Thermal Design Research Articles

Effect of the alloyed interlayer on the thermal conductance of Al/GaN interface

Understanding the interfacial phonon transport is essential for optimizing the thermal management of microelectronics, especially for high energy density devices. Some calculations have suggested that introducing interfacial defects or disorders will increase the interfacial thermal conductance, which helps heat dissipation, while some studies suggested otherwise. In this work, we introduced substitutional impurities in GaN by growing a ∼2-nm-thick AlxGa1−xN or InxGa1−xN alloyed interlayer at the Al/GaN interface and measured the interface thermal conductance by time-domain thermoreflectance at room temperature. Our results show that substituting Ga atoms near the interface with either lighter Al atoms or heavier In atoms at a nominal concentration of 20% or less will not necessarily change the thermal conductance of the Al/alloy interface but is detrimental to the thermal transport across the total Al/GaN interface, which provides an experimental guideline for the thermal design of GaN-based devices.

Four-phonon scattering and thermal transport in 2H–MoTe2

Among two-dimensional (2D) transition metal dichalcogenides, molybdenum ditelluride (MoTe2) is of great interest to achieving unique functionalities in diverse electronic, photonic, and phononic devices. Despite extensive studies over the past decade, the thermal transport properties and mechanisms of MoTe2 remain largely unclear. Here, we grow few-layer (7L, 10L, and 17L) single-crystalline 2H–MoTe2 and measure their in-plane thermal conductivities using an optothermal Raman technique. The room-temperature values range from 4 to 9 W m−1 K−1, substantially lower than theoretical predictions considering only three-phonon scattering. By employing first-principles calculations and machine learning-assisted molecular dynamics simulations, we reveal that four-phonon scattering could suppress the thermal conductivity of 2H–MoTe2 by 80%, which agrees with experimental measurement and is mainly attributed to the flexural acoustic modes. Thickness-dependent suppression is observed owing to the varying reflection symmetry. Our work highlights higher-order phonon-phonon interactions in 2D materials and offers guidance for the thermal design of emerging devices.

Comparing global sensitivity analysis methods for the joint thermal design parameters of a space manipulator based on the Sobol’ and PAWN

The Global Sensitivity Analysis method works well for screening and ranking impacting factors. It can help with thermal design optimization and thermal model calibration. This study compares the use of Sobol's variance-based method with PAWN's density-based method for 12 thermal design parameters (such as the properties of thermal control coating and the contact heat transfer coefficient between components) of a space manipulator joint thermal model. The two techniques have high consistent parameter sensitivity. The analysis's findings indicate that the junction electric box and shell's temperature variations are significantly influenced by the cooling surface's coating properties. The cooling surface's absorption rate has a bigger impact than emissivity does. Only the characteristics of the multilayer mask and the state of parcel coverage have a significant impact on the temperature of the joint shell. The outcomes offer technical backing for cooperative optimization in thermal design. The identical no-effect parameters are discovered to be recognized by Sobol' and PAWN at the same time, but they are ordered in different ways. The limitation of Sobol's technique, which uses variance as a sensitivity index, may be the cause of this outcome. Finally, the sensitivity response results are re-substituted into the finite element thermal model. The changing rule of temperature curve is used to further confirm the accuracy of the sensitivity results.

Effective thermal contact conductance of encased silicon ball from molecular dynamics simulations

The heat dissipation performance of point-contact structures in many micro/nano-devices is critical. In this study, the effects of the normal load and diameter of the silicon ball on the effective thermal contact conductance (Geff) of the encased silicon ball between two silicon plates were studied by using the non-equilibrium molecular dynamics method. The results show that increasing the normal load increases the real contact area between the silicon ball and silicon plate, and thus improves the transmission channel and the density of states of low-frequency phonons, resulting in the observation that the Geff increases with increasing normal load. It is also found that the Geff is not sensitive to the diameter of the silicon ball from 2.66 to 4.16 nm at low normal load due to the competition between the enhanced interfacial thermal conductance across the contact region and the reduced internal thermal conductance of the silicon ball. The variation of Geff in the encased silicon ball structure with the external load and diameter has important guidance for the packaging and thermal design of micro/nano-devices.

