Liquid film behavior in annular two-phase flow under flow oscillation conditions

Experiments were carried out to investigate the effects of sinusoidal forced oscillation of the inlet flow rate on the time variations of local liquid film thickness and the frequencies of large wave’s passing in steam–water annular two-phase flows. The liquid film thickness oscillated with the same period as the inlet flow rate. The mean film thickness in the thin film regions decreased and approached to an asymptotic value with an increase in the oscillation period of the inlet flow rate. This result was consistent with the experimental results of the occurrence of liquid film dryout under flow oscillation conditions reported in the literature. It was hence considered that the axial liquid transport from the thick to thin film regions mitigates the reduction of the critical heat flux caused by the flow oscillation. It was also found that the wave frequency in the thin film region increased with a decrease in the oscillation period. This observation suggested that the disturbance waves contribute to the enhancements of the liquid transport and consequently the critical heat flux associated with the liquid film dryout under flow oscillation conditions.     

Nonlinear combustion instability analysis based on the flame describing function applied to turbulent premixed swirling flames

Instability analysis of swirling flames is of importance in the design of advanced combustor concepts for aircraft propulsion and powerplant for electricity production. Thermoacoustic instabilities are analyzed here by making use of a nonlinear representation of flame dynamics based on a describing function. In this framework, the flame response is determined as a function of frequency and amplitude of perturbations impinging on the combustion region. This model is adapted to the case of confined swirling flames comprising an upstream manifold, an injection unit equipped with a swirler and a cylindrical flame tube. The flame describing function is experimentally determined and is combined with an acoustic transfer matrix representation of the system to provide growth rates and oscillation frequencies as a function of perturbation amplitude. These data can be used to determine regions of instability, frequency shifts with respect to the acoustic eigenfrequencies and they also yield amplitude levels when self-sustained oscillations of the system have reached a limit cycle. This equilibrium is obtained when the amplitude dependent growth rate equals the damping rate in the system. This requires an independent determination of this last quantity which is here based on measurements of the combustor resonance response curve, together with numerical estimates of the flame contribution to the system response. The geometrical parameters of the upstream manifold and flame tube are varied and the corresponding operating regimes are compared with those predicted with the FDF framework. The present demonstration of the FDF framework in a generic configuration indicates that this can be used in more general situations of technological interest.     

Experimental and numerical analysis of buoyancy-induced flow in inclined triangular enclosures

The buoyancy‐induced heat transfer and fluid flow in a triangular enclosure are investigated both numerically and experimentally. The enclosure is heated from one wall and the adjacent wall is insulated. Hypotenuse of the triangle is cooled isothermally. The numerical tests and experiments covered a range of Rayleigh number, Ra, from 1.5 × 10⁴ to 1.5 × 10⁵. The local and average Nusselt numbers are given for different orientation angles. A code was written based on finite difference method in Fortran platform to solve governing equations of natural convection. Experimental and numerical results show good agreement. It is observed that inclination angle can be used as a control parameter for heat transfer.     

Numerical analysis of turbulent swirling decay pipe flow

Turbulent swirling decay pipe flow has been investigated numerically in a vertical straight fixed pipe. The swirling flow is created by means of a rotating honeycomb which produces the solid body rotation at the inlet of the fixed pipe. Since there are no experimental data at the inlet of the fixed pipe; different axi-symmetric approaches may be considered to model the honeycomb effects at the downstream flow. Considering the appropriate approach and using the resulting flow field properties from the exit of the modeled swirl generator which are applied as the inlet boundary condition for the fixed pipe, several high Reynolds turbulence models are used to predict this type of the swirling flow. For wall treatments, both the standard wall function and the two-layer zone model are used. The comparison between the numerical and the existing experimental results shows that the RSM with two-layer zone model is generally more powerful than the others. Results show that the two-equation models with different near wall approaches are fairly well to predict the swirling flow in solid body rotation regions, but they fail to predict the pressure distribution along the pipe wall. Regarding the swirl intensity decay rate, irrespective of the inlet swirl type, the obtained decay rates from computations are in good agreement with the existing experimental results.     

