Influence of small-scale turbulence on internal flamelet structure

Direct numerical simulation data obtained from a highly turbulent (Kolmogorov length scale is less than a laminar flame thickness by a factor of about 20) lean hydrogen-air complex chemistry flame are processed, with the focus of the study being placed on flame and flow characteristics conditioned to instantaneous local values c F x , t of the fuel-based combustion progress variable. By analyzing such conditioned quantities, the following two trends are documented. On the one hand, magnitudes of fluctuations of various local flame characteristics decrease with increasing the combustion progress variable, thus implying that the influence of small-scale (when compared to the laminar flame thickness) turbulence on internal flamelet structure is reduced as the flow advance from unburned reactants to combustion products. On the other hand, neither local turbulence characteristics (conditioned rms velocities, total strain, and enstrophy) nor local characteristics of flame-turbulence interaction (flame strain rate) decrease substantially from the reactant side to the product side. To reconcile these two apparently inconsistent trends, the former is hypothesized to be caused by the following purely kinematic mechanism: residence time of turbulence within a large part of a local flamelet is significantly shortened due to combustion-induced acceleration of the local flow in the direction normal to the flamelet. This residence-time reduction with increasing c F is especially strong in the preheat zone ( c F < 0.3 ) and the residence time is very short for 0.3 < c F < 0.8 . Therefore, small-scale turbulence penetrating the latter zone is unable to significantly perturb its local structure. Finally, numerical results that indirectly support this hypothesis are discussed

Smallest scale of wrinkles of a Huygens front in extremely strong turbulence

By analyzing the statistically stationary stage of propagation of a Huygens front in homogeneous, isotropic, constant-density turbulence, a length scale l(0) is introduced to characterize the smallest wrinkles on the front surface in the case of a low constant speed u(0) of the front when compared to the Kolmogorov velocity u(K). The length scale is derived following a hypothesis of dynamical similarity that highlights a balance between (i) creation of a front area due to advection and (ii) destruction of the front area due to propagation. Consequently, the front speed is compared with the magnitude of the fluid velocity difference in two points separated by a distance smaller than the Kolmogorov length scale. Appropriateness of the smallest wrinkle scale is demonstrated by applying a fractal approach to evaluating the mean area of the instantaneous front surface. Since the scales of the smallest and larger wrinkles belong to different subranges (dissipation and inertial, respectively) of the Kolmogorov turbulence spectrum, the front is hypothesized to be a bifractal characterized by two different fractal dimensions in the two subranges. Both fractal dimensions are evaluated adapting the aforementioned hypothesis of dynamical similarity. Such a bifractal model yields a linear relation between the mean fluid consumption velocity, which is equal to the front speed u(0) multiplied with a ratio of the mean area of the instantaneous front surface to the transverse projected area, and the rms turbulent velocity u\u27 even if a ratio of u(0)/u\u27 tends to zero

Solenoidal and potential velocity fields in weakly turbulent premixed flames

Direct Numerical Simulation data obtained earlier from two statistically 1D, planar, fully-developed, weakly turbulent, single-step-chemistry, premixed flames characterized by two significantly different (7.53 and 2.50) density ratios {\sigma} are analyzed to explore the influence of combustion-induced thermal expansion on the turbulence and the backward influence of such flow perturbations on the reaction-zone surface. For this purpose, the simulated velocity fields are decomposed into solenoidal and potential velocity subfields. The approach is justified by the fact that results obtained adopting (i) a widely used orthogonal Helmholtz-Hodge decomposition and (ii) a recently introduced natural decomposition are close in the largest part of the computational domain (including the entire mean flame brushes) except for narrow zones near the inlet and outlet boundaries. The results show that combustion-induced thermal expansion can significantly change turbulent flow of unburned mixture upstream of a premixed flame by generating potential velocity fluctuations. Within the flame brush, the potential and solenoidal velocity fields are negatively (positively) correlated in unburned reactants (burned products, respectively) provided that {\sigma}=7.53. Moreover, correlation between strain rates generated by the solenoidal and potential velocity fields and conditioned to the reaction zone is positive (negative) in the leading (trailing, respectively) halves of the mean flame brushes. Furthermore, the potential strain rate correlates negatively with the curvature of the reaction zone, whereas the solenoidal strain rate and the curvature are negatively (positively) correlated in the leading (trailing, respectively) halves of the mean flame brushes.Comment: The work is accepted for oral presentation at the 38th Symposium (International) on Combustion. arXiv admin note: substantial text overlap with arXiv:2007.0833

