The joint influence of the hydrogen additions and the opening ratios on the propagation dynamics of premixed syngas/air flame

To further understand the combustion characteristics of syngas/air, the behaviors of premixed flame at various hydrogen additions and opening ratios were investigated. The combustion vessel is a half-open rectangular duct 50mm square by 900mm long. The shape of the flame is photographed by a high-speed camera and the pressure is measured by a pressure sensor at the closed end of the duct. The effect of the two variables on the flame dynamics are considered through the analysis of the flame shape evolution and the overpressure oscillation. The results show that the tulip flame appeared earlier and the overpressure peak reach the maximum under the condition of 30% hydrogen fraction; with increasing of the opening ratio, the tulip flame appeared later and the overpressure peak gradually decreased. Under the experimental conditions of the opening with the increase of the volumes of hydrogen blend ratio, the overpressure peak increased systematically, but the occurrence time fluctuated. The results show that the influence of the opening ratio and the hydrogen blend ratio pressures is mainly impacted when the hydrogen blend ratio is at least 30%,with a 0.25 opening ratio. At these critical parameters, the overpressure peak reaches a maximum of 0.9bar;when the opening ratio is 0.5, the overpressure peak suddenly dropped to 0.39bar.

Numerical simulation of the influence of pipe length on explosion flame propagation in open-ended and close-ended pipes

The pipeline length exerts great influence on flame propagation characteristics, Realizable [Formula: see text] model and Premixed combustion model were used to study the influence of pipe length on propane-air explosion flame in open-ended and close-ended pipes. Using the numerical model verified by experiments, the changes of flame structure and flame propagation speed are studied. The result showed that the Realizable model was in good agreement with the experimental results. It also proved that the reflected wave produced a strong interference on the flame front, which promoted the formation of tulip flame. Besides, some obvious vortices were usually generated in the burned gas after the tulip flame formed, which will affect the flow field around the flame front and thus exert influence on the flame structure. The formation mechanism of tulip flame as well as the flame self-acceleration is different in open-ended and close-ended pipes. In close-ended pipes, the reflection wave at the pipe end and the reflection-induced countercurrent both promote the formation of tulip flame. As the flame propagates to the pipe end, the flame propagation is inhibited by the compression wave formed by the rapid expansion of combustion products under high temperature. While, in open-ended pipes, the turbulence induced by the opening at the pipe end is the main cause of tulip flame formation. The flame acceleration depends on the combustion reaction of unburned gas, so the velocity of flame propagation continues to increase. Generally, the maximum flame propagation velocity in the open-ended pipe is larger than that in the close-ended pipe.

Two-Phase Flow Large Eddy Simulations of a Staged Multipoint Swirling Burner: Comparison Between Euler-Euler and Euler-Lagrange Descriptions

A two-staged swirling burner is numerically simulated through large eddy simulations. The impact of the liquid phase modeling approach is evaluated comparing the Eulerian and Lagrangian frameworks for two different operation points, full pilot injection and full multipoint injection. For the full multipoint injection, since the operation point is closer to a Lean Premixed Prevaporized (LPP) regime, both liquid phase models present similar flame structure (an M flame). For the full pilot injection, Eulerian and Lagrangian approaches result in different flames for equivalent boundary conditions: the Eulerian simulation produces a ‘tulip’ flame, while the Lagrangian spray forms a lifted flame. To assess the model sensitivity to boundary conditions parameters, complementary Lagrangian simulations are made varying injected droplets’ diameter and spray angle, this time resulting in a ‘tulip’ flame very similar to the Eulerian one. Finally, a last Eulerian simulation is made, where the injected droplets’ diameter is increased, still leading to a ‘tulip’ flame, showing that the strong interaction between liquid phase and flame highly impact the results.

The propagation of premixed flames in closed tubes

A nonlinear evolution equation that describes the propagation of a premixed flame in a closed tube has been derived from the general conservation equations. What distinguishes it from other similar equations is a memory term whose origin is in the vorticity production at the flame front. The two important parameters in this equation are the tube's aspect ratio and the Markstein parameter. A linear stability analysis indicates that when the Markstein parameter α is above a critical value αc the planar flame is the stable equilibrium solution. For α below αc the planar flame is no longer stable and there is a band of growing modes. Numerical solutions of the full nonlinear equation confirm this conclusion. Starting with random initial conditions the results indicate that, after a short transient, a at flame develops when α>αc and it remains flat until it reaches the end of the tube. When α<αc, on the other hand, stable curved flames may develop down the tube. Depending on the initial conditions the flame assumes either a cellular structure, characterized by a finite number of cells convex towards the unburned gas, or a tulip shape characterized by a sharp indentation at the centre of the tube pointing toward the burned gases. In particular, if the initial conditions are chosen so as to simulate the elongated finger-like flame that evolves from an ignition source, a tulip flame evolves downstream. In accord with experimental observations the tulip shape forms only after the flame has travelled a certain distance down the tube, it does not form in short tubes and its formation depends on the mixture composition. While the initial deformation of the flame front is a direct result of the hydrodynamic instability, the actual formation of the tulip flame results from the vortical motion created in the burned gas which is a consequence of the vorticity produced at the flame front.