Heating Power Density Research Articles

Comparison between 0D and 1D approaches for mechanical dissipation measurement during fatigue tests

AbstractFatigue in materials is generally associated with the production of heat, leading to the “self‐heating” of the tested material. The associated heat power density, named mechanical dissipation or intrinsic dissipation, can be deduced from the temperature changes captured on the tested specimen's surface by infrared thermography. When mechanical dissipation is spatially homogeneous in the tested specimen, the processing can be performed using a macroscopic approach, also named zero‐dimensional (0D) approach. The latter uses an averaged temperature over the whole specimen's measurement zone. The present study aims to analyse the error generated by the 0D approach in the assessment of mechanical dissipation. This error was measured with respect to a one‐dimensional (1D) approach, which is applicable for longitudinal specimens subjected to uniaxial loading. Experimental tests were performed on pure copper and acrylic glass. A model was also developed to analyse the influence of the material and of the heat exchanges with the specimen's environment. The results obtained show that the error generated by the 0D approach in mechanical dissipation measurement may not be negligible and that attention should be paid to the choice of approach for fatigue analysis.

Electrical resistance heating for deicing and snow melting applications: Experimental study

In this study, a novel electrical resistance heating method for deicing and snow melting applications was investigated. Three different forms of carbon fiber were embedded into concrete specimens and their heating performances were tested. To simulate the condition of concrete exposed to low temperatures, an environmental chamber was utilized. The effect of various parameters such as heat power density, ambient temperature, heating panel installment depth, concrete moisture and form of carbon fibers on temperature changes was investigated. Experimental results showed that carbon fiber electrical heating can offer viable solution for icing and snow accumulation problems. Compared to other heated pavement systems, it has relatively lower installment and material cost. The power densities of a heating system should be designed depending on a specific application. Otherwise, successful deicing operation may not be guaranteed. Using carbon fiber in cable form provides many advantages compared to using woven and unidirectional carbon fabrics. Placing heat panels close to concrete surface affected heating performance positively but long term integrity of heat panels may be affected by placing them very close to the surface. Even though some decrement was observed, moisture in concrete did not significantly degrade the heating performance.

Innovative Target for Production of Technetium-99m by Biomedical Cyclotron.

Technetium-99m (99mTc) is the most used radionuclide worldwide in nuclear medicine for diagnostic imaging procedures. 99mTc is typically extracted from portable generators containing 99Mo, which is produced normally in nuclear reactors as a fission product of highly enriched Uranium material. Due to unexpected outages or planned and unplanned reactor shutdown, significant 99mTc shortages appeared as a problem since 2008 The alternative cyclotron-based approach through the 100Mo(p,2n)99mTc reaction is considered one of the most promising routes for direct 99mTc production in order to mitigate potential 99Mo shortages. The design and manufacturing of appropriate cyclotron targets for the production of significant amounts of a radiopharmaceutical for medical use is a technological challenge. In this work, a novel solid target preparation method was developed, including sputter deposition of a dense, adherent, and non-oxidized Mo target material onto a complex backing plate. The latter included either chemically resistant sapphire or synthetic diamond brazed in vacuum conditions to copper. The target thermo-mechanical stability tests were performed under 15.6 MeV proton energy and different beam intensities, up to the maximum provided by the available GE Healthcare (Chicago, IL, USA) PET trace medical cyclotron. The targets resisted proton beam currents up to 60 µA (corresponding to a heat power density of about 1 kW/cm2) without damage or Mo deposited layer delamination. The chemical stability of the proposed backing materials was proven by gamma-spectroscopy analysis of the solution obtained after the standard dissolution procedure of irradiated targets in H2O2.

High power density two-phase cooling in microchannel heat exchangers

This paper reports two-phase cooling in compact cross-flow microchannel heat exchangers with high power density up to 180 W/cm3. The performance is enabled by high-speed air flow through microchannels and two-phase condensation of refrigerant R245fa. The heat exchangers were realized in 1 cm3 blocks of copper alloy, using micro-electrical-discharging machining. Two heat exchanger designs were analyzed, fabricated, and tested. The first device has 150 air-side channels of diameter 520 μm, and the second device has 300 air-side channels of diameter 355 μm. In both cases the refrigerant channels are 2.0 × 0.5 mm2. The heat exchangers were operated with Reynolds number between 7500 and 20,500 for the air flow and with mass flux between 330 and 750 kg/m2 s for the refrigerant flow. The refrigerant temperature at the channel entrance was 80 °C, which is near the maximum operating temperature for some electronic devices. For comparison purposes, the devices were also tested with single-phase refrigerant flows. This work demonstrates the potential of high power density heat exchangers that leverage advanced manufacturing technologies to fabricate miniature channels.

