Lift-off Distance Research Articles

Development of differential hall sensors for Pulsed Eddy Current Testing using Gaussian pulse excitation

This study presents the use of differential Hall sensors in Pulsed Eddy Current Testing with Gaussian pulse excitation. This method was selected for its ability to effectively distribute energy across the frequency spectrum, thereby enhancing inspection depth and precision compared to the conventional rectangular pulses. The design of the differential Hall sensors, which includes two Hall sensors, greatly minimizes interference from the primary magnetic field, resulting in improved accuracy in detecting defects, particularly corrosion, across varying lift-off distances. 3D-FEM simulations and experimental results demonstrate that differential Hall sensors outperform traditional single Hall sensors in reducing noise and maintaining corrosion signal. Compared to traditional single Hall sensors, the differential Hall sensors exhibit a significantly improved signal-to-noise ratio, particularly at larger lift-off distances. Additionally, the study employs the Full Width at Half Maximum method to accurately estimate the size of corrosion, offering a precise and non-invasive approach to assessing structural integrity.

Towards Advancing Real-Time Railroad Inspection Using a Directional Eddy Current Probe

In the field of railroad safety, the effective detection of surface cracks is critical, necessitating reliable, high-speed, non-destructive testing (NDT) methods. This study introduces a hybrid Eddy Current Testing (ECT) probe, specifically engineered for railroad inspection, to address the common issue of “lift-off noise” due to varying distances between the probe and the test material. Unlike traditional ECT methods, this probe integrates transmit and differential receiver (Tx-dRx) coils, aiming to enhance detection sensitivity and minimise the lift-off impact. The study optimises ECT probes employing different transmitter coils, emphasising three main objectives: (a) quantitatively evaluating each probe using signal-to-noise ratio (SNR) and outlining a real-time data-processing algorithm based on SNR methodology; (b) exploring the frequency range proximal to the electrical resonance of the receiver coil; and (c) examining sensitivity variations across varying lift-off distances. The experimental outcomes indicate that the newly designed probe with a figure-8 shaped transmitter coil significantly improves sensitivity in detecting surface cracks on railroads. It achieves an impressive SNR exceeding 100 for defects with minimal dimensions of 1 mm in width and depth. The simulation results closely align with experimental findings, validating the investigation of the optimal operational frequency and lift-off distance for selected probe performance, which are determined to be 0.3 MHz and 1 mm, respectively. The realisation of this project would lead to notable advancements in enhancing railroad safety by improving the efficiency of crack detection.

Performance Investigation of Multi-Turn Pickup for Circular Fractal Flexible Planar Eddy Current Probe

ABSTRACT This paper aims to compare and analyze the performance differences between a single-coil and multi-coil pickup coil layout in a flexible circular fractal planar eddy current probe. Flexible planar eddy current probes offer flexibility and adaptability, making them widely used in the detection of complex structures. The circular fractal flexible planar eddy current probe is a new type of probe that has a lower miss rate in detecting cracks at specific angles compared to traditional circular and rectangular coil eddy current probes. However, the single-coil structure of the probe’s pickup coil has the drawback of being less sensitive to weak signals. This paper investigates the detection performance of the flexible circular fractal probe by varying the number of turns in the original pickup coil. Experimental comparisons were conducted by preparing and installing single-coil and multi-coil circular fractal planar eddy current probes. The performance differences between single-coil and multi-coil configurations were compared and analyzed under various excitation frequencies, liftoff distances, and internal crack depths. The results showed that, compared to the single-coil probe, the multi-coil probe exhibited significantly improved detection signal values and sensitivity.

Circumferential Crack Detection in Ultra-High-Pressure Tubular Reactors with Pulsed Eddy Current Testing.

