Experimental Studies on Flame Stabilization in Backward Facing Step Micro-Combustors

2011 ◽  
Author(s):  
Anil Deshpande ◽  
Sudarshan Kumar
Keyword(s):  
Experimental Studies ◽  
Flame Stabilization ◽  
Backward Facing Step

scholarly journals Flame Stabilization in Case of Fuel Injection at Downstream of Backward-facing Step.

1992 ◽  
Vol 58 (545) ◽  
pp. 236-241
Author(s):  
Masashi TAKEBE ◽  
Tatsuro TSUKAMOTO ◽  
Susumu HASEGAWA ◽  
Takashi NIIOKA
Keyword(s):  
Fuel Injection ◽  
Flame Stabilization ◽  
Backward Facing Step

Absolute/Convective Instability Transition in a Backward Facing Step Combustor: Fundamental Mechanism and Influence of Density Gradient

2014 ◽  
Vol 137 (2) ◽  
Author(s):  
Kiran Manoharan ◽  
Santosh Hemchandra
Keyword(s):  
Stability Analysis ◽  
Flow Field ◽  
Flow Velocity ◽  
Convective Instability ◽  
Hydrodynamic Instability ◽  
Combustion Instability ◽  
Experimental Studies ◽  
Base Flow ◽  
Flow Density ◽  
Backward Facing Step

Hydrodynamic instabilities of the flow field in lean premixed gas turbine combustors can generate velocity perturbations that wrinkle and distort the flame sheet over length scales that are smaller than the flame length. The resultant heat release oscillations can then potentially result in combustion instability. Thus, it is essential to understand the hydrodynamic instability characteristics of the combustor flow field in order to understand its overall influence on combustion instability characteristics. To this end, this paper elucidates the role of fluctuating vorticity production from a linear hydrodynamic stability analysis as the key mechanism promoting absolute/convective instability transitions in shear layers occurring in the flow behind a backward facing step. These results are obtained within the framework of an inviscid, incompressible, local temporal and spatio-temporal stability analysis. Vorticity fluctuations in this limit result from interaction between two competing mechanisms—(1) production from interaction between velocity perturbations and the base flow vorticity gradient and (2) baroclinic torque in the presence of base flow density gradients. This interaction has a significant effect on hydrodynamic instability characteristics when the base flow density and velocity gradients are colocated. Regions in the space of parameters characterizing the base flow velocity profile, i.e., shear layer thickness and ratio of forward to reverse flow velocity, corresponding to convective and absolute instability are identified. The implications of the present results on understanding prior experimental studies of combustion instability in backward facing step combustors and hydrodynamic instability in other flows such as heated jets and bluff body stabilized flames is discussed.

scholarly journals Inlet Pressure Effects on Subatmospheric Flame Stabilization with an Optimum Size of a Cavity-Based Combustor

2020 ◽  
Vol 2020 ◽  
pp. 1-8
Author(s):  
Yakun Huang ◽  
Xiaomin He ◽  
Zhixin Zhu ◽  
Huanyu Zhu
Keyword(s):  
Equivalence Ratio ◽  
Experimental Studies ◽  
Flame Stabilization ◽  
Combined Cycle ◽  
Pressure Effects ◽  
Inlet Pressure ◽  
Flame Stability ◽  
Characteristic Time Scale ◽  
Optimum Size ◽  
Lean Blowout

Experimental studies are conducted to find an optimum size of the cavity flameholder, which is a new combustion concept of a turbine-based combined-cycle (TBCC) engine with an excellent flame stabilization. Besides, the effect of inlet pressure on the subatmospheric performance is investigated. The experimental results indicate that the increase of the cavity length improves the flame stability with an enlarged fuel/air mixture residence time, which suggests that the big length-height ratio in a proper range of the cavity with a stable dual-vortex should be chosen when designing the cavity-based combustor. In addition, the decrease in lean ignition and the lean blowout equivalence ratios can be attributed by either increase in the inlet pressure and temperature or decrease in the Mach number. The increase in inlet pressure will lead to a linear decrease in the lean blowout equivalence ratio with a slope of 0.66 per 0.1 MPa, whereas the lean ignition equivalence ratio has a rapid drop with the increase of pressure from 0.06 MPa to 0.08 MPa and reduces slowly with the growth of pressure in the range of 0.08 MPa to 0.1 MPa. The detailed analysis of the flow field indicates that the characteristic time-scale theory can ideally explain and predict the change of flame stability in the trapped vortex cavity.

