This work reports an original experimental investigation on the sooting characteristics of counterflow diffusion flames (CDFs) subject to sinusoidal oscillations of strain rates. The study was ...motivated by the necessity to consider transient responses of soot formation when using the laminar flamelet model to simulate soot formation in turbulent flames. Acoustic forcing was applied through loudspeakers installed at the inlet end of both the oxidizer and fuel nozzles of the counterflow burner to generate strain rate oscillations at the desired frequency and amplitudes. Particle image velocity and planar laser induced incandescence were used respectively to measure flow field and the soot volume fraction in a time-resolved manner. Counterflow flames of C2H4 and C3H8 fuels were tested with oxidizer streams of various oxygen mole fractions (XO). The experimental data revealed notable differences in unsteady soot responses between CDFs of C2H4 and C3H8 at the same XO. However, after matching the soot loadings of the two CDFs to have the same sensitivity to strain rate at steady states, their responses to unsteady strain oscillation became almost identical. We experimentally demonstrated that the unsteady response of soot formation to oscillating strain rates, as characterized by phase lag and amplitude attenuations, are strongly correlated with the sensitivity of the soot loading to strain rate under steady conditions. The results may have important implications for the development of transient flamelet models that incorporate unsteady effects on soot formation.
This paper presents a study of natural oscillations of a cantilever bar with five point masses with variable geometric and stiffness parameters (distances between locations of the masses, coefficients of ...variability of the bending stiffness of the bar sections). Using the exact method of forces based on the Mohr formula, there have been obtained expressions in general form for calculating the main unit coefficients of the secular equation, which makes it possible to perform calculations and to determine the oscillation frequency response of natural oscillations with a wide range of changes in the initial parameters of the physical and geometric state of cantilever bars. A numerical example has been given to illustrate the proposed theoretical approaches. The results have been compared with the results based on calculating a similar can tilever bar with one( reduced by masses) degree of freedom. A graphical dependence of the oscillation frequency response value on changing the value of the bending stiffness along the length of the cantilever bar gas been obtained. The theoretical provisions and applied results presented in the work will be widely used both in the practical design of bar systems and in scientific research in the field of mechanics of a deformable solid body.
From the existing literature, several variants of the differential equation solution of transverse vibrations of a plate, including approximate methods (variational methods of Ritz and Galerkin, ...Rayleigh, Leibenson, and numerous other scientists), are noted. In several cases, scientists proposed equations of the vibration form of a homogeneous plate that is satisfied when these equations are fulfilled. Thus, the solution is reduced to the Bessel equation (the first and second type). Several researchers investigated differential equation solutions in the form of a series (proposed in due time by Timoshenko, Theory of oscillations in engineering ONTI 1934, and by Cato Kenza, Iap. Soc.Mech.Eng.41.N: 347.1975. 1996-2000 c). At the end of the 20
th
century, theories on the functions of a complex variable and conformal mapping (Guz, Kubenko V.D, Aleksandrovich, Kosmodamiansky, etc.) were applied. By using the above method of inputting the function of a complex variable and conformal mapping, the oscillation problem of a polygonal plate was solved, and the solution was published in Moscow in the journal Izvestiya Rossiiskoi Akademii Nauk. Here, we propose a different approach for solving the oscillation problem of a polygonal, in which a connected plate was multiplied using rectilinear cuts.
Fire accidents might be caused by liquid fuel leakage, which can accumulate in a prescribed area under the action of physical barriers. Therefore, pool fires could be of an arbitrary geometry in less ...ideal configurations. The current objective is to uncover the underlying physics of flame structure and dynamics in pool fires of different geometries. In this work, pool fires with seven different geometries were examined. Flame base structure and instantaneous flame height in pool fires of different geometries were recorded. Temperature, velocity/vorticity fields and coherent vortical structures were further presented by Large Eddy Simulation (LES). Results show that mean flame height of circular pool fire is the highest, while that of right triangle fires is the lowest. Mean flame height of the other noncircular geometries lies in these two extremes. The presence of vertices and hypotenuses has a dramatic influence on the formation and development of vortical structures. Triangular pool fires have the highest oscillation frequency, followed by quadrangular and circular geometries. To account for the physics of air entrainment and vortex dynamics due to pool fire geometry, pool perimeter is used as a characteristic length scale to propose new scaling relations for describing flame height and oscillation frequency.
•Fuel geometry effect on flame structure and dynamics of pool fires is studied.•Flame length scales and oscillation frequencies of pool fires are compared.•Cross-sectional temperature, velocity and vorticity profiles are numerically revealed.•Results present the vortex dynamics of pool fires in different geometries.•Relations of flame height and oscillation frequency in different geometries are proposed.
•By analyzing the mechanism of SSCI, the AHCC method, consisting of a subsynchronous frequency detection part and harmonic current compensation part, is applied to the RSC current inner loop.•The ...frequency detection part of the AHCC method can real-time identify the SSCI frequency, ensuring its applicability in SSCI suppression under various operating conditions. Compared to the structural complexity of the suppression strategies, it is simpler and more achievable.•Considering various influencing factors such as wind speed, the number of wind turbines, and the degree of series compensation, multiple simulations with oscillation frequency variation were constructed to verify the robustness and adaptability of the proposed control strategy.