Electromagnetic Conditions in a Tundish with Channel Type Induction Heating

Numerical simulation is one of the effective methods to solve the magneto‐hydrodynamic problems in electromagnetic metallurgical reactor. A mathematical statement about magnetic vector potential and electric potential is developed to describe the three‐dimensional magnetic field, induced current field, Joule heating power field, and electromagnetic force field in a tundish with channel type induction heating. The resultant equations are solved numerically, and the predicted magnetic field agrees well with the experimental data from the previous reference. The research results show that the magnetic field, the induced current field, the electromagnetic force field, and the heating power are the strongest in the channels, and these fields are eccentric in the channel. In the channel, the induced current is along the axis of the channel, the magnetic field is along the circumferential direction of the channel, the electromagnetic force is along the radius of the channel, and Joule heat power density on the side facing the other channel is greater than that on the side facing away from the other channel.

Improvements of interfacial friction and heat transfer models for rectangular narrow channel reflood simulation based on RELAP5

Abstract Plate fuel assembly is widely used in reactor cores due to benefits of the narrow rectangular channels. Compared with rod assembly, the heat transfer area and power density can be increased in narrow rectangular channel, where little researches on post-CHF thermal-hydraulic characteristics have been done. Once the loss of coolant accident happens, post-CHF thermal-hydraulic characteristics are important for accident mitigation. This paper mainly evaluates the corresponding reflood models in RELAP5/MOD3 code. Combined with the post-CHF thermal-hydraulic characteristics in the narrow channel, interfacial friction model and post-CHF heat transfer models are modified. According to the comparison results with Saxena reflood experiments, conclusions can be obtained that the original RELAP5/MOD3 code overestimates quench front velocity and heat transfer in the film boiling region. The results from the code with new models agree better with the experimental data.

A Numerical Study on the Light-Weight Design of PTC Heater for an Electric Vehicle Heating System

As the market for electric vehicles grows at a remarkable rate, various models of electric vehicles are currently in development, in parallel to the commercialization of components for diverse types of power supply. Cabin heating and heat management components are essential to electric vehicles. Any design for such components must consider the requirements for heating capacity and power density, which need to reflect both the power source and weight reduction demand of any electric vehicle. In particular, design developments in electric heaters have predominantly focused on experimental values because of structural characteristics of the heater and the variability of heat sources, requiring considerable cost and duration. To meet the ever-changing demands of the market, an improved design process for more efficient models is essential. To improve the efficacy of the design process for electric heaters, this study conducted a Computational Fluid Dynamics (CFD) analysis of an electric heater with specific dimensions by changing design parameters and operating conditions of key components. The CFD analysis modeled heat characteristics through the application of user-defined functions (UDFs) to reflect temperature properties of Positive Temperature Coefficient (PTC) elements, which heat an electric heater. Three analysis models, which included fin as well as PTC elements and applied different spaces between the heat rods, were compared in terms of heating performance. In addition, the heat performance and heat output density of each analysis model was analyzed according to the variation of air flow at the inlet of the radiation section of an electric heater. Model B was selected, and a prototype was fabricated based on the model. The performance of the prototype was evaluated, and the correlation between the analysis results and the experimental ones was identified. The error rate between performance change rates was approximately 4%, which indicated that the reliability between the design model and the prototype was attained. Consequently, the design range of effective performance and the guideline for lightweight design could be presented based on the simulation of electric heaters for various electric vehicles. The fabrication of prototypes and minimum comparison demonstrated opportunities to reduce both development cost and duration.

