Measurement of the Groundwater Flow Velocity Based on Heat Pulse Method

Abstract

Groundwater flow is characterized by its small Darcy velocity generally in a magnitude of 10-6~10-3 m/s. Among the approaches of measuring groundwater flow velocity, the heat pulse method, which measures the groundwater flow velocity indirectly by monitoring the temperature response around a heated object, is considered as a simple and economic feasible method. In the previous heat pulse methods, the forced heat convection model is widely adopted, however, for a small Darcy velocity, the effect of natural convection may be non-negligible. To investigate the influence of natural convection in the heat pulse method, a series of experiments were conducted in the laboratory. The experimental system consists of a heated hollow copper sphere and 12 thermocouples distributed near the sphere surface. The fine glass spheres are used as the tested porous medium and filled up in a channel with a square cross profile. The experiments are performed at various heating powers and controlled groundwater velocities. The results show that the temperature response is less obvious with the Darcy velocity decreasing, which means that a larger heating power is needed for measuring a smaller Darcy velocity. The minimum groundwater velocity in an order of 10-5 m/s can be measured by using the experimental system. In addition, natural convection occurs when the Darcy velocity is small and the heating power is large, i.e., the buoyancy become significant and comparable with the pressure gradient.