Flow, Turbulence and Combustion最新文献

Flow instability such as rotating stall and even surge occurs when the centrifugal compressor stage operates under low flow conditions. This phenomenon is an extremely complex dynamic process, and it is closely related to the aerodynamic performance and internal flow of the stage. Therefore, it is necessary to study the flow development characteristics in the stage. This paper employs experimental measurement and full-annulus numerical simulation to investigate the effects of diffuser stall on the aerodynamic performance of the compressor and the internal flow of the impeller. The propagation direction, speed, evolution characteristics, and the number of the stall cell were obtained by experimental measurement, and the numerical simulation method was verified. The numerical results that there is a stall limit cycle with counter-clockwise rotation between the flow rate and total pressure ratio of the compressor when the diffuser stalls. Meanwhile, it is found that the stall limit cycle is closely related to the separation strength of the internal flow in the compressor. Finally, the coherent flow structure near the vane shroud side is identified by the modal decomposition methods when the diffuser stalls. The research results in this paper promote an in-depth understanding of the stall mechanism of centrifugal compressors.

Spatiotemporal wall temperature (T_wall) distributions resulting from flame-wall interactions of lean H₂-air and CH₄-air flames are measured using phosphor thermometry. Such measurements are important to understand transient heat transfer and wall heat flux associated with various flame features. This is particularly true for hydrogen, which can exhibit a range of unique flame features associated with combustion instabilities. Experiments are performed within a two-wall passage, in an optically accessible chamber. The phosphor ScVO₄:Bi³⁺ is used to measure T_wall in a 22 × 22 mm² region with 180 µm/pixel resolution and repetition rate of 1 kHz. Chemiluminescence imaging is combined with phosphor thermometry to correlate the spatiotemporal dynamics of the flame with the heat signatures imposed on the wall. Measurements are performed for lean H₂-air flames with equivalence ratio Φ = 0.56 and compared to CH₄-air flames with Φ = 1. T_wall signatures for H₂-air Φ = 0.56 exhibit alternating high and low-temperature vertical streaks associated with finger-like flame structures, while CH₄-air flames exhibit larger scale wrinkling with identifiable crest/cusp regions that exhibit higher/lower wall temperatures, respectively. The underlying differences in flame morphology and T_wall distributions observed between the CH₄-air and lean H₂-air mixtures are attributed to the differences in their Lewis number (CH₄-air Φ = 1: Le = 0.94; H₂-air Φ = 0.56: Le = 0.39). Findings are presented at two different passage spacings to study the increased wall heat loss with larger surface-area-to-volume ratios. Additional experiments are conducted for H₂-air mixtures with Φ = 0.45, where flame propagation was slower and was more suitable to resolve the wall heat signatures associated with thermodiffusive instabilities. These unstable flame features impose similar wall heat fluxes as flames with 2–3 times greater flame power. In this study, these flame instabilities occur within a small space/time domain, but demonstrate the capability to impose appreciable heat fluxes on surfaces.

The results of a two-dimensional direct numerical simulation of a lean n-octanol-ethanol fuel blend under Reactivity Controlled Compression Ignition (RCCI) conditions are presented in this paper. Stratified temperature and high reactivity fuel fields of Gaussian, bi-modal, and log-normal distributions are studied for uncorrelated and correlated scenarios. The pimple loop is made to run twice to achieve compression heating. A chemical mechanism with 171 species and 734 reactions was developed to capture autoignition characteristics and flame propagation reasonably well. Diagnosing techniques published in the literature are used to determine whether the flame fronts are spontaneously propagating or not. As reported previously for other fuel blends under RCCI conditions, both deflagration and spontaneous ignition flame fronts are observed to co-exist. Gaussian, bi-modal, and log-normal fields respectively move towards a spontaneously igniting mode. Correlating temperature and high reactivity fuel fields not only makes combustion more spontaneously igniting but also more premixed. The analysis reveals the sensitivity of the DNS results with respect to the initial conditions which accordingly should be chosen with care.

Thermal anemometry sensors for time-resolved velocity measurements average the measured signal over the length of their sensor, thereby attenuating fluctuations stemming from scales smaller than the wire length. Several compensation methods have emerged for wall turbulence, the most prominent ones relying on the small-scale universality in canonical flows or on the reconstruction based on two attenuated variance profiles obtained with sensors of different length. To extend these methods to non-canonical flows, the present work considers various adverse-pressure gradient (APG) turbulent boundary layer (TBL) flows in order to explore how the small-scale energy is affected in the inner and outer layer and how the two prominent correction methods perform as function of wall-distance, wire length and flow condition. Our findings show that the increased levels of small-scale energy in the inner, but also outer layer associated with APG TBLs reduces the applicability of empirical methods based on the universality of small-scale energy. On the other hand, a correction based on the relationship between the spanwise Taylor microscale and the two-point streamwise velocity correlation function, is able to correct the attenuated profiles of non-canonical cases. Combining the strength of both methods, a composite profile for the spanwise Taylor microscale is suggested, which then is used for the correction of probe-length attenuation effects across a multitude of flow conditions.

