Maximum Combustion Efficiency Research Articles

Unraveling the Nanosheet Zeolite-Catalyzed Combustion of Aluminum Nanoparticles-Doped exo-Tetrahydrodicyclopentadiene (JP-10) Energetic Fuel.

Nanosheet MFI zeolites (Zeolite Socony Mobil, five) have grown in popularity in cracking catalysis considering their tunability in porous topologies, acidic sites, and sheet thickness, thus allowing them to selectively adsorb molecules of specific sizes, shapes, and polarities, resulting in improved cracking performance for a specific fuel. Five different MFI zeolites in the form of a mesoporous nanosheet structure with a controlled concentration of acidic sites denoted as NSMFI(y), where y is Si/Al ratio, have been synthesized. The effects of the relative acidity content of these NSMFI(y) samples on the zeolite-catalyzed combustion of aluminum nanoparticles (AlNPs)-aided exo-tetrahydrodicyclopentadiene (JP-10) mixed energetic fuel droplets levitated in an oxygen-argon atmosphere were investigated using time-resolved imaging (optical and thermal infrared) and spectroscopic techniques (UV-vis and FTIR). The addition of 1.0 wt % of NSMFI(y) zeolites to AlNPs-JP-10 fluid fuel results in critically reduced ignition delays (9 ± 2 ms), elevated ignition temperatures (2800 ± 170 K), and prolonged burning times (60 ± 10 ms) with an enhanced combustion efficiency. The NSMFI(y) zeolites, which possess high acidity and significant mesoporosity, play a crucial role in improving the combustion efficiency by effectively catalyzing the chemical activation of JP-10 and prolonging the burning of the igniting droplet. The NSMFI (60) variant with the highest acidic site content achieved a maximum combustion efficiency of 80 ± 6%. A comprehensive catalytic combustion mechanism has been elucidated based on the detected reactive intermediates such as hydroxyl radical (OH) and aluminum monoxide (AlO). These findings will help to critically advance the development of next-generation, sustainable, and innovative mixed nanofluid fuels.

Thermodynamic and entropy generation analyses of Telsa-valve structured meso-scale combustors fuelled with hydrogen for thermophotovoltaic applications

The present work numerically examed the effects of hydrogen volume flow rates (Qv) and equivalence ratios (Φ) on thermal performances of meso-scale combustors structured with Tesla-vale type flow channels. Four distinct Tesla-valve configurations are implemented and then subsequently compared. The reverse-flow Tesla valve with counter-flow combustor (RC) structure shows a remarkable improvement of 72.6 % to the combustor wall temperature at Qv = 100 mL/min. The diodicity (Di) of the Tesla valve is found to be increased with a higher Qv, and a lower Φ contributes to a higher Di. Besides, Di is decreased when Φ goes up, stabilizing at Φ = 0.9. The reverse-flow Tesla valve exhibits a more uniform pressure distribution and entropy production than the forward-flow Tesla valve. At Φ = 0.9, the hydrogen-to-air ratio maximized heat release, producing the highest entropy. Tesla-valve structured combustors demonstrate near complete combustion before Φ reaching 0.9, the combustion efficiency (ηcombustion) is gradually decreased after Φ getting to 1.0. The RC construction achieved maximum combustion efficiency under conditions of insufficient oxygen. This present study demonstrates the feasibility of enhancing thermal and overall performances of meso-scale combustors structured with Tesla-valve channels, and revealing the key parameters effects on the combustion characteristics in these combustors.

Performance analysis of a planar shaped strut injector based supersonic combustion chamber

Abstract This current work presents the performance analysis of a supersonic combustor interface and flow construction through a scramjet engine with a planar shaped strut injector (PSSI) at the supersonic Mach. An important aspect of this study is discovering the fuel mixing mechanism inside the combustion chamber with PSSI. The novel PSSI configuration can enhance mixing and combustion performance. The scramjet configuration is incorporated with a supersonic inlet air temperature of 1250 K, where the vitiated air follows at Mach 3, and this technique is based on a high accelerating effect of the scramjet propulsion mechanism. Scramjet engines can maintain naturalistic high enthalpy conditions in minimum durations. It is observed that the maximum temperature of 3510 K is attained at the recirculating zones produced because of undulation enlargement and therefore the fuel jet losses concentration whereas the maximum combustion efficiency of 86 % is investigated from the current research work.

