Transient High Heat Flux Research Articles

On the intrusiveness of phosphor thermometry for metal surface temperature measurements in reciprocating engines: an experimental and heat conduction modeling study

Abstract Lifetime-based phosphor thermometry has been applied in a wide variety of surface temperature measurement applications due to its relative ease of implementation and robustness to background interference when compared to other optical temperature measurement methods. It is often assumed that the technique is minimally intrusive if the thickness of the applied phosphor coating is < 20 µm. To evaluate this assumption, high-speed phosphor surface temperature and thermocouple measurements were performed on 4140 steel substrates installed in the cylinder head of an optically-accessible internal-combustion engine for four operating conditions. For phosphor thermometry measurements, four substrates were studied, each coated with a phosphor layer of different thickness ranging between 6 µm and 47 µm. The phosphor thermometry measured temperature swings during combustion were shown to be heavily impacted by the presence of the phosphor coatings, increasing by roughly a factor of 2 - 2.5 when increasing the thickness from 6 µm to 30 µm. A technique was implemented which utilizes the temperature data in combination with a heat conduction model to provide estimates of the temperature swing and heat transfer flux in the absence of the phosphor coating. It was shown that even the 6 µm phosphor coating could lead to an order of magnitude increase in the temperature swing relative to the uncoated 4140 steel substrate. Despite the intrusiveness of phosphor thermometry for the surface temperature measurements, reasonable agreement was demonstrated between heat flux estimates determined with the heat transfer modeling technique and those deduced from temperature-swing measurements using two different high-speed thermocouples. The results indicate that phosphor surface thermometry can be a reliable surface temperature and heat flux diagnostic for transient high heat flux environments, as long as proper care is taken to account for the impact of the phosphor layer on the measurement.

Improved structure and enhanced properties of detonation sprayed W coating via hot isostatic pressing

Tungsten has garnered significant attention for its superior performance as a plasma-facing material in tokamak devices. However, its hardness and high ductile-to-brittle transition temperature present challenges for its application in plasma-facing components. To address these issues, this study employs the detonation spraying (DS) technique to prepare tungsten coatings on a TZM alloy substrate and further optimizes their properties through hot isostatic pressing (HIP) treatment. Advanced characterization techniques were utilized to conduct a detailed microstructural and compositional analysis of the coatings. The results indicated that the HIP treatment significantly reduced the porosity of the coatings, improving their density and overall quality. Additionally, HIP treatment led to a decrease in internal stress, a reduction in dislocations, and the grain growth within the coatings. The study also discussed how the HIP process improved the microstructure of the DS tungsten coatings through these mechanisms, thereby enhancing the coatings' thermodynamic properties. Mechanical property tests revealed that the coatings treated with HIP exhibited higher bonding strength and lower hardness, attributed to the increase in grain size and the reduction in dislocation density within the coatings. Thermal property tests demonstrated that the HIP-treated coatings possessed higher thermal diffusivity and specific heat capacity in the temperature range of 100 to 500 °C. Furthermore, the performance of the coatings under transient high heat flux conditions was significantly improved after the HIP treatment, showing better resistance to the thermal shock and cracking. These findings are of great importance for understanding the performance enhancement of DS tungsten coatings in nuclear fusion reactor applications via HIP, especially as candidate materials for the first wall of tokamak devices.

Performance and damage behaviors of detonation sprayed W/Fe/steel coatings under transient high heat fluxes

