| 1. |
- Kojourimanesh, M., et al.
(författare)
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Mobius transformation between reflection coefficients at upstream and downstream sides of flame in thermoacoustics systems
- 2021
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Ingår i: "Advances in Acoustics, Noise and Vibration - 2021" Proceedings of the 27th International Congress on Sound and Vibration, ICSV 2021. - : Silesian University Press.
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Konferensbidrag (refereegranskat)abstract
- An alternative approach to assess the conditions for the onset of thermoacoustics combustion instability is proposed. The method is based on the analysis of the relation between the reflection coefficient of combustor upstream section (subsystems) with respect to the burner/flame, Rup, and the input reflection coefficient, Rin, as observed from the inlet of the burner with flame and all downstream subsystems of a combustion appliance. The instability of the combustor can be determined by evaluating the system dispersion relation expressed in the terms of these two reflection coefficients. The properties of the relation between Rin, the reflection coefficient of the burner downstream subsystems, Rdn, and elements of the flame transfer matrix, TM, of the burner are investigated. This relation has the form of the well-known Mobius transformation. Using the well-developed theory of the transformation, we derive the necessary conditions of Rdn to ensure that the magnitude of Rin becomes less than 1 in a frequency range. This condition results in a passive thermoacoustics stability of the system's operation. Furthermore, an optimum value of Rdn is derived which provides a minimum value of Rin at given entries of the burner TM. The practical application of the developed theory provides suitable criteria and guidelines for designing passive acoustic properties at the downstream side of the combustion appliance.
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| 2. |
- Kojourimanesh, M., et al.
(författare)
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Thermo-acoustic flame instability criteria based on upstream reflection coefficients
- 2021
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Ingår i: Combustion and Flame. - : Elsevier BV. - 0010-2180 .- 1556-2921. ; 225, s. 435-443
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Tidskriftsartikel (refereegranskat)abstract
- A prospective method to assess thermo-acoustic instabilities based on two reflection coefficients measured from the upstream side of the burner is presented and experimentally validated. In order to compose a model which allows predicting the onset of thermo-acoustic instability of combustion in a practical appliance, one has to characterize the thermo-acoustic properties of the burner including the flame as an acoustically active element and acoustic properties of all other (usually passive) components of the combustion appliance both upstream as well as downstream of the burner. This kind of modeling strategy usually faces serious practical problems related to the need of measurements/modeling at the hot downstream part of the system. In the present work, we propose a measurement and a system modeling approach which relies on two acoustic measurements, namely reflection coefficients, only at the cold (burner upstream) part of the combustion appliance. Both reflection coefficients, termed upstream and input, can be readily measured using standard acoustic techniques. The need to measure the input reflection coefficient of an acoustically active subsystem may impose difficulties related to the acoustic instability of the measurement setup itself. The approach and technical solution to handle this problem via a special modification of the excitation source (loudspeaker box) is proposed. The dispersion relation to search for system eigen frequencies is represented in a form that couples the reflection coefficients of the upstream part of the appliance and input reflection coefficient from the downstream part as observed through the burner with flame. This form of the dispersion relation is commonly used in the theory of radio-frequency circuits and recently introduced for thermo-acoustic problems. The proposed method is applied to burners with premixed burner-stabilized Bunsen-type flames. The observed instability conditions and oscillation frequencies are compared with predictions of the proposed modeling approach and reveal good correspondence.
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| 3. |
- Raida, M. B., et al.