Исследование новой микрофакельной горелки с помощью программы Ansys Fluent

A study of a new micro-flame burner device (MFBD) for the combustion chamber of a gas turbine unit (GTU) using the Ansys Fluent program is presented. Research objective: determination of the optimal operating modes of the investigated burner, initial air parameters for maximum combustion efficiency, examination of the uniformity of the temperature field, and the concentration of nitrogen oxides in the combustion chamber of the gas turbine unit with the MFBD. The features of the design and operation of the micro-flame burner device are described, as well as the stages of modeling combustion processes in the Ansys Fluent software. The calculations conducted using various simulation parameters in this program allowed the evaluation of the efficiency of the new MFBD, determination of the temperature field distribution, and the nitrogen oxide content in the combustion products. The nitrogen oxide emissions in the combustion products during different modes of operation did not exceed 9 ppm. This data will be useful for further optimization and development of GTUs taking into account the requirements of efficiency and environmental safety. The presented study demonstrates the application of Ansys Fluent in the development of new technologies in the field of combustion and energy

Performance Evaluation of Ammonium Dinitramide-Based Monopropellant in a 1N Thruster

The development of propulsion systems based on green propellants, as an alternative to hydrazines, has been gaining interest within the space community. The study of Ammonium Dinitramide (ADN)-based liquid monopropellant, which is low-toxic and can deliver high performance, is the focal point of interest for Space Solutions Co., Ltd., Daejeon, Republic of Korea. A 1N ADN-based propulsion system was designed to evaluate the performance of the propellant. By combining a thermal and catalytic bed in a reactor, the performance of the propellant was examined in a designed thruster (chamber pressure of 11 bar). A total of 16 tests, with pulse mode experiments, were conducted; the accumulated firing time was 285 s. The preheating temperatures were maintained between 350 and 400 °C to achieve steady-state combustion. Notably, the maximum combustion efficiency was 91%. Test 9 recorded the highest decomposition temperature of propellant in the catalyst bed (1422 °C). Interestingly, the combustion instability observed throughout this study was ≤0.5%. This study could assist in the further development of ADN-based propulsion systems.

Computational Analysis of Shrouded Ejector Effect on Starting Vortex and Combustion Efficiency in Pulse Detonation Combustor with Different Fuels

ABSTRACT This research paper deals with the starting vortex of detonation combustion wave flow field in pulse detonation combustor (PDC). In order to achieve the potential pressure gain from PDC the shrouded ejectors are placed at exit section of detonation tube. Therefore, influence of shrouded ejector position plays a great role on leading vortex and reduction of NOx formation. Furthermore, n-butane (C4H10) and hydrogen (H2) fuel are used for stoichiometric (= 1) fuel-air mixture condition to simulate the thermal and prompt NOx formation at pulse time of 0.022s to 0.030s. The LES turbulence model is used to simulate the unsteady flow field in CFD based fluent platform. The one step irreversible chemical kinetics model analyzed the details of exothermic chemical reaction mechanism inside the combustor. The non-dimensional value of /d = 1.5 geometry position of shrouded ejector is better for leading vortex. So far, pickup detonation wave speed of 2306 ms−1 is obtained at shortest pulse time of 0.028s and this magnitude is higher than C-J velocity. The minimum pollutant number of 0.0000341 is obtained from hydrogen-air detonation, this magnitude is lesser compared to n-butane (C4H10)-air combustion. Furthermore, the maximum combustion efficiency of 85% is obtained from hydrogen-air mixture in detonation combustion process, which is comparatively higher than deflagration combustion process.

Experimental investigation on the electrospray counterflow flame in a small combustor with a porous media as the grounding electrode

An experimental study on electrospray counterflow combustion of ethanol in a new type combustor with a porous media as the grounding electrode is carried out. The electrospray process of ethanol is separated from the combustion system for visualization and four typical spraying modes are identified. The characteristics of electrospray counterflow combustion are discussed. Four flame shapes are classified and the distribution region is determined. The results demonstrate that the double-layer flame and dromedary flame are easier to be observed. The change of fuel mass fraction can significantly affect the flame diameter and temperature. When the fuel mass fraction YF > 0.075, the increase of carrier gas flow rate promotes the fuel evaporation. Ethanol vapor is carried into the porous media, and the flame diameter and temperature are incrensed. When YF < 0.075, the dilution of ethanol vapor by carrier gas is dominant, resulting in the decrease of fuel concentration. The combustion efficiency and ratio of CO2/CO is highest at the dromedary flame, and the maximum combustion efficiency and ratio of CO2/CO are 95.25% and 7.78 for Φ = 0.76 respectively, which means a very efficient conversation of fuel. The results gained insights for the design and operation of electrospray counterflow combustors.

