Jianhan Liang

Adaptive Mesh Refinement-Based Numerical Simulation of Detonation Initiation in Supersonic Combustible Mixtures Using a Hot Jet

An open-source program implementing a block-structured adaptive mesh refinement method was adopted for the fine structure numerical simulation of detonation initiation in supersonic combustible mixtures. Simulations were conducted on a nested parallel computing system. The initiation process was specified as three stages, and their respective flow field characteristics were analyzed. Results indicate that a hot jet under specific conditions can have a similar effect as a pneumatic oblique bevel for inducing periodical shock-induced detonative combustion by a bow shock. The interaction of bow shock–induced combustion with the local detonation wave, produced by the reflection shock on the upper wall, can create a structure with two triple-wave points. The hot jet not only plays a role in the detonation initiation but also acts as a stabilizing control mechanism for detonation propagation. In the simulations in this study, the detonation wave propagates in an overdriven state initially and achieves self-sustaining motion after the shutdown of the hot jet. Subsequently, the final pisiform structure of typical stable Chapman-Jouguet detonation cells is formed.

Detonation Initiation and Propagation in Nonuniform Supersonic Combustible Mixtures

Detonation initiation and propagation using a hot jet in nonuniform combustible mixtures was investigated through fine structure simulations. The Mach reflection on the upper wall resulted in the formation of the Mach stem finally near the interface, which was actually a local detonation wave. The continuous collisions of the lower triple-wave point between the lower wall and the upper triple-wave point played an important role in the successful detonation initiation. The dynamic stable structure propagated at the same speed as a whole in the nonuniform supersonic combustible mixtures. After the shutdown of the hot jet, a relatively new dynamic stable structure was formed eventually in the flow field. The hot jet not only could realize the detonation initiation, but also made an impact on the detonation propagation and the formation of the final dynamic stable structure.

Detonation Simulations in Supersonic Combustible Mixtures with Nonuniform Species

Adaptive high-resolution simulations of gaseous detonation using a hot-jet initiation were conducted in supersonic combustible mixtures with spatially nonuniform species. The two-dimensional Euler equations were used as the governing equations in combination with a detailed hydrogen–oxygen reaction model. Three different groups of mixtures, which represent various degrees of chemical reactivity, were investigated. The results show that, when the mixtures generally have a high degree of chemical reactivity, detonation initiation can eventually be realized successfully by Mach reflection as well as the deflagration-to-detonation transition mechanism, independent of the spatial distribution of the mixtures in the channel. A recurring four-stage sequence of detonation initiation, detonation attenuation, initiation failure, and detonation reinitiation can be identified. When the mixtures generally have an intermediate degree of chemical reactivity, detonation combustion can be fully realized in the channel, wh...

Investigation of the spark ignition enhancement in a supersonic flow

Ethylene spark ignition experiments were conducted based on an variable energy igniter at the inflow conditions of Ma = 2.1 with stagnation state T0 = 846 K, P0 = 0.7 MPa. By comparing the spark energy and spark frequency of four typical operation conditions of the igniter, it is indicated that the spark energy determines the scale of the spark and the spark existing time. The spark frequency plays a role of sustaining flame and promoting the formation and propagation of the flame kernel, and it is also the dominant factor determining the ignition time compared with the spark energy. The spark power, which is the product of the spark energy and spark frequency, is the key factor affecting the ignition process. For a fixed spark power, the igniter operation condition of high spark frequency with low spark energy always exhibits a better ignition ability. As approaching the lean fuel limit, only the igniter operation condition (87 Hz and 3.0 J) could achieve a successful ignition, where the other typical op...

Experimental and Numerical Study on Flame Stabilization in a Supersonic Combustor with Hydrogen Injection Upstream of Cavity Flameholders

Experimental observations and numerical simulations were conducted to study the flameholding characteristics and the flame stabilization mechanism in a supersonic combustor with hydrogen injection upstream of a cavity flameholder. OH radical distribution of the combustion flowfiled was obtained using OH spontaneous emission and OH-PLIF (Planar Laser-Induced Fluorescence). The supersonic combustion flowfield with L/D=7 cavity was calculated by large eddy simulations. The combustion model was based on a two scalar partially-premixed flamelet model with a level set approach. The results showed that hydrogen fuels were transported into the cavity shear; lean mixture and rich mixture were produced in internal cavity and declining injected jet respectively. An approximately steady partially premixed flame front exists in the cavity shear layer. The flame front propagates and extends to region around the fuel jet due to the counter-rotating vortices induced by the jet and the cavity shear layer. The flame front sustained in the shear layer likely penetrates through the jet core and ignites the whole jet. Behind the flame front most of the jet beam is ignited and burned as diffusion flames. The physical process of the flame stabilization demonstrated the similarity with a triple flame theory or edge flame concept, which indicted that triple flame theory might be the basic flameholding mechanism of the cavity flameholders.

Numerical Investigation on Detonation Control Using a Pulse Hot Jet in Supersonic Combustible Mixture

ABSTRACT To investigate the mechanism of detonation control using a pulse hot jet in the supersonic hydrogen-oxygen mixture, high-resolution simulations with a detailed reaction model were conducted using an adaptive mesh refinement method. After the successful detonation initiation, a contractive passway is generated between the hot jet and the main flow field behind the detonation front. Due to the contractive passway, the expansion of detonation products is prevented, hence, resulting in overdriven detonation. By setting up various contractive passways through adjusting the width of the hot jet, the overdrive degree of overdriven detonation also changes. It is suggested that the width of the hot jet has an approximately linear relation with the relative and absolute propagation velocities. When the contractive passway gradually disappears after the shutdown of the hot jet, overdriven detonation attenuates to the dynamically stable Chapman–Jouguet (CJ) detonation. When the contractive passway is re-established once again after the reinjection of the hot jet, the CJ detonation develops to the same overdriven detonation, indicating that the contractive passway controlled by the pulse hot jet can indeed control detonation propagation in the supersonic combustible mixture to some extent.

