No abstract is provided for this article.
Sealing smothering method is an important approach to control large fire in channels. This paper experimentally studied the fire behaviors after the sealing in a reduced-scale channel model, considering different initial sealing time under different sealing rates. A series of characteristic parameters, such as the temperature over fire source, mass loss rate of fuel, CO concentration at channel entrance as well as smoke temperature at channel entrance were measured. The results showed that for complete sealing, the earlier the initial sealing time is, the better the fire can be suppressed with a shorter burning duration and lower temperature. However, for incomplete sealing, although the sealing implemented before violent burning stage can reduce the temperature effectively, it causes a much longer burning duration and induces a higher CO concentration at the channel entrance. Especially during violent burning stage, the sealing not only does not restrict fire growth but also exacerbates the channel fire with an extremely high CO concentration at the channel entrance and a longer ceiling flame jet. Besides, as long as one of the channel entrances is not sealed completely, the temperature above the ceiling at channel entrance increases immediately after the sealing, which should be noticed for firefighters during fire rescue.
No abstract is provided for this article.
The single biggest market barriers of building materials is their fire risks, suffering from easily burning and generating a large amount of smoke and toxic gases. Fire behaviors of typical charring materials, including timber (Cherry) and polymer (acrylonitrile butadiene styrene, ABS), were investigated both experimentally and numerically. The proposed numerical model has considered both solid (the area inside sample slab) and gas (the area above sample surface) phases, addressing some previously usually-ignored fire processes, such as water evaporation, gases and liquids transportation inside solids, and volume change. Numerical results of timber and polymer were validated by cone calorimeter experiments. From both experimental and numerical results, three different stages of mass loss rate histories of charring materials were observed under external heat flux. In the gas phase, three high temperature areas were observed: two are near the walls of the two heaters due to the convection heat transfer; and one represents diffusion flame above sample surface, in which temperature increases with the intensity of external heat flux. This study provides not only an attempt to combine both solid and gas phases modeling for building materials, but also a platform for fire behavior modeling. Future research will focus on the validation of gas phase modeling and implementation of flame radiation.
Pyrolysis experiments were carried out on Kapur and Nyatoh hardwood species in ovens isothermally at low temperatures 160°C, 175°C and 190°C for extended durations up to 153 days in aerobic condition. Oxygen chemisorption and functionality of chars were analysed using thermogravimetric analysis (TGA) and Fourier transform infrared (FTIR) spectroscopy. Pyrolysis kinetic models for Nyatoh and Kapur wood under isothermal conditions in air were developed based on weight loss history. Thermal degradation of wood samples in the study followed a first-order reaction process after the initial period of fast degradation. The model can be used to estimate the heating period to reach different degrees of degradation under air condition as a function of temperature and to predict the duration to reach the final stage where self ignition is most likely at a particular low temperature. The study has shown that chars created at low temperature for long duration in aerobic condition were susceptible to oxygen chemisorption. Results indicated that they were reactive and prone to self ignition.
In this work, we utilized Al(OH)3 (AH) and Mg(OH)2 (MH) as dopants to reduce the flammability of hydrophobic silica aerogels (SA) and the related thermal properties and flame retardance were investigated detailedly. The TG-DSC analyses showed the thermostability of SA in MH/SA reached 512.4 °C and that for AH/SA was just 426.1 °C, both of which were higher than that of pure SA, 399.5 °C. It was known from cone calorimeter tests that the heat release rate, peak heat release rate and total heat release of AH/SA and MH/SA decreased significantly compared to that of pure SA. The time to ignition (TTI) of MH/SA was dramatically extended, reaching 20˜38 s, which was far longer than those of pure SA (˜6 s) and AH/SA (3˜8 s). The reduction in CO concentration, CO production rate and cumulative CO production verified the decreased smoke toxicity of AH/SA and MH/SA. It was further indicated that the flame-retardant effect of AH and MH correlated with their inhibitory effect on the pyrolysis of SA, while MH showed much better flame-retardant performance than that of AH. The research outcomes provide an inspiration to reduce the flammability of SA and benefit their expansion in thermal insulation field.
Thermal runaway (TR) propagation is considered the utmost safety issue of lithium-ion batteries (LIBs), which raised extensive concern. Using high-efficiency fireproof sheets to separate battery packs is one of the effective technologies to reduce the risk of TR propagation. Hence, we report a novel method, namely in-situ supercritical separation (ISS), to fabricate co-precursor aerogel sheets (CAS) based on an in-house device. ISS method can effectively reduce the preparation time of aerogel sheets to 3 h and greatly reduce the amount of solvent used without replacing or pressurizing additional solvent. More importantly, the proposed ISS method can effectively suppress the separation of the co-precursor heterogeneous substances, achieving homogeneous polymerization and improving the mechanical properties, high-temperature resistance and thermal insulation properties of the aerogel sheet. Specifically, CAS exhibits a compression strength of 638.5 kPa (50% strain), a low energy loss coefficient (0.238), and superior fatigue resistance (10,000 compressions). CAS has an ultralow thermal conductivity (i.e., 0.0197 W/(m·K)). The blocking functions of CAS are verified by a series of experiments where TR is triggered by abusive heating. Consequently, the TR propagation among fully charged LIBs with the highest temperature of up to 836.2 °C is successfully suppressed by 2-mm-thick CAS, yielding the maximum cell-to-cell temperature gap of 767 °C. Furthermore, it is proved that CAS with 35.7% wt aerogel is economical and capable of suppressing the TR propagation in the LIB module. The above results indicate that the CAS prepared by the ISS method is promising in applying to a safer LIB module.
