Baochun Fan

Investigations of secondary explosions induced by venting

A series of vented explosion tests were performed in a small‐scale cylindrical vessel using a short discharge duct. Overpressure–time profiles were recorded with five transducers both inside and outside of the vessel. Visualization of the flow field is provided by a high‐speed shadowgraph imaging system using sequential shadowgraphs. As is well known, two or more pressure peaks can occur during explosion venting, one from the vent rupture and the other from a secondary explosion induced by combustion of the vented combustible gases. Three typical distinct processes of vented explosions were found. Variations in intensity of the secondary explosion were measured as a function of vent burst pressure, ignition location, blockage ratio at the vent, and equivalence ratio of the fuel.

CONTROL OF VORTEX SHEDDING BEHIND A CIRCULAR CYLINDER USING ELECTROMAGNETIC FORCES

Both open and closed loop control algorithms have been developed for manipulating wake flows past a solid cylinder in an electrically low-conducting fluid. The intent is to avoid vortex shedding and flow separation from the body, which is achieved through the introduction of localized electromagnetic forces (Lorentz forces) in the azimuthal direction generated by an array of permanent magnets and electrodes on the surface of the circular cylinder. The array of actuators offers the advantage of making the Lorentz force time and space dependent. More specifically, one closed loop control method has been derived from the equations of motion capable of determining at all times the intensity of the Lorentz force in order to control the flow. This is accomplished first, independently of the flow (open loop algorithm) and second, based on some partial flow information measured on the surface of the solid body (closed loop algorithm).

Experimental and Numerical Study on Detonation Propagating in an Annular Cylinder

Gaseous detonation propagating in an annular cylinder was studied for hydrogen/oxygen/nitrogen mixtures numerically and experimentally. In experiments performed in an annular combustor, pressure gauges were used to measure the pressure and propagation velocity, and smoked foil measurement was performed to record the cellular structure. Meanwhile, based on the two-dimensional Euler equations with detailed finite-rate chemistry, the numerical calculations were performed by a fifth-order weighted essentially non-oscillatory (WENO) scheme and a semi-implicit Runge–Kutta method. The results show that the detonation is strengthened near the outer concave wall and weakened near the inner convex wall, so that the detonation front is continuously realigning itself to the local channel axis, to maintain the detonation steadily propagates with a planar front. In addition, the local explosion occurs on the inner wall periodically, which protects the detonation from quenches. The cellular size near the outer concave wall is smaller than that near the inner convex wall.

Wavelet Structure of Wedge-Induced Oblique Detonation Waves

An oblique detonation wave (ODW) for a Mach 7 inlet flow over a long enough wedge of 30° turning angle was simulated numerically using an Euler equation with one-step rection model. The fifth-order weighted essentially non-oscillatory (WENO) scheme was adopted to capture the shock wave. The numerical results show that three regions in the flow field behind ODW are defined: Zeldovich–von Neumann–Doering (ZND) model-like structure; single-sided, triple-point structure; and dual-headed, triple-point strucuture according to the wavelet structures. The first structure is smooth and straight. The latter two structures are very complicated. In the single-sided triple-point structure, all triple points facing upstream propagate dowanstream with almost the same velocities and have the character of temporal periodicity. Simultaneously, the triple-point traces are recorded to obtain cell structure of parallel straight lines. In the last structure, the triple points move down with two different velocities. The velocity of triple points facing downstream is obviously faster than that facing upstream, which leads to the periodic collisions of the triple point. This period has the character of temporal and spatial periodicity. The cell structure is inclining “fish scale” patterns due to the velocity component of the incoming flow tangential to the oblique detonation wave.

Investigation on External Explosions during Venting

The external explosions may occur in an explosion venting and give a potential risk. However, the basic dynamic process of external explosions during the venting to ambient air is yet not well understood. In this paper, a series of vented explosion tests in high failure pressure has been conducted in a cylindrical venting vessel. It has been demonstrated clearly from the pressure-time histories and the shadowgraphs of the external flow field that under some suitable venting conditions there exist two peak pressures in the external flow field, one is generated from the membrane rupture and the other is induced by the external explosion due to the violent combustion of the vented combustible gas. The effects on the flow patterns outside the venting vessel under different failure pressures, vent areas (blockage ratio) and chemical equivalent ratios are also discussed in the term of the experimental results. Moreover, the venting process was simulated numerically by using SIMPLE schemes in colocated grid, based on the k-e turbulent model and ‘eddy dissipation combustion model’. The calculated results are in good agreement with the measured results. The dominant mechanisms of the occurrence of the external explosion during the venting processes have been elucidated based on measured and calculated results.

Observations of flame behavior during flame-obstacle interaction

A series of experiments have been performed to investigate the premixed flame behavior during its propagation in a rectangle chamber with various obstacles mounted on its wall. A high speed shadowgraph system was used to visualize the flame front development. Three different obstacles were used, and their corresponding sequential images are presented and used to quantify the shape and location of the flame front during its propagation. Our experimental results for flame acceleration showed similar results as the literature: the flame accelerates as it passes over the obstacles and then decelerates due to the existence of a recirculation zone behind the obstacles. However, significant changes in flame structures and turbulent transitions were found for the three different obstacles. It is found that the passage, which is composed of the surface shape of the obstacle and the inner wall of chamber, is critical to the shape of the flame front and propagation of the flame. The obstacle with the largest volume results in the fastest flame acceleration.