Effect Of Radiative Heat Loss Research Articles

H2/CO 합성가스의 연소 특성에 관한 연구

Numerical study is conducted to predict effects of radiative heat loss and fuel composition in synthetic gas diffusion flame diluted with . The existing reaction models in synthetic gas flames diluted with are evaluated. Numerical simulations with and without gas radiation, based on an optical thin model, are also performed to concrete impacts on effects of radiative heat loss in flame characteristics. Importantly contributing reaction steps to heat release rate are compared for synthetic gas flames with and without dilution. It is also addressed that the composition of synthetic gas mixtures and their radiative heat losses through the addition of modify the reaction pathways of oxidation diluted with .ጀ䍆䭏㈰〲ㄱ㤲㈱㈷㌴㊀쀁x済Ĉ㠤⠀ጀ䩁䭏㈰〸〸㌴㤶㐷㜶㎀쀃x済ଋᴇ⠀ጀ䩁䭏㈰〲ㄱ㤲〹㠸㐴㊀쀂x済ᜌἩ⠀ጀ䩁䭏㈰〷㈴〶㜱ㄹㄵ㞀쀄x済ᘔ᠇⠀ጀ䩁䭏㈰〱ㄱ㤲ㄵ㠶㤹ㆀ쀁x済Ĉ㰨⠀ጀ䍆䭏㈰〰ㄱ㤲㈱㌵㔰㊀쀁x済ĉȡ⠀ἀ㘲ㄮ㌸ጀ䩁䭏ㄹ㤹ㄱ㤲〲㌹㔴㒀쀄x済ᔖⰏ⠀ጀ䩁䭏ㄹ㤶ㄱ㤲〲㌵〲㎀쀃x済᠇⤴⠀ጀ䩁䭏㈰〴ㄱ㤲㌰㐶㔴㊀쀃x済ᴓ⠞⠀ጀ䍆䭏㈰〶ㄶ㐱㤴㐲㜳㚀쀁x済ĉሧ⠀ጀ䍆䭏㈰〳ㄱ㤲㈱㌷㔱㢀쀁x済ĉᔥᢝ㊗⨀遹?⨀ﾖ⨀‣㒗⨀恂⒗⨀炖ޗ⨀䢏抗⨀ꢝ㊗⨀債⒗⨀㢖ޗ⨀㡎䚗⨀ᶗ⨀䀹஗⨀낃Η⨀ꣅ⢗⨀җ⨀頲⮗⨀⠀ጀ䍆䭏㈰〶ㄶ㐱㤴㐲㘸㢀쀁x済ĉᔭ⠀ጀ䩁䭏㈰〸〶㤴㈴㜰㘹㊀쀈x済Ἀ⬫਀䣌傗⨀梅?⨀梱㪗⨀䢠㞗⨀䂑ʗ⨀ʗ⨀林⨀ꢗ㆗⨀쀤㦗⨀�傗⨀ꣽ㺗⨀�侗⨀僺䶗⨀逸ᮗ⨀㠸ʗ⨀ꂀ亗⨀飲䪗⨀⠀ጀ䩁䭏㈰〷㈴㜳㜵㘲㘲㞀쀈x済ἐ㬫⠀ጀ䍆䭏㈰〳ㄱ㤲㈱㌷㔹㚀쀁x済ĉ⠀ጀ䩁䭏㈰〶〶ㄴ〷㈴㌸㢀쀂x済ܒ㬬⠀ጀ䩁䭏㈰〴ㄱ㤲㈹㜰㤳㒀쀂x済Ўท⠀ጀ䩁䭏㈰〴ㄱ㤲㈰㘴㤴㖀쀁x済ĉᴩ⠀ጀ䩁䭏㈰〴〳〴㌰㐷㔹㊀쀂x済ᄓب⠀ጀ䍆䭏㈰〳ㄱ㤲㈷〱ㄱ㞀쀂x済ĉ㔥⠀ጀ䍆䭏㈰〳ㄱ㤲㈱㌷㤴㒀쀁x済ĉḴ⠀ጀ䩁䭏㈰〸㌳㌳㠳㘰㈶む쀃x済ἓᨷ⠀ጀ䩁䭏㈰〱ㄱ㤲ㄱ〸ㄴ㖀쀁x済ĉ἟⠀ጀ䩁䭏㈰〲ㄱ㤲ㄸ㘶㜶ㆀ쀆x済