Ultra-high-pressure tubular reactors are crucial pieces of equipment for polyethylene production. Long-term operation under high temperature, high pressure, and other extremely harsh conditions can lead to various defects, with circumferential cracks posing a major safety risk. Detecting cracks is challenging, particularly when they are under a protective layer of a certain thickness. This study designed a pulsed eddy current differential probe to detect circumferential cracks in ultra-high-pressure tubular reactors, with the lift-off distance acting as a protective layer. Detection models for traditional cylindrical and semi-circular excitation differential probes were established using finite element simulations. Corresponding experiments under different lift-off conditions were carried out, and the model's accuracy was verified by the consistency between the simulation results and experimental data. The distribution of the eddy current field under different conditions and the disturbances caused by cracks at various positions to the detection signal were then calculated in the simulations. The simulation results showed that the cracks significantly disturbed the eddy current field of the semi-circular excitation differential probe compared with that of the traditional cylindrical probe. The designed differential probe effectively detected circumferential cracks of specific lengths and depths using the difference in the voltage signals. The experimental results were in agreement with the simulation results, showing that the designed probe could effectively detect 20 mm-long circumferential cracks at a lift-off of 60 mm. The experimental results also show that the probe's detection coverage area in the axial direction varied with the lift-off height. The probe design and findings are valuable for detecting cracks in ultra-high-pressure tubular reactors with protective layers.

Nondestructive evaluation of debonding in composites using air-coupled coda wave analysis and local defect resonance techniques

Abstract Low acoustic energy conversion efficiency is a major challenge for air-coupled ultrasonic technology. In the determination of the lift-off distance of air-coupled sensors, there is a balance between the acoustic energy attenuation and the difficulty of extracting defect information. In this study, an air-coupled local defect resonance (LDR) technique with coda wave analysis is proposed for the nondestructive evaluation of debonding in composites. A sensor consisting of 19 elements was used to simultaneously excite and receive ultrasonic waves. Air-coupled LDR experiments were conducted on the two types of composite structures. The effects of sensor lift-off distance and coda wave analysis on the performance of the LDR technique were investigated. It was found that the sensor lift-off distance and the coda wave analysis had a significant effect on the defect detection capability of the LDR technique. For composites, the optimal sensor lift-off distance was found to be between 3.5λ and 5.5λ, where λ is the wavelength. Compared to multiple reflection echoes, the coda waves are more suitable for identifying the damage in composites. The proposed non-contact ultrasonic technique effectively reduces the required incident acoustic energy and can be used for efficient detection of debonding in composites.

MAPOD analysis in eddy current testing of flaws considering multiple response signals and multiple flaw parameters

The reliability of the eddy current testing (ECT) in flaw detection is quantitatively evaluated by the probability of detection (POD). Precise and efficient modelling of POD gives direction for the implement of ECT on sites to avoid false or missing flaw detection. Traditional POD analysis focuses on single uncertain factor or single response signal with limited credibility in engineering. This paper considers multiple response signals and multiple flaw parameters to perform POD. The flaw length, the flaw depth, the coil impedance and the magnetic flux density are comprehensively studied under various lift-off distances. A finite element model (FEM) of ECT is established and verified with experiments to obtain sufficient simulation data for discrete POD modelling. The continuous POD function is then fitted based on the discrete values to show the superiority of integrating multiple factors. A comparison with conventional POD analysis further demonstrates the higher reliability of ECT flaw detection considering multiple flaw parameters and multiple response signals, especially for small flaws. Conflict of Interest Statement The authors declare no conflicts of interest.

Underwater quantitative thickness mapping through marine growth for corrosion measurement using shear wave EMAT with high lift-off performance

