circular orifice. In one case the jet effect is mostly contained within the boundary layer, while in the other case the jet effect extends beyond the boundary layer edge. Both cases have momentum flux ratios less than 2. Several numerical parameters are investigated, and some lessons learned regarding the CFD methods for computing these types of flow fields are outlined. Results in both cases are compared to experiment.

Rumsey, Christoper L.↗
NASA NTRS2007

Experimental Evaluation of an Isolated Synthetic Jet IN Crossflow

The second case for this workshop builds upon the isolated synthetic jet of Case 1 by adding a crossflow, with no streamwise pressure gradient, for the developing jet to interact with. Formally, Case 2 examines the interaction of a single, isolated, synthetic jet and a fully turbulent zero-pressure gradient boundary layer. The resulting flow has many of the characteristics that need to be modeled with fidelity if the results of the calculations are to serve as the basis for research and design with active flow control devices. These include the turbulence in the boundary layer, the time-evolution of the large vortical structure emanating from the jet orifice and its subsequent interaction with and distortion by the boundary layer turbulence, and the effect of the suction cycle on the boundary layer flow. In a synthetic jet, the flow through the orifice and out into the outer flowfield alternates between an exhaust and a suction cycle, driven by the contraction and expansion of a cavity internal to the actuator. In the present experiment, the volume changes in the internal cavity are accomplished by replacing one of the rigid walls of the cavity, the wall opposite the orifice exit, with a deformable wall. This flexible wall is driven by a bottom-mounted moveable piston. The piston is driven electro-mechanically. The synthetic jet issues into the external flow through a circular orifice. In the present experiment, this orifice has a diameter of 0.250 inches (6.35 mm). The flow is conceptually similar to that documented in Schaeffler [1]. To document the flow, several measurement techniques were utilized. The upstream boundary conditions (in-flow conditions), and several key phase-averaged velocity profiles were measured with a 3-component laser-Doppler velocimetry system. Phase-averaged velocity field measurements were made with both stereo digital particle image velocimetry and 2-D digital particle image velocimetry as the primary measurement system. Surface pressure measurements were made utilizing an electronically scanned pressure system.

Schaeffler, Norman W.↗
NASA NTRS2007

Time-Accurate Computations of Isolated Circular Synthetic Jets in Crossflow

Results from unsteady Reynolds-averaged Navier-Stokes computations are described for two different synthetic jet flows issuing into a turbulent boundary layer crossflow through a circular orifice. In one case the jet effect is mostly contained within the boundary layer, while in the other case the jet effect extends beyond the boundary layer edge. Both cases have momentum flux ratios less than 2. Several numerical parameters are investigated, and some lessons learned regarding the CFD methods for computing these types of flow fields are summarized. Results in both cases are compared to experiment.

Rumsey, C. L.↗
NASA NTRS2006

On the Mixing of Single and Opposed Rows of Jets With a Confined Crossflow

The primary objectives of this study were 1) to demonstrate that contour plots could be made using the data interface in the NASA GRC jet-in-crossflow (JIC) spreadsheet, and 2) to investigate the suitability of using superposition for the case of opposed rows of jets with their centerlines in-line. The current report is similar to NASA/TM-2005-213137 but the "basic" effects of a confined JIC that are shown in profile plots there are shown as contour plots in this report, and profile plots for opposed rows of aligned jets are presented here using both symmetry and superposition models. Although superposition was found to be suitable for most cases of opposed rows of jets with jet centerlines in-line, the calculation procedure in the JIC spreadsheet was not changed and it still uses the symmetry method for this case, as did all previous publications of the NASA empirical model.

Holdeman, James D.↗
NASA NTRS2008

Mixing of Multiple Jets with a Confined Subsonic Crossflow: Part III--The Effects of Air Preheat and Number of Orifices on Flow and Emissions in an RQL Mixing Section

