Combustion Chamber Walls Research Articles

Experimental study on transpiration cooling with phase change in rotating detonation engine

Transpiration cooling with phase change is an effective method for protecting high heat flux walls from ablation in combustion chambers and spaceflight vehicles. The combustion chamber walls of the rotating detonation engine (RDE) experience high heat flux, which poses significant challenges to thermal protection and limits its development. This study introduces a transpiration cooling thermal protection system for a kerosene/air RDE, where the liquid coolant absorbs heat with phase change, reducing wall temperature by more than 50% under certain conditions. The experiments investigated the effects of coolant flow rate, coolant types (water and kerosene), porosity of porous media, and fuel equivalence ratio on both transpiration cooling and detonation wave dynamics. The results show that water provides superior cooling effectiveness compared to kerosene, particularly with increased flow rates and porosity. During RDE operation, increasing flow rates of both water and kerosene initially enhances cooling efficiency but eventually leads to an overall reduction. Furthermore, higher coolant flow rates of both water and kerosene can disrupt the stability of detonation waves within the combustion chamber. Although enlarging the equivalence ratio improves cooling efficiency, maintaining continuous detonation wave generation remains challenging. The cooling efficiency in the detonation combustion mode is lower than that in deflagration. Additionally, carbon deposition was observed in the transpiration cooling system, particularly during prolonged operation. These findings provide valuable insights for the optimization and technical guidance of RDE design.

A Numerical and Experimental Performance Assessments of an Improved Natural Draft Biomass Cook Stove

ABSTRACT This article analyses the performance of an efficiently designed biomass cook stove using standard performance parameters. The investigation was first conducted using mathematical modeling developed in the ANSYS Fluent software. The results of primary air and the temperature profiles inside the combustion chamber were found through simulation. It revealed that the flue gas left the combustion chamber with an average velocity of 1.7 m/s. The average pot bottom and fuel bed temperatures were 650 K and 1100 K, respectively. A copper coil was wrapped around the combustion chamber wall to recover its waste heat. The recovered waste heat was utilized to warm the water flowing through the copper coil. The heated water could be used for other domestic applications. The fabricated design of the improved stove was experimented with water boiling test. The maximum thermal efficiency achieved was 42% with a firepower of 8.25 kW, after incorporating the waste heat recovery system, corresponding to 7.5 L test. The overall efficiency, which is the product of the modified combustion efficiency and the maximum thermal efficiency, was found to be 41.50%. The performance was found to be significantly better than that of a commercial biomass cook stove. The developed model was also evaluated for indoor CO, CO2 and PM emissions. The highest levels of CO2 and CO were 940 ppm and 23 ppm. The average amount of PM1, PM2.5 and PM10 found out to be 18 µg/m3, 20 µg/m3 and 35 µg/m3 respectively.

Unveiling the microstructure evolution and the short-time tensile creep behavior in the CuCrZr alloy

The creep behavior of the CuCrZr alloy, used in the combustion chamber wall of reusable launch vehicles, was investigated at 600-700 K and 150–250 MPa. Results demonstrate significant differences in the creep behavior, microstructure evolution including texture component, dislocation configurations and cavities at 600 K and 700 K, particularly at 250 MPa. At 600 K, the creep life exceeds 400 h, while at 700 K it drops below 0.5 h. The texture components evolved at 600 K-250 MPa are more balanced between hard texture (Brass texture) and soft texture (Goss texture and R-Goss texture). The texture components at 700 K-250 MPa are integrally biased toward the soft texture, implying that it is favorable to accelerate creep strain. The introduced high-density dislocations before creep act as an effective barrier to impede the motion of dislocations at 600 K, inducing long-time dynamic balance of work hardening and recovery softening, while the pre-creep dense dislocations act as an energy supplement at 700 K, promoting that recovery softening dominates the creep process. The dense dislocations can provide energetic support for cavity nucleation.

