Uncertainty Quantification of Time-Dependent Quantities in a System with Adjustable Level of Smoothness

We summarise the results of a computational study involved with Uncertainty Quantification (UQ) in a benchmark turbulent burner flame simulation. UQ analysis of this simulation enables one to analyse the convergence performance of one of the most widely-used uncertainty propagation techniques, Polynomial Chaos Expansion (PCE) at varying levels of system smoothness. This is possible because in the burner flame simulations, the smoothness of the time-dependent temperature, which is the study's QoI is found to evolve with the flame development state. This analysis is deemed important as it is known that PCE cannot accurately surrogate non-smooth QoIs and thus perform convergent UQ. While this restriction is known and gets accounted for, there is no understanding whether there is a quantifiable scaling relationship between the PCE's convergence metrics and the level of QoI's smoothness. It is found that the level of QoI-smoothness can be quantified by its standard deviation allowing to observe the effect of QoI's level of smoothness on the PCE's convergence performance. It is found that for our flow scenario, there exists a power-law relationship between a comparative parameter, defined to measure the PCE's convergence performance relative to Monte Carlo sampling, and the QoI's standard deviation, which allows us to make a more weighted decision on the choice of the uncertainty propagation technique.

Effect of Microwave Pulses on the Morphology and Development of Spark-Ignited Flame Kernel

Microwave-assisted spark ignition (MAI) is a promising way to enhance the ignition performance of engines under lean conditions. To understand the effect of microwave-induced flow during MAI, the development and morphology of spark-ignited methane-air flame kernel under various microwave pulse parameters are experimentally studied. Experiments are conducted in a constant volume combustion chamber, and flame development is recorded through a high-speed shadowgraph method. Flame area and deformation index are adopted to evaluate the flame characteristic. Results show that increasing the microwave pulse energy from 0 to 150 mJ exhibits a threshold process for expanding the flame kernel area under 0.2 MPa ambient pressure. When the pulse energy is below the threshold of 90 mJ, the microwave enhancing efficiency is much lower than that beyond the threshold. Increasing microwave pulse repetition frequency (PRF) changes the flow on flame surface and raises the absorption efficiency for microwave energy, and thus helps to improve the MAI performance under higher pressures. Hence, 1 kHz pulses cause more obvious flame deformation than those with higher PRF pulses under 0.2 MPa, while this tendency is reversed as the ambient pressure increases to 0.6 MPa. Besides, microwave pulses of different repetition frequencies lead to different flame kernel morphology, implying the various regimes behind the interaction between a microwave and spark kernel.

Unsteady characteristics of jet combustion in a supersonic combustor with a micro-vortex generator

To clarify the effect of the micro-vortex generator on the unsteady characteristics of jet combustion, a set of experiments had been carried out in a cavity-based supersonic combustor. Based on the advanced combustion diagnosis techniques, the ignition process, initial cavity-stabilized flame and dynamic flame development at the initial equivalence ratio of 0.20 are revealed in detail. Although the ignition processes are identical, the time for the flame propagation process in the cavity can be shortened when an MVG (micro-vortex generator) is located properly upstream of the injection. The initial flame cannot be stabilized in the combustor if the MVG is too close to the injection. After achieving initial stable combustion, the chemical reactions in the flame front are more vigorous and the shear layer can be lifted a little higher in the experiment with an MVG. At the same dynamic fuel adjustment method, the flame can be stabilized in the combustor without an MVG while the flame is blown out with an MVG. Based on numerous experimental results, it is found that the MVG dwindles the adjustment range of the dynamic injection, which makes against the stability of the flame when the engine decreases the thrust.

Transient Ignition of Premixed Methane/Air Mixtures by a Pre-chamber Hot Jet: a DNS Study

The present study investigates the transient processes controlling ignition by a hot jet issued from a pre-chamber. Direct numerical simulations (DNS) have been performed to study the characteristics of the turbulent jet flow and of the associated flame during the whole ignition process, quantifying the relevant physicochemical interactions between pre-chamber and main chamber. Thanks to a detailed analysis of the DNS results, the transient ignition is found to consist of three main sequential processes: (1) near-orifice local ignition in the main chamber; (2) further flame development supported by the jet flow; and (3) global ignition and propagation of a self-sustained flame in the main chamber, independently from the hot jet. The characteristic time-scale of the hot jet as well as jet-induced effects (local enrichment, supply of radicals and heat) are found to be essential for successful ignition in the main chamber. A more intense turbulence in the main chamber appears to support local ignition. However, it also induces local quenching, thus delaying global ignition. An ignition threshold based on a critical Damköhler number is a promising concept, but is not sufficient to describe the process in all its complexity.