3000 W tandem pumped all-fiber laser based on domestic fiber

In recent years, high power fiber laser has received great attention, leading to wide applications in numerous fields such as industry, biology and relevant research. Nevertheless, the output power of typical diode pumped fiber laser is limited by the thermal effect and brightness of pump source. Owing to the low quantum deficit, the tandem pumping employing ytterbium-doped fiber lasers (YDFLs) as the pumping source can effectively reduce the thermal issue and achieve high power output. With the much lower absorption coefficient at 1018 nm than at 976 nm, longer gain fiber is necessary in tandem pumped configuration to sufficiently absorb pump light, which in turn induces a more severe nonlinear effect such as the stimulated Raman scattering, bringing in more challenges in laser configuration design. In this paper, we demonstrate an all-fiber laser under master oscillator power amplifier configuration based on tandem pumping with domestic gain fiber produced by China Electronics Technology Group Corporation No. 46 Research Institute. The diameters of the core and inner cladding of the Yb3+ doped double cladding fiber are 25 m and 250 m, respectively. The modified chemical vapor deposition method with gas-solution co-doping method is adopted so that the fiber has a more uniform distribution of Yb ions, larger absorption cross section and higher absorption coefficient (0.41 dB/m@1018 nm). In the amplifier stage, a 40-m-long gain fiber is pumped by fourteen 1018 nm fiber lasers with a maximum total output power of 3511 W. A 67.8 W 1080 nm seed is amplified to 3079 W with a corresponding slope efficiency of 85.9%. The beam quality factor M2 is measured to be 2.14. In addition, no stimulated Raman scattering is found in output spectrum and the 3 dB band width of output laser is measured to be 1.4 nm. To the best of our knowledge, this marks the highest result ever reported for tandem pumping based on domestic gain fiber. Taking stimulated Raman scattering into account, the rate equations are built to calculate the properties and power evolution in the fiber amplifier. The numerical results are in good agreement with the experiment results. Besides, based on heat conduction equation, heat power density in the fiber core is analyzed, showing that the tandem pumping has great advantages in heat management and a huge potential to reach a higher power compared with the method of direct pumping. The theoretical and experimental results show that with ever-maturing fiber manufacturing technology, domestic fiber is capable of withstanding laser power as high as 3 kilowatts. Meanwhile, domestic fiber may achieve a higher output power by increasing the pump power, optimizing the gain fiber length and improving the cooling condition.

Strain‐induced crystallization in rubber: A new measurement technique

AbstractThe crystallinity of stretched crystallizable rubbers is classically investigated using X‐ray diffraction. In this study, we propose a new method based on temperature measurement and quantitative calorimetry to determine rubber crystallinity during mechanical tests. For that purpose, heat power density are first determined from temperature variation measurements and the heat diffusion equation. The increase in temperature due to strain‐induced crystallization is then deduced from the heat power density by subtracting the part due to elastic couplings. The heat capacity, the density, and the enthalpy of fusion are finally used to calculate the crystallinity from the temperature variations due to strain‐induced crystallization. The characterization of the stress–strain relationship and the non‐entropic contributions to rubber elasticity is not required. This alternative crystallinity measurement method is therefore a user‐friendly measurement technique, which is well adapted in most of the mechanical tests carried out with conventional testing machines. It opens numerous perspectives in terms of high speed and full crystallinity field measurements.

Light-driven self-assembly of hetero-shaped gold nanorods

Light-driven self-assembly and coalescence of two nearby hetero-shaped gold nanorods (GNRs) with different lengths are studied theoretically. The optical forces and torques, in terms of Maxwell’s stress tensor, upon these GNRs provided by a linearly polarized (LP) plane wave are analyzed using the multiple multipole (MMP) method. Numerical results show that the optical torque dominates their alignments and the optical force their attraction. The most likely outcome of the plasmon-mediated light–matter interaction is wavelength dependent. Three different coalescences of the two GNRs could be induced by a LP light in three different wavelength regimes, respectively. For example, the side-by-side coalescence of two GNRs with radius of 15 nm and different lengths (120 and 240 nm) is induced in water as irradiated by a LP light at 633 nm, the T-shaped one at 1064 nm, and the end-to-end one at 1700 nm. The plasmonic attractive force and heating power densities inside GNRs with different gaps are also studied; the smaller the gap, the larger the attractive force and heating power. The results imply that the plasmonic coalescence and heating of two discrete GNRs may cause the local fusion at the junction of the assembly and the subsequent annealing (even recrystallization). Because the heating makes the two discrete GNRs fused to become a new nanostructure, the plasmonic coalescence of optical manipulation is irreversible.

Toroidal inhomogeneity of plasma density fluctuations during ECR plasma heating in the L-2M stellarator

Correlation between short-wavelength (k⊥ ≈ 20–30 cm–1) and long-wavelength (k⊥ ≈ 1–2 cm–1) plasma density fluctuations in two poloidal cross sections of the stellarator chamber separated by 1/14 or 5/14 of the torus perimeter was studied using collective scattering of radiation of two 75-GHz gyrotrons and radiation of a 37-GHz Doppler reflectometer at an ECR heating power density of 1.6–3.2 MW/m3. It is found that excitation of turbulent fluctuations is bursty in character and that fluctuations excited in different L-2M cross sections are uncorrelated. It is shown that the energy of turbulent fluctuations is modulated by a low frequency of 5–20 kHz. An idea is put forward that anomalous transport is toroidally inhomogeneous.

