In-situ Thermal Transport Measurement of Flowing Fluid using Modulated Photothermal Radiometry

TL;DR

This study introduces a non-contact, in-situ method using modulated photothermal radiometry to measure the thermal conductivity of flowing fluids within pipes, accounting for flow effects and optimizing measurement conditions.

Contribution

It presents a novel application of MPR for in-situ thermal conductivity measurement of flowing fluids, including design considerations and flow condition guidelines.

Findings

Thermal conductivity can be accurately measured at flow velocities below 100 mm/sec.

Forced convection significantly affects measurements at higher flow velocities.

Optimal measurement conditions are identified based on flow and fluid properties.

Abstract

In situ thermal transport measurement of flowing fluid could be useful for the characterization and diagnosis of practical thermal systems such as fluid heat exchangers and thermal energy storage systems. Despite abundant reports on the ex-situ thermal conductivity measurement of stagnant fluids, a suitable technique for the thermal conductivity measurement of flowing fluid has been rarely reported. This paper presents the thermal conductivity measurement of flowing fluid within a pipe using a non-contact modulated photothermal radiometry (MPR) technique, where the surface of the pipe is heated by an intensity-modulated laser and the heat diffuses into the fluid with suitable modulation frequency. We design a tube section with small wall thickness suitable for the MPR measurements to maximize the sensitivity of the thermal response to the fluid properties while minimizing the lateral heat spreading effect. Intrinsic thermal conductivity of different fluids was obtained within a proper range of frequency and flow velocity where the forced convection effect is negligible. The forced convection effect became prominent at high flowing velocity and at low modulation frequency, leading to overestimated thermal conductivity of fluid. It is found that the intrinsic thermal conductivity could be obtained when the flow velocity is less than 100 mm/sec and ReD1/2Pr1/3 < 100 for DI water and Xceltherm oil under the specified experimental conditions, where Re_D is the Reynolds number and Pr is the Prandtl number.