Review of the application of neural network approaches in pedestrian dynamics studies.

In recent years, artificial intelligence methods have been widely used in the study of pedestrian dynamics and crowd evacuation. Different neural network models have been proposed and tested using publicly available pedestrian datasets. These studies have shown that different neural network models present large performance differences for different crowd scenarios. To help future research select more appropriate models, this article presents a review of the application of neural network methods in pedestrian dynamics studies. The studies are classified into two categories: pedestrian trajectory prediction and pedestrian behavior prediction. Both categories are discussed in detail from a conceptual perspective, as well as from the viewpoints of methodology, measurement, and results. The review found that the mainstream method of pedestrian trajectory prediction is currently the LSTM-based method, which has adequate accuracy for short-term predictions. Furthermore, the deep neural network is the most popular method for pedestrian behavior prediction. This method can emulate the decision-making process in a complex environment, and it has the potential to revolutionize the study of pedestrian dynamics. Overall, it is found that new methods and datasets are still required to systemize the study of pedestrian dynamics and eventually ensure its wide-scale application in industry.

On the mechanism of enhanced foam stability by combining carboxylated cellulose nanofiber with hydrocarbon and fluorocarbon surfactants.

The effect of carboxylated cellulose nanofiber (CCNF) on the firefighting foam stability and stabilization mechanism is investigated. The results show that equilibrium surface tension of CTAB/FC1157 solution decreases when CCNF concentration increases to 0.5 wt%, while CCNF has little effect on that of SDS/FC1157 solution. Besides, when CCNF concentration increases to 1.0 wt%, the foam initial drainage of SDS/FC1157 solution is delayed for about 3 min. Increasing CCNF concentration can slow down foam coarsening process and liquid drainage process of SDS/FC1157 and CTAB/FC1157 solutions, improving the foam stability. The foam stability enhancement of CTAB/FC1157-CCNF solution is due to the formation of bulk aggregates and the increase of viscosity. However, the foam stability enhancement of SDS/FC1157-CCNF solution may be caused by the increase of viscosity. CCNF significantly reduces the foaming ability of CTAB/FC1157 solution when CCNF concentration is >0.5 wt%. Nevertheless, the foaming ability of SDS/FC1157 solution decreases significantly when CCNF concentration reaches 3.0 wt%, and its foaming ability remains higher than CTAB/FC1157 solution. The foaming ability of SDS/FC1157-CCNF solution is mainly dominated by viscosity, while that of CTAB/FC1157-CCNF solution is dominated by viscosity and adsorption kinetics. Adding CCNF is expected to enhance the stability of firefighting foam and increase the efficiency of extinguishing fire.

Interfacial and rheological properties of long-lived foams stabilized by rice proteins complexed to transition metal ions in the presence of alkyl polyglycoside.

HYPOTHESIS: Transition-metal coordination complexes are hopeful to make advanced structural materials, since the metal-coordination bonds, unlike typical covalent bonds, can regenerate after rupture, allowing for dynamic, tunable, and reversible mechanical characteristics. Integration of metal-coordinate crosslinking in foam material has rarely been reported. EXPERIMENTS: We developed the hydrolyzed rice proteins (HRP) as the building block for amphiphilic transition-metal coordination complexes that could be used to make long-lived foams with high yield stress. Surface properties of the foaming solution were determined using equilibrium and dynamic tensiometers. Structural information of aggregates in the foaming solution was detected by small-angle X-ray scattering (SAXS) and Cryo-Tem. Visualization of liquid flow in the interfacial liquid film was studied by the reflective optical interference technique. Rheological response of liquid foam was characterized by a rheometer with amplitude-sweep and frequency-sweep modes. FINDINGS: In the presence of transition metal ions, HRP formed a mechanically strong rigid film. In the absence of transition metal ions or the addition of alkyl polyglycoside (APG), HRP was desorbed to produce a mobile film with a detergent state. The two interfacial states could be actively switched based on facile changes in bulk solution composition (metal ions or alkyl glycoside or chelating agent), and the switching between the two states led to the formation of extremely stable foam with high yield stress or the collapse of foam with a significant decrease in yield limit. The transition-metal coordination complexes adsorbed on the surface of the liquid film could increase the elastic modulus of liquid foam by more than an order of magnitude without increasing the viscosity of the foaming solution. We further revealed the origin of foam stability/instability and used other well-characterized proteins to prepare transition-metal coordination complexes to make long-lived foams. The cases described in this work illustrate the universal nature of the strategy, which in principle can be extended to many types of protein.

Research on Material Removal Mechanism of C/SiC Composites in Ultrasound Vibration-Assisted Grinding.

