Response of partially premixed flames to acoustic velocity and equivalence ratio perturbations

This article describes an experimental investigation of the forced response of a swirl-stabilized partially premixed flame when it is subjected to acoustic velocity and equivalence ratio fluctuations. The flame’s response is analyzed using phase-resolved CH^* chemiluminescence images and flame transfer function (FTF) measurements, and compared with the response of a perfectly premixed flame under acoustic perturbations. The nonlinear response of the partially premixed flame is manifested by a partial extinction of the reaction zone, leading to rapid reduction of flame surface area. This nonlinearity, however, is observed only when the phase difference between the acoustic velocity and the equivalence ratio at the combustor inlet is close to zero. The condition, Δφ_Φ^′-V^′≈0°, indicates that reactant mixtures with high equivalence ratio impinge on the flame front with high velocity, inducing large fluctuations of the rate of heat release. It is found that the phase difference between the acoustic velocity and equivalence ratio nonuniformities is a key parameter governing the linear/nonlinear response of a partially premixed flame, and it is a function of modulation frequency, inlet velocity, fuel injection location, and fuel injector impedance. The results presented in this article will provide insight into the response of a partially premixed flame, which has not been well explored to date.     

Simulations of combustion oscillation and flame dynamics in a strut-based supersonic combustor

This paper numerically investigated the dynamic characteristics of combustion in a model scramjet. Three-dimensional compressible large eddy simulation was performed on a hydrogen fueled combustor and pressure fluctuations were recorded. The analysis of pressure data showed that the combustion processes are intrinsically unstable under supersonic air inflow conditions. Flame dynamics were convinced by the fluctuations in flame lift-off distance away from the strut base. Combined with the corresponding time interval, instantaneous flame speed was calculated. Results indicated that pressure oscillations at different locations show difference in amplitude, frequency, and the underlying control mechanism. Flame front oscillation analysis showed that the flame–shock interaction in the strut recirculation zone was responsible for the combustion instability. Flame dynamics were compared with low-speed turbulent lifted flames. The transition between flame propagation just after the strut and shock-induced combustion in the subsonic bubble at the intersection of two wall-reflected oblique shocks made for the flame stabilization.     

Experimental and numerical investigation of the acoustic response of multi-slit Bunsen burners

Experimental and numerical techniques to characterize the response of premixed methane-air flames to acoustic waves are discussed and applied to a multi-slit Bunsen burner. The steady flame shape, flame front kinematics and flow field of acoustically exited flames, as well as the flame transfer function and matrix are computed. The numerical results are compared with experiments. The influence of changes in the mean flow velocity, mixture equivalence ratio, slit width and distance between the slits on the transfer function is studied, both numerically and experimentally. Good agreement is found which indicates the suitability of both the experimental and numerical approach and shows the importance of predicting the influence of the flow on the flame and vice versa. On the basis of the results obtained, the role and physical nature of convective flow structures, heat transfer between the flame and burner plate and interaction between adjacent flames are discussed. Suggestions for analytical models of premixed flame-acoustics interaction are formulated.     

A concurrent precursor inflow method for Large Eddy Simulations and applications to finite length wind farms

In order to enable simulations of developing wind turbine array boundary layers with highly realistic inflow conditions a concurrent precursor method for Large Eddy Simulations is proposed. In this method we consider two domains simultaneously, i.e. in one domain a turbulent Atmospheric Boundary Layer (ABL) without wind turbines is simulated in order to generate the turbulent inflow conditions for a second domain in which the wind turbines are placed. The benefit of this approach is that a) it avoids the need for large databases in which the turbulent inflow conditions are stored and the correspondingly slow I/O operations and b) we are sure that the simulations are not negatively affected by statically swept fixed inflow fields or synthetic fields lacking the proper ABL coherent structures. Sample applications are presented, in which, in agreement with field data a strong decrease of the power output of downstream wind-turbines with respect to the first row of wind-turbines is observed for perfectly aligned inflow.     

