Development and validation of models for turbulent reacting flows

This paper presents the first numerical evidence of an intermittency route to period-2 thermoacoustic instability in a subcritical single-element liquid rocket engine burning hydrogen peroxide/kerosene as we decrease the equivalence ratio (ϕ) from fuel-rich to fuel-lean. To achieve this, three-dimensional compressible large eddy simulation algorithms combined with the Euler–Lagrangian framework are used. A one-equation eddy sub-grid turbulence model with a partially stirred reactor sub-grid combustion model is employed to simulate the spray turbulent combustion process in a high-pressure liquid-fueled combustor based on open-source platform OpenFOAM. This paper focuses on examining the transition process of the dynamical states in the thermoacoustic system and the synchronization between multiple subsystems. The results indicate that, as the equivalence ratio reduces continuously (1.5 ≤ ϕ ≤ 0.5), the system dynamics shift from period-1 oscillations (ϕ = 1.5) to period-2 oscillations (ϕ = 0.5) via intermittency (1.3 ≤ ϕ ≤ 0.9). Under the equivalence ratio of 0.7 (ϕ = 0.7), a transient mode switching between period-1 and period-2 was also observed. The synchronization processes between the pressure and combustion subsystems in terms of phase-locking and frequency-locking are responsible for the emergence of complex dynamical states. The cycle snapshots analysis also provides more details on the synchronization processes between the pressure and the multiple subsystems, such as vortex dynamics, mixture fraction, and combustion heat release. In summary, this paper sheds light on the complex non-linear thermoacoustic oscillations and the underlying physical mechanisms related to the two-phase flow of spray combustion in liquid rocket engines using three-dimensional large eddy simulations, paving the way for developing passive or active control methods.

Large eddy simulation of combustion instability in a subcritical hydrogen peroxide/kerosene liquid rocket engine: Intermittency route to period-2 thermoacoustic instability

2023

Experimental and numerical comparisons of geometric scaling criteria for lean premixed swirl combustor

Xie,

Shi,

et al. 2023

This study experimentally and numerically investigates the applicability of the DaI and Re criteria for scaling the geometry of a lean premixed swirl combustor during a reaction and in the absence of it. We first set up an experimental system to test the loss of pressure, the flow field, and NOx emissions in a prototype combustor and two models of it scaled to 3/5 of its size. The results showed that the friction in the flow in the prototype decreased with an increase in its intensity, and the corresponding constant DaI model (M-D) exhibited a similar trend, while the constant Re model (M-R) exhibited an adverse trend to that of the prototype. The results of particle image velocimetry (PIV) of the flow field in the non-reactive state showed that regardless of the criterion used and the state of the reaction, the flow fields of the prototype and the models were similar under flows of different strengths. However, a quantitative comparison of their distributions of velocity showed that the peak velocity of the rotating jet of M-R was significantly lower than that of the prototype. PIV results of the flow field in the reactive state exhibited similar phenomena. Moreover, the NOx emissions of M-D were consistent with those of the prototype, while emissions from M-R were significantly higher. The numerical results also showed that the shape of the flame and the pattern of flow of M-R were significantly different from those of the prototype.

Numerical study on similarity performance of geometric scaling of lean premixed swirl combustor

Xie,

Shi,

et al. 2023

This paper describes the use of experimentally validated computational fluid dynamics methods to study the similarity performance of various models scaled by the DaI criterion. First, the numerical method is validated by particle image velocimetry and CH* chemiluminescence data under the reaction state. Combustor prototypes and models are then simulated under different equivalence ratios (ERs) and swirl numbers (SWs) with the geometric scaling factor (Q) ranging from 0.1 to 1. When Q < 0.3, the reaction zone is obviously stretched. Changes in Q produce large deviations in the velocity distribution. Increasing either ER or SW increases the deviation in the velocity distribution in the outer shear region in front of the combustor but reduces that in the recirculation zone and jet zone at the back of the combustor. The scaling law changes with ER and SW. To distinguish whether the reaction flow field of a model maintains similarity with respect to the prototype, a novel concept called “degree of similarity” is proposed. The “non-similarity range” for geometric scaling factors under different conditions is further clarified. When ER = 0.55, the range of non-similarity of the combustion flow field is Q ≤ 0.3. As ER increases, the range of non-similar intervals decreases, and when ER reaches 0.95, the non-similarity range is Q ≤ 0.1. When SW = 0.42, the non-similarity range is Q ≤ 0.4, and when SW ≥ 0.42, the non-similarity range is Q ≤ 0.3.