An Overview of Theoretical, Numerical and Experimental Studies on Clean Energy and Combustion

1. Introduction

This paper reviews recent theoretical, numerical and experimental studies on clean energy and combustion, focusing on advancements in combustion efficiency, emission reduction technologies and renewable energy integration. Specifically, this paper highlights key findings from 17 studies that address the modeling of combustion processes, energy conversion systems and clean energy sources. Such findings are significantly important in providing insights into improving the sustainability of energy systems while mitigating the environmental impacts of using traditional energy sources.

The need for cleaner energy sources and conversion mechanisms is increasing with a continuous increase in the global demand for energy. The aviation industry, in particular, is under pressure to reduce its carbon footprint and to explore sustainable alternatives to traditional jet fuel. Meanwhile, other industries, such as power generation, are investigating ways to optimize combustion efficiency and to reduce emissions. The studies reviewed in this paper focus on theoretical, numerical and experimental advances in clean energy and combustion technologies.

This review summarizes the key findings from 17 studies published between 2022 and 2025 in the MDPI topic “Theoretical, Numerical and Experimental Studies on Clean Energy and Combustion”, focusing on alternative fuels, combustion dynamics, reduced-order modeling, thermo-acoustic and turbulent combustion and emission control strategies. By integrating these diverse investigations, this paper aims to elucidate the interconnected advances in combustion science and their implications for sustainable energy applications.

2. Alternative Fuels in Aviation and Engines

The aviation sector, concerned with the escalating fuel consumption and greenhouse gas emissions, has shown interest in exploring the use of alternative fuels, as exemplified by Villette et al. [1], who evaluated the performance of Liquefied Natural Gas (LNG) and methanol in a geared turbofan engine, demonstrating that these non-drop-in fuels can reduce post-combustion temperatures by 1–3% and increase thrust by 3–10% compared to Jet-A, while achieving substantial reductions in CO₂, NO_x and CO emissions by up to 20%, 60% and 39%, respectively.

In another work conducted by Janovec et al. [2], the authors modeled hydrogen and battery-powered aircraft concepts, revealing that the power-to-weight ratio of existing battery technologies still need to be improved before electrified propulsion becomes a contender in the light-sport aircraft segment.

Complementing these aviation-focused studies, Aljabri et al. [3] conducted a computational comparison of spark-ignited (SI) and pre-chamber (PC) hydrogen engines, finding that lean mixtures (air–fuel ratio $\lambda =3.33$) with advanced spark timing in SI mode yield efficiencies exceeding 50% with minimal NO_x, whereas the PC configuration increases pressure rise rates and heat losses, necessitating further optimization—a challenge that echoes the trade-offs observed by Villette et al. [1], with low-energy-density fuels requiring larger tank volumes. On the ground, alternative fuels also show promise, as Garzón et al. [4] investigated straight soybean oil–diesel blends (50% and 80% v/v) in a compression ignition engine, noting that higher vegetable oil fractions delay combustion due to prolonged diffusive phases, but the oxygen content of the fuel improves combustion efficiency, a finding that parallels the emission reduction potential seen in aviation fuels.

Similarly, Wang et al. [5] explored co-firing coal with biomass syngas in a 300 MW boiler, observing that introducing 3 × 10⁴ m³/h of wood syngas reduces coal consumption by 5.1% and lowers the furnace exit temperatures due to diminished flame radiation, suggesting a synergy with Villette et al. [1] and Garzón et al. [4] in leveraging oxygenated fuels to curb emissions, albeit with thermodynamic adjustments to maintain system performance.

3. Combustion Dynamics and Modeling

Combustion modeling under varying conditions can provide critical insights into flame behavior, which could inform the system design requirements. Xu et al. [6] conducted thorough numerical simulations to investigate how an equivalence ratio can affect the detonation characteristics and performance of methane–oxygen combustion in rotating detonation rocket engines (RDREs). They showed that increasing the equivalent ratio reduces the number of detonation waves while boosting specific impulse, a trend that correlates with the enhanced reactivity observed by Wang et al. [7] in ammonia–air flames with higher cracking ratios.

In a different context, Gong and Tang [8] examined the high-gravity combustion characteristics of propane–air in a channel with backward-facing steps (in an ultra-compact combustor) utilizing large eddy numerical simulations.