Prediction of Maximum Combustion Efficiency and Thrust Force of DLR Scramjet Engine Using ANN Models

Prediction of Maximum Combustion Efficiency and Thrust Force of DLR Scramjet Engine Using ANN Models

Effects of Local Mixing Ratios and Mass Flow Rates on Combustion Performance of the Fuel-Rich LOX (Liquid Oxygen)/kerosene Gas Generator in the ATR (Air Turbo Rocket) Engine

This paper presents a numerical simulation analysis of the flow and combustion characteristics of a fuel-rich LOX (liquid oxygen)/kerosene gas generator in an ATR (air turbo rocket) engine, examining the effects of local parameters on the combustion flow field and performance. The analysis considers variations in unit injector mixing ratios and unit mass flow rates. The results indicate that as the mixing ratio in the inner-ring injectors increases (while the mixing ratio in the middle-ring injectors decreases), the oxygen concentration area near the axis zone and the 50% radius zone of the gas generator expands. Conversely, the kerosene concentration area near the axis zone decreases while gradually increasing near the 50% radius zone. In the flow direction section, there is an inverse relationship between the variation trend of local temperature and the oxygen concentration in the local area. As the oxygen concentration increases, the temperature decreases. The temperature distribution across the cross-section of the gas generator exhibits a circular pattern. When the mixing ratio (or mass flow rates) of the unit injector are perfectly balanced, the temperature distribution becomes highly uniform. A larger disparity in flow rate between the inner ring injector and the middle ring injector leads to a lower combustion efficiency. This effect differs from the effect of the mixing ratio difference between the two injector rings. Increasing the mixing ratio in the inner-ring injectors (or decreasing the mixing ratio in the middle-ring injectors) initially leads to a rise in combustion efficiency, followed by a subsequent decline. The maximum combustion efficiency of 89.10% is achieved when the mixing ratio is set to Km-1 = 0.7 and Km-2 = 2.79, respectively. Increasing the flow rate in the inner-ring injectors (or decreasing the flow rate in the middle-ring injectors) initially leads to an improvement in combustion efficiency, followed by a subsequent reduction. The maximum combustion efficiency of 86.13% is achieved when the mass flow rate is set to m-1 = m-2 = 0.1 kg/s.

Small Combustion Chamber for Marine Fuel Oil and Analysis of Exhaust Gas Characteristics of Marine Gas Oil

A small combustion chamber was developed and manufactured for empirical combustion testing of several alternative fuels developed to meet IMO emission limits for fuel oil used in ships. The combustion chamber consists of four independent tanks and a circulation system with a two-stage heating function owing to the high viscosity and temperature of ship fuel. A gun-type burner is mounted on the side of the combustion chamber, which possesses a cylindrical shape and a capacity of less than 300 L. This device was manufactured in accordance with several variables such that the basic stage performance and simulation tests of each fuel could be sufficiently completed before performing the combustion test to simulate the engines of large ships. To conduct an initial experiment using the developed combustion chamber, low-sulfur MGO with a sulfur content less than or equal to 0.05% was chosen, and ideal operating parameters were selected according to the measurement tests based on load control. The exhaust gas temperature differed by approximately 10.7% as a result of burning MGO at a burner load state of 80–100%. The use of a normal oxygen concentration of 4% helped remove approximately 14.31 ppm of nitrogen oxide and 1.91% of carbon dioxide. The maximum combustion efficiency was 70.17%, indicating the chamber’s potential for use in a variety of combustion tests of alternative fuels for ships in the forthcoming years.

Effects of recess ratio on combustion characteristics of LOX/methane swirl coaxial injectors

To explore the effects of recess ratio on the flame spatial structure and combustion efficiency of LOX/methane swirl coaxial injectors, visual combustion experiments with injectors featuring varied recess ratios were carried out based on an optically accessible combustor. The instantaneous spray and flame images were obtained using the red background light imaging technique. Liquid oxygen was tangentially injected into the center post of the injector at a temperature of 121.5 K, while gaseous methane was axially introduced into the coaxial annulus at a temperature of 290.5 K. With the recess ratio increasing from 0 to 0.83, the flame transforms from a steady state to an oscillating state at the frequency of 3144 Hz to 3281 Hz. A strong relationship between flame length and combustion efficiency is identified firstly. The combustion efficiency of stable flame is higher than that of the oscillating flame. There is a specific recess ratio between 0.33 and 0.54, corresponding to the maximum combustion efficiency, which causes the transition of the flame from a steady state to an oscillating state simultaneously. The combustion efficiency of the injector critical recess ratio (RR = 0.73) is the lowest. Both the spray and flame without recess appear to exhibit axial and transverse coupled oscillation. The transverse oscillation of spray and flame is gradually suppressed with larger recess ratio. As opposed to the oscillation region area, the amplitude of spray oscillation may be the main influence factor of flame oscillation scale. In addition, the relationship between self-pulsation, flame oscillation and combustion instability has been further verified.

