Lobed Jet Mixing Research Articles

Mechanisms of lobed jet mixing: Comparison of bilinear, rectangular, and circular lobed mixers

In this study, bilinear lobed mixers (BLMs), rectangular lobed mixers (RLMs), and circular lobed mixers (CLMs), with two sectional area ratios of the bypass channel to the core channel, were configured while maintaining the same lobe angle, lobe height, lobed nozzle length, mixing duct length, and core sectional area. Using three-dimensional steady Reynolds-averaged Navier–Stokes equations and the SST k–ω turbulence model, numerical simulations of jet mixing in these lobed mixers at three velocity ratios of the bypass stream to the core stream were performed. The jet mixing characteristics and performances of the three types of lobed mixers were analyzed. Through a comprehensive and in-depth analysis of the jet-mixing flow fields of the three types of lobed mixers, more universal mechanisms of lobed jet mixing were revealed. Systematic and meaningful conclusions were drawn from four aspects: the formation and evolution mechanisms of the cross flow, longitudinal vortices, heat and mass convection, and orthogonal vortex strip. The findings from this study hold significant implications for the development of lobed mixers and exploration of lobed jet-mixing mechanisms.

Mechanisms of lobed jet mixing: About circularly alternating-lobe mixers

Two configurations of circularly arranged alternating-lobe nozzles were adopted to form lobed mixers with/without a mixing duct. The jet mixing of each mixer was numerically simulated with the unchanged initial conditions of the primary and secondary streams, except the altered initial velocity of the secondary stream. The jet-mixing mechanisms of the circularly alternating-lobe mixers were synthetically analysed by combining the evolution of the flow field structures and the process of heat and mass transfer in the mixing field. It is found that the transverse flow is usually caused by the lobed geometry, and the entrainment of the primary stream also plays a role in certain circumstances. There are two mechanisms for the deflection of the transverse flow at the lobe peaks and troughs. One of these mechanisms is to be suppressed to deflect the flow while the other is deflected by the reaction force and induction effect. The primary and secondary streams deflect to bring the transverse interval between them, and subsequently, the streamwise vortex core appears at the transverse interval. The deflected flow consistently “digging” in the radial and circumferential radiation increases the dimension of the streamwise vortices. The transverse flow velocity decreases and the direction becomes unstable leading to the breakdown of the streamwise vortices. The transverse flow brings the heat and mass transfer. Under the two mechanisms, the frontiers of the primary and secondary streams deflect. Initially, the primary and secondary streams flow around the streamwise vortex core. Subsequently, the mixed stream flows around the vortex core, and the mixing stream area gradually expands outward. The heat and mass transfer decrease in scale when the streamwise vortices break down. Because of the velocity gradient at the interface, shear instability occurs to generate the normal vortex ring. The heat and mass transfer pushes the interface, which leads to the stretch of the normal vortex ring. The mixing speed varies due to the heat and mass transfer. The velocity gradient decreases fast in the rapid mixing segment, where the normal vortex ring breaks first.

Suggestions on investigations of lobed jet mixing

Jet mixings of different constructional lobed mixers comprised of lobed nozzles with and without a mixing duct were investigated numerically. Based on the heat and mass transfer investigation in the streamwise vortices and the “stretch” and “cut” of the normal vortex ring, suggestions on investigations of lobed jet mixing were provided. A more reasonable mechanism for lobed jet mixing was suggested to be revealed when combined fluid dynamics and heat and mass transfer were being investigated. Differences in fluid dynamics and heat and mass transfer with different constructional lobed mixers meant different mechanisms.

Numerical research on the mixing mechanism of lobed mixer with inlet swirl in linear radial distribution

A detailed numerical simulation is presented to investigate the effects of inlet swirl and its radial distribution on the mixing mechanisms of a turbofan mixer with 12 lobes, by using the commercial ANSYS CFX solver and k–ω SST model. The core-to-bypass temperature ratio and pressure ratio were set to 2.59 and 0.97, respectively, giving the Mach number of 0.66 and bypass ratio of 2.65 at mixing nozzle outlet. In the core inlet, the swirl angle was raised from 0° to 30° in a uniform or linear radial distribution manner. The inlet swirl and its radial gradient did enhance the development, interaction, and dissipation of the vortices downstream of lobed mixer, resulting in accelerating the lobed jet mixing. When the inlet swirl was less than 20°, the total pressure and thrust loss increments of lobed jet were acceptable and no more than 0.26% and 1.57%, respectively, compared with the baseline case. The results also showed that the three-dimensional separation bubble on center-body and the backflows along jet axis at the rail of center-body, resulting from the swirling flow between lobes’ trough and center-body, were the dominant sources of total pressure and thrust losses for all cases with inlet swirl. And, reasonable radial distribution of inlet swirl could inhibit the aforementioned 3D separation and backflow, and thus limited the increment of jet mixing loss favorably.

