Size Of Heat Source Research Articles

Quantitative assessment of thermal resistance in graphene field-effect transistors based on a phonon hydrodynamics and van der Waals coupling approach

In the progressive miniaturization of graphene field-effect transistors (GrFETs), pronounced thermal boundary resistance (TBR) and increasing ballistic-diffusive thermal resistance pose formidable challenges to thermal management. Quantitative analysis of thermal resistance is crucial for advancing efficient thermal designs in GrFETs using state-of-the-art technology. The inherently low-dimensional structure nature of graphene means that thermal conduction in GrFETs, governed by phonon hydrodynamic transport and van der Waals (vdW) interactions, cannot be accurately described by traditional Fourier's law-based models or the single-mode relaxation time approximation. This study introduces an innovative thermal simulation framework that addresses these challenges by incorporating phonon hydrodynamics equations and considering weak vdW coupling. Our findings indicate that thermal conduction in GrFETs is predominantly facilitated by flexural (ZA) phonons, and their thermal transport efficiency is significantly influenced by substrate perturbations. Specifically, the TBR in GrFETs is closely associated with the weak vdW coupling between ZA phonons and substrate atoms. Furthermore, this study employs dimensionless thermal resistance ratios and Knudsen numbers to explore the impact of silicon's geometric dimensions, heat source size, and ballistic phenomena on the substrate's thermal resistance. These insights provide substantial theoretical support for the development of efficient thermal management strategies in GrFETs.

Optimising liquid impingement jet arrays for concentrated heat sources

This work seeks to investigate jet array impingement liquid cooling of CPU processors that utilise Integrated Heat Spreaders. In this study, single and centralised heat sources of various sizes (10 mm to 20 mm) were investigated, and a series of single and multi-objective optimisation studies were carried out to determine the optimum arrangement of microjets for different heat source sizes based on variable geometric inputs of the jet orifice plate. The results indicated that the design of the optimum jet orifice strongly depends on the size of the heat source as well as whether the target optimisation strategy is to achieve minimum thermal resistance, or both minimum thermal resistance and minimum pumping power simultaneously. A clustering of quite different orifice designs in optimal zones is observed, where approximately the same hydraulic and/or thermal performance is observed.

Graph convolution network-based surrogate model for natural convection in annuli

This work develops a model for natural convection in annuli with internal heat sources based on Graph Convolution Network (GCN), achieving rapid prediction by directly establishing the mapping relationship between the geometric features and the temperature field. The GCN learns node features and connections between nodes in data structures. Compared with traditional machine learning networks, GCN exhibits greater advantages in handling complex geometric shapes and unstructured nodes. In this study, numerous data samples were constructed with various geometric features, e.g., varying sizes, positions, and quantities of internal heat sources. The results indicate that a fully trained GCN was capable to adaptively predict temperature field distributions with arbitrary heat source sizes, positions, and quantities. The mean prediction errors of GCN for the temperature field are around 0.17 %, 0.32 %, and 0.85 %, for the single, double, and triple heat sources cases, respectively. Furthermore, the GCN prediction performance was also compared with surrogate models by the convolutional, and fully connected neural networks (CNN and FNN), which shows an improvement of 88.1 % and 84.8 %, respectively. Therefore, compared with numerical results and other surrogate models, the proposed model presents superior geometric adaptability, high prediction accuracy and a remarkable enhancement in computational efficiency (three orders of magnitude).