Modeling of radiation and nitric oxide formation in turbulent nonpremixed flames using a flamelet/progress variable formulation

A model for the prediction of the nitric oxide (NO) formation in turbulent nonpremixed flames is proposed. Since the NO formation has a strong temperature sensitivity, the accurate prediction of the flame temperature under the consideration of radiative heat losses is required. The first part of the paper addresses the extension of a flamelet-based combustion model to account for radiative heat loss effects by introducing enthalpy as an additional parameter. A transport equation for enthalpy is solved, and the radiative sink term in this equation is obtained from unsteady flamelet solutions. The model is applied to a large-eddy simulation (LES) of Sandia flame D, and the importance of the interaction between turbulence and radiation on temperature and mixture fraction is investigated. Based on the radiative flamelet formulation, a consistent model for the prediction of NO formation is developed in the second part of the paper. In this model, an additional transport equation for the NO mass fraction is solved, and the chemical source term is obtained from a flamelet library. Since the consumption rate is dependent on the NO mass fraction, this term requires modeling, which is discussed in this paper. By employing a scale similarity argument, a closure model for application in LES is presented. After the analysis of the proposed model for the thermal, nitrous oxide, and prompt pathways for NO formation, the NO model is integrated into the extended flamelet/progress variable model and applied in LES of Sandia flame D and a Pratt & Whitney aircraft engine combustor configuration.

Theoretical analysis of the evolution from ignition kernel to flame ball and planar flame

Dynamics of flame kernel evolution with and without external energy addition has been investigated analytically and numerically. Emphasis is placed on the effects of radiation heat loss, ignition power and Lewis number on the correlation and transition between the initial flame kernel, the self-extinguishing flame, the flame ball, the outwardly propagating spherical flame and the propagating planar flame. The present study extends previous results by bridging the theories of the non-adiabatic stationary flame balls and travelling flames and allowing rigorous consideration of radiation heat losses. The results show that the effects of radiation heat loss play an important role in flame regimes and flame transition and result in a new isolated self-extinguishing flame. Furthermore, it is found that radiation heat losses significantly increase the critical ignition radius and result in three different dependences of the minimum ignition power on the Lewis number. Comparisons between the results from the transient numerical simulation and those from the quasi-steady state analysis show a good agreement. The results suggest that prediction of flame initiation without appropriate consideration of radiation is not acceptable.

Numerical study of the effects of radiative heat losses on diffusion flames

The effects of thermal radiation are numerically investigated for a methane-air counterflow diffusion flame, using ‘detailed’ chemistry. The radiative losses from combustion products (CO2 and H2O) were considered by using a thin gas approximation. The results show a significant effect of radiative losses causing extinction at low strain rates. On the basis of the radiative losses from gaseous combustion products, an extinction limit was found to be 0.7 s−1. The presence of soot will move this limit to higher strain rates. The radiation effects are relatively less at moderate and high strain rates, where they may cause a reduction in the peak temperatures by ∼ 10 per cent. In addition to decreasing peak temperatures and combustion products, the radiative losses also reduce the flame width. The results show the importance of including detailed chemical mechanism in correctly predicting the extinction limit and the influence of radiative losses on flame structure.

