Effects of entrainment and agglomeration of particles on combustion of nano-aluminum and water mixtures

The combustion of nano-aluminum and water mixtures is studied theoretically for a particle size of 80 nm and over a pressure range of 1–10 MPa. Emphasis is placed on the effects of entrainment and agglomeration of particles on the burning rate and its dependence on pressure. The flame thickness increases by a factor of ∼10, when particle entrainment is considered. This lowers the conductive heat flux at the ignition front, thereby reducing the burning rate. The pressure dependence of the burning rate is attributed to the changes in the burning time and velocity of particles with pressure. In the diffusion limit, the pressure exponent increases from 0 to 0.5, when the entrainment index increases from 0 to 1.0. A similar trend is observed in the kinetics-controlled regime, although the corresponding value exceeds the diffusion counterpart by 0.5. The kinetics-controlled model significantly over-predicts the burning rate and its pressure exponent, depending on the entrainment index. The present analysis suggests that nano-particles formed closely-packed agglomerates of diameter 3–5 μm, which may burn under diffusion-controlled conditions at high pressures.     

Combustion of micron-sized aluminum particle,liquid water,and hydrogen peroxide mixtures

The combustion of aluminum particle, liquid water, and hydrogen peroxide (H₂O₂) mixtures is studied theoretically for a pressure range of 1–20 MPa and particle sizes between 3 and 70 μm. The oxidizer-to-fuel (O/F) weight ratio is varied in the range of 1.00–1.67, and four different H₂O₂ concentrations of 0%, 30%, 60%, and 90% are considered. A multi-zone flame model is developed to determine the burning behaviors and combustion-wave structures by solving the energy equation in each zone and enforcing the temperature and heat-flux continuities at the interfacial boundaries. The entrainment of particles is taken into account. Key parameters that dictate the burning properties of mixtures are found to be the thermal diffusivity, flame temperature, particle burning time, ignition temperature, and entrainment index of particles. When the pressure increases from 1 to 20 MPa, the flame thickness decreases by a factor of two. The ensuing enhancement of conductive heat flux to the unburned mixture thus increases the burning rate, which exhibits a pressure dependence of the form r_b = ap^m. The exponent, m, depends on reaction kinetics and convective motion of particles. Transition from diffusion to kinetically-controlled conditions causes the pressure exponent to increase from 0.35 at 70 μm to 1.04 at 3 μm. The addition of hydrogen peroxide has a positive effect on the burning properties. The burning rate is nearly doubled when the concentration of hydrogen peroxide increases from 0 to 90%. For the conditions encountered in this study, the following correlation for the burning rate is developed: _rb[cm/s]=4.97(p[MPa])^0.37(_dp[μm])^-0.85(O/F)^-0.54exp(0.0066CH₂O₂).$r_b[\text{cm}/\text{s}]=4.97{(p[\text{MPa}])}^{0.37}{(d_p[\mathrm{\mu }\text{m}])}^{-0.85}{(\text{O}/\text{F})}^{-0.54}\exp (0.0066{\text{C}}_{{\text{H}}_2{\text{O}}_2})\text{.}$     

Effect of particle size on combustion of aluminum particle dust in air

The combustion of aluminum particle dust in a laminar air flow is theoretically studied under fuel-lean conditions. A wide range of particle sizes at nano and micron scales is explored. The flame speed and temperature distribution are obtained by numerically solving the energy equation in the flame zone, with the particle burning rate modeled as a function of particle diameter and ambient temperature. The model allows for investigation into the effects of particle size, equivalence ratio, and chemical kinetics on the burning characteristics and flame structures of aluminum-particle/air mixtures. In addition, the flame behavior with ultra-fine particles in the sub-nanometer range is examined by asymptotically treating particles as large molecules. Calculated flame speeds show reasonable agreement with experimental data. As the particle diameter decreases from the micron to the nano range, the flame speed increases and the combustion transits from a diffusion-controlled to a kinetically controlled mode. For micron-sized and larger particles, the flame speed can be correlated with the particle size according to a d−m relationship, with m being 0.92. For nano-particles, a d^−0.52 or d^−0.13 dependence is obtained, depending on whether the d^1.0- or d^0.3-law of particle burning time is implemented in the flame model, respectively. No universal law of flame speed exists for the entire range of particle sizes.     

