Study Of Solid-gas Two-phase Turbulent Boundary Layer Combustion Using Direct Numerical Simulation

Today,the combustion of fossil fuels is still the main way for energy production in the human society.The combustion processes of these fossil fuels are generally constrained in a volume-limited chamber in the form of solid-gas two-phase combustion.In the near-wall region of practical combustors,there exist extremely complex interactions between boundary layer turbulence,combustion,particles and wall.For example,flames have significant effects on quantities such as wall heat flux and lead to thermal erosion.The flames are wrinkled by turbulence influencing the combustion efficiency.Particle distributions are influenced by coherent turbulence in the boundary layer,which leads to near-wall particle aggregation and particle-wall collision.Flashback can occur in the near-wall low-velocity region.Flames unavoidably interact with the wall,which may lead to flame quenching.An improved understanding of solid-gas two-phase turbulent boundary layer combustion is,therefore,needed for a better design of efficient,clean and reliable combustion devices.However,due to strong shear,strong coupling and strong nonlinearity of two-phase turbulent combustion in the near-wall region.Even though,it is one of the most challenging frontiers in the international field of multiphase turbulent combustion,an improved understanding of this problem is urgently required.This paper develops a numerical experimental platform for high-precision gas-solid two-phase turbulent boundary layer combustion,and pioneeringly performed research on the interactions between boundary layer turbulence,combustion,reacting particles and wall.The results of this paper provide theoretical guidance for solving practical engineering problems and promote the theory of multiphase turbulent combustion.Firstly,in the present work,three-dimensional turbulent non-premixed oblique slotjet flames impinging at a wall were performed using direct numerical simulation(DNS)to investigate the interactions between turbulence,flame and wall.Two cases are considered with the same Reynolds number(Re)but different Damk?hler number(Da)to examine the extinction characteristics.It was observed that reignition occurred after the extinction of the lower branch flame in the case with large Da due to the decrease of scalar dissipation rate with turbulence relaxation.Reignition in the lower branch of combustion for the case with large Da occurs when the scalar dissipation rate relaxes,while no reignition occurs in the lower branch for the case with small Da due to excessive scalar dissipation rate.A method was proposed to identify the flame quenching edges of turbulent non-premixed flames in wall-bounded flows based on the intersections of mixture fraction and OH mass fraction iso-surfaces.The flame/wall interactions were examined in terms of the quenching distance and the wall heat flux along the quenching edges.The quenching distance is found to be negatively correlated with wall heat flux.The influence of chemical reactions and wall on flow topologies was identified.The vortex-dominant topologies appear when reignition occurs.The vortex-dominant topologies play an increasingly important role as the jet turbulence develops.The concept of flame edge was also introduced,and the influence of turbulence and wall on flame was explored.The flame edge speed is negatively correlated with the scalar dissipation rate in regions away from the wall,highlighting the role of turbulent mixing on the flame edge dynamics.During flame–wall interactions,the propagation speed of flame edges is mainly affected by the projection of edge flame normal in the wall-normal direction.In particular,the propagation speed increases with the increasing angle between the edge flame normal and the wall-normal in the near-wall region.Next,the lean premixed hydrogen-flame flashback in a turbulent boundary layer is investigated using DNS.It is found that backflow regions are always present immediately upstream of flame bulges that are convex towards the reactants in the buffer region.The flame bulges lead to an adverse pressure gradient and boundary layer separation.A budget analysis of the pressure transport equation is performed to explain the presence of the backflow regions.It is suggested that the positive dilatation and thermal diffusion terms near the leading edge of flame bulges are the main reasons for the pressure increase,leading to an adverse pressure gradient and the occurrence of backflow.The effects of the flame-induced adverse pressure gradient on the structures of the turbulent boundary layer are also investigated.The hairpin structures of the boundary layer turbulence are lifted by the adverse pressure gradient.The analysis showed that the ejection event is augmented by combustion while the sweep event is attenuated,which facilitates the occurrence of flame flashback.Thirdly,for the problem of particle accumulation and furnace slag-bonding,turbulent boundary layer combustion laden with inert particles over a flat plate is investigated using DNS.Particle accumulation due to the near-wall coherent vorticity is observed.To quantify the particle accumulation in the near-wall region,the concept of Shannon entropy is introduced which was originally used in statistical mechanics and information theory.The analysis of Shannon entropy showed the particle accumulation in the near-wall region which is more prominent in the cases with heavy particles than the light particles.Under the sweep event of the boundary layer quasi-streamwise vortex,outer-region particles are brought into the near-wall region.Although the ejection events throw particles away from the wall,the process is much slower than that of the inward sweep events.Therefore,the residence time of ejection is very long in the viscous sublayer for particles,and many particles are trapped in the low-speed streaks.The process of particle accumulation in the near-wall region is called turbophoresis.The turbophoresis effect is examined via the magnitude of streamwise vorticity.It is shown that the magnitude of streamwise vorticity is attenuated by heavy particles,and the attenuation increases with increasing mass loadings.Therefore,particle wall–accumulation is less prominent in cases with large mass loadings.Compared with the non-reacting cases,the distribution of particles is more inhomogeneous for the reacting cases,where the particles move faster due to intense reactions with increasing wall-normal distance.Subsequently,pulverized coal combustion in a hot turbulent environment is investigated using DNS.The effects of various factors,such as particle size,mass loading and preferential concentration on the combustion process of coal particles were explored to examine characteristics of turbulent pulverized coal combustion.The volatile matter is treated as a postulated substance(CaHbOc)and a two-step global reaction mechanism for volatile combustion is adopted.It was found that,the location of ignition appears more upstream and the volatile matter concentration is higher for the cases with smaller particles.The increase of particle mass loading results in an increase of volatile matter mass fraction,which leads to higher heat release rate and combustion temperature.It was found that the preferential concentration behavior of particles is more prominent when the particle Stokes number(St)is close to unity.The volatile matter concentration is higher when particles are preferentially concentrated,facilitating coal particles to ignite.Finally,this paper reports the first DNS of two-phase turbulent boundary layer combustion laden with pulverized coal.The interactions between boundary layer turbulence,combustion,reacting particles and wall are revealed.It was observed the combustion occurs first in the outer layer of the boundary layer.The combustion is more downstream as the wall-normal distance decreases.This is because the mean temperature of the inflow boundary layer decreases with decreasing wall-normal distance,and the ignition delay time of pulverized coal increases as the wall-normal distance decrease.It was shown that combustion expansion was mainly along the wall-normal direction,driving particles away from the wall.In addition,combustion depresses quasi-streamwise vorticity in the boundary layer,resulting in a decrease in particle accumulation in the near-wall region.Due to the quasi-streamwise vorticity,high-temperature gas and particles from the outer layer were swept toward the wall,raising the local wall heat flux.The characteristics of wall heat transfer were also explored.It was found that the radiation heat transfer to the wall significantly increased after coal particles ignite.