Lift-off Distance Research Articles

Metal film thickness measurement using phase linearity feature for immunity to lift-off effect

In eddy current (EC) thickness measurements, the presence of the lift-off effect will significantly impact the measurement signal, leading to inaccuracies. It has been reported that the phase signal demonstrates a certain level of immunity to the lift-off effect compared to the amplitude signal, as its variation in response to lift-off is relatively less prominent. However, it should be noted that the phase is not entirely unaffected by the lift-off effect. In this paper, a linear relationship between the tangent of the phase angle and the lift-off has been found, termed the phase linear feature. Firstly, the phase linear feature was validated through theoretical derivations and experimental investigations. Subsequently, a new method of thickness measurement based on the fixed lift-off deviation was proposed by using this feature, and a corresponding double-coil sensor was also designed. Finally, to validate the proposed method, measurements of aluminum and copper films were carried out using the developed measurement system under different lift-off distances. The experimental results demonstrate that the measurement relative error of the proposed method remains within 5% in the 0–3 mm lift-off range. This indicates that the proposed method can provide accurate thickness measurements and is independent of the lift effect. Furthermore, the proposed method also has the advantages of single-frequency excitation, simplicity of the sensor, and straightforward relationship between features and thickness. Therefore, this study presents an effective solution for practical online thickness measurement.

Flame stabilization characteristics of turbulent hydrogen jet flame diluted by nitrogen

Hydrogen is already widely produced and used in the industry. NOx formation during the combustion of hydrogen can be reduced by adding nitrogen. Nonetheless, this comes at the cost of decreased flame stability, which constrains the applicability of hydrogen. Therefore, the flame stabilization characteristics of nitrogen-diluted hydrogen jet flame at varying dilution concentrations (10%–50%) were investigated using thin-lipped nozzles with diameters of 2, 3, and 5 mm. The experimental results show that the nitrogen-diluted jet flame becomes bluer and the flame height and width decrease with the dilution concentration. Due to the incomplete combustion at downstream of the diluted jet flame, the classical prediction model using the dimensionless flame Froude number (Frf) overpredicts the flame height of nitrogen-diluted hydrogen jet flame. The model, ρeSLhμe=50(ueSL)g(ρeρ∞), can effectively predict the flame lift-off distance of diluted hydrogen jet flames, where SL is the corresponding laminar combustion speed in the lean mixture zone (Φ = 0.91). A unified prediction model of blowout limit based on the Damköhler number for diluted hydrogen and hydrocarbon jet flames was proposed for the first time. This study helps to better understand the instability mechanism of diluted hydrogen.

Optimal design of transducers based on the Halbach arrays permanent magnet structure

ABSTRACT For enhancing the static magnetic field and reducing the size of torsional guided wave magnetostrictive patch transducer (MPT) with permanent magnets, a MPT with Halbach arrays is proposed. The structural parameters design of the MPT can be guided by finite element simulation. It is found that increasing the central angle of the radially magnetised magnets can increase the strength of the static magnetic field, and it can be adjusted through designing the radial size and lift-off distance of the magnets. The Halbach magnet arrays composed of circumferentially magnetised magnets with a circumferential size of 6 mm and radially magnetised magnets with a circumferential size of 4 mm are selected to experimentally compare with the alternating magnetised magnet arrays composed of magnets with a circumferential size of 10 mm. The results show that the static magnetic field strength can be increased by 54.7% using Halbach magnet arrays. The proposed MPT composed of the Halbach magnet arrays and a linear solenoid array can excite and receive T(0,1) guided wave. Compared with the alternating magnetised magnet array MPT, the guided wave signal amplitude is increased by about 70% generated and received by the MPT with the Halbach magnet arrays.