Combustion characteristics of nanoaluminum, liquid water, and hydrogen peroxide mixtures

An experimental investigation of the combustion characteristics of nanoaluminum (nAl), liquid water (H₂O_(l)), and hydrogen peroxide (H₂O₂) mixtures has been conducted. Linear and mass-burning rates as functions of pressure, equivalence ratio (Φ), and concentration of H₂O₂ in H₂O_(l) oxidizing solution are reported. Steady-state burning rates were obtained at room temperature using a windowed pressure vessel over an initial pressure range of 0.24 to 12.4 MPa in argon, using average nAl particle diameters of 38 nm, Φ from 0.5 to 1.3, and H₂O₂ concentrations between 0 and 32% by mass. At a nominal pressure of 3.65 MPa, under stoichiometric conditions, mass-burning rates per unit area ranged between 6.93 g/cm² s (0% H₂O₂) and 37.04 g/cm² s (32% H₂O₂), which corresponded to linear burning rates of 9.58 and 58.2 cm/s, respectively. Burning rate pressure exponents of 0.44 and 0.38 were found for stoichiometric mixtures at room temperature containing 10 and 25% H₂O₂, respectively, up to 5 MPa. Burning rates are reduced above ∼5 MPa due to the pressurization of interstitial spaces of the packed reactant mixture with argon gas, diluting the fuel and oxidizer mixture. Mass burning rates were not measured above ∼32% H₂O₂ due to an anomalous burning phenomena, which caused overpressurization within the quartz sample holder, leading to tube rupture. High-speed imaging displayed fingering or jetting ahead of the normal flame front. Localized pressure measurements were taken along the sample length, determining that the combustion process proceeded as a normal deflagration prior to tube rupture, without significant pressure buildup within the tube. In addition to burning rates, chemical efficiencies of the combustion reaction were determined to be within approximately 10% of the theoretical maximum under all conditions studied.     

Modeling pressure loads during a premixed hydrogen combustion in the presence of water spray

This paper proposes a method for pressure evolution modeling during combustion process in presence of water spray. A simplified model based on empirical correlations is developed, which allows the estimation of the main factors influencing the pressure evolution, such as the combustion rate, the convective heat loss and the droplet evaporation rate. The results are then used as a guideline to adjust the parameters of a three-dimensional hydrodynamic code based on CREBCOM combustion model developed and validated for large-scale hydrogen combustion. This methodology provides an approach to estimate the important parameters for the determination of the pressure loads. Simulation results for hydrogen-air combustion in presence of water spray using the present model compare favorably to the experimental data of Carlson et al. [1].     

Evidence of nano-galvanic couple formation on in-situ formed nano-aluminum amalgam surfaces for passivation-bypassed water splitting

Reaction of Al metal with water is a well-known technique for large scale production of hydrogen. However, this method suffers from kinetic limitations due to formation of a passivation layer on Al, preventing optimal operations. Using high resolution Scanning Kelvin Probe Force Microscopy (SKPFM), we show the origin of formation of 'nano-galvanic couple' on in situ formed nano-aluminum amalgam surfaces in a water splitting system; passivation based limitations are completely bypassed in this approach. Furthermore, they offer an opportunity to beneficiate and recover mercury in contaminated water. The nano-galvanic corrosion due to substantial lateral variation in surface contact potential is responsible for the observed high throughput of hydrogen production (720 mL/min per 0.5 g Al salt). It may be noted that this process fares better than in situ prepared nano-Al based hydrogen production, wherein 600 mL/min of hydrogen is obtained for 0.5 g Al salt. Investigations using Cyclic Voltammetry (CV) and Electrochemical Impedance Spectroscopy (EIS) provide evidence for passivation-bypassed hydrolysis and favourable kinetics for in situ derived nano-AlHg hydrolytic agents (when compared to nano-Al). This study, to the best of our knowledge, reports the first direct proof of nano-galvanic couple formation on in-situ prepared nanoaluminum amalgam surface; paving a direct way to overcome the long standing passivation problem in Al hydrolysis. It is found that the hydrogen production rate and standard deviation (SD) of the contact potential of nanoaluminum amalgam are directly related to the rate of addition of the reducing agent, offering an opportunity for kinetic control for the in situ hydrolytic process.     

