Shahram Karami

Three turbulent lifted slot-jet flames of different fuel Lewis numbers are studied using direct numerical simulation (DNS). To reduce the computational cost, a one-step chemistry model is employed with a mixture-fraction dependent activation energy reproducing the dependence of the laminar flame speed on the equivalence ratio which is representative of hydrocarbon fuels. In addition to turbulent flames, axisymmetric laminar jet flames with a same chemistry model to that of turbulent flames are simulated. It is found that the maximum reaction rate decreases as the Lewis number increases in the laminar cases. Analysis of the turbulent cases reveals that the extinction limit is affected by the Lewis number of the fuel. The scalar dissipation rate of the extinction limit increases as the Lewis number decreases. For a given positive curvature, the conditional edge-flame propagation velocity on the curvature increases as the Lewis number decreases. This is similar to the observation in pr...

... The results indicated that the proposed design is more compact and efficient than the horizontal tube-type falling film absorber. Raisul Islam et al. [7] described the development of a novel film-inverting design concept for falling-film absorbers. ...

... The results indicated that the proposed design is more compact and efficient than the horizontal tube-type falling film absorber. Raisul Islam et al. [13] described the development of a novel film-inverting design concept for falling-film absorbers. ...

The spatiotemporal dynamics of the coherent structures in an under-expanded supersonic impinging jet are studied using a spectral proper orthogonal decomposition technique. For this analysis, a large eddy simulation of an under-expanded supersonic impinging jet at a pressure ratio of 3.4 and a stand-off distance of 2 jet diameters at a Reynolds number of 50,000 is performed. The mean flow fields illustrate some striking features of this flow, such as an oblique shock, a stand-off shock, a Mach disk, and a recirculation bubble. The spectral proper orthogonal decomposition method is applied to time-resolved three-dimensional flow fields. The accumulative energy of modes within each azimuthal mode number reveals that the first three azimuthal modes contain most of the energy of the flow. The spectra of these azimuthal modes show that the flow exhibits a low-ranked behaviour with discrete frequencies at the optimal symmetric azimuthal mode while other two azimuthal modes have negligible contributions in this behaviour. Three peaks are observed in the spectra of the optimal symmetric azimuthal mode. The spatial fields of the streamwise velocity and pressure of these peaks show that the complex structures are consequences of the under-expansion, Mach disk, and the impingement. Strong hydrodynamic instabilities exist in the shear layer of the jet in the optimal azimuthal mode at each of these dominant frequencies. High-amplitude acoustic waves are also present in the near-field of the jet. These acoustic waves are strong at the nozzle lip, suggesting that a feedback loop linking these two processes exists for dominant frequencies in the optimal mode. High cross-spectrum density of near-field pressure fluctuations and streamwise velocity fluctuations near the nozzle lip at these frequencies confirms the hydro-acoustic coupling, which is necessary to close the feedback loop.

Non-modal instability analysis of the shear layer near the nozzle of a supersonic under-expanded impinging jet is studied. The shear layer instability is considered to be one of the main components of the feedback loop in supersonic jets. The feedback loop is observed in instantaneous visualisations of the density field where it is noted that acoustic waves scattered by the nozzle lip internalise as shear layer instabilities. A modal analysis describes the asymptotic limit of the instability disturbances and fails to capture short-time responses. Therefore, a non-modal analysis which allows the quantitative description of the short-time amplification or decay of a disturbance is performed by means of a local far-field pressure pulse. An impulse response analysis is performed which allows a wide range of frequencies to be excited. The temporal and spatial growths of the disturbances in the shear layer near the nozzle are studied by decomposing the response using dynamic mode decomposition and Hilbert transform analysis. The short-time response shows that disturbances with non-dimensionalised temporal frequencies in the range of 1 to 4 have positive growth rates in the shear layer. The Hilbert transform analysis shows that high non-dimensionalised temporal frequencies (>4) are dampened immediately, whereas low non-dimensionalised temporal frequencies (<1) are neutral. Both dynamic mode decomposition and Hilbert transform analysis show that spatial frequencies between 1 and 3 have positive spatial growth rates. Finally, the envelope of the streamwise velocity disturbances reveals the presence of a convective instability.

A turbulent lifted slot-jet flame is studied using direct numerical simulation (DNS). A one-step chemistry model is employed with a mixture-fraction dependent activation energy which can reproduce qualitatively the dependence of laminar burning rate on equivalence ratio that is typical of hydrocarbon fuels. The basic structure of the flame base is first examined and discussed in the context of earlier experimental studies of lifted flame. Several features previously observed in experiments are noted and clarified. Some other unobserved features are also noted. Comparison with previous DNS modelling hydrogen flames reveals significant structural differences. The statistics of flow and relative edge-flame propagation velocity components conditioned on the leading edge locations are then examined. The results show that on average, the streamwise flame propagation and streamwise flow balance, thus demonstrating that edge-flame propagation is the basic stabilisation mechanism. Fluctuations of the edge locations and net edge velocities are, however, significant. It is demonstrated that the edges tend to move in an essentially two-dimensional elliptical pattern (laterally outwards towards the oxidiser, then upstream, then inwards towards the fuel, then downstream again). It is proposed that this is due to the passage of large eddies, as outlined in Su et al. (2006). However, the mechanism is not entirely two-dimensional, and out-of-plane motion is needed to explain how flames escape the high velocity inner region of the jet. Finally, the time-averaged structure is examined. A budget of terms in the transport equation for product mass fraction is used to understand the stabilisation from a time-averaged perspective. The result of this analysis is found to be consistent with the instantaneous perspective. The budget revealed a fundamentally two-dimensional structure , involving transport both in streamwise and transverse directions, as opposed to possible mechanisms involving a dominance of either one direction of transport. It featured upstream transport balanced by entrainment into richer conditions, while on the rich side, upstream turbulent transport and entrainment from leaner conditions balances the streamwise convection.