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. Here, 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 higher effective thermal conductivity of the 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 ReD is the Reynolds number and Pr is the Prandtl number.
Molten salts are being used or explored for thermal energy storage and conversion systems in concentrating solar power and nuclear power plants. Thermal conductivity of molten salts is an important ...thermophysical property dictating the performance and cost of these systems, but its accurate measurement has been challenging, as evidenced by wide scattering of existing data in literature. The corrosive and conducting nature of these fluids also leads to time consuming sample preparation processes of many contact-based measurements. Here, in this work, we report the measurement of thermal conductivity of molten salts using a modulated photothermal radiometry (MPR) technique, which is a laser-based, non-contact, frequency-domain method adopted for molten salts for the first time. By unitizing the advantages of front side sensing of frequency-domain measurements and the vertical holder orientation, the technique can minimize the natural convection and salt creeping effects, thus yielding accurate molten salt thermal conductivity . The MPR technique is first calibrated using standard molten materials including paraffin wax and sulfur. It is then applied on measuring pure nitrate salts (NaNO3 and KNO3), solar salt (NaNO3–KNO3 mixture), and chloride salt (NaCl–KCl–MgCl2). The measurement results are compared with data from literature, especially those obtained from laser flash analysis (LFA). Our results demonstrate that the MPR is a convenient and reliable technique of measuring thermal conductivity of molten salts. Accurate thermal conductivity data of molten salts will be valuable in developing the next-generation high-temperature thermal energy storage and conversion systems.
•Non-contact technique to handle high-temperature corrosive liquids.•Frequency controlled thermal penetration depth eliminates natural convection.•Front-side sensing and vertical holder orientation ...minimize salt creeping effect.•Accurate thermal conductivity data obtained for nitrate and chloride salts.•MPR is applicable for in-situ diagnostics of high-temperature thermal system.
Molten salts are being used or explored for thermal energy storage and conversion systems in concentrating solar power and nuclear power plants. Thermal conductivity of molten salts is an important thermophysical property dictating the performance and cost of these systems, but its accurate measurement has been challenging, as evidenced by wide scattering of existing data in literature. The corrosive and conducting nature of these fluids also leads to time consuming sample preparation processes of many contact-based measurements. Here, we report the measurement of thermal conductivity of molten salts using a modulated photothermal radiometry (MPR) technique, which is a laser-based, non-contact, frequency-domain method adopted for molten salts for the first time. By unitizing the advantages of front side sensing of frequency-domain measurements and the vertical holder orientation, the technique can minimize the natural convection and salt creeping effects, thus yielding accurate molten salt thermal conductivity . The MPR technique is first calibrated using standard molten materials including paraffin wax and sulfur. It is then applied on measuring pure nitrate salts (NaNO3 and KNO3), solar salt (NaNO3–KNO3 mixture), and chloride salt (NaCl–KCl–MgCl2). The measurement results are compared with data from literature, especially those obtained from laser flash analysis (LFA). Our results demonstrate that the MPR is a convenient and reliable technique of measuring thermal conductivity of molten salts. Accurate thermal conductivity data of molten salts will be valuable in developing the next-generation high-temperature thermal energy storage and conversion systems.
Isotropic polycrystalline graphite samples were irradiated with ~1 GeV 197Au and 238U ions of fluences up to 5 × 1013 ions/cm2. Beam-induced changes of thermophysical properties were characterized ...using frequency domain photothermal radiometry (PTR) and the underlying structural transformations were monitored by Raman spectroscopy. The ion range (~60 µm) was less than the sample thickness, therefore thermal diffusivity contributions of the irradiated as well as non-irradiated layer were considered when analyzing the PTR data. At the highest applied fluences, the thermal effusivity of the damaged layer degrades down to 20% of the pristine value and the corresponding calculated values of thermal conductivity decrease from 95 Wm−1K−1 for pristine material to 4 Wm−1K−1, a value characteristic for the glassy carbon allotrope. This technique provides quantitative data on thermal properties of ion-irradiated polycrystalline graphite and is very valuable for the prediction of lifetime expectancy in long-term applications in extreme radiation environments.
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In this study, the impact of nanographene incorporation on the thermal properties of mesoporous silicon (PSi) was evaluated using two complementary experimental methods: the temperature gradient (TG) ...and the photothermal radiometry (MPTR) methods. It is shown that the measured thermal conductivity of the mesoporous silicon (PSi) ranges from 0.10 to 0.68 W/m.K in the case of TG and from 0.37 to 3.02 W/m.K in MPTR and is strongly correlated to the electrochemical etching parameters. These values are much lower than that of crystalline silicon, estimated to be from 100 to 140 W/m.K, depending on the doping rate. They appear to be, however, in the order of magnitude range for the percolation models that also include the in-depth porosity and the crystallite mean radius. This set of experiments on the thermal conductivity was extended to investigate the effect of graphene incorporation in the PSi matrix (G-PSi) as it has seldom been reported in the literature. The results from both methods exhibit significantly higher values (1.7 ± 0.3 W/m.K for TG, and from 0.7 to 2.13 W/m.K for MPTR). This spread of the thermal conductivity values is attributed to the intrinsic working principle of the TG versus the MPTR method as highlighted in the last part of the present paper. Targeting the thermoelectric application of both matrices (PSi, G-PSi), the thermal conductivity remains sufficiently low for them to be considered as very promising materials, keeping in mind the enhancement of the power-factor attributed to the incorporation of graphene.
