Hydrogen Addition Ratio Research Articles

A numerical study on the combustion limits of mesoscale methane jet flames with hydrogen addition

A meso-scale jet flame model was established for the flame ports of domestic gas stoves. The influences of hydrogen addition ratio (β = 0%–25%) on the combustion limits were explored. The results show that with the increase of hydrogen addition ratio, the blow-off limit increases obviously, while the extinction limit decreases slightly, namely, the combustible range expands significantly. Quantitative analysis was carried out in terms of chemical effect and thermal effect. It was found that hydrogen addition will reduce O2 fraction in the pre-mixture for a constant equivalence ratio. Under near-extinction limit condition, since the flame is located at the nozzle exit, the external O2 cannot be entrained into or diffuse into the upstream of the flame, which leads to the decrease of reaction rate. However, for the near-blow-off cases, the external O2 can be entrained and diffuse into the flame, which compensates the difference of O2 content in the pre-mixture. Therefore, the combustion reaction is enhanced by hydrogen addition because more H radicals can be produced. In addition, as the flame is located closer to the tube with the increase of hydrogen addition ratio, heat transfer between flame and tube wall is augmented and the preheating of fresh mixture is strengthened by the inner tube wall. This heat recirculation effect becomes especially notable in low velocity cases. In conclusion, the extension of extinction limit by hydrogen addition is attributed to the thermal effect, while the increase of blow-off limit is mainly due to the intensification of chemical effect.

An Evaluation of the Conversion of Gasoline and Natural Gas Spark Ignition Engines to Ammonia/Hydrogen Operation From the Perspective of Laminar Flame Speed

Abstract Power generation systems will reduce carbon emissions primarily through the application of low or even zero carbon fuels under the global decarbonization trend. Ammonia is an ideal alternative fuel because it is cheap, readily available, and easy to store and transport. However, its mediocre combustion performance has raised concerns about its use in engines. The objective of this paper was to evaluate the amount of hydrogen that would need to be added to the ammonia from a laminar flame speed perspective if converting existing spark ignition engines to ammonia operation. The benchmark for determining the hydrogen blending ratio was to help ammonia achieve efficient combustion in the cylinder comparable to that of gasoline or natural gas. The results showed that hydrogen addition had the potential to greatly improve engine efficiency and emissions, although the combustion kinetics of ammonia-hydrogen mixtures were still dominated by ammonia with hydrogen addition levels below 60%. In addition, the hydrogen addition ratio was mainly determined by the kernel inception process, as this burning stage heavily influenced the repeatability of the combustion and the ease of combustion control. Also, at least 20% of hydrogen was required to be added to ammonia to adapt the engine to various operating conditions, while such a strategy still cannot help ammonia to obtain a rapid burning event compatible with gasoline or methane. Moreover, natural gas engines were more suitable for retrofitting to ammonia-hydrogen operation because they have a higher compression ratio and their combustion chambers are less demanding on the fuel laminar flame speed. Further, ammonia lean operation was recommended to be avoided in spark ignition configurations. Altogether, all of these findings support the need for additional efforts in ammonia engine optimizations and onboard ammonia dissociation system efficiency improvements.

Effect of hydrogen addition on the laminar burning velocity of n-decane/air mixtures: Experimental and numerical study

Hydrogen is a potential aviation alternative fuel. To explore the influence of hydrogen addition on the combustion of aviation kerosene, the spherical expanding flame method was used to measure the laminar burning velocity and Markstein length of n-decane/hydrogen/air mixtures at 1bar, 2bar and 470 K using experimental and numerical analysis. The experimental LBVs agreed well with the simulation at an initial pressure of 1 bar, but lower than the simulation at 2bar. The laminar burning velocity showed a linear relationship with the hydrogen addition ratio (RH) at the different effective fuel-air equivalence ratios (ϕF). The Markstein length decreased with increased RH and initial pressure, hence, increasing the susceptibility of flame front instability. Based on a one-step overall reaction assumption, sensitivity analysis of the premixture mechanism showed that the kinetic effect greatly influenced n-decane/hydrogen/air mixture combustion. Further kinetic analysis indicated that the maximum (H + OH) mole fraction of the chemical reaction is highly responsible for the linear relationship between laminar burning velocity and RH.

