Laboratory Testing of Heat-Pulse Flowmeter and its Application to the Distribution of Groundwater Flow
Date Issued
2012
Date
2012
Author(s)
Lee, Tsai-Ping
Abstract
Heat-pulse flowmeter can be used to measure low flow velocities in a borehole; however, bias in the results due to measurement error is often encountered. A carefully designed water circulation system was established in the laboratory to evaluate the accuracy and precision of flow velocity measured by heat-pulse flowmeter in various conditions. A movable diverter was also developed to extend the operation flow range assembled on the flowmeter. Test results indicated that the coefficient of variation for repeated measurements, ranging from 0.4% to 5.8%, tends to increase with flow velocity. The measurement error increases from 4.6% to 94.4% as the average flow velocity decreases from 1.37 cm/sec to 0.18 cm/sec. We found that the error resulted primarily from free convection and frictional loss. Free convection plays an important role in heat transport at low flow velocities. Frictional effect varies with the position of measurement and geometric shape of the inlet and flow-through cell of the flowmeter. Based on the laboratory test data, a calibration equation for the measured flow velocity was derived by the least-squares regression analysis. Our laboratory experimental results suggested that, to avoid a large error, the heat-pulse flowmeter measurement is better conducted in laminar flow and the effect of free convection should be eliminated at any flow velocities.
Field measurement of the vertical flow velocity using the heat-pulse flowmeter was then tested in a 23-m deep screened well in an alluvial aquifer to characterize the distribution of hydraulic conductivity along the screen. Measurement results indicate that groundwater flow is concentrated in two highly permeable sections. Their horizontal hydraulic conductivities are 3.7 to 6.4 times greater than the equivalent hydraulic conductivity of the whole aquifer, suggesting that contaminant migration rate could be underestimated in a heterogeneous aquifer. Two more field tests were conducted in open-holes in the fractured rock formation to characterize the preferential flow path. The field test results indicated that, with a proper calibration, the heat-pulse flowmeter measurement is capable of characterizing the vertical distribution of hydraulic conductivity or preferential flow. The position of the highly permeable fracture zone can be identified within the range of 25 cm and a single opened fracture was identified when most flow discharged in a specific location. However, the large aperture and high density of fractures were not certainly correlate well to the permeable zone. Comparing our test results with those obtained from other techniques, we found that heat-pulse flowmeter measurement is more efficient for locating permeable fractures.
Subjects
Hydraulic conductivity
Flowmeter
Aquifer
Fracture flow
Diverter
Type
thesis
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