Measurements of Markstein numbers and laminar burning velocities for liquefied petroleum gas-air mixtures

Experimental test for premixed laminar combustion of liquefied petroleum gas-air mixtures is conducted in a constant volume combustion bomb. Spherically expanding flames have been employed to measure laminar flame speeds over wide equivalence ratios, at the initial pressures of 0.05, 0.1 and 0.15 MPa, and preheat temperatures from 300 to 400 K. To study the effects of stretch on burning velocity, various Markstein numbers for both strain and curvature have been measured and the effects of initial temperature and pressure on these parameters have been discussed. Following the linear relation between flame speeds and flame stretches, one has then obtained the corresponding unstretched laminar burning velocity after omitting the effect of stretches imposed on these flames. Over the ranges studied, laminar burning velocities are fit by a functional form u_l=u_l0(T_u/T_u0)^α_T(P_u/P_u0)^β_P, and the dependencies of α_T and β_P upon the equivalence ratio of mixture are also discussed.     

Group combustion of staggeringly arranged heptane droplets at various Reynolds numbers, oxygen mole-fractions, and separation distances

The group combustion of interacting heptanes liquid droplets are numerically simulated by solving two dimensional unsteady laminar Navier-Stokes equations. The unsteady computations for the time-varying vaporization of multi-droplets are carried out with parameters of the Reynolds number (Re), the separation distance (S) between the droplets, and the oxygen mole-fraction. The n-heptane droplets initially at T₀ = 300 K are in hot air of 10 atm at T_g = 1250 K. Multi-droplets are staggeringly arranged at a separation distance ranging from 4 to 15 droplet radius. The Reynolds number, based on the droplet diameter and free stream velocity, is varied from Re = 10 to 50. The oxygen mole-fraction of the surrounding air is changed from 15% to 90%. The time variations of the flame structure, the combustion characteristics, and the burning rates are presented and discussed. These results indicated that the staggered arrangement of the multi-droplets induced combustion characteristics distinct from those of a single droplet. The burning rate of the interacting droplets in the staggered arrangement exhibited a relatively strong dependence on the Re, S, and oxygen mole-fraction. The burning rate of the interacting multi-droplets, non-dimensionalized by that of a single droplet, was found as a function of S and Re.     

The effect of temperature on the adiabatic laminar burning velocities of CH4−air and H2−air flames

The effect of temperature on the adiabatic laminar burning velocities of CH₄ + air and H₂ + air flames was analyzed. Available measurements were interpreted using correlation S_L = S_L0 (T/T₀)^α. Particular attention was paid to the variation of the power exponent α with equivalence ratio at fixed (atmospheric) pressure. Experimental data and proposed empirical expressions for α as a function of equivalence ratio were summarized. They were compared with predictions of detailed kinetic models in methane + air and hydrogen + air flames. Unexpected non-monotonic behavior of α was found in rich methane + air flames. Modeling results are further examined using sensitivity analysis to elucidate the reason of particular dependences of the power exponent α on equivalence ratio.     

Determination of laminar burning velocities for natural gas

Spherically expanding flames of natural gas-air mixtures have been employed to measure the laminar flame speeds, at the equivalence ratios from 0.6 to 1.4, initial pressures of 0.05, 0.1 and 0.15 MPa, and preheat temperatures from 300 to 400 K. Following Markstein theory, one then obtains the corresponding unstretched laminar burning velocity after omitting the effect of stretch imposed at the flame front. Over the ranges studied, the burning velocities are fit by a functional form u_l=u_l0(T_u/T_u0)^α_T(P_u/P_u0)^β_P, and the dependencies of α_T and β_P upon the equivalence ratio of mixture are also given. The effects of dilute gas on burning velocities have been studied at the equivalence ratios from 0.7 to 1.2, and the explicit formula of laminar burning velocities for dilute mixtures is achieved.     

Measurement of laminar burning velocity of dimethyl ether-air premixed mixtures

Measurement of laminar burning velocity of dimethyl ether-air mixtures was taken under different initial pressures and equivalence ratios using a constant volume bomb and high-speed schlieren photography. The stretched laminar burning velocity increases with the increase of stretch rate. At equivalence ratio of 1.0, low initial pressure gives high stretched flame speed. At initial pressure less than 0.1 MPa, the stoichiometric mixture gives the higher value of stretched flame speed than those at ? = 1.2 and ? = 0.8. The Markstein numbers decrease with the increase of equivalence ratio, and this reveals that lean mixture will maintain higher stability of flame front surface than that of rich mixture in dimethyl ether-air premixed flames.     