Relay-Race Combustion of Heterogeneous Systems

The structure of a combustion front in a multilayer system is analyzed taking into account radiative heat transfer in the gas interlayer. The effect of radiative heat loss on the structure and characteristics of the relay-race transformation regime is studied. The characteristics of the front and the transformation dynamics of the elements of the discrete system are determined. For the case of heat loss to the ambient medium, it is shown that the steady-state regimes are nonunique — high-temperature and low-temperature relay-race combustion fronts are detected. The absolute instability of the low-temperature regime and the dynamics of establishment of the high-temperature regime are studied in computational experiments under unsteady conditions. It is established that there is a critical heat loss above which there is flame extinction. The change in the thermokinetic characteristics during the process is analyzed, and it is shown that during combustion superadiabatic temperatures are reached.

Radiation effects on combustion and pollutant emissions of high-pressure opposed flow methane/air diffusion flames

Numerical simulations of one-dimensional opposed-flow laminar methane/air diffusion flames with detailed gas chemistry and global soot kinetics were conducted using the Sandia OPPDIF code to understand the interactions between radiation and chemistry. The gas-phase chemistry model was based on GRI-Mech 2.11, and Lindstedt's global soot kinetics model was used for soot formation calculations. The OPPDIF code was modified to include the effects of the radiation source term and the equations for soot mass fraction and soot particle number density based on soot kinetics. The radiation properties of the gas molecules and the soot particles, including a treatment for the self-absorption of radiation, were considered. Interactions between soot and gas-phase chemistry were considered. The effects of radiation heat loss, pressure, and injection velocity were studied for methane/air diffusion flames. The effects of radiation heat loss, thermophoresis, soot oxidation by O 2 and OH radicals, and depletion of acetylene by soot formation were all found to be important in determining soot concentrations, especially for high-pressure flames. Soot oxidation by OH radical dominates soot oxidation by O 2 at atmospheric pressure. However, the OH oxidation rate decreases with increasing pressure, and becomes comparable to the O 2 oxidation rate, which is very low compared with the overall soot formation rate at all pressures. The neglect of radiation heat loss leads to significantly higher estimates of soot and NO emissions compared with the estimates including radiation heat loss, particularly at high pressures. The effects of radiation, including those of self-absorption are found to be significant, particularly at lower stretch rates.

LIF MEASUREMENTS AND CHEMICAL KINETIC ANALYSIS OF NITRIC OXIDE FORMATION IN HIGH-PRESSURE COUNTERFLOW PARTIALLY PREMIXED AND NONPREMIXED FLAMES

We report quantitative, spatially resolved, linear laser-induced fluorescence (LIF) measurements of nitric oxide concentration ([NO]) in laminar, methane/air counterflow partially premixed and nonpremixed flames at six pressures up to 15 atm using excitation near 226.03 nm in the γ(0,0) band of NO. For partially premixed flames, fuel-side equivalence ratios (φB) of 1.45, 1.6, and 2.0 are studied at a global strain rate of 20 s−1. For nonpremixed flames, a complete set of NO measurements at global strain rates of 20 s−1, 30 s−1, and 40 s−1 is presented to supplement previously reported data in such flames. The quantitative NO measurements are compared with predictions from an opposed-flow flame code utilizing two GRI chemical kinetic mechanisms (versions 2.11 and 3.0). The effect of radiative heat loss on NO predictions is assessed by using a modified version of the code that considers radiation in the optically thin limit. The linear LIF measurements of [NO] are corrected for variations in the electronic quenching rate coefficient by using major species and temperature profiles generated by the opposed-flow flame code plus quenching cross sections for NO available from the literature. A pathway analysis provides the relative contribution of different NO formation mechanisms to the total amount of NO produced at various pressures. Quantitative reaction path diagrams are used to investigate pictorially species interactions during NO formation. Finally, we identify key reactions controlling NO concentrations in counterflow partially premixed and nonpremixed flames by using a sensitivity analysis. For the nonpremixed flames, NO measurements at pressures of 2–5 atm disagree substantially with predictions from either GRI mechanism. On the other hand, both mechanisms predict observed NO concentrations reasonably well beyond 6 atm. In general, for our nonpremixed flames, NO formation is dominated by the prompt route at lower pressures with an increasing contribution from the N2O pathway at higher pressures. For partially premixed flames, GRI 3.0 mimics the basic quantitative trends found in our LIF measurements at 1–15 atm. The kinetic analysis indicates that the prompt mechanism dominates at lower pressures, whereas the thermal and N2O pathways become more important at higher pressures. At any given pressure, a reduction in partial premixing results in kinetic behavior approaching that of the nonpremixed flames.

