Coaxial Jet Mixer Research Articles

Rotation effects on inhomogeneous mixing in axisymmetric sudden-expansion flows

Rotation effects on variable-density co-axial jet mixing in an axisymmetric sudden-expansion geometry are investigated experimentally. A central air jet surrounded by an annular helium/air jet is injected into a circular tube with an enlarged cross-sectional area. Two cases are examined in detail; one stationary and another with a constant speed of rotation of 840 r.p.m. about the tube axis. Results show that mixing between helium and air is greatly enhanced by rotation. Consequently, the recirculation region downstream of the sudden expansion is reduced and the reattachment length is decreased by about a factor of three. However, the flow at 30 step heights downstream of the sudden expansion is essentially identical for both the stationary and rotation cases.

Velocity predictions of contraswirling jets in a suddenly expanding confinement

Coaxial swirling jet mixing is relevant to combustion processes in double concentric-type burners. A detailed comparison of numerical predictions with experimental results for contraswirling jets in a suddenly expanding confinement indicates that theoretical predictions for the axial component of the velocity are in closer agreement with experimental values for weak swirls. Agreement deteriorates with increasing swirl intensity. 8 references.

Measurements of Confined, Coaxial Jet Mixing With Pressure Gradient

The mixing of a subsonic confined air jet with a coaxial secondary air stream is investigated experimentally. This work differs from previous studies in that emphasis is placed on the effects of an imposed adverse pressure gradient on mixing characteristics in the initial mixing (potential core) and transition regions. The results indicate that the presence of an adverse pressure gradient promotes more rapid mixing and spreading of the shear layer in the initial mixing region, as well as elevated turbulent normal stress and shear stress levels in the outer portion of the mixing layer after the primary and secondary streams have merged. Significant radial static pressure variations occur in both the initial mixing and transition regions as a result of turbulent normal stress gradients and streamline curvature effects induced by potential core entrainment.

Turbulent mixing of coaxial jets between hydrogen and air

Hydrogen-air mixing in coaxial jets was investigated using a one-equation model to simulate the velocity scale of turbulence. Combining the density fluctuation with other fluctuating components to define the eddy exchange coefficients, a set of empirical constants was derived for the closure of the partial differential equations that govern the mixing process of heterogeneous fluids. Comparing the numerical results with available experimental data, good agreements were obtained for distributions in velocity, shear stress, enthalpy, and species concentration along both axial and radial directions. The density fluctuation in hydrogen-air mixing, which is very difficult to measure, can be treated by the same method for compressible flows in a homogeneous fluid, provided that the kinetic energy of turbulence is considered simultaneously with all other governing equations.

Statistical Characteristics of Velocity, Concentration, Mass Transport, and Momentum Transport for Coaxial Jet Mixing in a Confined Duct

An experimental study of mixing downstream of coaxial jets discharging into an expanded circular duct was conducted to obtain data for the evaluation and improvement of turbulent transport models currently used for combustor flow modeling. A combination of turbulent momentum transport rate and two velocity component data were obtained from simultaneous measurements with a two-color LV system. A combination of turbulent mass transport rate, concentration and velocity data were obtained from simultaneous measurements with laser velocimeter (LV) and laser induced fluorescence (LIF) systems. These measurements were used to obtain mean, second central moment, skewness and kurtosis values for three velocity components and the concentration. These measurements showed the existence of countergradient turbulent axial mass transport where the annular jet fluid was accelerating the inner jet fluid. Results from the study are related to the assumptions employed in the current mass and momentum turbulent transport models.

Closure to “Discussion of ‘Turbulent Mixing of Coaxial Jets With Particular Reference to the Near-Exit Region’” (1976, ASME J. Fluids Eng., 98, p. 291)

Closure to “Discussion of ‘Turbulent Mixing of Coaxial Jets With Particular Reference to the Near-Exit Region’” (1976, ASME J. Fluids Eng., 98, p. 291)

Turbulent Mixing of Coaxial Jets With Particular Reference to the Near-Exit Region

Measurements, obtained with hot wire anemometry, are reported in a co-axiol jet flow where the inner and outer initial maximum velocities were the same. The hot-wire signal was processed with a modified, linearized procedure which is described. The downstream results show that the coaxial jet develops more rapidly than a single jet but that, for example, the Reynolds shear stress has not attained a self-preserving condition at a distance of 30 external diameters. Particular attention was paid to the region of the wake of the pipe wall and two dominant frequencies were observed, corresponding to two orthogonal velocity components. The high frequencies of these fluctuations indicate that they must be regarded as turbulent fluctuations and, as might be expected, lead to regions of almost constant Prandtl and Prandtl-Kolmogorov length scales.

