Research Highlights (selected)

In this work, the temperature homogenization characteristics of a vapor chamber-type heat spreader were investigated. A copper vapor chamber with dimensions of 102 mm × 102 mm × 5 mm (width × length × thickness) was tested, and the distributions of the radial temperature and cumulative effective thermal conductivity across the heat spreading surface were measured at different working fluid charge ratios and heat fluxes. Water was employed as the working fluid at charge ratios of approximately 30%, 40%, and 60%. The vapor chamber was heated at the center within a heat flux range from 100 W·cm⁻² to 200 W·cm⁻² with a heat sink located at the peripheral edge to induce one-dimensional radial outward heat flow. Based on the obtained results, it was found that the temperature homogenization characteristics of the vapor chamber were significantly influenced by the working fluid charge ratio combined with the applied heat flux and heat sink temperature uniformity. Thus, to improve temperature homogeneity, the vapor chamber should be operated with a proper amount of the working fluid at a high heat flux (lower than the critical value causing drying) together with a heat sink of a high-temperature uniformity. Regarding the tested vapor chamber operated at a 30% working fluid charge ratio, which produced the lowest level of temperature asymmetry, the overall effective thermal conductivity ranged from 2460 W·m⁻¹·K⁻¹ ± 240 W·m⁻¹·K⁻¹ to 2910 W·m⁻¹·K⁻¹ ± 290 W·m⁻¹·K⁻¹ at 180 W·cm⁻².

In this work, a flat-evaporator loop heat pipe (FELHP) having a flexible heat transport path was constructed and tested. The operating characteristics were investigated in terms of the operating temperatures and various thermal resistances using different working fluids (e.g. ethanol, acetone, and distilled water) and at different elevations. Regarding the effect of the working fluid, acetone showed the lowest thermal resistance at lower heat loads (e.g., 0.25 KW⁻¹±0.07 KW⁻¹ at 60 W), whereas the water-filled FELHP resulted in the widest operating range with the lowest thermal resistance of 0.25 KW⁻¹±0.05 KW⁻¹ at 320 W. A significant effect of elevation change was observed due to the prevailing hydrostatic head loss over the frictional pressure loss; the lowest thermal resistances at different elevations varied from 0.64 KW⁻¹±0.11 KW⁻¹ to 0.11 KW⁻¹±0.03 KW⁻¹ with decreasing elevation.

In this work, the transient responses of a heated infrared (IR) temperature sensor were investigated to improve the reliability of determined target temperatures obtained from IR-based medical thermometers. A medical-grade IR temperature sensor was heated at the lower edge of the sidewall of the sensor. To reduce the uncertainty due to the conversion factor of the thermal detector, the temperature of the target, which was a thermostatted blackbody source, was determined when the observed target temperature and the temperature of the detector coincided during the heating and cooling of the sensor. When the determined target temperature was compared with the blackbody source temperature, it was found that during heating, due to the produced temperature gradient in the sensor, the observed target temperature showed erroneous depressions, resulting in the determined target temperature being considerably lower than the true target temperature. In contrast, the determined target temperature during cooling of the heated sensor was consistent with the tested blackbody source temperatures within the claimed uncertainty at all heating conditions. Therefore, based on the obtained results, it was concluded that temperature measurements using an IR temperature sensor could be carried out with the least uncertainty by determining the target temperature when the observed target and detector temperatures coincided during cooling of the heated sensor.

The effect of the supplied heat load on the operating characteristics of the pneumatic temperature control technique was systematically investigated. A pressure-controlled loop heat pipe employing water as a working fluid was designed and constructed to provide a unique thermohydraulic linkage between temperature and pressure. The temperature of the high-speed vapor was precisely controlled by using helium (control gas) to actively control the compensation chamber pressure; the stability of the pneumatically controlled temperature was approximately 0.01 °C regardless of the heat load. Stable stepwise temperature increase was attained below a critical pressure step that rose with increasing heat load. This behavior implied that a stable range of pneumatic temperature control became larger with increasing heat load. In addition, the maximum pressurization rate for rapid, stable, and unlimited temperature rise increased with increasing heat load. The uniformity of the controlled temperature field was less than approximately 0.2 °C over 25 cm; effects of the heat load were insignificant at all tested conditions. Overall, pneumatic temperature control over extended pressure and temperature ranges was attained at higher heat loads with insignificant effects of the heat load on stability and uniformity.