Underwater inspection is important to ensure the safety, integrity and functionality of underwater structures. Although numerous conventional methods have been adopted for underwater inspection, successful application of most methods relies on the surface condition of the object, which, however, is typically covered by marine growth. Consequently, routine inspection requires thorough cleaning of marine growth, which is time-consuming and costly. Hence a method which can inspect objects without the need for extensive surface cleaning is necessary. Two methods have the potential to achieve this: pulse eddy current (PEC) and electromagnetic acoustic transducer (EMAT). PEC attains a significant lift-off distance, enabling inspection through marine growth. However, it suffers from high sensitivity to environmental conditions and low inspection accuracy due to ‘relative’ property which means its results are interpreted by comparing received signals to reference values. In contrast to PEC, EMAT provides ‘absolute’ measurements, ensuring precise results in the inspection. But it is limited by a small lift-off distance (<2∼3 mm), rendering it unsuitable for underwater applications with marine growth. Therefore, if the lift-off distance can be enhanced to a specific value, this method may offer a superior solution for underwater inspections. In this paper, a quantitative measurement method is proposed through employing a shear wave EMAT with high lift-off performance. A repelling configuration of magnets is introduced to achieve a significantly improved maximum effective lift-off distance of up to 5 mm in both air and seawater conditions with only 400 Vpp applied. This EMAT is then demonstrated to measure thickness through marine growth, showing excellent underwater performance in quantitative thickness mapping for corrosion inspection.

Effect of air distribution mode on jet flame and emission characteristics of high temperature gas-solid mixed fuel

The characteristics of the flame can not only intuitively reflect the flow, mixing and reaction processes of fuel and oxidant, but also provide detailed information on combustion efficiency and stability. The present study aims to delve into the jet flame characteristics and emission properties of high-temperature gas-solid mixed fuel. A comprehensive series of experiments were conducted on a custom-designed test platform. During the experiments, variations in both the position (h3 = 200, 600, 1000 mm) and velocity (v3 = 67, 138, 223 m/s) of the tertiary air were introduced. The study focused on analyzing the flame lift-off distance, the correlation coefficient of the flame root, the normalized root mean square temperature distribution within the flame body area, and the NOx emission characteristics across various air distribution scenarios. The experimental findings reveal that alterations in the position of tertiary air exert a greater influence on the flame lift-off distance than variations in its velocity. Furthermore, temporal fluctuations in the correlation coefficient of the flame root and the normalized root mean square temperature distribution within the flame body region serve as effective indicators of flame stability. Notably, an increase in both the tertiary air position and velocity leads to a notable reduction in NOx emissions. Additionally, through a quantitative analysis of flame stability, temperature distribution uniformity, and NOx emission characteristics, this study establishes a correlation between flame characteristics and pollutant emission profiles. This correlation offers a more comprehensive understanding of the combustion process involved in high-temperature gas-solid mixed fuel.

A lift-off suppression algorithm to improve the accuracy of ACFM in crack detection of ferromagnetic materials

Ferromagnetic pressurized components are widely used in petroleum, aerospace and nuclear industries, and their safety and stability are the key to the reliable operation of pressure equipment. In the process of crack detection of ferromagnetic materials by ACFM technology, the changes in lift-off distance caused by coating corrosion and irregularity of the surface will greatly affect the extraction of useful detection signals and reduce the detection accuracy. To overcome the drawback, a lift-off suppression algorithm for crack detection of ACFM is proposed. First, a differential strategy is proposed to discover the time-domain lift-off point of intersection (LOI) from the raw detection signals of ACFM for ferromagnetic materials. Then, the corresponding Bx and Bz signals are demodulated based on the amplitude of the LOI. The simulation and experimental results of the surface cracking of ferromagnetic materials by ACFM show that the novel Bx signals obtained according to the proposed method vary in a small range for different lift-off distances. The lift-off invariant property of the LOI enables the method to improve the interference of the lift-off effect on the conventional Bx signals and to enhance the detection accuracy of crack depth of ferromagnetic materials. The novel Bz signals perform consistently with the conventional Bz signals, and are virtually unaffected by the changes in lift-off distance.