Investigation of the cooling hole blockage induced by different thermal spray TBC deposition processes

High-temperature turbine airfoils and combustion chamber walls in jet engines require sufficient cooling via cooling holes and thermal barrier coating systems (TBCs) to protect them from hot combustion gases. As the demand for greater efficiency and higher firing temperatures in jet engines increases, there is a corresponding need for more advanced film cooling methods, such as the use of more complex hole geometries. The use of Additive Layer Manufacturing (ALM) techniques allows the production of such intricate cooling holes, enhancing the flow of cooling air onto component surfaces. Conventional TBC deposition techniques, for example Atmospheric Plasma Spraying (APS) or Electron Beam Physical Vapor Deposition (EB-PVD), often lead to partial or complete blockage of cooling holes. This study compares the blockage of TBCs deposited on conventionally-sheet alloys with standard cooling holes and ALM alloys with more complex cooling holes using APS as a baseline process. Additionally, alternative plasma spray deposition technologies such as Suspension Plasma Spraying (SPS) and Plasma Spray-Physical Vapor Deposition (PS-PVD) were explored. The aim was to determine the effectiveness of these processes in preventing blockage compared to the traditional APS process. The experimental results showed that the formation of the coating, whether originating from splats or from the vapor phase, the feedstock particle size, and the cooling hole geometry can all affect the blockage. It was demonstrated that PS-PVD, with its vapor-induced deposition, is highly effective in minimizing blockage, regardless of the cooling hole geometry.

A 3D CFD-FEA co-simulation study of low thermal effusivity TBCs applied to the piston and valves of an SI engine

When applied on combustion chamber walls, thermal barrier coatings (TBCs) with low thermal effusivity provide a pathway for reducing heat transfer and improving SI engine efficiency. A 3D CFD-1D FEA co-simulation routine was employed to study the effects of a proprietary TBC on SI engine performance under different permutations of coating the piston, exhaust valves, and intake valves. Marginal reductions (<0.1% points) in total heat transfer and improvements to efficiency were observed when all the three components were coated with the proprietary TBC. Two hypothetical TBC materials with ideally low thermal effusivities were formed by modifying the current material properties and their effect on engine performance was similarly studied at three engine loads at the same engine speed. It was found that coating all the three components with the lowest thermal effusivity TBC offers the largest improvements (∼0.5% points) in net fuel conversion efficiency accompanied by largest reduction (∼1.1% points) in total heat transfer, thus establishing expectations from future TBC materials.

Study on radiation heat transfer characteristics of aeroengine combustion chamber

The thermal radiation of gaseous combustion products in an aero-engine combustion chamber has obvious non-ash-body characteristics. In this paper, based on the OpenFOAM platform, a numerical study on multi-field coupled turbulence-combustion-radiation radiative heat transfer was carried out for a certain type of aero-engine combustion chamber using the mean weighted sum of grey gas (WSGG) gas radiation model, which can accurately describe the non-ash characteristics of the gas. The results show that the radiation heat transfer characteristics are influenced by several factors such as equivalent ratio, combustion pressure and wall emissivity, among which the equivalent ratio is dominant. Increasing the equivalence ratio from 0.11 to 0.4 resulted in a 189% increase in average incident radiation at the combustion chamber wall and a 233% increase in average incident radiation at the exit. Increasing the equivalent ratio, combustion pressure or wall emissivity will increase the average radiation intensity in the combustion chamber and the inhomogeneity of radiation distribution at the exit of the combustion chamber. The radial radiation distribution factor (RRDF) also increases with the increase of equivalent ratio, combustion pressure and wall emissivity.