Two-temperature principle for evaluating electrothermal performance of GaN HEMTs

Self-heating effects in Gallium nitride (GaN) high-electron-mobility transistors (HEMTs) can adversely impact both device reliability and electrical performance. Despite this, a holistic understanding of the relationship among heat transport mechanisms, device reliability, and degradation of electrical performance has yet to be established. This Letter presents an in-depth analysis of self-heating effects in GaN HEMTs using technology computer-aided design and phonon Monte Carlo simulations. We examine the differential behaviors of the maximum channel temperature (Tmax) and the equivalent channel temperature (Teq) in response to non-Fourier heat spreading processes, highlighting their respective dependencies on bias conditions and phonon ballistic effects. Our study reveals that Tmax, a crucial metric for device reliability, is highly sensitive to both heat source-related and cross-plane ballistic effects, especially in the saturation regime. In contrast, Teq, which correlates with drain current degradation, shows minimal bias dependence and is predominantly influenced by the cross-plane ballistic effect. These findings emphasize the importance of optimizing device designs to mitigate both Tmax and Teq, with a particular focus on thermal designs influenced by the heat source size. This work contributes to a deeper understanding of self-heating phenomena in GaN HEMTs and provides valuable insights for enhancing device performance and reliability.

Thermal spreading resistance from an isothermal source into a finite-thickness body

Thermal spreading resistance is encountered during two-dimensional thermal conduction from a hot surface into a body. Thermal spreading and the opposite problem of thermal constriction resistance are of much technological relevance in the design of heat sinks and thermal spreaders for thermal management of microelectronics and other heat-generating devices. Much of the past work in the theoretical analysis of thermal spreading is based on a source with given heat flux. In contrast, the isothermal source problem presents difficulties due to the resulting mixed nature of the boundary condition, due to which, only approximate solutions are available. This work derives the steady-state thermal spreading resistance from an isothermal source into a finite-thickness slab or cylinder. The mixed boundary condition is handled by posing it in the form of a spatially varying convective boundary condition, with a sufficiently large Biot number over the source to represent its isothermal nature. A series solution for the problem is derived, along a sufficient set of linear algebraic equations to determine the series coefficients. Results are shown to agree well with finite-element simulations. Results are compared with previously-reported approximate solutions within the parametric range of validity of the approximate solutions. The impact of key non-dimensional parameters on thermal spreading resistance is quantified. It is shown that thermal spreading resistance increases with decreasing size of the isothermal heat source, as expected. A third-order polynomial correlation with very good accuracy is proposed. This work advances the theoretical understanding of a problem for which only approximate solutions have been reported in the past. Results presented here offer a practical tool for thermal design and optimization of a variety of practical thermal management problems involving spreading or constriction.

General three-dimensional thermal illusion metamaterials

Abstract Thermal illusion aims to create fake thermal signals or hide the thermal target from the background thermal field to mislead infrared observers, and illusion thermotics was proposed to regulate heat flux with artificially structured metamaterials for thermal illusion. Most theoretical and experimental works on illusion thermotics focus on two-dimensional materials, while heat transfer in real three-dimensional (3D) objects remains elusive, so the general 3D illusion thermotics is urgently demanded. In this study, we propose a general method to design 3D thermal illusion metamaterials with varying illusions at different sizes and positions. To validate the generality of the 3D method for thermal illusion metamaterials, we realize thermal functionalities of thermal shifting, splitting, trapping, amplifying and compressing. In addition, we propose a special way to simplify the design method under the condition that the size of illusion target is equal to the size of original heat source. The 3D thermal illusion metamaterial paves a general way for illusion thermotics and triggers the exploring of illusion metamaterials for more functionalities and applications.

Influence of magnetic field on MHD mixed convection in lid-driven cavity with heated wavy bottom surface

This study investigates the influence of a rectangular heat source on magnetohydrodynamic hybrid convection flow in a lid-driven cavity. The effects of various parameters, such as the heat source size, magnetic field strength, and heat absorption/generation, are analyzed. The results show that increasing the heat source size decreases the average Nusselt number along the heated wall. The average Nusselt number also decreases with higher magnetic field strength and heat generation, while it increases with heat absorption. The major finding is to apply an important technique the Galerkin weighted residual technique of the finite element (FE) method to solve the non-dimensional equations and the associated boundary conditions. The isotherms are used to show the temperature distribution in a domain. Streamline present the flow field in the enclosure. However, it is easy to realize the direction and intensity of the heat transfer particularly in convection problems which the path of heat flux is perpendicular and the isotherm due to convection effect. Thus, the purpose of this research is to study the results of mixed convection. The effects of location and height of the partitions are considered for the various Richardson numbers. Fluid flow field, thermal field and heat transfer are presented through the streamlines and isotherms, respectively. Results are substantiated relating to the published work.