QUANTITATIVE LASER-INDUCED FLUORESCENCE MEASUREMENTS AND MODELING OF NITRIC OXIDE IN HIGH-PRESSURE (6–15 ATM) COUNTERFLOW DIFFUSION FLAMES

Laser-induced fluorescence (LIF) measurements of NO concentration ([NO]) have been obtained along the centerline of methane–air counterflow diffusion flames at 6 to 15 atm. This study is an extension of our previous work involving measurements of [NO] in similar flames at two to five atm, wherein we had used a counterflow premixed flame for calibration. For the flames studied here, a method based on computed overlap fractions is developed to calibrate [NO] measurements at higher pressures. The linear LIF measurements of [NO], which are corrected for variations in the electronic quenching rate coefficient, are compared with numerical predictions from an opposed-flow flame code utilizing two Gas Research Institute (GRI) chemical kinetic mechanisms (versions 2.11 and 3.0). The effect of radiative heat loss on code predictions is accounted for by using an optically thin radiation model. The revised GRI mechanism (version 3.0) offers a significant improvement in prompt-NO predictions for these flames compared to the older version (2.11), especially at pressures below eight atm. However, a consistent discrepancy remains in the comparisons, particularly at peak NO locations for pressures lower than six atm. The measurements display a continuing trend of decreasing NO concentration with increasing pressure at 6–15 atm as expected for flames dominated by prompt NO. The discrepancy between measurements and predictions decreases with rising pressure so that the revised GRI mechanism predicts [NO] with reasonable accuracy at pressures above six atm.

Evaluation of the Heat Leakage in the Thermal Diffusivity Measurement of Molten Metals by a Laser Flash Method

Evaluation of the radiative and conductive heat loss from a molten metal sample to the cell has been made in order to obtain accurate thermal diffusivities of molten metals at high temperature with a laser flash method. The results suggest that thermal diffusivity values of molten nickel can be determined in the temperature range from 1728 to 1928 K with an uncertainty of ±3% in comparison with case considering only the effect of radiative heat loss. The usefulness of a cell for a laser flash method has been confirmed by applying simulated results to evaluate the heat leakage in the thermal diffusivity measurement of molten metals.

Use of the conditional moment closure model to predict NO formation in a turbulent CH4/H2 flame over a bluff-body

The first-order conditional moment closure (CMC) model is applied to a CH4/H2 bluff-body flame with emphasis on NO prediction. The flow and mixing fields are calculated by assuming fast chemistry and a beta function pdf for mixture fraction. Reacting scalar fields are calculated by elliptic CMC formulation and the three different chemical kinetic mechanisms, Miller-Bowman, GRI Mech 2.11 and 3.0. Calculation results show good agreement with the measured conditional mean temperature and mass fractions of major species, although with some discrepancy on the fuel rich side. The predicted conditional mean OH mass fractions satisfy the partial equilibrium assumption for the fast shuffle reactions between H2 and O2. The predicted conditional mean CO mass fractions are in good agreement with the Two-Photon Laser Induced Fluorescence (TPLIF) data. The GRI Mech 2.11 and Miller-Bowman mechanism show reasonable agreement with the measurements of NO while the GRI Mech 3.0 results are about twice as high. Effects of radiative heat loss on NO formation are shown to be of no significance in the recirculation and neck zone of the bluff-body flame. The unconditional Favre mean temperature and species mass fractions are also in reasonable agreement with measurements.

