E. Gutmark

Noncircular jet dynamics in supersonic combustion

The mixing characteristics of circular, small-aspect-ratio elliptic and rectangular jets were studied in subsonic, sonic, and supersonic flows. The experiments were carried out in both cold and hot flows using hot-wire anemometry, thermocouples, and photography. The elliptic and rectangular jets had similar features, with a slightly better mixing performance of the elliptic jet in the subsonic and supersonic flows. The elliptic and rectangular jets had a higher spreading rate relative to the circular jet, epecially at the minor axis plane. In the subsonic jet, the spreading rate was limited to the first five equivalent diameters (De) from the nozzle. In the supersonic underexpanded jet, the spreading rate was three times higher in the entire range (30£>e) measured. The minor axis plane was also characterized by high intensity of near-field pressure fluctuations. The two phenomena can be related to each other when an acoustic feedback occurs. Nomenclature & = aspect ratio D = circular nozzle diameter De = equivalent diameter E = energy of the fluctuating pressure components in the power spectrum / = frequency M = Mach number r = radial coordinate Re = Reynolds number, U0De/v R05 = half-width of the jet; r at with U= U^ /2 Rw 5 = half-width of the reacting jet; r at which

Subsonic and supersonic combustion using noncircular injectors

Nonreacting and combustion tests were performed for subsonic, sonic, and supersonic conditions using noncircular injectors in a gas generator combustor. The noncircular injectors, including square, equilateral-, and isosceles-triangular nozzles, were compared to a circular injector. The flowfields of the jets were mapped with hot-wire anemometry and visualized using spark schlieren photography. The combustion characteristics were visualized by high-speed photography and thermal imaging, and the temperature distribution was measured by a rake of thermocouples . The present tests conducted at high Reynolds and Mach numbers confirmed earlier results obtained for the low range of these numbers, i.e., the combination of large-scale mixing at the flat sides with the fine-scale mixing at the vertices is beneficial for combustion. Large-scale structures provide bulk mixing between the fuel and air, whereas fine-scale mixing contributes to the reaction rate and to better flameholding characteristics.

Enhancement of mixing in reacting fuel-rich plumes issued from elliptical nozzles

The mixing of jets and reacting fuel-rich plumes issued from elliptical and circular nozzles was compared in the tests reported in this paper. In noiireactirig tests with hotwire probes, the jet issued from an elliptical nozzle mixed faster with a coaxial, ducted airstream than a jet issued from a circular nozzle. In combustion tests with thermocouples, mixing between the reacting fuel-rich plume and the coaxial air was also enhanced with elliptical nozzles. This resulted in higher local combustion temperatures and therefore higher reaction rates near the nozzle exit. In both cases, the tests were done at high Reynolds numbers and turbulent initial conditions.

Multistep dump combustor design to reduce combustion instabilities

Based on the understanding of the critical role of large-scale structures as drivers of pressure oscillations, a multistep dump was sucessfully tested to suppress pressure oscillations in a coaxial dump combustor. The multistep concept, which prevents development of large-scale structures, was studied in nonreacting air and water flows and in an annular diffusion flame before it was applied to the dump combustor burning gaseous fuel. The nonreacting tests in water and air showed that there is an optimal geometric configuration of the multisteps to achieve the highest level of turbulence. At this geometry the shear layer that is separated from the upstream step impinges on the next downstream step edge. When this geometry was tested in a dump combustor, the fuel injection pattern was found to be critical to obtain suppression of instabilities . With fuel injection distributed into the small-scale turbulence downstream of each step and not interfering with the flow impingement, the pressure oscillation in the dump combustor was suppressed.

Supersonic Flow Mixing and Combustion Using Ramp Nozzle

A special supersonic nozzle that features five swept ramps on its expansion side, called the RAMP nozzle, was used to enhance Mach 2 jet mixing with the surrounding air either at rest or flowing at Mach 1.3. The total pressure profiles of the supersonic jets were measured at several streamwise stations and the initial shearlayer growth rates were deduced from the measurements. The results showed that the special growth rate was significantly increased with the RAMP nozzle in comparison with axisymmetric supersonic jets discharging from reference circular nozzles. The increase was 44% at the convective Mach number of 0.23 and 110% at 0.86. Also, the supersonic jets were visualized using a planar Mie-scattering technique. Instantaneous images of the RAMP-expanded-jet showed large-scale well-organized structures that resembled axial vortices in the shear layers. The appearance of these structures was linked to the increase in the initial shear-layer growth rate. Lastly, for an afterburning fuel-rich supersonic jet, the RAMP nozzle changed the afterburning characteristics significantly. This suggests that the supersonic mixing is affected.

