Acoustic Pyrometry Research Articles

Measurement the 3D temperature distribution in the combustion zone using acoustic tomography and nolinear wavefont tracing

A non-invasive temperature measurement method in three dimensions is proposed to reconstruct the three-dimensional temperature. It relies on the use of nonlinear acoustic travel-time tomography, which incorporates acoustic path tracing, Gaussian kernel function, and modified Tikhonov regularization. Sound signals can be distorted due to large average temperature gradients and oscillatory release of heat in the combustion region. The effective acoustic signal is extracted based on spectral subtraction to evaluate the sound propagation time under distorted signals. The analysis of experimental errors indicates that the proposed acoustic pyrometry method has an average relative error of less than 15% compared to the time-averaged temperature measured by thermocouples. This result verifies the efficacy of the method when it comes to accurately reconstructing the temperature field in the combustion area. The reduction in error is attributed to the consideration of acoustic ray tracing within the proposed method.

Acoustic tomography for velocity estimation in high temperature flows

Accurately measuring the temperature of a gas flow is essential for monitoring purposes in many energy conversion applications. Typically, this is achieved using either contact measurement techniques, like thermocouple sensors, or radiation-based methods, like optic pyrometry. In harsh conditions, contact measurement techniques are prone to degradation due to the high oxidizing and high-temperature environment, thus reducing sensor lifespan. Radiation-based methods, on the other hand, rely on expensive and highly non-linear transduction systems. Acoustic pyrometry is attracting an increasing interest as it allows the estimation of the temperature distribution in a section by computing the time of flight of acoustic waves. If two measurement sections, at different axial positions, are considered, the same theoretical approach can be adapted to also compute the velocity map of the flow (acoustic tomography). However, the complexity of the mathematical problem to be solved for such a calculation needs a careful analysis. In this study, starting from a known temperature and velocity profile, a reconstruction algorithm was developed and tuned with a particular focus on velocity estimation. Relevant guidelines for a proper application of this measurement technique were also derived.

Precise temperature reconstruction in acoustic pyrometry: Impact of domain discretization and transceiver count

The present study delves into the complex relationship of domain discretization and transceiver allocation in enhancing the accuracy of acoustic pyrometry. Six distinct cases encompassing variations in resolution and transceiver numbers are examined. Each case involves the central and edge region cell size adjustments ranging from coarse to fine, achieved through bias factor. The temperature reconstruction process is evaluated with three different intricate temperature profiles. In the present study, two inverse methodologies namely, pseudo-inverse and Tikhonov regularization are critically evaluated in the presence of 5% and 10% additive noise. Findings highlighted the decisive role of the bias factor in influencing the accuracy of the reconstructed temperature profiles while understanding the significant impact of discretization error. Across various noise conditions, instances where the transceiver count matches the number of domain cells exhibit exceptional performance when employing the pseudo-inverse approach. Conversely, the strategic application of Tikhonov regularization emerges as a potent strategy, resulting in a substantial reduction of reconstruction errors across multiple orders of magnitude. This reduction along with the role of bias factor not only paves the way for a judicious reduction in transceiver count without compromising resolution and overall reconstruction accuracy, but also offers a new dimension to the advancement of acoustic pyrometry.

Acoustic pyrometry for flow velocity estimation: preliminary analysis

Acoustic pyrometry uses the variations of the speed of sound in a medium to determine its temperature. One of the assumptions at the base of this technique is that the flow velocity component in the direction of the acoustic path is negligible in comparison to the speed of sound. However, if the temperature of the medium is known, the acoustic pyrometry approach can be used to determine flow speed, as long as the flow is not perpendicular to the acoustic path. The combined approach of temperature and velocity determination is also known as acoustic tomography. This technique allows the determination of velocity and temperature maps in a flow region. Even though this is an interesting result, the mathematical approach to solve the velocity and temperature fields might be very complex and sensitive measurement set-up. In this study, a preliminary investigation is carried out to determine the flow velocity by starting from the time of flight of an acoustic signal moving along different acoustic paths in a constant temperature field. For this purpose, a simplified geometry is considered. The developed mathematical approach at the base of the methodology is presented in this study.