Fuel effects on diffusion flames at elevated pressures

This study addresses the influence of elevated pressures up to 1.6 MPa on the flame geometry and the flickering behavior of laminar diffusion flames and particular attention has been paid to the effect of fuel variability. It has been observed that the flame properties are very sensitive to the fuel type and pressure. The shape of the flame was observed to change dramatically with pressure. When the pressure increases, the visible flame diameter decreases. The height of a flame increases first with pressure and then reduces with the further increase of pressure. The cross-sectional area of the flame (A_cs) shows an average inverse dependence on pressure to the power of n (A_cs ∝ P⁻ⁿ), where n = 0.8 ± 0.2 for ethylene flame, n = 0.5 ± 0.1 for methane flame and n = 0.6 ± 0.1 for propane flame. It was observed that the region of stable combustion was markedly reduced as pressure was increased. High speed imaging and power spectra of the flame chemiluminescence reveal that an ethylene flame flickers with at least three dominant modes, each with corresponding harmonics at elevated pressures. In contrast methane flames flicker with one dominant frequency and as many as six harmonic modes at elevated pressures.     

Effects of temperature and composition on the laminar burning velocity of CH4 + H2 + O2 + N2 flames

This work summarises available measurements of laminar burning velocities in CH₄ + H₂ + O₂ + N₂ flames at atmospheric pressure performed using a heat flux method. Hydrogen content in the fuel was varied from 0% to 40%, amount of oxygen in the oxidiser was varied from 20.9% down to 16%, and initial temperature of the mixtures was varied from 298 to 418 K. These mixtures could be formed when enrichment by hydrogen is combined with flue gas recirculation. An empirical correlation for the laminar burning velocity covering a complete range of these measurements is derived and compared with experiments and other correlations from the literature.     

Effects of initial temperature on the structure of laminar CH4-air premixed flames

The growing popularity of natural gas as a eco-friendly fuel, is of paramount motivation of present investigation. In the present paper, the effect of initial temperature on the flame structure have been investigated in which laminar one-dimensional planar propagating flames of CH₄/air mixtures is simulated numerically using detailed chemical kinetic scheme and realistic transport models. The burning velocities are fundamentally important in developing models to predict progress of combustion. Hence, the burning velocities as a function of initial temperature of unburnt gas have been computed for stoichiometric mixture. The present predictions of burning velocities are compared with reported experimental data of Stone et al. [Combust. Flame. 114 (1998) 546], Hill and Huang [Combust. Sci. Technol. 60 (1980) 7] and Rallis and Garforth [Combust. Flame 31 (1978) 53]. The present prediction lies within the scatter of experimental data. A correlation in the form of S_u/S_u,0=(T_u/T_u,0)^1.575 has been developed to describe the dependence of initial temperature on the burning velocity for stoichiometric mixture. The structures of flame are investigated in details for initial temperature of 300 and 600 K which clearly indicate that detailed chemical kinetics are essential for prediction of the effects of initial temperature on the burning velocities. The present study will help in designing and developing the regenerative combustion systems.     

Ignition of methane and propane in high-temperature oxidizers with various oxygen concentrations

In this investigation, ignition processes of methane (CH₄ > 98%) and propane (C₃H₈ > 95%) using a high-temperature oxidizer (T_oxi > T_ai) with a varying oxygen concentration (z_O2=0.05÷0.21), applying two types of experimental installations, viz. a constant-volume bomb (CVB) and a co-flow reactor (CFR) were investigated. The influence of the initial temperature of the oxidizer (for methane T_oxi = 960 ÷ 1234 K and for propane T_oxi = 803 ÷ 1055 K), the equivalence ratio and oxygen concentration in the oxidizer on ignition of gaseous fuels is analyzed and discussed. It is shown that in order to achieve an effective reaction of ignition (taking into account the minimum value of ignition delay time τ_ig and maximal value of the increment of temperature ΔT) the oxidizer temperature need not be maximized. There are optimal values of temperature of the oxidizer (for methane T_oxi ≈ 1100 K and for propane T_oxi ≈ 950 K) in which the parameters mentioned above reach their extreme values.     

