arXiv subjects

H. Pitsch

Publications and source records attributed to H. Pitsch.

3 recordsLinked to original sources

arXiv2026

Scaling Laws for Thermodiffusively Unstable Lean Premixed Turbulent Hydrogen-Air Flames

Lean premixed hydrogen-air flames are strongly affected by thermodiffusive (TD) instabilities, which can alter the flame structure and enhance the local reactivity many-fold. Two recent models (Howarth et al. (Combust.~Flame 253, 2023) and Rieth et al. (MSC 2023)) describe the scaling of the stretch factor in turbulent hydrogen flames with the Karlovitz number using different parameters, i.e., the $\omega_2$ parameter from linear stability theory and the ratio of the Zel'dovich to the Peclet number (${Ze}/{Pe}$). Using a comprehensive set of 91 direct numerical simulation (DNS) cases spanning a wide range of pressures, equivalence ratios, turbulence intensities, and flow configurations, both formulations are systematically evaluated and an adapted formulation is proposed. The analysis of the governing non-dimensional groups reveals a scaling behavior characterized by two distinct regimes. In the first regime, typically relevant for burner and gas turbine conditions, both models reduce to an identical form that depends solely on the Karlovitz number and the stretch factor of laminar flames, independent of $\omega_2$ or ${Ze}/{Pe}$. In the second regime, characterized by ultra-low flame speeds, the explicit consideration of $\omega_2$ or the ratio ${Ze}/{Pe}$ is required for accurate scaling. In both regimes, the two models predict the DNS data reasonably well and reduce to the same functional form of non-dimensional groups, indicating their physical equivalence.

physics.flu-dyn↗

arXiv2026

Flow configuration and pressure effects on turbulent premixed hydrogen jet flames

Turbulent lean premixed hydrogen jet flames are simulated using direct numerical simulation employing detailed chemistry in both slot and round configurations at various pressures. All cases are simulated at a constant jet Reynolds number ($Re_j = 10000$) and a fixed ratio of characteristic length scales. While normalised macroscopic quantities (e.g., flame length, turbulent flame speed) appear comparable across configurations, fundamental discrepancies are observed that originate from the coupling of large- and small-scale effects. Mean local reactivity ($I_0$) decays monotonically downstream, driven by a decreasing Karlovitz number ($Ka^{*}$); however, this decay is modulated by geometry, with round jets exhibiting a faster decline due to mean negative curvature. Pressure is identified as a critical small-scale driver, fundamentally altering flame propagation by increasing the sensitivity of displacement speed to local curvature. At elevated pressures, this sensitivity induces higher flame stretch and accelerates wrinkling near the nozzle, which compounds with geometry-dependent effects, such as the slower decay of mean strain in slot configurations.

physics.flu-dyn↗

arXiv2021

A Reduced-order Model for Multiphase Simulation of Transient Inert Sprays

In global efforts to reduce harmful greenhouse gas emissions from the transport sector, novel bio-hybrid liquid fuels from renewable energy and carbon sources can be a major form of energy for future propulsion systems due to their high energy density. A fundamental understanding of the spray and mixing performance of the new fuel candidates in combustion systems is necessary to design and develop the fuels for advanced combustion concepts. In the fuel design process, a large number of candidates is required to be screened to arrive at potential fuels for further detailed investigations. For such a screening process, three-dimensional (3D) simulation models are computationally too expensive and hence unfeasible. Therefore, in this paper, we present a fast, reduced-order model (ROM) for inert sprays. The model is based on the cross-sectionally averaged spray (CAS) model derived by Wan (1997) from 3D multiphase equations. The original model was first tested against a wide range of conditions and different fuels. The discrepancies between the CAS model and experimental data are addressed by integrating state-of-the-art breakup and evaporation models. A transport equation for vapor mass fraction is proposed, which is important for evaporation modeling. Furthermore, the model is extended to consider polydisperse droplets by modeling the droplet size distribution by commonly used presumed probability density functions, such as Rosin-Rammler, lognormal, and gamma distributions. The improved CAS model is capable of predicting trends in the macroscopic spray characteristics for a wide range of conditions and fuels. The computational cost of the CAS model is lower than the 3D simulation methods by up to 6 orders of magnitude depending on the method. This enables the model to be used not only for the rapid screening of novel fuel candidates, but also for other applications, where ROMs are useful.

physics.flu-dyn↗