Flame characteristics of methane/air with hydrogen addition in the micro confined combustion space

This paper investigated methane/air flame characteristics with hydrogen addition in micro confined combustion space experimentally and computationally. The focus is on the effect of hydrogen addition on the methane/air flame stabilization, the onset of flame with repetitive extinction and ignition (FREI), and the global flame quenching in decreasing continuously combustion space. Furthermore, the effects of hydrogen addition on the flame temperature and the local equivalence ratio distribution were analyzed systematically using numerical simulations. In addition, the effects of hydrogen addition on the concentrations of OH and H radicals, and the critical scalar dissipation rate of local flame extinction were discussed. With a higher hydrogen ratio, the mixing is faster, and the flame is smaller. When the micro confined space is narrower, the heat loss to the combustor walls has a higher impact on the flames. The flames with higher hydrogen ratios have therefore lower peak flame temperatures and lower concentrations of H and OH radicals. The results show that hydrogen addition can effectively widen the stable combustion range of methane/air flames in the micro confined space by about 20% when the hydrogen addition ratio reaches 50%. The frequency and the maximum propagation velocity of FREI flames can be increased as well. The quenching distance of methane/hydrogen/air flames decreases nearly linearly with the increase of hydrogen ratio. This is attributed to the higher critical scalar dissipation rate of local flame extinction in flames with a higher hydrogen ratio.

Effect of nitrogen on deflagration characteristics of hydrogen / methane mixture

In order to study the influence of nitrogen on the deflagration characteristics of premixed hydrogen/methane, the explosion parameters of premixed hydrogen/methane within various volume ratios and different dilution ratios were studied by using a spherical flame method at room temperature and pressure. The results are as follows: The addition of nitrogen makes the upper limit of explosion of hydrogen/methane premixed gas drop, and the lower limit rises. For explosion hazard (F-number), hydrogen/methane premixed fuel with a hydrogen addition ratio of 10% has the lowest risk, and nitrogen has a greater impact on the dangerous degree of hydrogen and methane premixed gas whose hydrogen addition ratio does not exceed 30%. In terms of flame structure, the spherical flame was affected by buoyancy instability as the percentage of nitrogen dilution increased, but the buoyancy instability gradually decreased as the percentage of hydrogen addition increased. The addition of diluent gas reduces the spreading speed of the stretching flame and reduces the stretching rate in the initial stage of flame development. The laminar flame propagation velocity calculated by the experiment in this paper is consistent with the laminar flow velocity of the hydrogen/methane premixed gas calculated by GRI Mech 3.0. Considering the explosion parameters such as flammability limit, laminar combustion rate and deflagration index, when hydrogen is added to 70%, it is the turning point of hydrogen/methane premixed fuel.

Numerical investigation of laminar burning velocity for methane-hydrogen-air mixtures at wider boundary conditions

To extend the study related to laminar burning velocity (LBV) of methane mixed with hydrogen under different boundary conditions, the FFCM-MECH was selected for chemical kinetic analysis. The influencing mechanisms of LBV of methane mixed with hydrogen were investigated under similar operating conditions for the internal combustion engine, e.g., higher temperatures, higher pressures, CO2 dilution ratios and larger extent of equivalence ratios. The impact of boundary conditions on the concentration of important reactive radicals and the flame structure was also discussed. The results and findings are summarized as follows. The LBV increases as the hydrogen addition ratio and initial temperature increase, but showing opposite trends when the initial pressure and CO2 dilution ratio rise. Moreover, it shows an increasing trend followed by a decreasing trend as equivalence ratio increases. LBV is highly dependent on the concentration of some important radicals in the system, the higher the concentration of radicals, the faster the LBV. Methane mixed with hydrogen has a small rise in adiabatic temperature and an outstanding reduction in carbon dioxide and carbon monoxide content, which can significantly reduce the emission of harmful gases and greenhouse gases. This study not only gives a reference for the design on power machinery, but also is conducive to promoting the development of natural gas engines.

Experimental and numerical investigation of iC8H18 laminar combustion characteristics based on hydrogen – water complementary regulation

