Historical records from the past 2000 years indicate that the vegetative cover of the Loess Plateau has declined significantly with time. The decline in vegetative cover seems closely related to increased rates of soil erosion as well as the increased frequency of natural disasters such as floods, droughts and dust storms. The condition of the environment and its relationship with development is a serious problem as the Chinese government searches for ways to improve the living standard of people in the region. The purpose of this study is to outline historical changes in the vegetation and environmental conditions of the Loess Plateau. Although we believe that current population pressure of the Loess Plateau is becoming unsustainable, our objective is not to be overly pessimistic in regards to the future development of the region. Instead, our purpose is to increase the awareness of environmental conditions in the region and to encourage the implementation of policies that will promote sustainable development of the Loess Plateau. 相似文献
In industrial conditions, there are different kinds of installations endangered by an explosion of dust–gas mixtures. In order to prevent them from any consequences of potential explosions, active protection systems which use extinguishing powders, as the suppressing material, are more and more widely applied. It often happens that the industrial installations are additionally endangered by an action of mechanical vibrations. In the above-mentioned conditions, the extinguishing powder tends to aggregate, making the process of dispersing it in the protected areas more difficult.
The results of research into optimization of the shape, number of igniters and miniaturization of the explosive charge mass are presented. In case of occurrence of the mechanical vibrations in the installations that cause aggregation of the suppressing powder, the best solution was modification of the perforated combustion chamber located inside the extinguisher. As a result of that, a certain amount of gases generated in the course of combustion of the main charge is directed over the membrane, where the gases crash a layer of aggregated powder. Thus, the process of pushing the powder out from a container towards the dispersing head improves considerably and consequently increases the efficiency of the explosion-suppressing process. 相似文献
Secondary dust explosion is a serious industrial issue because it occurs under conditions corresponding to an increased quantity and concentration of dispersed, combustible dust when compared with the primary explosion. The problems of lifting and dispersion of a dust layer behind a propagating shock wave must therefore be understood to ensure safety regarding secondary dust explosion hazards. Using a new shock-tube facility for studying shock propagation over dust layers, limestone dust was subjected to Mach numbers ranging from 1.10 to 1.60. A shadowgraph technique was applied by using a high-speed camera (15,000 fps) for visualization of the dust-layer height change behind the moving shock wave. Also, the effect of dust-layer thickness on the entrainment process was observed by performing tests with two different layer depths, namely 3.2- and 12.7-mm thicknesses. New correlations were developed between the shock strength and the dust entrainment height as a function of time for each layer depth. In general, the results herein are in agreement with trends found in previous work, where there is a linear relationship between dust growth rate and shock Mach number at early times after shock passage. Also, new data were collected for image analyses over longer periods, where the longer observation time and higher camera framing rates led to the discovery of trends not previously observed by earlier studies, namely a clear transition time between the early, linear growth regime and a much-slower average growth regime. This second regime is however accompanied by surface instabilities that can lead to a much larger variation in the edge of the dust layer than seen in the early growth regime. In addition, for the linear growth regime, there was no significant difference in the dust-layer height growth between the two layer thicknesses; however, the larger thickness led to higher growth rates and much larger surface instabilities at later times. 相似文献
A correlation of the lower flammability limit for hybrid mixtures was recently proposed by us. The experimental conditions including ignition energy and turbulence which play a primary role in a gas or dust explosion were at fixed values. The sensitivity of such experimental conditions to the accuracy of the proposed formula was not thoroughly discussed in the previous work. Therefore, this work studied the effect of varying the ignition energy and turbulence intensity to the formula proposed in our previous paper. For ignition energy effect, results from methane/niacin mixture demonstrated that the MEC and LFL will not be affected by changing ignition energy. There is no distinguishable difference among gas explosion index (KG) and dust explosion index (KSt) derived from tests with every ignition energy (2.5 kJ, 5 kJ and 10 kJ) in a 36 L vessel. The proposed formula is independent of ignition energy. For turbulence effect, the proposed formula can have a good prediction of the explosion and non-explosion zone if the ignition delay time is within a certain range. The formula prediction is good as the ignition delay time increases up to 100 ms in this work. Propane/niacin and propane/cornstarch mixtures are also tested to validate the proposed formula. It has been confirmed that the proposed formula predicts the explosion and non-explosion zone boundary of such mixtures. 相似文献
This paper describes experiences and results of experiments with several metallic dusts within the nanometer range. The nano dusts (aluminium, iron, zinc, titanium and copper) were tested in a modified experimental setup for the test apparatus 20 L-sphere (also known as 20-L Siwek Chamber), that enables the test samples to be kept under inert atmospheric conditions nearly until ignition. This setup was already introduced in earlier papers by the authors. It was designed to allow the determination of safety characteristics of nano powders under most critical circumstances (e.g. minimisation of the influence of oxidation before the test itself). Furthermore the influence of passivation on explosion behaviour is investigated and additional tests with deposited dust were carried out to describe the burning behaviour of all dusts. For a better characterisation all samples were tested with a simultaneous thermal analysis (STA). To minimise the influence of oxidation all samples were handled at inert conditions until shortly before ignition or start of the test respectively. 相似文献