Numerical study on transient regression rate and combustion characteristics of segregated AP-based oxidizer/TAGN-based fuel

In the present study, a two-dimensional combustion model is established to simulate transient flow and combustion characteristics of a segregated oxidizer/fuel solid motor (SOFSM). A detailed gas-phase reaction kinetic mechanism with 35 species and 190-step reactions is developed to describe the gas-phase combustion process between AP-based oxidizer and TAGN-based fuel. The effects of mass flux and initial temperature of inflow oxidizer gas on the transient flow and combustion characteristics of the TAGN-based fuel are numerically investigated. With the mass flux of inflow oxidizer gas increasing from 249 kg·m−2·s−1 to 995 kg·m−2·s−1, the maximum spatially averaged regression rate of the burning surface of TAGN-based fuel increases by almost 200 % and the combustion efficiency first increases and then decreases with a maximum combustion efficiency of 94.3 % at the mass flux of 442 kg·m−2·s−1. With the initial temperature of inflow oxidizer gas increasing from 1000 K to 1750 K, the maximum average regression rate of the burning surface of TAGN-based fuel increases by around 10 %. Meanwhile, the combustion efficiency increases from 88.38 % to 95.8 %. During the combustion process (almost 3 s), the spatially averaged regression rate remains basically stable from 0.5 s to 2 s and increases slightly from 2.0 s to 3.0 s for given mass flux and initial temperature, indicating the combustion process is basically stable under the studied conditions. The results also show that during the transient combustion process, a large vortex first appears and then is split into a major vortex and a small one in the post-combustion chamber. With the increase of the mass flux and initial temperature of inflow oxidizer gas, the size of the vortex increases while the center position of the vortex moves towards the burning surface, which significantly enhances the mixing process and increases combustion efficiency.

Computational investigation to interpret the reacting flow conditions for two different fuels in the scramjet combustor

The scramjet engines use the atmospheric oxygen to complete the combustion process, therefore, known as Air breathing engines. It does not bear any active parts and operates at supersonic speed, so it becomes essential that the variables such as fuel, pressure, temperature, Mach number to create the necessary environment to initiate the combustion phenomenon. The supersonic combustion undergoes severe turbulence in the variables associated with combustion. Thus, the selection of the appropriate parameters in designing of a suitable combustor is indispensable. The research paper discusses the effects of two different fuel jets on the flow variables in a scramjet combustor under reacting flow conditions. The Computational Fluid Dynamics (CFD) simulations on the scramjet combustor are carried out using Ansys 15.0 software. To investigate the effect of fuels on the combustion characteristics, the effect of pressure, Mach number and species concentration are studied. The pressure distribution for both the fuels shows that the maximum pressure is seen for hydrogen fuel at Mach number 2.52. The Mach number contours for both fuels reveal that the recirculation zone formation is more pronounced for ethylene fuel. The performance assessment of the combustor show that the maximum combustion efficiency was 87.62% and 89.26% and mixing efficiency was 98.97% and 97.46% for hydrogen and ethylene fuel respectively at M-2.52. Thus, it is concluded that the current scramjet model operated more effectively at Mach 2.52 with hydrogen as a fuel.

The numerical investigation of combustion performance of scramjet combustor with variation in angle of attack

The present article involves computational investigation of parallel fuel injection based scramjet combustor. The wedge shaped strut face is used as a fuel injector. Selected geometry is first validated and then further investigation is performed. Steady-state, two dimensional, scramjet combustor model has been chosen to complete the numerical convergence through ANSYS Fluent software. Grid independence analysis has also been performed. Reynolds Average Navier Stokes equation in addition with k-epsilon turbulence modelling have been utilised to reach the convergence at a lower computational cost. Finite rate eddy-dissipation based Species transport modelling is chosen to solve the chemical kinetics between hydrogen and air. The incoming boundary condition of free-stream air has been optimized to improved combustion efficiency. There are three selected model is utilised for comparison i.e. zero degree, positive five and negative five degree air angle of attack model. To change the angle of attack of the incoming air, a Modified isolator is added ahead of the combustor with constant length. It is observed that the behaviour of shock waves and flow properties are dependent on the angle of attack. Comparative observation has been analysed with monitoring the combustion efficiency graph. Maximum combustion efficiency reaches up to 93% in negative five degree air angle of attack model moreover, mixing is also improved by 4%. It can be summarized that the scramjet combustor performance is primarily highly influenced by the geometry configuration and the nature of flow of incoming air.