Dynamics and flammability limit of stretched premixed flames stabilized by a hot wall

Dynamics and flammability limit of stretched, ultralean methane/air premixed flames stabilized by a hot wall were investigated theoretically and numerically using the large activation energy asymptotic approach and the detailed chemistry approach, respectively. An analytical expression describing the flame temperature and location was obtained. Several new findings on the flame regimes, bifurcation, and flammability limit are demonstrated. The analytical results showed that the appearance of hot wall results in new flame bifurcations. Except for the high-temperature flame, there exists a hot-wall-stabilized low-temperature flame which dramatically extends the flammable region of stretched flame to low stretch. Sublimit flame can be stabilized by a hot wall and has a non-zero flame speed even with a wall temperature less than its adiabatic flame temperature. Furthermore, the results also showed that the effects of radiation heat loss and the Lewis number effect are suppressed by the hot wall, and that stretched flames at a Lewis number below unity can extinguish at a finite distance from the wall. Numerical calculations with detailed chemistry well reproduced the predictions of the analytical results. In addition, numerical results showed that except for the low-temperature flame on the low-stretch-rate side, there exists another low-temperature flame regime on the high-stretch-rate side, which dramatically extends the flammable region to the high stretch rate and yields multiple flame extinctions. Theoretical analysis qualitatively agreed well with the numerical calculations, but failed in the prediction of the low-temperature flame at a high stretch rate. The present results provided useful guidance for the application of ultralean combustion in new combustion technology.

Global quenching of premixed CH 4/air flames: Effects of turbulent straining, equivalence ratio, and radiative heat loss

Global quenching of premixed CH4/air flames with turbulent straining, equivalence ratio, and radiative heat loss effects is explored in a cruciform burner. The burner equipped with a pair of counter-rotating high-speed fans and perforated plates provides downward propagating flames through near-isotropic intense turbulence, where flame-turbulence interactions are not influenced by ignition. Several CH4/air flames with different degrees of radiative heat loss, from small (N2-diluted) to large (CO2-diluted), are investigated. Each case covers a range of the equivalence ratio () with turbulent intensities (u/SL )a s much as 100, where SL is the laminar burning velocity, in which high rates of strain are achieved until, ultimately, global quenching of flames occurs. A Bradley’s Karlovitz number, defined as K 0.157(u/SL) 2 0.157Ka, is used to quantify global quenching boundaries of these turbulent flames, where ReT 0.5 ReT and Ka are the turbulent Reynolds and Karlovitz numbers, respectively. For pure CH4/air flames, the critical value of K for global quenching of rich/lean CH4 flames must be greater than 1.0/6.2. Values of Kc are very sensitive to , because Kc increases significantly as gradually approaches 1 from either lean or rich sides, with the maximum Kc occurring possibly near 1. By comparing N2- and CO2-diluted flames of the same SL, it is found that global quenching of lean/rich CH4 flames is/is not influenced by the radiatvie heat loss, respectively. The larger the radiative heat loss, the smaller the value of Kc for lean CH4 flames, in which values of Kc decrease from 4 (N2-diluted) to 3 (CO2-diluted) where SL 10 cm/s and 0.62. On the other hand, Kc 1.3 for both N2- and CO2-diluted rich CH4 flames where SL 10 cm/s and 1.20–1.45. These experimental results are important to the understanding of global quenching processes for turbulent premixed combustion.

Analysis of radiative heat-loss effects in thermal explosion of a gas using the integral manifold method

Semenov's classical model of thermal explosion in a combustible gas mixture is modified to include radiative (rather than conductive) heat-loss effects and gas-density changes. A geometrical asymptotic technique (the method of integral manifolds - MIM) is exploited to perform a qualitative analysis of the governing equations. The strength of this method lies in the compact, clear geometrical/analytical rendition and classification of all possible dynamical scenarios, in terms of the physico-chemical parameters of the system. It is found that there are two main dynamical regimes of the system: cooling regimes and fast explosive regimes. Peculiarities of these dynamical regimes are investigated and their dependence on physical system parameters is analyzed. A criterion for the occurrence of thermal explosion is disclosed. An estimate for the maximum mixture temperature is also derived analytically. It is found that, under certain operating conditions, the dynamics are such that the initial explosive stage of the process essentially behaves adiabatically before succumbing to the dominance of the radiative heat loss that brings the system down to the ambient temperature.

