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Under the condition that the gas composition constant equivalence ratio is Φ = 1, and the initial temperature and initial pressure are T 0 and P 0 , respectively, the experimental study of the premixed gas flames with different hydrogen doping ratios (φ = 10%–40%) is different. The behavior and shape change of propagation in the flaring rate pipe (ƞ = 1.0–0.25). The study found that the pre-mixed gas flame in the flared pipe has undergone more complicated shape changes than other studies. One of the outstanding findings is that the tulip flame appeared twice in this open pipe experiment. And through the high-speed camera and high-frequency pressure sensor to obtain the tulip flame picture and the pressure change in the combustion chamber, comprehensive analysis of the experimental results, and the results show that every appearance of the tulip flame is accompanied by the deceleration of the flame front and the increase of overpressure in the combustion chamber. • An interesting phenomenon in which the tulip flame appears twice in a row was discovered. • Independently designed solenoid valve control switch to ensure that the explosion vent opens or closes on time. • In-depth analysis of the tulip flame from multiple angles. [ABSTRACT FROM AUTHOR]

Highlights • NARX modeling based on radial basis functions. • Identification and prediction of nonstationary nonlinear time series in chemical processes. • Fast recursive identification algorithm with combined downdating and updating computation in a single learning step. • Necessary conditions for checking the singularity of the regressor matrices in the recursive alogorithm. • Sufficient conditions for ensuring the matrix invertibility. Abstract In the learning techniques based on kernel or orthogonal basis functions, the nonlinearity in the underlying complex dynamic process is modelled by a linear combination of a set of kernel or orthogonal basis functions. Once these functional parameters are selected, the learning task boils down to solving linear least squares (LS). This has motivated the development of various recursive learning algorithms, where matrix inversion is intrinsic in solving LS problems. However, what has not attracted much attention along this track is the analysis of the matrix invertibility conditions in the recursive algorithms. This analysis is especially important when a model is sequentially downdated from the data, which may lead to rank deficiency. The main contribution of this work is the analysis of these conditions, in the formulation of a recursive NARX algorithm based on radial basis functions (RBF-NARX). Aiming at identifying nonlinear and nonstationary time series, RBF-NARX also features a fast algorithm with combined downdating and updating in a single learning step. Both the necessary conditions for checking the singularity of the regressor matrices and the sufficient conditions for ensuring their invertibility are proved. The performance of RBF-NARX and the invertibility conditions are tested and verified by the data from chemical processes. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • Dynamic photovoltaic (PV) greenhouses with various PV densities are modeled. • Energy performance under different sun-tracking methods is analyzed. • No-shading sun tracking improves the performance of high-density PV greenhouses. • Mixed sun-tracking schemes can balance the production of food and electricity. Abstract Dynamic photovoltaic (PV) greenhouses integrate sustainable energy generation with plant cultivation, offering more possibilities of energy production and microclimate control by adjusting the sun-tracking angles. Previous studies on PV greenhouses barely paid attention to the PV partial shading effects, and rarely recorded the performance across the full range of rotation angles. In this study, we first build computer simulation models of typical greenhouses with high-density (1/2 roof area) and low-density (1/3 and 1/4 roof area) PV layouts. Then four special sun-tracking positions are found in the model of equivalent global irradiance, which is defined as the quotient of the total input power divided by the area of PV module under partial diffuse shadows. Simulation models are also built in terms of PV modules and interior irradiance. Simulations are conducted using the climate data of Delft, the Netherlands (52.01 ° N , 4.36 ° E). Results show that high-density PVs under no-shading sun tracking generate 6.91% more energy than that under conventional (quasi-perpendicular) sun-tracking. Meanwhile, no-shading sun tracking allows more diffuse sunlight to enter the greenhouse mounted with high-density PV panels, resulting in 10.96% and 10.68% improvement on the annual average global irradiance and uniformity on the target plane compared to the fixed PV panels in the closed position. Regarding low-density PV layouts, which barely suffer from partial shading problems, quasi-perpendicular sun tracking improves the annual energy generation by 7.40% relative to the closed position. However, the average global irradiance reaches the minimum in this position because more sunlight is blocked by PVs. Meanwhile, the average uniformity of global irradiance reveals good (but not the best) performance, resulting in up to 9.80% (1/3 coverage) and 4.70% (1/4 coverage) improvement respectively compared to the closed position. The proposed methods and simulation results provide guidelines for the initial design and daily operation of PV greenhouses, aiming to balance the PV power generation and food production. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • Rh/v is defined to estimate the potential of solar energy on the vertical area. • Sun-tracking PV shading elements integrated with windows are modeled and analyzed. • Novel optimum sun-tracking methods and cell layout are first proposed. • Annual energy generation and average efficiency are improved by 27.40% and 19.17%. • Optimum sun-tracking methods reveal great ability to protect glare from the sun. Abstract Vertical space bears great potential of solar energy especially for congested urban areas, where photovoltaic (PV) windows in high-rise buildings can contribute to both power generation and daylight harvest. Previous studies on sun-tracking PV windows strayed into the trade-off between tracking performance and mutual shading, failing to achieve the maximum energy generation. Here we first build integrated models which couple the performance of sun-tracking PV windows to the rotation angles. Secondly, one-degree-of-freedom (DOF) and two-DOF sun tracking are mathematically proven to be not able to gain either maximum power generation or non-glare daylighting under reasonable assumptions. Then we derive the optimum rotation angles of the variable-pivot-three-degree-of-freedom (VP-3-DOF) sun-tracking elements and demonstrate that the optimum VP-3-DOF sun tracking can achieve the aforementioned goals. When the restriction of the proposed model is relaxed, the same performance can be achieved by the optimum one-DOF sun tracking with extended PV slats and particular design of cell layout, requiring less complicated mechanical structures. Simulation results of nine global cities show that the annual energy generation and average module efficiency are improved respectively by 27.40% and 19.17% via the optimum VP-3-DOF sun tracking over the conventional perpendicular sun tracking. The proposed optimum sun-tracking methods also reveal better protection against sun glare. The optimum VP-3-DOF sun tracking is also demonstrated to be applicable to horizontal PV windows, as those applied in the sun roof of a glass greenhouse. [ABSTRACT FROM AUTHOR]