Discussion: “Turbulent Mixing of Coaxial Jets With Particular Reference to the Near-Exit Region” (Ribeiro, M. M., and Whitelaw, J. H., 1976, ASME J. Fluids Eng., 98, pp. 284–291)

Discussion: “Turbulent Mixing of Coaxial Jets With Particular Reference to the Near-Exit Region” (Ribeiro, M. M., and Whitelaw, J. H., 1976, ASME J. Fluids Eng., 98, pp. 284–291)

Turbulent Mixing of Multiple Co-Axial Gaseous Fuel Jets in a Supersonic Airstream

An experimental and analytical investigation of a strut-mounted, four-nozzle, downstream-facing, gaseous fuel-injector assembly was conducted in a Mach 4 airstream at 154 psia and 520 R. Helium was used as the injectant in order to simulate a low molecular weight fuel, and the interjet spacing (S/D) was the main parameter varied. The principal data are in the form of helium concentration profiles at seven axial stations, Mach number distributions at three axial stations, and Schlieren photographs of the flow field at different interjet spacings. An approximate analysis was developed based upon a linearization in the Von Mises plane, Crocco integrals for the temperature and concentrations fields, and an eddy viscosity model. Good agreement with the data was achieved. It was found that, at these conditions, interjet spacing has a significant effect on mixing only up to S/D of approximately 2.5, with the concentration of the injectant at a given point becoming smaller as S/D is increased.

Turbulent coaxial jet mixing in a constant area duct.

angle of attack from 7°-10° increases the pressure level in the 100-Hz frequency band by almost 30 db and that a change from 10°-12° decreases the level in the 1000-Hz band by about 20 db. It appears that a stall-warning system need not necessarily use the FPL spectrum in the entire audio-frequency range. One may generally obtain positive identification of a spectrum at a given air speed by observing the fluctuating-pressure signal in only two one-third octave bands. In the design of a warning system for stall at a wide range of speeds, the effects of flowspeed are also an important consideration. In scaling the FPL spectra, one takes the flow speed into account by referring the fluctuating pressure to the freestream dynamic pressure q = p Uoo/2 and referring the frequency to £/«>// (where / represents a characteristic length dimension). Figure 6 indicates how a spectrum may be expected to change with airspeed and also shows a number of possibilities as to how one might shift a level/frequency detection window of a stall-warning system to account for these speed -associated spectrum shifts. Thus, detection of incipient stall by means of microphones flush-mounted at properly selected locations on the upper surfaces of aircraft wings appears feasible. The greatest and most easily detectable changes in the fluctuating-pressure spectrum sensed by a microphone occur as the flow reattachment line passes over the microphone; therefore, microphones located where reattachment occurs at a critical angle of attack (at a given speed) are particularly well-suited for detecting when the aircraft reaches that critical angle. In designing a stall-detection system, one must also ensure that acoustic noise, such as results primarily from the propulsion system, does not mask the fluctuating-pressure signal used as the basis for stall detection. In many aircraft, the fluctuating-pressure spectrum at reattachment greatly exceeds that due to noise and thus the problem of masking is resolved; however, where this condition does not exist, additional signal processing (e.g., narrowband filtering, correlation, or averaging of signals from two sensors) may be used to extract the desired signal.

Comments on "Turbulent Mixing of Coaxial Jets"

Comments on "Turbulent Mixing of Coaxial Jets"

Turbulent mixing of coaxial jets

Experimental results on the turbulent mixing process between earbon dioxide and hydrogen central jets exhausting into a moving concentric stream of air are presented. The diffusion of mass and momentum only are considered, since both streams are at approximately equal temperature. The flow velocities are in the low to high subsonic range. Principal points of investigation are the condition of equal velocity between jets and the condition of equal mass flow per unit area between jets. Radial and axial distributions of concentration and velocity are presented. On the basis of the measurements, it is demonstrated that the product of local density and eddy kinematic viscosity coefficient can be assumed to be solely dependent upon the axial coordinate. Furthermore, it is concluded that no tendency toward segregation of the streams exists when either the velocities or the mass flows of the streams are equal. In agreement with previous results, it is found that mass appears to diffuse more readily than momentum. Finally, a formulation of the eddy viscosity and eddy diffusivity coefficients that adequately describe the results obtained is derived.