The effect of radiation reabsorption on NO formation in CH4/air counterflow diffusion flames

The effects of radiative heat loss on temperature and NO formation of CH4/air and that of CH4/CO2/air counterflow diffusion flames were numerically investigated by using detailed chemistry and transport properties with emphasis on assessing the importance of radiation reabsorption. Radiative heat transfer was calculated using the discrete ordinate method, and the radiative properties of the main emitting species, H2O, CO2 and CO, in these flames were represented using a statistical narrow-band model. Results show that radiation reabsorption has little influence on the NO formation of CH4/air flame. However, radiation reabsorption does have a significant effect on the NO formation and temperature of CH4/CO2/air flames, especially for low stretch rates.

Effects of radiative heat loss on the extinction of counterflow premixed H2–air flames

Radiation heat loss has an important impact on near-limit flames. It has been shown that radiation heat loss can make stretched CH4–air flames extinguish at a lower stretch rate. Numerical calculations of counterflow premixed H2–air flames were conducted using an accurate description of the chemical kinetics and transport properties. Radiation heat loss was considered. The results show that in addition to the stretch extinction limit, radiation heat loss also allows the lower and middle equivalence ratio counterflow premixed H2–air flames to extinguish at a lower stretch rate. For middle equivalence ratio counterflow H2–air flames, the closed temperature profiles become distorted O–shaped curves due to the lower Lewis number, being different from those of CH4–air flames. For higher equivalence ratio counterflow premixed H2–air flames, there are two stable flame branches—a normal flame branch and a weak flame branch. When the equivalence ratio is greater than a critical value, the closed temperature profile curve of every equivalence ratio flame opens and the normal flame curve can be extended to zero stretch rate. The calculation of the concentration limit of one-dimensional planar premixed H2–air flames was also conducted. The results show that the critical equivalence ratio corresponds to the concentration limit of the one-dimensional planar premixed H2–air flame. The extension of the flammable region due to the stretch is amplified for the counterflow H2–air flame because of its much lower Lewis number than that of the counterflow CH4–air flame.

Laser-induced fluorescence measurements and modeling of nitric oxide in counterflow partially premixed flames

Quantitative measurements of NO concentration ([NO]) have been obtained along the centerline of atmospheric pressure, methane–air, counterflow partially premixed flames using the laser-induced fluorescence (LIF) technique. The effect of partial premixing was studied by investigating flames with fuel-side equivalence ratios (φ B) of 1.45, 1.6, 1.8, and 2.0 at a constant global strain rate near 20 s −1. Linear LIF measurements of [NO] are corrected for variations in the electronic quenching rate coefficient by using major species profiles generated by an opposed-flow flame code and quenching cross-sections for NO available from the literature. Corrected linear LIF measurements of [NO] and temperatures measured using thin filament pyrometry are compared with numerical predictions from the opposed-flow flame code by utilizing the GRI (version 2.11) mechanism for the NO kinetics. The effect of radiative heat loss on code predictions is accounted for by using an optically thin radiation model. Reasonably good agreement exists between LIF [NO] measurements and predictions in all flames. In particular, all predictions fall within 10% of measurements at peak [NO] locations. Spatial separation was observed between regions where prompt-NO and thermal-NO dominate in the φ B = 1.45 flame. A previously modified rate coefficient for the prompt-NO initiation reaction improved agreement between predictions and measurements in the region dominated by prompt NO.

