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• Extraction of distant and soft defect time–frequency distribution surrounded by interference. • Instantaneous filtering may help detect extremely weak impedance discontinuities. • The proposed method has better location accuracy compared with traditional Wigner-Ville distribution methods. Time-frequency domain reflectometry is a highly sensitive method to locate cable defect. However, locating a potential defect far from the cable head may be less accurate because of the signal amplitude attenuation and the interference parts. To solve the problem, this paper proposes an accuracy improvement method based on instantaneous filtering in time-frequency domain. The method extracts the frequency spectrum of the time-frequency signal, and then filters the spectrum and reconstructs the signal based on the difference between the signal and interference spectra, which can eliminate the interference and improve the location accuracy. In simulation, the location error of this method improves from 5.47 % to 0.57 %. In experiment, the proposed method reduces the location errors and magnifies the location peaks of the breakage defect, thermal aging defect, and ground faults. For the defects studied in this paper, the method improves the defect location accuracy and defect amplitude detectability of TFDR. [ABSTRACT FROM AUTHOR]

• Novel rapid moisture test for oil-paper insulation. • Microstrip ring resonator analyzes dielectrics properties. • Precise moisture level distinction found. • New model assesses the moisture content accurately. Moisture ingress significantly accelerates the aging of oil-paper insulated equipment, making accurate assessment of moisture content critical for the operation and maintenance of oil-immersed insulated power equipment. In this paper, a novel method for rapid moisture content evaluation is proposed based on a microstrip ring resonator (MRR) designed with a fundamental frequency of 1.16 GHz. A calculation model for the dielectric response analysis using MRR is established to characterize dielectric properties of the material layered on the substrate. ANSYS HFSS electromagnetic simulation software was used to study the impact of dielectric material thickness and the substrate on the MRR test results. Experimental measurement demonstrated that the maximum deviation in resonance frequency was merely 0.542 %. The dielectric properties of uniformly and non-uniformly damp insulating pressboard were analyzed across the frequency band from 1 to 6 GHz. The MRR test results were compared with those of the standard resonant cavity method and frequency domain dielectric spectroscopy (FDS) test. Using the regression algorithm and curve analysis, moisture assessment models for both uniformly and non-uniformly damp oil-paper insulation were established. The experimental results show that the MRR test can clearly distinguish between the pressboard samples with varying moisture levels under uniform conditions, with a maximum moisture evaluation error of 8.19 %. The results demonstrate that the MRR test method, requiring only 56 s, can quickly and effectively evaluate the moisture content of insulating pressboard. [ABSTRACT FROM AUTHOR]

• A data fusion approach with high spatiotemporal resolution for wall temperature measurement is reported. • This approach is tested at different number of thermocouples and the IR camera sampling rate. • This approach is validated against different experimental conditions. A high spatial and temporal resolution of wall temperature measurement is beneficial for liquid film cooling design for thermal protection of rocket engine combustor chamber. The conventional thermocouple method has a high sampling frequency up to 50 kHz, but it is based on point measurement. The infrared (IR) thermal technique can achieve a two-dimensional surface temperature measurement, but the current sampling frequency is limited to a few hundreds of Hz. Here we report the application of the data fusion approach, which is integrated with proper orthogonal decomposition (POD) and compressed sensing (CS) to reconstruct the temperature field of a wall with high spatiotemporal resolution. In this approach, the low-frequency IR thermal camera data was analyzed by POD to acquire the POD modes. The high-frequency thermocouple data with the POD modes were utilized to get the high-frequency mode coefficients by CS. Subsequently, the temperature information reconstruction was completed with POD modes and the mode coefficients. The data fusion method was applied to reconstruct the 250 × 400 pixels temperature field with a frequency of 100 Hz. Using 10 thermocouples and the IR camera with sampling frequency f = 2 Hz, the root mean square error (RMSE) of the reconstructed temperature field is 1.11%, which shows the reliability of the present approach. The less thermocouple and the lower IR camera sampling frequency leads to poorer reconstruction quality, though the effect of IR camera sampling frequency is less significant than the number of thermocouples. The optimized sensor number and sampling frequency were determined by the RMSE in this paper. The RMSE of the data fusion method with the initial temperature T 0 from 40 °C to 120 °C and the flow rate Q from 45 mL/min to 120 mL/min are less than 1.4%, which shows wide applicability of the present approach. A data fusion method which integrates the high spatial resolution IR camera data with the high temporal resolution thermocouple data for wall temperature measurement is reported. The typical steps are shown in above figure. The coherent structure is extracted from the IR camera data based on proper orthogonal decomposition (POD). The coefficients of POD modes are then optimized by the compressed sensing from thermocouple data. The average root mean square error (RMSE) of the reconstructed result with 105 points and 100 Hz is around 1.11%. This method is also validated against different wall temperature and jet flow rate conditions and largest RMSE is found to be less than 1.4%. [Display omitted] [ABSTRACT FROM AUTHOR]

