Lift-off Distance Research Articles

MILD combustion of methane in a model combustor with an inverse-diffusion flame configuration

MILD (moderate or intense low-oxygen dilution) combustion is a promising technology for mitigating NOx emission from fuel combustion, but the combustion stability becomes an issue when using low-preheat air due to the slower reaction at the burner exit. In this study, an inverse-diffusion flame (IDF) burner configuration is proposed to enhance the combustion stability of MILD regime. The burner features an earlier preheat of fuel stream instead of usually adopted air stream, and can be also operated in traditional diffusion swirling combustion mode. Experiments are conducted in a model combustor to examine the effect of varying burner load (P) and equivalence ratio (φ) on flame topology, pollutant (CO and NO) emission performance, as well as flame emitting spectrum for this burner. It shows that increasing P can enhance the combustion stabilities of both swirling mode and MILD mode, and MILD combustion is found unable to be sustained when P is lower than 2 kW. Increasing φ enlarges the flame liftoff distance under swirling combustion, but has a much smaller effect on MILD combustion. Emission data shows a nearly 50 % reduction of NO by MILD combustion comparing to swirling combustion in all operating cases. Spectra analysis suggests a higher sensitivity of the flame response to the operating parameters under swirling combustion in respect to MILD combustion. A longer combustion chamber is found to produce a lower CO emission but has negligible effect on NO emission. Flow field and temperature distributions by further numerical simulation clearly demonstrate the effectiveness of fuel preheat by adopting IDF burner configuration for enhancing the flame stability when adopting normal temperature air in MILD combustion.

Experimental large-scale jet flames’ geometrical features extraction for risk management using infrared images and deep learning segmentation methods

Jet fires are relatively small and have the least severe effects among the diverse fire accidents that can occur in industrial plants; however, they are usually involved in a process known as the domino effect, that leads to more severe events, such as explosions or the initiation of another fire, making the analysis of such fires an important part of risk analysis. This research work explores the application of deep learning models in an alternative approach that uses the semantic segmentation of jet fires flames to extract the flame’s main geometrical attributes, relevant for fire risk assessments. A comparison is made between traditional image processing methods and some state-of-the-art deep learning models. It is found that the best approach is a deep learning architecture known as UNet, along with its two improvements, Attention UNet and UNet++. The models are then used to segment a group of vertical jet flames of varying pipe outlet diameters to extract their main geometrical characteristics. Attention UNet obtained the best general performance in the approximation of both height and area of the flames, while also showing a statistically significant difference between it and UNet++. UNet obtained the best overall performance for the approximation of the lift-off distances; however, there is not enough data to prove a statistically significant difference between Attention UNet and UNet++. The only instance where UNet++ outperformed the other models, was while obtaining the lift-off distances of the jet flames with 0.01275 m pipe outlet diameter. In general, the explored models show good agreement between the experimental and predicted values for relatively large turbulent propane jet flames, released in sonic and subsonic regimes; thus, making these radiation zones segmentation models, a suitable approach for different jet flame risk management scenarios.

A Review of Magnetic Flux Leakage Nondestructive Testing.

Magnetic flux leakage (MFL) testing is a widely used nondestructive testing (NDT) method for the inspection of ferromagnetic materials. This review paper presents the basic principles of MFL testing and summarizes the recent advances in MFL. An analytical expression for the leakage magnetic field based on the 3D magnetic dipole model is provided. Based on the model, the effects of defect size, defect orientation, and liftoff distance have been analyzed. Other influencing factors, such as magnetization strength, testing speed, surface roughness, and stress, have also been introduced. As the most important steps of MFL, the excitation method (a permanent magnet, DC, AC, pulsed) and sensing methods (Hall element, GMR, TMR, etc.), have been introduced in detail. Finally, the algorithms for the quantification of defects and the applications of MFL have been introduced.

Efficient Model Assisted Probability of Detection Estimations in Eddy Current NDT with ACA-SVD Based Forward Solver.