Nonlinear interaction between a precessing vortex core and acoustic oscillations in a turbulent swirling flame

The interaction of a helical mode with acoustic oscillations is studied experimentally in a turbulent swirl-stabilized premixed flame. In addition to a precessing vortex core (PVC), the helical mode features perturbations in the outer shear layer of the burner flow. Measurements of the acoustic pressure, unsteady velocity field and flame emission are made in different regimes including self-sustained combustion oscillations and stable regimes with and without acoustic forcing. The acoustic oscillation and the helical mode create a pronounced rotating heat release rate perturbation at a frequency corresponding to the difference of the frequencies of the two individual mechanisms. Measurements over a wide range of operating conditions for different flow rates and equivalence ratios show that while the helical mode is always present, with a constant Strouhal number, self-excited thermoacoustic oscillations exist only in a narrow region. The interaction can be observed also in cases of thermoacoustically stable conditions when external acoustic modulation is applied to the system. The evolution of the helical mode with the forcing amplitude is examined. High-speed imaging from the downstream side of the combustor demonstrates that the heat release rate perturbation associated with the nonlinear interaction of the helical mode and the acoustic oscillations produces a ”yin and yang” -type pattern rotating with the interaction frequency in the direction of the mean swirl. At unstable conditions, the oscillation amplitude associated with the interaction is found to be significantly stronger in the heat release rate than in the velocity signal, indicating that the nonlinear interaction primarily occurs in the flame response and not in the aerodynamic field. The latter is, however, generally possible as is demonstrated under non-reacting conditions with acoustic forcing. Based on a second-order analysis of the G-equation, it is shown that the nonlinear flame dynamics necessarily generate the observed interaction component if the flame is simultaneously perturbed by a helical mode and acoustic oscillations.     

Predictions of buoyancy-induced flow in various across-shape concave enclosures

The present study was conducted to numerically investigate the steady laminar buoyancy-driven and convection heat transfer characteristics within three different across-shape concave enclosures for the Prandtl number of 0.71 and 4, the Grashof number range 10⁴ ≤ Gr ≤ 2 × 10⁵_, and the gap range 0 ≤ H₁/H₂ ≤ 0.25. The steady Navier-Stokes equations, governing the flow under Boussinesq approximation, are solved with the dimensionless stream function-vorticity formulation in terms of curvilinear coordinates using the finite difference method. The results show that the effects of various shapes, the strength of the vortex is relatively bigger in the rectangular-rectangular concave enclosure than in the rectangular-circular concave enclosure at the same Grashof number. Heat transfer from the different across-shape concave enclosures is evaluated, and flow and heat transfer characteristics are discussed.     

合成气稀释旋流扩散火焰稳定性研究

通过激光诱导荧光(PLIF)和照相等方法研究了合成气稀释旋流扩散燃烧特性。研究了空气、燃料旋流强度及它们相互配合对火焰稳定性的影响,发现燃料空气反向旋流情况中在扩张段出口上方具有较高的OH浓度,说明这种流动组织方式加强了初始阶段的混合,强化了化学反应,从而有利于燃烧的稳定;在所实验的范围内,强化空气旋流和燃料旋流都起到稳定燃烧的作用;在燃料和空气出口附加扩张段能起到稳定火焰的作用,在一定范围内,扩张段张角对火焰形态影响较大,扩张段张角小,火焰细长,扩张段张角大,火焰粗壮。     

A critical review of buoyancy-induced flow transitions in horizontal annuli

The main mechanisms of transition of buoyancy-induced flows in the horizontal annulus between circular cylinders are reviewed, based on the available literature. Both experimental and theoretical studies are considered. The different scenarios for the evolution of the flow regimes and temperature patterns are tracked, for increasing values of the Rayleigh number, Ra. The occurrence of various instability and bifurcative phenomena is pointed out, and linked to other relevant parameters, such as the radius ratio R and the Prandtl number, Pr. Although most of the relevant literature is on 2D cases, the effect of the third dimension is considered as far as possible. Studies on the influence of the eccentricity of the inner cylinder on the laminar flow and the thermal asset are also reviewed. Finally, open questions and topics for future research are hinted at.     

