2.1. Experimental Setup

Figure 1 shows the schematics of (a) the CVCC testing platform and (b) the ion current detection module established in this study. The CVCC is forged from stainless steel and features a rectangular cavity with dimensions of 300 mm × 50 mm × 50 mm. On the cavity surface, two square glass observation windows with a size of 55 mm × 55 mm are symmetrically arranged. In addition, a spark plug is installed 69 mm away from the left side of the cavity for ion current detection, and a pressure sensor is installed 171 mm away from the same side. The models of experimental equipment for the remaining subsystems are listed in Table 1, including the ignition system, signal acquisition system, data acquisition system, and gas intake-exhaust system.

Figure 1. Schematics of (a) the CVCC test system and (b) the ion current detection module.

To ensure the accuracy and repeatability of the experimental data, this study designed a standardized procedure for gas mixing, ignition triggering, and signal acquisition. After evacuating the CVCC to a vacuum state, the experimental gases were sequentially introduced into the chamber until the target pressure was reached, followed by a three-minute standing period to ensure no gas flow was observable in the schlieren images. Upon activation, the signal trigger sends pulse signals to synchronously initiate ignition and signal acquisition. Specifically, the data acquisition card recorded the pressure and ion current signals synchronously at a sampling frequency of 50,000 Hz.

Experimental conditions are presented in Table 2. Equivalence ratio (Φ) was varied from 0.6 to 1.2 to cover lean, stoichiometric, and rich combustion conditions. Meanwhile, comparative experiments were conducted using 100% NH₃ and 90% NH₃ + 10% H₂ blend as the fuel. The comparison was designed to reveal the impact of hydrogen addition on cation concentration as well as ion current signals. Specifically, each operating condition was repeated at least three times to avoid deviations in a single experiment that might be caused by operational fluctuations, instrument noise, or environmental interference.

2.2. Numerical Setup

To investigate the combustion mechanism, this study analyzed cation concentration variations under different initial conditions (equivalence ratio and hydrogen blending ratio) and identified the dominant elementary reactions for each combustion condition. Chemical kinetic analysis of ammonia combustion under oxygen-enriched conditions was carried out using CHEMKIN-PRO (ANSYS, Canonsburg, PA, USA), adopting the ammonia-hydrogen combustion mechanism proposed by Cherepanov et al. [20]. The premixed laminar flame burner model was mainly employed to analyze the initial flame propagation behavior, with the simulation conditions listed in Table 3. Considering that the electrical mobility of free electrons in flame plasma is orders of magnitude higher than that of bulky cations, electrons act as the primary charge carriers governing the macroscopic ion current. Consequently, the simulated electron density was selected as the most direct proxy for electrical conductivity [22]. Within the investigated range of equivalence ratios, the simulated electron density matched well with the experimentally measured maximum ion current signal, as shown in Figure 2, which verified that the adopted reaction mechanism can effectively predict the evolution of ion and electron concentrations during ammonia combustion, and provided theoretical support for the waveform analysis of ion current signals.

Figure 2. Validation of the simulation.

3.1. Ion Current Characteristics of Ammonia Combustion

Figure 3 presents the pressure and ion current signals of pure-ammonia oxygen-enriched combustion. Previous studies have demonstrated that the characteristic oscillation frequencies of ammonia combustion are approximately 2000 Hz, 4000 Hz, and 6000 Hz. Therefore, a low-pass filter with a cutoff frequency of 1500 Hz is applied to ensure that the smoothed curves can clearly reflect the general trend of signal variations. The processed figures keep the raw signals in light color to clearly show the experimental results. It is worth noting that the pressure signals show an abnormal drop at approximately 5 ms under all conditions. This fluctuation is caused by a local low-pressure vortex formed as the flame front passes through the pressure sensor cavity, and it does not affect the overall analysis of pressure signals. It can be seen that the overall waveform of pressure and ion current signals exhibits high similarity under different equivalence ratios. The ion current signal remains at the baseline level during the initial ignition stage. As the flame propagates to the ion current sensor, charged species such as cations and electrons produced by chemical ionization in the flame migrate directionally under the bias voltage applied to the measuring electrodes. Consequently, the ion current rises instantaneously and forms a distinct chemical ionization peak, followed by a short plateau. Subsequently, as the pressure signal approaches its peak, the mixture is fully combusted and releases a large amount of heat. After this point, the temperature gradually decreases, leading to a decline in both pressure and ion current signals.

Figure 3. Pressure and ion current signals (a) Φ = 0.6; (b) Φ = 0.8; (c) Φ = 1.0; (d) Φ = 1.2.