Enhancement of fine-scale turbulence for improving fuel-rich plume combustion

A passive method to enhance fine-scale mixing was developed and studied in cold flows. Its effect on the combustion intensity and flame stability was then studied in reacting flows. Hot-wire anemometry was used to map the mean and turbulent flowfields of the nonreacting flows. Reacting flows were studied in a free flame and ducted gas-generator fuel-rich plume using planar laser-induced fluorescence imaging, a rake of thermocouples , and high-speed photography. A modified circular nozzle having several downstream-facing steps upstream of its exit was used to introduce numerous inflection points in the initial mean velocity profiles, thus producing multiple corresponding sources of fine-scale turbulence generators and reducing the strain rates in the initial region. Cold flow tests showed turbulence increases of up to six times the initial turbulence level relative to a circular nozzle and a substantial decrease of the mean velocity gradient. The flame of this nozzle was more intense with a homogeneous heat release in the free-flame experiments and ducted-plume combustion experiments, even when the gas-generator exhaust velocity was supersonic. Secondary plume ignition was obtained under conditions that prevented sustained afterburning using the circular nozzle.

Mixing characteristics of a ducted elliptical jet

Mixing between elliptical ducted air jets discharged at a sudden expansion and nitrogen, which was radially injected through the larger duct walls, was experimentally studied using hot-wire anemometry and gas sampling techniques. Mixing for this flowfield, which simulates a combustor with fuel addition through the side walls, increased considerably when the air jet was produced by an elliptical, rather than a circular, cross section. Elliptical jets discharging from orifices provide better mixing than jets discharging from elliptical and circular pipes. Additional mixing enhancement was achieved when the elliptical jets were acoustically forced by excited resonant pressure waves of the duct. The mean and turbulent velocity measurements provide insight into the mechanism of the observed mixing enhancement for this simulated boundary-layer combustion process.

Feedback Control of a Dump Combustor with Fuel Modulation

Closed-loop control tests were performed to suppress the combustion instability of a 0.7-MW dump combustor and to extend its flammability limits. The pressure oscillations originating from the unstable combustion were measured at the dump plane and at the exhaust nozzle. The signals were used as a reference to lock on and to produce an acoustic signal that modulated the fuel flow at a predetermined phase shift relative to them. At a certain range of phase shift angles, the amplitude of the combustion oscillations was reduced to 50% of its unforced level. The control system was most effective when the reference signal was picked up at the dump. The amount of reduction was proportional to the acoustic forcing level, but leveled off for high forcing amplitudes. The effectiveness of the control system was reduced as the mass flow rate of the air was increased. The combustion instability became bimodal with multiple unstable frequencies, and a more sophisticated lock-on and phaseshift system is required to suppress effectively oscillations with more than a single dominant frequency. However, even for the high flow rates, the amplitude of the instability was reduced by nearly 40%.

Feedback control of an unstable ducted flame

Active control of a naturally unstable ducted flame was realized using acoustic forcing of either the shear layer of the flame jet or the duct itself. The feedback signal was derived from either the duct pressure signal or the CH intensity (related to the flame heat release rate), and delayed in time to produce cancellation of the natural resonant oscillations of the system. Direct driving of the shear layer using the duct pressure signal feedback produced the best control with the lowest power requirements. The controller was able to reduce the acoustic power in the duct at the resonant frequency from 19 Pa to about 0.7 Pa, or nearly 30 dB. When operating in the cpntrolled mode, the driving speaker is producing a sound pressure level more than three orders of magnitude below the natural duct uncontrolled level (both measured in the duct), so the effect is clearly not just acoustic cancellation.

Mixing and Combustion in a Spinning Combustor

The effect of spin on mixing and combustion in a circular diict with a primary air jet and secondary gas injection through the duct wail was studied using a spin rig, with spin rates up to 10,000 rpm. The spinning ducted flowfield was compared to stationary conditions. Spin enhanced the spreading rate of the pipe and orifice jets, resulting in a larger radial velocity component and an onset of a tangential component in the jets core for the pipe jet and in the jets shear layer for the orifice jet. The turbulence amplification rate was enhanced predominantly in the pipe jet. The tests showed that the duct-spin affects the flow even for a relatively short entrance duct. Measurement of the mixture ratio distribution using gas sampling probes in nonreacting tests were done with inert gas injection. Centrifugal forces affected the mixing depending on the weight of the gas relative to air. The mixing was enhanced for lighter-than-air gas and suppressed for heavier. Combustion tests included both ethylene as secondary gas and solid-fuel at the wall, In gaseous fuel combustion, the spin increased the combustion temperature due to improved mixing caused by low-molecular -weight products and reduced density. For solid-fuel combustion the spin had an adverse effect. Combustion temperatures were decreased due to reduced mixing caused by centrifugal forces acting on soot and heavy hydrocarbon products.