Adiabatic laminar burning velocities of CH4 + H2 + air flames at low pressures

Experimental measurements of the adiabatic burning velocity in methane + hydrogen + air flames using the Heat Flux method are presented. The hydrogen content in the fuel was varied from 0 to 20%. Non-stretched flames were stabilized on a perforated plate burner from 20 to 100 kPa. The equivalence ratio was varied from 0.8 to 1.4. Adiabatic burning velocities of CH₄ + H₂ + air mixtures were found in good agreement with the literature results at atmospheric pressure. Also low-pressure measurements in CH₄ + air flames performed earlier were accurately reproduced. The effects of enrichment by hydrogen on the laminar burning velocity at low pressures have been studied for the first time. Calculated burning velocities using the Konnov mechanism are in satisfactory agreement with the experiments over the entire range of conditions. Pressure dependences of the burning velocities for the three fuels studied could be approximated by an empirical exponential correlation.     

A simplified correlation for fixed bed pressure drop

An experimental study was conducted on the pressure drop characteristics of a variety of vertical packed beds in turbulent flow of air. The materials of different particle diameter, D_p, with a range of sphericity Φ, 0.55 ≤ Φ ≤ 1.00 were used in random loose packing to produce beds of different lengths, L, with a range of porosity, ε, 0.36 ≤ ε ≤ 0.56. In the covered test cases the cross-sectional velocity distribution at the exit plane of the packed beds and the pressure drop ΔP_Bed were measured in a particle Reynolds number range of Re_p, 675 ≤ Re_p ≤ 7772. The particular emphasis of the study was given to determine the influence of ε, Φ, D_p, L, Re_p on ΔP_Bed. In this respect the measurements of ΔP_Bed were compared with the well-known Ergun's Equation and the data were expressed in terms of correlations through introduced dimensionless parameters of pressure coefficient, ΔP^? and exit Reynolds number Re_exit. The proposed correlations of ΔP^? = ΔP^?(εRe_pD_p / L) and Re_exit = Re_exit(Re_pD_p / L) are found to be appropriate for the determination of ΔP_Bed and mean exit velocity, U, respectively with an acceptable fit of experimental data in an error margin less than ± 20%. The methodology is presented in this paper as an alternative approach to the available literature on packed beds.     

A study on the determination of burning velocities of LFG and LFG-mixed fuels

Numerical and experimental studies on the determination of burning velocities of landfill gas (LFG) fuel and LFG-mixed fuel with LPG have been conducted. The propriety of numerical results is inspected for the adopted GRI-v2.11 and two C3 reaction mechanisms separately developed by Sung et al. and Qin et al. The predicted burning velocities using the C3 reaction mechanism suggested by Sung et al. are in good agreement with the present and previous experimental data. Based on numerical results, the correlation formula of maximum burning velocities of LFG fuel and LFG-mixed fuel with LPG is proposed as a function of the blending percentage of CH₄ and LFG at Φ=1.0. In addition, the correlations of burning velocities of LFG fuel and LFG-mixed fuel with LPG are obtained over a wide range of equivalence ratio. The proposed correlation formulas of burning velocities for LFG fuel and LFG-mixed fuel with LPG are quite well in accord with numerical results.     

The structure of a cyanobiphenyl side chain liquid crystalline poly(silylenemethylene)

The structure of a side chain liquid crystalline poly(silylenemethylene) (-(SiCH₃R-CH₂)-: R=O(CH₂)₁₁O-Ph-Ph-CN, Ph=phenyl) (CN-11) has been studied by X-ray diffraction and differential scanning calorimetry (DSC). The DSC results showed that CN-11 has transitions at ∼92 °C (T₂) and ∼147 °C (T₁) during both cooling and immediate heating. A third transition occurred at ∼50 °C (T₃) during heating after annealing at room temperature. The X-ray fiber pattern of the CN-11 annealed at room temperature showed several wide and small angle reflections which were indexed by a monoclinic unit cell with parameters a=16.8 Å, b=7.42 Å, c=43.6 Å and β=102.1° (b: fiber direction), representing a crystal structure with layer thickness of ∼43 Å. Upon heating at T₃, the crystal structure became less ordered (but somewhat more ordered than smectic A (S_A) and smectic C (S_C)). This was followed by S_A (or S_C) phase at T₂, and ultimately an isotropic state (I) at T₁. The observed layer thickness (∼43 Å) is about ∼1.5 times the most extended side chain length, indicating a double-layer structure with tilted or interdigitated side chains. The X-ray fiber pattern had a four-point pattern at d=4.52 Å, suggesting that the side chains in the crystal are likely to be tilted by 56° from the polymer fiber axis.     

