Initial Ambient Temperatures Research Articles

Methane-unconventional oxidant flames. Laminar burning velocities of nitrogen-diluted methane–N 2 O mixtures

Abstract The flame propagation in nitrogen-diluted CH4–N2O mixtures was monitored by pressure measurements during explosions in a spherical vessel with central ignition. The burning velocities were obtained from experimental measurements of pressure variation during closed vessel explosions and from the detailed modelling of free laminar premixed flames. Lean- and stoichiometric methane–nitrous oxide mixtures (equivalence ratios: 0.8 and 1.0) diluted by various amounts of nitrogen between 40 and 60 vol% were studied at various initial pressures between 0.3 and 1.8 bar and ambient initial temperature. Nitrogen addition to each CH4–N2O mixture results in the decrease of laminar burning velocity and flame temperature, along with the increase of flame width. Examination of burning velocity variation against average flame temperature in experiments at constant initial pressure and various inert concentrations allowed the determination of the overall activation energy; examination of burning velocity dependence on pressure, at constant inert concentration, allowed the determination of the overall reaction orders. For all CH4–N2O–N2 mixtures, the temperature, volumetric rate of heat release and reactive species concentration profiles across the flame front were examined versus similar data characteristic to stoichiometric methane–air mixtures. The most important elementary reactions in CH4–N2O–N2 and CH4–air mixtures were identified by means of sensitivity analysis.

Large eddy simulation of the low temperature ignition and combustion processes on spray flame with the linear eddy model

Large eddy simulation coupled with the linear eddy model (LEM) is employed for the simulation of n-heptane spray flames to investigate the low temperature ignition and combustion process in a constant-volume combustion vessel under diesel-engine relevant conditions. Parametric studies are performed to give a comprehensive understanding of the ignition processes. The non-reacting case is firstly carried out to validate the present model by comparing the predicted results with the experimental data from the Engine Combustion Network (ECN). Good agreements are observed in terms of liquid and vapour penetration length, as well as the mixture fraction distributions at different times and different axial locations. For the reacting cases, the flame index was introduced to distinguish between the premixed and non-premixed combustion. A reaction region (RR) parameter is used to investigate the ignition and combustion characteristics, and to distinguish the different combustion stages. Results show that the two-stage combustion process can be identified in spray flames, and different ignition positions in the mixture fraction versus RR space are well described at low and high initial ambient temperatures. At an initial condition of 850 K, the first-stage ignition is initiated at the fuel-lean region, followed by the reactions in fuel-rich regions. Then high-temperature reaction occurs mainly at the places with mixture concentration around stoichiometric mixture fraction. While at an initial temperature of 1000 K, the first-stage ignition occurs at the fuel-rich region first, then it moves towards fuel-richer region. Afterwards, the high-temperature reactions move back to the stoichiometric mixture fraction region. For all of the initial temperatures considered, high-temperature ignition kernels are initiated at the regions richer than stoichiometric mixture fraction. By increasing the initial ambient temperature, the high-temperature ignition kernels move towards richer mixture regions. And after the spray flames gets quasi-steady, most heat is released at the stoichiometric mixture fraction regions. In addition, combustion mode analysis based on key intermediate species illustrates three-mode combustion processes in diesel spray flames.

Inoculation with a psychrotrophic-thermophilic complex microbial agent accelerates onset and promotes maturity of dairy manure-rice straw composting under cold climate conditions

The objective was to determine the effects of psychrotrophic-thermophilic complex microbial agent (PTCMA) comprised of a psychrotrophic bacterium consortium (PBC) and a thermophilic cellulolytic fungi consortium (TCFC), on composting in a cold climate. Mixtures of dairy manure and rice straw were inoculated with PTCMA, PBC, TCFC and sterile water (control) and composted at an initial ambient temperatures of −2 to 5°C. In compost piles inoculated with PBC or PTCMA, temperatures reached the thermophilic phase (>55°C) faster (8–11d) than piles inoculated with TCFC or control. Furthermore, compost inoculated with TCFC or PTCMA had greater decreases in total organic carbon and carbon-to-nitrogen ratios, as well as significant increases in total nitrogen, degradation of cellulose and lignin and germination index than PBC inoculation or Control compost. Consequently, inoculation with both (i.e. PTCMA) accelerated the onset and promoted maturity of composting under cold-climate conditions.

