Flow characteristic of highly underexpanded jets from various nozzle geometries

Flow characteristics of highly underexpanded jets at the same nozzle pressure ratio of 5.60 but issuing from four different nozzles, i.e., the circular, elliptic, square, and rectangular nozzles, are studied using large eddy simulations. The results show that the square jet penetrates fastest, although the turbulence transition is similar for different jets. The penetration rates of different jets show the similar linear dependency on the square root of time, but the penetration constant Gamma for the noncircular jets deviates more than 5% from the theoretical value of 3.0. The circular and square jets both correspond to a three-dimensional helical instability mode, while the elliptic and rectangular jets haveatwo-dimensional flapping instability in their minor axis planes. All the jets undergo a Mach reflection forming the Mach disk, but the Mach disk in the elliptic and rectangular jets is not easily visible. The intercepting shocks in the square jet originate at the four corners of the nozzle exit at first, while the formation of the intercepting shocks is only observed in the major axis planes for the elliptic and rectangular jets. In addition, great differences are observed on the mixing characteristics between different jets. In particular, the elliptic jet penetrates slowest, has the shortest length of jet potential core, and takes the largest mixing area. (C) 2017 Elsevier Ltd. All rights reserved.</p

Numerical investigation of characteristic frequency excited highly underexpanded jets

A highly underexpanded jet with a nozzle pressure ratio of 5.60 is excited simultaneously by the inherent characteristic frequencies in the steady jet of 14.569 kHz 37.086 kHz and 45.695 kHz as well as other two reference frequencies of 1.0 kHz and 40.0 kHz. The flow characteristic of the excited jets is revealed by comparing to the steady jet using large eddy simulation technique. For low-frequency excitation (fe = 1.0 kHz) the flow and acoustic fields of the forcing jet are similar with the steady jet. However when the jet is excited by high frequencies the acoustic source moves to the nozzle exit and the jet potential core together with the near-field shocks oscillate periodically at the excitation frequency. The excitation at fe = 1.0 kHz increases the mixing area since y/D = 24 from the nozzle exit which is contrary to the effect of other high frequencies that enhances the mixing in the near-field region but decreases the mixing area since y/D = 18. The peak frequency of the excited jets generally becomes identical to the excitation frequency once being excited except the fe = 1.0 kHz and h = 40.0 kHz jets. High-order harmonics of the dominant frequency are observed in the pressure spectrum of jets excited by high frequencies and the dominant mode turns into the axisymmetric mode from the original helical one accordingly. In particular forcing the jet with the axisymmetric mode of h = 14.569 kHz provides the fewest shock cells but the largest amplitude in shock oscillation the most harmonics in the spectrum and the largest mixing area within 8 &lt; y/D &lt; 12. 2017 Elsevier Masson SAS. All rights reserved.</p

Combustion of Vaporized Kerosene in Supersonic Model Combustors with Dislocated Dual Cavities

Supersonic combustion of vaporized kerosene in a Mach 2.5 model combustor with a total temperature of 1500 K and a total pressure of 1.3 MPa was experimentally investigated for an optimal integration of the cavity-based flameholder and the fuel injection scheme. A novel design of a supersonic model combustor consisting of a two-staged fuel injection system and dislocated dual cavities was proposed to improve the combustor performance, including the combustion efficiency, flame stabilization, combustor "unstart," and heat release distribution. Specifically, a large number of experiments were performed to systematically investigate the effects of fuel injection distribution, which is controlled by varying the injector spacing and the fuel equivalence ratio, on the static pressure distribution, thrust increment; lean blowout limit, wall temperature distribution, and combustor unstart characteristics. The results show that there exists an optimal range of injector spacing to obtain enhanced combustion performance while avoiding the combustion unstart. Furthermore, the equal fuel injection with an overall equivalence ratio of 0.5 for the two injectors was found to result in the optimal static pressure distribution and hence the largest thrust increment

Study on Flame Stabilization in a Dual-Mode Combustor Using Optical Measurements

Flame stabilization in a dual-mode scramjet combustor was studied using simultaneous detection of flowfield and reaction zone. The instantaneous reactive flowfield was clearly visualized using an improved pulsed schlieren system, whereas the reaction zone was marked by CH* chemiluminescence. Experiments were performed in a Mach 2.5 model combustor with the total temperature of 1275 +/- 25 K and the total pressure of 1.0 MPa. Ethylene was used as fuel with equivalence ratios varied from 0.258 to 0.411. Four typical flame stabilized locations and corresponding flowfields are presented. When no choke occurs, the flame is stabilized in the cavity or the shear layer. Schlieren images show that the flow in the reaction zone of the latter case is supersonic. When the combustor is choked at the injection location, the flame oscillates between the shear layer and the jet wake, and the flow in the reaction zone becomes subsonic. When the flame is stabilized in the jet wake, autoignition can occur due to the compression of precombustion shock train. The short-lived aerodynamic throat formed near the injection location may be the possible cause for triggering the unsteady flame oscillation. The effects of heat release on flame stabilization modes are discussed

A comparative study of highly underexpanded nitrogen and hydrogen jets using large eddy simulation

Three-dimensional large eddy simulations (LES) of highly underexpanded hydrogen and nitrogen jets at the same nozzle pressure ratio (NPR) of 5.60 and at a Reynolds number around 105 are performed. The classical near-field structures of highly underexpanded jets are well captured by LES, especially the shape and size of Mach barrel for both jets are very similar and agree well with the available literature data. However, the flow field and the shock structures after the Mach disk differ significantly. The density in the annular shear layer of H-2 jet is much lower because of its smaller molecular weight. Meanwhile, the H-2 jet has a much longer jet core and more shock cells. The dominant instability mode is helical for the N-2 jet, but is axisymmetric for the H-2 jet. There are two discrete peaks of f(s) = 37.086 kHz and f(2s) = 45.695 kHz in the spectrum of the N-2 jet, while the spectrum of the H-2 jet is characterized by a fundamental screech frequency of f(s) = 47.020 kHz and its high-order harmonics. The H-2 jet mixes more rapidly with the ambient air but has a much smaller mixing area on cross-section planes. Mixing between the ambient air and fuel still takes places at the jet boundary defined according to the mixture fraction of Z = 0.02, and the area of fully turbulent region of the highly underexpanded jets seems to be less predicted based on the traditional vorticity T/NT (turbulent/non-turbulent) interface for both jets. Copyright (C) 2016, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved

Blowout Limits of Cavity-Stabilized Flame of Supercritical Kerosene in Supersonic Combustors

Blowout limits of cavity-stabilized flame of supercritical kerosene were experimentally studied by using Mach 2.5 and 3.0 direct-connect supersonic model combustors operated under various air and fuel conditions. Specifically, the effects of the stagnation temperature and the stagnation pressure on the blowout limits were investigated for supercritical kerosene injected from the wall upstream of a cavity flameholder in a Mach 2.5 combustor. Experiments were performed under the same conditions for supercritical kerosene injected from the rear part of the cavity bottom to study the influence of the location of fuel injection. The blowout limits of supercritical kerosene injected from the wall upstream of the cavity were further investigated in a Mach 3.0 combustor. Besides the effects of the stagnation temperature and stagnation pressure, the effect of the divergence angle of the combustor on the lean-fuel blowout limit was studied. Results show that there exist two blowout limits corresponding to the lean- and rich-fuel conditions for a given stagnation temperature. The location of fuel injection has substantial influence on the blowout limits, whereas the influence of the stagnation pressure and the divergence angle of the combustor can be neglected in the range of interest