Speciation studies of methyl butanoate ignition

The current work presents the results of an experimental study of the intermediates formed during ignition of methyl butanoate (C₅H₁₀O₂) and air mixtures. A rapid-sampling system and the University of Michigan rapid compression facility were used to acquire gas samples at conditions of P = 10.2 atm and T = 985 K using mixtures of χ_mb = 0.96%, χ_O2 = 20.79%, χ_N2 = 52.89%, and χ_Ar = 25.25% (mole fraction, percent basis); corresponding to ? = 0.30 and an inert gas to O₂ molar ratio of 3.76. The samples were analyzed using gas chromatography. Quantitative measurements of mole fraction time-histories of methane, ethane, propane, ethene, propene, and 1-butene are compared with model predictions based on a reaction mechanism developed in previous work. The methane and ethene time-histories are in excellent agreement (within ∼20%), while propene and ethane are underpredicted by the model. Sensitivity analysis shows ignition is controlled primarily by competition between H₂O₂ and HO₂ kinetics at these conditions. Reaction path analysis shows the methyl butanoate fuel consumption is dominated by H-atom abstraction by OH.     

Experimental determination of laminar burning velocity for butanol and ethanol iso-octane blends

The potential of butanol as an additive in iso-octane used as gasoline fuel was characterized with respect to laminar combustion, and compared with ethanol. New sets of data of laminar burning velocity are provided by using the spherical expanding flame methodology, in a constant volume vessel. This paper presents the first results obtained for pure fuels (iso-octane, ethanol and butanol) at an initial pressure of 0.1 MPa and a temperature of 400 K, and for an equivalence range from 0.8 to 1.4. New data of laminar burning velocity for three fuel blends containing up to 75% alcohol by liquid volume are also provided. From these new experimental data, a correlation to estimate the laminar burning velocity of any butanol or ethanol blend iso-octane-air mixture is proposed.     

Empirical determination of equation of state for zero internal pressure in rare gas solids and semi-crystalline polymers

An equation of state for zero internal pressure in rare gas solids and semi-crystalline polymers has been determined based on the empirical functions of thermal pressure coefficient γ_V with respect to volume at constant pressure. The experimental data of PVT over wide range of temperature and pressure published by Anderson and Swenson and Syassen and Holzapfel for rare gas solids and Olabisi and Simha and Zoller for semi-crystalline polymers are used to evaluate γ_V. The function of γ_V with respect to volume determined at constant pressure is given by where V₀ is the volume at 0 K, A, ? and c are constants. The function of internal pressure P_i = γ_VT − P with respect to temperature at constant pressure is determined by converting the function of γ_V(V) to a function of temperature γ_V(T). An empirical equation of state for zero internal pressure determined by pressure P^∗, volume V^∗ and temperature T^∗ at which P_i = 0 is expressed by P^∗V^∗/RT^∗=C^∗−D^∗V^∗ for rare gas and semi-crystalline polymer where C^∗ and D^∗ are constants. The practical meaning of the equation of state for P_i = 0 in the semi-crystalline polymers has been discussed.     

