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In situ thermal transport measurement of flowing fluid could be useful forthe characterization and diagnosis of practical thermal systems such as fluidheat exchangers and thermal energy storage systems. Despite abundant reports onthe ex-situ thermal conductivity measurement of stagnant fluids, a suitabletechnique for the thermal conductivity measurement of flowing fluid has beenrarely reported. This paper presents the thermal conductivity measurement offlowing fluid within a pipe using a non-contact modulated photothermalradiometry (MPR) technique, where the surface of the pipe is heated by anintensity-modulated laser and the heat diffuses into the fluid with suitablemodulation frequency. We design a tube section with small wall thicknesssuitable for the MPR measurements to maximize the sensitivity of the thermalresponse to the fluid properties while minimizing the lateral heat spreadingeffect. Intrinsic thermal conductivity of different fluids was obtained withina proper range of frequency and flow velocity where the forced convectioneffect is negligible. The forced convection effect became prominent at highflowing velocity and at low modulation frequency, leading to overestimatedthermal conductivity of fluid. It is found that the intrinsic thermalconductivity could be obtained when the flow velocity is less than 100 mm/secand ReD1/2Pr1/3 < 100 for DI water and Xceltherm oil under the specifiedexperimental conditions, where Re_D is the Reynolds number and Pr is thePrandtl number.

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