The fabrication of high-quality W/steel joints for the blanket first wall presents a challenge in the field of nuclear fusion due to the inherent differences in properties between W and steel. In this study, detonation sprayed (DS) pure iron coatings were utilized as interlayers between DS W coatings and steel to improve the performance of the W/steel joints. The morphology of the DS W/DS Fe/steel coatings indicated that both DS W and DS Fe coating materials were of high quality. Axial tensile tests showed lower bonding strength of the DS W/DS Fe/steel substrate coating system than the DS W/steel coatings. However, the three-layer coating system exhibited much better performance in transient high heat flux (HHF) loading tests compared to the two-layer coating system. Moreover, the addition of DS Fe interlayers changed the failure mode of DS W/steel coatings from W coating detachment to partial melting of the W coating surface. The damage behaviors of the three-layer coating system under 45 MJ/m2 transient HHF were investigated to comprehend the stress-relieving effect of the DS Fe interlayers. These results unequivocally illustrate the possibility of employing DS W/DS Fe/steel coatings as a viable approach for the manufacturing and in-situ repairing of the blanket first wall.

A transient heat flux sensor for PbTe thin films based on transverse Seebeck effect

Among thermoelectric materials with thermoelectric effect, lead telluride (PbTe) is widely used because of its high performance and chemical stability in the medium temperature region. In this study, PbTe was creatively employed to develop a transient thin films heat flux sensor (THFS) using magnetron sputtering technique based on the transverse Seebeck effect. After static and dynamic calibration, the rise time of the THFS is 35 μs and the sensitivity is up to 7.9 μV(kW·m−2)−1, it can be measured in transient high heat flux testing environments without the need for a signal amplifier. In the experiment of measuring transient heat flux with explosion driven shock tube, the THFS has the advantages of high resolution and high dynamic response, which provides scientific basis for the study of explosion thermal damage effect and has important significance.

Transient temperature control performance evaluation of a novel hybrid PCM-based active heat sink

In this study, a novel hybrid PCM-based active heat sink is proposed for temperature control management of transient high heat flux shock electronic devices. The temperature control features of the novel hybrid PCM-based active heat sink are numerically studied under the periodic heat flux condition. Impacts of parameters, including phase-change temperatures, heat transfer coefficient, heat sink parameters, and porosity of the porous metal structure embedded in PCM, are numerically investigated. Then the thermal control performance ratio is evaluated. Results reveal that the innovative hybrid PCM-based active heat sink achieves an outstanding temperature management performance when compared to a heat sink without PCM. And the temperature is reduced by more than 10 K. Increasing the height and width of the heat sink can make the temperature distribution more consistent, but an excessive increase seems unnecessary. Compared with pure PCM, When the porosity of porous metal structure decreases from 0.96 to 0.76, the initial melting time of PCM is delayed by 38.7%, and the solidification time is accelerated by 30.2%.

A transient heat flux sensor based on the transverse Seebeck effect of single crystal Bi2Te3

A novel transient heat flux sensor (THFS) was developed to measure the heat flux of an explosive driven shock tube. The working principle of the THFS is based on the transverse Seebeck effect (TSE) of a single crystal Bi2Te3. The sensor has a rise time of 51 μs and sensitivity of up to 69.8 μV/(kW/m2), with no signal amplifier required for the transient high heat flux test. This sensor allows for highly time-resolved measurements of heat flux in the explosion field, thereby supporting the study of the thermal damage effect of an explosion.

Micromachined Thermocouple for Rapid Detection of Ultrahigh Heat Flux at High Temperature

This article presents a novel high temperature heat flux (HTHF) sensor based on micromachined thermocouple (MTC) to meet the needs of transient high heat flux detection in ultrahigh temperature environments, such as fire research, high-power pulsed lasers testing, and aerospace engineering. The targeted design, fabrication, and package of an MTC-based HTHF sensor with excellent heat dissipation capacity are proposed, and relevant experiments were conducted in two aspects. First, powerful laser tube-based tests at ambient temperature show that such a sensor can detect a heat flux high to 10.8 MW/m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> with an extremely fast response; its response time, in this case, is 1.9 ms. And then, turbine engine test pipe based tests at high temperature reveal that the sensor with water cooling could survive at 1200 °C for at least 100 s. Compared to conventional heat flux sensors, it is more sensitive to transient heat flux changes and has a faster response speed. Such satisfactory performance makes the sensor extremely competitive in ultrahigh heat flux detection in extremely harsh environments.

