Akira Umemura

Rapid flame propagation in a vortex tube in perspective of vortex breakdown phenomena

A theoretical investigation is made to identify the mechanism responsible for the phenomena of rapid flame propagation in a vortex tube. The consideration is based on the inviscid, three-dimensional flow structure that results from two identical flames steadily propagating away from an (igniting) plane of symmetry within an infinitely long straight Rankine vortex, in which the core is filled with a uniform flammable gas mixture of the same density and temperature as the surrounding unbounded inert gas. A new mechanism is proposed in Part I. Because the radial expansion of vortex tube through thermal expansion across the flame causes the spiraling of axial vortex filaments, a torus of azimuthal vorticity is produced at the shoulder part of the parabola-like flame and the flame tip is convected forward by its induced velocity. A closed form of expression for the steady flame propagation speed is derived from the requirement of single-valued pressure field, which agrees with experimental measurements. In Part II, the proposed mechanism and flame propagation speed expression are validated by numerical simulation.

Atomization Regimes of a Round Liquid Jet with Near-Critical Mixing Surface at High Pressure

Among a large number of reports on atomization, little is known about the characteristic feature of atomization occurring in a gas whose temperature and pressure exceed the critical values of the injected fuel. In the present study, microgravity experiments were conducted to examine the high-pressure atomization regimes of a round, SF 6 liquid jet injected through a 0.1 mm diameter nozzle into N 2 gas. The temperatures of SF 6 liquid and N 2 gas were fixed at 0.94 and 0.95 times the SF 6 critical temperature, respectively, while the pressure of N 2 gas was elevated from 1.3 to 2.4 times the SF 6 critical pressure, so that the surface of the injected jet was in near-critical mixing state and the value of surface tension was smaller than one hundredth of the standard water value. The following were found. As pressure P increases, surface tension decreases and surface gas density increases, which makes turbulent atomization emerge at lower jet speed. The atomization regime of low-speed jet changes drastically at a certain pressure Pt =7.6 MPa. At P Pt , hydrodynamic-assisted capillary instability emerges immediately downstream of the nozzle exit and the liquid jet is disintegrated at a wavelength much shorter than one of Rayleigh instability. The analysis of this breakup mechanism identifies a new hydrodynamic-assisted capillary instability, which has the maximum growth rate proportional to jet speed and, therely, is more unstable at low jet speed than the well-known Taylor instability. The disappearance of this type of capillary instability at P > Pt demonstrates that there exists a threshold value of surface tension for the capillary instability to be excited. At P > Pt , the liquid jet breaks into droplets with short spacing at a relatively large distance from the nozzle exit.

Wave nature in vortex-bursting initiation

A new mechanism responsible for rapid flame propagation in a flammable vortex tube (vortex bursting) has been previously proposed by one of the authors. (1) The vortex tube is radially expanded due to thermal expansion at the flame surface. (2) For the conservation of angular momentum, vortex filaments which constitute the original vortex tube are deformed into spiral form, thus producing a torus of azimuthal-vorticity distribution of the shoulder part of the parabola-like flame surface. (3) By Biot-Savarts law, the azimuthal-vorticity distribution induces an axial velocity at the flame-tip location, so that the flame is convected forth with a speed corresponding to the maximum rotational speed of the vortex tube. In the present study, this theory is extended to examine the transient behavior of a flame propagating in a vortex tube. The most important findings are a wavelike nature of the azimuthal-vorticity distribution, which is produced by the spiraling of axial filaments, and its effect on flame extinction. The azimuthal-vorticity distribution propagates as a wave with its own speed, which is determined from its circulation and form. When the rotational speed of vortex tube is too high relative to the flame heat-release rate, the azimuthal-vorticity torus passes the flame tip, and the burned gas region is immersed in a straining flow which is produced by the circulating flow around the azimuthal-vorticity torus. Thus, the elongated burned gas region is rapidly cooled by the surrounding cold gas, and the tapered flame front suffers from the danger of extinction. The steady flame propagation state is established when the propagation speed of the azimuthal-vorticity wave coincides with the flame-tip propagation speed, which is consistent with the previously proposed expression for steady flame propagation speed.

Characteristics of supercritical droplet gasification

A liquid-fuel droplet exposed to a supercritical ambient gas (its thermodynamic state exceeding the fuel-critical point) may experience the transition to continuous phase change during its lifetime. This transition can be analyzed in numerical simulation by using a fuel-related diffusion coefficient that vanishes at the droplet surface when the surface reaches the critical state of the mixture involved. Based on the numerical simulation of spherically symmetric gasification process, a droplet gasification regime map is constructed that predicts which of the following three gasification regimes is realized for the given values of system pressure, ambient gas temperature, and initial droplet temperature: (1) subcritical gasification regime in which the droplet maintains its surface throughout its lifetime, (2) transitional regime in which the droplet experiences the transition to continuous phase change during its lifetime, and (3) supercritical gasification regime in which continuous phase change occurs from the onset of gasification. The characteristic physics underlying each gasification regime are revealed. The pressure dependence of gasification lifetime for fixed ambient gas temperature and initial droplet temperature shows that there is a pressure corresponding to minimum gasification lifetime for which the droplet surface reaches the mixture-critical state as soon as the droplet is exposed to the ambient gas. The gasification lifetime is reduced significantly by increasing initial droplet temperature. The effects of initial droplet temperature are discussed in detail. Implication of the gasification regime map when applied to sprays is discussed to address future investigations.

