C102 回転流中に形成されるブンゼン火炎先端部の振動挙動に対するスワール数の効果(オーガナイズドセッション16 : 新世代エンジンのための燃焼改善と排気低減技術)

In most modern-day lean gas turbine combustors, flame is stabilized by swirl flows. A central toroidal recirculation zone (CTRZ), developed by these swirl flows, helps to stabilize the flame. The current study is focused on the effects of swirl number and a central rod on the flow behavior in a lean premixed swirl-stabilized combustor. Turbulence is modeled using the two-equation, realizable k-ϵ model of Reynolds-Averaged Navier–Stokes (RANS), and flamelet generated manifold (FGM) technique is used to model combustion. Both swirl number and central rod have a very significant effect on the flow and the flame in the combustor. The CTRZ moves upstream along with the flame, creating a flashback, with an increase in the swirl number. Also, the presence of a central rod accelerates the flame flashback at high swirl numbers. Higher turbulence is observed in inner shear layers, especially at high swirl numbers. N O x emissions are higher in the regions of high temperature, especially in the CTRZ. These emissions decrease with the increase in the swirl intensities. At low swirl numbers, the case with the central rod exhibits lower N O x production, and at high swirl numbers, the case without the central rod yields better results in the N O x mitigation.

Effect of Thermal Buoyancy on the Two-Dimensional Upward Flow and Heat Transfer Around a Square Cylinder

In this paper, the effect of swirl number on mixing structure and ignition probability in turbulent non-premixed natural gas/air flame has been investigated. The flames are visualized by a digital camera and inverted by Abel’s transformation. Maps of ignition probability are generated by changing the spark position axially and radially while the flow field and mixing topology are established using numerical simulation. Three zones were defined based on the probability of successful ignition. The first one was the ineffective zone where the probability of a successful ignition was less than 20%. The second one was called the transition zone where the probability of a successful ignition was between 20% and 80%, and a successful ignition was highly dependent on the spark location. The third one was called the high-probability zone where the probability of a successful ignition was more than 80%. It was found that although the increment in swirl number improves the rate of mixing, it does not result in a better distribution of successful ignition probability. In high swirl numbers, the high-probability region is very small but at lower swirl numbers, the high-probability region is distributed at the burner exit, resulting IN better ignition characteristics.

This paper presents an experimental investigation of combustion induced vortex breakdown (CIVB) flashback propensity for flames yielded from Hydrogen (H2) - Carbon Monoxide (CO) fuel blends and actual synthesized gas (syngas) mixtures. A two-fold experimental approach, consisting of a high definition digital imaging system and a high speed PIV system, is employed. The main emphasis was on the effect of concentration of different constituents in fuel mixtures on flashback limit. In addition, the effect of Swirl Number on flashback propensity was discussed. The percentage of H2 in fuel mixtures played the leading role to cause CIVB flashback. For a given air mass flow rate, the mixture containing higher percentage of H2 underwent flashback at much leaner condition than that containing less H2. Flashback map for actual syngas fuel compositions showed distinct behavior due to the presence of various diluents in the mixture. CO had significant dominance over H2 that helped retarding flashback propensity. Of the two major diluents carbon dioxide (CO2) and nitrogen dioxide (NO2), CO2 was more dominant. It appeared that the flashback propensity decreased with an increase in Swirl Number. The analysis of flow field of reacting flames revealed the complex vortex-chemistry interaction leading to vortex breakdown and flashback. Based on the experiential results a parametric model similar to Peclet Number approach was developed employing flame quenching concept. A value of the quench parameter, Cquench was obtained from the correlation of flow Peclet Number and flame Peclet Number, which was observed to be dominated by the fuel composition rather than Swirl Number.