An Explainable Evaluation Model for Building Thermal Comfort in China

The concentration of atmospheric greenhouse gases is being amplified by human activity. Building energy consumption, particularly for heating and cooling purposes, constitutes a significant proportion of overall energy demand. This research aims to establish a smart evaluation model to understand the thermal requirements of building occupants based on an open-access dataset. This model is beneficial for making reasonable adjustments to building thermal management, based on factors such as different regions and building user characteristics. Employing Bayesian-optimized LightGBM and SHAP (SHapley Additive exPlanations) methods, an explainable machine learning model was developed to evaluate the thermal comfort design of buildings in different areas and with different purpose. Our developed LightGBM model exhibited superior evaluation performance on the test set, outperforming other machine learning models, such as XGBoost and SVR (Support Vector Regression). The SHAP method further helps us to understand the interior evaluation mechanism of the model and the interactive effect among input features. An accurate thermal comfort design for buildings based on the evaluation model can benefit the carbon-neutral strategy.

Experimental Investigation on Boiling Heat Transfer Characteristics of Liquid Methane in Mini Channel

LOX/LCH4 rocket engine has been recognized as the ideal power choice for future space vehicles due to the merits of low cost, non-toxic and pollution-free, convenient maintenance, suitable for reuse and high specific impulse. In the process of wide range variable thrust of LOX/LCH4 rocket engine, the coolant methane is in a subcritical state due to the low combustor pressure under low operation conditions. The instability of two-phase flow is easy to occur in regenerative cooling channel (RCC), and it is urgent to investigate the heat transfer performance of methane with phase change in RCC. Experiments have been conducted to investigate the flow boiling characteristics of liquid methane in the single mini channels with the diameters of 1.0, 1.5 and 2.0 mm. Effects of the mass flux (266.75~1781.26 kg/m2·s), inlet pressure (0.56~4.24 MPa), heat flux (53.25~800.07 kW/m2) and channel diameter (1.0~2.0 mm) on the flow boiling heat transfer coefficients are tested. Results show that there are two regions with different heat transfer mechanism, one is the nucleate boiling dominated region for low mass quality and the other is the convection evaporation dominated region for high mass quality. A new correlation expressed by Bo, We, Kp, X, Co, Ftg is proposed, which yields good fitting for 355 experimental data with a mean absolute error (MAE) of 10.9%. Present experimental results can provide reference for the thermal protection prediction and optimal design of RCC in LOX/LCH4 rocket engine.

Design and Numerical Analysis of Recuperator for a Liquid Carbon Dioxide Energy Storage System

A liquid carbon dioxide energy storage (LCES) system has the characteristic of compact structure and easy liquefaction. As a component of heat recovery in the LCES system, the recuperator plays a crucial role in influencing the round trip efficiency (RTE) of the energy storage system, but very little attention has been paid to it even though its operation conditions are quite different from other thermal systems. In this case, the thermal and hydraulic design of the recuperator in the LCES system was completed. The flow characteristics and thermal performance of the recuperator under design conditions were analyzed, and the effects of operating at various loads on the flow characteristics and thermal performance of the recuperator were investigated. It was found that from the inlet to the outlet, the resistance coefficient of CO2 on the cold side increased gradually while decreasing gradually on the hot side. Down the flow direction, the average temperature of CO2 on the cold side increased sharply, while decreasing slowly in the hot side. When the systems discharged with varying loads, the pressure drop increased along both channels, but the resistance coefficient decreased gradually on both channels. The heat transfer coefficient (HTC) increased gradually on both sides too. The Nusselt number (Nu) in the first half of the cold side did not change much, while increasing gradually in the second half, but it continued to increase on the hot side.

Development and Validation of a Segregated Conjugate Heat Transfer Procedure on a sCO2 Dry Gas Seal Test Bench