Non-premixed fluidized bed combustion of C1-C4n-alkanes

The non-premixed combustion of C₁-C₄n-alkanes with air was investigated inside a bubbling fluidized bed of inert sand particles at intermediate temperatures: 923 K ? T_B ? 1123 K. For ethane, propane and n-butane, combustion occurred mainly in the freeboard region at bed temperatures below T₁ = 923 K. On the other hand, complete conversion occurred within 0.2 m of the injector at: T₂ = 1073 K. For methane, the measured values of T₁ and T₂ were significantly higher at 1023 K and above 1123 K, respectively. The fluidized bed combustion was accurately modeled with first-order global kinetics and one PFR model to represent the main fluidized bed body. The measured global reaction rates for C₂-C₄n-alkanes were characterized by a uniform Arrhenius expression, while the global reaction rate for methane was significantly slower. Reactions in the injector region either led to significant conversion in that zone or an autoignition delay inside the main fluidized bed body. The conversion in the injector region increased with rising fluidized bed temperature and decreased with increasing jet velocity. To account for the promoting and inhibiting effects, an analogy was made with the concept of induction time: the PFR length (b_i) of the injector region was correlated to the fluidized bed temperature and jet velocity using an Arrhenius expression. These results show that the conversion of C₂-C₄n-alkanes can be estimated with one set of critical bed temperatures and modeled with one Arrhenius kinetics expression.     

Fluidized bed gasification of Kingston coal and marine microalgae in a spouted bed reactor

Steam gasification experiments were performed using a low-rank coal from South Australia, a marine microalga, and a blend of leached microalgal biomass and coal, in a spouted, fluidized bed reactor. The effect of different operating conditions – air-to-fuel ratio (A/F), steam-to-fuel ratio (S/F) and bed temperature (T_b) – on the producer gas composition was investigated. Producer gas compositions were analyzed and samples of bed material were also examined to identify ash components formed during each experiment. The optimum operating conditions for coal gasification, in this system, were identified to occur with A/F = 1.82, S/F = 0.75 and T_b = 850 °C. These conditions resulted in a producer gas with the highest heating value (per mass of fuel fed), the highest extent of carbon conversion and the optimum H₂:CO ratio for Fischer–Tropsch synthesis. In addition, preliminary attempts to gasify a sun-dried marine microalga are reported. The dried biomass, sieved to 1.0–3.35 mm, was gasified with air and steam. Preliminary experiments, utilizing the as-received biomass, proved unsuccessful due to rapid bed sintering. Leaching of the algal biomass to remove the extra-cellular salt and co-gasification of the resultant biomass (10 wt%) with low-rank coal also proved unsuccessful due primarily to blockages of the downstream product lines most likely due to attrition of the algae feed in the screw feeder and elutriation from the bed.     

Burning velocity of methane/diluent mixture with reformer gas addition

The motivation of this study is to explore the feasibility of extending the EGR (exhaust gas recirculation) diluent tolerance for methane/air mixtures with reformer gas (CO and H₂). A preheated cylindrical combustion chamber was used to measure the laminar burning velocity of methane/air mixture with variations of EGR diluent, reformer gas, temperature and pressure. The experiments were carried out at the range of initial temperature from 298 K to 498 K and initial pressure from 1 atm to 5 atm. The maximum EGR fraction is 40%. Reformer gas was introduced to raise the burning velocity of methane/EGR mixture to the undiluted level. Results indicate that the reformer gas has potential to improve the burning velocity while reducing the nitric oxide emission.     

The influence of axial pressure on relaxor properties of BaBi2Nb2O9 ceramics

On the basis of our studies it results that dielectric properties of BaBi₂Nb₂O₉ ceramics are sensitive to axial pressure applied. The pressure causes an increase of dispersion in the real part of dielectric permittivity ?′(T,f) and a rise in the temperature T_m at which the maximum in ?′(T,f) dependence occurs. The applied pressure induces in the ?′(T) dependence an additional step-like anomaly, which appears at the temperature T_A < T_m. The applied pressure shifts both T_m and T_A at the same rate, i.e. dT_A/dX = dT_m/dX = +14 °C/kbar at high axial pressure range, above the threshold pressure X_thresh. The Vogel–Fulcher relationship is employed to determine the axial pressure influence on relaxor properties of BBN ceramics. The simulated order parameter q takes non-zero values below Burn‘s temperature T_B, where the polar clusters appear on cooling. For pressures higher than 0.8 kbar, the T_B changes at the rate dT_B/dX = −200 °C/kbar. The decrease in the difference between Burn's T_B and the freezing T_f temperatures induced by the applied axial pressure is observed. This could be ascribed to the narrowing of temperature range of relaxor behavior.