Orbital instability and prediction of a Bunsen flame tip motion with burner rotation

Thermal runaway of chemical reactions often lead to secondary decomposition of the material, especially with autocatalytic decomposition characteristics. Autocatalytic decompositions are often characterized by drastic decomposition, intense heat release, high thermal risk and difficultly in control. Quantitative classification of different autocatalytic strengths is significant for the formulation of risk control measures and recognition of the most dangerous scenarios in relief system. In this work, the factor, which is the ratio of the heat released by the initial stage to that of the total reaction, has been defined to effectively and quantitatively assess the strength of autocatalytic decomposition based on the Perez-Benito model that includes two-stage competitive reactions. According to “3σ principle” of the normal distribution, the factor was classified into five grades: 0–5%, 5–18%, 18–82% 82–95%, 95–100% corresponding to the autocatalytic strength: strong, less strong, moderate, less weak and weak, respectively. This method is validated using cumyl hydro-peroxide and dicumyl peroxide, the autocatalytic strength of which are well known to be less strong (7.92%) and weak (100%), respectively. The results are consistent with the profile width, heat-quench-reheat test and the isothermal tests. This method was used to classify the autocatalytic strength of dimethyl sulfoxide, 29.5 mass% nitro-guanidine aqueous solution, and Benz aldoxime. The results show that the autocatalytic strengths of these three materials are strong (0.01%), moderate (31.52%) and less weak (82.67%), respectively. Through the classification assessment, it helps to get a more intuitive understanding of the degree of substance danger from the perspective of kinetics and thermodynamics.

The outcomes of numerical and experimental studies performed on a laminar circular swirling jet impinging on a flat surface is presented. In the numerical studies the computational domain is three-dimensional, which aids in predicting the complexity of the swirl flow field. The impact of inlet velocity profile is also studied. The Reynolds number is between 250 and 1500, with normalized jet outlet to surface spacing ranging from 2 to 12, and swirl number varying from 0 to 0.8. The configuration of the inlet velocity profile greatly influences the cooling characteristics. It was found that the cooling diminishes with rise in normalised nozzle to plate distance, due to the negative effect of increased entrainment and reduction in impingement velocity. The optimum distance from the nozzle to the plate is twice the diameter of the nozzle. In the range of Reynolds number and normalised nozzle to plate distance investigated extreme cooling happens at the point of stagnation. The cooling is superior with a parabolic velocity profile, compared to uniform profile. At a Reynolds number of 1000, the Nusselt number at the stagnation point for the parabolic velocity profile is 30% higher than that of the uniform profile. The stagnation Nusselt number rises by 275% as the Reynolds number increases from 250 to 1500. The relationship between the stagnation Nusselt number (Nus) and the Reynolds number (Re) can be expressed as Nus ∝ Re0.7. From a cooling perspective, pipe nozzle is less responsive to the introduction of swirl than orifice nozzle. Introduction of tangential velocity to a cylindrical jet can either improve cooling or reduce it, contingent on the swirl number and the Z/D. In the scenario of parabolic velocity, the average Nu rises with the swirl number, peaks at a swirl number of 0.4, and then declines thereafter. The average Nusselt number rises by 20% upon the introduction of a swirl with a swirl number of 0.4. With a uniform profile, swirl has a detrimental effect on the heat transfer on the entire area.

The compositional structure of swirl-stabilised turbulent nonpremixed flames

This work experimentally investigates the dynamics of interaction between a propagating vortex ring and density stratified interface of finite thickness. The flow evolution has been quantified using a high speed shadowgraph technique and particle image velocimetry. The spatial and temporal behaviours of the vortex in the near and far field of the interface and the plume structure formed due to buoyancy are investigated systematically by varying the vortex strength (Reynolds number, Re) and the degree of stratification (Atwood number, At). Maximum penetration length (Lpmax) of the vortex ring through the interface is measured over a range of Reynolds (1350 ≤ Re ≤ 4600) and Richardson (0.1 ≤ Ri ≤ 4) numbers. It is found that for low Froude number values, the maximum penetration length varies linearly with the Froude number as in the study of Orlandi et al. [“Vortex rings descending in a stratified fluid,” Phys. Fluids 10, 2819–2827 (1998)]. However, for high Reynolds and Richardson numbers (Ri), anomalous behaviour in maximum penetration is observed. The Lpmax value is used to characterize the vortex-interface interactions into non-penetrative, partially-penetrative, and extensively penetrative regimes. Flow visualization revealed the occurrence of short-wavelength instability of a plume structure, particularly in a partially penetrative regime. Fluid motion exhibits chaotic behaviour in an extensively penetrative regime. Detailed analyses of plume structure propagation are performed by measuring the plume length and plume rise. Appropriate scaling for the plume length and plume rise is derived, which allows universal collapse of the data for different flow conditions. Some information concerning the instability of the plume structure and decay of the vortex ring is obtained using proper orthogonal decomposition.