Time-of-flight estimation in acoustic pyrometry: sensitivity to pulse characteristics

Acoustic pyrometry is a non-intrusive measurement technique that may have several applications in turbomachinery. This methodology estimates the gas temperature by measuring the time of flight of an acoustic wave moving through a medium. It can be accomplished by placing a sound source (emitter) and a set of microphones (receivers) on opposite sides of a section. The emitter generates a sound pulse, and the receivers detect it. Since the emitter-receiver distances are known and fixed, the average temperatures of the paths traversed by the acoustic pulse can be computed by estimating the time-of-flight through deconvolution techniques. However, despite the straightforward principle, an acoustic wave suffers a variation of amplitude when propagating within a medium because of energy losses and ambient noise. Hence, time-of-flight estimation becomes a critical task, especially when considering high-frequency waves or short distances between sensors. It is then fundamental to select proper acoustic waves to maximise the cross-correlation between the signals of the emitter-receiver couples, thus improving the accuracy of the time-of-flight measurements and, consequently, the estimation of the spatial temperature distribution within a specific area. This study is a preliminary investigation, based on a modelling approach, to estimate the impact of different acoustic waves on the accuracy of the time-of-flight measurement. The results of this analysis will be useful to design and setup an acoustic pyrometry application.

Sensor fusion of pyrometry and acoustic measurements for localized keyhole pore identification in laser powder bed fusion

In-situ process monitoring as an aid to part qualification for laser powder-bed fusion ( L -PBF) technology is a topic of increasing interest to the additive manufacturing community. In this work, airborne acoustic and inline pyrometry measurements were recorded simultaneously with the laser position. X-ray radiography imaging was used to spatio-temporally register keyhole pore locations to the pyrometry and acoustic signals, enabling binary labeling of data partitions based on pore formation. These labeled partitions were subsequently featurized using a highly comparative time-series analysis toolbox. Acoustic data was found to be much more effective than the pyrometry data for keyhole pore identification. However, when the information contained in both sensing modalities was combined in a sensor fusion strategy, the error rates of the top performing models were significantly reduced.

Evaluation of regularization methods for acoustic pyrometry

Measurement of temperature distribution is vital in boilers, heat exchangers, and other industrial applications. Acoustic pyrometry offers the advantage of measuring the temperature in the entire domain in a non-intrusive manner. Acoustic pyrometry involves estimating the temperature of the domain using the time of flight information between the transceivers. Since acoustic pyrometry is an inverse problem, it is sensitive to noise and the number of cells in the domain. Regularization methods help in obtaining feasible solutions. Therefore, the performance of four regularization methods, namely, Tikhonov, modified Tikhonov, Total variation (TV), and iterative reweighted least-squares (IRLS) in reconstructing three different temperature profiles, is studied and compared against the pseudo-inverse method. The effect of noise and the cell ratio (CR) on temperature reconstruction is also studied. Overall, the best results are observed for the coarsest cell ratio and modified Tikhonov with sharp filter regularization.

Influence of emitter-receiver number on measurement accuracy in acoustic pyrometry

