In-situ Thermal Transport Measurement of Flowing Fluid using Modulated Photothermal Radiometry. (arXiv:2105.00047v1 [physics.flu-dyn]) Leave a comment

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.

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