Spark Ignition of SPP Injector Under Sub-Atmospheric Conditions

The sub-atmospheric ignition performance of an SPP (Stratified Partially Premixed) injector and combustor is investigated experimentally on the high-altitude test facility. In order to explore the influence of sub-atmospheric pressure on reignition performance and flame propagation mode, experiments are conducted under different pressures ranging from 19 kPa to 101 kPa. The inlet temperature and pressure drop of the injector (ΔPsw/P3t) are kept constant at 303 K and 3% respectively. The transparent quartz window mounted on the sidewall of the model combustor provides optical access of flame signals. Ignition fuel-air ratio (FAR) under different inlet pressures are experimentally acquired. The spark ignition processes, including the formation of flame kernel, the flame development and stabilization are recorded by a high-speed camera at a rate of 5kHz. Experimental results indicate that the minimum ignition FAR grows rapidly as the inlet air pressure decreases. An algorithm is developed to track the trajectory of flame kernels within 25ms following the spark during its breakup and motion processes. Results show that the calculated trajectory provides a clear description of the flame evolution process. Under different inlet air pressures, the propagation trajectories of flame kernels share similarities in initial phase. It is pivotal for a successful ignition that the initial flame kernel keeps enough intensity and moves into CTRZ (Center-Toroidal Recirculation Zone) along radial direction. Finally, the time-averaged non-reacting flow field under inlet pressure of 54kPa and fuel mass flow of 8kg/h is simulated. The effects of flow structure and fuel spatial distribution on kernel propagation and flame evolution are analyzed.

Formalization of the Burning Process of Virtual Reality Objects in Adaptive Training Complexes

Within the scope of this article, the problem of the formalization of physical processes in adaptive training complexes is considered on the example of virtual objects burning. Despite a fairly complete study of this process, the existing mathematical models are not adapted for the application in training complexes, which leads to a significant increase in costs and lower productivity due to the complexity of the calculations. Therefore, an adapted mathematical model is proposed that allows us to formalize the structure of virtual objects of burning, their basic properties and the processes of changing states, starting from the flame development of an object and ending with their complete destruction or extinguishment. The article proposes the use of threshold value diagrams and rules for changing the states of virtual reality objects to solve the problem of the formalization of burning processes. This tool is quite multi-purpose, which allows you to describe various physical processes, such as smoke, flooding, the spread of toxic gases, etc. The area of the proposed formalization approach includes the design and implementation of physical processes in simulators and multimedia complexes using virtual and augmented reality. Thus, the presented scientific research can be used to formalize the physical processes in adaptive training complexes for professional ergatic systems.

Combustion Modes and Unsteady Characteristics during the Condition Transition of a Scramjet Combustor

To investigate the combustion modes and unsteady characteristics during the condition transition of a scramjet combustor, a series of experiments were carried out under the condition of Mach 2.52 supersonic incoming flow, the corresponding stagnation pressure and temperature of which were 1.6 MPa and 1486 K, respectively. A fuel supply system that could dynamically adjust the injection pressure was adopted to simulate the condition transition stage of a scramjet. Based on the advanced combustion diagnosis technique, the transient chemiluminescence image and the wall pressure were recorded during the whole combustion process. Three typical modes of turbulent combustion occurred when the injection pressure drop gradually increased. The jet flame was stable after the condition transition when the injection pressure drop was relatively low. An unstable combustion phenomenon accompanied by intermittent local extinction and reignition could be found near the blowout limits. With a further increase in the injection pressure drop, the flame was blown out quickly during the transition process. In addition, the flame development characteristics during condition transition under stable combustion mode and the effect of injection pressure drop were studied in detail. During the process of switching between the two conditions, the area and light intensity of the flame decreased over time, and the wall pressure was accordingly reduced. As the increase in injection pressure dropped, the intensity of chemical reactions deceased obviously and the transition time became longer.