The freezing temperatures of two different, high-purity (nominal purity of 99.9999 %) zinc samples were realized, and the dissolved impurities were analyzed at three independent Glow Discharge Mass Spectrometry (GD-MS) laboratories to correct the impurity effects using the sum of individual estimates (SIE) method. Comparisons were made between the difference in the realized freezing temperatures and the three differences in the SIE corrections based on the individual assay results. Among them, the impurity compositions analyzed at one laboratory resulted in the difference in the SIE corrections matching the measured temperature difference; that is, the SIE-corrected zinc freezing temperatures based on only one assay laboratory coincided with the measurement result. Therefore, the best approximation to the freezing temperature of ideally pure zinc was attained with the aid of the consistent assay result, and the physical measurement of the difference in the realized freezing temperatures could be used as a practical method verifying the consistency and the reliability of the chemical assay results.

The International Temperature Scale of 1990 (ITS-90) is commonly expressed in terms of the ratio of the resistance of a standard platinum resistance thermometer (SPRT) to the resistance at the triple point of water. Most high-precision thermometry relies on the alternating current (AC) method due to its manifold practical benefits. However, there are some negative consequences of using AC methods, and it is of particular interest to quantify the effect of the measuring current frequency on the realized temperature of the ITS-90 fixed points. In this work, the effect of the measuring current frequency was investigated by measuring the differences between the AC and direct current (DC) resistance ratios at various fixed-point temperatures. Two AC at 30 Hz and 90 Hz and one DC at a reversal frequency of 3 Hz were used to measure the resistance ratios at four ITS-90 metal fixed points, namely, mercury, water, zinc, and silver; seven SPRTs having different nominal resistances (e.g., 0.25 Ω, 0.6 Ω, 2.5 Ω, and 25 Ω) were employed to explore the effect of frequency on the measured resistance ratio. It was found that the frequency dependence was most noticeable at the freezing point of silver where the insulation leakage and its transient dielectric polarization were expected to manifest themselves the most significantly. For other fixed points, the frequency dependence was found to be insignificant, implying that under normal conditions the benefits of the AC measurements can be fully utilized without affecting the calibration uncertainties of the tested types of SPRTs.

In this work, the pressure dependence of the temperature readings of reference deep‐ocean thermometers (i.e. SBE 35, Sea‐Bird Electronics) was investigated in a thermostatically controlled and pressurized environment. To this end, a deep‐ocean environment emulator was constructed to reproduce the extremely stable temperature and high‐pressure nature of the deep ocean; it consisted of a pressure vessel, a pressure controller and a thermostatic water bath. With the aid of this emulator, the pressure effect on three SBE 35 thermometers was investigated at various hydrostatic pressures from atmospheric pressure to 68 MPa, equivalent to the pressure at a depth of 6 800 m. In addition, the effect of external thermal disturbances was corrected by measuring the real‐time temperature variation around the SBE 35 thermometer in order to achieve the highest possible accuracy. From extensive testing of the pressure dependence of the temperature readings of the three SBE 35 thermometers, it was found that two of the three thermometers exhibited clear and repeatable pressure dependences, although the respective tendencies were different from each other. The third thermometer showed no repeatable pressure dependence. All the measured pressure dependences of the three thermometers amounted to changes which were within ±1 mK. The results indicated that the pressure dependence of these reference deep‐ocean thermometers is significant, and the pressure dependence is not the same for all SBE 35 devices but specific to each individual device. It was concluded that the pressure dependence of the individual reference deep‐ocean thermometers needs to be accounted for during calibration.