ИССЛЕДОВАНИЕ ОСНОВНЫХ ХАРАКТЕРИСТИК УМНОЖИТЕЛЕЙ ЧАСТОТЫ В ДИАПАЗОНЕ 120-220 ГГЦ

The features of the design and the operation principle of frequency doublers in range 120-220 GHz are considered. The results of input and output power measurements of frequency doublers are given, the values of them efficiency factors are calculated. A technique for measuring and estimating the results of the deviation of the efficiency is developed. The dependence of the transmission coefficients from the value of the output power of the frequency doublers is investigated.Рассмотрены особенности конструкции и принципа работы удвоителей частоты в диапазоне частот 120-220 ГГц. Приведены результаты измерений входной и выходной мощности удвоителей частоты, рассчитаны значения их коэффициентов передачи по мощности. Разработана методика измерений и оценки результатов отклонения коэффициента передачи по мощности удвоителей частоты. Исследованы зависимости коэффициентов передачи по мощности от значения выходной мощности удвоителей

Адресный волоконно-оптический датчик для измерения относительной влажности в комплектных распределительных устройствах

A number of governing documents and by-laws of the Russian Federation, branch ministries, departments and companies have introduced the use of measuring relative air humidity, elements insulation, and SF6 into operation and maintenance process of complete switchgear. A wide range of high-precision laboratory instruments has been developed to implement these measurements. However, as a rule, these are scheduled measurements to be carried out once or twice a quarter, although the constant on-line monitoring of humidity is concerned in both the production and scientific circles of the energy industry. The possibility of on-line monitoring appeared with the advent of fiber-optic object-based passive networks for collecting information and the possibility of forming interrogation channels in them, which is provided for by the development of the Smart Grid Plus concept. Fiber optic sensors, single in their physical layer structure with passive optical networks, are highly robust and resistant to high electromagnetic fields, typical of those generated in a switchgear, and are designed to operate in harsh environments. Among their broad class, fiber optic sensors on Bragg gratings, which differ from others by direct measurement methods, have significant advantages. In particular, an increase or decrease in relative humidity will lead to a corresponding change in the wavelength of the sensing source reflected from the grating, which can be measured with an accuracy of sixth place from its absolute value.This paper proposes to consider a two-element sensor of relative humidity of a parallel structure, which differs from the existing ones by using address fiber Bragg gratings made in SMF-28 fiber. One of the gratings has a polyimide-replaced quartz shell, synthesized using a reductant fiber coating, and a completely multiplicative response to temperature and deformation caused by humidity. The second grating is recorded in a standard fiber and responds only to temperature. It is possible to include an additional third grating with a partially etched cladding, which can be used for refract metric measurements of the amount of condensed moisture on the elements of a complete switchgear. All the gratings are identical, have, as a rule, the same Bragg wavelength after manipulating their claddings, but they have differing unique addresses, which are formed by recording two transparency windows in each of the gratings with different difference frequency space. The transparency windows correspond to phase p-shifts symmetrically located at the same distance from the center of each grating. The structure obtained makes it possible to record information of the measurement conversion at the said difference frequencies in the radio range, which significantly increases the speed of relative humidity measurements and their accuracy by an order of magnitude more. In addition to what has been said, it is possible to note the capability for building a network of these sensors in series arranged in switchgear devices, with a different radiofrequency address group being used in each of them.Рядом руководящих документов и подзаконных актов Российской Федерации, отраслевых министерств, ведомств и компаний измерение относительной влажности воздуха, изоляции элементов, элегаза введено в практику процесса эксплуатации и обслуживания комплектных распределительных устройств. Разработан широкий спектр высокоточных лабораторных приборов, которые используются для реализации указанных измерений. Однако, как правило, данные измерения проводятся планово, один-два раза в квартал, хотя о постоянном онлайн мониторинге влажности речь идет как в производственных, так и научных кругах энергетической отрасли. Возможность онлайн мониторинга появилась с появлением волоконно-оптических объектовых пассивных сетей сбора информации и возможности формирования в них сенсорных каналов, что предусмотрено также развитием концепции «Smart Grid Plus». Волоконно-оптические датчики, единые по структуре физического уровня с пассивными оптическими сетями, обладают высокой помехоустойчивостью, не подвержены влиянию мощных электромагнитных полей, характерных для создаваемых в комплектных распределительных устройствах, предназначены для работы в жестких условиях эксплуатации. Среди их широкого класса существенными преимуществами обладают волоконно-оптические датчики на брэгговских решетках, которые отличаются от других прямыми методами измерений. В частности, увеличение или уменьшение относительной влажности приведет к соответствующему изменению отраженной от решетки длины волны зондирующего источника, которая может быть измерена с точностью до шестого знака от ее абсолютного значения.В данной работе предложен к рассмотрению двухсенсорный датчик относительной влажности параллельной структуры, отличающийся от существующих использованием адресных волоконных брэгговских решеток, выполненных в волокне SMF-28. Одна из решеток имеет замененную полиимидом кварцевую оболочку, синтезированную с помощью восстановителя покрытия волокна, и полный мультипликативный отклик к температуре и деформации, вызванной влажностью. Вторая – представляет собой решетку, записанную в стандартном волокне, и реагирует только на температуру. Возможно включение дополнительной третьей решетки с частично вытравленной оболочкой, которая может быть применена для рефрактометрических измерений количества конденсированной влаги на элементах комплектного распределительного устройства. Все решетки идентичны, имеют, как правило, одинаковую длину волны Брэгга, после манипуляции над их оболочками, но отличаются уникальным адресом, который формируется записью двух окон прозрачности в каждой из решеток с различным разностным частотным пространством. Окна прозрачности соответствуют фазовым p-сдвигам, симметрично расположенным на одинаковом расстоянии от центра каждой из решеток. Полученная структура позволяет регистрировать информацию измерительного преобразования на указанных адресных разностных частотах в радиодиапазоне, что существенно повышает быстродействие измерений относительной влажности и их точность еще на порядок. В дополнение к сказанному можно отметить возможность построения сети указанных датчиков в последовательно расположенных комплектных распределительных устройствах, при этом в каждом из шкафов будет использована другая радиочастотная адресная группа