Computational Investigation of Backward-Facing Step Flow Preceding a Porous Medium

2009 ◽  
Author(s):  
C. Krishnamoorthy ◽  
K. C. Ravi ◽  
S. Yao ◽  
F. W. Chambers
Keyword(s):  
Porous Medium ◽  
Experimental Studies ◽  
Turbulence Models ◽  
Reynolds Numbers ◽  
Darcy Law ◽  
Recirculation Region ◽  
Backward Facing Step ◽  
Air Filters ◽  
Step Flow ◽  
Medium Permeability

Optimal performance of air filters and heat exchangers requires uniform inlet flow, but flow separation produces nonuniformity. The backward-facing step flow has a separation resembling those found in industrial flows. Flow resistance of the devices is a parameter which alters upstream pressure gradients, thereby affecting separation and device performance. Air filters often are modeled as porous media using an extended Darcy Law. The present work applied Computational Fluid Dynamics (CFD) to examine the changes in the step flow resulting from the resistance of a downstream air filter. Computations were performed for a backward-facing step with a 2:1 expansion ratio for a case without a filter (reattachment at ∼6 step heights) and for filters located 4.25 and 6.75 step heights downstream. FLUENT commercial CFD software was used and results were compared to many no-filter case results in the literature and our own experimental studies for the step with downstream filters. The simulations were performed for Reynolds numbers based on approach channel mean velocity and hydraulic diameter of 2000, 3750, 6550 and 10000. The different turbulence models available in FLUENT were evaluated and the Realizable k-ε model was used for the final computations. Grid independence studies were conducted. The effects of different values of the filter modeling permeability, inertial constant and thickness also were investigated for Re = 10000 with the filter at 4.25 step heights. It was found that the computational results did not compare well to no-filter cases or the experiments with filters at the lower Reynolds numbers. It is believed that the turbulence models were unsuitable for these flows at transitional Reynolds numbers. Good agreement for no-filter results and for the experiments with filters was observed for Re = 10,000. The CFD model seems to capture the physics of the separation better at the higher Reynolds numbers. The CFD velocity profiles at Re = 10,000 with the filters agree with those of the experiments. When the filter is placed at 4.25 step heights, the flow reattaches upstream of the filter with a reduction in recirculation area. When the filter is at 6.75 step heights, the separated flow tends to reattach and the opposite side tends to separate. At Re = 10,000 and the filter at 4.25 step heights, the variations of porous medium permeability, inertial constant and the filter thickness have negligible effects on the recirculation region over the parameter ranges considered.

scholarly journals Numerical and Experimental Studies on a Syngas-Fired Ultra Low NOx Combustor

2017 ◽  
Vol 139 (11) ◽  
Author(s):  
S. Krishna ◽  
R. V. Ravikrishna
Keyword(s):  
Mixture Fraction ◽  
Experimental Studies ◽  
Flame Stabilization ◽  
Mixing Enhancement ◽  
Temperature Pattern ◽  
Mean Velocity ◽  
Velocity Magnitude ◽  
The Mean ◽  
Trapped Vortex ◽  
Exhaust Measurements

