Zhang Cao

In many industrial scenarios, the object to be imaged is binary valued, and its composition can be known in advance. Typical X-ray computed tomography (XCT) methods are challenged to achieve high-quality images from limited projections. An image reconstruction method for high-precision XCT of binary-valued distributions is proposed for projections of limited views. A binary matrix of only two elements, i.e., 0 and 1, is introduced for target distributions with two known linear attenuation coefficients. Projection histograms along ray paths from different projection views are used to derive the binary matrix. The Sigmoid function is utilized to achieve the binary matrix in which each row has exactly one nonzero item of 1. The distribution of the imaged object is obtained by mapping the linear attenuation coefficient at each grid one-by-one. Compared with the simultaneous algebraic reconstruction technique (SART) and filtered back-projection (FBP) methods, the proposed method yields lower normalized mean distances and higher image similarity from limited views. In noisy cases, the normalized mean absolute distance of the proposed method was reduced by more than 40%. In experiments, an aluminum column and a carbon rod were imaged with clear edges and fewer artifacts by the proposed method. Also, it is used to reconstruct triple-valued distributions, with image errors lower than SART and FBP.

A single -path laser absorption spectroscopy (LAS) is fused to enable electrical capacitance tomography (ECT) for quantitative imaging of flame gas temperatures. A temperature histogram is achieved along the single path from LAS, while the permittivity values along the same path are obtained via using the Calderon method in ECT. In cases of dynamic flames, these histograms vary over time. An online mapping from permittivity histograms to temperature histograms is constructed via the modified Transformer model. Permittivity histograms of more virtual laser paths are derived from the ECT image of permittivity and the mapping. Temperature histograms along these paths are achieved from the mapping and the permittivity histograms. Temperature distributions are reconstructed from these histograms and enable a quantitative imaging of gas temperature from only a single-path LAS, which is regarded to have no spatial resolution along the path. A self -pressurized alcohol lamp was used in experiments for flame imaging. Also, a compact sensor of both LAS tomography and ECT was used to monitor alcohol lamp flames to verify the proposed method. LAS tomography was utilized to reconstruct the temperature, and the mean temperature relative deviation is less than 5 % for 90 % of the measured samples, with a maximum relative deviation of 7.46 %.

Doppler impact intensity measurement is widely applied in railway transportation and aerospace engineering for enhancing equipment robustness, reliability, and accelerometer accuracy. Data acquisition and processing still present challenges due to high frequency, non-stationarity, and large data volume. This study presents an in-situ transient velocity profile measurement system that segments Doppler intensity in the frequency domain and identifies the effective component on-chip. The system adopts a four channels acquisition structure that achieves dynamic data capture even when a single channel’s sampling rate is relatively low, while its method supports expansion to higher frequencies in future applications. A impact dynamics up to 63 MHz can be captured by data acquisition in four channels of only 31.25 mega sampling per second, and the computations is reduced by a half. For different velocities of mechanical impacts, the proposed method yields a smaller deviation compared with commercial LDV and accelerometer.

Lateral shearing interferograms of only one single frame are proposed to capture transient 3D images of dynamic flames. Interferometric phase at each pixel in one interferogram is derived from the intensity and two calibrated parameters. The phase is extracted pixel by pixel from the light intensity of interferograms of the flames. The refractive index and temperature distribution are reconstructed after unwrapping. The feasibility of light intensity parameter calibration from the extinction interferograms was validated in numerical simulations, and the analysis of error sources for the proposed method was conducted, with the relative uncertainty of temperature determined to be 6.8%. A Bunsen burner was used to confirm the feasibility of the proposed method, with temperature consistent with thermocouple and an average error of 26.73 K. The burner was acoustically excited and measured at frame rates of 2k to 20k fps. Temperature fluctuations were detected at different frame rates, and higher accuracy of the proposed method was verified by comparison with the five-step method. The proposed method achieves temperature images in a pixel-wise way, and it has the potential to increase the imaging frame rate up to the highest frame rate of the camera.

