Speaker
Description
The inherent heterogeneity of the subsurface strongly affects the heat transport behaviour and remains a critical challenge in geoscience and related industrial applications. Capturing this behaviour requires resolving the interplay between advection, conduction, and the structural complexity of porous media. In this study, we investigate pore-scale thermal dynamics through laboratory experiments relevant to applications such as geothermal energy production, aquifer thermal energy storage, and heat tracing hydrogeology. Conventional heat tracer laboratory setups face practical limitations: thermocouples provide high temporal resolution but only point-based information. Alternatively, infrared thermography offers spatial coverage but cannot reliably measure absolute fluid temperature due to strong infrared absorption by water. To overcome these constraints and achieve both high spatial and temporal resolution, we present a novel application of luminescence thermometry as a non-invasive method for resolving temperature fields in analog porous media.
The method relies on the temperature-sensitive emission of luminescent nanoparticles, whose characteristic emission decay time decreases with increasing temperature. Here, we employ a zirconium(IV) complex, Zr(mesIPDPt-BuPh)2, exhibiting bright, photostable emission and a strong temperature-lifetime sensitivity of approximately 2.5%/K. The approach is based on resolving in 2D the luminescence decay curve following pulsed LED using a high-speed camera. Decay time constants are extracted from the recorded decay curves and converted into temperature values using an experimentally established calibration function. Heat transport experiments are conducted in an optically transparent flow cell housing a synthetic quasi-2D porous medium (15 × 6 cm) with a porosity of 0.36 and a minimum pore size of approximately 0.5 mm and equipped with Type-T thermocouples at the inlet and outlet to provide reference temperature measurements for validation. The solid matrix is made of a material with a thermal conductivity comparable to that of natural aquifers. The flow cell is initially saturated with a batch of the nanoparticles dispersed in water at room temperature, followed by the injection of a heated batch at 50 °C under controlled flow conditions. During injection, emission decays are captured and processed to yield pixel-wise lifetime measurements across the pore space, which are subsequently transformed into temperature fields.
The achieved temperature measurement precision is approximately ±0.1 K, with spatial and temporal resolutions of 0.25 mm and 130 ms, respectively. By varying the flow rate, the Péclet number is tuned across ranges representative of natural aquifer conditions. The resulting temperature fields reveal clearly visible thermal mixing and the progression of a thermal front, demonstrating the high precision afforded by this method. These fields show non-uniform thermal front propagation and spatially variable temperature gradients, indicating regimes of Local Thermal Non-Equilibrium (LTNE), where fluid and solid phases maintain distinct temperatures. These features, which cannot be captured by point-wise measurements alone, highlight luminescence thermometry as a robust tool for quantitatively resolving localized heat exchange and coupled flow and heat transport in heterogeneous porous media. Experimental results are compared with numerical simulations that replicate porosity, grain size, and Darcy flux. Ongoing work aims to extend this approach to other geological structures and flow regimes relevant to a broad range of hydrogeological settings.
| Country | France |
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