A thickness reduction testing method for ferromagnetic materials based on variable intensity DC magnetization

The assessments of internal thickness reduction of ferromagnetic components are extremely important. Traditional nondestructive testing (NDT) methods are usually non-linear and sometimes the electromagnetic parameters of components are required. In this paper, a thickness reduction testing method based on variable intensity DC magnetization (VIDC-MAG) is proposed. Both theoretical modeling analysis and simulation modeling analysis show that a pole point of magnetic permeability μm appears with the increase of DC magnetization intensity, which corresponds to an inflection point magnetizing current im. The im decreases with the increase of the defect depth of the wall thickness reduction and shows a linear relationship. The experimental results show that the method performs well for the inner wall thickness thinning and the relative error of the measurement results is small. Additionally, the measurement results are related to the width of the thickness reduction, the probe excitation frequency, the magnetic properties of ferromagnetic material and the lift-off distance of the magnetizer. The method is applicable to the thickness variation of the cover layer and does not need to know the electromagnetic parameters of the components in advance, which has greater practical application value.

Inductance-to-digital converters (LDC) based integrative multi-parameter eddy current testing sensors for NDT&E

Sensors’ signal conditioning and digital interface are important for sensor technologies, digital transformation, and the Internet of Things (IoT) in particular. Frequency output sensors and inductance-to-digital converters (LDC) offer distinct advantages over Analog-to-Digital Converter (ADC) interfaces. To miniaturize the eddy current testing system and achieve multi-parameter measurements, this paper reviews eddy current signal conditioning and digital interfaces in the field of displacement sensors and non-destructive tests & evaluation (NDT&E). After review and comparison of different signal conditioning and digital interfaces, the LDC chip is used in eddy current testing for multiple parameter measurements including lift-off distance and defect detection. An LDC-based integrative eddy current non-destructive testing method is proposed. The LDC chip provides AC to drive the LC resonator composed of the sensor coil and the parallel capacitor. By measuring the resonance frequency and parallel resistance (Rp) of the LC resonator, the simultaneous measurement of the lift-off distance and the defect is achieved. Based on the method, the testing experiments of two kinds of specimens made of aluminium and steel were carried out under different lift-offs. Experimental results show that both parameters of frequency and Rp of the LC resonator can intuitively reflect the defect depth and lift-off. Frequency- Rp curves can be used to quantify defect depth and lift-off distance and differentiate the material. This work also illustrates an integrated approach of signal conditioning and digital interface for multiple parameters or feature measurement for integrated eddy current NDT&E. The work opens a new way for addressing multiple parameter measurements of eddy current NDT & E including lift-off estimation and defect characterization via digital interfaces. It has the potential to bridge the gaps in metrology of geometry, displacement measurement, and NDT&E for defect detection and material characterization.

Research on non-destructive testing of stress in ferromagnetic components based on metal magnetic memory and the Barkhausen effect

In this paper, the relationship between the effective field of metal magnetic memory (MMM) emission and the structural stress of the ferromagnetic components is studied. The relationship between the voltage of the received magnetic Barkhausen noise (MBN) and the excitation source is also studied. Moreover, the characteristics of the emission spectrum of the MMM and MBN detection signals, and the influences of the lift-off distance on the stress detection accuracy of the two detection methods are studied. The advantages and disadvantages of the two magnetic non-destructive testing (NDT) techniques are analyzed separately, and their functions are combined to enhance the advantages and weaken the disadvantages. Modern signal analysis methods such as wavelet packet transform and signal feature extraction are introduced to process MMM signals, and a classifier for stress grading is established in combination with machine learning methods. On this basis, graded detection of the temperature stress of the continuous welded rail (CWR) was carried out. The detection accuracies of the 30–50 MPa, 50–70 MPa and 70–90 MPa stress grade ranges of CWR reach 71.43%, 82.76% and 75.00% respectively. The rapid grading detection of the temperature stress of the CWR and the rapid judgment of the structural stability of the CWR are realized based on the MBN-MMM detection technique.