An experimental study of the effect of water content on combustion of coal tar/water emulsion droplets

Isolated high asphaltene droplets of coal tar/water emulsion were studied to investigate the non-steady behavior of the burning droplets. Data on size and temperature histories were obtained. Coke residues were analyzed by scanning electron microscope. Lower and upper limits for ignition time delay were established. The error, defined as the time lag between these two limits, was less than 8 ms. Ignition time delays of emulsions were longer than for ordinary coal tar (CT) droplets of the same size but the peak temperature of emulsions occurred much earlier. A steeper temperature rise observed in the emulsions during portions of their combustion history is evidence not only of soot reduction but also the extent of burnout of the cenospheres. The latter is an important aspect in the reduction of pollutant emissions. The emulsion droplets indicated swelling of considerable magnitude compared with that of CT. Coke particles formed from emulsions were more porous, with thinner and fragile shells. The CT residues were harder and more resistant to burning. Excess burnout time or the ratio of burnout time of the emulsions depended on the water concentration, indicating that longer oxidation time was required for coke particles from coal tar than from emulsions.     

Numerical study on laminar flame velocity of hydrogen-air combustion under water spray effects

In the context of hydrogen safety and explosions in hydrogen-oxygen systems, numerical simulations of laminar, premixed, hydrogen/air flames propagating freely into a spray of liquid water are carried out. The effects on the flame velocity of hydrogen/air flames of droplet size, liquid-water volume fraction, and mixture composition are numerically investigated. In particular, an effective reduction of the flame velocity is shown to occur through the influence of water spray.To complement and extend the numerical results and the only scarcely available experimental results, a “Laminar Flame Velocity under Droplet Evaporation Model” (LVDEM) based on an energy balance of the overall spray-flame system is developed and proposed. It is shown that the estimation of laminar flame velocities obtained using the LVDEM model generally agrees well with the experimental and numerical data.     

Numerical investigation of water injection quantity and water injection timing on the thermodynamics,combustion and emissions in a hydrogen enriched lean-burn natural gas SI engine

Water direct injection into the cylinder is one of effective ways to suppress the combustion rate and knocking combustion in turbocharged SI engine. In this study, a detailed one-dimensional model coupled with the water direct injection was built by using the GT-Power according to the real tested hydrogen-enriched lean-burn natural gas (NG) SI engine, and validated against the experimental data. Then, a series of cases with various water injection quantity and injection timing were comprehensively investigated on the thermodynamics, combustion and emissions characteristics of the NGSI engine. The impact of the thermo-physical of the water were discussed in detailed by sweeping various water injection quantity and water injection timing. The results indicated that peak combustion pressure and peak heat release rate decreased with the increasing the water injection quantity. In addition, the 50% combustion location and peak combustion pressure location were retarded with the increasing the water injection quantity. As for the water injection timing, the peak combustion pressure and peak combustion temperature were slightly decreased with retarding the water injection timing. Apart from that, the indicated thermal efficiency decreased 4.03% and the equivalent fuel consumption increased 3.56% with injecting 60 mg water into the cylinder compared the case without water injection. Furthermore, the indicated thermal efficiency decreased 4.68% and the equivalent fuel consumption increased 4.66% by sweeping the water injection timing from the 150 CA to 50 CA before top dead center. However, the volumetric efficiency slightly ascended with increasing the water injection quantity and retarding the water injection timing. Finally, the NOx emissions declined with increasing the water injection quantity and retarding the water injection timing. However, CO emission and unburned HC emissions increased with increasing the water injection quantity and retarding the water injection timing. The main aim of this paper is expected to provide a comprehensively assessment of the thermo-physical of water on the thermodynamics, combustion, and emissions of the hydrogen enriched NGSI engine.     

Experimental study of inlet manifold water injection on combustion and emissions of an automotive direct injection Diesel engine

This paper describes an experimental study conducted on a modern high speed common-rail automotive Diesel engine in order to evaluate the effects on combustion and pollutant emissions of water injected as a fine mist in the inlet manifold.     

The combustion of large particles of char in bubbling fluidized beds: The dependence of Sherwood number and the rate of burning on particle diameter

Particles of char derived from a variety of fuels (e.g., biomass, sewage sludge, coal, or graphite), with diameters in excess of , burn in fluidized bed combustors containing smaller particles of, e.g., sand, such that the rate is controlled by the diffusion both of O₂ to the burning solid and of the products CO and CO₂ away from it into the particulate phase. It is therefore important to characterize these mass transfer processes accurately. Measurements of the burning rate of char particles made from sewage sludge suggest that the Sherwood number, Sh, increases linearly with the diameter of the fuel particle, dchar (for ). This linear dependence of Sh on dchar is expected from the basic equation Sh=2εmf(1+dchar/2δdiff)/τ, provided the thickness of the boundary layer for mass transfer, δdiff, is constant in the region of interest (). Such a dependence is not seen in the empirical equations currently used and based on the Frössling expression. It is found here that for chars made from sewage sludge (for ), the thickness of the boundary layer for mass transfer in a fluidized bed, δdiff, is less than that predicted by empirical correlations based on the Frössling expression. In fact, δdiff is not more than the diameter of the fluidized sand particles. Finally, the experiments in this study indicate that models based on surface renewal theory should be rejected for a fluidized bed, because they give unrealistically short contact times for packets of fluidized particles at the surface of a burning sphere. The result is the new correlation