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•Thermal conductivity measurement in p + doped PSi.•Temperature gradient (TG) and photothermal radiometry (MPTR) methods are employed.•Significant thermal conductivity changes induced by nanographene insertion in PSi.•Very low thermal conductivity values are measured comparatively to the crystalline Si.
•Modulated photothermal radiometry for bulk and coatings.•Measured bulk materials with thermal conductivity 1 - 30 W m−1 K−1 up to 973 K.•First thermal conductivity measurement of thin ...solar-absorbing coatings.•Thermal conductivity of the coatings of 0.4 ~ 0.8 W m−1 K−1 up to 973 K.
This paper presents the development of instrumentation for the measurement of high-temperature thermal conductivity of bulk and coatings using a modulated photothermal radiometry (MPR) method, where a sample is heated by an intensity-modulated laser to probe into different layers of the sample. While MPR has been previously established, most of the prior studies only focus on the measurement at room temperature. The MPR has not been well studied for measurements of bulk and coating materials at high temperatures, which are increasingly important for a multitude of applications, such as materials used in the concentrating solar power (CSP) plants and the nuclear reactors. MPR is a non-contact technique that utilizes the intrinsic thermal emission from the specimens for thermometry, which is favorable for measurements at high temperatures in harsh environment. The authors designed and utilized a sample holder suitable for high temperature measurement up to 973 K with good temperature uniformity within the sample. The high-temperature MPR setup was validated by measuring bulk materials with known thermal conductivity. The setup and technique were then extended to the measurement of black solar-absorbing coatings of 10 to 50 μm thick on various substrates by modulating the frequency of the laser heating beam and the thermal penetration depth. The studies showed that thermal conductivities of typical solar-absorbing coatings are 0.4 ~ 0.8 W m−1 K−1, indicating a possibly large temperature drop within the coating under high solar irradiation flux, such as over 1000-sun for central solar towers in CSP plants.
Thermal and structural properties of aged silicone rubber composite insulators widely used in high-voltage transmission power systems were characterized by photothermal radiometry (PTR) to evaluate ...the degree of aging. A two-layer PTR theoretical model was developed to extract the thermal diffusivity and the aged layer thickness from the PTR frequency-scan data. Results show that aging generated an aged layer with a reduced thermal diffusivity at the surface of the composite insulator. Further investigation into the depth dependence of the thermal diffusivity of aged composite insulators found a monotonically increasing thermal diffusivity along the depth direction, approximating to a hyperbolic tangential profile, which indicates a spatially continuous variation from surface to bulk during the aging process. The PTR depth-profiling of thermal and structural properties provides a deeper understanding of the aging process in composite insulators.
Assess the suitability of the technique for objective monitoring of laser tattoo removal by an extended treatment protocol.
One half of the tattoo in the first volunteer was treated with nanosecond ...and the other half with picosecond laser pulses at 1064 nm. In the second subject, four test areas were treated repeatedly using different radiant exposures from 1.5 to 6 J/cm
. Measurements of diffuse reflectance spectra and photothermal radiometric transients were performed 4-20 weeks after each treatment session. Inverse Monte Carlo analysis based on a three-layer model of tattooed skin was applied to assess the tattoo characteristics and analyze their changes.
The results clearly indicate a gradual reduction of the ink content and an increase of the subsurface depth of the tattoo layer with all treatments at a radiant exposure of 3 J/cm
or higher. The observed dependences on laser pulse duration, radiant exposure, and a number of treatments are in excellent agreement with visual fading of the tattoo.
The presented methodology enables noninvasive characterization of tattoos in human skin and objective monitoring of the laser removal treatment.
Acting like thermal resistances, ferroelectric domain walls can be manipulated to realize dynamic modulation of thermal conductivity (k), which is essential for developing novel phononic circuits. ...Despite the interest, little attention has been paid to achieving room‐temperature thermal modulation in bulk materials due to challenges in obtaining a high thermal conductivity switching ratio (khigh/klow), particularly in commercially viable materials. Here, room‐temperature thermal modulation in 2.5 mm‐thick Pb(Mg1/3Nb2/3)O3–xPbTiO3 (PMN–xPT) single crystals is demonstrated. With the use of advanced poling conditions, assisted by the systematic study on composition and orientation dependence of PMN–xPT, a range of thermal conductivity switching ratios with a maximum of ≈1.27 is observed. Simultaneous measurements of piezoelectric coefficient (d33) to characterize the poling state, domain wall density using polarized light microscopy (PLM), and birefringence change using quantitative PLM reveal that compared to the unpoled state, the domain wall density at intermediate poling states (0< d33<d33,max) is lower due to the enlargement in domain size. At optimized poling conditions (d33,max), the domain sizes show increased inhomogeneity that leads to enhancement in the domain wall density. This work highlights the potential of commercially available PMN–xPT single crystals among other relaxor‐ferroelectrics for achieving temperature control in solid‐state devices.
Thermal conductivity modulation in PMN–xPT single crystals is demonstrated with a maximum enhancement of more than 20% at the intermediate poling state (intermediate d33). The enhancement is attributed to the reduction in domain wall density due to alternating current poling. The thermal conductivity decreases at optimized poling (maximum d33) due to increasing domain size inhomogeneity.
•First non-contact in-situ thermal conductivity measurement of flowing fluid.•Identify experimental conditions to obtain intrinsic thermal conductivity of fluid.•Demonstrate the technique on water, ...ethanol, and two types of oils.•Measure oil from room temperature to 170 °C.
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 higher effective thermal conductivity of the 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 ReD is the Reynolds number and Pr is the Prandtl number.
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