Influence of precessing vortex core on flame flashback in swirling hydrogen flames

This study examines the influence of vortex core precession on flame flashback of swirl-stabilised hydrogen flames. Theoretical considerations suggest that the angular velocity of a swirling flow is reduced as vortex precession causes it to acquire an eccentric motion around the central axis of the burner. The eccentric motion of the vortex generates a secondary flow, which is thought to reduce the angular velocity and tangential momentum available to the primary flow, and thereby reduce the flashback propensity at the centre of the vortex core. Experiments measuring the influence of the eccentric motion of the flame tip on flame flashback behaviour were conducted using high-speed sequences of OH*-chemiluminescence images. Temporal analysis of a large sample of images revealed the existence of a systematic rotational frequency of the flame tip around the central axis of the burner. Analysis of the radial position of the flame tip in relation to its axial propagation velocity showed that flashback velocity increased as the flame tip eccentricity approached zero and flashback velocity decreased as the eccentricity amplitude of the flame tip reached larger values. This suggested that flame eccentricity caused by vortex core precession may be detrimental to upstream flame propagation and may be effective in inhibiting flame flashback in swirl-stabilised flames.     

The prediction of turbulent swirling jet flow

A space marching integration procedure is used to solve the Reynolds equations governing the axisymmetric incompressible turbulent swirling jet flow. Turbulence is modelled by the k−ε model with an isotropic turbulent viscosity. Besides mean velocity field, turbulent properties—such as Reynolds stresses, turbulent kinetic energy and dissipation rate—are obtained and the results are compared with experimental data. Agreement is quite encouraging and shows that the assumption of isotropic turbulent viscosity seems plausible.     

Thermal and emission characteristics of a turbulent swirling inverse diffusion flame

The thermal and emission characteristics of a swirl-stabilized turbulent inverse diffusion flame (IDF) burning liquefied petroleum gas (LPG) were studied experimentally and the results of visible flame lengths, flame temperatures, in-flame gaseous species concentrations and global pollutant emissions were reported.The flame shape and length of the swirling IDF and a non-swirling IDF were compared. The swirling IDF is featured by a large internal recirculation zone (IRZ), which plays an important role in stabilizing and shortening the flame. Compared with the non-swirling IDF, the swirling IDF is shorter, wider and more stable. For the swirling IDF, both temperature and species distributions are uniform in the IRZ. Comparison of the radial NO_x/temperature distributions indicates that the thermal NO mechanism plays a leading role in NO_x formation, since the high-temperature IRZ favors thermal NO production. The effects of air jet Reynolds number (Re) and overall equivalence ratio (Ф) on centerline temperature and emission index were examined. The main finding is that the IRZ which is large in size and high in temperature dominates the thermal and emission characteristics of the swirling flame.Efforts were made to compare the global NO_x and CO emissions of the swirling and non-swirling IDFs. It was found that strong swirl and lean combustion are two key factors for reducing NO_x emission. However, the decreasing NO_x emission is compromised by increasing CO. Under stoichiometric and rich conditions, EINO_x of the swirling IDFs is slightly higher, but the EICO is significantly lower. Further comparison of EINO_x with other studies indicates that the swirling IDF can achieve low NO_x emission.     