Model assisted probability of detection (MAPoD) is crucial for quantifying the inspection capability of a nondestructive testing (NDT) system which uses the coil or probe to sense the size and location of the cracks. Unfortunately, it may be computationally intensive for the simulation models. To improve the efficiency of the MAPoD, in this article, an efficient 3D eddy current nondestructive evaluation (ECNDE) forward solver is proposed to make estimations for PoD study. It is the first time that singular value decomposition (SVD) is used as the recompression technique to improve the overall performance of the adaptive cross approximation (ACA) algorithm-based boundary element method (BEM) ECNDE forward solver for implementation of PoD. Both the robustness and efficiency of the proposed solver are demonstrated and testified by comparing the predicted impedance variations of the coil with analytical, semi-analytical and experimental benchmarks. Calculation of PoD curves assisted by the proposed simulation model is performed on a finite thickness plate with a rectangular surface flaw. The features, which are the maximum impedance variations of the coil for various flaw lengths, are obtained entirely by the proposed model with selection of the liftoff distance as the uncertain parameter in a Gaussian distribution. The results show that the proposed ACA-SVD based BEM fast ECNDE forward solver is an excellent simulation model to make estimations for MAPoD study.

Experimental study of horizontally lifted high-speed nonpremixed flame jets

This paper presents an experimental study on horizontally jet flames with rather high velocities (58 m/s-167 m/s, Reynolds number up to 3.0 × 105), which mainly fall in the momentum-controlled region. Both high-speed schlieren and color imaging techniques have been adopted to obtain the time-resolved images, based on which the flame and flow dynamics has been discussed comprehensively. Under the test conditions, all the flames have shown lift-off characteristics. The dynamic mode decomposition (DMD) and proper orthogonal decomposition (POD) analysis has been performed to reveal the flame and flow fluctuating frequency and the corresponding spatial distribution. The results have shown two high frequency modes at 269 Hz and 537 Hz in the lifted-off jet region, which may come from the test bench vibration. The statistics of flame length, height and lift-off distance have been presented, which have been compared with previously reported data at lower velocities. The comparison indicates that the flame behaviors have shown quite dramatic differences in the buoyancy and momentum-controlled regions. The present experiments have extended the research of horizontally placed jet flames to a wider velocity range, which has greatly broadened our knowledge of the lift-off jet flame behaviors under high velocity conditions.

Pulverized biomass flame under imposed acoustic oscillations: Flame morphology and emission characteristics

Forced intermittent combustion with periodical variations of pressure, velocity, and air-fuel ratios is a promising method to increase efficiency and reduce emissions from combustion and gasification applications. In this work, flame characteristics and emissions from a pulverized biomass burner are investigated under oscillations induced by an acoustically-driven synthetic jet. Instantaneous images of incandescent light emitted from flame were captured using high-speed cameras. The images were analyzed to identify the liftoff distance, flame length, and shape. The flame liftoff distance decreased under excited conditions, notably at high forcing amplitude applied to small particle size distribution (63-112 μm). In such conditions, acoustic forcing increases particle dispersion as presented in the previous work, providing conditions for earlier ignition due to enhanced fuel-air mixing besides reducing CO emissions. Flue gas emissions were influenced mainly by the particle size distribution, from which the 63-112 μm particle size presented the lowest values of CO and highest levels of NO emissions. The results presented stable flame edge positions for the particle size of 63-112 μm, while wide range particle distributions (0–600, 0-400 μm) had strong fluctuations, indicating high flame instability. The experimental work adds new insights regarding acoustic excitation in swirl burners, which could be used to optimize pulverized fuel combustion.

High-Precision Thickness Measurement of Cu Film on Si-Based Wafer Using Erasable Printed Eddy Current Coil and High-Sensitivity Associated Circuit Techniques

The eddy current coil is widely adopted to measure the thickness of copper (Cu) film on the silicon (Si) based wafer. However, the lift-off distance (LOD) variation between the coil and Cu film has a strong negative effect on the accuracy and repetitiveness of the thickness measurement. Therefore, it is necessary to eliminate the impact of LOD variation. In this article, a high-precision thickness measurement method is proposed to determine the thickness of Cu film on the Si-based wafer by using the techniques of the erasable printed eddy current coil (EPECC) and the high-sensitivity associated circuit. The theoretical feasibility and sensitivity improvement of the conceptual design are analyzed in detail. As a proof of concept, a prototype of the proposed method is fabricated. The EPECC is prepared on the back surface of the wafer, and the LOD is unvaried during the measurement. Experimental results show that comparing with the traditional eddy current coil, the measurement stability, precision, and accuracy of the Cu film thickness by the proposed method are significantly improved even under severe vibration conditions. The relative error is 2% in repeated measurements.