Limited projections from actual optical paths yield an underdetermined projection equation for tunable diode laser absorption spectroscopy tomography. An iterative reconstruction method via adaptive grid partitioning is proposed to solve the equation. A partition of grids is used to determine equality constraints on grids for the same pixel value, and the unknowns are decreased. Projection equation of the gridding at high spatial resolution is decomposed into several equations with fewer unknowns. This is verified by using asymptotic point spread functions. The shape proximity of the point spread function to the Dirac function with the proposed method is 21.1% smaller than that of simultaneous algebraic reconstruction technique (SART). In addition, smaller image errors and higher structural similarity index values are achieved in both noise-free and noisy cases compared to SART. In temperature imaging on a high-temperature wind tunnel, a relative deviation of 2.55% from thermocouple values verified the accuracy of the proposed method.

A fast tunable diode laser absorption spectroscopy (TDLAS) sensor is proposed for gas temperature measurement in transient flows of a scramjet. Two distributed feedback (DFB) lasers emitting near 7185.58 and 7444.34 cm⁻¹ for H₂O detection were sinusoidally tuned to achieve two-line thermometry. Electro-optic modulators were used to perform envelope modulations for frequency division multiplexing and radiation noise immunity. A quantitative gas temperature uncertainty analysis model has been established for high-speed TDLAS sensors to optimize the trade-off between accuracy and measurement speed of the sensor. The sensor rate was optimized to 1 MHz. The uncertainty model showed that a time resolution of 1 μs produced a temperature uncertainty of 8.8%, which was acceptable for monitoring high-frequency unstable combustion. Both simulation and experiments were conducted to evaluate the performance of the proposed sensor. In case of harsh scramjet exhaust flow, the proposed TDLAS sensor with a temperature range of 300-2200 K verified that the fluctuation frequency of exhaust flow was below 150 kHz. And there is consistency between the uncertainty model and the sensor, with the faster the rate, the greater the deviation.

A multiple line thermometry (MLT) method is proposed for high-precision gas temperature measurement by extracting the integral absorbance from dual-comb spectroscopy (DCS). The integral absorbance of each spectral line in DCS is derived from the inner products of the Voigt function basis and the time-domain signal of the absorption spectra. The ratio of integral absorbances is a temperature-dependent constant, independent of pressure and concentration. The proposed MLT method is insensitive to pressure and concentration fluctuation. Numerical simulations demonstrate that the temperature precision of the proposed MLT method exhibits ultralow correlation with fluctuation in pressure and concentration for precise temperature calculation. The absorption spectra of acetylene were detected in the wavenumber range of 6523.12-6545.36 cm⁻¹ and the temperature range of 295.15-453.15 K using dual phase-unlocked optical combs. Compared to the multispectral fitting method, the relative temperature deviation of the proposed MLT method is reduced from 0.03%-2.05% to 0.02%-0.92%.

Tunable diode laser absorption spectroscopy (TDLAS) tomography utilizes various laser paths from different views to image combustion flames. Usually, one data acquisition (DAQ) channel is required for the projected intensity along each laser path, and a DAQ hardware with complex channel alignment and huge storage capacity is used for the collection and cache of laser intensities from multiple projections to image 2-D dynamic distributions. A cost-effective tomography system is achieved with an eightfold increase in data storage capacity by on-chip data compression and a half decrease of channel number by channel multiplexing (CM) of time division, and it enables fast response and long-period flame monitoring without hardware modifications. The proposed system can achieve a monitoring time of 40 s at a framerate of 2 k FPS. The complete ignition and extinguishing of a Bunsen burner were captured, and the imaged flame frequency matches that of a photodetector of visible light. Also, nine working cases were monitored at the exit of a two-stage combustion chamber, and relative deviations of measured temperatures by the proposed method and thermocouple are less than 3%, which verifies the robustness of the proposed system in harsh environments.