Based on Figure 3, the peak values and the time at which the signals reach the peaks (defined as peak timings) of the pressure and ion current signals are extracted from the processed curves, as presented in Figure 4. Notably, range error bars are included in Figure 4 to indicate the maximum, minimum, and average values obtained from three repeated tests. The peak values of both pressure and ion current signals show a similar variation trend. Specifically, the peak values increase initially and then decline as the equivalence ratio increases, with the maxima occurring under near-stoichiometric combustion. Similarly, the peak timings advance first and then delay as the equivalence ratio increases. These phenomena can be explained by the fact that near-stoichiometric combustion provides a stoichiometric mixture of fuel and oxidizer, yielding the most intense combustion and the highest reaction temperature. Consequently, a higher heat release rate produces the maximum pressure peak. Meanwhile, the high-temperature atmosphere significantly promotes the chemical ionization of species and the generation of charged particles, which enhances the intensity of the ion current signal.

Figure 4. (a) Peak values and (b) peak timings of pressure and ion current signals.

The above results demonstrate that the ion current signal during oxygen-enriched ammonia combustion correlates strongly with the equivalence ratio. Additionally, the ion current can effectively characterize the combustion behavior inside the CVCC, which validates the feasibility of ion current detection technology for combustion diagnosis and optimization.

3.2. Analysis of Cation Reaction Kinetics in Oxygen-Enriched Ammonia Combustion

Essentially, an ion current arises from the directional migration of electrons in a flame under an external electric field. The mobility of cations is much lower than that of electrons under low voltage conditions. However, the high mobility of electrons is restricted by the electrostatic force of cations, leading to a uniform spatial distribution of the two species [22]. Therefore, the analysis of the cation concentration during ammonia combustion can directly reflect the core characteristics of the ion current, providing a key basis for interpreting ammonia combustion behavior.

During the combustion of pure ammonia or ammonia/hydrogen blends, the dominant cations include NH₄⁺, NO⁺, as well as H₃O⁺, and seven key elementary reactions are involved, as listed in Table 4. Although the adopted mechanism may underestimate the relative abundance of NO⁺ under fuel-lean conditions, as noted in the original kinetic study [20]. The model is expected to capture the variation trends of the dominant cations and the total cation concentration, providing a reliable foundation for the qualitative analysis of the ion current signals. Therefore, this study employs CHEMKIN-PRO to conduct a chemical kinetic analysis of pure ammonia combustion across various equivalence ratios. By identifying the dominant pathways for cation formation and consumption, the underlying mechanisms governing the formation and evolution of the ion current signal can be comprehensively clarified.

Figure 5 shows the concentration evolutions of the cations NH₄⁺, NO⁺, and H₃O⁺, as well as the total concentration of these three cations. Specifically, the fuel is 100% NH_3, and the oxidizer is 90% O₂ + 10% N₂ under the initial condition of 293 K and 100 kPa. The curves at each equivalence ratio in the figure are divided into three stages according to the variation trends of the cations, where yellow, green, and blue respectively correspond to the first, second, and third stages. It can be seen from the figure that under lean-burn and stoichiometric conditions, the dominant cations are NH₄⁺, NO⁺, and H₃O⁺, and the variation trends of cation concentration are similar at different equivalence ratios. In contrast, the dominant cation under rich-burn conditions includes only NH₄⁺. In Figure 5d, the total cation concentration curve visually overlaps with the NH4⁺ curve. It can be attributed to the reason that NH4⁺ constitutes nearly the entirety of the total cations under rich combustion conditions. Therefore, to reveal the mechanisms behind the two distinct ion- dominated patterns, namely the multi-cation dominance under lean/stoichiometric combustion and the single-cation dominance under rich combustion, this study performs a chemical kinetic analysis for typical operating conditions of the two patterns. In particular, combustion at Φ = 0.6 and Φ = 1.2 is chosen as the research cases.

Figure 5. Cation concentration of pure ammonia combustion at (a) Φ = 0.6; (b) Φ = 0.8; (c) Φ = 1.0; (d) Φ = 1.2.

Figure 6 shows the rate profiles of the dominant elementary reactions in the first two stages at Φ = 0.6. Specifically, the absolute reaction rates for the formation and consumption of the three cations are presented for sensitivity analysis, which enables the identification of the dominant reaction pathways. The first stage corresponds to the early combustion period, when the concentrations of reactants and radicals are relatively high, leading to intense chemical ionization. Most NH₃ is oxidized by OH to produce N atoms, which further participate in chemical ionization via reaction R[5] N + O NO⁺ + e⁻ to generate NO⁺. Meanwhile, NH₂ produced by the oxidation of a small fraction of NH₃ is converted into NH₄⁺ through reaction R[1] 2H + NH₂ NH₄⁺ + e⁻. The rapid accumulation of NO⁺ and NH₄⁺ produced by chemical ionization gives rise to a sharp increase in the ion current signal. In this stage, the total cation concentration curve gradually forms a peak, which is consistent with the chemical ionization peak of ion current signals in Figure 3. In addition, the proton-transfer reaction R[6] NO⁺ + H₂O + H NO + H₃O⁺ consumes a small amount of NO⁺ to produce H₃O⁺, which is further converted into NH₄⁺ via R[4] H₃O⁺ + NH₃ H₂O + NH₄⁺. The two reactions proceed at similar rates, so H₃O⁺ can be regarded as a proton transfer bridge between NO⁺ and NH₄⁺ in this stage, with its concentration maintained at an extremely low level.

