High-pressure Combustion Research Articles

Experimental and Numerical Investigation of Laminar Burning Velocities of Artificial Biogas Under Various Pressure and CO2 Concentration

As a renewable and sustainable fuel made from digestion facility, biogas is composed predominantly of methane (CH4) and carbon dioxide (CO2). CO2 in biogas strongly affects its combustion characteristics. In order to develop efficient combustors for biogas, fundamental flame characteristics of biogas require extensive investigation. In understanding the influence of CO2 concentration and mixture pressure on biogas combustion, the effects of CO2 concentration on the laminar burning velocity of methane/air mixtures were studied at different pressures. The studies were conducted using both numerical and experimental methods. The experiment was conducted using a constant volume high pressure combustion chamber. The propagating flames were recorded with a high speed digital camera by employing Schlieren photography technique. The numerical simulation was carried by utilizing CHEMKIN-PRO with GRI-Mech 3.0 employed as the chemical kinetics model. The results show that the laminar burning velocity of methane-air mixtures decreased with an increase in CO2 concentration and mixture pressure. Therefore, the burning velocity of biogas mixtures may decrease as the amount of CO2 in the gas increases.

Optical Diagnostics in a High-Pressure Combustor with Gaseous Oxygen and Kerosene

This paper presents analysis of results from optical diagnostics in a high-pressure combustor burning gaseous oxygen (GOX) and liquid kerosene RP-2 fuel through a jet-swirl coflow injector. The objectives of the experiment were to measure flame stabilization and position under high-pressure conditions. Data were obtained at pressures from 2 to 16.5 MPa and mixture ratios from 2.9 to 20. High-speed cameras captured side-on chemiluminescence and infrared images of the flame. Results show that the flame-spreading angle from the injector ranges from approximately 3 to 6 deg, varying with pressure and propellant mass flow rate. A novel borescope was used to image the flame from upstream of the GOX post, enabling visualization of the flame near its stabilization location. The flame stabilization characteristics change significantly with the fuel flow velocity (which is proportional to pressure). At low pressures and fuel flow velocities, the flame appears to be distributed azimuthally in a nearly axisymmetric manner. At higher velocities, helical spirals of luminosity develop near the GOX post. In addition, the side-on views reveal concentrated streaks of, presumably, fuel entering the combustion chamber. These results suggest that computations must resolve the individual fuel injection orifices, to capture the flame stabilization.

Pressure effects and transition in the stabilization mechanism of turbulent lifted flames

This study reports novel measurements on the effects of pressure on the lift-off behavior and the stabilization mechanism of turbulent non-premixed methane jet flames. A high-pressure combustion duct (HPCD) was operated within the range of pressure P = 1–7 bar using jet velocities of 4 m.s−1 ≤Uj≤30 m.s−1 and co-flow velocities of 0.23 m.s−1 ≤Uc≤0.60 m.s−1. Lift-off heights were measured from chemiluminescence pictures while joint images of hydroxyl and velocity, performed using joint PLIF-OH/PIV, were used to extract information about the stabilization mechanism. It is shown that while the lift-off height generally increases with pressure, the impact of pressure depends on the magnitude of the co-flow velocity. For Uc = 0.30 m.s−1, the flame's base remains near the nozzle over the entire pressure range and the measured flame speeds indicate that edge-flame stabilization is dominant. The slope of the lift-off height vs. jet velocity curves is positive. For Uc = 0.60 m.s−1 and P > 2 bar, the flame stabilizes further downstream and a transition to turbulent premixed flame propagation appears to have occurred. At these conditions, the slope of the lift-off height vs. jet velocity curves becomes negative. This reversal at high pressure is a new result for methane. More importantly, the transition in the stabilization mechanism with increasing Uc is consistent with results reported earlier for ethylene and appears to be independent of the fuel.