Flexible anisotropic magnetoresistive sensors for novel magnetic flux leakage testing capabilities

Rigid magnetic field sensors such as anisotropic magnetoresistive (AMR), giant magnetoresistive (GMR) and Hall sensors have been used for years and have become industry standard for electromagnetic non-destructive testing (NDT). Recent technological developments in the field of flexible electronics allow for the fabrication of reshapeable magnetic field sensors on flexible substrates via thin-film deposition or printing. The magnetic properties of these sensors have comparable characteristics to industry-standard rigid magnetic field sensors, with the added ability of adapting to the surface of complex components and scanning in contact with the sample surface. This improves defect detectability and magnetic signal strength by minimizing the scanning lift-off (LO) distance. In this article flexible AMR sensors mounted on a rotative mechanical holder were used to scan a semi-circular ferromagnetic sample with 3 reference defects via magnetic flux leakage (MFL) testing, thus demonstrating the applicability of this type of sensors for the scanning of curved samples. In order to benchmark the performance of these sensors in comparison to industry standard rigid magnetic field sensors, a ferromagnetic sample with 10 reference defects of different depths was scanned employing flexible AMR and rigid GMR sensors. Defects with depths ranging from 110μm up to 2240μm were detected with an signal-to-noise ratio (SNR) of 2.7 up to 27.9 (for flexible AMR sensors) and 6.2 up to 72.3 (for rigid GMR sensors), respectively. A 2D magnetometer mapping of the sample with a spatial scanning step of 10×50μm2 (flexible AMR) and 16×100μm2 (rigid GMR) was obtained. The results show that this type of sensor can be used for high-resolution and high-detail mapping of defects on the surface of planar and non-planar ferromagnetic samples since the scanning lift-off distance is equal to the substrate thickness of 20μm for in-contact scanning. The SNR comparison between flexible and rigid sensors shows that the performance of the flexible AMR sensors employed is not very far behind the performance of the rigid GMR sensors used.

Lift-Off Effect of Koch and Circular Differential Pickup Eddy Current Probes

A flexible or planar eddy current probe with a differential structure can suppress the lift-off noise during the inspection of defects. However, the extent of the lift-off effect on differential probes, including different coil structures, varies. In this study, two planar eddy current probes with differential pickup structures and the same size, Koch and circular probes, were used to compare lift-off effects. The eddy current distributions of the probes perturbed by 0° and 90° cracks were obtained by finite element analysis. The analysis results show that the 90° crack can impede the eddy current induced by the Koch probe even further at relatively low lift-off distance. The peak-to-peak values of the signal output from the two probes were compared at different lift-off distances using finite element analysis and experimental methods. In addition, the effects of different frequencies on the lift-off were studied experimentally. The results show that the signal peak-to-peak value of the Koch probe for the inspection of cracks in 90° orientation is larger than that of the circular probe when the lift-off distance is smaller than 1.2 mm. In addition, the influence of the lift-off distance on the peak-to-peak signal value of the two probes was studied via normalization. This indicates that the influence becomes more evident with an increase in excitation frequency. This research discloses the lift-off effect of differential planar eddy current probes with different coil shapes and proves the detection merit of the Koch probe for 90° cracks at low lift-off distances.

On ammonia/diesel dual-fuel combustion in optical engine

Ammonia-diesel dual-fuel combustion mode is one of the potential ways to achieve lower carbon emissions in compression ignition engines. In this work, the effects of ammonia addition on diesel combustion characteristics were experimentally investigated in an optical CI engine. The gaseous ammonia is introduced to the engine through a port injection while diesel is directly injection into the cylinder. A color high-speed camera was utilized to detect and differentiate between diffusion burning and lean-premixed ammonia combustion. Additionally, computational models were conducted and the findings were coupled with the experimental data. According to optical investigations, the lift-off distance is created during the diesel spray and progressively grows as the ammonia energy ratio (AER) rises. The findings indicate that, in comparison to pure diesel combustion mode, combustion in an ammonia environment exhibits prolonged ignition delay duration and lesser flame luminosity. Secondly, a multi-injection strategy for diesel fuel was used where the diesel pre-injection timing (SOPI) values adjusted from −30 °CA ATDC to −60 °CA ATDC). Advancing SOPI is an effective approach to suppress diesel diffusion flame and reduce soot production in the cylinder. For advanced SOPI, findings reveal that rich pockets have decreased drastically as the combustion progresses. With the advancement of SOPI, there is a notable a connection between the flame speed and the peak heat release rate, where a high flame speed correlates to a greater heat release rate peak. Finally, the ratio of pre-injection of diesel fuel (ROPI) varied from 0 to 100% as a mean to improve flame propagation. There is a crucial threshold of pre-injection (ROPI) mass ratio of 45%, at which the initial flame features alter considerably. As the ROPI exceeds 45%, the flame kernel appears as a visible flame near to the main injection timing (SOMI), indicating that the ignition delay duration is decreasing.