The Effect of Avgas and Ethanol Fuel Mixture on Fuel Spray Temperature

This research aims to examine the effect of the avgas-ethanol fuel mixture on the fuel spray temperature. The research was carried out using a pressurized tube which was sprayed into the mock up which was assumed to be an aircraft combustion chamber with a pressure of 2 bar and a distance of 7,73 cm which would later be sprayed using pure avgas, a mixture of avgas and ethanol namely A80E20 and A60E40 then added with a temperature sensor at the end nozzle, center of jet, and impact wall. The spray temperature can result in incomplete combustion and result in higher exhaust emissions and also damage to the engine if it does not match the temperature capacity of the combustion chamber walls. The spray temperature results for the A60E40 fuel mixture are the lowest temperature, namely 25o-18.2oC and knowing that the more fuel is mixed, the lower the temperature of the fuel.

Effect Of Wavy Structure Of Liquid Film On Flow Characteristics Of Impingement Jet Flowing On Fuel Liquid Film

In recent years, diesel engines have been downsized in order to improve combustion efficiency by using high-pressure injection to atomize the fuel and to increase fuel economy. As a result, the diesel spray flame inevitably impinges on the combustion chamber wall, resulting in heat loss due to heat transfer between the flame and the wall. According to Newton's cooling law, the heat transferred from the flame to the wall is proportional to the heat transfer coefficient, which is closely related to the flow of the spray flame. However, there are few reports on the flow of a spray flame impinging on a wall. The flow characteristics of a non-vaporized spray impinging on a wall surface, assuming that the flow of a spray flame impinging on a wall surface is equivalent to that of a non-vaporized spray. The fuel film formed on the wall affected on the flow characteristics of the spray. When the spray flame impinges on the wall in the engine cylinder, no liquid film is formed on the wall. Still, during cold start, a liquid fuel film may form on the piston wall due to the low temperature in the engine cylinder, resulting in HC and soot emissions. As a result, the spray may flow over the formed film. Therefore, it is necessary to investigate the flow characteristics of the spray flowing over the fuel film. In this study, the experiments using an impingement gas jet were conducted on a wall surface with a fuel film, and the velocity field of the jet after impingement of the wall surface was measured by the time-series PIV. In addition, the thickness of the wavy liquid film formed by the gas jet impingement was measured by using the laser-induced fluorescence (LIF) method, discussing the relationship between the velocity of the jet after wall impingement and the characteristics of the wavy liquid film.

The Experiment and Simulation on Impact Characteristics of AVGAS Spray Based on Impact Phenomenon

Avition gasoline (avgas) is one of the fuels used for combustion of piston-type aircraft engines and uses ignition in the form of spark plugs or internal combustion engines. In recent years, the use of avgas fuel itself is very high used in high- wing light aircraft engines, namely Cessna. avgas spray phenomenon that collides with the cylinder wall may occur during fuel injection, so it will produce a changed radius and spray height, which will affect the mixing of fuel and air, later engine performance and exhaust emissions will also be affected. Therefore, it is important to know and study the spray impact phenomenon in avgas-fueled aircraft engines. This research is a spray experiment with a specified pressure and sprayed on the combustion chamber wall so that the results of the image can be studied coupled with data from experimental research, it can make research from computational fluid dynamics (CFD) simulations, providing valuable insights for future research in this field.

Hydrocarbon fuels, combustion characteristics & insulating refractories in industrial furnace

Liquid fuels like Furnace Oil, Light distillate oil, Diesel & gaseous fuels like PNG (Piped Natural Gas), LPG (Liquefied Petroleum Gas) are predominantly used at present in industrial applications. Single fuel, Dual fuel & Multi-fuel options are available in the market. All these fuels are called hydrocarbon fuel. A loss of drop of oil in every second can waste about 4000 liters in a year. Selection of right type of fuel depends on various factors like availability, storage, handling, Pollution & landed cost of the fuel. These different fuels used for combustion in industrial furnace are discussed herewith. Complete combustion in industrial furnace enhances efficiency, control pollution as well as global warming. Efficient use of fuel leads to complete combustion. This paper deliberates about combustion of fuel and how complete combustion is to be achieved in industrial furnace. Stoichiometric ratio ensures complete combustion. Industrial furnace uses refractories to form a combustion chamber with proper insulation to ensure temperature within the combustion chamber is as per requirement of the job. The outside skin temperature of industrial furnace is about 35ºC to 45ºC from safety point of view. To maintain this temperature difference with minimum wall thickness needs proper refractory selection which must withstand high temperature. The main objective of this research paper is to propose strategies to select the right fuel, proper insulating material to achieve complete combustion & minimum heat losses through the walls of combustion chamber. This will help in making an efficient design and optimize combustion controls to keep heat losses at minimum level.