The liquidus temperatures of two tin fixed-point ingots having different impurity compositions have been determined using the adiabatic heat pulse-based melting technique; the employed samples were analyzed at three different chemical assay laboratories for characterization of the impurities present. The shape of the melting curves, which showed a rapid increase in the early part of the melt and subsequent gradual and nearly linear increase in the later part, were found to be qualitatively well described by the Scheil-Gulliver solidification model corresponding to the detected impurities of the two samples; it was understood that the impurities having distribution coefficients (k_i) less than 1 resulted in a rapid melting temperature change in the early part of the melt, and impurities having k_i > 1 manifested themselves in the later part as a gradual increase in the melting temperature. The liquidus temperatures of the different tin samples were then determined by extrapolating the melting temperatures in the later parts of the melting range, where the variations of the melting temperatures were satisfactorily described by the corresponding parts of the Scheil-Gulliver model, to the melted fraction of unity. The determined liquidus temperatures of the two samples were corrected for the effect of the detected impurities, resulting in an agreement between the corrected temperatures within the claimed uncertainties. It was concluded that with the aid of the consistent chemical assays, the liquidus temperatures determined by the heat pulse-based melting and corrected for the impurity effect were able to produce the closest approximation to the ideal definition of the fixed-point temperature of the ITS-90, which is the melting temperature of the pure fixed-point substance.

Recently, a novel temperature control technique utilizing the unique thermo-hydraulic operating principles of the pressure-controlled loop heat pipes (PCLHPs) was proposed and proved its effectiveness, by which a faster and more stable temperature control was possible by means of the pressure control. However, due to its recent emergence, the proposed hydraulic temperature control technique has not been fully characterized in terms of the various operating parameters including the sink temperature. In this work, the effect of the sink temperature on the LHP-based hydraulic temperature control was investigated to improve the stability of the proposed technique. Start-up characteristics and transient responses of the operating temperatures to different pressure steps and sink temperatures were examined. From the test results, it was found that there was a minimum sink temperature, which ensured a steady-state operation after the start-up and a stable hydraulic temperature control with the increasing pressure steps, due to the unstable balance between the heat leak and the liquid subcooling in the compensation chamber at low sink temperatures. In addition, the range of the stable hydraulic temperature control was extended with the increasing coolant temperature due to the decreased heat leak, which resulted in the increased pressure difference between the evaporator and the compensation chamber. Therefore, it was found and suggested that for a stable hydraulic temperature control in an extended range, it was necessary to operate the PCLHP at higher sink temperatures than the low limit.

Deep supercooling of tin is a characteristic sign of a high-purity sample having fewer nucleation sites. However, due to the risk of oxidation during the cell fabrication (especially during the initial sample filling process), oxidized tin can be present in the sample, which manifests itself as a nucleation agent and significantly affects the realization of the freezing temperature of tin. In this work, the effect of the oxidation of high-purity tin (nominally 99.9999 % pure) and an effective means of the reducing the oxidized sample were investigated. We fabricated an open-type tin freezing point cell using 1.15 kg of high purity tin shot, which was poured into the crucible in air, giving rise to a high risk of oxidation. The oxidized tin sample showed abnormally small supercools of about 0.1 °C which were caused by additional nucleation sites introduced by the oxidation. The oxidized sample also exhibited a melting temperature inversion over the entire range of the melting with a temperature depression of about 1.9 mK. The oxidized sample was carbothermally reduced by melting the sample in its graphite crucible and holding at a temperature of 750 °C for a total duration of 365 hours. The carbothermal reduction of the oxidized sample caused the satisfactory recovery of the deep supercool, and the recovered supercool was about 10.6 °C after about 100 hours of reduction, and 10.7 °C after 365 hours of reduction. At the end of the reduction process, the melting curve inversion was significantly alleviated, having 0.1 mK temperature depression over less than 10 % of the melting range. Importantly, the carbothermal reduction at 750 °C was found to be an effective means of removing the oxide nucleation sites and of recovering the characteristic deep supercool of tin.

In this work, the consistency of the heat pulse-based melting technique, which was used to determine the liquidus temperature of tin, was examined by comparing the liquidus temperatures of tin at 101 325 Pa and at the vapor pressure of tin (i.e. the triple-point temperature), both of which were realized by the heat pulse-based melting. Periodic square wave-type temperature steps with an amplitude of 0.7 °C were generated in the isothermal region of the pressure-controlled loop heat pipe, and the tin sample, having a segregated impurity distribution established by the prior outward slow freezing, was melted by application of the temperature step-based heat pulses. The triple point temperature was found to be lower than the liquidus temperature of tin at 101 325 Pa by 3.23 mK with an expanded measurement uncertainty of 0.24 mK (i.e. a coverage factor of k = 2), while the ideal temperature difference calculated from the ITS-90 given pressure coefficient (i.e. 3.3 × 10^-8 K/Pa) is about 3.34 mK. The difference between the measured temperature difference and the ideal temperature difference was attributed to the incomplete removal of the gases in the tin triple-point cell. Overall, these results further corroborated the notion that the heat pulse-based melting technique was shown to yield results consistent with the prescription of the ITS-90, and to be a reliable method in terms of the realization of the fixed-point temperatures.