This study was motivated by a goal to understand the mixing and emissions in the Rich-burn/Quick-mix/Lean-burn (RQL) combustor scheme that has been proposed to minimize the formation of oxides of nitrogen (NOx) in gas turbine combustors. The study reported herein was a reacting jet-in-crossflow experiment at atmospheric pressure. The jets were injected from the perimeter of a cylindrical duct through round-hole orifices into a fuel-rich mainstream flow. The number of orifices investigated in this study gave over- to optimum to underpenetrating jets at a jet-to-mainstream momentum-flux ratio of J = 57. The size of individual orifices was decreased as the number of orifices increased to maintain a constant total area; the jet-to-mainstream mass-flow ratio was constant at MR = 2.5. The experiments focused on the effects of the number of orifices and inlet air preheat and were conducted in a facility that provided the capability for independent variation of jet and main inlet air preheat temperature. The number of orifices was found to have a significant effect on mixing and the distributions of species, but very little effect on overall NOx emissions, suggesting that an aerodynamically optimum mixer might not minimize NOx emissions. Air preheat was found to have very little effect on mixing and the distributions of major species, but preheating both main and jet air did increase NOx emissions significantly. Although the air jets injected in the quick-mix section of an RQL combustor may comprise over 70 percent of the total air flow, the overall NOx emission levels were found to be more sensitive to main stream air preheat than to jet stream air preheat.

Holdemann, James D.↗
NASA NTRS2010

Spreadsheet Calculations for Jets in Crossflow: Opposed Rows of Inline and Staggered Holes and Single and Opposed Rows with Alternating Hole Sizes

The primary purpose of this jet-in-crossflow study was to calculate expected results for two configurations for which limited or no experimental results have been published: (1) cases of opposed rows of closely-spaced jets from inline and staggered round holes and (2) rows of jets from alternating large and small round holes. Simulations of these configurations were performed using an Excel (Microsoft Corporation) spreadsheet implementation of a NASA-developed empirical model which had been shown in previous publications to give excellent representations of mean experimental scalar results suggesting that the NASA empirical model for the scalar field could confidently be used to investigate these configurations. The supplemental Excel spreadsheet is posted with the current report on the NASA Glenn Technical Reports Server (http://gltrs.grc.nasa.gov) and can be accessed from the Supplementary Notes section as TM-2010-216100-SUPPL1.xls. Calculations for cases of opposed rows of jets with the orifices on one side shifted show that staggering can improve the mixing, particularly for cases where jets would overpenetrate slightly if the orifices were in an aligned configuration. The jets from the larger holes dominate the mixture fraction for configurations with a row of large holes opposite a row of smaller ones although the jet penetration was about the same. For single and opposed rows with mixed hole sizes, jets from the larger holes penetrated farther. For all cases investigated, the dimensionless variance of the mixture fraction decreased significantly with increasing downstream distance. However, at a given downstream distance, the variation between cases was small.

Holdeman, James D.↗
NASA NTRS2014

Effects of Forward- and Backward-Facing Steps on the Crossflow Receptivity and Stability in Supersonic Boundary Layers

The effects of forward- and backward-facing steps on the receptivity and stability of three-dimensional supersonic boundary layers over a swept wing with a blunt leading edge are numerically investigated for a freestream Mach number of 3 and a sweep angle of 30 degrees. The flow fields are obtained by solving the full Navier-Stokes equations. The evolution of instability waves generated by surface roughness is simulated with and without the forward- and backward-facing steps. The separation bubble lengths are about 5-10 step heights for the forward-facing step and are about 10 for the backward-facing step. The linear stability calculations show very strong instability in the separated region with a large frequency domain. The simulation results show that the presence of backward-facing steps decreases the amplitude of the stationary crossflow vortices with longer spanwise wavelengths by about fifty percent and the presence of forward-facing steps does not modify the amplitudes noticeably across the steps. The waves with the shorter wavelengths grow substantially downstream of the step in agreement with the linear stability prediction.

Balakumar, P.↗
DOE OSTI2025

Characterization of Flashback and Flame-Holding in a Jet-in-Crossflow Mixing Configuration with Methane-Hydrogen Fuel Blends

An approach that combines experimental and numerical analyses has been implemented to characterize the fundamentals of flashback events and flame-holding phenomena during high-hydrogen combustion in a jet-in-crossflow (JICF) configuration. Such flame dynamics are visualized experimentally using nanosecond (ns)-based hydroxyl planar laser-induced fluorescence (OH-PLIF) and chemiluminescence diagnostics techniques. The JICF burner has an optically accessible pre-mixing tube allowing the optical diagnostics. The testing was conducted for varied pre-mixer velocities (V) and equivalence ratio (ϕ) for 90%-100% (H2, by mole) H2/CH4 reactant mixtures at atmospheric temperature and pressure conditions. Two distinct flashback events were identified – conventional rich flashback and lean flashback, recorded while increasing ϕ and decreasing ϕ, respectively. The cause of lean flashback was attributed to the lower momentum flux ratio which bends the jet sharply, closer toward the injection plane. The mean OH-PLIF images characterized the flame-holding behavior where the flame was found to be stabilized on the leeward side only or on both windward and leeward sides as a lifted flame near the fuel port. A Large Eddy Simulation (LES) with detailed chemistry combustion modeling approach was implemented along an OH* sub-mechanism, and it showed qualitative agreement with the integrated line-of-sight chemiluminescence results as well as the planar OH-PLIF measurements.