     

Effects of fuel properties on the combustion behavior of different types of porous beds soaked with combustible liquid

Experimental studies were conducted to investigate the behaviors of non-spread diffusion flames of liquid fuel-soaked porous beds. The effects of the properties of the porous bed and the fuel on combustion behaviors, including flame temperature profiles, combustion duration time, fuel consumption and the amount of fuel residue in the porous beds, were studied in the experiments. Additionally, a heat transfer model was used to predict the amount of fuel consumption in the porous beds. Consistency between the predicted and experimental results confirmed that heat conduction is the controlling mechanism in the combustion behavior of liquid fuel-soaked porous beds.     

颗粒组合角度对近壁颗粒传热特性的影响

为了探究颗粒堆积结构变化对近壁面颗粒传热过程的影响,构建了近壁面两颗粒的非稳态传热模型,研究了不同初始温度条件下颗粒组合角度变化对近壁面颗粒传热特性的影响规律。结果表明：初始温度越高,所需换热时间越长,颗粒组合角度增加,使得换热时间明显减少。初始温度为1073K时,换热面平均热流密度值呈先迅速下降后缓慢下降的趋势。同一时刻,组合角度越大,颗粒平均温度越低。不同组合角度颗粒的固相传热率均达到0.8以上。颗粒组合角度越大,固相传热占比越小,辐射传热占比越大。在换热前期,辐射传热率最高可达0.57,对传热过程的影响不可忽略。     

Influence of functional group structures on combustion behavior of pulverized coal particles

The ignition and combustion behavior of pulverized coal was studied with respect to coal rank in a custom-designed visual drop tube furnace. The results showed that low-rank coals were ignited in a shorter time, mainly due to the presence of larger amounts of functional groups, while the ignition delay time of high-rank coals was longer. With increasing temperature and particle size, the ignition mode of coals shifted from heterogeneous into homogeneous, which was related to the increased yield of volatile matter. The chemical percolation devolatilization analysis results showed a clear relationship between the yield and composition of volatile matter and the amount and type of functional groups in coal. In addition, the tar yield was consistent with the amount of aliphatic hydrocarbons and the length of aliphatic chains, which explained the tailing combustion mode of the bituminous coal. The findings of the study showed that the yield and composition of volatiles in coal had a significant impact on the ignition behavior, which depended on the composition of functional groups, particle size, and the combustion environment.     

Impact of hydrogen generated by splitting water with nano-silicon and nano-aluminum on diesel engine performance

Efficient utilization of hydrogen generated during the reactions of nano-silicon/water and nano-aluminum/water in internal combustion engine has been investigated in the current work. Engine performance and emission studies of formulated and stabilized nanoemulsion fuels (water in diesel W/D), nano-aluminum in water/diesel (W/DA) and water in nano-silicon/diesel (W/DS) have been compared with those of diesel. Experimental investigations showed reduction in brake specific fuel consumption (BSFC) by 21% and 37%; rise in brake thermal efficiency (BTE) by 16% and 14% when engine was fueled with W/DA and W/DS respectively. For nanoemulsion fuels an increase in induced power was also recorded. Brake mean effective pressure, BTE and NOx emission dropped for W/D due to reduced exhaust gas temperatures. Nevertheless due to elevated peak cylinder pressures and exhaust gas temperatures a marginal rise in NOx, CO, HC and radiative heat emissions was observed with W/DA and W/DS.     