In this work, the liquidus temperature of tin was determined by melting the sample using the pressure-controlled loop heat pipe. Square wave-type pressure steps generated periodic 0.7 °C temperature steps in the isothermal region in the vicinity of the tin sample, and the tin was melted with controllable heat pulses from the generated temperature changes. The melting temperatures at specific melted fractions were measured, and they were extrapolated to the melted fraction of unity to determine the liquidus temperature of tin. To investigate the influence of the impurity distribution on the melting behavior, a molten tin sample was solidified by an outward slow freezing or by quenching to segregate the impurities inside the sample with concentrations increasing outwards or to spread the impurities uniformly, respectively. The measured melting temperatures followed the local solidus temperature variations well in the case of the segregated sample and stayed near the solidus temperature in the quenched sample due to the microscopic melting behavior. The extrapolated melting temperatures of the segregated and quenched samples were 0.95 mK and 0.49 mK higher than the outside-nucleated freezing temperature of tin (with uncertainties of 0.15 mK and 0.16 mK, at approximately 95 % level of confidence), respectively. The extrapolated melting temperature of the segregated sample was supposed to be a closer approximation to the liquidus temperature of tin, whereas the quenched sample yielded the possibility of a misleading extrapolation to the solidus temperature. Therefore, the determination of the liquidus temperature could result in different extrapolated melting temperatures depending on the way the impurities were distributed within the sample, which has implications for the contemporary methodology for realizing temperature fixed points of the International Temperature Scale of 1990 (ITS-90).

Recently, a new type of a hydraulic temperature control technique, which was based on thermo-hydraulic characteristics of a pressure-controlled loop heat pipe (PCLHP), was suggested and proved its effectiveness in terms of the stability, precision, and predictability. However, the hydraulic operating temperature control of a PCLHP showed a temporary operation failure when a large-scale temperature increase was attempted with a rapid increase in the control gas pressure due to a possible pressure inversion between the evaporator and the compensation chamber. Although the PCLHP was soon recovered from the temporary instability, an accompanied sudden temperature drop made the hydraulic temperature control inadequate for applications where fast, large-scale, yet stable temperature controls were required. In this work, transient responses of the PCLHP to various increase rates of the control gas pressure were tested to obtain an optimum increase rate of the control gas pressure at which a large-scale temperature increase could stably be achieved without any instabilities. The tested pressure increase rates were from 25 Pa/s to 100 Pa/s, and the obtained optimum rate was 50 Pa/s for the tested PCLHP at 800 W. At this optimum rate of the control gas pressure increase, the PCLHP showed stable temperature increases with maximum increase rates of about 2 K/min whereas the control gas pressure increases at 100 Pa/s consistently resulted in temporary operation failures. Details on the experiments and the analyses on the obtained results were provided.

The APMP bilateral key comparison APMP.T-K3.4 was initiated by the request of RCM-LIPI (Indonesia) to link their national standards to the average reference values (ARVs) of the CCT-K3. Korea research institute of standards and science (KRISS, Republic of Korea) was requested to provide the linkage to the CCT-K3 for the temperature range from -38.8344 °C to 419.527 °C. In the APMP.T-K3.4, two standard platinum resistance thermometers (SPRTs) were chosen as the artifacts, and they were calibrated at the ITS-90 fixed-points in the comparison range. The fixed-points in this comparison included Zn FP (419.527 °C), Sn FP (231.928 °C), In FP (156.5985 °C), Ga MP (29.7646 °C), and Hg TP (-38.8344 °C). The comparison was carried out in a participant-pilot-participant sequence where KRISS served as the pilot. The linkage was from the fixed-point resistance ratios of RCM-LIPI to the ARVs of the CCT-K3 through the difference between the fixed-point resistance ratios of KRISS and the ARVs of the CCT-K3. The temperature differences between the national standards of RCM-LIPI and the ARVs of the CCT-K3 were agreed within the comparison uncertainties of the ATPM.T-K3.4. This report provides the detailed information on the linkage mechanism, results of the comparison, and bilateral differences between RCM-LIPI and the laboratories participated in the CCT-K3.