Experimental study of the initial pressure effect on methane‐air explosions in linked vessels

This study experimentally addresses the pressure evolution during the closed and vented explosion progress of a methane‐air mixture ([CH4 = 10%]) in different vessel‐pipeline configurations at ambient initial temperature and different initial pressures. Based on the experimental measurements, it is found that the maximum values of both closed and vented explosion pressure increase linearly with the initial pressure in isolated and linked vessel scenarios. Compared with a vented explosion, the initial pressure has a greater impact on the explosion overpressure in an isolated vessel and a single spherical vessel connected to a pipe during a closed explosion. For the configuration of two spherical vessels connected by a pipe, the explosion overpressure in the primary vessel is always lower than that in the secondary vessel during both closed and vented explosions at each initial pressure. According to Chapman‐Jouget Detonation Theory (C‐J detonation theory), when the ignition is located in a large vessel, detonation is observed in the secondary vessel at an initial pressure of 0.08 MPa. The vent position and ignition position both have an influence on the extent of variation of the pressure differential between two vessels with a given initial pressure. For the configuration of a single vessel connected to a pipe, the initial pressure influences the maximum explosion pressure and vented explosion overpressure least in a small vessel when the ignition position is located in the small vessel. These conclusions can provide references for the safety design of chemical equipment. © 2017 American Institute of Chemical Engineers Process Process Saf Prog 37:86–94, 2018

Propagation indices of methane-air explosions in closed vessels

The peak explosion pressure, the maximum rate of pressure rise and the time necessary to reach the peak explosion pressure are important flammability indices of fuel–air combustion in closed vessels, characteristic for the laminar propagation stage of the process. In the present paper, these indices were examined using methane of various concentrations within the flammability limits, at variable initial pressure between 50 and 200 kPa and ambient initial temperature. For each composition, the experimental explosion pressures were compared with the adiabatic explosion pressures, computed under the assumption that chemical equilibrium is reached in the flame. The experimental explosion pressures and the rates of pressure rise are examined in comparison with literature data, the fluctuations being attributed to differences of heat lost by the flammable gas to the explosion vessel, during flame propagation. Using the differences between the adiabatic and experimental explosion pressures, the amount of heat lost to the walls during the explosion propagation in a closed vessel and the fraction of the transferred heat from the total released heat have been determined.

Optical emission spectrum of filamentary nanosecond surface dielectric barrier discharge

Streamer-to-filament transition is a general feature of high pressure high voltage (HV) nanosecond surface dielectric barrier discharges. The transition was studied experimentally using time- and space-resolved optical emission in UV and visible parts of spectra. The discharge was initiated by HV pulses 20 ns in duration and 2 ns rise time, positive or negative polarity, 20–60 kV in amplitude on the HV electrode. The experiments were carried out in a single-shot regime at initial pressures P > 3 bar and ambient initial temperature in air, N2, H2:N2 and O2:Ar mixtures. It was shown that the transition to filamentary mode is accompanied by the appearance of intense continuous radiation and broad atomic lines. Electron density calculated from line broadening is characterized by high absolute values and long decay in the afterglow. The possible reasons for the continuous spectra were analyzed.

Computational Investigation on Soot Mechanism of Diesel and Diesel/n-Butanol Blend in Constant Volume Chamber with Various Ambient Temperatures

Computational investigation was carried out in an optical constant volume chamber to explore the combined effects of initial ambient temperatures (800, 900, and 1000 K) and n-butanol additive (20% volume fraction of n-butanol) on diesel combustion and soot characteristics. An improved phenomenological soot model integrated with a reduced n-heptane/n-butanol/polycyclic aromatic hydrocarbon (PAH) mechanism was developed and implemented into the KIVA-3V R2 code to study the soot formation and oxidation mechanism. The predicted chamber pressure and heat release rate as well as soot mass trace and distribution showed good agreements with the experimental data. Results indicated that the ignition delay was retarded and total soot mass was reduced with the decrease of initial ambient temperature and with the n-butanol additive. The heat release rate of pure diesel demonstrated a transition from diffusion-dominated combustion at 1000 K to premix-dominated combustion at 800 K. Diesel/n-butanol blend showed no obvi...