This work presents a detailed investigation on the stability, flame macrostructure and combustion characteristics of oxy-methane/hydrogen (H2CH4O2CO2) flames in a premixed swirl-stabilized model gas turbine combustor. The effects of equivalence ratio, inlet Reynolds number and inlet swirl number on stability and combustion characteristics were examined experimentally and numerically. A base operation case was selected as a reference to study the effects of the different parameters with equivalence ratio of 1.0, bulk mean throat velocity of 5.2 m/s, and swirl number of 0.98. Fixed compositions of the oxidizer (30%O2/70%CO2) and fuel (50%H2/50%CH4) streams were considered throughout the study. Large eddy simulation (LES) technique was adopted to compute the premixed turbulent flame regime. Experimentally captured flame images were compared with the flame contours obtained from the numerical simulations, and good conformations were observed. The results showed that near-stoichiometry flames exhibit strong inner recirculation zone (IRZ) and are stabilized between two shear layers. At lower equivalence ratios, the flames are corner-stabilized, and the flow is dominated by a single primary eddy in the IRZ. The calculated strain rates predict flame quenching at equivalence ratio of 0.5, which was also confirmed experimentally. It was observed that the effect of Reynolds number on the flame shape is not significant; however, the flow is dominated by larger primary eddy with strong IRZ as the Reynolds number increases. The IRZ also gets stronger with the increase in the flow swirl number. At higher swirl numbers, the flame stabilization occurs at the inner shear layer and the outer recirculation zone (ORZ) diminishes. It was found that the chemical time scale for all cases under investigation is shorter than the Kolmogorov time scale, signifying that the chemical reactions are dominated by the laminar flame speed.

The numerical simulation of multiphase combustion flow field in a pulverized coal gasifier with one single nozzle is conducted by using the method of two dimensional asymmetric swirl model. Based on the eddy-dissipation-concept model, the calculation process includes volume reaction and surface reaction which can simulate the turbulent combustion with multiple mechanisms. By changing the swirl number of the nozzle, a series of parallel simulations are conducted to collect the information on shape and distribution characteristics of reversed flow and flame. The results show that: the flow field inside this gasifier is mainly composed of a central recirculation zone and an external recirculation zone; a lager swirl number can lead to a sharp decline on axial velocity, which means for the one hand it can enhance the particle residence time, but for the other hand it cause a formation of the no reaction filled with coal on the top of gasifier; when the swirl number is under 1.4, the flame showed in an awl shape and the temperature of entire gasifier is too high to gasify; when the swirl number is over 1.6, the high- temperature area exists near the top of gasifier and the flame shows in a flat petal shape, which is easy to turn the flame back to the nozzle; by using the Gauss curve to fit the scatter points, we can obtain a perfect swirl number 1.4.