Acoustic pyrometry is an interesting technique that may find several useful applications in turbomachinery. As the speed of sound is directly related a medium temperature, this measurement technique estimates the temperature of a gas by considering the time of flight of an acoustic wave moving through it. If only an acoustic emitter-receiver couple is used, only the average temperature along the acoustic path can be determined. If multiple emitter-receiver couples laying on the same plane are used, a reconstruction of the temperature map in the section is possible. This estimation is performed by considering that multiple acoustic paths travel across the same sub-portions of the section and, therefore, the temperature of each sub-portion affects the time of flight along several sound paths. Many parameters affect the accuracy of the measurement, and they are related to the physic of the phenomena involved in the measurement, the accuracy of the instrumentation used, the interaction between the acoustic wave and the flow velocity and the hardware set-up. In this study, the impact of some set-up parameters on the accuracy of the measurement was investigated and, in particular, the number of sound emitter-receiver couples and the number of investigation sub-portions in which the section is divided. A reference temperature map has been considered as a benchmark. This study, which is a preliminary investigation on this technique, was useful to assess the capability of this methodology to correctly describe a temperature distribution in an ideal condition. Therefore, it represents a first step in the set-up of an experimental investigation with an acoustic pyrometer..

Measurement of GT exhaust gas temperature by acoustic pyrometry: Preliminary error investigation

Acoustic pyrometry is an interesting technique that may find several useful applications in turbomachinery. It is well known that the speed of sound in a medium is directly related to its temperature. Acoustic pyrometry estimates the temperature of a gas by considering the time of flight of an acoustic wave moving through it. If one acoustic emitter-receiver couple is used, only the average temperature along the acoustic path can be determined. If multiple emitter-receiver couples laying on the same plane are used, a reconstruction of the temperature map in the section is possible. In this last case, the analysis is based on the fact that the temperature of each sub portion of the section affects the time of flight of all the acoustic paths travelling across it. Many parameters affect the accuracy of the measurement. They are mainly related to the physic of the sound propagation in a medium, the accuracy of the instrumentation used, the interaction between the acoustic wave and the flow velocity and the hardware set-up. In this study, the impact of the measurement set up of an acoustic pyrometry for the measurement of the exhaust gas temperature in a gas turbine was investigated to determine the optimal solution in terms of accuracy and robustness to uncertainties.

Temperature-field reconstruction algorithm based on reflected sigmoidal radial basis function and QR decomposition

Information on temperature-field distribution can reflect the combustion state within utility boiler furnaces, which is conducive for the formulation of control strategies to ensure safe boiler operation; therefore, it is significant for the realization of automatic boiler control. Ultrasonic thermometry is a noncontact acoustic pyrometry technique that utilizes the time-of-flight of ultrasonic waves on limited wave propagation paths to continuously reconstruct distributed temperature fields, and thus overcomes the shortcomings of traditional physical temperature measurement methods. To address the issue of low reconstruction accuracy of the temperature-field edges caused by traditional acoustic pyrometry algorithms and realize high-accuracy reconstruction of two-dimensional temperature fields, this study proposes a temperature-field reconstruction algorithm based on a combination of the reflected sigmoidal radial basis function and QR decomposition (referred to as the RSQR algorithm). The results of simulation experiments indicate that the accuracy of the reconstruction results obtained using the proposed algorithm increase by 2.03% on an average compared to the commonly used temperature-field reconstruction algorithms, with an increase of 1.88% in the reconstruction accuracy of the temperature-field edges.

A Brief Review of the Combustion Diagnosing Techniques for Coal-Fired Boilers of Power Plants in China

China has a large number of coal-fired power plants which are commonly suffered with many problems like uncertain coal source, frequent variation of coal quality, blended coal combustion and great fluctuation of the load. It is dire need of a system that can conduct combustion diagnosis, which can enables new horizon in optimization and to ensure the safety and efficiency of a unit. The contents of the combustion diagnosis include flame stability evaluation and temperature measurement. The latter provides quantitative information of the furnace, which can be used for unburnt carbon and pollutant emissions prediction. Diagnosing techniques are generally based on flame radiation intensity detect, flame image analysis, acoustic pyrometry and artificial intelligence analysis. The performance and engineering application of these techniques are reviewed and compared in this study. Artificial intelligence analysis based on the flame radiation intensity signal is recommended for flame stability evaluation as it is easily implemented in practical boilers with good accuracy and low cost. The challenge of the method based on flame image is thought to be that the reliability of the hardware system for flame image collection remains to be improved as they are easily affected by the high temperatures and fouling in engineering applications. The particles produced in the furnace is the hinder for application of the acoustic pyrometry in coal-fired boilers.