FEM thermal and stress analysis of bonded GaN-on-diamond substrate

A three-dimensional thermal and stress analysis of bonded GaN on diamond substrate is investigated using finite element method. The transition layer thickness, thermal conductivity of transition layer, diamond substrate thickness and the area ratio of diamond and GaN are considered and treated appropriately in the numerical simulation. The maximum channel temperature of GaN is set as a constant value and its corresponding heat power densities under different conditions are calculated to evaluate the influences that the diamond substrate and transition layer have on GaN. The results indicate the existence of transition layer will result in a decrease in the heat power density and the thickness and area of diamond substrate have certain impact on the magnitude of channel temperature and stress distribution. Channel temperature reduces with increasing diamond thickness but with a decreasing trend. The stress is reduced by increasing diamond thickness and the area ratio of diamond and GaN. The study of mechanical and thermal properties of bonded GaN on diamond substrate is useful for optimal designs of efficient heat spreader for GaN HEMT.

Investigation on an innovative resorption system for seasonal thermal energy storage

An innovative resorption system is established and investigated for seasonal thermal energy storage. Solar energy is stored in form of chemical potential in summer whereas the stored energy could be released in form of sorption heat in winter. Working pair of MnCl2-CaCl2-NH3 is selected and composite sorbents are developed with expanded natural graphite treated with sulfuric acid as the matrix for heat and mass transfer intensification. It is indicated that the highest effective heat storage density, heat power density and system COP are able to reach 1047kJkg−1, 402Wkg−1 and 0.58 under the condition of 30°C heat output temperature and 15°C ambient temperature. Novel resorption thermal energy storage system verifies the feasibility for seasonal energy storage at high ambient temperature in winter, which reveals great potentials for solar energy utilization. Also worth noting that two possible solutions i.e. temperature upgrade mode and sorption-compression mode are analyzed and compared when ambient temperature is relatively low i.e. below 0°C. Results demonstrate that heat could be supplied in term of −15°C ambient temperature and 50°C heat output temperature. Two methods could deal with the issues at low ambient temperature, which have their respective advantages for different applications.

Two-slope soft X-ray spectra observed in experiments on electron cyclotron resonance plasma heating in the L-2M stellarator

In experiments on the generation and electron cyclotron resonance heating (ECRH) of plasma in the L-2M stellarator, non-Maxwellian two-slope soft X-ray (SXR) spectra were observed. The temperatures of the thermal and epithermal components of the spectra were measured as functions of the heating power and plasma density. A hypothesis based on the experimental results is suggested to explain the formation mechanism of two-slope SXR spectra in the ECRH experiments at the L-2M stellarator. The measured SXR spectra are compared with the results of numerical simulations.

Experimental investigation of n-eicosane based circular pin-fin heat sinks for passive cooling of electronic devices

Efficient thermal management (TM) based on phase change material (PCM) is adopted for the cooling of portable electronic devices. PCM namely n-eicosane is employed to absorb thermal energy released by such electronics. Four different configurations of circular pin-fin heat sinks with fin thickness of 2mm, 3mm and 4mm including a no fin heat sink (used as a reference heat sink) were adopted. Pin-fins were made of aluminum due to light weight and good thermal conductivity to act as a thermal conductivity enhancers (TCEs). Pin-fin heat sinks of constant (9%) volume fraction of TCE are filled with four volumetric fractions of PCM to explore the best amount of PCM volume. A wide range of heat flux is provided at the heat sink base and the effect of fin configuration, PCM volume, latent heat phase, power densities, thermal capacity and thermal conductance are reported in this study. Three different critical set point temperatures (SPTs) are selected for this investigation. Enhancement ratios are reported against various PCM fractions to illustrate the thermal performance for passive cooling. The results show that 3mm fin thickness heat sink has best enhancement in operation for TM module controlling temperature of electronic devices.

Extension of the double-ellipsoidal heat source model to narrow-groove and keyhole weld configurations

The double-ellipsoidal heat power density model proposed by Goldak, has been widely used as the basis for modelling heat transfer in arc welding operations for more than thirty years. This approach has proved to be extremely effective for a wide range of arc welding operations. However, the application of a double-ellipsoidal heat power density distribution is less appropriate for keyhole-laser or electron-beam welding operations, or in situations where arc welding takes place within deep narrow grooves. In this paper the double-ellipsoidal distribution is extended to a double-ellipsoidal-conical heat power density model in order to accurately describe transient temperature fields for a wider range of geometries and welding processes. The new extended model was validated through comparing predicted welding thermal cycles with those measured for a single pass electron beam weld, as well as those measured in a multi-pass narrow groove gas-tungsten-arc weld. In both cases, excellent agreement was obtained between predicted and measured thermal transients.

