Numerical evaluation of alternative heat pulse probe designs and analyses

Abstract

The heat pulse probe (HPP) has received more attention as it allows in situ, simultaneous, and automated measurements of soil hydraulic and thermal properties, as well as soil water fluxes. Although the currently used design allows many applications, changes in HPP design and analyses are needed to increase the sensitivity to smaller water fluxes. In this study, numerical simulations were used to evaluate the effects of (1) sensor locations and thermal properties of the HPP sensor body material, (2) the heater diameter and heat pulse intensity, and (3) vapor flow on HPP performance. A numerical study was carried out using the HYDRUS code for the HPP consisting of three parallel needles, spaced 6 mm apart. A heater of varying diameter in the center is surrounded by a pair of thermistor needles. Our results show that significantly different temperature responses will be obtained depending on the axial location of thermistors, and that only temperature measurements near the middle of the 33‐mm‐long heater fulfill the assumption of an infinite line heat source. It is shown that larger heater needle diameters allow larger heat pulses, leading to larger temperature differences between upstream and downstream thermistor needles and a higher sensitivity to water flux measurements. For example, for a standard 1‐mm‐diameter heater needle, ten times greater heat pulse input results in ten times greater sensitivity, but with the maximum temperature at the heater exceeding 100°C. This maximum temperature can be reduced to 80°C by increasing the heater diameter to 4 mm, while maintaining equal sensitivity. Consideration of vapor transport significantly reduced temperature increases near the heater caused by latent heat of vaporization. Larger heat pulses are beneficial for estimation of liquid water fluxes in the 1 cm d−1 range, provided vapor transport is considered.