Online monitoring of furnace exit gas temperature in power plants

Furnace exit gas temperature (FEGT) is an important physical parameter of energy conservation and environmental protection in a coal-fired boiler furnace; however, to date, there is no effective method for monitoring it continuously online. The recently developed acoustic pyrometry is a comparatively advanced method of temperature measurement which possesses the essential characteristics of traditional temperature measurement approaches. We experimentally implemented online FEGT monitoring using acoustic pyrometry, and we could successfully replace the flue gas temperature probes successfully. Two key problems of the acoustic pyrometry technique, i.e. the acoustic signal and time delay estimation, were solved. We adopted the linear sweeping frequency signal as a sound source signal; its sweeping frequency ranged from 500 to 4000 Hz and the sweeping cycle was 0.1 s. We used a generalized cross-correlation technique with PHAT (Phase Transformation) and ML (Maximum Likelihood) as the time delay estimation method when the boiler was in different states. The actual operation of the monitor indicated that the acoustic pyrometry system was accurate and reliable.

A method for two-dimensional temperature field distribution reconstruction

The information of temperature field distribution is significant for industrial applications because it can reflect the internal running state of industrial equipment and assist to develop control strategy and ensure safety in operation of industrial equipment. Ultrasonic thermometry is a kind of acoustic pyrometry and it has been evolving as a new temperature measurement technology for various environment. The principle of ultrasonic thermometry is based on the dependence of ultrasonic sound velocity on temperature. In this paper, a reliable method for two-dimensional temperature field distribution reconstruction via ultrasonic thermometry is investigated. The results of actual experiment demonstrate that the proposed reconstruction method possesses a relatively better information reflection of temperature field distribution and higher reconstruction accuracy than common reconstruction algorithm, the least square method.

Study of ultrasonic thermometry based on ultrasonic time-of-flight measurement

Ultrasonic thermometry is a kind of acoustic pyrometry and it has been evolving as a new temperature measurement technology for various environment. However, the accurate measurement of the ultrasonic time-of-flight is the key for ultrasonic thermometry. In this paper, we study the ultrasonic thermometry technique based on ultrasonic time-of-flight measurement with a pair of ultrasonic transducers for transmitting and receiving signal. The ultrasonic transducers are installed in a single path which ultrasonic travels. In order to validate the performance of ultrasonic thermometry, we make a contrast about the absolute error between the measured temperature value and the practical one. With and without heater source, the experimental results indicate ultrasonic thermometry has high precision of temperature measurement.

A validation of computational fluid dynamics temperature distribution prediction in a pulverized coal boiler with acoustic temperature measurement

The main objective of this work was to examine the capability of CFD (Computational Fluid Dynamics) on properly predicting temperature distribution in the combustion chamber. Numerous approaches were employed to verify CFD models of large-scale utility boilers. Furnace Exit Gas Temperature is one of the key values used for verification studies. Harsh environment and large dimensions inside the furnace make temperature measurement a complex task. Traditionally used suction pyrometry provides only local information. With this technique, while extremely accurate, it is practically impossible to obtain a representative temperature distribution at the furnace exit as measurements in different locations are not taken at the same time. Acoustic Pyrometry technique is the most appropriate for comprehensive CFD flame shape prediction verification. Not only average temperature value in a certain boiler cross-section can be continuously measured but also its complete two-dimensional distribution. CFD code was used to simulate the OP-650 front-fired boiler operation. The boiler is equipped with Acoustic Gas Temperature Measuring system located in a horizontal plane approximately 4 m under the furnace exit. Comparison of simulation results with measurements proves good accuracy of CFD results.