Laser spectroscopic investigation of diesel-like jet structure using C8 oxygenates as the fuel

Di-n-butyl ether (DNBE) and n-octanol have very low sooting tendencies in diesel-like combustion, as demonstrated in previous engine studies. This finding is not fully understood for pure DNBE, because it has a very high cetane rating (∼100). In order to investigate the underlying mechanisms, the structure of diesel-type jets is analyzed by a number of optical diagnostics, such as spontaneous Raman scattering (SRS), laser-induced fluorescence (LIF), OH* luminescence imaging, Mie scattering, and shadowgraphy. Pure DNBE and a tailor-made blend of 50% DNBE and 50% n-octanol as well as neat n-heptane are used as the fuel in separate experiments. The jets are probed in a simulated engine-like environment in a high-pressure combustion vessel. In particular, the inner flame structure is analyzed by SRS and LIF. This yields information on the local temperature and the concentrations of O2, CO, and polycyclic aromatic hydrocarbons (PAH). For the first time, O2 is quantitatively detected in the core of a diesel-like flame by resonance-enhanced SRS. Thereby, air entrainment into the inner flame core is assessed. Results show that air entrainment is particularly strong for pure DNBE, explaining its high soot oxidation rate and overall low sooting tendency. High entrainment is primarily attributed to the low heat-release rate of DNBE, which is likely an effect of its high ignitability. Thus, it can be concluded that the high cetane rating of pure DNBE does not only lead to relatively poor pre-combustion mixture preparation and consequently considerable soot formation but seemingly also to particularly strong soot oxidation. Moreover, the jet structure turns out to be very similar for the DNBE/n-octanol blend and neat n-heptane, indicating that the net effect of volatility and fuel oxygenation is weak.

Quantitative measurement of temperature in oxygen enriched CH4/O2/N2 premixed flames using Laser Induced Thermal Grating Spectroscopy (LITGS) up to 1.0 MPa

The application of laser diagnostics to high pressure combustion phenomena is particularly challenging, especially in practical combustors such as rocket motors. In this study, temperature measurements using Laser Induced Thermal Grating Spectroscopy (LITGS) are demonstrated in oxygen enriched CH4/O2/N2 premixed laminar flames at pressures up to 1.0 MPa. We use a previously developed OH absorption LITGS technique to determine product gas temperatures from 0.3 to 1.0 MPa, for both high temperature oxygen-enriched and pure-oxygen flames, for measurements up to 3000 K. Further, we demonstrate how it is necessary to correct the measurements for the local absorption of laser light to obtain accurate temperatures, and offer a technique for producing the correction by using different laser energies. Once the correction is applied, we demonstrate that the measurements at 0.5 MPa are within 1.6% of the adiabatic non-strained flame temperatures, with a standard deviation of about 160 K, thus offering a competitive method for the challenging conditions at high pressures and temperatures. The values obtained at derived temperatures at 1.0 MPa were lower than the adiabatic unstrained flame temperatures, which could possibly be attributed to loss mechanisms.

Soot formation in diluted laminar ethene, propene and 1-butene diffusion flames at elevated pressures

Soot formation characteristics of ethene, propene, and 1-butene, the most abundant unsaturated intermediates in thermal decomposition of paraffinic hydrocarbons, were investigated in laminar diffusion flames stabilized on a co-flow burner installed in a high-pressure combustion chamber with optical access. All three olefins were diluted with nitrogen to produce sooting but non-smoking diffusion flames at desired pressures. Pressure range was 1–2.5 bar with 1-butene, and 1–8 bar with propene and ethene. Upper pressure limits of 1-butene and propene were established by their respective vapour pressure characteristics. The spectral soot emission technique, in which radiation emitted by the soot within the flame was collected as line-of-sight intensity and spectrally resolved over the range 690–945 nm, was used to measure radially-resolved temperature and soot volume fraction. The carbon mass flow rates of the three fuels were kept constant at 0.505 mg/s to facilitate direct comparison among the fuels at elevated pressures. With the same dilution level, the sooting propensity increased from ethene to 1-butene as expected; however, the pressure sensitivity of propene and 1-butene differed significantly from that of ethene. Soot yields in both propene and 1-butene flames showed a much weaker dependence on pressure than the soot in ethene flames. In the decomposition of propene and 1-butene, allyl radical and 1,3-butadiene are known to form in critical quantities leading to formation of higher molecular growth species specifically six-membered ring aromatics, and presence of these simple aromatics is argued to play a role in lowering the pressure sensitivity of the soot in C3 and C4 olefin flames.