The present study examines using water dilution and hydrogen as control measures to regulate the laminar burning velocity of iso-octane/air mixtures in a constant volume combustion bomb. Experimental results are obtained by processing schlieren photographs and simulation results are calculated by Chemkin software. The water dilution rates of the mixtures were between 0 and 5% and the hydrogen addition ratio was between 5 and 55%. The initial pressure was set at 101.3 kPa with the preheating temperature ranging from 400 K to 500 K. According to the results, laminar burning velocity varied almost linearly with the dilution of water and the addition of hydrogen, respectively. This study also distinguishes the thermal and kinetic effects of water on the combustion processes. The thermodynamic effect of water vapour on laminar burning velocity was higher than the kinetic effect. In kinetics, O, H, and OH were the most affected free radicals by the addition of water vapour. The main free radical change affected the rate of the elementary reaction. Besides, the kinetic effect of water vapour promoted combustion. However, this effect was offset by the thermodynamic effect. The introduced hydrogens of premixed mixtures did not lead to measurable improvements in laminar burning velocity. Additionally, the change in the reaction rate of the elementary reaction was not obvious, which was inconsistent with the change of laminar burning velocity. Lastly, a laminar burning velocity model using hydrogen and water vapor content as control parameters was proposed. Accordingly, the designated laminar burning velocity of the iso-octane/air mixture was achieved by altering hydrogen and water vapour ratios in the mixture. BP (Back-Propagation) neural network algorithm (a multi-layer feed forward neural network trained by error back propagation algorithm) and empirical data were used to verify the model, and the model’s maximum error was less than 5%.

Hydrogen effect on flame extinction of hydrogen-enriched methane/air premixed flames: An assessment from the combustion safety point of view

Hydrogen-enriched natural gas is a promising low-carbon fuel in combustion devices. To better assess its feasibility from the extinction prevention point of view, the extinction of near-limit premixed hydrogen-methane/air flames over a wide range of equivalence ratios was measured using the single-flame counterflow configuration. Results showed that the hydrogen addition resulted in a greater extinction stretch rate. Further analysis demonstrated that the extinction stretch rate of premixed hydrogen-methane/air flames linearly correlated with the corresponding reference flame speed. Thereby a combustion regime map diagram, separating the burning and extinction, was sketched. In addition, critical extinction Damkӧhlor number of premixed hydrogen-methane/air flames was investigated and it was found to be insensitive to the hydrogen addition and equivalence ratio. Finally, the hydrogen addition effect and the corresponding extinction response were assessed for two scenarios, 1) constant thermal load (gas turbine, gas-fired boilers, etc.), and 2) constant fuel injection pressure (gas stove, oven, etc.). The results indicated that H2 addition promoted the combustion safety by increasing the extinction stretch rates, but an allowable H2 variation window existed when considering the unexpected high-temperature damage of the burners. The allowable H2 variation window of Scenario 1 was found much broader than that of Scenario 2.

Role of hydrogen addition in propane/air flame characteristic and stability in a micro-planar combustor

For combustion occurring under micro-scale conditions, how to enhance the flame stability is of significance. For this, a micro-planar quartz combustor with transparent main walls is designed to experimentally shed lights on the flame stabilization mechanism of premixed propane flame with hydrogen added in terms of flame dynamics. Experimental results show that as far as the pure propane combustion is concerned, there are six types of flame structures with varying the gaseous inlet velocity, i.e., flame repetitive extinction and ignition (FREI flame), cellular flame, planar flame, U-shaped flame, inclined flame and spinning flame. Furthermore, blending hydrogen is found to significantly extend the flame stability. This is due to the fact that the addition of hydrogen can overcome the inlet velocity inertia and restrain flame instability caused by hysteresis. For the inlet velocity considered in this work, hydrogen addition can inhibit transformation of flame type from U-shaped flame to inclined flame. Finally, the flame position, thickness and curvature of U-shaped flame are shown to be significantly affected by hydrogen addition ratio. Specifically, the larger the hydrogen addition ratio, the smaller the flame thickness, indicating that hydrogen addition is conducive to maintaining symmetry of the flame back and forth. In general, this work has an implication of designing a high stability micro-power system by reasonably considering the inlet parameter.

Large eddy simulation of premixed hydrogen-rich gas turbine combustion based on reduced reaction mechanisms

This study numerically investigates the combustion characteristics in a swirl burner for CO-H2 syngas by large eddy simulation. Firstly, experimental data are used to verify the applicability of the Boivin mechanism and the Sandia mechanism. The simulation results show that the Boivin mechanism predicts a more reasonable flame structure. Then, Syngas25, Syngas45, and Syngas100 (composed of 25%, 45%, and 100% H2 by volume, respectively) are set to numerically investigate the effect of hydrogen addition ratio on combustion characteristics by using the dynamic Smagorinsky model and the partially stirred reactor model. The detailed simulation data shows that the addition of hydrogen greatly influences the flame structure and flow field; with an increase in hydrogen addition, the flame length shortens, the temperature rises, and flashback caused by combustion induced vortex breakdown occurs. Furthermore, the precessing vortex core cannot exist stably in Syngas100, though it can be clearly observed in other cases.