Study on combustion characteristics and particulate emissions of a common-rail diesel engine fueled with n-butanol and waste cooking oil blends

Biodiesel is a promising alternative fuel because of its renewability and extensive source of raw materials. Butanol can be blended in biodiesel to reduce the kinematic viscosity and promote the fuel atomization. In this respect, biodiesel was blended with 10% and 20% n-butanol, and the combustion characteristics and particulate emissions of the fuel blends were tested in a turbocharged, 6-cylinder, common rail diesel engine at a constant speed of 1400 rpm under seven engine loads. The experimental results show that under various engine loads, all of the butanol and biodiesel fuel blends provide faster combustion than diesel due to the higher oxygen content of n-butanol and the lower cetane number of butanol which results in stronger premixed combustion. The addition of butanol is beneficial to concentrating the heat release and thus shorten the combustion duration. With an increased proportion of butanol, soot emissions of butanol and biodiesel fuel blends decrease, the number concentration and volume concentration of ultrafine particles (UFPs) reduce noticeably. Meanwhile, the geometric mean diameters of UFPs decrease with an increase in butanol. With an increase of the engine loads, the number concentration peaks of UFPs gradually transfer from the size range of nucleation mode particles (NMPs) to the size range of accumulation mode particles (AMPs) due to the elevated combustion temperatures and high equivalence ratios. Moreover, biodiesel and fuel blends exhibit a higher percentage of NMPs as compared to diesel because of the fuel-bound oxygen, zero aromatics, and low sulfides.     

Effects of plate vibration on the mixing and combustion of transverse hydrogen injection for scramjet

Transverse injection is an effective mixing enhancement technique for the combustor of scramjets. Vibration of the plate structure in combustor will easily be induced due to aerodynamic load and harsh aerothermodynamic load simultaneously. Effects of the plate vibration on the mixing and the combustion of the transverse hydrogen injection have been investigated numerically in this study. Finite rate chemistry model is used as combustion model. The supersonic jet experimental model of the Stanford University is modified slightly and used as the analysis model. Effects of the frequency and the amplitude of the plate vibration on combustion performance and flow field structure have been investigated in detail. The results show that the plate vibration increases the mixing efficiency, the combustion efficiency and the total pressure loss coefficient. Besides, it can change the flame structure and the shock wave structure, as well as increase the shock wave intensity at downstream of the injection. The vibration frequency has relatively little effect on the combustion efficiency and the total pressure loss coefficient. When the vibration frequency is large, it presents some high frequency pulsations for the total pressure loss coefficient. However, the vibration amplitude has large effect on combustion efficiency and the total pressure loss coefficient. When the vibration amplitude is small, the combustion efficiency presents regular periodic change with time. When the vibration amplitude is large, it diverges with time, and the flow tends to be unstable. The large vibration amplitude changes the stability of the original flow. Consequently, the combustion with large amplitude fluctuation can critically damage the combustion stability.     

Analysis and modeling of char particle combustion with heat and multicomponent mass transfer

A char combustion model suitable for a large-scale boiler/gasifier simulation, which considers the variation of physical quantities in the radial direction of char particles, is developed and examined. The structural evolution within particles is formulated using the basic concept of the random pore model while simultaneously considering particle shrinkage. To reduce the computational cost, a new approximate analytical boundary condition is applied to the particle surface, which is approximately derived from the Stefan–Maxwell equations. The boundary condition showed reasonably good agreement with direct numerical integration with a fine grid resolution by the finite difference method under arbitrary conditions. The model was applied to combustion in a drop tube furnace and showed qualitatively good agreement with experiments, including for the burnout behavior in the late stages. It is revealed that the profiles of the oxygen mole fraction, conversion, and combustion rate have considerably different characteristics in small and large particles. This means that a model that considers one total conversion for each particle is insufficient to describe the state of particles. Since our char combustion model requires only one fitting parameter, which is determined from information on the internal geometry of char particles, it is useful for performing numerical simulations.     

Conjugated processes of bulk aluminum and hydrogen combustion in water-oxygen mixtures

The article deals with the investigation of the combustion of aluminum bulk samples in water vapor, an aqueous solution of hydrogen peroxide (33.1 wt%), and water-oxygen mixture at uniform heating of the reactor (1 K/min) up to 773 K. It is revealed that the major portion of hydrogen peroxide is decomposed directly in aqueous solution. The resulting oxygen, as well as oxygen added to the water, provided oxidation of only a part of hydrogen (≈25%) released during the complete oxidation of aluminum by water. The time dependences of the reactants’ temperature and pressure, as well as the temperature corresponding to the onset of the H₂ release were determined. The most intense oxidation of aluminum in water vapor was noted within the temperature range of 593–769 K, as well as in a hydrogen peroxide solution at 548–693 K, and in H₂O/O₂ mixture at 567–742 K. It is revealed that the oxidation of hydrogen with oxygen intensifies water oxidation of aluminum to a greater extent than it follows from the heat effects of H₂ oxidation. As a result of oxidation, a loose powder of aluminum oxide nanoparticles was obtained.