Study On Flow Field Characteristics Of Shear Jet In Confined Space

So-called "coherent structures" of the turbulence, it is that shearing turbulence irregularly triggers an ordered movement, uncertain starting time and location. But once triggered, it a particular development state of motion in somehow certain sequence. This article is relied on the National High Technology Research and Development Program "The key technologies research of jointly removalling respirable particulate matter and heavy metals of difficult capture coal based on the dry method ". In the platform of the subject research team, it has studies the flow mechanism of round turbulent shear jet in confined space, the flow characteristics of concentric shear jet in the limited space and the entrainment properties of different mouth concentric shear jet in the confined space. And it comes to the following conclusions:The instability process of shearing jet can be divided into three phases:growth stage, instability stage and mixed stage. First, due to the Kelvin-Helmholtz instability it induces the upper and lower shear layer and simultaneously rolled a pair of vortex of upper and lower symmetry and contrary rotation direction; With the eddy to continue moving forward, the up and down rotation and opposite direction vortex appears unstable staggered in the middle of trajectory development the eddy by the interaction between each other and gradually grow and affect the formation and arrangement of subsequent vortex; With the interaction between the vortex of upper and lower rotation and opposite direction, it is constantly away from the midline and finally reaches a relatively stable stage of development. After this stage, it regenerates the larger size vortex, similar to the Karman street, of opposite direction of rotation and up and down staggered. It can be seen that the generation, merger and annexation to small vortices of large-eddy, and interactions between large eddies of opposite direction of rotation cause the movement of large eddy away from the jet center line,which is the main reason for thickening the jet flow field.In the field of engineering applications, pulse jet bag filter has been a very wide range of applications to dust cleaning. Although cleaning technology is more mature, the key issue is cleaning mismanagement leads to poorly regenerative capacity of the filter bag, dust covering or saturating into the filter bag-fiber layer which is difficult to remove, cause the filtration performance of filter bag in a sharp decline and the system running in too much resistance, affect the safety and normal operation of the system. The bag-house soot cleaning technology has a very closely relationship with optimal design of the system and ensuring the filter service life. Therefore, it is critical to study pulse jet cleaning mechanism. The author proposes the soot cleaning concept that the whirlpool and pressure caused by a jet contemplated continued structure is the soot cleaning driving force, that is, jet produces large eddy coherent structures in confined spaces in the process of the vortex generating, pairing, merging, crushing and restructuring. Concentric shear jet continuously and irregularly produces different pressure zones, and even generates negative pressure zones. These different pressure zones are presented and affected by restricted near the wall, Vortex in constant forward transferring and changing, make the same location of the filter bag near the wall at different times by changing force, and we can analyze the power source of reverse acceleration generated by filter bag vibration is the process vortex continuously passing, then in nature conclude the soot cleaning mechanism of pulse jet bag filter. The excellent characteristics of concentric shear jet fit the soot cleaning requirement of pulse jet bag filter, can protect the center jet of concentric jet and reasonably adjust the action length of main jet. To achieve the same jet action radius, concentric shear jet takes less time and the center jet length of concentric shear jet main jet is much longer.In the tubular blowing ash removal process of pulse jet bag filter, it induces the upper air of pocket and soot cleaning compressed air to jointly enter inside the bag; makes filter bag sidewall along the length direction of the pressure positive peak and reasonable distribution of pressure positive peak arrival time by adjusting the injection parameters. According to these two factors to study on the effect of pulse cleaning, it can achieve the purpose of dust cleaning. If the design of blowback dust cleaning is unreasonable and total volume of blowback wind and secondary induction wind is not enough, it can’t reach the cleaning effect; if the blowback wind is too large, it will cause the waste of compressor energy. Therefore, the entrainment’s properties of jet directly restrict the cleaning effect of the pulse jet bag filter. In the text, under the prerequisite of blowing the same mass and flow rate of air, through the comparative analysis of entrainment characteristics of shear jet pulse jet port and the traditional single injection port (i.e., the amount of entrainment and surface velocity and pressure inside the bag) it can be obtained shear jet injection port can entrainment more air than the traditional single injection nozzle when pulse jet, and improve the dust cleaning efficiency of the bag-house. When the nozzle’s length of the central jet blow port is more than the surrounding jet injection blow mouth tube, it will increase the mass and flow rate of the gas entrainment, and also increase the pressure inside the bag in the meantime, but the velocity of reflux from the bag side wall back to the clean room is relatively small increased; while the length of the blowing tube of the central jet inlet pipe is shorter than the surrounding jet injection blow mouth tube, it decreases the amount of air entrainment, while the bag internal pressure is also reduced accordingly. Under the premise of the same mass and flow rate of compressed air, the shear jet sucks air into the bag more than the single-jet, in which the most entrainment volume is Shinya shear jet, the entrainment rate of8.174; As injection pipe of central jutting shear flow can get more air entrainment, while increasing the internal pressure of the filter bag and the air regurgitation velocity in filter bag side wall correspondingly increases a little, the dust cleaning role of extended pulse vents in the bag filter is more significant.