Numerical Simulations of a Lifted Hydrogen Jet Flame Using Flamelet Generated Manifold Approach

Abstract A turbulent lifted H2/N2 jet flame in a vitiating coflow environment is numerically investigated, using the Flamelet generated manifold (FGM) combustion model with large eddy simulations (LES). Due to the hot vitiated H2/air coflow, the primary stabilization mechanism is the auto-ignition followed by a premixed flame. In addition to using H2 as a fuel, this flame poses two other modeling challenges: (i) the auto-ignition, which is a transient chemistry-driven phenomenon; (ii) the existence of multiple combustion regimes, e.g., diffusion at auto-ignition location but premixed in the postflame. A series of LES/FGM simulations are completed in this work by reducing the coflow temperature from 1045 K to 1000 K. The FGM model can predict the characteristics of the flame by showing a lifted flame. It also accurately predicts the trend in the flame lift-off distance with a change in the coflow temperature. The current results are compared for mixture fraction, temperature, and OH mass fraction at multiple locations, which have also been correctly captured. It is noted that for a high coflow temperature (and hence a low lift-off distance), the flame's lift-off is highly sensitive to the inlet boundary conditions and the mesh resolution near the jet entry. A relatively coarse mesh is used for all the simulations, which is generated using a careful strategy that not only resolves the jet instabilities near the fuel inlet but also keeps the overall mesh count low and allows for a large computational time step. A systematic sensitivity analysis of the computational speed is also performed. This work provides some useful guidelines for simulating the H2 diluted flames using the FGM model, which may be valuable to the gas turbine industry.

Lattice-Boltzmann modeling of lifted hydrogen jet flames: A new model for hazardous ignition prediction

This numerical study deals with the hazardous ignition of a jet flame in a vitiated co-flow. A novel formulation, based on a passive scalar variable, will be presented to predict hydrogen auto-ignition events. The model, derived from the theoretical analysis of the Jacobian, correctly describes the appearance and absence of auto-ignition in complex configurations based on initial thermodynamic and mixture conditions. No chemical reaction and species equations are required to perform the simulations. Results of Lattice Boltzmann Methods (LBM) simulations of a 3D H2/N2 Cabra flame will be presented using a detailed H2-Air mechanism. Validation against experimental and numerical results will be provided for the lift-off (distance to auto-ignition). The passive scalar predictions are successfully compared with the reactive simulations. The results show a potential extension of this model to an extensive spectrum of hydrogen safety and large-scale turbulent combustion applications.

Weak Signal Processing Methods Based on Improved HHT and Filtering Techniques for Steel Wire Rope

As one of the most important processes in steel wire rope inspection, defect signal processing is of great significance in guaranteeing safety and precision measurement. Aiming at the weak signal detection of steel wire rope with mixed strands and noise, a combined signal processing method based on magnetic flux leakage testing and multi-step filtering techniques are proposed in this paper. The experiments are first introduced and performed on three typical types of steel wire rope with diameters of 28 mm, 32 mm, 45 mm, and different wire broken defects detected under liftoff distances of 13 mm and 20 mm; the acquired signals are then analyzed both in time and frequency domain. According to the weak signal characterizations, the principle of the proposed methods and algorithm are given concretely. Afterwards, comparison of signal processing results between the traditional lowpass filtering, wavelet denoising, median filtering, and the proposed method are presented. Finally, the influence factors of smoothing types and moving average span of the proposed methods are investigated. The processing results of the proposed methods are shown through short-time Fourier transform and signal-to-noise ratio analysis, which not only demonstrates the validity and feasibility of the combined methods with the highest signal to noise ratio of 90.37 dB, but also exhibits a great potential of precision defect detection and practical application in steel wire rope inspection.