Non-contact Ultrasonic Sonar-based Ranging Technique for In-motion 3D Railroad Tie Deflection Measurements

This paper presents an ultrasonic technique for non-contact surface deflection mapping of railroad ties using concepts of sonar-based ranging. The technique utilizes an array of capacitive ultrasonic transducers arranged along the length of the railroad tie at a lift-off distance of 3 in. from the rail surface to ensure contactless measurements. The transducer array is used in pulse-echo mode and distances from the transducers to the tie surface are measured by tracking the time-of-flight of the waves reflected from the tie surface. A reference-based cross-correlation operation is introduced to compute the time-of-flight, wherein one of the transducers is used as a reference for the distance measurements. The reference-based cross-correlation ensures accurate peak-detection for time-of-flight based differential distance measurements. An acoustic signal strength-based technique is utilized to differentiate between signals reflected from ties and those reflected from ballast. Laboratory scale tests were first performed as a proof-of-concept on a slender wooden beam to measure deflections in loaded and unloaded conditions. Dynamic tests were also performed on this beam to determine the ability to track time-varying positions. Field tests on a replica test track with wood ties were performed at the Rail Defect Testing Facility at University of California San Diego by mounting the prototype on a test-cart moved at walking speed. A dynamic assessment of the prototype was also performed to determine natural frequencies of the mounting beam that may be relevant for future uses at higher speeds. The results indicate the potential of this non-contact system to measure full-field tie deflections in 3D in-motion. This ability may potentially be used to detect conditions of poor tie support that may cause derailments.

Numerical Investigation on the Characteristics of a Novel Multi-Swirl Lean Direct Injection Burner With Large Eddy Simulation-Flamelet Generated Manifold Method

Abstract The lean direct injection (LDI) concept can eventually replace the current combustion systems in aviation engines because of its low NOx emissions without compromising other parameters. A novel multiswirl LDI burner with distributed fuel injection surrounded by air- flow through multiple hexagonal swirlers of vane angle 45 deg was developed. In this study, large eddy simulation (LES), using the dynamic Smagorinsky-Lilly subgrid model together with the flamelet generated manifold (FGM) combustion model, has been used to analyze the correlation between aerodynamics, mixing, and combustion characteristics of the burner. The agreement between the experimental data and the LES simulation was good, and it can accurately predict the trend of variables like velocity, temperature, and mixture fraction at different Reynolds number and equivalence ratios. The LES-FGM results demonstrated the rapid formation of a homogeneous fuel-air mixture over a short distance, which results in excellent flame stability, and it has ultralow NOx emission due to low flame temperature and negligible internal recirculation. It was discovered that going from a global equivalence ratio of 0.7 to 0.9 increases the flame liftoff distance because the high fuel volume flowrate causes the mixing of swirling air and fuel jet to become more and more delayed. The research in this paper provides an insight into the ability of the LES-FGM method to capture the complex flow field structures and the flame behavior globally of a novel low NOx multiswirl burner that might be a competitor in the combustors of commercial gas turbine engines.

Shaking Noise Exploration and Elimination for Detecting Local Flaws of Steel Wire Ropes Based on Magnetic Flux Leakages

Nowadays, steel wire rope (SWR) plays a more and more crucial role in modern facilities including but not limited to goods transmission, elevators, and dams. Due to the external environment, local flaws (LFs) on the SWR may cause deadly accidents. Thus, its health condition should be monitored. When applying the magnetic flux leakage (MFL) based nondestructive testing method to collect the LF signals in multichannel, the shaking noise inevitably exists, and greatly influences the accuracy of LF detection. This article focuses on analyzing shaking sources and the representation of shaking noise in MFL signals based on the lift-off distance. Then, after analyzing morphological features of shaking noise, strand noise, and LF signals, the morphological image processing based method is proposed to suppress shaking noise, especially when LF signals are covered and surrounded by strong shaking noise. In comparison with the state-of-the-art denoising method, this proposed method not only suppresses both strand noise and strong shaking noise but also improves the signal-to-noise ratio of MFL signals for better LF detection.

Improving Specific Absorption Rate Efficiency and Coil Robustness of Self-Decoupled Transmit/Receive Coils by Elevating Feed and Mode Conductors.