Cellulose material is a dielectric with intricate microscopic relaxation processes due to its complex structure. However, conventional models and curve fitting methods used for tracing and analyzing these processes often fail to capture crucial dielectric information. This paper aimed to extract the Distribution of Relaxation Time (DRT), the most fundamental and effective dielectric information providing the time scale and relative contribution of all microscopic relaxation processes. First, a distributed extended Debye model with infinite branches was constructed based on the microscopic nature of dielectric relaxation. Then, an implicit equation of the DRT function was established, inspired by the mathematical principles of infinite subdivision and summation. To obtain the numeral solution of the DRT function, a regularization method was proposed and validated. Finally, the approach was applied to cellulose insulating paper with varying degradation degrees. The relaxation process with a long time constant played a significant role, and variations during the degradation process were attributed to reduced activation energy. With clear physical interpretation and robust mathematical foundation, our method sheds light on the intricate dielectric relaxation processes in cellulose. This not only enhances the theoretical understanding and practical application of cellulose materials but also provides valuable insights for the analysis and application of other materials. [Display omitted] [ABSTRACT FROM AUTHOR]

Ammonia received increased attention as a carbon-free fuel and hydrogen carrier. Poor combustion stability and high NOx emission are the main issues for ammonia combustion. Effects of rotating gliding arc (RGA) discharge plasma on the flame stability and NOx emission of NH 3 /air premixed swirling flames were investigated. The discharge characteristics of the RGA were studied. The flame stabilization mechanisms by plasma were revealed and analyzed according to the flame structure transition and the distribution of key intermediate OH radicals. The NOx emissions were measured by the Gasmet DX4000 Fourier Transform Infrared (FTIR) gas analyzer. Results showed that the RGA discharge plasma can enhance the stability and extend the lean blow-off (LBO) limits from 0.6 to approximately 0.43–0.57. A hysteresis phenomenon about the transition between the lift-off flame and the attached flame was observed without plasma and disappeared with plasma activation. The plasma reduces NO and NO 2 emissions mainly due to the kinetic and cracking effects of plasma. Extremely low NO emission below 300 ppm can be observed for very lean ammonia flames with an equivalence ratio (φ) lower than 0.6, which are stabilized by plasma, while a large amount of unburned NH 3 emission is accompanied. • A rotating gliding arc discharge plasma-assisted ammonia combustion in a swirl combustor is proposed. • A hysteresis phenomenon about the transition between the lift-off and attached flame disappears with plasma. • Applying plasma extends the lean blow-off limits and enhances flame stability due to the continuous ignition effect. • A rotating gliding arc discharge plasma can enhance flame stability and reduce NO and NO 2 emissions simultaneously. • NO emission below 300 ppm can be observed for extremely lean ammonia flames stabilized by plasma. [ABSTRACT FROM AUTHOR]

Electric assisted combustion for hydrogen enriched hydrocarbons may even extend the lean burn limit and provide the further improvement on combustion stability. This study investigates the effect of hydrogen enrichment and DC electric field on lean CH 4 /air flame propagation. Electric field inside the chamber was generated by mesh and needle electrodes. Effect of hydrogen enrichment on the ion mole fraction in the flame was discussed based on reaction mechanism included neutral and ion reactions. The flame propagation images, flame displacement speed were used to evaluate the combined influences of hydrogen enrichment and electric field on propagating flame. Results showed that the hydrogen addition would increase positive ions mole fraction and the peak value is mainly determined by H 3 O+. This would be due to that CH increases with hydrogen fraction, which is the main species in the initial reaction for the ion reactions. Electric field effect about flame propagation was suppressed with hydrogen addition due to the competition between the increment in ion mole fraction and the decrement in flame time. Electric assisted combustion is more evident at leaner conditions and elevated pressure. The ratio of ionic wind velocity to flow velocity may be the determined factor to predict the electric field effect about propagating flame. The tendency based on this ratio is in accordance with the experimental results for various hydrogen fraction and equivalence ratio at elevated pressure. • Hydrogen enrichment combined with electric assisted combustion was studied at elevated pressure. • Electric assisted combustion will be suppressed with hydrogen addition. • Electric assisted combustion will be more efficient at elevated pressure. • Ratio of ionic wind velocity to flow velocity may be the determined factor of electric assisted combustion. [ABSTRACT FROM AUTHOR]