Novel Coil Transducer Induced Thermoacoustic Detection of Rail Internal Defects Towards Intelligent Processing

A novel resonant tri-coil transducer is proposed to induce thermoacoustic (TA) signals for detecting rail internal defects. It consists of a ferrite plate-backed tri-coil and its associated circuits, a shielding layer, and a three-dimensional printed enclosure for supporting and securing. General TA detection theory is illustrated to understand how to detect rail internal defects. For achieving electromagnetic (EM) field shaping and focusing, the physical structure of the proposed transducer is optimized. It reduces backward EM fields and interference. Then, an equivalent circuit model between the proposed transducer and the rail is constructed to analyze optimal power transfer efficiency as a function of distance and frequency. To validate the design concept, the proposed transducer is fabricated for inducing TA signals, and experiments are performed. Several engineered rail specimens (UIC-60E) with different types of defects are prepared. The induced TA waves are found to be compressional. With selected features, a support vector machine algorithm-based multinomial classifier model is trained and employed for intelligent processing. Then, the proposed transducer is integrated into the self-designed automatic rail monitoring system. Experimental results show that the self-designed system can achieve an accuracy rate of 96.4%, successfully identifying defects in defect classification. The proposed transducer has low cost in fabrication and maintenance, focused magnetic fields with a large lift-off distance and enhanced acoustic wave intensity. Compared to earlier version of the system, the spacing between the transmitter and the rail has been further increased.

Characterization on multiphase microstructures of carbon steels using multi-frequency electromagnetic measurements

Phase composition is dominant in determining mechanical properties of carbon steels therefore is one of important microstructural elements that needs to be characterized and monitored in the steel production process. However, the characterization method usually used in steel production is off-line with destructive inspection. Microstructure characterization using electromagnetic signals can be applied in real time with in-line measurement that can meet requirements of steel continuous productions. The key is to establish the accurate electromagnetic responses to steel microstructure variations. This paper studied responses of electromagnetic signals on carbon steels with different phase compositions using a U-shaped and a cylindrical electromagnetic sensor. Relationships between steel microstructures and electromagnetic signals were established using the multi frequency electromagnetic system for steel samples with low to high carbon grades. Influences of phases, phase fractions, grain size and grain shapes on the relative permeability values were investigated. Results show that the low frequency inductance of electromagnetic signals can be used to distinguish the phase composition and the phase fraction of carbon steels. Effects of sensor lift-off distance and sample edge effects are also studied as requirements of industrial application.

A Novel Defect Detection Method for Overhead Ground Wire.

Overhead ground wires typically have strong axial tension and are prone to structural defects caused by corrosion and lightning strikes, which could lead to serious safety hazards. Therefore, it is important to detect defects accurately and quickly to avoid those problems. Existing defect detection methods for overhead ground wires are mainly traditional metal defect detection methods, including eddy current detection, ultrasonic detection, and manual visual inspection. However, those methods have problems of low detection efficiency, high environmental requirements, and insufficient reliability. To solve the above problems, this paper studies a novel type of defect detection technology for overhead ground wire. Firstly, the magnetic leakage characteristics around the defects of overhead ground wires are analyzed, and the defect detection device is designed. Then, the influence of air gap, lift-off distance, defect width, and cross-sectional loss rate on the magnetic flux leakage signal is studied, a novel defect detection method for overhead ground wire is proposed, and experimental verification is carried out. The results show that the proposed method can accurately locate and quantify the defect, which has the advantages of good reliability and high efficiency and lays the foundation for preventing accidents caused by defective overhead ground wires.