(författare)
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Laminar burning velocity measurements of ethanol+air and methanol+air flames at atmospheric and elevated pressures using a new Heat Flux setup
- 2021
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Ingår i: Combustion and Flame. - : Elsevier BV. - 0010-2180. ; 230
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Tidskriftsartikel (refereegranskat)abstract
- A new setup for burning velocity measurements of liquid fuels at elevated pressures using the Heat Flux method has been constructed. The burner design has been improved comparing to previous studies in this and other laboratories. Laminar burning velocities were determined experimentally for ethanol+air and methanol+air mixtures over a range of conditions including equivalence ratio (0.8 to 1.3), inlet gas temperature (318–338 K), and pressure (1–5 bar). Experimental uncertainties have been determined and analyzed. The overall accuracy of the burning velocity, SL, was evaluated to be better than ±1 cm/s for atmospheric pressure and ±1.5 cm/s at elevated pressure conditions. Experimental results at 1 atm are in good agreement with recent literature data. To compare the present measurements with the literature data often obtained at other initial temperatures and pressures, an empirical expression SL = SL0 (T/T0)α (p/p0)β, which correlates the burning velocity at a specific temperature, T, and pressure, p, with that at standard conditions indicated by superscripts 0, was implemented. At a fixed temperature, pressure dependence at each equivalence ratio can be analyzed. It was found that power exponents β derived in the present work and from the literature data possess large scattering. The best agreement was found between the present results and experimental literature data from spherical flames using non-linear stretch correction models. Burning velocities at atmospheric as well as elevated pressures and power exponents β have been also compared with kinetic modeling results using several detailed kinetic mechanisms, showing a fair to a good agreement. As the new results are accurate and free from stretch effects, they form a new source of reliable data for validation of the reaction mechanisms.
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| 4. |
- Saxena, V., et al.
(författare)
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Designing an acoustic termination with a variable reflection coefficient to investigate the probability of instability of thermo-acoustic systems
- 2021
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Ingår i: "Advances in Acoustics, Noise and Vibration - 2021" Proceedings of the 27th International Congress on Sound and Vibration, ICSV 2021. - : Silesian University Press.
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Konferensbidrag (refereegranskat)abstract
- This paper presents results of the development of an acoustic device to be utilized as a duct termination with variable reflection coefficient. This study is motivated by the idea to experimentally evaluate the probability of instability of a thermo-acoustic system where combustion acts as an active acoustic element, and this termination acts as a passive acoustic element that can be configured to a desired value of the reflection coefficient at the upstream side of the flame and burner for lab-scaled physical modelling of, for instance, domestic boilers. This termination consists of a cylinder containing a stack of truncated hollow cones with narrow gap in between and a telescopic tube. The gap between the adjacent cones, and sound-absorbing fibrous material (“Acotherm”) placed in the cavity of these cones produce a low reflection coefficient in the frequency range between 40 and 800 Hz. Longitudinal displacement of these cones inside the cylinder generates a reflection coefficient with magnitude ranging from 0.2 to 0.9. The telescopic tube with an adjustable length (between 0.85 - 1.38 m) allows to achieve a wide range of phases of reflection coefficient. The steps taken to optimize the design and performance of this termination in presence of flame are presented here.
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| 5. |
- Saxena, Vertika, et al.
(författare)
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Designing variable reflection coefficient for upstream and downstream terminations to study their effect on flame thermoacoustics
- 2022
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Ingår i: INTERNATIONAL JOURNAL OF SPRAY AND COMBUSTION DYNAMICS. - : SAGE Publications. - 1756-8277 .- 1756-8285. ; 14:3-4, s. 251-265
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Tidskriftsartikel (refereegranskat)abstract
- In this paper, the design, construction and results of experiments performed on a generic combustion system are presented. The setup is supplemented by various weakly frequency-dependent variable reflection coefficient (RC) devices as upstream and downstream acoustic terminations. The main objective of building such terminations is to provide a method to study burner/flame stability when it is placed between various acoustic configurations (RC: 0.1-0.9) and to determine the figure of merit of a burner based on the evaluation of its map of (in-)stability. Furthermore, burner design parameters such as the burner perforation pattern (holes diameter, pitch, perforation area, etc.) which will provide combustion stability for the widest range of burner's acoustic embedding conditions are identified. The experimental setup comprises of an upstream acoustic termination, a telescopic tube with adjustable length is placed after the upstream termination followed by the burner and the quartz tube. On the top of the quartz tube, the replaceable downstream terminations are installed. Nine downstream terminations are constructed by stacking plates of 0.25 mm thickness separated by spacers ranging from 0.1 to 1 mm thickness. Particularly, for the burners tested in this setup, the smallest hole diameter burner (with the largest perforation area) results in the largest stable region on the stability map in the parameter space. An increase in the flow velocity leads to an increase in the frequency of instability and makes a stable system tend to become unstable, while an increase in the equivalence ratio contributes to stabilizing system instability
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