| The flow disorder in intake shaft caused by natural wind pressure is a complex thermal dynamic phenomenon. It will hinder the natural wind pressure in closed mesh existing in the intake shaft connected with surface of mechanical extraction mine, under the existing average temperature difference or the density difference of air column. When the natural wind pressure changes in a larger scope, it can cause the wind speed decrease, stagnation and even reverse. Especially in lower temperature of mine, the instability of flow happens repeatedly, which threatens the mine safety production seriously. This phenomenon is not only difficult to heat the intake shaft in winter but also leading to gas accumulation in bottom coal bunker roadway area that hides troubles in safety production. In addition, the reverse flow can bring the bottom-hole gas, dust and vapor into wellhead room that pollute operation environment and shorten the service life of equipment. Aiming at this problem, many scholars did some researches on flow disorder in intake shaft on the basis of measuring mine ventilation resistance and natural wind pressure. According to the measured data and derivation discriminant of shaft flow disorder, the main factors that caused flow changes in intake shaft were found out, and the corresponding control measures were put forward. However, the current researches mainly focus on emergency analysis of flow disorder in shaft and passive treatment of natural wind pressure. As time goes on, the phenomenon of flow stagnation or reverse appears repeatedly and cannot get effective control and prevention. Therefore, it is necessary to make a further discussion of this phenomenon development process, reveal the evolution rules, and get effective control method fundamentally, so that this problem can be solved completely.As mentioned above, theoretical analysis laboratory experiments and field tests were adopted to research the relationship between the flow temperature in intake shaft, the flow velocity and natural wind pressure in detail by numerous data tests and system analysis. The evolution mechanism of air disorder was also studied, and the effective prevention of air disorder was obtained.Firstly, statistical analysis was carried out on flow disorder occurred mine to research the common features of flow disorder in intake shaft through literature analysis. The reasons of natural wind pressure formation were summarized based on definition of natutal wind pressure and the actual situation. By simplifying the model of ventialation system, the occurrence of this phenomenon was did theoretical analysis. Based on the features and theoretical analysis of air disorder in intake shaft, the ventilation system model was established through the approximate similitude method. The combination of electric heating tapes and refrigerant was adopted to change the temperature of intake flow, forming the natural wind pressure in this model. The wind multi-parameter testers, pitot tubes, electronic thermometers and precision barometer were used to monitor the flow parameters in the experimental system. The influence rules of both internal and external factors on the intake flow state were obtained. Based on the experimental results, the flow parameters of actual ventilation systems were tested on site to verify the result of theoretical and experimental analysis. The correlation between air quantity, temperature and natural wind pressure was studied in the intake shaft. The evolution mechanism of air disorder was revealed by data calculation analysis. The method was proposed by which the hot air was used to balance the natural wind pressure and control the air disorder. We adopted both numerical simulation and field test to investigate the hot air control law of flow disorder. The technical proposal was obtained with the formation of control system, which was verified by field test finally. The research results have great significance on the improvement of flow stability in the intake shaft, shaft freeze prevention and safety production guatantee.Through the above research, the main contents and conclusions of this paper are as follows:(1) Characteristics and theoretical analysis of flow disorder in intake shaft zoneThrough literature analysis, the three common characteristics of air disorder were summarized in the intake shaft, namely occurrence season characteristic, satellite phenomenon characteristic and mine characteristic. According to definition of natural wind pressure and actual conditions on site, eight seasons of natural wind pressure formation were summarized. In addition, criterion of flow state change in intake shaft and number axis description of flow disorder theory caused by local natural wind pressure were obtained, the specific known is as follows: 1) This phenomenon usually occurs in late autumn, the whole winter and early spring, especially in sudden drop in air temperature in winter night, but it scarcely appears in summer. 2) The occurrence of flow disorder usually accompany with the phenomenon of emitting thick fog, shaft freeze, harmful gases accumulation or dust float. 3) Flow disorder in the intake shaft often appears in these large-scale coal mines with yearly output of 1.2~3 million tons. It rarely occurs in the coal mines with depth > 1200 m, but frequently appears in the coal mines with 300~800 m depth, especially for central ventilation system. Air disorder is less seen for partition diagonal ventilation and regional ventilation. 