摘要:

Most of the previous work on periodic limit cycles in linearly unstable combustors has been carried out for the case of purely longitudinal classical modes whose natural frequencies are integral multiples of the fundamental. The contrary situation of transverse modes in a cylindrical chamber has led to unexpectedly high amplitudes in the periodic limit cycles, the reasons for which have not been thoroughly understood. Some results reported here serve to clarify the two chief consequences of nonlinear coupling between modes: the first is obvious, for the nonlinear coupling causes excitation of higher modes, and hence energy transfer, when, say, the first mode is unstable; the second has long been known, namely that the coupling must also cause shifts of the frequencies from their linear non-integral values to the integral values required for a periodic limit cycle, but part of the significance of this property has been unclear. In particular, the necessity for the frequency shifts reduces the efficiency at which energy is transfered between modes and leads to higher amplitude oscillations. Our analysis shows that the larger are the differences of the linear frequencies from perfect integral values, the larger are the amplitudes in the limit cycle. These conclusions suggest a way to reduce the amplitudes of transverse oscillations, and examples are presented showing this possibility.

ISSN:0010-2202    

DOI:10.1080/00102209908924195

出版商:Taylor & Francis Group    

年代:1999

数据来源: Taylor

摘要:

Premixed-gas flames subject to steady spatially-varying straining flows were studied to examine one aspect of premixed flames in strongly turbulent flows where strain rate gradients are present and local strain rates may be high enough to cause local flame-front extinguishment. The spatially-varying straining flows were created using a counterflow slot-jet burner with slightly non-parallel jet exits. When the flow configuration was premixed combustible gas vs. cold inert gas, so that only a single flame was produced, steady flame “edges” could be created where the flame would exist in the low-strain region but would be extinguished in the high-strain region. When the flow configuration was premixed gas vs. premixed gas, twin flames would exist in the low-strain region that converged to a corner-like tip in the high-strain region. For both configurations the local strain at the location of the stationary flame edge was somewhat lower than the strain required to extinguish flames in the same mixture subject to a spatially uniform strain. The difference was greater for the twin-flame configuration, particularly at high Lewis number (Le). Due to diffusive-thermal instabilities, cellular flames were observed at low Le and travelling-wave patterns were observed at high Le. Le effects also led to the formation of isolated “flame tubes” rather than continuous fronts at sufficiently low Le and high strain rates. All of these results are consistent with recent theoretical predictions. These results indicate that “laminar flamelet” models of premixed turbulent combustion may be reasonably accurate for single flames over a wide range of Le and twin flames with Le close to unity, even at conditions approaching those where local flame quenching occurs, but may not be accurate for twin-flames except for Le near unity. This finding is somewhat different from previous experimental and theoretical results fornonpremixededge-flames, where more substantial differences between uniform flames and edge-flames were found for all Le.

ISSN:0010-2202    

DOI:10.1080/00102209908924196

出版商:Taylor & Francis Group    

年代:1999

数据来源: Taylor

摘要:

A turbulent combustion model based an the probability density function (PDF) approach has been extended for the spray combustion computations under simulated diesel engine conditions. This approach accounts for the effects of turbulence and of random dynamics of evaporating droplets on the mean rate of chemistry. The paper begins with formulation of the equation for the joint PDF for the gaseous mixture variables where evaporating droplets are viewed as point sources. Then the modified micromixing model that involves evaporation process is described and details of a Monte-Carlo modeling of PDF-equation are given. After that, the results of numerical studies dealing with diesel spray combustion are discussed. Three different examples are considered. First, the computations of the evaporating spray injected into heated nitrogen atmosphere are carried out. The contribution of evaporating droplets dynamics to the mean and variance distributions of temperature and vapor concentration is demonstrated. Next, the spray combustion under light-duty and heavy-duty diesel conditions is simulated, and the results are compared with calculations using the Eddy-Break-Up combustion model, and with experimental data. It is shown that the PDF equation model is able to predict experimental data significantly better than the Eddy-Break-Up model. The last part of computations concerns the diesel spray autoignition governed by the strong turbulence effects. Spatial probability distributions of self-ignition sites are displayed and compared with experimental observation. It is shown that the region with high probability of ignition sites occurs at the level of dense spray and is displaced towards the nozzle hole if the inlet air temperature and pressure increase.

ISSN:0010-2202    

DOI:10.1080/00102209908924197

出版商:Taylor & Francis Group    

年代:1999

数据来源: Taylor

摘要:

Planar laser-induced fluorescence (PLIF) measurements of nitric oxide (NO) have been obtained in atmospheric pressure, laminar, partially premixed ethane-air flames. The objective was to quantify PLIF measurements in partially premixed flames across which there exists a significant range of quenching environments. In particular, we investigated two flame configurations with an overall equivalence ratio of 0.5 and fuel-tube equivalence ratios of 1.33 and 2.22, respectively. Each PLIF image was made semi-quantitative by scaling the entire image based on a single laser-saturated fluorescence (LSF) point measurement. The quantified PLIF measurements were then assessed by comparison with separate two-dimensional arrays of LSF measurements. Good agreement is found between quantified PLIF and LSF measurements in both flames at all elevations. In addition, a majority of the PLIF measurements fall within the uncertainty of the LSF measurements.

ISSN:0010-2202    

DOI:10.1080/00102209908924198

出版商:Taylor & Francis Group    

年代:1999

数据来源: Taylor

摘要:

The propulsive potential of a reactive mixture that uses detonation as a combustion process is studied. An experimental set up is built up to determine the thrust and the impulse developed in single and multi-operating cycles by the detonation products of a reactive mixture contained in a cylindrical combustion chamber (CC). One end of the CC, called the thrust wall (TW), is closed and supports the thrust. The other end is open into atmosphere for the exhaust of the detonation products. The detailed flow features are experimentally investigated by means of pressure gauges inside the CC and in the immediate vicinity of open-ended of CC. The specific impulse Ispreached in our device, with C2H4= 3 O2as a detonative mixture, is about 200 s. The overpressure profiles recorded on the TW show clearly that the flow inside the CC is self-similar. Consequently, a relationship between Ispand the Chapman Jouguet's characteristics of the detonative mixture is established. The maximum operating frequency can be linked to the scale of the reactive mixture contained in CC and the maximum averaged thrust is deduced. The atmospheric shock wave produced is found to be equivalent to that of a strong point source of explosion of energy EOliberated by the reactive mixture. For the pulsed detonation results, the same thrust and impulse profiles as in single shot was obtained, but it was found a deficit about 30% in the level of thrust and consequently on the specific impulse. This deficit is mainly due to the quality of the filling of the CC and of the renewing the reactive mixture because of low value of length to diameter ratio of CC.

ISSN:0010-2202    

DOI:10.1080/00102209908924199

出版商:Taylor & Francis Group    

年代:1999

数据来源: Taylor