A combined combustion and conjugate heat transfer analysis of a hybrid draft biomass cookstove

A cookstove comprises both fluid and solid regions, with combustion gases representing the fluid domain and the combustion chamber wall and insulation forming the solid regions. These solid regions are made of different materials and make significant contributions to stove performance. So far, most studies conducted on biomass stoves have only analysed the fluid domain, neglecting solid regions. The current study integrates combustion and conjugate heat transfer analysis, encompassing both fluid and solid domains, to investigate the fluid flow and heat transfer behaviour of a hybrid draft biomass cookstove (HDBC) using CFD. This analysis aims to gain insights into the impact of stove materials on its performance. The study analysed six cases of HDBC, testing different combinations of materials: two for the combustion chamber (ceramic-cement and stainless-steel) and three for insulation (ceramic wool, perlite, and vermiculite). The combustion of biomass and the formation of soot are modelled using reaction kinetics and a one-step soot model, respectively. The CFD findings for thermal efficiency, CO, and PM2.5 emissions derived using combustion and soot model demonstrated closed alignment with the experimental result with variations of 3%, 19%, and 23%, respectively. It was observed that stoves with stainless-steel combustion chamber material exhibit better thermal and emission performance compared to ceramic-cement stoves, with perlite taking the edge over other insulation materials. The success of these materials is attributed to their superior thermophysical properties, which play a significant role in material selection.

Numerical and experimental analysis of thermal and flow operating conditions of waterwall tubes connected by fins

This paper presents a method for determining the temperature distribution in the cross-section of waterwall tubes connected by fins using an in-house numerical algorithm prepared in the MATLAB environment, based on differential equations with separable variables. In order to verify the correctness of the algorithm operation, the temperature values obtained from it, determined for the frontal area of the tubes, are compared with the temperatures found in the Ansys Fluent environment and those measured on the test stand. A system corresponding to a fragment of the combustion chamber wall of a supercritical steam boiler was selected to perform the analysis. The system consists of three tubes connected by fins. The temperature distributions in the cross-sections of the tubes were compared for the case when each of the tubes was heated on one side with the same heat flux and when the heat flux falling on the central tube was by 50% higher than the heat flux incident on the neighbouring tubes. Experimental verification was carried out on a stand equipped with three vertical tubes connected by fins, heated on one side by infrared radiators.

Exploring combustion characteristics of a novel micro combustor with a fan swirler for thermophotovoltaic system

The combustion and thermal characteristics of non-premixed methane (CH4)–air flames in a swirl micro combustor are numerically investigated for potential use in a thermophotovoltaic (TPV) system. Swirling flows associated with methane–air combustion are simulated using a Reynolds stress model and detailed chemistry of GRI-Mech 3.0. The optimal condition for improved emission and combustion performance is established by taking into account six blade angles and three fuel-lean equivalence ratios, to consider among other geometric features of the fan swirler and operating conditions.The reactive swirling flows in a micro combustor with a fan swirler are modified by altering the blade angle (θb). For θb = 5°, 10°, and 15°, the swirl micro combustor with a two-vortex structure exhibits a significant enhancement of the fuel–air mixing and thermal performance compared to no-swirl combustor. As θb increases, inner vortices close to the fuel stream and outer vortices near the combustor wall behind the fan blades are developed according to the interaction of swirling flows and recirculating flows due to the blockage effect of the fan blades. For θb = 15°, the strong interaction between two vortex regions maximizes combustion efficiency. The combustion efficiency is highest at θb = 15° and lowest at θb = 75°. For three fuel-lean conditions of θb = 15°, the flame zone is enlarged towards the combustion chamber wall, resulting in a high and uniform wall temperature. The resulting radiant emitter efficiencies for a TPV system are increased by 28.3 %–35.1 % compared to that of the no-swirling micro combustor. The results indicate that the fan swirler has recirculation flows for the best emitter and combustion efficiency unlike vane-type swirlers.