Metal Thickness Measurement System Based on a Double-Coil Eddy-Current Method With Characteristic Ratio Detection

Eddy-current sensor systems have been employed to determine the real-time thickness distribution of metal films and the thickness at endpoint detection in metal chemical mechanical polishing (CMP) processes, to achieve global planarization and avoid over-polishing or under-polishing. However, the large lift-off distance of 3.5 mm between the sensor coil and metal film gradually decreases with continuous wear of the polishing pad, which has a strong negative impact on the sensitivity and accuracy of the thickness measurement. In this study a double-coil eddy-current sensor system, with novel characteristic ratio detection capability, is proposed for metal film thickness measurement and compared with a traditional amplitude-based method. The linear relationship between the characteristic ratio and metal film thickness that is immune to lift-off distance is demonstrated both theoretically and experimentally. The experimental results show that the proposed method performs better than the amplitude-based method, reducing the effect of lift-off distance variation and obtaining an excellent linear measurement range from 24 to 2095 nm. The measurement accuracy is as high as 2.1 nm with an excitation frequency of 1.39 MHz and a large lift-off distance of 3.5 mm. The relative measurement error is less than 1.2% and the endpoint thickness ranges from 100 to 180 nm in the Cu-CMP process, when the lift-off distance varies from 3.6 to 3.3 mm.

Development of dual-main excitation mechanism modes electromagnet EMAT for testing ferromagnetic materials

ABSTRACT In the ultrasonic testing of the mechanical properties of ferromagnetic materials, the main excitation mechanism modes of ultrasonic waves are Lorentz force mechanism ultrasonic and magnetostriction mechanism ultrasonic. Lorentz force mechanism ultrasonic mainly detects the conductive characteristics of materials, while magnetostriction mechanism ultrasonic mainly detects the hysteresis characteristics of materials. However, conventional electromagnetic acoustic transducers (EMATs) cannot simultaneously characterise these two characteristics of ferromagnetic materials. In this study, we conclude from the finite element simulation that a quantitative lift-off distance (i.e. the distance between EMAT and the specimen) can change the main mechanism mode of the excited ultrasonic wave. Based on this conclusion, an electromagnet electromagnetic acoustic transducer with dual main excitation modes (DM-E-EMAT) is designed, which can realise the dual mode switching of the main Lorentz force mechanism and main magnetostriction mechanism by adding or removing hollow square gasket. The ultrasonic testing of ferromagnetic materials based on the two main mechanisms of DM-E-EMAT can simultaneously characterise the conductivity and magnetostriction properties of ferromagnetic materials, which enables the characterisations of two types of characteristic parameters with one transducer, improves the efficiency of electromagnetic ultrasonic non-destructive testing of the mechanical properties and reduces the detection errors of ferromagnetic material.

Experimental study on jet fire characteristics of hydrogen-blended natural gas

Blending hydrogen into natural gas can increase fuel reactivity and the risk of jet fires in case of pipeline leakage though it is an effective delivery method. In this work, horizontal jet fires of hydrogen-blended natural gas at various operating pressures and hydrogen content were investigated experimentally. The flame temperature, lift-off distance, and flame length were measured for the scenarios with 0%–50% hydrogen content (in volume fraction) and 200–800 Pa. Results show that with the increase of hydrogen content, the flame temperature rises while the lift-off distance and flame length decrease. The flame length is slightly affected when the hydrogen content is less than 10% and a maximum reduction in flame length is 13.7% as the blended hydrogen content increases to 50%. Further theoretical analysis suggested the dimensionless correlations of the temperature distribution along the jet axis, lift-off distance, and flame length with different hydrogen content, respectively. The research results may provide reference for the risk assessment of hydrogen-blended natural gas pipelines.

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.