Evaluation on combustion and lean-burn limit of a medium compression ratio hydrogen/methanol dual-injection spark-ignition engine under methanol late-injection

Lean-burn is an effective way to optimize the fuel economy of spark-ignition methanol engines. Hydrogen-assisted combustion can improve the stability of combustion and increase the lean-burn limit of such engines. In this study, a medium compression ratio of port injection-type hydrogen/direct-injection methanol was used in a dual-injection spark-ignition engine to investigate the combustion and lean-burn limit under different ratios of addition of hydrogen, excess air ratios, and ignition timings based on methanol late-injection. The results show that the timing of ignition of the hydrogen-enriched methanol engine could be retarded without worsening combustion to a greater extent compared with that in the methanol engine. The timing of ignition had little effect on the coefficient of variation of the indicated mean effective pressure after the addition of hydrogen under lean-burn conditions. Its addition increased the lean-burn limit. The greater the amount of hydrogen added was, the higher the lean-burn limit was. The influence of excess air ratio on the parameters of combustion for 9% of added hydrogen was much smaller than that without hydrogen addition. The critical hydrogen addition ratio from “hydrogen-assisted combustion” to “dual-fuel combustion” was 6%.The optimal ignition timing corresponding to the lean-burn limit at different ratios of added hydrogen was in the range of 33°–42° of the crank angle before the top dead center. Thus, hydrogen-assisted combustion can significantly improve the stability of combustion and increase the lean-burn limit to an excess air ratio of approximately three in a direct-injection spark-ignition methanol engine with a medium compression ratio.

Dynamic modeling and operation strategy of natural gas fueled SOFC-Engine hybrid power system with hydrogen addition by metal hydride for vehicle applications

Abstract SOFC-Engine hybrid power system is a promising energy conversion technology with high efficiency. However, its dynamic behaviors are still unclear. Herein, the dynamic modeling of this hybrid system is performed. Besides, the control strategies of hydrogen addition and power distribution are further investigated to enhance the system performance. The results show that the relatively slow dynamics of the SOFC component is dominating in the hybrid system. The reason is mainly attributed to that the autonomy of the engine with fast dynamics is partly restricted without hydrogen addition. When hydrogen is fed to the engine as a part of inlet fuel by metal hydride and waste heat recovery unit, the dynamics of the hybrid system can be improved. Moreover, the efficiency can also be improved to 67.6% with the hydrogen addition ratio χ = 2.0. After that, the control strategy for power distribution is proposed to achieve the optimal overall performance for the hybrid system. The SOFC provides most of the output power as a stable power baseline and the engine copes with the dynamic part. In such a strategy, the hybrid system enables to respond to the change of power load within 1 s and to achieve the overall energy conversion efficiency up to 75%, which is promising for the vehicle applications. In brief, this work can provide an insight into the dynamic behaviors of the SOFC-Engine hybrid energy conversion system to obtain the feasible operation strategy for its vehicle applications.

Study of soot functional groups and morphological characteristics in laminar coflow methane and ethylene diffusion flames with hydrogen addition

The soot in laminar coflow methane and ethylene diffusion flames with different hydrogen addition ratios is sampled by a capillary soot sampling system and analyzed by Fourier transform infrared spectroscopy (FTIR) and field emission scanning electron microscopy (FESEM). Soot functional groups and morphological characteristics are studied. The results show that with hydrogen addition, the relative contents of aromatic and aliphatic functional groups in the soot of methane flames increase, while the relative contents of the same functional groups in the soot of ethylene flames mostly show different trends, indicating that H2 has a chemical promotion and inhibition on soot in methane and ethylene flames, respectively. The relative content of oxygen-containing functional groups in the soot of both methane and ethylene flames with hydrogen addition increases. By adding H2, the coagulation and condensation of soot low in the methane flames are greatly enhanced, indicating that H2 promotes the formation and growth of PAH molecules. This is an important reason for the chemical promotion effect of H2 in methane flames. For the soot of C2H4 hydrogen-added flames, oxidation is relatively thorough such that a large number of soot branches are oxidized. This is an important reason for the chemical inhibition of H2.