Simulation of stray radiation from optical window with temperature-dependent spectral properties.

Stray radiation analysis coupled with a temperature field is performed for a semitransparent window, focusing on the temperature-dependent optical properties. The transient temperature response of the optical window-based encapsulation structure is first investigated under an external transient high-heat flux loading. The spectral selectivity of the window to thermal radiation is involved. Subsequently, several typical cases for stray radiation are conducted, considering the inhomogeneous optical properties caused by the temperature heterogeneity. It is found that the stray radiation distribution is more chaotic compared to the results with optical properties independent of temperature. In addition, the stray radiation power has a large deviation (150%) if one neglects the temperature dependence. Meanwhile, the difference in wave band power decreases with the wavelength rising. Additionally, the stray radiation power generated by the window is far less in the visible wave band than that in the near-infrared wave band. The results reveal that the temperature dependence in optical properties of a semitransparent window should be seriously considered when calculating the stray radiation, especially in high-precision detection devices.

Recrystallization suppression through dispersion-strengthening of tungsten

Tungsten is the material of choice for the divertor region of future nuclear fusion reactors, an environment that will expose plasma-facing components (e.g. divertor, etc...) to high temperatures and transient high heat flux events. Under these conditions, recrystallization and grain growth of tungsten can occur, leading to undesirable microstructural and mechanical property changes. Therefore, there is a need to raise the recrystallization temperature of tungsten and limit the kinetics of the recrystallization and grain growth processes. In this work, we examine the effect of different types (TiC vs. TaC vs. ZrC) and different concentrations (1.1 vs. 5 vs. 10 wt.%) of dispersed second phase particles in a tungsten matrix on the high temperature performance. The addition of second-phase particles effectively increases the temperature of and time for recrystallization and slow grain growth; however, the addition of a high weight fraction of particles alters the surface chemistry, which may impact subsequent plasma-surface interactions. These results show that the addition of small concentrations of dispersed particles can be effectively employed in tungsten to raise the upper operating temperature limit for tungsten in a fusion reactor.

Numerical analysis of fatigue behaviors for tungsten armor of ITER divertor

Abstract Thermal responses and fatigue behaviors of the tungsten armor of monoblock divertor under steady-state and transient high heat flux (HHF) loads as well as the coupling of the two were simulated and analyzed by the finite element method. It was found that, under the action of thermal cycle loads, the earliest fatigue failure area of tungsten armor is on the surface, and with the increasing of HHF load, the damage area of the armor extends also, while the transient load only causes damage on surface of the armor. The fatigue lifetime of tungsten armor decreases rapidly with the increase of load, whether under steady-state load or transient load. Different transient load under the same steady-state load is researched. When two loads are coupled, the fatigue life is shorter as the load value is high, while the fatigue lifetime is basically unchanged when the transient load is the same under different steady-state loads, that is to say, the steady-state load has little influence on the results, and the transient load during ELMs is the leading factor for the fatigue lifetime.

Active Radiative Liquid Lithium Divertor for Handling Transient High Heat Flux Events

The extreme heat flux anticipated in fusion reactor divertor plasma facing components (PFCs) is perhaps the most challenging technology issue for fusion energy development. Most divertor PFCs are designed based on the maximum steady-state operational limits. Application of lithium (Li) in NSTX resulted in improved H-mode confinement, H-mode power threshold reduction, and reduction in the divertor peak heat flux while maintaining essentially Li-free core plasma operation even during H-modes. These promising Li results in NSTX and related modeling calculations motivated the passive and active radiative liquid lithium divertor concepts (RLLD and ARLLD) (Ono et al. in Nucl Fusion 53:113030, 2013; Fusion Eng Des 89:2838, 2014). This radiative process has the desired effect of spreading the localized divertor heat load to the rest of the divertor chamber wall surfaces, facilitating divertor heat removal with relatively small amount of lithium ~ few moles/s to handle the expected steady-state high divertor heat load. However, in addition to the high steady-state heat flux, the fusion reactor divertor PFCs could also experience significant transient heat flux such as ELMs and/or other magnetic reconnection events which can deposit large transient heat flux onto the divertor PFCs. If unprotected, it could damage the divertor PFC surfaces which could lead to a highly undesirable unplanned shutdown for PFC repair and/or replacement. In this paper, we explore feasibility of LLD and ARLLD concepts in handling such transient heat flux to protect the divertor PFCs from the extreme transient heat flux while maintaining the normal plasma operations. We also suggest a possible implementation technique using inductive pellet injector for the reactor PFC protection from transient heat flux which can be tested on NSTX-U.