Numerical simulation is carried out to study the combined effects of rotation and induced swirl on the fully developed turbulent pipe flow at Reynolds number Reb = 4900, based on the bulk velocity and the pipe diameter. The swirl is induced by a body force in the tangential momentum equation, which produces a tangential velocity field in the near wall region. Results from a single swirl along with five different strengths of the rotation are considered. The effects of rotation and swirl on turbulent structures are investigated in detail. Also instantaneous axial velocity fluctuations are provided to visualize the effects on the turbulent structures. Introduction Turbulent flows of liquids or gases through long straight pipes occur in a variety of different industrial applications. Such flows have received considerable attention throughout the years and are fairly well understood today, although some uncertainties still prevails at very high Reynolds numbers; see e.g. Hultmark et al. (2012). Under certain circumstances, however, the streamlines are helical rather than straight lines and the mean flow becomes twocomponential rather than one-componential. This happens if a swirling motion arises or if the pipe is subjected to axial rotation. Swirl may result from a swirl generator or an upstream elbow, whereas axially rotating pipes are found in turbo machinery cooling systems. In both cases, a circumferential componentUθ of the mean velocity vector coexists with the axial mean velocity componentUz. The presence of a circumferential mean velocity component tends to orient the coherent near-wall structures with the local mean flow direction. Besides the tilting of the near-wall structures, the structures may be strengthened or weakened in a two-componential mean flow. Fully developed turbulent flow in axially rotating pipes has been studied experimentally by Murakami & Kikuyama (1980) and Imao et al. (1996) and by means of large-eddy simulations (LES) by Eggels & Nieuwstadt (1993) and direct numerical simulations (DNS) by Eggels et al. (1994) and Orlandi & Fatica (1997). It is observed that rotation results in drag reduction. Recent DNS studies of swirling pipe flow by Nygard & Andersson (2010) showed the same influence of the induced swirl on the axial mean velocity as axial rotation. However, the presence of swirl turned out to have less clear-cut effects on the turbulence field. In the cases with stronger swirl, even drag reduction was reported whereas weak swirl gave rise to excess drag. Swirl and axial rotation both give rise to helical streamlines and it is therefore not unexpected that similarities between these two circumstances can be found. The aim of the present study is to examine how an originally swirling pipe flow reacts to axial rotation. For a given swirl number, five different rotation rates (N = −1,−0.5,0,0.5,1) with both senses of rotation will be studied. To this end, the full Navier-Stokes equations are solved in three-dimensional space and in time on a computational mesh sufficiently fine to resolve the energetic large-eddy structures. Governing equations The governing equations are solved in cylindrical coordinates θ , r, and z. For practical reasons, the variables qθ = ruθ , qr = rur and qz = uz are introduced. Here uθ , ur, and uz are velocity components in the respective coordinate directions. All variables in the governing equations are non-dimensionalized with the centerline velocity of the Poiseuille profile, Up and the pipe radius, R. To simplify, the non-dimensionalized total pressure, ptotal is divided in to three parts as follows: ptotal = P(θ)+ P(z)+ p(θ ,r,z, t). (1) The first part, P(θ), is the artificial transverse pressure component. In order to introduce a swirl in the pipe flow, the azimuthal pressure gradient, dP/dθ , is introduced. The second part of Eq. (1) is the mean axial pressure, P(z) and finally, p(θ ,r,z, t) represents the remaining part of the total pressure. The Navier-Stokes equations in terms of the new variables, in a reference frame rotating with the pipe wall

Steady-state laminar mixed convection of Newtonian fluids in a square cross-sectioned cylindrical annular enclosure with rotating inner wall and heated top cover has been numerically analysed based on axisymmetric incompressible flow simulations. Richardson number, Reynolds number and r_i/R effects on heat and momentum transport have been investigated for the range of Richardson number 0 ≤ Ri ≤ 1, Reynolds number 500 ≤ Re ≤ 2000 and 0.25≤r_i/R≤8 at a representative value of Prandtl number (i.e. Pr=1.0). A scaling analysis has been also carried out in order to elucidate the possible influences of Reynolds, Richardson and Prandtl numbers and r_i/R on the mean Nusselt number. It has been found that the mean Nusselt number (Nu) ̅ demonstrates a monotonically decreasing trend with increasing Ri whereas (Nu) ̅ increases with increasing r_i/R and Re which is consistent with scaling estimation. It is also observed that the flow pattern in the case of purely forced convection (i.e. Ri = 0) is significantly different from those in mixed convection (i.e. Ri> 0). In the case of Ri = 0 (i.e. purely forced convection), a one-cell flow structure with two small vortexes on the top corners is observed for r_i/R≤1, whereas a second cell appears in the flow field for r_i/R> 1 at Re = 1000. On the other hand, in the case of mixed convection (i.e. Ri> 0), two-cell and four-cell flow structures occur in the flow field depending on Ri and r_i/R for the range of Ri, Re and r_i/R considered here at Pr⁡=1.0. Based these observations, a flow regime diagram has been proposed here for mixed convection (i.e. Ri> 0) for the range of Ri, Re and r_i/R analysed in this study.

The influence of swirl on the reattachment length in an abrupt axisymmetric expansion

Experimental study of pre-swirl flow effect on the heat transfer process in the entry region of a convergent pipe