Linear acoustic characteristics of supercritical droplet vaporization

Our previously developed numerical model of supercritical droplet vaporization is applied to examine the linear acoustic characteristics of liquid-fuel droplets vaporizing in various supercritical environments. The model assumes the hold of phase equilibrium at the droplet surface and incorporates the property of vanishing diffusion coefficient at the critical mixing surface so that the transition to continuous phase change might be simulated. The rate of work conducted by a vaporizing droplet on its surrounding gas provides a fundamental quantity (volume source strength) to examine the way of acoustic interaction between droplet vaporization and ambient gas state fluctuations. In a dilute spray confined in a chamber, the natural pressure oscillation may be excited or attenuated when the amplification rate, which is derived by differentiating the volume source strength with respect to pressure, takes positive or negative values, respectively. The computed amplification rate exhibits distinct behavior in the three regimes of supercritical droplet gasifications, namely: (1) subcritical gasification regime in which the droplet has the surface throughout its lifetime, (2) transitional gasification regime in which the transition to continuous phase change takes place at a finite radius of droplet surface and (3) supercritical gasification regime in which continuous phase change occurs from the beginning. In the subcritical gasification regime, the value of amplification rate changes from positive to negative in a period of time comparable to the droplet temperature relaxation time, and its magnitude is relatively small. The transition to continuous phase change causes the amplification rate to increase abruptly, whereas in the supercritical regime, the amplification rate takes a vanishingly small value. Thus, unusual pressure oscillations may occur for a spray in the transitional gasification regime. The underlying physics are also explained.

Nonlinear instabilities leading to rapid mixing and combustion in confined supersonic double-shear-layer flow

Direct numerical simulations conducted in the present study show that a slow fuel gas stream issued between supersonic high-temperature air streams confined in a constant-area channel can mix with air quickly to cause explosive combustion along the following processes: (1) linear flaw instability excitation, (2) eddy formation without shocks, fuel flow acceleration to supersonic speed and enhanced mixing with air, associated with fuel layer meandering, (3) explosive combustion, and (4) thermally choked burnt gas flow. The underlying physics of the supersonic instabilities involved are revealed by interpreting the simulation results in an attempt to find an effective mixing enhancement technique. The basic flow configuration consists of a confined, plane, double shear/mixing layer flow with forcing fluctuations at the inlet. The difference in velocities between inlet air and fuel streams is supersonic. The reflection condition imposed at the walls serves to disturb acoustically the double shear layer flow in such a way that the walls reflect Mach waves radiated from the inlet disturbance. The most unstable wave excited downstream is skew-symmetric with respect to the centerline, thus leading to the meandering of fuel layer accompanied by Karman-vortex-like eddies. A series of instability excitations couples with the fuel layer meandering in a confined flow region, enhances the exchange of momentum and species between the fuel and air streams, thus accelerating the mixture to a supersonic speed within a short distance prior to explosive combustion. The behavior of the flame front resembles that of lifted turbulent-jet flames. Flame flashback, stationary flame front and flame blowout take place, depending on the inlet condition. Their criteria are provided in terms of the Chapman-Jouguet detonation wave speed.

Spray Group Combustion

Starting from a consideration of microscopic flame propagation modes between neighboring droplets and macroscopic flame propagation modes in spray elements, the excitation mechanism of group combustion (diffusion flame enclosing droplets) is described for an example of atomizing liquid fuel jet issuing into an otherwise stagnant oxidizing atmosphere.

Microgravity experiments on ISS in order to examine a new atomization theory discovered through normalgravity and microgravity environments

In order to elucidate turbulent atomization processes, many studies by the use of a liquid jet issuing from a circular nozzle have been conducted for a long time. Although Rayleighs instability has been regarded as the only determinant for the breakup of the liquid jet, the source of the disturbances has been unclear and thus the physical explanation of experimental results was impossible. From our experimental and numerical approaches under normalgravity and microgravity environments, it was found that the breakup by the short-wave mode occurs around the tip of the liquid jet without any disturbances. The long-wave mode is caused by the existence of a nozzle exit through a self-closed breakup cycle sustained inherently by the capillary waves emanated from the tip of the liquid jet after every breakup. Our further experiments revealed the existence of the relaxation region which gives a reasonable explanation of the extremely large breakup length. In addition, the two-valueness of the breakup length was found through a lot of experimental results. Establishment of a new breakup theory enable to explain all of experimental results requires long-period microgravity environments and the currently-projected experiments on ISS are introduced in the present paper.

Numerical and experimental study on flame propagating mechanism of a fuel droplet array

The major goals of the researches in combustion science are to provide predictive and controlling capabilities to enhance combustion technology and fire safety. In other words, the practical motivations for combustion study and application research are, owed to the widespread dependence on combustion processes in modern societies. Furthermore, environmental concerns recently dominate needs of combustion research for the realization of low-emissive, efficient energy generation and utilization. The utilization of microgravity is an exceedingly useful tool for the scientific research for realization of physics and dynamics of combustion phenomena, such as spray combustion. Many experiments have been conducted in order to investigate combustion phenomena without natural convection. Now in addition, numerical simulation becomes useful method for further understanding of combustion phenomena. From the fundamental viewpoints, the interactions among fluid dynamics, scalar transport, thermodynamics and chemical kinetics that are characteristic of combustion phenomena have been investigated by above experimental and numerical methods. This paper reports about our efforts on microgravity combustion study. In order to describe the characteristic mode of droplet-to-droplet flame propagation, we conducted the numerical simulations and dropshaft experiments comparing with theoretical prediction models of flame propagation.