Self-decoupling technology was recently proposed for radio frequency (RF) coil array designs. Here, we propose a novel geometry to reduce the peak local specific absorption rate (SAR) and improve the robustness of the self-decoupled coil. We first demonstrate that B1 is determined by the arm conductors, while the maximum E-field and local SAR are determined by the feed conductor in a self-decoupled coil. Then, we investigate how the B1, E-field, local SAR, SAR efficiency, and coil robustness change with respect to different lift-off distances for feed and mode conductors. Next, the simulation of self-decoupled coils with optimal lift-off distances on a realistic human body is performed. Finally, self-decoupled coils with optimal lift-off distances are fabricated and tested on the workbench and MRI experiments. The peak 10 g-averaged SAR of the self-decoupled coil on the human body can be reduced by 34% by elevating the feed conductor. Less coil mismatching and less resonant frequency shift with respect to loadings were observed by elevating the mode conductor. Both the simulation and experimental results show that the coils with elevated conductors can preserve the high interelement isolation, B1+ efficiency, and SNR of the original self-decoupled coils.

Optimal design of electromagnetic metamaterial electronic device sensor with specific performance based on multivariate big data fusion

As the basic unit structure of electromagnetic metamaterial, the structure size of open resonant ring directly determines the constitutive parameters of SRR (dielectric constant ε and permeability μ). In order to improve the energy exchange efficiency of the electromagnetic acoustic transducer, the coil backplane of the electromagnetic ultrasonic shear wave sensor was optimized. First, the influence of the thickness of the coil backplane on the ultrasonic signal is studied by the experimental method, and then, the magnetic field distribution of the coil backplane is simulated by the finite element simulation software. Finally, the SNR and lift distance of the shear wave sensor before and after the coil backplane optimization are compared by experiments. In this article, a multi-hypothesis data fusion method in distributed detection system is proposed, which extends the multi-sensor data fusion rules to more general cases. The results show that the optimum thickness of the coil back plate in the electromagnetic ultrasonic transverse wave sensor is 1.5–2.0 mm. Using iron powder with the same length and width as the working area of the coil as the coil backplane can significantly increase the magnetic field strength in the working area of the sensor. Compared with the coil backplane made of non-magnetic materials, the optimized backplane can increase the signal-to-noise ratio of the sensor by about one time and the lift-off distance by about 1 mm.

Study on the Influence of Coil Placement on Pulsed Eddy Current Testing

The pipeline system of plant is installed with an insulation layer on the outer wall of the pipeline. Currently, the primary methods of detecting pipeline system is ultrasound and uradiographic testing, which require the removal of the outside layer before testing. As a result, detecting time extends, and labor costs increase, increase the risk of accidental exposure. Applying the PEC technique for steam pipelines can eliminate the disassembly of insulation and achieve non-stop online screening. The defects testing by coil placement is an essential indicator of the PEC technique, which is the main influence factor of PEC accuracy. The article intensifies the modeling and simulation of the pipelines by applying Ansys Maxwell and designs coaxial and vertical detection coils to simulate the detection capability of PEC on flat bottom defects in case of consistency in the lift-off distance, materials, and other conditions. It selects nuclear power plant sample pipes for coaxial and vertical PEC testing. Recording the test result of PEC and UT methods, and the effects of two coil placement methods on PECT are compared. Finally, the study proposes development trends.

Experimental Study on Geometrical Characteristics of a Square Turbulent Buoyant Jet Flame

This paper gives the experimental results for the buoyant jet diffusion flame in square burners. The flame height and lift-off were measured and discussed. The results show that the normalized flame height and lift-off height of square flames are lower than that of round flames with the same experimental conditions, which indicates enhanced air entrainment in square flames. The model of flame height based on the flame Froude-number, which integrates a correction factor to account for the enhancement of air entrainment, reasonably collapses the square flame data under different experimental cases. The critical Froude number for the transition from buoyancy to momentum controlled regime of a square jet flame is highly dependent on the fuel type and burner size. The lift-off height of the square jet flames increases linearly when the initial velocity increases, but also depends on the fuel type and burner size. A unified correlation using a dimensionless flow number derived from the mixedness-reactedness flamelet model is established, which reasonably predicts the lift-off distances of square jet flames.