Numerical investigation on the effect of variation of upper wall divergence angle of parallel fuel injection scramjet combustor performance

Numerical investigation of Scramjet combustor is performed with the help of center lobed strut to act as a parallel fuel injector. The geometry of the combustor plays a major role to get the enhanced combustion performance in scramjet engine due to the absence of rotating parts. Keeping this idea, the behavioral impact of upper wall divergence angle inside the supersonic combustor and their outcome have been studied by allocating three models. The supersonic free stream air is used as an oxidizer and sonic speed hydrogen as a combustible fuel to complete the chemical reaction during the combustion. Experimental model has been selected for the purpose of comparison and validation. The grid independence analyze has also been done. Ansys 14.0 - Fluent solver is opted for numerical investigation. The computational results show substantial conclusions. The mixing behavior follow the similar pattern in all the models and reaches up to 100% but the behavior of mixing length is found lesser in case of one and two degree upper wall divergence angled combustor model. The mixing length is found 41 mm for both one and two degree angle model whereas 51 mm mixing length has been captured in case of three degree divergence angled model. Initially the rapid combustion is observed in one degree angle of divergence model among all cases. However, the maximum combustion efficiency reaches 95.3% in case of two degree angle of divergence at the upper wall. Which signifies the greater impact of combustor geometry on the scramjet engine performance.

Experimental investigation on the thrust regulation of a Mg–CO2 Martian ramjet

Experimental research was conducted to evaluate the thrust regulation feasibility and investigate thrust regulation characteristics of Mg–CO2 Martian ramjet. Before the combustion experiment, a mass flow calibration experiment was carried out to assess the performance of fuel mass flow rate regulation. Moreover, a constant chamber pressure combustion experiment verified the feasibility of the Mg–CO2 ramjet. Then, the tests of thrust regulation performance were carried out in a mainstream temperature of 800 K and a mass flow rate of 100 g/s to obtain the performance of ramjet on different conditions. It was observed that the ramjet achieved stably self-sustaining combustion under different chamber pressure phases, which preliminarily proved that the ramjet has thrust regulation performance. The detailed thrust estimating results showed that the maximum thrust is 76.4 N and the minimum is 50.5 N with a 1.51 thrust regulation ratio. Furthermore, the pressure regulation response is quick, and the thrust regulation ratio is adjustable. A parameter (the regulation sensitivity) for better evaluation of regulation performance was proposed, and the regulation sensitivity is 0.08 MPa/s in this experiment. What's more, the combustion efficiency of magnesium powder in the combustor improved with the progress of adjustable thrust experiment sequence due to the increase in the internal wall temperature of the ramjet, maximum combustion efficiency achieved is 71.6%.

Investigation on combustion characteristics and thermal performance of a three rearward-step structure micro combustor fueled by premixed hydrogen/air

In order to achieve high and uniform wall temperature distribution in the micro combustor, a three-dimensional mathematical model of the premixed hydrogen/air mixtures in the micro combustors with rearward-step structure is established in this work. The effects of rearward-step structure, inlet velocity on combustion characteristics and thermal performance in the micro combustor are investigated. The results indicate that the rearward-step structure enhances the combustion intensity and stabilizes the flame in micro combustor. When the inlet velocity is 7.0 m/s, with the increase in the rearward-steps from one to three, the maximum combustion efficiency difference is about 2.28% and the percentages of improvements in lower and upper flammability limits are 20% and 3.5%, respectively. The achieved results show that the rearward-step structure increases the mean outer wall temperature and deteriorates the outer wall temperature uniformity in the micro combustor. When the inlet velocity is 5.0 m/s, the mean outer wall temperature and the temperature difference of the combustor with 3-steps are 6.3 K and 39.6 K higher than that of the combustor with 1-step, respectively. The maximum synergy degree in the micro combustor with 3-steps is 2.99% when the inlet velocity and equivalence ratio are 7.0 m/s and 1.5, respectively.