The working fluids of loop heat pipes (LHPs) play an important role in the operation of the LHPs by influencing the operating temperatures and the heat transfer limits. Therefore, the proper selection of a working fluid is a key practice in LHP fabrication, and there has been a high demand for an appropriate index that enables the quantitative comparison of the steady-state thermal performance of the working fluids. In this work, a figure of merit for LHPs was theoretically derived and experimentally verified. In particular, the pressure losses in the LHP operation were balanced with the saturation pressure difference between the evaporator and the compensation chamber to derive the figure of merit. This derived figure of merit for LHPs successfully predicted the steady-state thermal performance of the tested working fluids within the variable conductance regime. In the constant conductance regime, the differences in the condenser cooling capacity and in the liquid subcooling for different working fluids determined the thermal performance of each working fluid. The limitations and prospects of the proposed figure of merit were discussed in detail.

In this work, the freezing point of tin (Sn FP) was realized by the inside nucleation where the supercooling of tin and the reheating of the sample after the nucleation were achieved without extracting the cell from an isothermal apparatus. To this end, a novel hydraulic temperature control technique, which was based on thermo-hydraulic characteristics of a pressure-controlled loop heat pipe (LHP), was employed to provide a slow cooling of the sample for a deep supercooling and a fast reheating after the nucleation to minimize the amount of initial freeze of the sample. The required temperature controls were achieved by the active pressure control of a control gas inside the compensation chamber of the pressure-controlled LHP, and the slow cooling at -0.05 K/min for deep supercooling of tin and a fast heating at 2 K/min for reheating the sample after the nucleation were attained. Based on this hydraulic temperature control technique, the nucleation of tin was realized at a supercooling of around 19 K, and the satisfactorily fast reheating of the sample to the plateau producing temperature (i.e., 0.5 K below the Sn FP) was achieved without any temperature overshoots of the isothermal region. The inside nucleated Sn FP showed many desirable features compared to the Sn FP realized by the conventional outside nucleation method. The longer freezing plateaus and the better immersion characteristics of the Sn FP were obtained by the inside nucleation, and the measured freezing temperature of the inside nucleated Sn FP was as much as 0.37 mK higher than the outside nucleated Sn FP with an expanded uncertainty of 0.19 mK. Details on the experiment were provided and explanations for the observed differences were discussed.

In this work, a hydraulic operating temperature control of loop heat pipes (LHPs) was proposed to achieve a precise, stable, and theoretically predictable operating temperature control. To this end, a pressure controlled LHP was devised to control the saturated vapor temperature at the evaporator by applying hydraulic action on the compensation chamber with an immiscible control gas. In particular, by forming an isothermal region in the vapor transport line, it was attempted to control the temperature of the isothermal region, and the resulting operating temperature controllability was investigated in terms of stability, precision, and predictability. Theoretical basis and limitations of the proposed method were established based on the thermo-hydraulic operating principles of the LHPs, and experimental validation was performed using Dowtherm A as a working fluid and helium as the control gas. Test results showed that the devised pressure controlled LHP was able to control the isothermal region temperature within the stability of 25 mK, and the resolution of the temperature control was around 60 mK at 100 Pa change in the control gas pressure. Large scale operating temperature change was also possible, and a guide to achieve an effective operating temperature control was suggested. Details of the design and fabrication of the devised pressure controlled LHP was also provided.

An isothermal region of a finite size is an important requirement for precision metrology and its related industries. In this work, a loop heat pipe (LHP)-based isothermal region generator was devised in which a high speed vapor flow region of the LHP was utilized to achieve the isothermal region. To this end, a stainless steel LHP employing water as a working fluid was designed and fabricated. In particular, an annular type vapor flow region was configured in the vapor transport line to accommodate the isothermal region with dimensions of 50 mm ´ 360 mm (diameter ´ height). Temperature uniformity testing demonstrated that the maximum temperature difference and the effective thermal conductivity of the isothermal region at the maximum heat load were 0.56 °C and 311 000 W/(m K), respectively. The temperature uniformity improved with increasing mass flow rate and showed a dependence on the speed of the working fluid. The steady state operating temperature of the isothermal region ranged from 47.1 °C to 64.0 °C with temperature hysteresis at low heat loads. Details of the design and operating characteristics of the LHP-based isothermal region generator are provided and discussed.