In-line inspection technology of oil and gas long-distance pipeline based on alternating current field measurement

Abstract Alternating current field measurement (ACFM) has advantages of fast inspection speed, high efficiency, sensitive inspection, and easy quantification. In this study, a probe based on ACFM has been carried out, trying to inspect cracks on the inner wall of pipeline with high precision. An inspection tool which is working according to ACFM is developed in order to determine the stress corrosion cracking. The lift-off distance and excitation frequency parameters are optimized, and the influence of uneven magnetic permeability of the pipeline on the test results is analyzed. In addition, experiments are carried out to test the practicality of the tool. The minimum axial crack length that the probe can detect is 15 mm and the minimal depth is 1 mm. The research results show that the probe has the ability to quantitatively evaluate the axial cracks on the inner surface of the pipeline, which has certain guiding significance for the design and development of the AC electromagnetic field inspection probe.

Enhancing the Lift-Off Performance of EMATs by Applying an Fe3O4 Coating to a Test Specimen

Electromagnetic acoustic transducers (EMATs) are noncontact ultrasonic transducers. The transduction efficiency of a particular EMAT on a given specimen is dependent on the lift-off distance, which is the distance between the EMAT coil and the specimen surface. The transduction efficiency drops dramatically with increased lift-off distance, requiring EMATs to be in close proximity to the specimen, usually within a few millimeters. This article proposes a new EMAT method of applying an Fe3O4 coating to the test specimen and quantitatively studying the enhancement effect of Fe3O4 coating on lift-off distance. To eliminate the interference of the electrical and magnetic properties of the tested specimen, a nonmagnetic and nonconductive glass specimen is selected. The experimental results on a glass substrate coated with Fe3O4 demonstrate the feasibility of EMATs generating and receiving ultrasonic waves through the coating, by a magnetoelastic mechanism. The transduction efficiency of EMATs on an Fe3O4 coating does not increase linearly with the bias static magnetic field, and the maximum measured signal amplitude value occurs at a relatively low flux density of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\sim 0.12$ </tex-math></inline-formula> T. More specifically, it has been shown that the Fe3O4 coating can significantly enhance the lift-off distance of EMATs operating at 4 MHz to 8 mm on coated stainless steel. The performance of the Fe3O4 coating can be optimized, showing considerable potential to expand the application range of EMATs.

Measurement of Lift-Off Distance and Thickness of Nonmagnetic Metallic Plate Using Pulsed Eddy Current Testing

Eddy current testing (ECT) is an efficient method to estimate the thickness of sample but it is affected by other parameters information including electromagnetic properties and lift-off. In the previous researches, the lift-off point of intersection (LOI) feature has been found in pulsed eddy current (PEC), which can overcome the fluctuation of lift-off. In this paper, another feature, i.e. conductivity point of intersection(CPI), is proposed for non-coaxial T-R sensor in PEC. Specifically, the response signals of different sample conductivity almost intersect with each other at the certain time, which can be regarded immune to the conductivity of sample. The analytical solution and numerical solution are conducted, and the results indicate that the intersection point magnitude is mainly related with parameters of sensor and thickness of samples. Considering the elements related with conductivity in the Hessian matrix exists ill-conditioning, it will produce the magnification of measurement error under iterative inversion process. By referring to the CPI feature, the thickness of the sample and lift-off of sensor can be accurately inverted based on the modified Newton–Raphson method. The experiments are carried out to verify the proposed method, and the error is only 2.9%.