Reducing emissions of an SI engine by alternative spark plugs with hydrogen addition and variable compression ratio

As a consequence of the emissions-cheating scandals and more strict emission regulations enforce researchers to reduce emissions out and find alternative fuels for SI engines. For this purpose, various spark plugs are available in the market with different electrode materials. However, they have not been tested together with different engine parameters. Hence, emissions out from a variable compression spark-ignited engine with different spark plugs and hydrogen enrichment were the scope of this study. The tests were conducted with a four-stroke, single-cylinder, naturally aspirated, variable compression ratio (VCR) engine. Two different compression ratios (CR) of 8.5:1 and 10:1 at maximum brake torque (MBT) spark timing applied to assess the effects of different spark plugs and hydrogen usage at different engine loads. Copper, iridium and platinum spark plugs were tested for each experiment condition. Also, hydrogen was added through the intake manifold with flow rates of 0, 2 and 4 l/min to enhance the combustion of the VCR engine. Carbon monoxide (CO), carbon dioxide (CO2), nitrogen oxides (NOx) and unburned hydrocarbons (UHC) emission values were measured in this study. According to test results, with iridium and platinum spark plug usage, hydrogen addition and higher CR, the engine emitted lower CO and UHC at all engine loads. However, a higher amount of CO2 was emitted because of increased completeness of the combustion and the amount of NOx emissions rose due to increment in-cylinder temperatures. These variances were more apparent with platinum spark plug usage compared to the iridium spark plug. As a result, the usage of iridium and platinum spark plugs were shown lower incomplete emissions products out, except NOx emissions.

Effect of exhaust gas recirculation and hydrogen direct injection on combustion and emission characteristics of a n-butanol SI engine

The n-butanol as a clear and renewable biofuel is considered an ideal alternative fuel for SI engines. Hydrogen as a partial alternative fuel can improve the combustion and emissions of SI engine because of its advantages of low ignition energy, fast flame propagation and so no. In order to decrease the additional NOx caused by hydrogen addition, the cooperative effect of EGR and hydrogen direct injection on combustion and emission in an SI n-butanol engine was investigated in this paper. Experiments are conducted at five hydrogen addition ratios (0%, 5%, 10%, 15%, 20%), five EGR ratios (0%, 5%, 10%, 15%, 20%) and three excess air ratios (1, 1.1, 1.2) with 1500 rpm of engine speed. The results indicate that Ttq, Pmax, dQmax and Tmax decrease with the increasing ηEGR and increase with the increasing φH2, while the APmax, AdQmax, θ0–10 and θ10–90 show the contrary tendency. The 15% EGR rate cooperate with 10%~15% hydrogen addition ratio can keep emissions (HC, CO, NOx) better than the original engine. Compared to the original pure butanol engine, the n-butanol engine with hydrogen direct injection and EGR could obtain better comprehensive performance through the coordinated control of φH2 and ηEGR. More specifically, 15% EGR rate cooperated with 10% hydrogen addition ratio can obtain better comprehensive performance of combustion and emission, but there is a little expense on combustion parameters compared to the original engine under λ = 1. So the 15% EGR ratio is considered a superior EGR ratio cooperated with hydrogen addition.

Kinetic study of the effects of hydrogen blending to toluene reference fuel (TRF)/air mixtures on laminar burning velocity and flame structure

Because of its high reactivity, rapid flame speed, and low ignition energy, hydrogen is an excellent additive in gasoline to improve the combustion performance and reduce pollutant formation. In this study, we conducted a numerical study of the effect of hydrogen blending into TRF (toluene/iso-octane/n-heptane)/air mixtures on laminar burning velocity and flame structure at the initial pressure of 1 bar, initial temperature of 453 K, equivalence ratio of 0.8–1.4 and hydrogen addition ratio range from 0% to 50%. The adiabatic flame temperature increases slightly and the thermal diffusion coefficient increases significantly with hydrogen addition. The peak values of H + OH + O mole fractions are correlated nearly linearly with the laminar burning velocity, indicating a direct correlation relationship between the peak mole fraction of H + OH + O and the laminar burning velocity. The increase of reaction rate of R361 (H + O2 = O + OH) is one of the main reasons for the enhanced laminar burning velocity due to hydrogen addition. The rate-of-production analysis of fuel shows that hydrogen promotes the consumption of hydrocarbons. The thermal/dilution and chemical effects of hydrogen addition were numerically quantified. The chemical effect is the dominant effect on laminar burning velocity. In the last part of the study, we separated the thermal, chemical and dilution effects of hydrogen addition and the results demonstrate that the relative importance of these effects follow the order of chemical > dilution > thermal under the conditions of our study.