In the second stage, reactants and radicals are substantially consumed, making it difficult to sustain the chemical ionization reactions, which rely on the participation of radicals. As a result, the increase in NO⁺ concentration gradually slows down. In addition, as the reactant NH₃ is nearly depleted and the concentration of H₂O increases, the equilibrium of reaction R[4] H₃O⁺ + NH₃ H₂O + NH₄⁺ shifts toward the reverse direction, leading to a rapid rise in H₃O⁺ concentration. Although a small amount of NH₄⁺ is produced by minor side reactions during this process, the NH₄⁺ concentration still decreases rapidly to an extremely low level. Generally, the total cation concentration in this stage shows a slowly decreasing trend, which corresponds to the plateau region of the ion current curves in Figure 3.

Figure 6. Sensitivity analysis of formation and consumption reactions of NH₄⁺, NO⁺, and H₃O⁺ at Φ = 0.6.

Based on the above analysis, the dominant reaction pathways of the three main cations can be obtained, as shown in Figure 7. Under lean-burn and stoichiometric combustion conditions, the dominant cations include NH₄⁺, NO⁺, and H₃O⁺. In the early stage of combustion, the main cations contributing to the ion current are NH₄⁺ and NO⁺, both of which originate from chemical ionization reactions. However, in the middle and late stages of combustion, NO⁺ and H₃O⁺ become the dominant cations determining the ion current intensity. In this stage, R[4] plays a dominant role in this stage, accompanied by substantial consumption of NH₄⁺ and production of H₃O⁺. In addition, the waveform of the ion current corresponds well to the variation trend of cation concentration.

Figure 7. Dominant reaction pathways of oxygen-enriched ammonia combustion during (a) the first stage and (b) the second stage at Φ = 0.6.

Compared with the lean-burn and stoichiometric combustion, the dominant cation under rich-burn combustion is solely NH₄⁺, whose concentration remains at a high level of 10⁻¹¹ throughout the process. In contrast, the concentrations of H₃O⁺ as well as NO⁺ only fluctuate on the order of 10⁻¹³. Similarly, this study divides the combustion into three stages, and the formation/consumption rates of the main cations under these conditions are shown in Figure 8.

In the first stage, the NH₄⁺ concentration rises extremely rapidly to a peak at the early stage of combustion, which arises from chemical ionization reactions, namely reaction R[1]–R[3]. Meanwhile, NO⁺ is also produced by the corresponding chemical ionization reaction R[5], which is consistent with the trend observed under lean-burn and stoichiometric combustion conditions. However, the reaction rate of R[1] is two orders of magnitude higher than that of R[5]. This can be attributed to the lower oxygen concentration under rich-burn conditions compared with lean-burn and stoichiometric combustion conditions. From the reaction pathways shown in Figure 7, the formation of N atoms required for R[5] highly depends on the oxidation of NH₃ by OH radicals. The production of OH radicals relies on the reaction H + O₂ O + OH. Therefore, the poor-oxygen atmosphere under rich-burn conditions restricts the formation of N atoms, resulting in a consistently low concentration of NO⁺. This explains why NH₄⁺ is the only dominant cation under rich-burn combustion.

Figure 8. Sensitivity analysis of formation and consumption reactions of NH₄⁺, NO⁺, and H₃O⁺ at Φ = 1.2.

After the first stage, the concentration of the dominant cation NH₄⁺ shows a slow and steady downward trend from its earlier high peak, remaining generally on the order of 10⁻¹¹. This can be attributed to the reversal of Reactions R[1]–R[3], corresponding to chemical recombination. However, the rate of chemical recombination at this stage is only about 1/10 that of the chemical ionization reactions in the first stage. Subsequently, the reactions approach equilibrium. Based on the above analysis, the dominant formation pathways of cations under rich-burn combustion are illustrated in Figure 9. Given that NH₄⁺ is absolutely dominant while the concentrations of NO⁺ and H₃O⁺ are extremely low, only the dominant formation pathway of NH₄⁺ is presented here.

Figure 9. Dominant reaction pathways of oxygen-enriched ammonia combustion at Φ = 1.2.