• A 2K-Fold-Net is proposed to generalize U-Net and its stacked or cascaded variants, with K sub-U-Nets and hence the name. • The influence of the fold-pair number K on its performance of segmenting medical images is studied. • A special case with K=2 is realized and empowered with the feature enhancing functionalities of the attention-aware feature enhancement method, and is thus named as "Enhanced-Feature-4-Fold-Net", or EF3-Net in short. • The proposed networks are tested by four medical image segmentation datasets, and show superior performance as compared with some other popular variants of U-Net. For segmenting medical images, U-Net has become a popular and effective tool. However, it also has some shortcomings in segmenting fuzzy boundaries and eliminating interferences. Improvements of the original U-Net have been proposed by many authors, resulting in many variants such as MultiResUNet, DoubleU-Net and W-Net. Based on the common characteristics of these structures, we propose in this work a generalized structure by multiplying the folds of a fully convolutional network (FCN) for even more times, and thus name it as "2K-Fold-Net". The more folds in this structure provide more freedoms to create cross links between the neighboring folds. The influence of the fold-pair number K on its performance is also studied. The realizations with K up to 6 are compared to three other variants of cascaded U-Nets using the CVC-ClinicDB dataset. Then the special case "4-Fold-Net" is further empowered with the feature enhancing functionalities recently seen in the attention-aware feature enhancement method. This new net is hence named as "Enhanced-Feature-4-Fold-Net", abbreviated as "EF 3 -Net". Finally, 2K-Fold-Net and EF 3 -Net have been compared with U-Net, SegNet, DoubleU-Net, MultiResUNet and its variants using four challenging medical image datasets. The results have demonstrated that the proposed nets outperform the other variants of U-Net, even with slightly lower amount of parameters. The code is available on: https://github.com/raik7/EF3-Net. [ABSTRACT FROM AUTHOR]

• A method is proposed to design flexible optical waveguides with high uniformity and efficiency for delivering light in medical therapies. • The waveguides consist of the arrays of light-scattering dots and inverted trapezoidal lenses. • A backpropagation neural network incorporated with particle swarm optimization has been proposed to optimize the structural parameters. • Through easily accessible process techniques, a prototype of the optimized waveguide has been fabricated and tested. Light therapies are new modalities of treatment applicable to various diseases, which rely on optical devices to transmit light to target tissues. However, most existing devices are made of hard materials, and are usually applied at a distance from human body. Wearable optical waveguides that can achieve high planar optical performance are still barely seen. This work is devoted to developing a new design method of such flexible waveguides incorporating arrays of light-scattering dots and inverted trapezoidal lenses to achieve high uniformity and efficiency, which are the key performance criteria. To optimize these criteria, a backpropagation neural network model between the design parameters and the performance is established, which is then incorporated with particle swarm optimization. Furthermore, the effects on the performance of the other elements that are difficult to be designed via numerical optimizations have been studied in simulations, such as the depth and shape of the scattering dots and the transmitting and reflective character of the surfaces. Through easily accessible process techniques, a prototype of the optimized waveguide has been fabricated, and assembled with a light source. The measured irradiances on multiple target points on the emitting surface show the agreement with their simulated counterparts, with some undesirable mismatches in the real prototype from the ideal model taken into account. [ABSTRACT FROM AUTHOR]