A filtered-laminar-flame PDF sub-grid-scale closure for LES of premixed turbulent flames: II. Application to a stratified bluff-body burner

Large eddy simulation of the two stratified nonswirling configurations of the Cambridge burner studied by Sweeney et al. (2012) is presented. The sub-grid-scale combustion closure relies on a physical space filtering operation with a filter size determined locally depending on the resolved and sub-grid-scale flame properties, which is discussed in a companion paper. Similarly to the premixed configuration of the same burner, the modeling reproduces the differential diffusion effects leading to accumulation of carbon and an enhancement of mixture fraction in the recirculation zone, an effect that is less pronounced than in the fully lean premixed case, because of the modification of the topology of the reaction zone that is induced by the mixture stratification. The study of the LES combustion regimes shows that the reaction zones develop under a quite large range of flame topologies, from wrinkled flamelets up to thin reaction zones. Instantaneous and time-averaged LES data were analyzed to extract information concerning the degree of stratification and the orientation of flame and mixing vectors. A decomposition of the flame response into premixed, diffusion, and partially premixed flamelets is performed, to conclude that the premixed mode dominates close to the burner, with a partially premixed burning regime further downstream. Overall, the length scales associated with stratification were found to be much larger than that of the reaction zone and flame, resulting in a quasi-homogeneous propagation, predominantly in a back supported stratified combustion regime. Overall good agreement between simulation and measurements was obtained for either configurations.     

Large eddy simulation of flow and heat transfer in a rotating ribbed channel

The internal cooling passage of a gas turbine blade equipped with ribs is modeled as a rotating ribbed channel. The flow and heat transfer in the ribbed channel have been investigated by conducting large eddy simulations with a dynamic subgrid-scale model. The Reynolds number considered is 30,000 and rotation numbers are 0, 0.1 and 0.3. The time-averaged results show good agreement with the experimental data. By comparing the present data with those of the smooth channel, it is observed that the vortices shed from the rib induce strong wall-normal motions, and they are augmented on the trailing-wall side by the rotation, resulting in a significant increase in the heat transfer due to rotation. It is also shown that the similarity between the streamwise velocity and temperature is significantly destroyed by both the rotation and the rib itself.     

Coupled large eddy simulations of turbulent combustion and radiative heat transfer

Radiative heat transfer is known to play an important role in combustion processes but is often neglected in simulations because of its complexity and the related numerical costs. An original approach is proposed here to perform large eddy simulations of turbulent combustion including radiation: unsteady combustion and radiative heat transfer are computed by two independent codes that exchange data when needed through a specialized language, CORBA, working on an internal computer network or over the Internet. The radiation code gets temperature and mass-fraction fields from the combustion code and returns radiative energy source terms. This coupling technique is easy to implement, portable, flexible, and versatile. Each code keeps its own structure and may be developed and optimized independently, especially when running on massively parallel machines. Preliminary results show that taking radiative heat transfer into account strongly modifies the flame dynamics, probably because the burnt gas temperature decreases, making the flame stabilization weaker and increasing the flame sensitivity to turbulent motions. Comparisons with experimental data are very encouraging.     

煤粉炉的燃烧分析与燃烧器改造

通过对煤粉锅炉的燃烧分析，从运行角度提出了降低锅炉飞灰可燃物的途径及改进措施，供相关运行人员参考。     

Analysis of acoustic pressure oscillation during vented deflagration and proposed model for the interaction with the flame front

Explosion relief panels or doors are often used in industrial buildings to reduce damages caused by a gas explosion. Decades of research produced a significant contribution to the understanding of the phenomena involved. Among the aspects that need further research, interactions between acoustic oscillations and the flame front is one of the most important. Interactions between the flame front and acoustic oscillations has raised technical problems in lots of combustion applications as well and has been studied theoretically and experimentally in such cases. Pressure oscillations have been observed in vented deflagrations and in certain cases they are responsible for the highest pressure peak generated during the event. At Scalbatraio laboratory of Pisa University a test facility (CVE) was built in order to investigate vented hydrogen deflagrations. This paper presents an overview of the results obtained during several experimental campaigns. The tests are analysed with the focus on the investigation of flame acoustic interaction phenomena. Qualitative and quantitative analysis is presented and the generated possible physical phenomena are investigated.     

Testing of gas and liquid fuel burners for power and process industries

The object of the present paper is a review of issues related to the testing of gas and liquid fuel burners which are amongst the most important items of equipment in materials processing and energy producing industries. The text gives basic information about fuels, types of burners and their testing and also about modelling of combustion and formation of pollutants, mainly nitrogen oxides. The first two sections of the text provide an overview of fuels and burner types. The most part of the paper deals with an assessment of conditions and equipment required for testing of gas and liquid fuel burners. Conditions that must be satisfied in burner tests in order to preserve comparable operating conditions as in the real application are stressed. Last part provides an outline of the utilisation of statistical analysis methods and modelling by computational fluid dynamics (CFDs) including the formation of pollutants.     