Study on optimal combustion strategy to improve combustion performance in a single-cylinder PCCI diesel engine with different combustion chamber geometry

To determine the optimal operating conditions in a homogeneous charge compression ignition (HCCI) engine, early and multiple injections were conducted to reduce wall wetting or promote the ignition of the premixed charge in a single-cylinder diesel engine. The baseline engine was modified to prevent wall wetting as follows: (1) Reduction of the geometric compression ratio (from 17.8:1 to 15:1) to expand the ignition delay, by modifying the piston (2) Reduction of the spray-cone angle (from 156° to 60°) to reduce fuel deposition on the cylinder wall and top of the piston.The experimental results show that a high combustion pressure was caused by rapid premix combustion in the conventional engine. The modified engine configuration maintained a high thermal efficiency, whereas the injection timing became too early. The two-stage injection strategy significantly increased the combustion pressure and advanced the ignition compared to single injection because of reduced spray penetration and improved combustion efficiency. The induction of cooled-exhaust gas recirculation increased the ignition delay of the mixture and decreased the maximum combustion pressure. A comprehensive analysis of experimental results, it was founded that changes in combustion chamber geometry and application of two-stage injection and exhaust gas recirculation resulted in significant reductions in NOx and improved combustion performance.

Study of influence of boundary conditions on deformation and stresses in a cooled piston of a diesel engine – part B - calculations

The paper is continuation of the part A and presents influence of boundary conditions on mechanical and thermal stresses of an oil cooled piston in diesel engine Scania DC09 074A and its deformation due to high combustion pressure and variable temperature. Calculations of the piston loads were carried out by using 0D mathematical model at assumption of the Vibe combustion model and the Woschni heat transfer model for homogenous mixture. For precise determination of the gas temperature in the combustion chamber the CFD technique was used and for that case the simulation of compression, combustion and expansion processes was carried out by using the program Fluent. The numerical calculations were done for two kinds of fuel: diesel oil and CNG. In order to proper work of CNG engine the compression ratio was decreased. This was done by reducing the volume of the combustion chamber. Calculations done in the program Ansys showed that the modification of the piston do not influence significantly on the total stresses. The paper shows the differences of the total stresses and piston deformation at fastened piston hub and at “floating” piston pin. Most calculations are taken that the piston hubs are fixed, but it is not right assumption. In summary the paper gives indications how to set boundary conditions for “floating” piston pin. Due to different fixing of the pin in the piston hub the deformation of the piston shape and von Misses stresses differ at the same boundary conditions The analysis has shown that FEM calculations of the piston should be done together with the pin for giving the engineers right information how to design the piston shape, usually “barrel-oval”, in order to enable a proper clearance between the piston and cylinder walls at high loads.

Study of influence of boundary conditions on deformation and stresses in a cooled piston of a diesel engine – part A - mathematical models

The paper presents influence of boundary conditions on mechanical and thermal stresses of an oil cooled piston in diesel engine Scania DC09 074A and its deformation due to high combustion pressure and variable temperature. Due to different fixing of the pin in the piston hub the deformation of the piston shape and von Misses stresses differ at the same boundary conditions. This part of paper describes mathematical model of the piston load especially of heat transfer to wall taking into account three elements: convection, radiation and conductivity of the piston material. Calculations of the piston loads were carried out by using 0D mathematical model at assumption of the Vibe combustion model and the Woschni heat transfer model for homogenous mixture. For precise determination of the gas temperature in the combustion chamber the CFD technique recently is used and for that case the simulation of compression, combustion and expansion processes was carried out by using the program Fluent. The numerical calculations were done for two kinds of fuel: diesel oil and CNG due to possibility of changing the oil fuelling system into gaseous fuelling system. In order to proper work of CNG engine the compression ratio should be decreased. This was done by reducing the volume of the combustion chamber. Most calculations have done by researchers at assumption that the piston hubs are fixed, but it is not right assumption. In summary the paper gives indications how to set boundary conditions for “floating” piston pin. The paper presents shortly mathematical model of heat balance in the piston and FEM solution of stresses and deformation which are based on strains. This study is an introduction to calculations showed in the part B.