4) The natural wind pressure in shaft zone is energy difference caused by many factors such as atmospheric environment changes, strata heat release and heat source of producing activity, which has great randomness and complexity. 5) Mechanical ventilation pressure, ventilation resistance distribution and natural wind pressure are the main influencing factors of flow disorder in intake shaft. Among these factors, the change of local natural ventilation pressure between the intake shafts is the key factor for airflow disorder.(2) Simulation study on mine ventialation systemAccording to the characteristics of flow disorder mine, the prototype is abstracted as simplified ventilation system, which is "2 intake shafts and 1 air-return shaft". The experimental system is established by adopting the principle of selective similarity. In the model, the diameters of two intake shafts and one air-return shaft are 70 mm, 102 mm and 70 mm respectively. The height is 12.3m, linear scale 71.4ld(28); velocity scale 1vd(28), density scale 1.11rd(28). Through setting the heat source to simulate the natural wind pressure and dynamic monitoring of air flow parameters, the influence of external temperature factors of mine ventilation system, resistance distribution and flow static pressure on flow state in intake shaft zone was studied. The results indicated: 1) The local natural wind pressure has two kinds of basic forms, namely cold and hot wind pressure. The hot wind pressure prevents the ventilation in main shaft and promotes the ventilation in parallel shaft. However, cold wind pressure prevents the ventilation in parallel shaft, while the hot air pressure is opposite. 2) Continuous effect of hot and cold wind pressure in the parallel circuit of main and auxiliary shafts is the cause of the constant increase of local natural wind pressure.3)The resistance difference of intake shafts has great influence on flow disorder in ventilation system. The flow reverse is not easy to occur in shafts with low resistance and their contact tunnels, while the shaft with large resitance does the contrary. 4) The resistance increase in air-return tunnels can greatly reduce the negative pressure assigned to intake shaft area by fan and promote the flow disorder. The mines that prone to have flow disorder should avoid large ventilation resistance in air-return shafts. When flow reverse occurs in intake shaft area, flow may not reverse by reducing ventilation resistance in air-return shafts. 5) In the experiment, the method of increasing fan static pressure is approximate to the method of reducing ventilation resistance in air-return tunnels, which can increase the air pressure assigned to intake shaft area by fan and reverse its flow direction. But at the same time, the air volume of other shafts will increase obviously. 6) External factors of mine ventilation system, namely flow temperature, is the key factors of the formation of natural wind pressure in shaft area and the change of flow state. Internal factors of mine ventilation system, namely resistance distribution and fan static pressure, determine the difficulty of natural wind pressure to change the flow state in shaft area. However, it has great limitation to the control flow disorder.(3) Field tests on site and study on the mechanism of flow disorderVentilation system of actual mines with "2 intake shafts and 1 air-return shaft" was selected as analysis object. Intake shaft area was divided and flow parameters and mine ground atmospheric parameters were tested. Through the analysis of a large number of test data, the flow reverse process in loading chamber crossheadings of main shaft was studied under the condition of supplying hot air and not supplying hot air. The results indicated: 1) when the temperature is low, the thermal environment of intake shaft is easy to be different. When environment temperature changes, natural wind pressure is easy to form between intake shafts, which results in flow disorder. When the environment temperature is high, the thermal environment of intake shafts tends to be uniformed and their flow state is more stable. 2) When the natural wind pressure leads to the flow disorder in the intake shaft, the fluctuation range of the total flow rate is small. When the main shaft and crossheading flow was decreased or stagnant, the speed of air-return shaft flow may be increased. 3) The thermal environment of the intake shaft is not consistent, which will lead to the change of the wind speed of the shaft and the crossheading with the change of the surface air temperature. When the natural wind pressure is larger, it will cause the flow disorder in the shaft area. 4) Without shaft hot flow conditions, due to the small amount of auxiliary heat radiating, the auxiliary flow temperature change rate are higher, and the natural wind pressure is to promote the cold wind pressure of auxiliary shaft with the mine air temperature decreases. After the air volume increasing, thermal environment of auxiliary shaft on wind heat exchange ability is abating, resulting in lower wind temperature and higher natural wind pressure. When it is near the mine ventilation resistance of auxiliary shaft, leading to the main shaft crossheading appeared stagnant or reversed. 