Natural Convection in Open Square Enclosure with Different Heat Source Sizes

In this study, a real-time experimental investigation is performed by a nonintrusive technique using a Mach-Zehnder interferometer. Experiments are performed in a partially open square enclosure with an internal heat source located at the center of bottom wall. The enclosure contains two openings at the bottom and top of the left and right vertical walls, respectively. The openings occupy 25% of the wall’s height. The experimental interferograms give temperature distribution in the investigating domain. The effect of heat source size and Rayleigh number on natural convection is evaluated experimentally, and the results are compared with numerical results obtained using ANSYS Fluent 2020. A dense fringe pattern is observed at larger heat source size and Rayleigh number. An air velocity transducer is used to measure inlet velocity and observe that high velocity air enters the cavity from bottom region of opening. Heat transfer and volume flow rate in the enclosure increase with increase in heat source size and Rayleigh number. Within the range of Rayleigh number 107 to 108, an increase in the heat source size by four times leads to an average increment in Nusselt number by about 33%, whereas the increment in volume flow rate is about 1%.

Mixed Convection in a Double Lid-Driven Wavy Shaped Cavity Filled with Nanofluid Subject to Magnetic Field and Internal Heat Source

A numerical investigation is carried out to analyze the impacts of internal heat source size, solid concentration of nanoparticles, magnetic field, and Richardson number on flow characteristics in an oppositely directed lid-driven wavy-shaped enclosure. The left and right vertical walls of the enclosure are cooled isothermally and moving with fixed velocity in upward and downward directions, respectively. The bottom wall is wavy shaped and isothermally cooled as the vertical walls while the top wall is kept adiabatic. A rectangular heater is placed horizontally in the center of the cavity. The physical problems are characterized by 2D governing partial differential equations accompanying proper boundary conditions and are discretized using Galerkin’s finite element formulation. The study is executed by analyzing different ranges of geometrical and physical parameters, namely, internal heat source length 0.2 ≤ CH ≤ 0.6 , solid concentration of nanoparticles 0 ≤ φ ≤ 0.09 , Hartmann’s number 0 ≤ Ha ≤ 70 , and Richardson’s number 0.1 ≤ Ri ≤ 10 . The results indicate that the overall heat transfer rate declines with the increasing length of internal heat source. The presence and rising values of solid concentration of nanoparticles cause the augmentation of heat transfer whereas the magnetic field has a negative influence and the Richardson number has a positive influence on heat transfer.

Numerical Investigation of the Hypersonic Inlet under Throttling with Heat Source

The influence of using a heat source to manage the shock wave boundary layer interactions (SWBLI) at the hypersonic inlet under throttling were studied numerically. This hypersonic inlet was created for a fluid flow Mach number of 5. The throttling was induced by a plug placed near the intake isolator's outlet. The study's parameters included the heat source power and size. The intake performance indicators were the total pressure recovery and the flow distortion. The position of the heat source was determined by studying the interplay of the shock waves from the compression ramp. The results demonstrated the existence of the shock waves at the heat source, and its influences on the SWBLI inside the isolator. This behaviour, led to an increase in the total pressure recovery and reduction of the flow distortion.

A simplified modelling approach for thermal behaviour analysis in hybrid plasma arc-laser additive manufacturing