flame holding↗
DOE OSTI2025

Characterization of Flashback and Flame-Holding in a Jet-In-Crossflow Mixing Configuration with Methane-Hydrogen Fuel Blends

An approach that combines experimental and numerical analyses has been implemented to characterize the fundamentals of flashback events and flame-holding phenomena during high-hydrogen combustion in a jet-in-crossflow (JICF) configuration. Such flame dynamics are visualized experimentally using nanosecond (ns)-based hydroxyl planar laser-induced fluorescence (OH-PLIF) and chemiluminescence (CL) diagnostics techniques. The JICF burner has an optically accessible pre-mixing tube allowing the optical diagnostics. The testing was conducted for varied pre-mixer velocities (V) and equivalence ratio (ϕ) for 90%-100% (H2, by mole) H2/CH4 reactant mixtures at atmospheric temperature and pressure conditions. Two distinct flashbacks were identified – conventional rich flashback and lean flashback, recorded while increasing ϕ and decreasing ϕ, respectively. The cause of lean flashback was attributed to lower flux ratio which bends the jet sharply, closer toward the injection plane. The flashback was found to occur along the boundary layer, assisted by the inner wall of the pre-mixer tube. The mean OH-PLIF images characterized the flame-holding behavior where the flame was found to be stabilized on the leeward side only or on both windward and leeward sides as a lifted flame near the fuel port.

flame holding↗
DOE OSTI2025

Mineral Scale Formation during Crossflow Reverse Osmosis at Constant Flux and Constant Transmembrane Pressure Conditions

Mineral scale formation on membrane surfaces is a significant challenge in reverse osmosis water purification. Laboratory fouling experiments are typically run such that the transmembrane pressure (TMP) is fixed, and the permeate flux decreases over time as scales accumulate on the membrane surface. However, this change in flux means that the hydrodynamic conditions at the membrane surface are continuously changing, which could affect crystallization and foulant deposition processes. Operating under constant permeate flux conditions, in contrast, is advantageous because it keeps the hydrodynamic conditions relatively consistent, making it possible to compare how membrane properties (e.g., surface chemistry) affect fouling propensity. Industrial reverse osmosis operations are not run strictly in either constant TMP or constant flux mode; while they may start at a constant TMP, feed pressure may be periodically adjusted to maintain permeate water production within a specified range. Here, the scarcity of constant permeate flux reverse osmosis scaling experiments reported in the literature frustrates efforts to compare membrane fouling processes under constant TMP and constant flux conditions. For the first time, the evolution of the fouling layer resistance was compared as a function of cumulative permeate volume per membrane area during constant TMP and constant flux reverse osmosis filtrations. Scaling experiments were conducted by challenging commercial reverse osmosis membranes with a model feed solution nearly saturated with calcium sulfate dihydrate (gypsum). At low fluxes, the increase in fouling layer resistance was quantitatively similar for the two operational modes. In contrast, at high fluxes, the fouling layer resistance increased more rapidly in constant flux filtration than in constant TMP filtration. The mechanism of scale formation in constant TMP and constant flux operation was self-limiting and self-reinforcing, respectively.

36 MATERIALS SCIENCE↗
NASA NTRS1972

Gas crossflow effects on airflow through a wire-form transpiration cooling material

An experimental analysis was conducted to determine the effects of gas stream flow parallel to the discharging surface on the flow characteristics of a wire-form porous material. Flow data were obtained over a range of transpiration airflow rates from 0.129 to 0.695/grams per second-centimeter squared and external gas stream Mach numbers from 0 to 0.46. The conclusion was drawn that the flow characteristics of the wire cloth were not significantly affected by the external gas flows.

Kaufman, A. S.↗