This study analyzes the effect of both ignition position and hydrogen volume fraction on the flame propagation structure and overpressure wave. The interpretations are based on the propagation dynamics of the premixed syngas/air explosions at different ignition positions with different hydrogen volume fractions investigated at a constant equivalence ratio of 1 in a semi-open rectangular combustion chamber experimentally. The initial temperature T 0 and pressure p 0 were 298 K and 0.1 MPa, respectively. The experimental results are consistent with those obtained using the proposed analytical theory, indicating that the proposed theory predicts the early flame acceleration stage and the flame front motion well under some limited conditions. Flame–wall interaction and combustion instability decrease the theoretical flame front position than the experimental value and increase the wall contact time than the experimental value in some cases. At the same ignition location, both the overpressure growth rate and the maximum overpressure increase with increasing hydrogen volume fraction. When ignited at 0 mm off the closed end, the increased volume fraction of hydrogen would cause the initial location of the tulip flame to approach the opening end of the duct, but at an earlier time. The closer the ignition position to the opening end is, the more intense the overpressure and flame leading edge oscillation are, and the period of overpressure is similar to that of the flame oscillation. Finally, the ignition position and hydrogen volume fraction have obvious effects on overpressure, the structure and development of the flame front. [ABSTRACT FROM AUTHOR]

The effect of laminar flame speed on the flame dynamics in two-dimensional half open tubes is investigated through a two-dimensional laminar combustion model. Several single-step reaction mechanisms are established to alter the corresponding laminar flame speed. The simulation results show that the similarity of flame propagation only holds during the acceleration phase. In the flame acceleration phase, the flame shape and flame tip position corresponding to each dimensionless moment are almost the same. This similarity gradually disappears after the flame touches the wall. When the flame decelerates, the periodic oscillation of the flame tip velocity is related to the laminar flame speed. The higher the laminar flame speed, the longer the corresponding dimensionless oscillation period. The destruction of this similarity is also reflected in the correlation between the flame surface area and the growth rate of burnt gas volume. In addition, the dimensionless evolution correlation of the flame surface area during the acceleration phase is given in this paper. The moving speed of the flame skirt at different laminar flame speeds and the time interval of flame inversion in the two-dimensional half open tubes are also presented. • Simulation of flame dynamics in a two-dimensional tube under different laminar flame speed. • Simplified mechanisms were established to ensure that only laminar flame speeds were changed in the simulation. • The similarity of flame propagation is destroyed after the flame touches the wall. • The relationship between the volume growth rate of the burned gas and the flame surface was found to be different in the four stages. [ABSTRACT FROM AUTHOR]

This paper explains the basic knowledge to understand the relationship between overpressure wave behavior and the structure of flame propagation, especially the effects of the ignition position and opening ratio on the combustion dynamics. The explanations are based essentially on the experimental investigations of premixed syngas/air explosions carried out at a constant equivalence ratio of 1 and the hydrogen volume fraction of 0.3 in a rectangular half-open combustion chamber. The initial temperature T 0 and pressure p 0 were 298 K and 0.1 MPa, respectively. The four ignition positions were set at 0, 150, 450 and 750 mm off the left of the combustion chamber. Four different opening ratios of the duct were employed, including 0.25, 0.5, 0.75, and 1. The obtained results show that ignited at IP 0 , the increased opening ratio would cause the distorted tulip flame to disappear, accompanied by the propagation direction change of flame front. At the same ignition position, when the opening ratio increased, the oscillation amplitude of overpressure, and the velocities of the right-hand and left-hand flame all rose, but the overpressure growth rate and maximum overpressure decreased. As the ignition position approached the opening end, the overpressure and the flame front oscillations became more intense. It is indicated that different ignition positions and opening ratios exerted obvious effect on the flame front propagation and the overpressure. [ABSTRACT FROM AUTHOR]