Direct numerical simulation and analysis of a hydrogen/air swirling premixed flame in a micro combustor

A three dimensional spatially developing hydrogen/air premixed flame in a micro combustor with a moderate Reynolds number and a high swirl number is studied using direct numerical simulation. The inflow mixture is composed of hydrogen and air at an equivalent ratio of 1.0 in the jet core region, and pure air elsewhere. The maximum axial velocity at the inlet is 100 m/s. A fourth-order explicit Runge–Kutta method for time integration and an eighth-order central differencing scheme for spatial discretization are used to solve the full Navier–Stokes (N–S) equation system. A 9 species 19-step reduced mechanism for hydrogen/air combustion is adopted. Vortex and turbulence characteristics are examined. Two instabilities, namely Kalvin–Helmholtz instability and centrifugal instability, are responsible for the transition from laminar flow to turbulence. A cone-like vortex breakdown is observed both in the isothermal swirling flow and in the swirling flame. One dimensional premixed laminar flame is studied, the structure of which is compared with that of the multi-dimensional one. Probability density functions of the curvature and tangential strain rate are presented. It is shown that the flame curvature has a near zero mean, and the flame aligns preferentially with extensive strain. Finally, the turbulent premixed flame regime diagram is used to characterize the flame. It is found that most of the flame elements lie in the laminar flame regime and the thin reaction zones regime.     

Liquid film dryout in a boiling channel under flow oscillation conditions

A simple theory was developed to elucidate the influence of sinusoidal oscillation of the inlet flow rate on the occurrence of liquid film dryout in an annular two-phase flow regime in a boiling channel. The theory assumes that the critical heat flux (CHF) under an oscillatory condition can be calculated from values in steady states provided that the effect of axial mixing of the liquid film is appropriately considered. The trends of CHFs calculated using a one-dimensional three-fluid model and those experimentally measured under atmospheric pressure were in reasonable agreement with the proposed theory. However, the CHF values measured under oscillatory conditions were usually higher in the experiment than in the numerical simulation, which indicated that axial liquid transport induced by disturbance waves might enhance axial mixing of the liquid film.     

Nonlinear dynamic behavior of non-convective zone in salt gradient solar pond

Non-Convective Zone (NCZ) of salt gradient solar pond is a typical double diffusive system of salinity and temperature, and it is subjected to instable effects of adverse temperature gradient. The onset of instability may occur as an oscillatory motion because of the stabilizing effect of the salinity. In this paper, the marginal state between the steady state and the convection of the NCZ is studied. The stability of the Boussinesq approximation of the Navier-Stokes equations is analyzed by a perturbation approach. The marginal states for the onset of convection are obtained by analytical method, which is based on the linearization of the ordinary differential equations, and then numerical method is used to solve the nonlinear ordinary differential equations. Numerical results provide the trajectories of the temperature and velocity coefficients in the three-dimensional phase space, as well as the two-dimensional temperature, salinity and velocity fields in NCZ. The results demonstrate that the numerical study is in agreement with the marginal stability and the critical Rayleigh number derived from linear stability analysis. Both the linear and nonlinear studies indicate that oscillation is a narrow region above the stable region; however, the nonlinear numerical results indicate that the linear stability analysis leans to a larger upper boundary in the oscillatory regions.     

Combustion characteristics of a swirling inverse diffusion flame upon oxygen content variation

The combustion characteristics of a swirling inverse diffusion flame (IDF) upon variation of the oxygen content in the oxidizer were experimentally studied. The oxidizer jet was a mixture mainly composed of oxygen and nitrogen gases, with a volumetric oxygen fraction of 20%, 21% and 26%, and liquefied petroleum gas (LPG) was used as the fuel. Each set of experiment was conducted with constant oxygen content in the oxidizer. When the oxygen was varied, the changes in flame appearance, flame temperature, overall pollutant emission and heating behaviors of the swirling IDF were investigated. The swirling IDFs with different oxygen content in the oxidizer have similar flame structure involving a large-size and high-temperature internal recirculation zone (IRZ) which favors for thermal NO formation, and the thermal mechanism dominates the NO production for the swirling IDFs. The use of nitrogen-diluted air (with 20% oxygen) allowed the IDFs to operate at lower temperature with reduced NO_x formation, compared to the case of air/LPG combustion (with 21% oxygen). Meanwhile, an increase in CO emission is observed. With oxygen-enriched air (26% oxygen), the increase in temperature and EINO_x under lean conditions is more significant than under rich conditions. With 26% oxygen in the oxidizer stream, the IDF produces: (1) a shorter and narrowed navy-blue flame ring located closer to the burner exit, (2) highly luminous yellow flame extending into the central IRZ and above the blue flame ring, (3) a low CO emission, especially under lean conditions, (4) an increase in temperature at low Ф while a decrease in temperature at high Ф, and (5) an increase in EINO_x at all Ф. The heating test using the swirling IDFs in flame impingement heat transfer reveals that the heating rate can be monotonically increased as oxygen content in the oxidizer jet increases under the lean condition (Ф = 1.0). The oxygen enrichment does not contribute to the heating rate under the rich condition (Ф = 2.0), because for the non-premixed combustion of an IDF, the enrichment in oxygen means a lower oxidizer jet Reynolds number and thus less complete combustion occurs as a result of reduced amount of entrained ambient air.     