Numerical Predictions of Scarfing on Performance of S-Shaped Nozzle with Asymmetric Lobe

Numerical predictions of a lobed jet mixing flow within an S-shaped nozzle were conducted with respect to the influence of lobe scarfing on performance of the lobed S-shaped nozzle. The simulation results presented in this paper were all obtained by solving compressible steady three-dimensional Reynolds-averaged Navier–Stokes equations with a realizable turbulence model. The effect of scarfing optimization was mainly addressed by both the flowfield downstream of the lobed mixer and the interested variables characterizing the performance of the lobed S-shaped nozzle. Due to the studied S-shaped nozzle, a sharp decrease of total pressure recovery coefficient, as well as a reduced thermal mixing efficiency, within the flowfield close to the outlet is shown. The model with a bigger scarfing angle is suggested to induce the forced mixing process ahead of schedule, but with less mixing strength as compared with the baseline model. However, the model with a more aggressive scarfing angle is shown to be the case with both a decreased thermal mixing efficiency and an augmented total pressure recovery coefficient within the flowfield downstream of the cross section, except the 20 deg scarfing model. Both the thrust coefficient and thrust mixing efficiency are all found to decrease as a result of the tested model with a bigger scarfing angle.

A study on a lobed jet mixing flow by using stereoscopic particle image velocimetry technique

In a continuing effort to study the mixing enhancement by large-scale streamwise vortices generated by a lobed nozzle, a high-resolution stereoscopic particle image velocimetry (PIV) system was used in the present study to conduct three-dimensional measurements of air jet flows exhausted from a lobed nozzle and a conventional circular nozzle. The three-dimensional instantaneous and ensemble-averaged velocity fields, instantaneous and ensemble-averaged streamwise vorticity distributions and turbulent kinetic energy distributions were used to analyze the characteristics of the mixing process in the lobed jet flow compared with conventional circular jet flow. The existence of large-scale streamwise vortices in the lobed jet mixing flow is revealed clearly from the stereoscopic PIV measurement results. The instantaneous streamwise vorticity distributions revealed that the large-scale streamwise vortices generated by the corrugated trailing edge of the lobed nozzle break into smaller, but not weaker streamwise vortices gradually as they travel downstream. This is the proposed reason why a lobed nozzle would enhance both large-scale mixing and small-scale mixing reported by other researchers. The overall effect of the lobed nozzle on the mixing process in the lobed jet mixing flow was evaluated by analyzing the ensemble-averaged streamwise vorticity distributions. It was found that ensemble-averaged streamwise vortices in the lobed jet mixing flow grew up and expanded radially over the first diameter of the test nozzle, then began to break down into smaller and weaker vortices further downstream. The averaged turbulent kinetic energy profile indicated that most of the intensive mixing between the core jet flow and ambient flow, due to the special geometry of the lobed nozzle, occurred within the first two diameters of the lobed nozzle, which corresponds to the upstream region where the ensemble-averaged streamwise vortices break down.

Dual-plane stereoscopic particle image velocimetry: system set-up and its application on a lobed jet mixing flow