Experimental and large eddy simulation results for the purging of salt water from a cavity by an overflow of fresh water

This paper presents the results of an experimental and numerical investigation of a flow in which salt water is purged from a square cavity by an overflow of fresh water. Two numerical simulations are presented, one two-dimensional simulation and one three-dimensional large eddy simulation. The results are used to describe the important transport mechanisms that occur during the purging process. In particular, we propose a mechanism for the formation of the streamers observed in the experiment. We also discuss the performance of the numerical models for flows of this type.     

Evaluation of log‐of‐power extremum seeking control for wind turbines using large eddy simulations

The extremum seeking control (ESC) algorithm has been proposed to determine operating parameters that maximize power production below rated wind speeds (region II). This is usually done by measuring the turbine's power signal to determine optimal values for parameters of the control law or actuator settings. This paper shows that the standard ESC with power feedback is quite sensitive to variations in mean wind speed, with long convergence time at low wind speeds and aggressive transient response, possibly unstable, at high wind speeds. The paper also evaluates the performance, as measured by the dynamic and steady state response, of the ESC with feedback of the logarithm of the power signal (LP‐ESC). Large eddy simulations (LES) demonstrate that the LP‐ESC, calibrated at a given wind speed, exhibits consistent robust performance at all wind speeds in a typical region II. The LP‐ESC is able to achieve the optimal set‐point within a prescribed settling time, despite variations in the mean wind speed, turbulence, and shear. The LES have been conducted using realistic wind input profiles with shear and turbulence. The ESC and LP‐ESC are implemented in the LES without assuming the availability of analytical gradients.     

A filtered-laminar-flame PDF sub-grid scale closure for LES of premixed turbulent flames. Part I: Formalism and application to a bluff-body burner with differential diffusion

A sub-grid scale closure for Large Eddy Simulation (LES) of turbulent combustion based on physical-space filtering of laminar flames is discussed. Applied to an unstructured grid, the combustion LES filter size is not fixed in this novel approach devoted to LES with refined meshes, but calibrated depending on the local level of unresolved scalar fluctuations. The context is premixed or stratified flames, the derived model relies on four balance equations for mixture fraction and its variance, and a progress variable and its variance. The proposed formalism is based on a presumed probability density function (PDF) derived from the filtered flames. Closures for the terms of the equations that are unresolved over LES grids are achieved through the PDF. The method uses flamelet tabulated detailed chemistry and is first applied to the simulation of laminar flames (1D and 2D) over various grids for validation, before simulating a turbulent burner studied experimentally by Sweeney et al. (2012). Since this burner also features differential diffusion effects, the numerical model is modified to account for accumulation of carbon in the recirculation zone behind the bluff-body. A differential diffusion number based on the gradient of residence times is proposed, in an attempt to globally quantify differential diffusion effects in burners.     

Comparison of pneumatic jets and tabs for Active Aerodynamic Load Control

A fast, efficient way to control loads on utility scale wind turbines is important for the growth of the wind industry. Microtabs and microjets are two Active Aerodynamic Load Control devices, which address this need. Both act perpendicular to the surface of the airfoil, and these actively controlled devices are used to mitigate changes in aerodynamic loading experienced by wind turbine rotors due to wind gusts, wind shear, or other atmospheric phenomena. This work explores the aerodynamic effects of microjets and then compares them to those of microtabs. Flow around an airfoil with an activated microjet at the trailing edge has been simulated using the Reynolds‐averaged Navier–Stokes solver OVERFLOW‐2. Using a Chimera overset grid topology, a microjet has been placed near the trailing edge of the lower surface of a NACA 0012 airfoil. For a jet velocity about half of the freestream velocity, the microjet can change the lift up to ΔC_L = 0.2, but the amount of change varies with the momentum coefficient of the jet. The change in lift is not symmetric for positive and negative angles of attack due to changes in the boundary layer thickness with angle of attack. Increasing the Reynolds number reduces the effectiveness of the microjet only slightly. The effects of jet velocity, jet activation time, and airfoil angle of attack on airfoil lift, drag, and pitching moment are compared with previous work, which illustrates the deployment of a microtab at the 95% chord location of a NACA 0012 airfoil. This study shows that microjets and microtabs have very similar responses in lift and pitching moment, but the drag for the microjet is noticeably lower. Copyright © 2013 John Wiley & Sons, Ltd.     