Thermal shock resistance of tungsten with various deformation degrees under transient high heat flux

Tungsten (W) is considered as the most promising plasma facing material in fusion device, which will be exposed to steady and transient heat loads. Usually, the deformation degree of W influences its thermal shock properties. So we evaluated the thermal shock resistance of rolled pure tungsten (PW) with 60%, 90% deformation degrees and W-1.0wt%La2O3 (WL10) with 52%, 88% deformation degrees using electron beam at an absorbed power density of 0.22 GW m−2 for 5 ms and further discussed the relationship between the thermal shock resistance and microstructures, thermal-mechanical properties. The absorbed beam current (30 mA), electron beam acceleration voltage (120 kV) and loaded area (4 × 4 mm2) is used to estimate the absorbed power density. The results indicated that PW-90%, LW-88% exhibited smaller grain size, higher relative density, microhardness but lower strength, thermal conductivity compared to PW-60%, LW-52%. The cracking threshold was <0.22 GW m−2 for PW-60%, LW-52% and >0.22 GW m−2 for PW-90%, LW-88%. Elliptic left pores in PW-60% and LW-52% aggravated the effect of stress concentration cracking during the transient high heat flux test and decreased the cracking threshold altough they exhibited high strength and thermal conductivity.

Lunar Cumulate Mantle Overturn: A Model Constrained by Ilmenite Rheology

AbstractLunar cumulate mantle overturn has been proposed to explain the abundances of TiO2 and heat‐producing elements (U, Th, and K) in the source region of lunar basalts. Ilmenite‐bearing cumulates (IBCs) that were formed near the end of lunar magma ocean solidification are the driving force for overturn. IBCs are enriched with dense TiO2 and FeO contents and have lower viscosity and solidus than those of the underlying lunar cumulate mantle. We investigate the effects of temperature‐ and ilmenite‐dependent mantle rheology on the dynamic process of lunar cumulate mantle overturn and conditions for long‐wavelength downwellings of an IBC layer in a 3‐D spherical geometry. Our results show that the ilmenite‐induced rheological weakening is necessary to decouple the IBC layer from the top stagnant lid and facilitate overturn. Models with IBC viscosity derived from the experimental scaling can only produce short‐wavelength downwellings (spherical harmonic degree &gt;3) and show an overturn timescale more than 100 Ma. A viscosity of the IBC layer at least 10−4 lower than that of the ambient mantle can produce the long‐wavelength (spherical harmonic degree ≤3) downwellings in ~10 Ma and even a hemispheric downwelling. Such low IBC viscosity requires additional weakening mechanisms, such as remelting or/and water enrichment. During the overturn, the cold downwellings displace upward the materials from hot lower mantle and produce partial melting in upper mantle, which may serve as a viable mechanism for early lunar magmatisms. The settling of cold downwellings on the core‐mantle boundary stimulates a transient high heat flux, which may contribute to generating an early lunar dynamo event.