This paper proposes a novel open-winding brushless doubly-fed reluctance generator (OW-BDFRG) with dual two-level converters in order to reduce the converter rating and switching frequency for large-scale wind turbine applications. The new converter topology is equivalent to a three-level converter directly connected to the control winding of typical BDFRG. The OW-BDFRG system with this topology structure requires lower converter rating and switching frequency, and has a more flexible control mode, better operation performance, and fault redundancy capability. For the OW-BDFRG, this paper also proposes a new control scheme combining direct power control (DPC) with sliding mode variable structure (SMVS) control to implement the power tracking. The voltage-vector switching table of DPC is redesigned according to the error signals of active and reactive powers of the power winding, as well as the sector location of control winding flux. The active and reactive powers of the OW-BDFRG can be directly decoupled and independently controlled by properly selecting the switching voltage vectors. The novelty of this paper lies in an OW-BDFRG topology driven by dual two-level converters to improve the system characteristics, and the use of SMVS control to improve the DPC accuracy and robustness to parameter variations. Finally, the effectiveness of the proposed system is verified through simulation and experimental studies.
The brushless doubly-fed wind power system based on conventional power control strategies lacks 'inertia' and the ability to support grid, which leads to the decline of grid stability. Therefore, a control strategy of brushless doubly-fed reluctance generator (BDFRG) based on virtual synchronous generator (VSG) control is proposed to solve the problem in this paper. The output characteristics of BDFRG based on VSG are similar to a synchronous generator (SG), which can support the grid frequency and increase the system 'inertia'. According to the mathematical model of BDFRG, the inner loop voltage source control of BDFRG is derived. In addition, the specific structure and parameter selection principle of outer loop VSG control are expounded. The voltage source control inner loop of BDFRG is combined with the VSG control outer loop to establish the overall architecture of BDFRG-VSG control strategy. Finally, the effectiveness and feasibility of the proposed strategy are verified in the simulation.
Tensile properties of a single-crystal Co–Al–W–Ni–Cr–Ta alloy with low tungsten content have been studied within the temperatures ranging from 20 to 1000°C at a constant strain rate of 1.0×10−4 s−1. The alloy exhibits comparable yield strength with that of Co–Al–W-base alloys containing more tungsten. From 600°C to 800°C, a yield strength anomaly is observed, probably due to the cross-slip of superdislocations from the octahedral plane to the cube plane. TEM analysis demonstrates that stacking faults (SFs) appear both in γ channels and γ′ precipitates in a wide temperature range. These SFs are responsible for the obvious strain hardening observed in stress–strain curves. From room temperature to 900°C, the deformation is dominated by dislocations shearing γ′ particles. At 1000°C, the main deformation mechanism is dislocations bypassing γ′ particles.
A series of numerical simulations were carried out to investigate the effectiveness of a fly-wing smoke screen in improving smoke exhaustion performance in a naturally ventilated urban road tunnel with vertical shafts. The fly-wing smoke screens with different included angles ( 0 ° < θ ≤ 180 ° ) were installed under the tunnel ceiling. The numerical results showed that plug-holing was prevented effectively by the fly-wing smoke screen, which stops, gathers, and then guides the smoke to the shaft vent. The velocity field under the tunnel ceiling showed that the optimal included angle with better smoke exhaustion performance was 60 ° ≤ θ ≤ 120 ° . The curve of CO mass flow rate through vertical shaft can be divided into three regions. The best included angle of fly-wing smoke screen is found at 90° with the best smoke exhaustion efficiency. An empirical formula is proposed to describe the exhaustion improvement effectiveness of fly-wing screen. The variation of smoke exhaustion improvement is mainly dominated by the coupling effect of smoke gathering and flow-guidance of the fly-wing smoke screen under different included angles. The practical engineering application of the proposed fly-wing smoke screen in urban tunnels with vertical shaft is also presented. This research offers a basic reference for the design of those naturally ventilated road tunnels with vertical shaft and fly-wing smoke screen.
This paper reviewed the state of the art in designing renewable energy systems specifically solar-based energy system, ground source-based system and day-lighting system, to gain optimum performances in sustainable buildings. Efficiency of each of these systems in reducing resource consumption was evaluated. Geometric conditions have a determining effect on the performances of solar-based energy system and day-lighting system. In solar-based energy system, designing factors, such as system selection, building's orientation, installation location, area of installation, tilt angle and surface temperature, are needed to be considered. Factors of day-lighting system, such as fenestration option, material, area or size, shape, orientation, position, ceiling and shading devices, are needed to be designed carefully to optimize the quality of the luminous environment for occupants. For ground source-based energy system, season condition, operating condition, mode of system, selection of compressor, ground heat exchanger, pump, are important to improve system's performance and reduce cost.
The brushless doubly-fed generator (BDFG) has the advantages of brushless structure, variable-speed constant-frequency operation, flexibly controllable active and reactive powers, low converter capacity and cost, and so on, which is very suitable for the wind power generation, especially the large wind turbines and offshore wind farm [1].