Laser-Induced Fluorescence Measurements and Modeling of Nitric Oxide in High-Pressure Counterflow Diffusion Flames

Quantitative laser-induced fluorescence (LIF) measurements of NO concentration ([NO]) have been obtained along the centerline of prompt-NO dominated, methane-air counterflow diffusion flames at two to five atm, Global strain rates of 20, 30 and 40 s−1 were investigated at each pressure, with the addition of a 15 s−1 case at three and four atm. Linear LIF measurements of [NO] are corrected for variations in the electronic quenching rate coefficient by using major species profiles generated by an opposed-flow flame code and quenching cross-sections for NO available from the literature. Corrected linear LIF measurements of [NO] are compared with numerical predictions from the opposed-flow flame code by utilizing the GRI (version 2.11) mechanism for the NO kinetics. The effect of radiative heat loss on code predictions is accounted for by using an optically thin radiation model. A modest decrease in predicted temperature owing to radiative heat loss causes a significant decrease in predicted [NO], indicating the temperature sensitivity of the prompt-NO kinetics. Comparisons between [NO] measurements and predictions show that the GRI mechanism underpredicts prompt-NO by a factor of two to three at all pressures. The underprediction peaks at 2 to 3 atm, and decreases with pressure from 3 to 5 atm. Although the GRI mechanism does not display this trend, predictions with a modified rate coefficient for the prompt-NO initiation reaction give qualitative agreement with the experimentally observed variation. However, modifying the prompt-NO initiation reaction is not sufficient to account for the differences between measurements and predictions, thus indicating a need for refinement of the CH chemistry.

Asymptotic analysis of radiative extinction in counterflow diffusion flames of nonunity Lewis numbers

The effects of radiation heat loss and variation in near-unity Lewis numbers on the structure and extinction of counterflow diffusion flame established near the stagnation plane of two opposed free streams of fuel and oxidizer are analyzed using the asymptotic method of large activation energy. Radiation heat loss from the reaction zone is accounted for using the optically thin assumption. The main concern of this study is the thermal effects of radiation heat loss and Lewis numbers on diffusion flame extinction, particularly at small stretch rates. The existence of two extinction limits, the radiation extinction limit at a small stretch rate and the conventional quenching limit at a large stretch rate, is theoretically reproduced. This simplified analysis is able to predict the existence of inflammable limits of counterflow diffusion flames in terms of the near-unity Lewis numbers and concentrations of the fuel and oxidizer streams.

Probability density function calculations of local extinction and no production in piloted-jet turbulent methane/air flames

The intense nonlinear interaction between turbulent fluctuations and finite-rate chemistry can cause local extinction and has a strong influence on NO production in non-premixed turbulent flames. Accurate predictions of local extinction and NO formation in turbulent flames require a rigorous means of representing such a strong coupling of turbulence and chemistry and hence are substantial challenges for turbulent combustion models. In this study, a self-contained joint velocity-composition-turbulence-frequency probability density function (PDF) method is used to make calculations of a series of piloted-jet nonpremixed flames of methane/air. The ingredients of the present model include the simplified Langevin model for velocity, a stochastic model of turbulence frequency, and the Euclidean minimum spanning tree (EMST) mixing model. An augmented reduced mechanism (ARM2) for methane oxidation, which involves 19 species and 15 reactions (including NO chemistry), is incorporated into the joint probability density function (JPDF) calculations using the in situ adaptive tabulation (ISAT) algorithm. The effects of radiative heat loss are studied using an optically thin limit model. The calculation results show good agreement with the experimental data, including the minor species NO and CO. The increase of local extinction with increasing jet velocity is accurately represented by the calculations.

Two-Dimensional Effects on Measurement of Thermal Diffusivity by AC Calorimetric Method: II. Effects of Heat Loss

In our previous paper [I. Hatta et al..: Jpn. J. Appl. Phys. 38 (1999) 2988.], the condition required for measuring thermal diffusivity along the surface of a thick plate-like sample using an ac calorimetric method was proposed in which the sample is subjected to an adiabatic condition. In measurement at high temperatures, we cannot avoid the effects of radiation heat loss. Therefore, it is important to extend the analysis to include a case where there is heat dissipation from the sample to surroundings. We analyze two-dimensional temperature wave propagation in a thick plate-like sample during heat loss between the plate-like sample and a thermal bath. In such a case, we propose the conditions for measuring thermal diffusivity to an accuracy of ±1%.