| At present, the frequent dust explosion accidents were attracted more attention of social and national government, and need much deeper investigation on the cause, regularity and dangerous results of dust explosion. With the development of ultra-fine processing technology, the probability of dust explosion increased with the decreasing of particle size. Particularly, when the particle diameter reduced from the micro-size to nano-size, the mechanism and characteristics of combustion and explosion would be converted greatly. Now the dissimilarity of explosive characteristics and dust explosion venting for nano-and micro-dust were inconclusive, and needed plenty of experiments for supporting and verifing. Based on the above condition, this paper conducted the experimental investigation on the explosive characteristics in confined case and pressure-vented flame of the micro and nano-PMMA dust in venting. The main work and conclusion are as follows:(1) The measurement results of Malvern laser diffraction and Scanning Electron Microscope (SEM) showed that the worse microscopic decentralized state of 100 nm generated the serious agglomeration which small particles integrate into large particles.(2) Comparative experiments were conducted in the standard 20 L spherical vessel to explore the explosive characteristics of 30 ?m,800 nm and 100 nm PMMA dust. It showed that nano-size dust had shorter explosion persistent period, higher pressure rise rate and depressurization rate with respect to micron dust. For the fast pyrolysis rate and violent surface reaction of nano-dust, the maximum explosion pressure and the maximum pressure rise rate of 100 nm and 800 nm PMMA appeared at the concentration of 250 g·m-3. The values of maximum explosion pressure were respectively 0.821 MPa and 0.865 MPa, and the maximum pressure rise rate were 100.494 MPa·s-1 and 95.205 MPa·s-1. On account of lager particle diameter and smaller specific surface area, the explosion pressure and pressure rise rate reached the maximum at the concentration of 750 g·m-3, and the values were respectively 0.708 MPa and 37.024 MPa·s-1, which were much less vs nano-dust.(3) Theoretically, the explosion intensity gradually increased with the decrease of particle size. However, in present experimental research, it showed that different explosion law. When the dust concentration were higher than 250 g·m-3, the maximum explosion pressure and the maximum pressure rise rate of 100 nm dust were always lower than that of 800 nm dust for the prominent agglomeration effect. Meanwhile, the explosion pressure Pm was more close to that of the micron dust. When the dust concentration were lower than 250 g·m-3, the maximum explosion pressure of 100 nm dust exceeded that of 800 nm dust. In consequence, more consideration should be taken to that finer granularity of nano-dust had the coagulation at high dust concentration (>250 g·m-3) and the more intense explosion pressure comparing to large-size dust particles at low concentration (<250 g·m-3). According to the assessment of dust explosion by explosion index Kst, the nanosize PMMA were divided into the strong explosion grade St2 but the micro PMMA was belong to the weakest level Stl.(4) The comprehensive explosion protection method for all sorts of enclosed system containing flammable and explosive dust is venting. Multi-venting membranes and venting devices in various combinations vent devices were set on 20 L apparatus. It conducted the venting experiments without ducts for investigating the characteristics of the micro and nano-PMMA dust under different vented conditions. Experimental results showed that with increase of the vent area, the reduced pressure decreased and the rate of explosion pressure inner container increased. Because of higher pressure rise rate, the overpressure in vessel of 100 nm PMMA dust was still high after venting, and the pressure ratio ?p of reduced pressure and the maximum explosion pressure linearly decreased slowly with the increase of vent area Av. Nevertheless, the pressure ratio ?p of 30 ?n dust decreased significantly as vent area increasing, and the application of venting was obvious to reduce the overpressure for micron dust. As for uniform vented area and static activation pressure, the degree of overpressure for nano-dust was higher than micron dust, so larger vented area should be used for nano-dust explosion venting to reach the same decompression extant. According to current discharge forecast results, it shows that the design standard NFPA 68 has a higher accuracy for nano and micro PMMA dust discharge design versus EN 14491, so the micro/nano dust explosion discharge design can be referrer to NFPA 68. However, as for the same internal overpressure, the required vent area of nanometer dust dust is greater than that of the microns.(5) High-speed cameras simultaneously recorded the external vented flame propagation, exploring the external vent morphology and evolution of the micro and nano-PMMA dust under different vented conditions. For high static activation overpressure (Pstat=0.4MPa) and smaller vent diameter, the vented flame appeared the highly and moderated under-expanded jet configuration, and vented flame die out prematurely versus explosion pressure. It showed that the risk of flame outside was less towards this conditions. With regard to lower static activation overpressure (Pstat=0.14MPa) and large vent diameter, the vented flame of 100 nm dust evolved from higher under-expanded jet to atmospheric flame burning with unbroken vented configuration. However, the vented flame of 30 ?m PMMA evolved from moderately under-expanded jet to atmospheric flame burning, and configurations were irregular for its obvious effect of sedimentation and turbulent dissipation. In addition, dust pressure relief finished early than the flame relief. According to the influence of reduced pressure on the external vent flame, experimental results show that the decrease of reduce pressure for 100 nm PMMA will lead to significant increase of the maximum vented flame length, but that relation of 30 ?m is weaker.(6) According to laws of nanosize and micron dust explosion in confied vessel and open venting, it founded that although there were significant agglomeration effects for 100 nm PMMA dust, it had high pressure rise rate and risk, and fast-response venting devices must be used for dust explosion protection. Special attention must be taken to that the increase of discharge area will bring out the increases of an external venting flame propagation distance and hazardous areas. |
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