Numerical investigation on hydrogen-enriched methane combustion in a domestic back-pressure boiler and non-premixed burner system from flame structure and pollutants aspect

In this paper, the non-premixed hydrogen-enriched methane-air combustion was investigated numerically with the use of a CFD code. In the first part of the study, the combustion experiments were performed in a back- pressure boiler using natural gas. The intake rate of fuel was kept constant as 45 Nm3/h while the coefficient for the air excess ratio was changed between 1.2 and 1.35. After the experiments, the numerical analyses were performed. The Fluent code was utilized as the simulation instrument. The eddy dissipation combustion model was selected to be used in the numerical analyses, since it is known that this combustion model can save computational time and fairly predict the combustion flame structure and emissions. Pure methane and natural gas were taken as fuels in the numerical analyses. The obtained results from the numerical analyses were validated with the experimental flue gas temperature and emission measurements. Then, the hydrogen-enrichment of pure methane fuel was investigated numerically in such a way that the boiler capacity (432 kW) was kept constant. The coefficient for the air excess ratio was 1.2 for all the considered combustion simulation cases. The hydrogen addition ratio was 25%, 50% and 75% by mass, respectively. The thermal NO emissions and temperature distributions in the combustion chamber were obtained according to the different hydrogen-enriched methane fuel combustion cases. In addition, the emissions contained in the flue gas together with the temperature values were calculated. The obtained results from the numerical studies indicate that the hydrogen-enrichment of methane reduces the carbon emissions, while it substantially augments the formation of the thermal NO emissions. The calculated thermal NO emission value in the flue gas is 147 ppm for the pure methane combustion case, and it is 566 ppm for the combustion case with 75% by mass of hydrogen addition ratio. Therefore, it is determined that hydrogen fuel is a pollutant from the thermal NO emission aspect for the considered enrichment ratios in the studied domestic boiler-burner system.

Experimental study of propane/air premixed flame dynamics with hydrogen addition in a meso-scale quartz tube

The combustion process of propane/air premixed flame in meso-scale quartz tubes with different hydrogen additions was investigated experimentally to explain the flame-wall interaction mechanism. The ranges of different flame regimes were obtained by changing the flow rates of propane and hydrogen. The effects of hydrogen addition, inlet velocity and equivalence ratio were analyzed. The results show that the hydrogen addition broadens the operation ranges of fast flame regime and slow flame regime significantly. The flame propagation speed is in the same order of the thermal wave speed in solid wall for the slow flames. In fast flame regime, the flame propagation speed has an inverse correlation with the inlet flow velocity irrespective of the equivalence ratio. With the increase of the equivalence ratio, the maximum flame speed in fast flame regime decreases gradually, while the maximum flame speed in slow flame regime increases continually. It indicates that rich fuel condition suppresses the fast flame and promotes the slow flame. In slow flame regime, the output thermal efficiency is dominated by the inlet velocity and equivalence ratio.

Effects of Hydrogen Addition Ratios on Cycle-by-Cycle Variations of a Duel-Fuel Spark Ignition Engine with Ethanol Intake Port Injection and Hydrogen Direct Injection under Various Excess Air Ratios

In order to improve the combustion stability of ethanol engine under lean-burn condition, the experiment was carried out on a spark ignition engine with ethanol intake port injection and hydrogen d...

Study on the propagation characteristics of hydrogen/methane/air premixed flames in variable cross-section ducts

The flame propagation characteristics of hydrogen/methane/air mixtures with different hydrogen addition ratios (φ=0, 10 %, 20 %, 30 %, 40 % and 50 %) in different variable cross-section ducts were studied. A high-speed camera and pressure sensors were used to collect flame images and determine the overpressure dynamics. The results show that the smooth flame front will be twisted and folded, when the flame propagates to the abrupt position of the cross-section area of the duct. The larger the abrupt change rate of the duct cross-section is, the more obvious the disturbance to the flame and the more severe the turbulence. With increasing hydrogen addition ratio, the flame propagation speed and overpressure in the four kinds of variable cross-section ducts studied increase. The time for the flame front to reach the downstream end is gradually shortened, and the flame propagation time when the flame propagates from the smaller cross-section tube to the larger cross-section tube is more severely shortened with increasing hydrogen addition ratio than that when the flame propagates from the larger cross-section tube to the smaller cross-section tube. The increase of the overpressure caused by the addition of hydrogen is more significant when the flame propagates from the smaller cross-section tube to the larger cross-section tube. When the flame propagates from the smaller cross-section tube to the larger cross-section tube or from the larger cross-section tube to the smaller cross-section tube, the larger the abrupt change rate of the duct cross-section is, the larger the maximum overpressure.