Abstract Carbondioxide at supercritical state shows particularly favorable thermodynamic properties for closed-loop Brayton and Rankine cycles. High density, close to a liquid, and low viscosity, close to a gas, drive to achieve higher energy conversion efficiency with smaller size turbines and components. Since minimizing the sCO2 leakage flows drives to a further increasing of overall efficiency, dry gas seals (DGS) are considered as a key enabling technology for achieving this purpose and for achieving a carbon footprint in line with the increasingly stringent targets. DGSs are gas-lubricated, mechanical, noncontacting, end-face seals, consisting of a mating (rotating) ring and a primary (stationary) ring. The operating equilibrium clearance of the seal is determined by the balance of opening dynamic forces, mainly depending on angular velocity and grooves shape, and closing forces caused by pressure gradients and spring force. Due to high rotational speeds, typical small size sealing gaps (order of magnitude of Micron), high fluid pressure and density, the heat generated by friction through the seal has a large impact on the temperature distribution, therefore a thermal design is needed to stay below the seal allowable temperature; this requirement has been even amplified in the last years with the DGS application to hot turbomachinery. Nowadays, numerical conjugate heat transfer (CHT) analysis is a good industrial practice to quantify the thermal distribution in turbomachinery components. On the other hand, due to different order of magnitude of secondary flows cavity sizes and DGS seal gaps, simulating the whole fluid domain with three-dimensional (3D) computational fluid dynamic (CFD) calculation could drive to prohibitive computational costs. In this regard, this paper presents a fast numerical iterative procedure based on a commercial one-dimensional flow network modeler (altair flow simulator), with real gas effects included, coupled with a commercial finite element solver (ansys mechanical). Additional 3D CFD simulations are carried out to enhance the predictability of the fluid solver in specific critical areas. The proposed procedure is applied and validated in a DGS specific test bench designed, owned and operated by Flowserve. The validation dataset has been generated operating the DGS in the test bench at 15 different conditions in terms of angular velocity and housing temperature with sCO2 as working fluid aligned with those expected on the unit. Test rig consists of a rotor and housing where the turbomachine operating condition (pressure, speed, temperature and sealing fluid) are mirrored to verify the seal performance. The rig is equipped with test seal and a plug seal mounted in a back-to-back configuration. Data set used for this comparison is composed by thermocouples on the statoric rings, retainers and housing of each seal, operating at five different angular speeds and three minimum levels of housing temperature (100 °C, 210 °C, and 250 °C). These temperatures are designed to mirror turbomachinery conditions around the seal, and, if friction heat is not enough to guarantee them, a series of 18x1 kW heating elements circumferentially distributed in the test bench housing, are activated. The nominal operating condition has been considered as the starting point of the model predictions and, there, several sensitivity analyses have been carried out to align the model to test bench measurements; then, the selected configuration has been frozen and used to assess the model performance on other operating conditions. The numerical results have shown a good agreement with experimental data at each operating condition in terms of punctual temperatures previously described and with extremely low computational times.

Conceptualization of a novel battery thermal management system based on capillary-driven evaporative cooling

In conventional heat pipe based battery thermal management systems the thermal contact between the battery and the heat pipe is enhanced by means of heat conductive elements. These additional elements introduce multiple layers of thermal resistance and contribute to increased weight. This paper aims to address this issue by minimizing the contact thermal resistance and potentially reduce this additional weight. The proposed solution relies on capillary-driven evaporative cooling (CDEC), wherein a wick structure is directly integrated onto the battery's surface to enable direct cooling. To demonstrate this concept, an experimental study was conducted by affixing a Copper foam to an emulated battery block, and using ethanol and Novec 7000 as cooling media. The CDEC system's thermal performance was assessed under three heating conditions, and different operating conditions. The results indicated that the copper foam with higher pore density outperformed the other due to its greater wetting height. The maximum cell surface temperature was maintained around 40 °C for a continuous 50 W heat input. Furthermore, the thermal resistance of the system was lowered by a factor of 6 compared to an air-cooled system. The thermal resistance ranged from a minimum of 0.32 to a maximum of 1.5 K/W, which were comparatively low compared to some existing battery thermal management system designs. This paper introduces an innovative battery cooling concept, presents experimental evidence of its feasibility, and demonstrates its ability to effectively regulate battery temperature within acceptable limits even under high heat loads, while minimizing overall thermal resistance.

Comprehensive analysis method of acquiring wall heat fluxes in rotating detonation combustors

Accurate perception of the combustor thermal environment is crucial for thermal protection design of a rotating detonation combustor (RDC). In this study, a comprehensive analysis method is established to calculate the non-uniform heat flux distribution of the RDC by utilizing the measured temperature distribution of the combustor outer wall obtained by the high-speed infrared thermal imager. Firstly, a physical model based on the geometric characteristics of the RDC is constructed and its thermal conductive process is simulated, given by different heat flux boundary conditions. The analysis results indicate that the method used in current research for computing the heat flux of RDC leads to significant discrepancies, particularly in regions of obvious axial heat flux gradients near the top of the combustor. Then the Levenberg-Marquardt (L-M) method for inversely solving non-uniform heat flux is developed based on the two-dimensional heat conduction equation, and the wall heat fluxes are inversely calculated by the L-M method based on the above numerical data. Results show that the L-M method can obtain more accurate heat flux distribution even in the zones with large heat flux gradients, considering the axial heat conduction within the combustor outer wall caused by the non-uniform heat flux. Finally, the wall heat flux distribution is analyzed coupling the L-M method together with the experimental measurements in kerosene two-phase RDC. The analyses show that the wall heat flux exhibits non-uniform distributions both circumferentially and axially, with axial non-uniformity being particularly pronounced. The highest temperature of combustor outer surface and the highest wall heat flux occurs within the region of 20 mm from the combustor head, which corresponds to nearly 14 % of the combustor length. The heat flux peak and thermal heat rate are positively correlated with the combustor equivalence ratio in the range between 0.44 and 0.64. In this study, the distribution of wall temperature field in the two-phase RDC is obtained for the first time and the wall heat flux is calculated inversely by L-M method. This work provides a more accurate method for calculating heat flux in RDC and offers references for thermal protection designs in RDCs.