Isolation and identification by 18S rDNA sequence of high lipid potential microalgal species for fuel production in Hainan Dao

To exploit indigenous microalgal species with the potential for biodiesel production, 101 algal cultures were isolated from partial waters in Hainan province. Eight cultures were selected based on their high biomass, high lipid content and ease of cultivation, then identified based on morphology and 18S rDNA sequence analysis. These isolates were identified as Tetranephris brasiliensis DL12, Ankistrodesmus gracilis DL25, Ankistrodesmus sp. CJ02, Desmodesmus subspicatus WC01, A. gracilis CJ09, Chlorella vulgaris CJ15, Desmodesmus sp. WC08, Chlorella sorokiniana XS04, respectively. Desmodesmus sp. WC08 reached the highest biomass concentration (2.32 g L⁻¹) with the lipid content of 31.30%. Higher lipid content of 47.90% and 47.39% were gained by A. gracilis CJ09 and C. vulgaris CJ15, respectively. However, C. vulgaris CJ15 and Desmodesmus sp. WC08 had higher lipid productivity (117.37 mg L⁻¹ d⁻¹and 115.73 mg L⁻¹ d⁻¹, respectively) in terms of comprehensive consideration. The fatty acid compositions of these microalgal species were mainly palmitic, palmitoleic, stearic, oleic with GC–MS (gas chromatography–mass spectrometer) analysis. A. gracilis CJ09, T. brasiliensis DL12, A. gracilis DL25 and Desmodesmus sp. WC08 had the higher oleic acid content (over 50% of the total fatty acids) than the others. The results suggest that marine microalgae strain Desmodesmus sp. WC08 can be the most appropriate candidate for producing oil for biodiesel, based on its higher biomass productivity, lipid productivity and fatty acid profile.     

Review and tests of methods for the determination of the solar cell junction ideality factors

Up to 22 methods for the determination of the solar cell ideality factor, n, have been presented in this paper. Most of them use the single-exponential diode model, static mode, dc regime, forward bias and a single I–V data set.Mostly, n is estimated simultaneously with other parameters and the methods apply to p–n and/or MS and MIS diodes. The disparities in model assumptions, structures investigated and operation conditions are too large to allow simple comparison of the efficiencies of those methods.Thus, the authors have expressed some considerations on those assumptions and operation conditions, together with comments on strengths and weaknesses of the different techniques. Thirteen methods have been assessed experimentally using a commercial c-Si solar cell. The resulting n values range from 1.26 to 1.50 for the temperature and light intensity intervals of [295; 328] K and [0; 1000] W m⁻², respectively. A discussion on the overall results of this work has also been made, showing notably that (i) the assumption of a constant n for any temperature, light intensity and biasing voltage has been found inappropriate by various workers; (ii) dark condition methods give slightly lower n than light condition ones; (iii) n ranges from 1 to 2 for most of the structures published in this work, except for some MIS structures, hetero-junction semiconductors solar cells, metal contacts with amorphous semiconductors, texturized Si devices, and c-Si (second diode in the two-exponential diodes model); (iv) n, I_s and thus Φ_B for Schottky diodes can be determined by different techniques on dark condition, and may be commonly used as evaluating criteria of metal/semiconductor (MS) and metal/insular/semiconductor (MIS) interfacial properties.     

Comparison of Reynolds averaged Navier-Stokes based simulation and large-eddy simulation for one isothermal swirling flow

The flow structure of one isothermal swirling case in the Sydney swirl flame database was studied using two numerical methods. Results from the Reynolds-averaged Navier-Stokes (RANS) approach and large eddy simulation (LES) were compared with experimental measurements. The simulations were applied in two different Cartesian grids which were investigated by a grid independence study for RANS and a post-estimator for LES. The RNG k-ε turbulence model was used in RANS and dynamic Smagorinsky-Lilly model was used as the sub-grid scale model in LES. A validation study and cross comparison of ensemble average and root mean square (RMS) results showed LES outperforms RANS statistic results. Flow field results indicated that both approaches could capture dominant flow structures, like vortex breakdown (VB), and precessing vortex core (PVC). Streamlines indicate that the formation mechanisms of VB deducted from the two methods were different. The vorticity field was also studied using a velocity gradient based method. This research gained in-depth understanding of isothermal swirling flow.     

Average Droplet Diameter Measurement and Results for Fuel Aerosol Injected by Certain Types of the Turbojet Burners

INTRODUCTIONMeasurementofthedropletdiameteroffuelaerosolisverydifficult,becauseofthefastchangeofitssize,whichiscausedbyevaporationandassociation.Itisalsoverydifficulttogetaknowledgeofaerosolstreamsinthecourseofconstructionofnewtypeoffuelburn-ersanddiagnosticprocessaswell.Diffractionmethodisoneofthemostusefulmea-suringproceduresinthiscase.Thedropletsizeofmonodispersivemediumiseasytorecognise.Analy-sisofadiffeactionpatternofpolydispersivemediumisconsiderablymoredifficult.Thisworkpresentsam…