Analysis of the relationship between near-wall velocity distribution and wall heat flux under diesel spray flame impingement in an RCEM

To clarify the relationship between the near-wall flow and the heat flux through the wall, the velocity distribution of the diesel spray flame and the heat flux at the combustion chamber wall were measured in a rapid compression and expansion machine (RCEM), which has a twodimensional combustion chamber. The velocity distribution of the diesel spray flame near the wall was measured by the particle image velocimetry (PIV), and the heat flux through the wall where the diesel spray flame impinges was measured using a newly developed multi-point heat flux sensor. Furthermore, the velocity gradient tensor was calculated based on the measured velocity distribution results, and the effect of the shear flow characteristics on wall heat transfer was analyzed. As a result, the higher the injection pressure leads the higher the peak value of the heat flux. A fluctuation of the heat flux exhibits the same trend as the fluctuations of the velocity, and a high heat flux appeared in the high-velocity gradient tensor region, which suggests that the characteristics of shear flow near the wall can affect the heat flux.

The Simulation of Biogas Combustion in Top Open Burner

The top open furnace is commonly used for the heating processes. The efficiency of the top open furnace is low due to the amount of heat wasted through the top open. This study was using ANSYS Fluent, a two-dimensional computational fluid dynamic (CFD) model that was used to build the top open furnace. Exhaust gas recirculation (EGR) was installed in the boiler in order to collect the exhaust gas and reuse it to dilute the oxygen supply. To check the combustion of low calorific gas (LCV) gas, which is biogas composed of 60% methane and 40% carbon dioxide by mass fraction, the computational study started with a standard combustion with EGR. Medium-sized mesh is used for the smoothing grid. This mesh uses inflation to enable the size of the cells and the stack element. The findings of the numerical sensitivity test show that the boundary conditions along the combustion chamber wall can affect the flame temperatures. The MILD regime was reached using biogas fuels when the right parameters were used.

Minimally-intrusive, dual-band, fiber-optic sensing system for high-enthalpy exhaust plumes

<abstract> <p>The propulsion research lab at Utah State University has developed a minimally-intrusive optical sensing system for high-temperature/high-velocity gas-generator exhaust plumes. For this application glass fiber-optic cables, acting as radiation conduits, are inserted through the combustion chamber or nozzle wall and look directly into the flow core. The cable transmits data from the flame zone to externally-mounted spectrometers. In order to capture the full-optical spectrum, a blended dual-spectrum system was employed, with one spectrometer system tuned for best-response across the visible-light and near-infrared spectrum, and one spectrometer tuned for best-response in the near- and mid-infrared spectrum. The dual-band sensors are radiometrically-calibrated and the sensed-spectra are spliced together using an optimal Wiener filtering algorithm to perform the deconvolution. The merged spectrum is subsequently curve-fit to Planck's black-body radiation law, and flame temperature is calculated from associated curve maxima (Wien's law). The presented fiber-optic sensing systems performs a function that is analogous to Raman spectroscopy. The system non-contact, high-temperature measurement, and does not interfere with the heat transfer processes. In this report data collected from a lab-scale (200 N) hybrid rocket system are analyzed using the described method. Optically-sensed flame-temperatures are correlated to analytical predictions, and shown to generally agree within a few degrees. Additionally, local maxima in the optical spectra are shown to correspond to emission frequencies of atomic and molecular oxygen, water vapor, and molecular nitrogen; all species known to exist in the hybrid combustion plume. Presented data demonstrate that selected fiber-optics can survive temperature greater than 3000 ℃, for durations of up to 25 seconds.</p> </abstract>