3.3. Impact of Hydrogen Blending on Ion Current in Ammonia Combustion

To investigate the effect of hydrogen addition on oxygen-enriched ammonia combustion, the 10% hydrogen-blended ammonia combustion is investigated and compared with pure ammonia combustion. Figure 10 shows the ion current signals of two fuels under four equivalence ratios. Overall, the two signals exhibit similar waveform characteristics. In addition, it can be observed that the ion current intensity of ammonia-hydrogen blended combustion is significantly higher than that of pure ammonia throughout the entire combustion. Figure 11 presents (a) the peak values and (b) peak timings at various equivalence ratios. Similarly, it can be seen that, for both fuel systems, the peak signal intensity first increases and then decreases, while the time to peak occurs earlier and then later as the equivalence ratio increases from 0.6 to 1.2. This trend is consistent with that observed in the above analysis in pure ammonia combustion. Compared with pure ammonia combustion, the 10% hydrogen-blended combustion exhibits a higher peak value and a shorter time to reach the peak under all equivalence ratios. This can be attributed to the higher chemical reactivity and faster burning velocity of hydrogen. As a result, the addition of hydrogen significantly accelerates the overall combustion reaction rate, thereby greatly increasing the formation rate of charged particles.

Figure 10. Comparison of ion current signals before and after hydrogen blending at (a) Φ = 0.6; (b) Φ = 0.8; (c) Φ = 1.0; (d) Φ = 1.2.

Figure 11. The (a) peak values and (b) peak timings of the ion currents before and after hydrogen blending.

To clarify this phenomenon, a kinetic analysis of ammonia-hydrogen blended combustion is performed in this study. Figure 12 presents the cation concentration during the 10% hydrogen-blended combustion at different equivalence ratios. It can be seen that NH₄⁺, NO⁺, and H₃O⁺ are the dominant cations under lean-burn and stoichiometric conditions, while only NH₄⁺ dominates under rich-burn conditions. This trend is identical to that of pure ammonia combustion, and the concentration evolution of the three main cations at different combustion stages also shows similar behavior. From the viewpoint of cation concentration variation, the similar ion current waveforms between pure ammonia and ammonia-hydrogen blended combustion can be well explained. Essentially, ammonia is an effective hydrogen carrier and can decompose to produce a small amount of hydrogen during combustion. Therefore, the overall waveform characteristics of the ion current and the fundamental mechanisms governing the waveform variation in ammonia-hydrogen blended combustion are basically consistent with those in pure ammonia combustion.

Figure 12. Cation concentration of 10% hydrogen-blended combustion at (a) Φ = 0.6; (b) Φ = 0.8; (c) Φ = 1.0; (d) Φ = 1.2.

The 10% hydrogen-blended case presents a higher cation concentration throughout the combustion. Compared with pure ammonia, the concentration of the dominant cation during blended combustion is increased by 1–2 orders of magnitude overall, accompanied by a faster growth rate and a higher peak value. This remarkable increase in cation concentration directly matches the experimental observation that the ion current signal intensity of hydrogen-blended fuel is higher than that of pure ammonia combustion. Thus, it is confirmed that hydrogen blending has a significant promoting effect on the ion formation reactions.

Furthermore, under the condition of 10% hydrogen-blended, the relative concentration proportion of NH₄⁺ at the early combustion stage is significantly higher than pure ammonia combustion under lean-burn and stoichiometric combustion. To quantify this phenomenon, this study defines the ratio of the NH₄⁺ concentration peak to the total concentration of all cations as the NH₄⁺ peak proportion coefficient $R_{{\text{NH}}_{\text{4}}^{\text{+}}}$ , as shown in Equation (1):

Figure 13 shows the $R_{{\mathrm{N}\mathrm{H}}_4^{+}}$ under lean and stoichiometric combustion conditions. It can be clearly observed that the proportion of NH₄⁺ at its peak moment in the 10% hydrogen-blending combustion is higher than that in pure ammonia combustion. This study selects the operating point at Φ = 1.0 and 2.5 ms for analysis, because the concentration proportion of NH₄⁺ is the highest at this condition, which yields the most obvious analytical results. Figure 14 presents the formation and consumption reaction rates of H₂. It is found that H atoms are involved in the products of several major H₂-consuming reactions, which proceed at high rates, leading to a high H-atom concentration in the early reaction stage. Consequently, reaction R[1], namely 2H + NH₂ NH₄⁺ + e⁻, tends to proceed toward the direction of chemical ionization. In addition, the chemical ionization process for NH₄⁺ formation is endothermic, and the higher reactivity and faster burning velocity of hydrogen raise the overall temperature, which in turn further promotes the generation of NH₄⁺. Consequently, the ion concentration proportion of NH₄⁺ in the ammonia–hydrogen blended combustion is significantly higher than that in pure ammonia combustion.

Figure 13. Concentration proportion of NH₄⁺ at the peak moment.

Figure 14. Sensitivity analysis of H₂ consumption and formation reactions under 10% hydrogen-blending condition.