Hybrid plasma transferred arc (PTA)-laser additive manufacturing (AM) has the potential to build large-scale metal components with high deposition rate and near-net shape. However, the process is complex with many parameters adjustable for process control, which determine the thermal behaviour and thus the final structure and properties of the deposited components. In this study, a three-dimensional steady-state finite element model with two independent circular surface heat sources was developed, validated, and used to analyse the thermal behaviour in hybrid PTA-laser AM of Ti-6Al-4V. Artificial conductivity in three orthogonal directions was applied in the melt pool to compensate for the melt pool convection effect. The predicted melt pool geometry, heat-affected zone and thermal cycles had good agreement with the corresponding experimental data. This model has advantages over the widely used volumetric heat source model, since it is more representative of the energy sources used, giving accurate thermal prediction for a wide range of process parameters. As the heat source parameters in this model are directly linked to the actual arc/laser size, it enables to capture heat source size effect on the hybrid process. In addition, it is easier to calibrate compared to the model with volumetric heat sources due to the fewer empirical parameters involved. It was found that in the investigated ranges of all the parameters, the melt pool geometry is more sensitive to laser power and travel speed compared to arc-laser separation distance and laser beam size. The full-field distributions of the cooling rate and temperature gradient in the hybrid process were obtained and the roles that different process parameters played on them were also studied, which provided useful thermal information for metallurgical analysis.

Study of Mixed Convection and Surface Radiation in Flush Mounted Vented Cavities

In the present work, a computational study of mixed convection in air-filled vented cavities is presented. The experimental work is also introduced in this study in order to enrich the computational results. The parametric study of the enclosure is presented for different ranges of Reynolds numbers, ; Richardson numbers, ; and vent ratio, , and the heat input provided to the right wall, . The continuity, momentum, and energy equations are discretized by the finite volume method and solved using SIMPLEC algorithm. In the present study, optimization of the cavity vent ratio, heat source size, and location found as a novelty in the present work is introduced based on the combined numerical and experimental investigation to achieve better cooling. Empirical correlations for Nusselt number and maximum right wall temperature are developed as functions of , , and .

Experimental and analytical investigation of a 0.3-mm-thick loop heat pipe for 10 W-class heat dissipation

In this study, a 0.3-mm-thick ultra-thin sheet-type loop heat pipe (LHP) for high heat flux (>5 W/cm2) for cooling high-performance mobile devices was designed, fabricated, and its heat transfer characteristics were experimentally and analytically evaluated. The LHP was designed after a steady-state numerical model specialized for ultra-thin copper LHPs was developed. The heat source size was 10 × 10 mm, and the overall size of the LHP was designed to be mounted on smartphones. Two copper sheets were half-etched and soldered together to form a prototype model of the LHP during the fabrication process. To evaluate the LHP's heat transport performance, we measured the temperature of each part of the LHP in response to heat input. Furthermore, the same test was conducted in five orientations (Horizontal, Roll ± 90°, Pitch ± 90°) to verify the effect of operating orientation on performance, assuming that the LHP is mounted on a mobile device. In the horizontal operation, the fabricated LHP exhibited a maximum heat transport performance of 10 W (10 W/cm2) and a minimum thermal resistance and effective thermal conductivity of 2.51 K/W and 2438 W/mK, respectively. The orientation change test resulted in a change in the thermal resistance of the LHP under thermal loads greater than 6 W. At a 10 W heat load, the Roll −90° orientation obtained a minimum thermal resistance of 2.41 K/W and an effective thermal conductivity of 2538 W/mK. On the other hand, the LHP achieved a thermal resistance of 3.54 K/W and an effective thermal conductivity of 1728 W/mK at the Pitch +90° orientation under the same heat load. On the other hand, the effect on the evaporator temperature and the heat source temperature is small, and the proposed LHP can be expected to be implemented in mobile devices that take any orientation. The proposed steady-state numerical model agreed well with the experimental results, and the estimated heat loss and heat dissipation due to LHP operation showed that 71.3% of the maximum heat load was dissipated in the condenser in the horizontal orientation, and the evaporation efficiency changed with the change of the orientation due to the ease of liquid reflux.