The technical basis and system set-up of a dual-plane stereoscopic particle image velocimetry (PIV) system, which can obtain the flow velocity (all three components) fields at two spatially separated planes simultaneously, is summarized. The simultaneous measurements were achieved by using two sets of double-pulsed Nd:Yag lasers with additional optics to illuminate the objective fluid flow with two orthogonally linearly polarized laser sheets at two spatially separated planes, as proposed by Kaehler and Kompenhans in 1999. The light scattered by the tracer particles illuminated by laser sheets with orthogonal linear polarization were separated by using polarizing beam-splitter cubes, then recorded by high-resolution CCD cameras. A three-dimensional in-situ calibration procedure was used to determine the relationships between the 2-D image planes and three-dimensional object fields for both position mapping and velocity three-component reconstruction. Unlike conventional two-component PIV systems or single-plane stereoscopic PIV systems, which can only get one-component of vorticity vectors, the present dual-plane stereoscopic PIV system can provide all the three components of the vorticity vectors and various auto-correlation and cross-correlation coefficients of flow variables instantaneously and simultaneously. The present dual-plane stereoscopic PIV system was applied to measure an air jet mixing flow exhausted from a lobed nozzle. Various vortex structures in the lobed jet mixing flow were revealed quantitatively and instantaneously. In order to evaluate the measurement accuracy of the present dual-plane stereoscopic PIV system, the measurement results were compared with the simultaneous measurement results of a laser Doppler velocimetry (LDV) system. It was found that both the instantaneous data and ensemble-averaged values of the stereoscopic PIV measurement results and the LDV measurement results agree well. For the ensemble-averaged values of the out-of-plane velocity component at comparison points, the differences between the stereoscopic PIV and LDV measurement results were found to be less than 2%.

Particle image velocimetry and planar laser-induced fluorescence measurements on lobed jet mixing flows

An experimental investigation of the vortical and turbulent structures in lobed jet mixing flows was conducted. The techniques of planar laser-induced fluorescence (PLIF) and particle image velocimetry (PIV) were used to accomplish flow visualisation and velocity filed measurements of the lobed jet mixing flows. Compared with a conventional circular jet flow, the lobed jet mixing flows were found to have a shorter laminar region, a smaller scale of spanwise Kelvin–Helmholtz vortices, quicker transition to turbulence and earlier appearance of small-scale vortical and turbulent structures. The intensive mixing of the core jet flow with ambient flow was found to concentrate within the first two nozzle diameters in the lobed jet mixing flow. More rapid growth of the shear layer at the near field and quicker decay of the central line velocity were also found in the lobed jet mixing flow. All these indicated a better mixing enhancement performance of the lobed nozzle compared with the conventional circular nozzle in the near-field region.

Research on the vortical and turbulent structures in the lobed jet flowusing laser induced fluorescence and particle image velocimetrytechniques

The changes in vortical and turbulent structure in the near field(X / D < 3.0) of jet mixing flow caused by a lobed nozzle were investigatedexperimentally in the present study. The techniques of planar laser inducedfluorescence and particle image velocimetry were used to accomplishflow visualization and velocity field measurements. The experimental resultsshowed that, compared with a circular jet flow, lobed jet mixing flow wasfound to have a shorter laminar region, a smaller scale of the spanwiseKelvin-Helmholtz vortices, earlier appearance of small scale turbulentstructures and a larger region of intensive mixing in the near field of the jetmixing flow. The central line velocity decay of the lobed jet mixing flow wasalso found to be much faster than that in a conventional circular jet. Allthese indicated that the lobed nozzle provides better mixing performance thandoes a conventional circular nozzle.

244ステレオPIVの噴流解析への適用

The “classical” PIV technique is only capable of recording the projection of velocity into the plane of the laser sheet, i.e the out-of-plane velocity component is lost while the in-plane components are affected by an unrecoverable error due to the perspective transformation. For highly three-dimensional flow fields like lobed jet mixing flow, this may lead to substantial measurement error of the local velocity vector. In order to get the velocity three-components simultaneously to reveal the interaction and evolution of various vortices in the lobed jet mixing flow, a high-resolution stereoscopic PIV system is used in the present study to measure the near flow field of a lobed jet mixing flow. Based on the analyzing of the three-components and three-dimensional velocity fields of the stereoscopic PIV measurement results, the mixing characteristics in the lobed jet mixing flow is studied and the mechanism of the mixing enhancement in lobed jet mixing flow is discussed.

B212 噴流の混合促進のPIVとLIFによる計測

An experimental investigation of vertical and turbulent structure change near a jet mixing flow caused by the lobed nozzle has been conducted. The technique of the Laser Induced Fluorescence (LIF) and Particle Image Velocimetry (PIV) were used to accomplish the flow visualization, instantaneous quantitative concentration measurements and velocity field measurements of the lobed jet mixing flows. Compared with circular jet flow, the lobed jet flow were found to have shorter laminar region, its spanwise Kelvin-Hemholtz vortices paring and combining process was accelerated, small scale turbulent vortices appeared earlier, and the bigger turbulent intensity was found in the lobed jet than that in circular jet flow. All these indicated the mixing enhancement performance of lobed nozzles over the circular nozzle.