Interfacial thermal conductance in graphene/black phosphorus heterogeneous structures

Graphene, as a passivation layer, can be used to protect the black phosphorus (BP) from the chemical reaction with surrounding oxygen and water. However, BP and graphene heterostructures have low efficiency of heat dissipation due to its intrinsic high thermal resistance at the interfaces. The accumulated energy from Joule heat has to be removed efficiently to avoid the malfunction of the devices. Therefore, it is of significance to investigate the interfacial thermal dissipation properties and manipulate the properties by interfacial engineering on demand. In this work, the interfacial thermal conductance between few-layer BP and graphene is studied extensively using molecular dynamics simulations. Two important parameters, Pcr, the critical heat power density of maintaining thermal stability, and Pmax, the maximum heat power density with which the system can be loaded, are identified. Our results show that interfacial thermal conductance can be effectively tuned in a wide range by external strains and interfacial defects. The compressive strain can enhance the interfacial thermal conductance by one order of magnitude, while interface defects give a two-fold increase. These findings could provide guidelines in heat dissipation and interfacial engineering for thermal conductance manipulation of BP-graphene heterostructure-based devices.

A novel polyimide based micro heater with high temperature uniformity

MEMS based micro heaters are a key component in micro bio-calorimetry, nondispersive infrared gas sensors, semiconductor gas sensors and microfluidic actuators. A micro heater with a uniform temperature distribution in the heating area and short response time is desirable in ultrasensitive temperature-dependent measurements. In this paper we propose a novel micro heater design to reach a uniform temperature in a large heating area by optimizing the heating power density distribution in the heating area. A polyimide membrane is utilized as the substrate to reduce the thermal mass and heat loss which allows for fast thermal response as well as a simplified fabrication process. A gold and titanium heating element is fabricated on the flexible polyimide substrate using the standard MEMS technique. The temperature distribution in the heating area for a certain power input is measured by an IR camera, and is consistent with FEA simulation results. This design can achieve fast response and uniform temperature distribution, which is quite suitable for the programmable heating such as impulse and step driving.

Consistent neutron-physical and thermal-physical calculations of fuel rods of VVER type reactors

For modeling the isotopic composition of fuel, and maximum temperatures at different moments of time, one can use different algorithms and codes. In connection with the development of new types of fuel assemblies and progress in computer technology, the task makes important to increase accuracy in modeling of the above characteristics of fuel assemblies during the operation. Calculations of neutronphysical characteristics of fuel rods are mainly based on models using averaged temperature, thermal conductivity factors, and heat power density. In this paper, complex approach is presented, based on modern algorithms, methods and codes to solve separate tasks of thermal conductivity, neutron transport, and nuclide transformation kinetics. It allows to perform neutron-physical and thermal-physical calculation of the reactor with detailed temperature distribution, with account of temperature-depending thermal conductivity and other characteristics. It was applied to studies of fuel cell of the VVER-1000 reactor. When developing new algorithms and programs, which should improve the accuracy of modeling the isotopic composition and maximum temperature in the fuel rod, it is necessary to have a set of test tasks for verification. The proposed approach can be used for development of such verification base for testing calculation of fuel rods of VVER type reactors

Extension of electron cyclotron heating at ASDEX Upgrade with respect to high density operation

The ASDEX Upgrade electron cyclotron resonance heating operates at 105 GHz and 140 GHz with flexible launching geometry and polarization. In 2016 four Gyrotrons with 10 sec pulse length and output power close to 1 MW per unit were available. The system is presently being extended to eight similar units in total. High heating power and high plasma density operation will be a part of the future ASDEX Upgrade experiment program. For the electron cyclotron resonance heating, an O-2 mode scheme is proposed, which is compatible with the expected high plasma densities. It may, however, suffer from incomplete single-pass absorption. The situation can be improved significantly by installing holographic mirrors on the inner column, which allow for a second pass of the unabsorbed fraction of the millimetre wave beam. Since the beam path in the plasma is subject to refraction, the beam position on the holographic mirror has to be controlled. Thermocouples built into the mirror surface are used for this purpose. As a protective measure, the tiles of the heat shield on the inner column were modified in order to increase the shielding against unabsorbed millimetre wave power.