Thermal rectification effect of pristine graphene induced by vdW heterojunction substrate

As an effective means of thermal management, thermal rectification (TR) has been studied in a variety of complex asymmetric or heterogeneous structures. Due to experimental conditions, many complex structures cannot be realized easily, and the low-temperature thermal bath temperature far below room temperature does not conform to the actual application environment of the device. In this paper, we studied the thermal rectification inside graphene supported by a heterogeneous substrate of SiO2 and GaN alternately. Heat flux is easier to transfer from the graphene covered on the SiO2 side to the GaN side. When the temperature of the cold bath is maintained at 300 K, only the temperature of the high-temperature hot bath is changed, and the maximum TR rate is about 52%. TR rate can be tuned by heat source temperature and size, with heat source size of 2 nm and temperature above 450 K, the highest TR rate can be obtained. The phonon density of states and participation ratio reveal the influence of the substrate and position on the phonon transport in graphene. Besides, the spectral energy density analysis confirmed that the substrate effect caused the shift of the graphene flexural phonon mode and the reconstruction of the phonon spectrum, which was in good agreement with PDOS, and further explain the basic mechanism of TR from the scattering rate and the degree of localization.

Thermo-magnetic convection of nanofluid in a triangular cavity with a heated inverted triangular object

The present study aims to explore the buoyancy-driven convective phenomena of nanofluid flow in a cavity of an isosceles triangle with an inverted triangular heat source under the effect of a sloped magnetic field. The cases of different heating loads are analyzed considering various sizes of the heating triangle. The outer triangular cavity is kept at a lower temperature; whereas the inner inverted triangular is maintained as the isothermal heat source. Space in-between the inner and other triangle is filled with nanofluid (Al2O3-water). Finite element methodology is adapted to compute the coupled transport equations numerically. The convective phenomena and associated heat transfer characteristics are examined systematically for a range of control parameters like Rayleigh numbers (103–106), Hartmann number (0–70), and magnetic field inclination angle (0−180°), and the size of the inner inverted triangular heat source. The illustrations of local distribution of streamlines, isotherms, entropy generation are presented along with the average Nusselt number, Nu. It shows that the size of the inner triangular heat source controls the overall thermal behavior. The maximum convective heat transfer is obtained with the least flow obstruction (i.e. smaller size of the inner triangle). The study of magnetic field inclination shows that maximum heat transfer is obtained at an inclination of 90° and minimum flow obstruction.

Directed Energy Deposition-Arc (DED-Arc) and Numerical Welding Simulation as a Hybrid Data Source for Future Machine Learning Applications

This research presents a hybrid approach to generate sample data for future machine learning applications for the prediction of mechanical properties in directed energy deposition-arc (DED-Arc) using the GMAW process. DED-Arc is an additive manufacturing process which offers a cost-effective way to generate 3D metal parts, due to its high deposition rate of up to 8 kg/h. The mechanical properties additively manufactured wall structures made of the filler material G4Si1 (ER70 S-6) are shown in dependency of the t8/5 cooling time. The numerical simulation is used to link the process parameters and geometrical features to a specific t8/5 cooling time. With an input of average welding power, welding speed and geometrical features such as wall thickness, layer height and heat source size a specific temperature field can be calculated for each iteration in the simulated welding process. This novel approach allows to generate large, artificial data sets as training data for machine learning methods by combining experimental results to generate a regression equation based on the experimentally measured t8/5 cooling time. Therefore, using the regression equations in combination with numerically calculated t8/5 cooling times an accurate prediction of the mechanical properties was possible in this research with an error of only 2.6%. Thus, a small set of experimentally generated data set allows to achieve regression equations which enable a precise prediction of mechanical properties. Moreover, the validated numerical welding simulation model was suitable to achieve an accurate calculation of the t8/5 cooling time, with an error of only 0.3%.

Исследование тепловых свойств изотермических плит при спекании капиллярного сердечника