Temperature Dependence of Solar Light Assisted CO2 Reduction on Ni Based Photocatalyst

Methanation of CO2 by H2 can be in the future an important reaction to store the surplus of renewable electricity during production peaks. The catalytic thermal CO2 methanation (the Sabatier reaction) can be carried out at temperatures above 250 °C using Ni supported on silica-alumina (Ni/SiO2–Al2O3). Recently it has been observed that this exothermic reaction can be promoted by solar light irradiation of Ni/SiO2–Al2O3 at initial near ambient temperatures. In the present work we provide a study of the influence of the initial temperature on the photoassisted Ni/SiO2–Al2O3 methanation of CO2, under conditions in which the dark reaction is not observed. An increase of the photoassisted methanation rate with the initial temperature in the range from ambient to 150 °C has been observed. The reaction kinetics for lower initial temperatures exhibited an induction period not observed for reactions performed at higher temperatures. The results are discussed in terms of the operation of plasmon photo activation in which the energy of photons is thermalised in a confined space of the active nanoparticles leading to locally high temperatures and the simultaneous photogeneration of electrons and positive holes.

Accumulation of formamide in hydrothermal pores to form prebiotic nucleobases

Formamide is one of the important compounds from which prebiotic molecules can be synthesized, provided that its concentration is sufficiently high. For nucleotides and short DNA strands, it has been shown that a high degree of accumulation in hydrothermal pores occurs, so that temperature gradients might play a role in the origin of life [Baaske P, et al. (2007)Proc Natl Acad Sci USA104(22):9346-9351]. We show that the same combination of thermophoresis and convection in hydrothermal pores leads to accumulation of formamide up to concentrations where nucleobases are formed. The thermophoretic properties of aqueous formamide solutions are studied by means of Infrared Thermal Diffusion Forced Rayleigh Scattering. These data are used in numerical finite element calculations in hydrothermal pores for various initial concentrations, ambient temperatures, and pore sizes. The high degree of formamide accumulation is due to an unusual temperature and concentration dependence of the thermophoretic behavior of formamide. The accumulation fold in part of the pores increases strongly with increasing aspect ratio of the pores, and saturates to highly concentrated aqueous formamide solutions of ∼85 wt% at large aspect ratios. Time-dependent studies show that these high concentrations are reached after 45-90 d, starting with an initial formamide weight fraction of[Formula: see text]wt % that is typical for concentrations in shallow lakes on early Earth.

Combustion and soot emission characteristics of soybean biodiesel in constant volume chamber

ABSTRACTAn optically accessible constant volume chamber was adopted to investigate the combustion and soot emission characteristics of soybean biodiesel under 800, 900, 1000, and 1200 K initial ambient temperatures. Results showed that with the decrease of initial ambient temperature ignition delay prolonged and heat release rate transited from mixing controlled combustion to premixed dominate combustion mode. As the initial temperature decreased, natural flame luminosity gradually decayed, which indicated lower soot emission at lower initial ambient temperature.

Propagation of ethylene–air flames in closed cylindrical vessels with asymmetrical ignition

A study of explosions in several elongated cylindrical vessels with length to diameter L/D=2.4–20.7 and ignition at vessel's bottom is reported. Ethylene–air mixtures with variable concentration between 3.0 and 10.0vol% and pressures between 0.30 and 1.80bara were experimentally investigated at ambient initial temperature. For the whole range of ethylene concentration, several characteristic stages of flame propagation were observed. The height and rate of pressure rise in these stages were found to depend on ethylene concentration, on volume and asymmetry ratio L/D of each vessel. High rates of pressure rise were found in the early stage; in later stages lower rates of pressure rise were observed due to the increase of heat losses. The peak explosion pressures and the maximum rates of pressure rise differ strongly from those measured in centrally ignited explosions, in all examined vessels. In elongated vessels, smooth p(t) records have been obtained for the explosions of lean C2H4–air mixtures. In stoichiometric and rich mixtures, pressure oscillations appear even at initial pressures below ambient, resulting in significant overpressures as compared to compact vessels. In the stoichiometric mixture, the frequency of the oscillations was close to the fundamental characteristic frequency of the tube.

Numerical analysis of ignition and flame stabilization in an n-heptane spray flame

Unsteady Reynolds-averaged Navier–Stokes (URANS) and large-eddy simulations (LES) of an n-heptane spray flame have been performed with efficient chemistry calculation via dynamic adaptive chemistry and uniformly random distribution parallelization. Predictions for such key parameters as ignition delay time and flame lift-off length are validated against the experimental data from the engine combustion network. The transient, convection, diffusion and chemical reaction terms in species transport equations are analyzed to gain insight into flame stabilization mechanisms, showing the dominant effects from the auto-ignition process. The influence of the turbulence–chemistry interaction on the ignition and flame stabilization is studied for two cases with different initial ambient temperatures by reconstructing probability density function of mixture fraction. For the case with an initial ambient temperature of 1000K, the analysis shows the fluctuation in mixture fraction is significant, but it has negligible influence on the ignition process. For the case with an initial ambient temperature of 850K, the turbulence–chemistry interaction plays a significant role on ignition and consequently the stabilization process. In addition, large eddy simulations with a third-order Monotone Upstream-centered Schemes for Conservation Laws are performed for a series of cases with different oxygen concentrations. The results show that LES predict the instantaneous flame dynamics and flame lift-off lengths more accurately than URANS.