A New Transient High Heat Flux Convection Calibration Facility for Heat Transfer Gauges in High Enthalpy Flows

This paper presents a novel transient method for calibrating heat transfer gauges for convective wall heat flux measurements in high enthalpy flows. The new method relies on the transient heating of sensor substrates under rapid exposure to a hot flow in order to obtain the necessary reference heat flux. Compared to previous calibration facilities, the new facility is simple, inexpensive, easy to adapt for different flow configurations and sensor geometries, and quick to run across a wide range of conditions. In this paper, the design of the new calibration facility is described and the transient calibration method explained. The method is demonstrated by calibrating a Gardon gauge at convective heat transfer coefficients between 300 and 600 W/m2 K. Typical facility data and calibration results are presented in terms of voltage-heat flux sensitivity and calibration correction ratio. These results are shown to agree with theoretical estimates within the estimated calibration uncertainty.

Surface Ablation and Melting of Fusion Materials Simulated by Transient High Heat Flux Generated in an Electothermal Plasma Source

An electrothermal (ET) plasma source has been used to produce intense transient high heat flux to evaluate surface erosion and melting of plasma-facing materials. The source produces high heat flux relevant to expected conditions during disruption event in future large fusion devices. Surface vaporization has been evaluated for selected fusion-relevant materials under radiant energy deposition. Additional simulation included the effect of melting and splattering. Vapor and droplet formation and their associated shielding effect have been investigated in this paper to assess the difference between a developed boundary layer from only vapor or melt layer or the mixed vapor-melt layer with possible ejection of molten droplets away from the surface. Fully self-consistent erosion models are developed and implemented in the ET ETFLOW code in a new version ETFLOW-boundary layer to model the response of plasma-facing materials and their erosive behavior under transient ET plasma high heat fluxes closely similar to expected ones in future fusion large tokamaks.

Thermal Shock Performance of Sintered Pure Tungsten with Various Grain Sizes Under Transient High Heat Flux Test

Pure tungsten samples with multimodal grain size of 1.85 ± 0.84/0.47 ± 0.2 μm (PW1) and grain size of 0.33 ± 0.1 μm (PW2) were prepared by resistance sintering under ultra high pressure. Then the thermal shock performance of both W was evaluated using the electron beam facility with 1 cycle and 5 ms pulse duration at 0.22 GW/m2. PW1 exhibited higher relative density, thermal conductivity but lower microhardness and bending strength compared with PW2. Furthermore, PW1 displayed slight higher major crack density (smaller major crack distance) while slight smaller major crack width compared with PW2. Besides, the microcracks that only formed in PW2 have the potential to detach W grains from the matrix. Moreover, both samples showed close major crack depth and surface roughness. Thus it can be concluded that PW1 with large grain size showed better thermal shock performance compared with PW2 with small grain size.

High heat flux testing of divertor plasma facing materials and components using the HHF test facility at IPR

The High Heat Flux Test Facility (HHFTF) was designed and established recently at Institute for Plasma Research (IPR) in India for testing heat removal capability and operational life time of plasma facing materials and components of the ITER-like tokamak. The HHFTF is equipped with various diagnostics such as IR cameras and IR-pyrometers for surface temperature measurements, coolant water calorimetry for absorbed power measurements and thermocouples for bulk temperature measurements. The HHFTF is capable of simulating steady state heat load of several MW m−2 as well as short transient heat loads of MJ m−2. This paper presents the current status of the HHFTF at IPR and high heat flux tests performed on the curved tungsten monoblock type of test mock-ups as well as transient heat flux tests carried out on pure tungsten materials using the HHFTF. Curved tungsten monoblock type of test mock-ups were fabricated using hot radial pressing (HRP) technique. Two curved tungsten monoblock type test mock-ups successfully sustained absorbed heat flux up to 14 MW m−2 with thermal cycles of 30 s ON and 30 s OFF duration. Transient high heat flux tests or thermal shock tests were carried out on pure tungsten hot-rolled plate material (Make:PLANSEE) with incident power density of 0.49 GW m−2 for 20 milliseconds ON and 1000 milliseconds OFF time. A total of 6000 thermal shock cycles were completed on pure tungsten material. Experimental results were compared with mathematical simulations carried out using COMSOL Multiphysics for transient high heat flux tests.