5) Under hot conditions, the thermal environment of intake shaft in inconsistencies still exists, leading to the auxiliary heating problems and formation of cold wind pressure that hinders flow in main shaft. When the main shaft inlet flow is reduced, flow temperature increases and hot wind pressure hindered flow in intake shaft occurres because of the same amount of hot air. The combined effect of main and auxiliary shaft of cold and hot wind pressure results in faster accumulation rate of the natural wind pressure and shorter the flow state changes time. 6) The variation of natural wind pressure between the intake shaft is mainly determined by the temperature change of ground and the thermal environment difference of the intake shaft. If the natural wind pressure increases or decreases, the flow state will appear repeatedly stagnation or reversal. 7) The natural wind pressure changes the flow intake volume, through the interaction between hot and cold air pressure. Air volume change will react to the thermal environment of the shaft, which leads to a further increase in the temperature difference between flow in shaft. Thus, it promotes the natural wind pressure gradually increased, and the circulation mechanism is formed, eventually leading to the flow reversal.(4) Research on the flow control method of the intake shaftBased on the mechanism study, the flow control method of the intake shaft was discussed, especially in hot flow condition. The followings were concluded: 1) Under the circumstance without hot flow, the proper flow control method should be determined by the specific season. In spring, the temperature getting warmer, the warm circumstance of the shaft house should be adjusted; while approaching in winter, the hot flow should be supplied in advance. Under the circumstance with hot flow, the proper flow control method should be determined by monitoring the flow temperature, speed, and other parameters and adjusting the hot flow amount. 2) Under the normal inlet flow condition, the cold and hot flow mix at the beginning of the intake shaft, and the wind temperature raises rapidly, and fell dramatically later; both the humidity and the density of the flow drop firstly, and then increase gradually. In general, the hot flow has a strong influence on the flow temperature, humidity and density within 100 meters. Under the circumstance with hot flow, the flow temperature distribution on the same level of shaft is uneven, and then reaches unanimity with the increase of the shaft depth. 3) Based on the hot flow influence on the flow parameters, the positions of the flow temperature sensors were determined. The sensor in the main shaft was located in the center of the guide beam 25 meters depth, while the sensor in the auxiliary shaft was located in the guide beam 50 meters depth near the hot flow shaft sidewall. 4) By increasing the hot flow in the auxiliary shaft and decreasing in the main shaft, the inlet flow amount of the main shaft could increase dramatically, at the same time, the stagnant flow of the main shaft crossheading would return to normal in a short time. When the average temperature of the auxiliary shaft is 3.0 to 4.5℃ degrees higher than the one in the main shaft, natural wind pressure that can promote inlet flow in main shaft will be formed. And the range of natural wind pressure is from-90.80 to-51.82 Pa. It can also ensure the loading chamber having an inlet flow amount from 1100 to 1750 m3/min.(5)The air flow disorders control system with its application in the intake shaft zone.Based on the research results, technical approach was proposed to solve this problem by monitoring the wellbore airflow parameters and controlling the amount of hot air. Meanwhile, the air flow disorders control system was developed in the intake shaft zone, and the field test has also been conducted. The results show that: 1) for the coal mines prone to appear airflow disorder, the monitoring of wellbore wind flow parameters and control of the hot air flow method can balance the natural wind pressure formed between the intake shafts, and enable the airflow status in the wellbore and its underground connections to become normal. 2) Through the reasonable arrangement of sensors, and the real-time monitoring of both the air temperature and wind speed in the ventilation shaft area under low temperature, based on the changing situation, the combination of the frequency conversion controller, PLC, electric control valves etc. was used to control the wellbore heating hot air volume. This can reduce the temperature difference in short time, achieving the prevent and control of airflow disorder phenomenon. 3) For the coal mines with many intake shafts located in the low-temperature area, a larger temperature difference exists between day and night. Thus, the monitoring and control of ventilation shaft wind flow parameters should be strengthened and the convenient control of wellbore heating should be also taken, in order to avoid the large temperature difference of airflow between the wellbores, further to reduce the appearance of airflow disturbance, thus improving the reliability of ventilation system. |
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