Effect of fluorine-containing nano-sized multi-functional additives on thermal decomposition behavior and gaseous products of double-base propellants

To study the effect of fluorine-containing nano-sized multi-functional additives on the thermal decomposition behavior and gaseous products of propellant, the thermal decomposition process and gaseous products of double-base propellants were studied by TGA/DSC-IR instruments. The high-pressure combustion of double-base propellants was conducted in a closed vessel, and the gaseous products were characterized by FT-IR spectroscopy. The effect of pressure, TiO2-trifluoroacrylate additives and temperature on the gaseous products was discussed. Results show that NO2 and NO gases were produced in the thermal decomposition rather than in the high-pressure combustion. The fluorine-containing nano-sized multi-functional additives do not affect the species of gaseous products of double-base propellants, but it decreases the concentration of CO toxic gas. Also, the thermal stability of double-base propellant is improved after the addition of TiO2-trifluoroacrylate additives. Furthermore, the FT-IR peak intensity of gaseous products was improved by increasing temperature in the temperature range 30–150 °C. Therefore, the application of fluorine-containing nano-sized multi-functional additives can reduce the damage of toxic gases on the environment and personnel.

Towards the development of an efficient low-NOx ammonia combustor for a micro gas turbine

Recent studies have demonstrated stable generation of power from pure ammonia combustion in a micro gas turbine (MGT) with a high combustion efficiency, thus overcoming some of the challenges that discouraged such applications of ammonia in the past. However, achievement of low NOx emission from ammonia combustors remains an important challenge. In this study, combustion techniques and combustor design for efficient combustion and low NOx emission from an ammonia MGT swirl combustor are proposed. The effects of fuel injection angle, combustor inlet temperature, equivalence ratio, and ambient pressure on flame stabilization and emissions were investigated in a laboratory high pressure combustion chamber. An FTIR gas analyser was employed in analysing the exhaust gases. Numerical modeling using OpenFOAM was done to better understand the dependence of NO emissions on the equivalence ratio. The result show that inclined fuel injection as opposed to vertical injection along the combustor central axis resulted to improved flame stability, and lower NH3 and NOx emissions. Numerical and experimental results showed that a control of the equivalence ratio upstream of the combustor is critical for low NOx emission in a rich-lean ammonia combustor. NO emission had a minimum value at an upstream equivalence ratio of 1.10 in the experiments. Furthermore, NO emission was found to decrease with ambient pressure, especially for premixed combustion. For the rich-lean combustion strategy employed in this study, lower NOx emission was recorded in premixed combustion than in non-premixed combustion indicating the importance of mixture uniformity for low NOx emission from ammonia combustion. A prototype liner developed to enhance the control and uniformity of the equivalence ratio upstream of the combustor further improved ammonia combustion. With the proposed liner design, NOx emission of 42 ppmv and ammonia combustion efficiency of 99.5% were achieved at 0.3 MPa for fuel input power of 31.44 kW.

Resolution Requirements in Stochastic Field Simulation of Turbulent Premixed Flames

The spatial resolution requirements of the Stochastic Fields probability density function approach are investigated in the context of turbulent premixed combustion simulation. The Stochastic Fields approach is an attractive way to implement a transported Probability Density Function modelling framework into Large Eddy Simulations of turbulent combustion. In premixed combustion LES, the numerical grid should resolve flame-like structures that arise from solution of the Stochastic Fields equation. Through analysis of Stochastic Fields simulations of a freely-propagating planar turbulent premixed flame, it is shown that the flame-like structures in the Stochastic Fields simulations can be orders of magnitude narrower than the LES filter length scale. The under-resolution is worst for low Karlovitz number combustion, where the thickness of the Stochastic Fields flame structures is on the order of the laminar flame thickness. The effect of resolution on LES predictions is then assessed by performing LES of a laboratory Bunsen flame and comparing the effect of refining the grid spacing and filter length scale independently. The usual practice of setting the LES filter length scale equal to grid spacing leads to severe under-resolution and numerical thickening of the flame, and to substantial error in the turbulent flame speed. The numerical resolution required for accurate solution of the Stochastic Fields equations is prohibitive for many practical applications involving high-pressure premixed combustion. This motivates development of a Thickened Stochastic Fields approach (Picciani et al. Flow Turbul. Combust. X, YYY (2018) in order to ensure the numerical accuracy of Stochastic Fields simulations.