Molecular dynamic simulation of tungsten ablation under transient high heat flux

Molecular dynamic (MD) method is used to simulation the tungsten ablation under transient high heat flux generated by energetic ions. A model including 363,600W atoms was built based on Finnis–Sinclair potential. The results show that the ablation threshold is much lower than the one of boiling. So the ablation effects might be underestimated if using energy threshold of boiling instead of that of ablation. Particle size distribution of ablation products follows a power decay law with an exponent around −2.5, which does not affect by the incident heat flux. The transverse velocities of particles obey normal distribution, and a stream speed is added to the random movement for the longitudinal velocity. As the ablation start up, the recoiled impulse can induce shock wave in remained target, which is supported by experimental pressure wave measurements.

Micro/nano composited tungsten material and its high thermal loading behavior

Tungsten (W) is considered as promising candidate material for plasma facing components (PFCs) in future fusion reactors attributing to its many excellent properties. Current commercial pure tungsten material in accordance with the ITER specification can well fulfil the performance requirements, however, it has defects such as coarse grains, high ductile–brittle transition temperature (DBTT) and relatively low recrystallization temperature compared with its using temperature, which cannot meet the harsh wall loading requirement of future fusion reactor. Grain refinement has been reported to be effective in improving the thermophysical and mechanical properties of W. In this work, rare earth oxide (Y2O3/La2O3) and carbides (TiC/ZrC) were used as dispersion phases to refine W grains, and micro/nano composite technology with a process of “sol gel – heterogeneous precipitation – spray drying – hydrogen reduction – ordinary consolidation sintering” was invented to introduce these second-phase particles uniformly dispersed into W grains and grain-boundaries. Via this technology, fine-grain W materials with near-full density and relatively high mechanical properties compared with traditional pure W material were manufactured. Preliminary transient high-heat flux tests were performed to evaluate the thermal response under plasma disruption conditions, and the results show that the W materials prepared by micro/nano composite technology can endure high-heat flux of 200MW/m2 (5ms).

Recent progress in the NSTX/NSTX-U lithium programme and prospects for reactor-relevant liquid-lithium based divertor development

Developing a reactor-compatible divertor has been identified as a particularly challenging technology problem for magnetic confinement fusion. Application of lithium (Li) in NSTX resulted in improved H-mode confinement, H-mode power threshold reduction, and other plasma performance benefits. During the 2010 NSTX campaign, application of a relatively modest amount of Li (300 mg prior to the discharge) resulted in a ∼50% reduction in heat load on the liquid lithium divertor (LLD) attributable to enhanced divertor bolometric radiation. These promising Li results in NSTX and related modelling calculations motivated the radiative LLD concept proposed here. Li is evaporated from the liquid lithium (LL) coated divertor strike-point surface due to the intense heat flux. The evaporated Li is readily ionized by the plasma due to its low ionization energy, and the poor Li particle confinement near the divertor plate enables ionized Li ions to radiate strongly, resulting in a significant reduction in the divertor heat flux. This radiative process has the desired effect of spreading the localized divertor heat load to the rest of the divertor chamber wall surfaces, facilitating the divertor heat removal. The LL coating of divertor surfaces can also provide a ‘sacrificial’ protective layer to protect the substrate solid material from transient high heat flux such as the ones caused by the edge localized modes. By operating at lower temperature than the first wall, the LL covered large divertor chamber wall surfaces can serve as an effective particle pump for the entire reactor chamber, as impurities generally migrate towards lower temperature LL divertor surfaces. To maintain the LL purity, a closed LL loop system with a modest circulating capacity (e.g., ∼1 l s−1 for ∼1% level ‘impurities’) is envisioned for a steady-state 1 GW-electric class fusion power plant.