In this paper, an adaptive harmonic current compensation (AHCC) method is proposed to mitigate subsynchronous oscillation (SSO) in series-compensated doubly-fed induction generator (DFIG)-based wind farms. The AHCC method, which consists of the subsynchronous frequency detection part and harmonic current compensation part, possesses the ability to mitigate SSO with wide-band frequency variation. The frequency detection part adopts the triple spectral line interpolation FFT method to detect and track oscillation frequency. The harmonic current compensation part adopts a quasi-proportional integral resonant (Q-PIR) controller. The Q-PIR adds a damped remodeling branch of oscillation frequency and blocks the sub-synchronous component's propagation. The center frequency of Q-PIR is adaptively adjusted according to the oscillation frequency to ensure stable operation under various conditions. Simulation results demonstrate the effectiveness and robustness of the AHCC method in mitigating SSO and enhancing the stability of wind power systems.
The results of a series of bench-scale pool fire experiments to explore the burning characteristics under a sealed environment are presented in this work.Although the oxygen fraction in the space ...gradually decreases by burning consumingin comparison to open conditions, the transient heat release rate can still be calculated using the PER approach using the in-situ measured oxygen fraction. The dimensionless flame height can still be calculated using the McCaffrey's model for open conditions, but a greater coefficient should be used because the flame burns in a ‘hunger’ mode when the oxygen fraction is low. The current non-sealed fire model for centerline temperature rise of pool fires is unsuitable for the sealed environment: the centerline temperature rise decreases more quickly than that predicted by Nasr's model developed for cases with a limited ventilation in the middle and later stages of the fire. A new engineering model has been developed in this study by curve fitting sealed fire test results. Pagni's law can still be suitable to describe the relationship between the frequency of flame oscillation and the diameter of a pool fire in a sealed environment, but the modified coefficients are required.
•A series of pool fire experiments in sealed ship engine room were conducted.•New model of characteristics concerned sealed pool fires was developed.•Differences of combustion parameters between sealed and open spaces were compared.•Abundant experimental data of sealed pool fires were collected.
Terahertz (THz) silicon-based electronics is undergoing rapid developments. In order to keep this momentum high, an accurate and optimized on-wafer characterization procedure needs to be developed. ...While evaluating passive elements, the measured s-parameter data can be verified by a direct use of EM simulation tools. However, this verification requires to precisely introduce part of the measurement environment such as the probes, pads, and access lines to accurately predict the impact of calibration and layout for on-wafer measurements. Unfortunately, this procedure is limited to passive elements. Hence, in this work, we propose a new procedure to emulate the measurement of active devices using an electromagnetic SPICE cosimulation. By this method, one can clearly highlight that a measurement artifact that was observed for the transistor measurement can be reproduced. One of the most representative examples of measurement artifact involves the measurement and estimation of <inline-formula> <tex-math notation="LaTeX">{f}_{\text {MAX}} </tex-math></inline-formula> which is not constant over all frequency bands. Also, the measurement is difficult to perform above 40 GHz. This typical problem is now undoubtedly attributed to the probe-to-substrate coupling and probe-to-probe coupling which are strongly dependent on the probe geometry. Finally, this cosimulation procedure evidently underlines the need for an optimized deembedding procedure above 200 GHz.
Biodiesel is an emblematic energy of green power and developing vigorously, which is conducive to an important strategic significance covering the sustainable development of the global economy, the ...promotion of energy substitution, the reduction of environmental pressure, and the control of atmospheric contamination. In order to study the biodiesel energy burning characteristics, a series of experiments were carried out in an ISO9705 full-scale room, and the effects of pool size on the burning rate, flame height, flame oscillation frequency, and flame radiation fraction of biodiesel energy fire were systematically analyzed. This study uses genetic algorithm-back propagation neural network (GA-BPNN) algorithms for real-time prediction of transient fire mass loss rates. Three parameters (pool size, liquid depth, and burning time) are paired with the fuel mass loss rate and trained using a GA-BPNN algorithms model. The results show that the GA-BPNN algorithms predictions have a good correlation with the validated experimental values and the relative is less than 15%. Furthermore, the ratio of intermittent flame height and continuous flame height to their mean value is calculated respectively at about 1.72 and 0.58. The flame oscillation frequency decreases following the increase in oil pool size, the correlation can be calculated by f=0.3(D/g′)1/2. Finally, a new correlation χrad=0.2Q˙*−2/3 is proposed to predict the flame radiation fraction by increasing the flame viewing factor coefficient. The proposed correlation can be used to describe its evolution under different oil pool sizes, which can be essential to estimate the flame radiation impact on surroundings for such biodiesel energy fires.
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•The combustion characteristics of biodiesel pool fire is investigated.•GA-BPNN algorithm is used to predict the real-time biodiesel burning rate.•A new correlation of flame radiation fraction prediction is introduced.
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