Despite the increasing interest on the loop heat pipes, the uncertainty due to the paucity of reported data on the transient responses makes the widespread application of these devices limited. In this work, a flat evaporator loop heat pipe (FELHP) was developed using a specially devised vapor-liquid separator, and the transient responses of the FELHP were thoroughly examined. Included are the transient responses to start-ups, sinusoidal heat input with various periods and amplitudes, abrupt heat load decreases, and termination of the operation. The FELHP developed herein showed a stable steady state operation from the minimum start-up heat load of 40 W to 140 W. The thermal resistance variation, which varied from 1.14 ºC/W to 0.54 ºC/W, showed the existence of the variable conductance regime. As for the transient responses to the sinusoidal heat input, the FELHP showed the minimum effective period below which it practically did not respond to the heat load variation, and the amplitude dependence of the minimum effective period was observed. In addition, from the comparison of the sinusoidal heat input and the abrupt heat load decrease responses, the mode of heat load decrease was found to be an important influential factor to the operation failure of the FELHP as was the magnitude of the heat load decrease. The FELHP also showed the sleeping mode operation at a very low heat load of 10 W and the stable recovery to the steady state operation was quickly achieved when the heat load was recovered.

Despite the importance of resistance bridges in thermometry as fundamental instruments measuring resistance ratios of SPRTs, an accurate assessment of their nonlinearity and investigation of their behavior under various operating conditions have not been investigated much. Among various methods for nonlinearity assessment, a Hamon-type resistance network known to be RBC (resistance bridge calibrator) is widely used. However, due to finite temperature coefficients of base resistors used in the RBC, the evaluated nonlinearity of the resistance bridges under normal operating environment have been in doubt. In this work, to accurately evaluate the nonlinearity of the resistance thermometry bridge, an air medium thermostatted chamber having peak-to-peak temperature stability of around 0.1 ºC was devised, and using this chamber, the nonlinearity of the resistance bridges at KRISS (two ASL F900s) was assessed. In addition to this, the behavior of the resistance bridge in terms of the nonlinearity was thoroughly investigated under different operating conditions of various gain and bandwidth combinations. The results showed that the resistance thermometry bridge marked the minimum nonlinearity of 11 ppb at 10⁵ gain and 0.1 Hz bandwidth, and showed noticeable dependence on the gain.

Recent advances in electronics technology have led to growing demand for highly efficient heat transfer devices with operating temperature controllability. Although capillary pumped loops are excellent candidates for this requirement and have been successful in various applications, their typical cylindrical evaporator shape has been a major drawback for applications to flat heat sources. Moreover, operating characteristics and the corresponding physical understanding of the capillary pumped loops are still uncertain, and lack of data for their operation makes their application unreliable. Thus, in this work, a capillary pumped loop with a flat evaporator was devised, and its operating characteristics were investigated.

There is growing demand for highly efficient heat transfer devices having excellent performance, operational stability and low power consumption. Although loop heat pipes satisfy these requirements, conventional loop heat pips have limited application to flat heat sources due to their mostly cylindrical evaporator shape. To overcome this limitation, various types of flat evaporator loop heat pipes have been developed, though their operational reliability is still uncertain. In this work, we focused on the development of a flat bifacial evaporator loop heat pipe, and its operating characteristics at transient and steady states are discussed in detail.

Despite the outstanding performance of LHPs over conventional heat pipes, they have limited applications to heat sources with flat surfaces due to their cylindrical thermo-contact surfaces. In order to resolve this problem, we devised a novel planar wick structure, and we examined the operating characteristics at a horizontal orientation for different fluid inventories. Also, as a means of thermally controlling the fuel cells, we evaluated the performance of the planar LHP. Steady state as well as transient state responses were studied in detail, and the relationship between fluid inventory and the operational characteristics of the planar LHP is discussed.