Effect of swirling flow on fluidized bed drying of wheat grains

In this paper, batch drying of wheat grains in a fluidized bed dryer, which had a swirling flow field in its drying medium, was experimentally investigated. In the experiments, a laboratory scaled fluidized bed type dryer was used. The effects of the swirling flow field on the drying performance were investigated by using an axial guide vane type swirl generator. The effects of the mass flow rate and temperature of the air on the drying performance were also investigated. The goal of this work is to present the experimental results on the drying process of wheat and the effects of the swirling flow field on the drying performance.     

The effect of surface mass transfer on buoyancy-induced darcian flow adjacent to a horizontal heated surface

The effect of uniform surface mass transfer on the buoyancy-induced flow in a porous medium adjacent to a horizontal heated surface with a power law variation of wall temperature, T_wαx^λ, is considered in this paper. It is found that a similarity solution exists for the problem when $\text{λ = 2}$. Approximate solutions based on the local nonsimilarity method are obtained for other values of λ. Numerical solutions were carried out up to the third-level of truncation and results are presented for a wide range of massflux parameter and wall temperature distributions.     

Effect of hydrogen enrichment on combustion characteristics of methane swirling and non-swirling inverse diffusion flame

Influence of hydrogen addition on appearance of swirling and non-swirling inverse diffusion flame (IDF) along with emissions characteristics are investigated experimentally. The combustion characteristics including flame length, axial and radial temperature variation, and noise level are analysed for hydrogen addition in methane by mass basis for constant energy input and by volume basis for constant volumetric fuel flow rate. Hydrogen addition in methane IDF produces shorter flame by compressing entrainment zone, mixing zone, reaction zone, and post-combustion zone. Hydrogen addition shift these zones towards fuel and air exit from the burner. Enrichment of methane with hydrogen on a mass basis up to 6% reduces CO emission considerably and increases NO_x emission moderately. Effect of H₂ addition on combustion and emission characteristics is more prominent in non-swirling IDF. Combustion noise is augmented with the hydrogen addition and the magnitude of sound level depends on the hydrogen concentration.     

Study on the dynamic process of in-duct hydrogen-air explosion flame propagation under different blocking rates

To effectively analyses the flame propagation of premixed hydrogen-air explosion, this paper carries out a numerical study on the dynamics of flame propagation during hydrogen explosions in a closed duct under different blocking rates. The study shows that flame structure is roughly the same when the flame passes through an obstacle under different blocking rates. The difference in blocking rates only shows a slight difference in the degree of flame deformation. When the flame passes through the obstacle, Rayleigh -Taylor (R-T) instability accompanies the entire flame propagation process and corresponds to each stage flame acceleration. Kelvin-Helmholtz(K-H) instability has a more prominent influence on the tip flame propagation. When the explosion flame propagates, instabilities lead to difference in density gradient and pressure gradient in the duct. Interaction between density gradient and pressure gradient leads to formation of baroclinic torque, which is the main cause of the vorticity. During the flame propagation, the vorticity at the front of the flame is roughly zero, whereas the vorticity formed at the obstacle or in the burned gases is more apparent. The larger the blocking rate, the more prominent the turbulence intensity during the flame propagation.