Spray and flame characteristics of wall-impinging diesel fuel spray at different wall temperatures and ambient pressures in a constant volume combustion vessel

The qualitative mixture concentration distributions and the flame characteristics of wall-impinging diesel fuel spray at different wall temperatures and ambient pressures were investigated in a high-temperature high-pressure constant volume combustion vessel. The techniques of laser induced exciplex fluorescence (LIEF) and flame natural luminosity imaging have been used to visualize the liquid and vapor phases of the spray and the flame development process respectively. Results reveal that, as the wall temperature increases, the vapor-rich field extends to the region close to the wall and the area of high vapor fluorescence intensity increases. However, the wall temperature only has a little influence on the liquid phase of the spray. The flame luminosity intensity increases and the ignition delay decreases as the wall temperature increases. Cases at higher wall temperature show more soot emissions, higher soot formation and oxidation rate. In the condition of Pa = 4 MPa, the fuel has evaporated completely before impinging on the wall. The flame area and height at different wall temperatures are nearly the same. The ignition position is observed near the impingement point. However, in the condition of Pa = 2 MPa, there are some liquid phase of the fuel impinging on the wall. The flame area and height increase as the wall temperature increases. The ignition position is always observed in the wall jet region. The distance between ignition position and the axis of the spray decreases as the wall temperature increases.

Preparation and Experimental Evaluation of Phase-Change Characteristics in Carbon-Based Suspensions.

In this study, micro/nanocarbon-based materials (MNCBMs) were prepared using the high-pressure combustion method (HPCM) with an isoperibol oxygen bomb calorimeter at different oxygen pressures (0.5–3.0 MPa). The prepared MNCBMs were added to water to form carbon-based suspensions (CBSs); sodium dodecyl benzene sulfonate (SDBS) and defoamer were added to the CBSs to enhance their stability. The thermal conductivity, viscosity, density, and contact angle of the CBSs were measured using appropriate instruments to determine their fundamental characteristics. The phase-change characteristics of the CBSs were measured and analyzed using a differential scanning calorimeter (DSC) to evaluate the feasibility of employing them as phase-change materials in ice-storage air-conditioning systems. The results revealed that the maximal change ratios of thermal conductivity, viscosity, density, and contact angle of the samples were −3.15%, 6.25%, 0.23%, and −57.03%, respectively, as compared with the water. The CBS of S5 (oxygen pressure of 2.0 MPa) had the lowest melting temperature and subcooling degree (SD) and the highest freezing temperature in the experiments conducted using the DSC; thus, S5 was determined to be the most suitable CBS for use as a phase-change material of cold energy storage in this study.

Large-eddy simulation of direct injection natural gas combustion in a heavy-duty truck engine using modified conditional moment closure model with low-dimensional manifold method

This article describes the application of a modified first-order conditional moment closure model used in conjunction with the trajectory-generated low-dimensional manifold method in large-eddy simulation of pilot ignited high-pressure direct injection natural gas combustion in a heavy-duty diesel engine. The article starts with a review of the intrinsic low-dimensional manifold method for reducing detailed chemistry and various formulations for the construction of such manifolds. It is followed by a brief review of the conditional moment closure method for modelling the interaction between turbulence and combustion chemistry. The high computational cost associated with the direct implementation of the basic conditional moment closure model was discussed. The article then describes the formulation of a modified approach to solve the conditional moment closure equation, whose reaction source terms for the conditional mass fractions for species were obtained by projecting the turbulent perturbation onto the reaction manifold. The main model assumptions were explained and the resulting limitations were discussed. A numerical experiment was conducted to examine the validity the model assumptions. The model was then implemented in a combustion computational fluid dynamics solver developed on an open-source computational fluid dynamics platform. Non-reactive jet simulations were first conducted and the results were compared to the experimental measurement from a high-pressure visualization chamber to verify that the jet penetration under engine relevant conditions was correctly predicted. The model was then used to simulate natural gas combustion in a heavy-duty diesel engine equipped with a high-pressure direct injection system. The simulation results were compared with the experimental measurement from a research engine to verify the accuracy of the model for both the combustion rate and engine-out emissions.

