Investigation On The Performance Of The Heating Probe To Measure The Seepage Velocity And Temperature

A novel method for simultenously measuring the velocity and tempurature of ground water will be proposed based on the mixed convection characteristics of a Joule heated probe buried in porous media.To this end,a complicated mixed convection model around a heated probe will be established accounting for variable geometrical and fluid flow parameter.First of all,the shortcomings of several exsiting measurement methods of seepage velocity are reviewed and summaried.A few conclusions can be drawn that(1)Frist,there is no method at present that can simultaneously measure velocity,tempurature and direction of ground water flow;(2)the measurement process is complicated,incapable of in situ and real-time measurement,and insensible to the change of groundwater flow velocity;(3)In general,the low cost instrument has a relatively low accuracy,and its responding time is long,while an accurate measurement method is very expensive;(4)recyclability is poor,or abandon in use results in the contamination of groundwater.Therefore,the development of a simple,accurate and inexpensive measurement probe is still desirable,which composes the main content of this paper.First of all,a spherical probe was designed,of which the outer surface of the ball was grooved and filled with platinum wire to achieve the effect of surface heating through Joule effect.After a series of experiments,it is found that the optimal current input of the spherical probe is about 0.2 A.The experimental results show that the temperature difference between the ball surface temperature and the incoming water temperature when the water is at rest is larger than the temperature difference when the water is flowing,and the temperature increase from the water flowing state to the static state is proportional to the incoming flow velocity.However,the probe measurement is only suitable to the case of water velocity of 0.003 m/s or larger.Then,a cylindrical probe was designed,of which the outer surface was attached with a few vertical heating strips.According to the calibration experiments,it is found that the cylindrical probe can also measure the water temperature and velocity,and the minimum error is about 0.04.After a series of experiments under various conditions of flow velocity and heating current,there are two regions that are clarcified.In one region,the proposed method is effective,but in the other region it is non-effective or fail in measurement.An equation for dividing the two regions was given.The experimental results of flow direction and velocity of 1.5 A were used to obtain the symmetrical axis of the incoming stream passing the center of the cylinder.The measurement accuracy for flow direction is about 30~o.The experimental results show that the relative temperature difference between the platinum plate and the incoming stream is directly proportional to the incoming flow volecity.The relative temperature difference matches the incoming flow volecity,and the minimum flow volecity that can be detectable currently is about 0.0015 m/s.Finally,an improved cylindrical probe was designed basing the original cylindrical probe,in which the heated fluid flow away faster from the strip surface of the probe,and the interference in between the heating strips was alleviated.Similarly,the calibration experiment was carried out,The minimum error in measuring temperature is about 0.08?;therefore,the cylindrical probe adopts 12 divisions,the precision in measuring the flow direction was increased to 15~o;the experimental results show that the relative temperature difference of the platinum plate on the flow-in surface of the probe is directly proportional to the flow velocity of the incoming flow,The flow volecity is matched by the relative temperature difference,and the minimum flow volecity that can be detectable currently is about 0.0005 m/s.The design of the above three kinds of probes follows the concepts of simplicy,quick response ability and accuracy in measuring temperature,magnitude and direction of seepage velocity.However,there still remains a large room for improvement.The preliminary experiments using the proposed methods for measuring seepage velocity in aquifers show that these methods are promising in the future,which has both theoretical and practical significances for petroleum exploitation,geothermal energy utilization and underground pollution control.