Combustion temperature analysis in a fluidized-bed reactor by utilizing palm oil biomass for a renewable energy

Biomass from palm oil is a renewable energy source that can be utilized and has very promising availability. Biomass energy is a renewable and sustainable energy that can replace conventional (fossil) fuels. The main objective of the experiment in this article is to analyze the combustion temperature, emissions, and efficiency of palm oil biomass fuel to use and applied in rural/remote areas. The palm oil biomass used in this study is palm kernel shells, empty fruit bunches, oil palm midrib, and oil palm fibers. The experiments in the research carried out in a fluidized-bed combustion chamber designed explicitly with capacities of up to 5 kg of biomass. The results of operations on fluidized-bed when the valve is open 100%, 75%, and 50% with overall palm oil biomass show a high combustion temperature. The highest combustion temperature was recorded in the TC test for 100% open valves with 3 kg biomass of 943°C. While the minimum combustion temperature obtained on TF2 at 50% open valve with 1 kg biomass of 619°C, overall combustion temperatures in this experiment showed high results. The maximum emission for O2 is 20.4% which is obtained at 50% open valve, while for CO2 the maximum emission is produced when 100% open valve is 19.9% with a biomass weight of 1 kg and 3 kg, respectively. The yield for maximum combustion efficiency when using 1 kg of biomass recorded at 50% open valve was 94.9%. While the minimum efficiency of 87.7% is obtained when the valve is 100% open with biomass of 2 kg. As the biomass fuel used in fluidized-bed increases, the combustion temperature also increases significantly.

Numerical modeling of heat transfer characteristics in a two-pass oxygen transport reactor for fire tube boilers under oxy-fuel combustion

Among the available mitigation options for CO2 emissions, oxy-fuel combustion technology has been gaining significant attention as a promising technology for curtailing the CO2 emission. Currently the technology is at the demonstration phase on existing coal and gas power plants and some new pilot plants are being tested. In spite of the prospect, to integrate oxy-combustion to fire-tube boilers, heat transfer challenges need to be addressed due to fragility of the membrane at the hostile operating conditions. In the current study, numerical modeling of a two-pass oxygen transport reactor for fire-tube boiler was conducted to investigate the heat transfer characteristics to the saturated water and steam at various operating pressures, emissivities and thermal conductivities. The results indicate that the effect of varying the boiler thermal load resulted in slight decrease in heat transferred by only about 2% at 50 bar, at fixed fuel firing rate. Despite this insignificant increase in the total heat transferred, the combustion component of the total heat transferred decreases significantly by about 16% at 50 bar. The combustion efficiency was also found to decrease by only about 0.03%, which is considered as insignificant. The total heat transferred to the load slightly decreases as the emissivity of pipes increases up to the emissivity of 0.95 beyond which the heat transferred decreased drastically at emissivity of 1 due to the flame quenching. The optimum thermal conductivity of the inner pipe to deliver highest heat to the load by ensuring flame availability, maximum combustion efficiency of 98.58% as well as optimum heat transfer is found to be 0.1 W/m-K. There were no significant variations in heat transferred and combustion efficiency when the emissivity of the membrane was varied between 0.5 and 1.

Improvement of the Combustion Completeness of Hydrogen Jet Flames within a Mesoscale Tube under Zero Gravity

Microjet hydrogen flames can be directly used as micro heat sources or can be applied in micro propulsion systems. In our previous study, under zero gravity and without an active air supply, the combustion completeness of hydrogen jet flames within a mesoscale tube with an inner diameter of 5 mm was very low. In this study, we were dedicated to improving the combustion efficiency by using a convergent nozzle (tilt angle was around 68°) instead of the previous straight one, and the exit diameter was 0.8 or 0.4 mm. The numerical results demonstrate that the maximum combustion efficiency in the case of d= 0.8 mm was only around 15%; however, the peak value for the case of d = 0.4 mm was around 36%. This happened because with d = 0.4 mm, the fuel jet velocity was around four times that of the d = 0.8 mm case. Hence, the negative pressure in the combustor of d = 0.4 mm decreased to a much lower level compared to that of d = 0.8 mm, which led to an enhancement of the air entrainment ratio. However, the highest combustion efficiency of d = 0.4 mm was still below 36%; therefore, a slightly larger tube or an even smaller nozzle exit diameter will be necessary for further improvements to the combustion efficiency.