High-Precision Resistivity Measurement of Silicon Wafer Under Unstable Lift-Off Distance Using Inductive and Laser Sensors-Integrated Probe

High-Precision Resistivity Measurement of Silicon Wafer Under Unstable Lift-Off Distance Using Inductive and Laser Sensors-Integrated Probe

Flame morphologic characteristics of horizontally oriented jet fires impinging on a vertical plate: Experiments and theoretical analysis

Flame morphological characteristics of horizontal impinging jet fires are of practical importance in predicting and controlling the undesirable energy transfer to the nearby obstacle, which usually results in an escalating accident accompanied by severe consequences. But up to now, the relevant research is still very limited. The length of flame spread on the obstacle's surface for various nozzle diameters as well as the lift-off distance restricted by a vertical plane surface downstream the flame has not been quantified yet. In this work, the evolutions of flame spread length and lift-off distance of horizontal jet fires impinging on a vertical plate for different nozzle diameters at various energy (heat) release rates and nozzle-plate spacings have been quantified comprehensively. Experiments were conducted with three nozzles with different inner diameters of 2.0, 3.0 and 4.2 mm. The heat release rates of the fire source ranged between 5.7 and 32.2 kW, and the nozzle-plate spacings were varied from 0.20 to 0.40 m with a corresponding free condition. The results showed that the length of flame spread along the vertical plate increased with increasing the heat release rate and decreasing the nozzle-plate spacing for a given nozzle diameter. In addition, the large nozzle had a relatively greater increase in the flame spread length than the small one. A new correlation was proposed on physically the coupling effects of the flux ratio of buoyancy-induced air entrainment and jet momentum and the dimensionless heat release rate, showing good agreement with the experimental results. It was also found that the normalized lift-off distance under impingement can be correlated with the modified dimensionless flow number, and the correlation can well collapse all the lift-off data of this work. The present findings contribute to a better understanding of horizontal impinging jet fires, allowing predictions to be made regarding the possible threat and the establishment of the necessary safety distance for the pipeline to reduce the potential risk of such fire disasters.

A parametric investigation of methane jets in direct-injection compression-ignition conditions

This work investigated the effects of ambient and injection conditions on the autoignition, flame evolution and combustion characteristics of the natural gas jets at direct-injection (DI) compression-ignition (CI) conditions. Methane (CH4, as a natural gas surrogate) was directly injected into an optically accessible constant-volume combustion chamber (CVCC). For the experiments, the ambient gas density within the CVCC was fixed at 24kgm−3, but other ambient conditions were varied, including ambient gas temperature (1060–1200K) and ambient oxygen concentration (10–21 vol.%). The effects of injection pressure (10–20MPa reservoir pressure) were also assessed at a fixed ambient temperature and ambient oxygen conditions of 1200K and 21 vol.%, respectively. High-speed schlieren imaging, heat release rate analysis and flame luminosity measurement were applied to the CH4 jet flames. The results show that the ignition delay decreases with increasing ambient temperature or ambient oxygen concentration but does not display an apparent trend with changing injection pressure. Optical images reveal that the CH4 jet combustion typically starts from a localised kernel before propagating downstream of the jet volume in most cases, other than the lower ambient oxygen concentration case. The optical results also reveal that after ignition, the CH4 jet flame recesses back towards the nozzle before stabilising at a lift-off distance. The jet becomes more lifted with a decreasing ambient temperature, a reducing ambient oxygen concentration, and an increasing reservoir pressure. The results also reveal that the heat release rate and flame luminosity profiles roughly correspond to the ignition and combustion characteristics of the CH4 jet flames.

A Surface Crack Assessment Method Unaffected by Lift-Off Based on ACFM

Alternating current field measurement (ACFM) has been widely used to detect and evaluate metal surface cracks. However, the lift-off caused by various attachments significantly affects the accuracy of crack size assessment. In this article, a simple method is proposed to evaluate surface cracks without the limitation of lift-off. The theoretical models of ACFM and the lift-off effect are developed. The influence of lift-off on magnetic field response signals is studied with the aid of the finite-element method (FEM). The ACFM test system is set up, and the experiments are carried out to evaluate surface cracks of different dimensions at different lift-off distances. The test results show that the length and depth of surface cracks can be accurately evaluated at an unknown lift-off distance by the proposed method.