Experimental analysis and semicontinuous simulation of low-temperature droplet evaporation of multicomponent fuels

Low-pollutant and efficient combustion not only in internal combustion engines requires a balanced gaseous mixture of fuel and oxidizer. As fuels may contain several hundred different chemical species with different physicochemical properties as well as defined amounts of biogenic additives, e.g., ethanol, a thorough understanding of liquid fuel droplet evaporation processes is necessary to allow further engine optimization. We have studied the evaporation of fuel droplets at low ambient temperature. A non-uniform temperature distribution inside the droplet was already considered by including a finite thermal conductivity in a one-dimensional radial evaporation model (Rivard and Bruggemann in Chem Eng Sci 65(18):5137–5145, 2010). For a detailed analysis of droplet evaporation, two non-laser-based experimental setups have been developed. They allow a fast and relatively simple but yet precise measurement of diameter decrease and composition change. The first method is based on collecting droplets in a diameter range from 70 to 150 µm by a high-precision scale. A simultaneous evaluation of mass increase is employed for an accurate average diameter value determination. Subsequently, a gas chromatographic analysis of the collected droplets was conducted. In the second experiment, evaporation of even smaller droplets was optically analyzed by a high-speed shadowgraphy/schlieren microscope setup. A detailed analysis of evaporating E85 (ethanol/gasoline in a mass ratio of 85 %/15 %) and surrogate fuel droplets over a wide range of initial droplet diameters and ambient temperatures was conducted. The comparison of experimental and numerical results shows the applicability of the developed model over a large range of diameters and temperatures.

Modeling, Analysis, and Simulation of Paste Freezing in Freeze-Form Extrusion Fabrication of Thin-Wall Parts

During the freeze-form extrusion fabrication (FEF) process for aqueous-based pastes, the subzero temperature (in Celsius) environment aids the part in maintaining its shape by freezing the water present in the paste. The first few layers of paste freeze very quickly when deposited; however, as the part's height increases, the freezing time increases as the rate of heat conduction to the substrate decreases rapidly. The freezing time can substantially exceed the time required to deposit one layer of paste due to water's high latent heat, leaving the extruded paste in its semiliquid state, and causing the part to deform or even collapse. Therefore, dwell time may be required between layers. A method is needed to predict paste freezing time in order to fabricate a part successfully while minimize the part build time. In this paper, a simplified one-dimensional (1D) heat transfer model was introduced for fabricating thin-wall parts by the FEF process. The simplified model, which could reduce computation times from days to minutes, was validated by the commercial finite element software Fluent. The paste temperature and paste freezing time for various process parameters were computed via numerical simulation using this model. As the layer number increases, the paste freezing time reaches a steady state. The relationship between the steady-state freezing time and the total time, which is the sum of the deposition time for the current layer and the dwell time between the current and next layers, was studied for various convection coefficients, paste materials, paste solids loadings, initial paste temperatures, ambient temperatures, and layer thicknesses.

Experimental and computational investigation of temperature effects on soot mechanisms

Effects of initial ambient temperatures on combustion and soot emission characteristics of diesel fuel were investigated through experiment conducted in optical constant volume chamber and simulation using phenomenological soot model. There are four difference initial ambient temperatures adopted in our research: 1000 K, 900 K, 800 K and 700 K. In order to obtain a better prediction of soot behavior, phenomenological soot model was revised to take into account the soot oxidation feedback on soot number density and good agreement was observed in the comparison of soot measurement and prediction. Results indicated that ignition delay prolonged with the decrease of initial ambient temperature. The heat release rate demonstrated the transition from mixing controlled combustion at high ambient temperature to premixed combustion mode at low ambient temperature. At lower ambient temperature, soot formation and oxidation mechanism were both suppressed. But finally soot mass concentration reduced with decreasing initial ambient temperature. Although the drop in ambient temperature did not cool the mean in-cylinder temperature during the combustion, it did shrink the total area of local high equivalence ratio, in which soot usually generated fast. At 700 K initial ambient temperature, soot emissions were almost negligible, which indicates that sootless combustion might be achieved at super low initial temperature operation conditions.