Combustion performance of dual-injection using n-butanol direct-injection and gasoline port fuel-injection in a SI engine

In this study, combustion performance of dual-injection using n-butanol direct-injection (DI) and gasoline port fuel-injection (PFI) was evaluated. Dual-injection with various mass fraction of gasoline PFI and n-butanol DI were examined in a single cylinder SI engine operating at 1500 r/min, wide-open-throttle and stoichiometric air-fuel ratio (λ = 1). Indicated mean effective pressure (IMEP), knock behaviors, cylinder pressure and fuel consumption performance of dual-injection were compared with those of gasoline single-injection. At maximum brake torque (MBT) spark timings, dual-injections can produce higher engine IMEP when compared with gasoline PFI single-injection. Due to the increased engine IMEPs, dual-injections of 80% gasoline PFI-20% n-butanol DI (G80B20) and 50% gasoline PFI-50% n-butanol DI (G50B50) exhibited higher knock propensity and heavier knock intensity. When the mass fraction ratio of n-butanol DI reached to 80%, in-cylinder cooling effects of n-butanol vaporization dominated and led to a decrease of knock occurrence. Through in-cylinder pressure measurement, relatively high maximum combustion pressure with earlier crank angle were observed under dual-injection modes. Increase of the mass fraction of n-butanol DI, dual-injections resulted in higher fuel consumption rates. Nevertheless, comparisons of the indicated specific energy consumption rates of each injection mode indicated that n-butanol gasoline dual-injection had superior fuel conversion efficiencies.

Soot aggregate morphology in coflow laminar ethylene diffusion flames at elevated pressures

The effect of combustion pressure on soot aggregate morphology and on primary soot particle diameter was investigated by thermophoretic sampling in nitrogen-diluted ethylene flames. Soot aggregate samples were collected using a high-pressure thermophoretic sampling system installed inside the high-pressure combustion chamber. Collected samples were imaged by transmission electron microscopy followed by an automated image analysis process to yield aggregate morphology information. The experiments covered pressures from 3 to 6 bar, and samples were collected at heights of 2 and 5 mm above the burner exit in nitrogen-diluted ethylene (ethylene to nitrogen mole ratio 2:1) flames with nominal visible flame heights of about 20 mm. It was observed that average primary soot particle diameter increased with increasing pressure within the pressure range considered, at both measurement locations within the flame. Fractal parameters of the soot aggregates were inferred directly from the experimental data (as opposed to the usual practice of assuming a priori values for fractal dimension and pre-factor). Near to the soot nucleation and growth regions of the co-flow laminar diffusion flames, at 2 mm above the burner exit, the fractal dimension of the aggregates was not as high as the accepted universal value of about 1.9, in spite of the fact that number of primary particles per aggregate is quite high and pressure is well above atmospheric. At 5 mm above the burner exit, however, the fractal dimension of the soot aggregates approached the accepted universal value of 1.9, at high pressures; at relatively lower pressures, the fractal dimension was about 1.6 at the same location within the flame. In view of the inferred fractal results, caution should be exercised when applying fractal analysis to low heights in flames at lower pressures where soot inception and growth are relatively dominant.

Large-eddy simulations of transcritical injection and auto-ignition using diffuse-interface method and finite-rate chemistry

The need for improved engine efficiencies has motivated the development of high-pressure combustion systems, in which operating conditions achieve and exceed critical conditions. Associated with these conditions are strong variations in thermo-transport properties as the fluid undergoes mixing and phase transition, and two-stage ignition with low-temperature combustion. Accurately simulating these physical phenomena at real-fluid environments remains a challenge. This study examines a diffuse-interface method for simulating the injection and ignition of n-dodecane at transcritical conditions. To this end, a compressible solver with a real-fluid state equation and finite-rate chemistry is employed. Simulations of an ECN-relevant diesel-fuel injector are performed for both inert and reacting conditions. For the spray ignition, four specific operating points (corresponding to ambient temperatures between 900 K and 1200 K) are investigated to examine effects of the real-fluid environment and low-temperature chemistry. Comparisons with available experimental data demonstrate that the presented numerical method adequately captures the diesel fuel injection and auto-ignition processes under transcritical conditions.