Waste heat regeneration from thermoelectric generator based improved biomass cookstove (TIBC): Modelling of TEG system utilizing DC-DC converter with fuzzy logic MPPT

Most of the people in the rural areas cook their food in traditional biomass cookstove which emits a lot of emission that leads to poor health of the women and children. This paper points out a technology that utilises a thermoelectric generator (TEG), which converts thermal energy into electrical energy directly. A TEG is a small plate made of semiconductor p-n junctions connected electrically in series and thermally in parallel. The difference in temperature produced on the hot side and cold side of the TEG is proportional to the voltage generated. The TEG is integrated into the wall of the inner combustion chamber of the biomass cookstove with the help of heat plate and the cold side of the TEG is connected to a flower-type heat sink to maintain the temperature difference. The power generated by the TEG is used to drive a 12 V DC fan which helps to keep the cold side of the TEG relatively cooler and direct the same air inside the inner combustion chamber of the cook stove through holes to increase the air-to-fuel ratio. As a result, it improves the cookstove's combustion, thereby reducing emissions and increasing thermal efficiency. For testing the efficiency of the TEGs available in the market, a hit and trial method is used by taking TEGs of different specifications and connecting them to a hot side and a cold side test bed and then finally the performance is realized. Therefore, for every different TEG, a hardware has to be made for final conclusion of the result. To eliminate the hit and trial method, the electrical characteristics of thermoelectric generator module has been analysed by modeling a TEG module in MATLAB/Simulink and implementing it with fuzzy logic control (FLC) based MPPT utilizing a DC-DC boost converter with a constant as well as variable load. This fuzzy logic-based scheme exhibits fast dynamic response and precisely tracks the maximum power point (MPP) from TEG under varying load conditions. Based on the robust performance of the obtained simulation results, the TEG module is integrated and tested in the laboratory using a thermoelectric generator based improved biomass cookstove (TIBC). The novelty lies on the concept of using MATLAB/Simulink platform by modeling the FLC based MPPT incorporating boost converter to test the performance of TEG module for its integration with TIBC and eliminating traditional hit and trial method. This TIBC is able to power the fan directly within just 2–3 min using the electricity generated by the TEG module. The maximum power generated from the TIBC is 6.25 W using a single TEG. The fan requires 1.4 W and rest of the power was used to charge auxiliary battery.

Impact of Preheating on Flame Stabilization and NOx Emissions From a Dual Swirl Hydrogen Injector

Abstract Flame stabilization, flame structure, and pollutant emissions are investigated experimentally on a swirled injection system operating with globally lean air/hydrogen mixtures at atmospheric conditions and moderate Reynolds numbers. This injector consists of two coaxial ducts with separate injection of hydrogen into a central channel and of air into an annular channel. Both streams are swirled. The resulting flames exhibit two stabilization modes. In one case, the flame takes an M-shape and is anchored to the hydrogen injector lips. In the second case, the flame is aerodynamically stabilized above the injector and takes a V-shape. Regions of existence of each stabilization mode are determined according to the operating conditions. For low air flow rates, the flame can be either anchored or lifted above the hydrogen injector lips depending on the path followed to reach the operating condition. At high air flow rates, the flame is always lifted regardless of the trajectory followed. The impact of air inlet temperature on these stabilization regimes is then evaluated from T= 300 K up to 770 K. Flame re-attachment is shown to be controlled by edge flame propagation and the impact of preheating is well reproduced by the model. Unburnt hydrogen and NOx emissions are finally evaluated. Unburnt hydrogen is only observed for global equivalence ratios below 0.4 and at ambient inlet temperature. NOx emissions decrease when the global equivalence ratio is reduced. Furthermore, at fixed global equivalence ratio, NOx emissions decrease as the thermal power increases, regardless of air preheating and the flame stabilization regime. At high power, NOx emissions reach an asymptotic value that is independent of the thermal power. The impact of flame shape, air preheating, and combustion chamber wall heat losses on NOx production is also evaluated. NOx emissions are shown to scale with the adiabatic flame temperature Tad at the global equivalence ratio and the residence time inside the combustor.