Investigation about Temperature Effects on Soot Mechanisms Using a Phenomenological Soot Model of Real Biodiesel

A phenomenological soot model of real biodiesel was proposed to investigate the effects of initial ambient temperatures on combustion and soot emission characteristics of soybean biodiesel. Validat...

A nanosecond surface dielectric barrier discharge at elevated pressures: time-resolved electric field and efficiency of initiation of combustion

We study a nanosecond surface dielectric barrier discharge (SDBD) initiated by negative or positive polarity pulses 10–15 kV in amplitude in a cable, 25–30 ns FWHM, 5 ns rise time, in the regime of a single shot or 3 Hz repetitive frequency. Discharge parameters, namely spatial structure of the discharge and time- and space-resolved electric field are studied in a N2 : O2 = 4 : 1 mixture for P = 1–5 atm. The possibility of igniting a combustible mixture with the help of an SDBD is demonstrated using the example of a stoichiometric C2H6 : O2 mixture at ambient initial temperature and at 1 atm pressure. Flame propagation and ignited volume as a function of time are compared experimentally for two discharge geometries: SDBD and pin-to-pin configurations at the same shape and amplitude of the incident pulse. It is shown that the SDBD can be considered as a multi-point ignition system with maximum energy release near the high-voltage electrode. Numerical modeling of the discharge and subsequent combustion kinetics for the SDBD conditions is performed. The discharge action leads to the production of atoms and radicals as well as to fast gas heating, due to the relaxation of electronic and vibrational degrees of freedom. The calculated ignition delay time is in reasonable agreement with the experimental results.

Explosion of gaseous ethylene–air mixtures in closed cylindrical vessels with central ignition

Explosions of gaseous ethylene–air mixtures with various concentrations between 3.0 and 14.0vol.% and initial pressures between 0.20 and 1.10bar were experimentally investigated at ambient initial temperature, using several elongated cylindrical vessels with length to diameter ratio between 1.0 and 2.4. The maximum explosion pressures pmax, the explosion times θmax, the maximum rates of pressure rise, (dp/dt)max and the severity factors of centrally ignited explosions KG are examined in comparison with similar data obtained in a spherical vessel. The measured deflagration indices are strongly influenced by the length to diameter ratio of the vessels, initial pressure and composition of the flammable mixtures. Even when important heat losses are present, linear correlations pmax=f(p0) and (dp/dt)max=f(p0) were found for all examined fuel–air mixtures, in all closed vessels. The heat losses appearing in the last stage of explosions occurring in asymmetrical vessels were estimated from the differences between the experimental and adiabatic maximum explosion pressures. These heat losses are higher when the asymmetry ratio L/D is higher and were found to depend linearly on the initial pressure.

The effects of initial conditioning and ambient temperatures on abrasion loss and temperature change of porous asphalt

This paper investigates the effects of initial conditioning and ambient temperatures on abrasion loss and temperature change of porous asphalt after subjected to 300 rotations in the Los Angeles drum. Identical specimens were tested at Universiti Sains Malaysia (USM) and Delft University of Technology (TU Delft), the Netherlands. Specimen temperature change decreased as the initial conditioning temperature increased. Temperature inside the drum remained unchanged. The abrasion loss of samples tested at TU Delft were higher than those tested at USM. Therefore for meaningful comparison of test results worldwide, the ambient temperature at which a Cantabro test is performed must be specified.

Additive Effects on the Burning Velocity of Ethylene–Air Mixtures

Using a simple correlation based on the cubic law of early pressure rise during a closed vessel explosion, the laminar burning velocities of the stoichiometric ethylene–air mixture diluted with several additives (Ar, N2, and CO2) (concentrations between 4 and 40 vol %) were calculated from pressure–time records, at total initial pressures between 0.2 and 1.2 bar and ambient initial temperature. These are important input properties for numerical modeling of explosion propagation in closed or vented enclosures and for safety recommendations concerning the necessary inert gas addition. The decrease of burning velocities determined by the increase of the total initial pressure and inert gas addition is examined. A computational study of free laminar premixed flames of ethylene–air in the presence of the same additives based on the use of 53 species and an extended reaction mechanism with 592 elementary reactions delivered the temperature and species profiles in the flame front, together with the flame width a...