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The objective of this paper is to use fiber optic sensors embedded in a tube wall to measure local convective heat transfer coefficients of a single-phase fluid. By using Rayleigh backscatter and an interferometer technique, mechanical changes in a fiber sensor that are proportional to temperature can be detected. This allows the location and magnitude of the temperature along the fiber to be measured. Using these fibers, we can measure axial profiles of the wall temperature in a heated tube with an internal fluid. By using multiple sensors spaced circumferentially around the tube, we can then generate axial and circumferential temperature maps of the tube wall. When combined with a known uniformly applied heat flux, these measurements can be used to determine the local heat transfer coefficients for single-, two-phase, and supercritical flows. In this study we consider a horizontal tube with internal diameter of 4.57 mm and heated length of 0.4 m. Using the fibers, wall temperature is measured every 0.7 mm in the streamwise direction at eight evenly spaced axial locations with an uncertainty of 2 °C. Co-located, calibrated thermocouples will verify the fiber temperature readings. Electric heaters provide a heat flux up to 20 W/cm2. Using this setup, heat transfer coefficients in the developing and fully developed region are obtained for water in laminar flow regimes and compared with established convective heat transfer correlations and models. The measured heat transfer coefficients in agreement with what is expected. In future work, this test section will be used to study nearcritical carbon dioxide convective heat transfer in both steady and transient conditions. 

In this study, we use infrared thermography to calculate local heat transfer coefficients of top and bottom heated flows of near-critical carbon dioxide in an array of parallel microchannels. These data are used to evaluate the relative importance of buoyancy for different flow arrangements. A Joule heated thin wall made of Inconel 718 applies a uniform heat flux either above or below the horizontal flow. A Torlon PAI test section consists of three parallel microchannels with a hydraulic diameter of 923 μm. The reduced inlet temperature (TR = 1.006) and reduced pressure (PR = 1.03) are held constant. For each heater orientation, the mass flux (520 kgm−2s−2 ≤ G ≤ 800 kgm−2s−2) and heat flux (4.7 Wcm−2 ≤ q″ ≤ 11.1 Wcm−2) are varied. A 2D resistance network analysis method calculates the bulk temperatures and heat transfer coefficients. In this analysis, we divide the test section into approximately 250 segments along the stream-wise direction. We then calculate the bulk temperatures using the enthalpy from the upstream segment, the heat flux in a segment, and the pressure. To isolate the effect of buoyancy, we screen the data to omit conditions where flow acceleration may be important or where relaminarization may occur. In the developed region of the channel, there was a 10 to 15 percent reduction of the local heat transfer coefficients for the upward heating mode compared to downward heating with the same mass and heat fluxes. Thus buoyancy effects should be considered when developing correlations for these types of flow. 

This study is part of the preliminary experimental investigations designed to assess the feasibility of using supercritical carbon dioxide (sCO2) in the vicinity of its critical point for thermal management applications. In the present study, sCO2 was used as a working fluid in a diffusion bonded 316/316L stainless steel test section having staggered micro pin fin array flow passages of hydraulic diameter 679 µm (0.679 mm). The test section was subjected to a single wall non-uniform heat flux boundary condition and was operated in a horizontal orientation. The primary objective was to characterize the heat transfer performance of sCO2 as it flows through the staggered pin fin array for experimental conditions that span its critical and pseudocritical point. Data analysis methods employing 2-D and 3-D heat transfer models of the test section were used to calculate the average heat transfer coefficients for a given set of experimental conditions. Experiments were conducted by varying the inlet temperature (18 ≤ T_in ≤ 50 °C) and for fixed mass flux (300 kg m-2 s-1), heat flux (40 W cm-2), and reduced pressure (1.1). Experimental data were also compared against the predictions of a correlation proposed for single phase flows in microchannel staggered diamond pin arrays. The correlation predicted the data within 4.3 % when the ratio, T_Bulk/T_PC exceeded 1. It was also found that the enhancement in the heat transfer, a result of employing staggered pin array flow geometry instead of microchannels, carries a commensurate penalty in pressure drop.