C/SiC composites are the preferred materials for hot-end structures and other important components of aerospace vehicles. It is important to reveal the material removal mechanism of ultrasound vibration-assisted grinding for realizing low damage and high efficiency processing of C/SiC composites. In this paper, a single abrasive particle ultrasound vibration cutting test was carried out. The failure modes of SiC matrix and carbon fiber under ordinary cutting and ultrasound cutting conditions were observed and analyzed. With the help of ultrasonic energy, compared with ordinary cutting, under the conditions of ultrasonic vibration-assisted grinding, the grinding force is reduced to varying degrees, and the maximum reduction ratio reaches about 60%, which means that ultrasonic vibration is beneficial to reduce the grinding force. With the observation of cutting debris, it is found that the size of debris is not much affected by the a p with ultrasound vibration. Thus, the ultrasound vibration-assisted grinding method is an effective method to achieve low damage and high efficiency processing of C/SiC composites.

Cu-Mn-Ce ternary oxide catalyst coupled with KOH sorbent for air pollution control in confined space.

For air pollution control in confined space such as submarine and spacecraft, copper-manganese-cerium ternary oxide catalysts coupled with KOH sorbent were synthesized through the wet impregnation method, solid-state impregnation method A and B, and wet/solid-state impregnation method. The samples were tested for CO and CO2 removal dynamically and isothermally from 30 °C to 150 °C using two fixed bed reactors, and then characterized by XRD, nitrogen adsorption and desorption, and FE-SEM/EDS. The results showed that all the coupled CuMnCe/KOHs were able to catalyze CO and capture the produced CO2 in situ. While the coupling treatments affected the CO oxidation and CO2 absorption performance of the samples significantly and differently. Among all samples, CuMnCe/KOH-WSI with the large KOH bulk phase exhibited the outstanding CO catalytic activity and CO2 sorption efficiency, higher than the uncoupled CuMnCe/KOH. While for CuMnCe/KOH-WI and CuMnCe/KOH-SI-I samples demonstrating high-dispersed KOH species in the catalyst, the addition of the sorbent could inhibit the catalyst activity due to the occupation of the surface site and pore structure. Furtherly, the effect of the temperature was varied for CO conversion and CO2 capture performances of the sample, while they achieved an optimization balance at 150 °C for CuMnCe/KOH-WSI.

Experimental study of the effect of internal pressure on oscillating behavior of pool fires.

A series of experiments have been conducted to study the flame behavior of ethanol pool fires in a closed chamber. The effect of internal pressure and the size of the pool burner is considered. Tests include pressure conditions ranging from 50 kPa to 350 kPa and 5 circular pool burners with different diameters (2 cm, 4 cm, 6 cm, 8 cm, and 10 cm). Measurements such as gas temperature, internal pressure, oxygen concentration, and video record for all tests are obtained. Steady-state burning period is identified to facilitate a quantitative analysis of flame behavior. Image processing is carried out to obtain time average appearance of pool fires. The concept of oscillation intensity is introduced. Oscillation behaviors of pool fires in a closed system as a function of internal pressure and pan diameter are correlated with oscillation intensity. Four flame structures are observed: laminar, tip flicking, sinuous meandering, and turbulent flame. Relationships between oscillation intensity to flame structure and Grashof number to flame structure are established. Effect of internal pressure and gravitational force to oscillation frequency is also accessed. Simple theoretical model is developed. An empirical expression using the relationship of Strouhal number and Grashof number is established. Two distinct behaviors on oscillation frequency as a function of pressure are observed. Results obtained from this work will facilitate the understanding of oscillation behavior of ethanol pool fires in different sizes with various internal pressure conditions in a closed chamber.

Effect of pressure on the heat transfer of pool fire in a closed chamber.

A combined experimental and analytical study was performed to determine the effect of pressure on the heat transfer of pool fire in a closed chamber. A series of ethanol pool fires with nominal diameters from 4 cm to 10 cm were conducted for a wide range of pressure conditions in between ~ 60 kPa to 250 kPa. Considering the effect of pressure, a theoretical formulation was proposed to estimate the radiation flux received by the pool surface from the chamber wall and hot gases. The relationship between the modified mass burning flux of pool fire and pressure was obtained. It was found that external radiation heat transfer received by pool surface depends significantly on gas temperature and its radiative properties and wall temperature. Results showed that these parameters are highly coupled in determining the effect of pressure to the external radiation. Furthermore, the average mass burning flux modified by the obtained external radiation was fitted with pressure by a power function and the fitted exponent value was consistent with theoretical analysis.

Modeling and analysis of bench-scale pyrolysis of lignocellulosic biomass based on merge thickness.

