Optical thermometry to characterize heat transport in permeable porous media

The study of heat transport in porous media has recently attracted a lot of attention due to the wide range of industrial and geological applications, yet the impact of the structural heterogeneity of naturally occurring aquifers on their hydraulic and thermal properties is often disregarded. In that regard, a novel application of phosphor thermometry to porous media is proposed with the aim of examining under which conditions the validity of existing thermal transfer models in complex natural saturated porous media can be questioned. This experimental technique relies on monitoring the temperature-dependent luminescence properties of solid phosphor particles seeded into the fluid as tracers, using light sources and cameras. It offers the possibility of characterizing quantitatively the interaction between flow and heat transport processes at the pore scale in transparent analog porous media, with minimal interference and from spatially-resolved measurements, hereby overcoming the technical limitations of current experimental techniques, which are constrained to point temperature measurements.

Here, as proof of concept, we present a demonstration experiment performed on a slow-moving flow in a synthetic porous medium with a heterogenous size distribution, and using YAG:Cr³⁺, a thermographic phosphor with a temperature sensitivity exceeding 0.3%/K [1]. The measurements are performed using a modulated light source and are recorded at a sampling rate of 1 kHz during continuous injection of an aqueous solution which is initially at a constant temperature, different from that of the resident solution. The results show the dynamics of the spatial temperature distribution in the porous medium with a precision of ±0.3°C.

[1] J. L. Bonilla and B. Fond, "Phosphor thermometry using the phase-shift method: optimization and comparison with decay time method," 2022.