Combustion characteristics, engine performances and emissions of a diesel engine using nanoparticle-diesel fuel blends with aluminium oxide, carbon nanotubes and silicon oxide

This research investigates the effect of nanoparticle-diesel fuel blends on combustion characteristics, performances and exhaust emissions of a four-stroke single-cylinder diesel engine. Three types of nanoparticles employed in this experiment were aluminium oxide, carbon nanotubes and silicon oxide. The nanoparticles were dispersed in a dosage of 25 ppm, 50 ppm and 100 ppm with pure diesel and were labelled as DA25, DA50, DA100 (aluminium oxide blends), DC25, DC50, DC100 (carbon nanotubes blends) and DS25, DS50, DS100 (silicon oxide blends). The ultraviolet-visible spectrophotometer analysis was done for 200 h to quantify the stability of the nanoparticle-diesel fuel blends. The blended fuels were experimentally tested with YANMAR TF120M diesel engine at engine loads of 0%, 25%, 50%, 75% and 100% at a constant 1800 rpm engine speed. The results revealed that the brake specific fuel consumption reduced by up to 19.8% and brake thermal efficiency enhanced by 18.8%. Silicon oxide blends show better results than aluminium oxide blends in many aspects, such as higher combustion pressure, lower brake specific fuel consumption and lower carbon monoxide emissions. Aluminium oxide and silicon oxide show stable blend conditions with 28.1% and 22.0% absorption ratio at 200 h sedimentation time, while carbon nanotube blends were least stable with absorption ratio reducing to less than 30% after 10 h. Combustion analysis of carbon nanotube blends with diesel show that carbon nanotubes have the potential to be further researched as an additive for diesel fuel due to significant combustion improvement in NOx emissions. However, the issue on the stability of carbon nanotube blends must be resolved before its potential can be fully utilized.

Study on Fuel Distribution of Wall-Impinging Diesel Spray under Different Wall Temperatures by Laser-Induced Exciplex Fluorescence (LIEF)

Spray wall-impingement has large effects on the pollutant emissions and thermal efficiency of engines. Different wall temperatures can affect the gas–liquid phase transition of the spray, and the temperature gradient from the ambient to the wall will cause a different mixture process in the region nearby the wall. However, there are few studies on spray wall-impingement with different wall temperatures, particularly on the liquid–gas phase transition of the spray. Therefore, in this paper, the effects of different wall temperatures on spray-impingement have been investigated in a high-temperature, high-pressure constant volume combustion vessel. Cooling equipment was used to adjust the temperature difference between the wall and the ambient gas. n-Dodecane was chosen as the diesel surrogate to study the spray process. The spray wall-impingement was tested by changing the injection pressures (Pi) and wall temperatures (Tw). The ambient temperature (Ta) and ambient pressure (Pa) were kept constant at 773 K and 4 MPa, and the distance (L) between wall and injector was set to 35 mm to mimic the radius of the combustion chamber in heavy-duty diesel engines. A laser-induced exciplex fluorescence (LIEF) technique was used to probe the vapor and liquid phases of the injected fuel. Results show that the liquid phases of the spray do not reach the wall except in the condition of low wall temperature and high injection pressure. The liquid penetration develops and then becomes constant after 1 ms from the start of injection. With the increase of injection pressures (600–1600 bar), the liquid concentration of the spray decreases; however the liquid penetration decreases insignificantly. The wall temperature and the injection pressure have little influence on the liquid interpenetration process. For the vapor phase of the spray, the high concentration regions (equivalence ratio (φ) > 1) mainly distribute in the area of 10 mm away from the impact point on the wall. With the decrease of wall temperatures, the high-concentration regions are enlarged at the near wall regions. However, at the injection pressure of 1600 bar, the influence of wall temperatures on the equivalence ratio is small. The decreasing wall temperature deteriorates the mixing process of the fuel and ambient gas, but the effect is weakened with the increase of injection pressure.