Simulations and exhaust measurements of temperature and pollutants in a syngas-fired model trapped vortex combustor for stationary power generation applications are reported. Numerical simulations employing Reynolds-averaged Navier–Stokes (RANS) and large eddy simulations (LES) with presumed probability distribution function (PPDF) model were also carried out. Mixture fraction profiles in the trapped vortex combustor (TVC) cavity for nonreacting conditions show that LES simulations are able to capture the mean mixing field better than the RANS-based approach. This is attributed to the prediction of the jet decay rate and is reflected on the mean velocity magnitude fields, which reinforce this observation at different sections in the cavity. Both RANS and LES simulations show close agreement with the experimentally measured OH concentration; however, the RANS approach does not perform satisfactorily in capturing the trend of velocity magnitude. LES simulations satisfactorily capture the trend observed in exhaust measurements which is primarily attributed to the flame stabilization mechanism. In the exhaust measurements, mixing enhancement struts were employed, and their effect was evaluated. The exhaust temperature pattern factor was found to be poor for baseline cases, but improved with the introduction of struts. NO emissions were steadily below 3 ppm across various flow conditions, whereas CO emissions tended to increase with increasing momentum flux ratios (MFRs) and mainstream fuel addition. Combustion efficiencies ∼96% were observed for all conditions. The performance characteristics were found to be favorable at higher MFRs with low pattern factors and high combustion efficiencies.

scholarly journals Direct numerical simulation of backward-facing step flow at and expansion ratio 2

2019 ◽  
Vol 863 ◽  
pp. 341-363 ◽  
Author(s):  
A. Pont-Vílchez ◽  
F. X. Trias ◽  
A. Gorobets ◽  
A. Oliva
Keyword(s):  
Incompressible Flow ◽  
Experimental Studies ◽  
Turbulence Models ◽  
Separated Flow ◽  
Numerical Data ◽  
Expansion Ratio ◽  
Backward Facing Step ◽  
Recirculation Bubble ◽  
Practical Applications ◽  
Recirculation Flow

Backward-facing step (BFS) constitutes a canonical configuration to study wall-bounded flows subject to massive expansions produced by abrupt changes in geometry. Recirculation flow regions are common in this type of flow, driving the separated flow to its downstream reattachment. Consequently, strong adverse pressure gradients arise through this process, feeding flow instabilities. Therefore, both phenomena are strongly correlated as the recirculation bubble shape defines how the flow is expanded, and how the pressure rises. In an incompressible flow, this shape depends on the Reynolds value and the expansion ratio. The influence of these two variables on the bubble length is widely studied, presenting an asymptotic behaviour when both parameters are beyond a certain threshold. This is the usual operating point of many practical applications, such as in aeronautical and environmental engineering. Several numerical and experimental studies have been carried out regarding this topic. The existing simulations considering cases beyond the above-mentioned threshold have only been achieved through turbulence modelling, whereas direct numerical simulations (DNS) have been performed only at low Reynolds numbers. Hence, despite the great importance of achieving this threshold, there is a lack of reliable numerical data to assess the accuracy of turbulence models. In this context, a DNS of an incompressible flow over a BFS is presented in this paper, considering a friction Reynolds number ($Re_{\unicode[STIX]{x1D70F}}$) of 395 at the inflow and an expansion ratio 2. Finally, the elongation of the Kelvin–Helmholtz instabilities along the shear layer is also studied.

3-D reconstruction and analysis of mammalian mitotic spindle structure

1992 ◽  
Vol 50 (1) ◽  
pp. 578-579
Author(s):  
Kent McDonald ◽  
David Mastronarde ◽  
Rubai Ding ◽  
Eileen O'Toole ◽  
J. Richard McIntosh
Keyword(s):  
Cross Sections ◽  
Mitotic Spindle ◽  
Experimental Studies ◽  
Video Camera ◽  
Three Dimensions ◽  
Mitotic Spindles ◽  
Black And White ◽  
Reconstruction Methods ◽  
Spindle Structure ◽  
Tissue Culture Line

Mammalian spindles are generally large and may contain over a thousand microtubules (MTs). For this reason they are difficult to reconstruct in three dimensions and many researchers have chosen to study the smaller and simpler spindles of lower eukaryotes. Nevertheless, the mammalian spindle is used for many experimental studies and it would be useful to know its detailed structure.We have been using serial cross sections and computer reconstruction methods to analyze MT distributions in mitotic spindles of PtK cells, a mammalian tissue culture line. Images from EM negatives are digtized on a light box by a Dage MTI video camera containing a black and white Saticon tube. The signal is digitized by a Parallax 1280 graphics device in a MicroVax III computer. Microtubules are digitized at a magnification such that each is 10-12 pixels in diameter.

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