This paper mainly introduces the sintering process of the monolithic capillary wick and analyzes the influence of different copper powder particle size, filling rate, copper powder shape and heat source size on the heat transfer performance of the isothermal plate. The experimental results show that: (1) For the isothermal plate sintered with spherical copper powder, the capillary force of large particle size copper powder is small, but the flow resistance is also small, and the performance of the isothermal plate sintered with large particle size copper powder is better. (2) In the case of low filling rate, the isothermal plate is dried due to insufficient return fluid. In the case of high filling rate, on the one hand, the thickness of the liquid film at the evaporation end of the isothermal plate is large, resulting in additional thermal resistance. On the other hand, the thin film evaporation mode will be transformed into pool boiling mode, which will reduce the heat transfer performance. (3) Spherical copper powder sintered plate with regular shape has the best performance, while dendritic copper powder sintered plate has relatively high thermal resistance. (4) The heat source area has a great influence on the thermal resistance of the plate. Under the same heating power, the thermal resistance of the small area heat source is much higher than that of the large area heat source; The thermal resistance of sintered copper plate is lower than that of pure copper plate under two heat source areas.

Investigation of the Parameter-Dependence of Topology-Optimized Heat Sinks in Natural Convection

The thermal design of natural convection heat sinks is critical to the heat management of electronic devices. In this paper, topology optimization (TO) with a new parameter advancing scheme is employed to study the effect of the heating power, heat source size, allowed volume and material thermal conductivity on the optimized design of natural convection heat sinks. Except for the heating power, the other three factors also play an important role on the optimization results. For the small heating power, TO predicts the tree-like heat sink with many secondary-branches to improve heat conduction, but the specific structure is highly dependent on the heat source size, allowed volume and material thermal conductivity. For the large heating power, TO prefers to produce the taper-like heat sink where the tiny branches fade away to adapt to the strong flow, and the influences of the other three factors are reduced. However, two primary branches that connect the heat source and top corners of the design domain always exist, indicating that they are the most effective ways to improve heat transfer. This work has highlighted the impact of different parameters in TO of natural convection heat sinks and provides a more in-depth understanding of the design guidelines.

Reduced Thermal Conductivity in Ultrafast Laser Heated Silicon Measured by Time-Resolved X-ray Diffraction

We investigate the effect of free carrier dynamics on heat transport in bulk crystalline Silicon following femtosecond optical excitation of varying fluences. By taking advantage of the dense 500 MHz standard fill pattern in the PLS-II storage ring, we perform high angular-resolution X-ray diffraction measurements on nanosecond-to-microsecond time-scales with femtometer spatial sensitivity. We find noticeably slowed lattice recovery at increasingly high excitation intensities. Modeling the temporal evolution of lattice displacements due to the migration of the near surface generated heat into the bulk requires reduced thermal diffusion coefficients. We attribute this pump-fluence dependent thermal transport behavior to two separate effects: first, the enhanced nonradiative recombination of free carriers, and, second, reduced size of the effective heat source in the material. These results demonstrate the capability of time-resolved X-ray scattering as an effective means to explore the connection between charge carrier dynamics and macroscopic transport properties.

Modeling temperature rise in multi-track reciprocating frictional sliding

As in various manufacturing processes, in sliding tests with scanning motions to extend the sliding distance over fresh countersurface, temperature rise during any pass is bolstered by heating during prior passes over neighboring tracks, providing a “heat accumulation effect” with persisting temperature rises contributing to an overall temperature rise of the current pass. Conduction modeling is developed for surface temperature rise as a function of numerous inputs: power and size of heat source; speed and stroke length, and track increment of scanning motion; and countersurface thermal properties. Analysis focused on mid-stroke location for passes of a square uniform heat flux sufficiently far into the rectangular patch being scanned from the first pass at its edge that steady heat accumulation effect response is adopted, focusing on maximum temperature rise experienced across the pass' track. The model is non-dimensionalized to broaden the applicability of the output of its runs. Focusing on practical “high” scanning speeds, represented non-dimensionally by Peclet number (in excess of 40), applicability is further broadened by multiplying non-dimensional maximum temperature rise by the square root of Peclet number as model output. Additionally, investigating model runs at various non-dimensional speed (Peclet number) and reciprocation period values, it appears these do not act as independent inputs, but instead with their product (non-dimensional stroke length) as a single independent input. Modified maximum temperature rise output appears to be a function of only two inputs, increasing with decreasing non-dimensional values of stroke length and scanning increment, with outputs of models runs summarized compactly in a simple chart.