The bench-scale pyrolysis of lignocellulosic biomass was investigated based on effect of thickness by both the experiment and numerical simulation. A critical thickness at which the two peaks of mass loss rate start to merge and the pyrolysis process is significantly accelerated, is paid attention in the fire propagation apparatus experiment at N2 atmosphere. A new method is put forward to predict the merge thickness by coupling the Gpyro pyrolysis model with the optimized chemical reaction parameters, moisture and changed volume in OpenFOAM. Eventually, the predicted equation of merge thickness at various external heat fluxes is obtained, which is basically the same with that of thermal thickness.

Estimation of beech pyrolysis kinetic parameters by Shuffled Complex Evolution.

The pyrolysis kinetics of a typical biomass energy feedstock, beech, was investigated based on thermogravimetric analysis over a wide heating rate range from 5K/min to 80K/min. A three-component (corresponding to hemicellulose, cellulose and lignin) parallel decomposition reaction scheme was applied to describe the experimental data. The resulting kinetic reaction model was coupled to an evolutionary optimization algorithm (Shuffled Complex Evolution, SCE) to obtain model parameters. To the authors' knowledge, this is the first study in which SCE has been used in the context of thermogravimetry. The kinetic parameters were simultaneously optimized against data for 10, 20 and 60K/min heating rates, providing excellent fits to experimental data. Furthermore, it was shown that the optimized parameters were applicable to heating rates (5 and 80K/min) beyond those used to generate them. Finally, the predicted results based on optimized parameters were contrasted with those based on the literature.

Burning rate of merged pool fire on the hollow square tray.

In order to characterize fire merging, pool fires on hollow trays with varying side lengths were burned under quasi-quiescent condition and in a wind tunnel with the wind speed ranging from 0m/s to 7.5m/s. Burning rate and flame images were recorded in the whole combustion process. The results show that even though the pool surface area was kept identical for hollow trays of different sizes, the measured burning rates and fire evolutions were found to be significantly different. Besides the five stages identified by previous studies, an extra stage, fire merging, was observed. Fire merging appeared possibly at any of the first four stages and moreover resulted in 50-100% increases of the fire burning rates and heights in the present tests. The tests in wind tunnel suggested that, as the wind speed ranges from 0 m/s to 2 m/s, the burning rates decrease. However with further increase of the wind speed from 2 m/s to 7.5 m/s, the burning rate was found to increase for smaller hollow trays while it remains almost constant for larger hollow trays. Two empirical correlations are presented to predict critical burning rate of fire merging on the hollow tray. The predictions were found to be in reasonably good agreement with the measurements.

The effect of azeotropism on combustion characteristics of blended fuel pool fire.

The effect of azeotropism on combustion characteristics of blended fuel pool fire was experimentally studied in an open fire test space of State Key Laboratory of Fire Science. A 30 cm × 30 cm square pool filled with n-heptane and ethanol blended fuel was employed. Flame images, burning rate and temperature distribution were collected and recorded in the whole combustion process. Results show that azeotropism obviously dominates the combustion behavior of n-heptane/ethanol blended fuel pool fire. The combustion process after ignition exhibits four typical stages: initial development, azeotropic burning, single-component burning and decay stage. Azeotropism appears when temperature of fuel surface reaches azeotropic point and blended fuel burns at azeotropic ratio. Compared with individual pure fuel, the effect of azeotropism on main fire parameters, such as flame height, burning rate, flame puffing frequency and centerline temperature were analyzed. Burning rate and centerline temperature of blended fuel are higher than that of individual pure fuel respectively at azeotropic burning stage, and flame puffing frequency follows the empirical formula between Strouhal and Froude number for pure fuel.

Initial fuel temperature effects on burning rate of pool fire.

The influence of the initial fuel temperature on the burning behavior of n-heptane pool fire was experimentally studied at the State Key Laboratory of Fire Science (SKLFS) large test hall. Circular pool fires with diameters of 100mm, 141 mm, and 200 mm were considered with initial fuel temperatures ranging from 290 K to 363 K. Burning rate and temperature distributions in fuel and vessel wall were recorded during the combustion. The burning rate exhibited five typical stages: initial development, steady burning, transition, bulk boiling burning, and decay. The burning rate during the steady burning stage was observed to be relatively independent of the initial fuel temperature. In contrast, the burning rate of the bulk boiling burning stage increases with increased initial fuel temperature. It was also observed that increased initial fuel temperature decreases the duration of steady burning stage. When the initial temperature approaches the boiling point, the steady burning stage nearly disappears and the burning rate moves directly from the initial development stage to the transition stage. The fuel surface temperature increases to its boiling point at the steady burning stage, shortly after ignition, and the bulk liquid reaches boiling temperature at the bulk boiling burning stage. No distinguished cold zone is formed in the fuel bed. However, boiling zone is observed and the thickness increases to its maximum value when the bulk boiling phenomena occurs.