Ash fouling monitoring based on acoustic pyrometry in boiler furnaces

Unexpected fouling of heat transfer surfaces has always been one of the main operational concerns in coal-fired utility boilers. A cost-effective way to address this difficulty is the continuous monitoring of fouling tendencies. This article has adopted acoustic pyrometry to monitor the changes of the flue gas temperature near the heat surface and has proposed a new clean factor, &, based on this acoustic method to monitor the ash fouling. A laboratory study and field research using a boiler were conducted. The results indicated that the acoustic system could be adopted to monitor temperature changes near the water-cooling wall in the boiler furnace. The new clean factor, &, could be used to monitor the changes of the ash fouling on the heat surface. This study may help to develop a more intelligent use of soot blowers, save power station boiler furnace energy and reduce discharge.

Monitoring ash fouling in power station boiler furnaces using acoustic pyrometry

Monitoring the real-time pollution status of the ash fouling produced by power station boiler furnaces and properly using the soot blower system of the boiler can significantly improve the safety and efficiency of such coal-fired boilers. This article examined the application of acoustic pyrometry close to the water-cooling wall by monitoring the flue gas temperature online and proposed a new clean factor. This article also studied the acoustic wave transmission rule near the water-cooling wall, and carried out experiments and research on a power station boiler furnace under a high-temperature operating environment. The studies showed that when boilers ran at a high temperature, the sound rays close to the water-cooling wall would bend toward the high temperature area inside the furnace due to the temperature gradient effect. The acoustically measured flue gas temperatures close to the wall were no longer a close approximation of the real flue gas temperatures of the water-cooling wall. After the sound rays were bent, the measured temperatures increased and would gradually increase as ash fouling began to accumulate on the wall. The clean factor ξ can be directly determined for real-time online monitoring of the condition of the local ash fouling at the water-cooling wall.

Application of Improved LMS Adaptive Algorithm in Acoustics Pyrometry

Acoustic pyrometry is a comparatively advanced method of temperature measurement developed in recent years, which possesses the essential characteristics of traditional temperature measurement approach. Considering the interferences, like strong background noise, reverberation and so on, in boiler furnace, the LMS (least mean square) adaptive filter algorithm should be improved to meet certain environment above. In order to make the LMS algorithm have the characteristic of fast convergence and small steady state error, an improved, power-normalized and variable step-size discrete cosine transform LMS algorithm is proposed, which combines the power-normalized discrete cosine transform LMS algorithm with the variable step size LMS algorithm that uses the sliding forgetting-weighted window. The time delay estimation simulation in the strong-noise environment verifies the improved DCT-MVSS LMS algorithm can achieve good performance.

Research on Flue Gas Temperature Monitoring in Acoustic Pyrometry of 200 MW Boiler Furnace

An online temperature monitoring system for flue gas based on acoustic pyrometry was developed and installed in a boiler of 200 MW. Hot-state experimental research of acoustic source signals included linear sweeping frequency signal and pseudo random binary sequence were tested, and the temperature was measured by K ceramic thermocouples which were specially made for verification. Research indicates that those signals can be used as sound source signals due to the treatment. The error of temperature in the measurement used acoustic pyrometry which is stable and reliable can be less than 1%, and it provides important consultation for this technology popularizing application.

Acoustic Pyrometry System for Environmental Protection in Power Plant Boilers

On-line monitoring of the distribution of the temperature field in a boiler furnace is important to reduce emissions that cause pollution and damage to the environment in power plants. Acoustic pyrometry, which is best suited for high temperature and dusty conditions, has been a very active research area recently. The present study provides a reconstruction technique of a two-dimensional temperature field through the measurements of multiple paths, which is used to monitor the combustion process in the boiler furnace. The geometric pixel division and the regularization algorithm were used to solve the reconstruction problem of temperature profile in many cases. Then, adopting this method, a temperature field monitoring system with multiple paths was developed and applied to a 330 MW unit (opposed wall firing). Through the long-term operations, it was proved that the acoustic pyrometer system could realize the visualization of temperature profile in the furnace.