Scaling of reaction progress variable variance in highly turbulent reaction waves

Self-propagation of a reaction wave, which consists of an infinitely thin reaction zone (front) and a thick inert mixing layer adjacent to the front, in constant-density statistically stationary, homogeneous isotropic turbulence unaffected by the wave is analytically studied. In the asymptotic case of a high turbulent Reynolds number, high Karlovitz number, and low Damk_ohler number Da, the scalar variance c02 is shown to be proportional to Da for the statistically stationary stage of the wave evolution. This scaling is supported by newly analyzed Direct Numerical Simulation data discussed in detail by Sabelnikov et al. ["Thin reaction zones in constant-density turbulent flows at low Damk_ohler numbers: Theory and simulations," Phys. Fluids 31, 055104 (2019)]. The obtained analytical results also show that, under conditions of the present study, spatial gradients of reactant concentration non-uniformities due to the reaction and spatial gradients of reactant concentration non-uniformities due to the turbulence are of the same order of magnitude. Accordingly, major statistical characteristics of the scalar field c(x, t) such as the mean area of an iso-scalar surface c(x, t) = const, the mean molecular flux through this surface, etc., can be found adopting results known in the theory of inert and passive turbulent mixing. Nevertheless, the reaction indirectly affects these characteristics by controlling the mean thickness of the reaction wave and, consequently, the spatial gradient of the mean reaction progress variable. Published under an exclusive license by AIP Publishing

Evaluation of mean species mass fractions in premixed turbulent flames: A DNS study

Direct Numerical Simulation (DNS) data obtained by Dave and Chaudhuri (2020) from a lean, complex-chemistry, hydrogen-air flame associated with the thin-reaction-zone regime of premixed turbulent burning are analyzed (by adapting fiv e alternati v e definitions of combustion progress variable c) in order to examine three different models that (i) are based on the flamelet paradigm and (ii) aim at evaluating mean concentrations of various species in applied CFD research into turbulent combustion. Mean mole fractions of all considered species and mean density are predicted if the laminar-flame profiles of species mole fractions and density, respectively, are directly averaged using a Probability Density Function (PDF) P (c). The best predictions are obtained by extracting P (c) from the DNS data and defining c based on hydrogen mass fraction.These predictions suggest that mean mole fractions of various species in a premixed turbulent flame can be evaluated at a post-processing stage of a CFD study by adopting P (c), obtained at the major stage of the simulations, to average a flamelet library. When applied in such a way, the flamelet paradigm is useful even for lean hydrogen-air flames and even at Karlovitz number as large as 13. If the same PDF is applied to average reaction rates from the same flamelet library, the mean rates of production/consumption of species n are poorly predicted, e.g. for radicals H, O, OH, HO2 , and H2 O2 if c is defined using hydrogen mass frac- tion. A hypothesis that conditioned rates (Wn | c) can be predicted using conditioned mole fractions (Xn| c) , temperature (T | c) , and density (ρ| c) is not supported either, e.g. for radicals O and OH. These differences between predictive capabilities of the first approach (directly averaging concentration profiles) and two other approaches (averaging reaction rates) are attributed to weakly (highly) non-linear dependencies of the concentrations (rates, respectively) on c