Numerical simulation on detonation initiation and propagation in supersonic combustible mixtures with nonuniform species

High-resolution numerical simulations of gaseous detonation initiation by means of a hot jet injection were conducted on detonation initiation and propagation in supersonic combustible mixtures with a spatially nonuniform species distribution adopting the adaptive mesh refinement open-source program AMROC. Three different groups of mixtures, which represent different rates of chemical activity, were investigated in total. The results show that when the mixtures in general have a high chemical activity, detonation initiation can be finally realized successfully both through the Mach reflection and the DDT mechanism in the flowfield, no matter how the mixtures are distributed in the channel. Four processes of detonation initiation, detonation attenuation, initiation failure and detonation re-initiation together make up periodical transition process with the help of the lateral expansion of the detonation. When the mixtures in general have an intermediate chemical activity, detonation combustion can be fully realized in the whole channel with different overdrive degrees in the upper half part and lower half part. After the shutdown of the hot jet, the overdriven detonation attenuates gradually, and finally a slightly strong detonation and a slightly weak detonation are formed together, which can be regarded as a new stable “CJ” state. However, whether detonation initiation can be realized or not in this case is determined by the distribution of different mixtures. When the mixtures in general have a low chemical activity, detonation initiation cannot be realized successfully. The reliable approach for the successful realization of detonation initiation in this case should be the application of a stronger hot jet.

Experimental investigation of combustion stabilization modes in a cavity-based supersonic combustor with different wall divergence angles

Optical diagnostics and pressure measurements are carried out to investigate the characteristics of cavity-assisted hydrogen jet combustion in a Mach 2.52 supersonic flow which simulates flight Mach 5.5 ∼ 6 conditions. Two combustor configurations with wall divergence angles being 2.25° and 3.5°, and two injection schemes are designed. Main attention is focused on the influences of the combustor wall divergence angles and injection schemes on combustion stabilization modes. The experimental results show that two totally different combustion stabilization modes and transition processes between combustion stabilization modes are detected. Heat release and jet–cavity interactions are supposed to play an important role in the transition of combustion stabilization modes. In addition to injection schemes and the cavity configuration, the combustor wall divergence angle is demonstrated to obviously affect the combustion stabilization modes in the supersonic cavity flows, suggesting that the divergence angle should be seriously considered in the supersonic combustion organization. The contrastive experiments also show that as long as combined cavity shear layer/recirculation combustion stabilization mode is achieved, the divergence angle should be smaller and the injection distance should be shorter with all the possible means to obtain more stable combustion in present condition. Meanwhile, it needs to be more cautious to design injection schemes if the combustor wall divergence angle is larger.

Detonation interaction with cavity in supersonic combustible mixture

Two-dimensional adaptive simulations of detonation are carried out in a cavity embedded channel to investigate detonation interaction with the cavity in supersonic combustible mixtures. The reactive Euler equations with a detailed reaction model are solved using the second-order MUSCL-TVD scheme based on the open-source program AMROC. The results show that when the detonation wave propagates backward and crosses over the cavity, an oblique shock wave is first induced originated from the left edge of the cavity in the detonation front, which is demonstrated to actually be an oblique shock-induced combustion and further induces an unburned jet behind the oblique shock. As the oblique shock wave grows, the detonation wave further propagates backward with the front height gradually reduced and together the enlargement of the unburned jet. Rather than the speculated detonation failure, the detonation wave realizes relatively dynamic sustainment due to the pressure oscillation in the subsonic combustion in the cavity which can contribute significantly to the formation of highly unstable shear layers. The rapid turbulent mixing resulting from the large-scale vortices along the shear layers can enhance the consumption of the unburned jet and the subsequent chemical energy release, which plays a significant role in the detonation sustainment. A contractive passway is formed due to the highly unstable shear layers along the unburned jet resulting from hydrodynamic instabilities, which further induce the formation of overdriven detonation and its forward propagation once again. A periodical process of forward detonation propagation, detonation attenuation, detonation sustainment is formed in supersonic combustible mixtures due to detonation interaction with the cavity.

Detonation attenuation and re-initiation in expanding channels

Two-dimensional adaptive simulations are carried out solving the reactive Euler equations with a detailed reaction model based on the open-source program AMROC to investigate detonation evolution using a hot jet initiation in expanding channel filled with supersonic combustible mixtures. The results show that the supersonic mixture experiences a Prandtl-Meyer expansion with according flow acceleration in the expanding channel, which eventually leads to the nonuniformity of the incoming flow. The nonuniformity characteristic of the mixture results in the generation of a curved detonation directly behind the expansion region. Due to the low numerical diffusion for high grid-resolution, the unburned jet directly behind the expansion fan is consumed slowly, hence resulting in some chemical heat loss when it is released after the sonic plane behind the detonation front. This can further weaken the detonation front, which might be one of the reasons that result in the rapid attenuation of the detonation front. Because of highly oscillating near the upper wall resulting from the expansion fan, the shear layer along with the unburned jet becomes very unstable, hence leading to unsuccessful construction of hydrodynamic throat. As a result, the detonation cannot sustain but continuously attenuate, and finally fails, which is expected to realize re-initiation utilizing the hot jet injection once again before the shock front is totally blew out of the channel.