A method to compensate for the lift off effect of ACFM in crack estimation of nonferromagnetic metals

Alternating current field measurement (ACFM) is one of the most important techniques for detecting metal cracks. During measurement, however, the changes in lift-off distance caused by coating corrosion and irregularity of the surface greatly affect the accuracy of crack sizing. To overcome this drawback, an advanced algorithm is proposed in this paper to compensate for the lift-off effect. Benefitting from this method, the dimensions of cracks can be inversed directly without considering the lift-off distance. More importantly, the algorithm has been verified efficiently as estimating the cracks of an aluminum workpiece. The results show that the sizing errors based on this algorithm can be limited to 10%.

Effects of spacing on flaming and smoldering firebrands in wildland–urban interface fires

Firebrand (ember) attack has been shown to be one of the key mechanisms of wildfire spread into wildland–urban interface communities. After the firebrands land on a substrate material, the ignition propensity of the material depends on not only the attributes (e.g. shape, size, and numbers) but also the distribution of the firebrands. To help characterize this process, this study aims to investigate the effects of gap spacing on the burning behaviors of a group of wooden samples. Experiments are conducted using nine wooden cubes, 19 mm on each side. These samples are arranged in a 3 × 3 square pattern on suspension wires and are ignited by hot coils from the bottom surface. The gap spacing (s) between the samples varies in each test (ranging from 0 to 30 mm). After ignition, the samples are left to burn to completion. The burning process is recorded using video cameras. Sample mass loss and temperatures are monitored during the flaming and smoldering processes. The results show that the flame height and the sample mass loss rate have non-monotonic dependencies on the gap spacing. When the gap spacing reduces, the flame height and the mass loss rate first increase due to enhanced heat input from the adjacent flames to each sample. When s ≤ 10 mm, flames from individual samples are observed to merge into a single large fire. As s further decreases, the air entrainment at the flame bottom decreases and the flame lift-off distance at the flame center increases, resulting in an increased flame height, decreased flame heat feedback to the solid samples, and a decreased mass loss rate. The decreased mass loss rate eventually leads to a decrease in the flame height as well. The gaseous flame height is correlated to the solid burning rate. The correlation generally follows previous empirical equations for continuous fire sources. For the smoldering combustion, compared to a single burning sample, the smoldering temperature and duration significantly increase due to the thermal interactions between adjacent burning samples. To help interpret the results of the burning experiments, thermogravimetric analysis is also performed in air and nitrogen, resulting in heating rates ranging from 10 to 100 K/min.

Instantaneous lift-off distance estimation in magnetic flux leakage testing of steel wire ropes

Abstract Steel wire rope (SWR) defects plagued its application in many important fields, such as cranes, ports, etc. However, the accuracy of SWRs’ local flaws detection based on magnetic flux leakage (MFL) method is susceptible to the lift-off distance. This paper proposes an instantaneous lift-off distance estimation method to study the effect of lift-off distance on MFL detection for SWR. We firstly derive the relation between the lift-off distance and the MFL field strength based on the magnetic dipole model. After obtaining the time-frequency representation of the MFL signal by using multi-synchrosqueezing transform, we extract the instantaneous amplitude (IA) of the strand signal without other noise interference. Finally, the nonlinear least-squares method is used to fit the relation between the IAs of the strand signals and the lift-off distances to achieve the estimation of the instantaneous lift-off distance. A SWR with three local flaws is tested to validate the proposed method. This work provides instantaneous monitoring of lift-off distances, and it can be used in future noise suppressing methods development or quantitative analysis of local flaws.

Inspection of Defect Under Thick Insulation Based on Magnetic Imaging With TMR Array Sensors