Microstructural Stability of a Metallic Thermal Barrier Coatings for Rocket Engine

Metallic thermal barrier coatings (TBCs) consisting of a bond coating and a top coating have been extensively utilized for protecting the walls of rocket combustion chambers. However, standard coating systems often encounter failures due to the significant differences in coating composition and thermal expansion coefficient compared to the substrate under high heat flux conditions. To protect liquid rocket combustion chamber walls, a novel metallic multilayer TBC system applied with atmospheric plasma spraying is developed in the present work. It attempted to deposit dense Ni-based alloy and Cu-based bonding coatings with low oxide contents achieved by introducing boron as a deoxidizer element through atmospheric plasma spraying. The structural stability of the TBC was assessed through high temperature thermal exposure experiments, while the thermal cycle life is evaluated using laser thermal shock. Results show that the NiCrCu2B and CuNi2B bonding coatings prepared through in situ deoxygenation effect of boron exhibit dense structures, low oxide content, and excellent bonding quality. The high temperature thermal exposure experiment reveals that the multilayer structural TBC can withstand 850 °C for 10 hours without the formation of Kirkendal effect pores. Moreover, the thermal cycling life results indicate that the multilayer structural TBC designed in this study, employing a composition gradient transition and the in situ deoxygenation effect of boron, possesses a significantly improved thermal cycle lifetime compared to traditional structural TBCs.

Study on combustion stability and flame development of ammonia/n-heptane dual fuel using multiple optical diagnostics and chemical kinetic analyses

Ammonia is considered as one of the most potential clean energy due to its carbon-free structure, which ensures the extensive decrease in global CO2 emissions. However, it has difficulty in using ammonia as a single fuel in actual apparatus due to the low reactivity and slow laminar flame speed. The dual fuel combustion of ammonia and high reactivity fuel is a potential solution, which is expected to address the issues raised by ammonia. Nevertheless, the investigations on the influence of boundary conditions on combustion stability and related mechanisms in dual fuel approach of ammonia and high reactivity fuel are still scarce at present. Therefore, in current study, n-heptane was used as the direct-injected fuel, and the investigations on combustion stability and relevant mechanisms of ammonia/n-heptane dual fuel approach were conducted by using optical diagnostics and chemical kinetic analyses. Results indicate that in n-heptane-only combustion, the premixed blue flames are found in the initial combustion stage. The auto-ignition sites dominated by orange chemiluminescence are generated near the combustion chamber wall in ammonia/n-heptane dual fuel combustion and then the flames develop towards the combustion chamber center. The addition of ammonia inhibits the combustion of n-heptane. The ammonia/n-heptane dual fuel combustion is more sensitive to the change of intake temperature compared with n-heptane-only combustion. The increase of directed-injection pressure from 300 bar to 600 bar and compression ratio from 11 to 14.5 favors the flame development speed. The increase of compression ratio is an effective mode to improve the ammonia/n-heptane dual fuel combustion stability compared with increasing intake temperature and modulating directed-injection strategies. The chemical kinetic analyses indicate that the ignition delay of dual fuel combustion of ammonia and n-heptane is more sensitive to the variations of ambient temperature compared with ambient pressure. Consequently, the changes of ambient temperature on the dual fuel combustion stability of ammonia and n-heptane play a dominant role. In a word, the dual fuel combustion stability of ammonia and n-heptane can be achieved through the collaborative optimization of multiple boundary conditions to reduce the dependence on single factor.