Intrinsic thermo-acoustic instability criteria based on frequency response of flame transfer function

A study of Intrinsic Thermo-Acoustic (ITA) instability behavior of flames anchored to a burner deck is performed by introducing a mapping between the Flame Transfer Function, FTF(s), defined in the complex (Laplace) domain and the experimentally measured Flame Frequency Response, FFR(iω). The conventional approach requires a system identification procedure to obtain the FTF(s) from the measured FFR(iω). Next, root-finding techniques are applied to define the complex eigenfrequencies. The common practice is to fit the FTF(s) by a rational function that may lead to artifacts like spurious poles and zeros. The purpose of the present work is to establish instability criteria which are directly applicable in the frequency domain. The particular case is considered where the acoustic boundary conditions at both sides of the flame are anechoic. Therefore, the pure ITA mode is treated. First, the causality of the measured FFR(iω) is checked. Then, the criteria of the ITA mode instability applicable to the FFR(iω) phase and magnitude, are derived. Causality properties are used to find the unstable frequency, growth rate, and even the maximum possible value of the linear growth rate. In addition, a procedure is explained to reconstruct the flame transfer function in the complex plane s from the measured flame frequency response which could be an alternative method to study the FTF behavior in the complex domain instead of its estimation with a rational function

Designing an acoustic termination with a variable reflection coefficient to investigate the probability of instability of thermoacoustic systems

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

CFD analysis of mucous effect in the nasal cavity

This research aims to investigate the airflow patterns and particle deposition in a healthy human upper airways. A realistic 3-D computational model of the upper airways including the vestibule was developed using a series of CT scan images of a healthy human. Simulations of the airflow fields in the upper airway passages were performed by solving the Navier-Stokes and continuity equations for breath rate 20 L/min. The trajectory analysis approach was applied to study the particle transport and deposition for the model with and without mucous lining. The presented results revealed that the mucous layer can have significant impact on airflow analysis and there were noticeable differences in the amount of particle deposition in each models

Thermo-acoustic flame instability criteria based on upstream reflection coefficients

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 modelling strategy usually faces serious practical problems related to the need of measurements/modelling at the hot downstream part of the system. In the present work, we propose a measurement and a system modelling 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 modelling approach and reveal good correspondence