Analyzing their simulation result, they concluded that the centrifugal forces double the flame propagation speed by triggering Rayleigh–Taylor and Kelvin–Helmholtz instabilities, which enhance mixing and reduce combustion time—an effect that resonates with the turbulence-driven flame enhancements noted by Martinez-Sanchis et al. [9] in methane–oxygen rocket flames. Zhang et al. [10] further explored the flame stabilization by simulating lean premixed methane flames piloted by rich premixed flames, showing that the pilot flame adjusts stretch and heat transfer to anchor the main flame closer to the wall, a mechanism that could inform the micro-combustion stability challenges addressed by Kutkut et al. [11].

Micro-scale combustion introduces additional complexities, as reviewed by Dias et al. [12], who highlighted phenomena such as flashback, quenching diameter and flames with repeated extinctions and ignition (FREI) cycles, which Kutkut et al. [11] substantiated through simulations of premixed methane–air combustion in micro-channels, noting that higher inlet velocities stabilize flames while lower velocities trigger FREI modes influenced by equivalence ratio and channel size.

Huang et al. [13] utilized numerical simulations to investigate the impact of thermal gas on the boundary layer flame flashback in channels. They demonstrated that thermal gas expansion globally modifies flow in narrow channels but remains localized in wider ones, challenging the model critical gradient assumptions while aligning with Gong and Tang [8] in emphasizing the role of flow–flame interactions under confinement.

Modeling efforts to reduce computational costs are advanced by Gitushi and Echekki [14], who compared different species selection methods, including the principal component analysis (PCA), directed relation graphs (DRGs), the global pathway selection (GPS) and the manifold-informed species selection methods, to reduce the complexity of the chemical mechanisms describing complex hydrocarbon fuel combustion. They suggested that these approaches effectively capture key species across the oxidation stages, with sensitivity to variance and target species as also implied in parametric sensitivities in Xu et al. [6].

Building on this, Alqahtan et al. [15] developed a hybrid chemistry framework using neural networks to model the low-temperature oxidation of n-heptane, coupling it with foundational chemistry to achieve acceptable accuracies at reduced computational expense. Such a strategy could enhance the detonation simulations of Xu et al. [6] or the micro-flame analyses of Kutkut et al. [11].

4. Thermoacoustic and Turbulent Combustion

Thermoacoustic oscillations and turbulent combustion significantly influence the stability and performance of a combustion system. Tao et al. [16] performed a theoretical analysis of these effects in a lean premixed combustion regime, revealing that extending the length of the combustion chamber from 1.2 m to 6.0 m can reduce the amplitude of the sound pressure by 34% and delay the trigger time from 0.32 s to 0.91 s, which could mitigate the pressure fluctuations noted by Gong and Tang [8] under high gravity conditions.

In a different study, Martinez-Sanchis et al. [9] simulated turbulent non-premixed methane–oxygen flames at 20 bar, observing that despite a non-premixed setup, premixed combustion occurs in lean and rich regions, with turbulence shifting products toward lean conditions, an effect that parallels the flow modifications in the study performed by Huang et al. [13] and suggests a link to the flame stabilization mechanisms explained in the work of Zhang et al. [10].

5. Catalyst and Emission Control

Emission control through catalysts and fuel modifications is pivotal for clean combustion, as Liu et al. [17] demonstrated with Zr-modified Cu-Ce/SAPO-34 catalysts for NH₃-SCR, where Zr reduces acid sites and denitration efficiency yet achieves over 99% NO removal at 400–500 °C, contrasting with the results of Wang et al. [5], who found that higher ammonia cracking ratios in turbulent flames increase NO formation despite enhanced reactivity, highlighting a trade-off between combustion intensity and emissions, akin to the findings of Villette et al. [1]. These studies collectively underscore the need for integrated fuel and catalyst strategies to balance efficiency and emission goals.

6. Conclusions

The collective works from the studies presented in the MDPI topic “Theoretical, Numerical and Experimental Studies on Clean Energy and Combustion” illuminate the multifaceted advancements in clean energy and combustion, where alternative fuels such as LNG, methanol, hydrogen and biomass syngas offer substantial emission reductions but necessitate careful engine and system design adjustments to address efficiency and stability challenges, as seen in Villette et al. [1], Janovec et al. [2] and Wang et al. [5]. Combustion dynamics, from detonation waves [2] to micro-flames [12], reveal the critical interplay of flow and heat transfer, while modeling innovations [5,6] and catalyst developments [3] provide tools to optimize these processes, suggesting a future where integrated approaches could fully realize the potential of sustainable combustion technologies.