A lot of industrial structures are covered by insulation. Defects under insulation (DUI) frequently distress the structure maintenance and may lead to serious safety concerns. In this article, a probe with tunneling magnetoresistance (TMR) array sensors is proposed to inspect defects under thick insulation. The probe contains 64 TMR sensors with the size of each sensor length <inline-formula> <tex-math notation="LaTeX">$\times $ </tex-math></inline-formula> width &#x003D; 0.45 mm <inline-formula> <tex-math notation="LaTeX">$\times0.45$ </tex-math></inline-formula> mm, which can generate a magnetic field image with high spatial resolution. Excitation current with extremely low frequency (in the order of a few hundred Hertz) is utilized for inspection of defects with large liftoff distances. In this low-frequency range, the TMR sensors have much better sensitivity than conventional induction coils. A data processing method based on discrete wavelet transform (DWT) is utilized to reduce noise and highlight defect indication. Experimental results demonstrate that the probe is capable of detecting a machined rectangular defect with dimensions of 6 mm (length), 3 mm (width), and 1 mm (depth) under 10 mm thick insulation. The minimum dimensions of the machined defect under 20 mm insulation that can be detected by the probe are 9 mm (length), 3 mm (width), and 1 mm (depth). As a comparison, for a commercial pulsed eddy current probe (PECA-HR-SM), the length of minimum detectable detect is 19 mm under 18 mm thick insulation. If the thickness of the insulation is 30 mm, a machined rectangular defect with dimensions 21 mm (length), 3 mm (width), and 7 mm (depth) can be detected by the proposed probe. The experimental results demonstrate that the proposed probe is a promising alternation with excellent sensitivity for inspection of defects with large liftoff distances.

Edge Effects of an Eddy-Current Thickness Sensor During Chemical Mechanical Polishing

Accurate and in-situ thickness measurements of copper films on Si-based wafers are employed to improve global planarization, adjust polishing pressure, and avoid over-polishing and under-polishing during chemical mechanical polishing (CMP). However, edge effects cause inaccurate thickness measurements at the wafer edge, and greatly decrease the effective area of integrated circuit (IC) manufacturing. In this study, edge effects were studied using both a trans-dimensional simulation model of electromagnetic fields and circuit coupling in eddy-current thickness measurement and an experimental nanoscale thickness-measurement sensor system with a resolution of 2.07 &#x00C5; /mV. The simulation and experimental results were in close agreement when the sensor coil was located at different edge positions. The influence of different factors such as copper film thickness, edge positions, calibrated thickness, lift-off distance, and excitation voltages on thickness-measurement performance in the edge region were also examined. Copper film surface current density was calculated to clearly evaluate eddy-current distortion at different edge positions. The determined influences of these parameters on edge effects could be used to improve the edge-measurement performance of sensor systems by adjusting coil and circuit parameters or propose an edge signal compensation algorithm during CMP.

Thickness Measurement Based on Eddy Current Sensor With Coaxial Double Coil for Chemical Mechanical Polishing

Real-time and accurate thickness measurements of copper films on 300-mm silicon wafers are utilized to improve global planarization, control polishing pressure, and avoid dishing and erosion caused by under-polishing and over-polishing during chemical mechanical polishing (CMP) of metals. Nevertheless, the requirements of nanometer-scale measurement sensitivity, wide measurement range from tens of nanometers to 2 μm, and high edge measurement performance make eddy-current thickness measurement extremely challenging. Therefore, we proposed a concise and effective theoretical model to analyze the measurement range and sensitivity in both the amplitude- and phase-based methods. The advantages and disadvantages of the two methods are compared and discussed systematically through simulations and experiments. The effects of the lift-off distance and frequency of excitation signal on measurement sensitivity, range, and edge measurement performance are also examined. The results show that the proposed theory can be used to improve measurement range and sensitivity. And the double-sensor system demonstrated better edge measurement performance than a single coil. Finally, a double-coil eddy current thickness measurement system designed with an average sensitivity of 1.9 mV/nm, a measurement range of 60 nm to 2 μm, and an edge attenuation position of 141.5 mm at a lift-off distance of 3.5 mm can be utilized in the amplitude-based method for advanced Cu-CMP.

Physics-Based Inversion of a Millimeter-Wave Diagnostic for Liquid Metal Additive Manufacturing

This article presents a physics-based inversion technique for reliable characterization of droplets deposited by a liquid metal jetting additive manufacturing system, using an in-situ millimeter wave diagnostic. The overall approach is demonstrated in a custom droplet-on-demand liquid metal jetting system with effective droplet diameters ranging from 0.8 to 1.7 mm. The inversion results demonstrate that this approach can provide information about the droplet size and lift-off distance between the droplet and the diagnostic by monitoring the amplitude and phase of the reflection coefficient at the input port.

