| As is known,air volume of roadway is one of the most important basic parameter in mine ventilation.If air volume of all underground roadways is real-time controled,we can detect the existence of the system security risks,the development of appropriate measures to reduce the risk of accidents to the probability of transformation.Mine air volume on-line monitoring technology is an important way to grasp the distribution of underground air volume in real time.The working form is to collect the local wind speed by using the sensor in the tunnel,and then calculate the air volume according to the section size of the tunnel.However,this approach to obtain the air volume accuracy is generally low,the reason for the two aspects.One is the wind speed sensor only monitors a fixed point of data,and can not represent the average wind speed of the entire tunnel;the other is the wind speed sensor monitoring indication is always in a random fluctuation in the "uncertainty" state,so it can not provide accurate measurement of wind speed value.The problem is how to measure the wind speed accurately in the tunnel,and transform it to the average wind speed of the section,which is the single point test of the average wind speed.In this paper,a series of studies on the single point precision measurement method of tunnel average wind speed are carried out by means of perspective of turbulence randomness,indoor experiment and field test.This work has the advantages of improving the on-line monitoring accuracy of mine air volume important theoretical and practical value.In order to prove the conjecture of turbulence random pulsation induced "uncertainty",the experimental model of average straight tunnel with no external disturbance is designed.The wind speed and wind pressure in the tunnel model is controlled by Laser Doppler Anemometry and Pitot tube Was observed.It is found that the wind speed and wind pressure still with large irregular fluctuations,the maximum fluctuation range is 30%,and the time series obeys the normal distribution and has the regular statistical average result.It is suggested that the phenomenon of "uncertainty" is absolute and not simply determined by external disturbance.The random fluctuation of turbulent flow is the essential reason of the uncertainty of mine ventilation parameters.Based on the theory of turbulence statistics,this paper studies the fine measurement method of tunnel wind speed,establishes the statistical measurement mathematical model of single point averagely hourly wind speed and tunnel average wind speed,and puts forward the principle of uniform wind speed sampling time scale is larger than the turbulence state traversal time scale.Based on the principle of sampling,this paper studied the homogenization method of single point wind speed.When the single point wind speed is proposed,the homogenization process belongs to the simple sampling in the simple random sampling,and the averagely hourly wind speed sampling unit is the sample capacity.The relationship between the unit number of the wind speed sampling unit and the turbulence intensity,the error and the confidence interval is established.This paper studied the conversion mechanism between the single point averagely hourly wind speed and the average wind speed of the tunnel by using the fluid mechanics theory and the Laser Doppler Anemometry experiment.The results show that single point averagely hourly wind speed is theoretically non-linear relationship with the average wind speed of the tunnel,but it can be simplified as the proportional relationship within the wind speed range.The proportional coefficient can be used as the conversion coefficient between the point wind speed and the average wind speed.The spatial distribution reflects the dimensionless velocity field structure.Based on this,the near-constant principle of velocity field structure is proposed,that is,the dimensionless velocity field structure of the section is approximately invariant when the average wind speed changes.According to the statistical measurement method of tunnel wind speed,the calibration steps of velocity field structure coefficient are given.In practice,only a speed field structure of the calibration can be used at any point when the wind speed is converted out of the average wind speed in the section.This paper studied the distribution law of turbulence intensity and the mechanism of turbulence generation.In the average straight tunnel where the turbulence is fully developed,the distribution of turbulence intensity along the vertical line in the tunnel is "parabolic",which is the smallest at the center of the tunnel.With the distance from the center of the tunnel,the turbulence intensity increases gradually and reaches the maximum in the near wall area.Turbulence can be simply considered to be formed by the shear instability between the flow layers.The greater the velocity gradient,the stronger the shear effect between the flow layers,the higher the turbulence intensity,and the more obvious the "uncertainty".Therefore,in the sensor arrangement,not only to select the wind speed distribution relative to the rules of the section,but also to understand the velocity gradient on the distribution of this situation.Turbulence random pulsation can cause the monitoring indication to fluctuate inaccurate.The experimental results show that the probability of instantaneous wind speed and average wind speed is not more than 5% when the turbulence intensity is 4%?6.8% and 9.8%,respectively79.6%?58.4% and 38.2% respectively.The actual downhole turbulence is more significant,so the accurate measurement of tunnel wind speed must be based on statistical measurement methods. |
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