In this work, numerical investigations were performed using large eddy simulations and validated against detailed measurements in the CeCOST swirl stabilised burner. Both cold and reactive flow have been studied and the model has shown a good agreement with experiments. The verification of the model was done using the LES index of quality and a single grid estimator. The cold flow simulations predicted results closely to experiments setting baseline for the reactive simulations. Coherent structures like the vortex rope above the swirler and a precessing vortex core in the combustion chamber were identified. The reactive conditions were modelled with the Flamelet generated manifold and artificially thickened flame models. Simulations were performed for an experimental syngas composition from black liquor gasification at three different CO₂ dilution levels. Three different Reynolds numbers were investigated with the model matching closely to experimentally detected 2D flow field and OH for the most CO₂ diluted mixture. It was found that the opening angles of the flames differ by a maximum of 13% between experiments and simulations. The most diluted fuel investigated experienced a liftoff distance of 23.5 mm at the Re 25 k. This was also the highest liftoff distance experienced in this cohort of fuels. The same fuel also proved to have the thickest flame annulus at 78.5 mm. Overall, in cases with no experimental data available the predictions made by the model follow the same trends which hints its applicability to higher Re cases.

A Direct Numerical Simulation (DNS) database of statistically planar flames ranging from the wrinkled flamelets to the thin reaction zones regime and DNS data for a Bunsen premixed flame representing the wrinkled flamelets regime have been utilised to evaluate the fractal dimensions of flame surfaces using the filtering dimension method, the box-counting algorithm and the correlation dimension approach. The fractal dimension evaluated based on the fully resolved three-dimensional data has been found to be reasonably approximated by adding unity to the equivalent fractal dimension evaluated based on two-dimensional projections irrespective of the methodology of extracting fractal dimension. This indicates that the flame surface can be approximated as a self-similar fractal surface for the range of Karlovitz and Damköhler numbers considered here. While all methods, provide results identical to each other for benchmark problems, it has been found that the fractal dimension evaluation based on box-counting method provides almost identical results as that obtained using the filtering dimension method for both three and two dimensions, while the fractal dimensions based on the correlation dimension tend to be slightly smaller. The findings of the current analysis have the potential to be used to reliably estimate the actual fractal dimension in 3D based on experimentally obtained 2D binarised reaction progress variable field. The inner cut-off scales estimated based on all three methodologies yield comparable results in terms of order of magnitude with the box-counting method predicting a smaller value of inner cut-off scale in comparison to other methods. The execution times for fractal dimension extraction based on filtering dimension and box-counting methodologies are found to be comparable but the correlation dimension method is found to be considerably faster than the two alternative approaches and provides results consistent with theoretical bounds in all cases.

The filtered tabulated chemistry (FTACLES) approach utilizes data from pre-tabulated explicitly filtered 1D flame profiles for closure of the LES-filtered transport terms. Different methodologies are discussed to obtain a suitable progress variable c from detailed chemistry calculations of a methane/air flame. In this context, special focus is placed on the analytical modeling of the reaction source term using series of parameterized Gaussians. For increasing effective filter sizes in LES (i.e. including the flame thickening) the precise shape of the reaction rate profile becomes less and less relevant. In particular, it is shown that for one-step chemistry, a single Gaussian is sufficient to derive an explicitly expressible 1D flame profile with a prescribed laminar flame speed and thermal flame thickness. The resulting artificial flame profile is shown to have similarities with profiles based on carbon chemistry and detailed reaction mechanisms. Next, the behavior of the filtered c-transport equation is analyzed and several possible closure methods are compared for a wide range of filter widths. It is shown that the unclosed contribution of the filtered diffusion term can be combined with the subgrid convection term, thus simplifying the FTACLES formulation. The model is implemented in OpenFOAM and validated in 1D for a variety of LES filter sizes in combination with artificial flame thickening. A power-law-based wrinkling model is modified for use with artificial flame thickening and combined with the FTACLES model to enable 3D simulations of a premixed turbulent Bunsen burner. The comparison of 3D Large Eddy Bunsen flame simulations at increasing levels of turbulence intensity shows a good match to experimental results for most investigated cases. In addition, the results are mostly insensitive to the variation of the mesh size.

Aerodynamic and structural analysis was conducted for a generic supersonic combustion demonstrator designed to operate under flight conditions at an altitude of 23 km and a speed corresponding to Mach number 5.8. Optimization methodologies were applied to the compression section of the model to ensure the required temperature and Mach number conditions at the combustion chamber entrance for the spontaneous combustion of hydrogen fuel, as well as to the expansion section to meet the Brayton thermodynamic cycle. In the aerodynamic analysis, both analytical and numerical approaches were considered for cases without fuel injection and with fuel burning, treating air as a calorically perfect gas without viscous effects. In the structural analysis, only the case with fuel burning was evaluated due to its higher structural demands. Additionally, cases with different plate thicknesses (6 mm, 4 mm, 3 mm, and 2.5 mm) were considered, and the components of the scramjet consisted of Stainless Steel 304 (beams and ribs), Aluminum 7075 (side panels and ramps), Inconel 718, or Tungsten (leading edges and combustion chamber entrance). The results of the aerodynamic numerical simulation demonstrated that the designed scramjet was capable of meeting both on-lip and on-corner shock conditions, ensuring maximum atmospheric air capture. In the structural numerical simulation, for sheets thicker than 2.5 mm, the maximum equivalent von Mises stress in the structure was lower than the yield stress of the materials used, indicating that the deformations were within the elastic regime and thus reversible.