Accurate Thickness Measurement Using Eddy Current System Based on Novel Transformer Model

Eddy current testing has been widely regarded as a popular tool for thickness measurement due to the advantages of non-contact, low cost and efficiency. Phase signature is believed to change linearly as a function of specimen thickness using the conventional transformer model, which, however, does not take into account the effect of eddy current diffusion and reflection. In this work, a novel transformer model is developed, and an accurate method for thickness measurement is proposed subsequently. Firstly, true skin depth is formulated with a simple but accurate function, and an equivalent thickness is introduced to characterize eddy currents considering the difference between the standard and true skin depth. Secondly, the use of the thickness of equivalent eddy currents develops the novel transformer model, and a novel method is figured out to improve the accuracy of thickness measurement. Basically, the difference between the true and standard skin depth is closely related to excitation frequency, liftoff distance and the electromagnetic properties of a specimen. Therefore, a correction parameter is needed and it should be re-determined for different cases. Finally, an eddy current system was built, and the experiments were carried out to evaluate the novel method. The results show that it outputs more accurate specimen thickness, especially for low conductive materials.

In-line detection of defects in steel pipes using flexible GMR sensor array

Steel pipes serve as the main source for transporting water, gas, and other petrochemical substances for longer distances. These pipes were able to withstand extreme weather conditions and hostile environments because of their remarkable properties such as higher strength, durability, lower cost, and improved wear and corrosion resistance. However, prolonged usage of these pipes in such environments may lead to the initiation of defects in their inner surface such as leak holes, cracks, corrosion, etc. In overtime, these defects may become more severe, resulting in component failure and property losses. Hence, earlier detection of defects is highly recommended to avoid these failures. In this work, an in-line robot system has been proposed for detecting the defects in the steel pipes. This robot utilizes a non-destructive way for evaluating the flaws by means of the magnetic flux leakage (MFL) technique. A 3D finite element model has been developed with the aid of ANSYS Maxwell 3D software for evaluating the generated magnetic field and optimizing the lift-off distance. The permanent magnet is preferred as the magnetizing material for implementing local magnetization in the inspection area. The magnetic flux leakage from the defect region is sensed by using a flexible GMR sensor array of six sensors. Artificial defects were introduced in a 6-inch diameter steel pipe in various shapes and the Arduino UNO controls the overall process. The data from the sensor array were collected using the Arduino and plotted as the waveform graph. From this graph, the voltage variations among the sensors represent the defect region. In addition, the higher peak in amplitude denotes that the flux is influenced by the defect’s depth. Thus, the waveform graph for the introduced defects was analyzed and all graph represents a better signal to noise ratio (SNR) for identifying the defects.

Laser ignition of iso-octane and n-heptane jets under compression-ignition conditions

This work aims to investigate the effect of laser-induced plasma ignition (LI) on combustion behaviours of iso-octane (a gasoline surrogate) at compression-ignition (CI) conditions. A high-energy laser was used to force the fuel ignition at a quiescent-steady environment inside an optically accessible constant-volume combustion chamber with 900K ambient gas temperature, 22.8kg/m3 ambient gas density and 21 vol.% O2 concentration. A diesel surrogate (n-heptane) was tested at a lower charge temperature of 735K to offset its higher fuel reactivity than the iso-octane, such that the flames of both fuels can have a similar lift-off length. Forced laser ignition was introduced either before or after the natural autoignition timing of the fuels. The laser was focused at the jet axis 15mm and 30mm from the nozzle. High-speed schlieren imaging, heat release analysis and flame luminosity measurement were applied to the flames. The high-speed schlieren imaging was used to monitor the flame structure evolution of the natural ignition and LI cases. Due to laser ignition, the flame lift-off lengths decrease, with which the uncertainties in the lift-off distances reduce by more than 80%. The laser-affected flame bases return back to the natural flame base locations. The uncertainties in the lift-off lengths also increase, as the flame stabilisation locations approach the natural lift-off distances. Under the test conditions of this work, the rates at which the iso-octane flames shift downstream are slower than the n-heptane cases. The heat release rate profiles show high heat release from the flames following the LI events, before transitioning to lower steady values. The flame luminosity measurements indicate a strong correlation between the LI affected lift-off length and increased soot formation. The luminosity levels decrease as the flame base shifts downstream over time.