Investigation on the thermal and hydrodynamic performances of a micro-pin fin array heat sink for cooling a multi-chip printed circuit board

Multi-chip printed circuit boards (PCBs) with integrated functionalities are widely used in large servers. This study investigates the thermal design of a PCB with nine different chips by using a micro-pin fin array heat sink. Four types of micro-pin fin arrays, namely, the square-pin fin array (SPFA), round-pin fin array (RPFA), truncated-pyramid-pin fin array (TPPFA), and truncated-conical-pin fin array (TCPFA), are considered, with the TPPFA providing the best thermal and hydrodynamic performances. The fabricated TPPFA heat sink was experimentally studied, achieving a maximum temperature of 63.3 °C, a temperature nonuniformity of 6.5 °C, and a pressure drop of 3.1 kPa at a total heat transfer rate of 540 W. Numerical simulations are conducted to examine the detailed temperature and flow distributions in the heat sink, exhibiting relatively uniform temperatures in the areas with micro-pin fin arrays and beneficial secondary flows in the downstream of the micro-pin fins. Further numerical simulations are performed to reveal the importance of the type of the coolant and the temperature dependence of the thermophysical properties, as well as the validity of the field synergy principle in analyzing the thermal performance of the heat sink considered in this work.

Thermal illusions for thermal displays: a review.

Thermal illusions, a subset of haptic illusions, have historically faced technical challenges and limited exploration. They have been underutilized in prior studies related to thermal displays. This review paper primarily aims to comprehensively categorize thermal illusions, offering insights for diverse applications in thermal display design. Recent advancements in the field have spurred a fresh perspective on thermal and pain perception, specifically through the lens of thermal illusions.

Thermal safety boundary of lithium-ion battery at different state of charge

Thermal runaway (TR) is a critical issue hindering the large-scale application of lithium-ion batteries (LIBs). Understanding the thermal safety behavior of LIBs at the cell and module level under different state of charges (SOCs) has significant implications for reinforcing the thermal safety design of the lithium-ion battery module. This study first investigates the thermal safety boundary (TSB) correspondence at the cells and modules level under the guidance of a newly proposed concept, safe electric quantity boundary (SEQB). A reasonable thermal runaway propagation (TRP) judgment indicator, peak heat transfer power (PHTP), is proposed to predict whether TRP occurs. Moreover, a validated 3D model is used to quantitatively clarify the TSB at different SOCs from the perspective of PHTP, TR trigger temperature, SOC, and the full cycle life. Besides, three different TRP transfer modes are discovered. The inter-conversion relationship of three different TRP modes is investigated from the perspective of PHTP. This paper explores the TSB of LIBs under different SOCs at both cell and module levels for the first time, which has great significance in guiding the thermal safety design of battery systems.

Ab initio investigations on hydrodynamic phonon transport: From diffusion to convection

In classical theory, heat conduction in solids is regarded as a diffusion process driven by a temperature gradient, whereas fluid transport is understood as convection process involving the bulk motion of the liquid or gas. In the framework of ab initio theory, which is directly built upon quantum mechanics without relying on measured parameters or phenomenological models, we observed and investigated the fluid-like convective transport of energy carriers in solid heat conduction. Thermal transport, carried by phonons, is simulated in graphite by solving the Boltzmann transport equation using a Monte Carlo algorithm. To capture convective transport, with phonon distributions deviating significantly from equilibrium Bose-Einstein distribution, we determined phonon interactions using ab initio approaches that go beyond relaxation time approximations. The presence of strong momentum-conserved Normal scatterings in graphite introduces a regime for hydrodynamic phonon transport. Fluid-like features, such as vortex and jet flow, are visualized and compared with classical theories on heat diffusion and fluid convection. Our study on phonon convection enhances fundamental understandings of heat conduction in solids from both atomic scale and quantum aspects, innovating thermal designs for future microelectronic devices and other thermal management applications. This potentially offers solutions for heat dissipation challenges in the post-Moore era.

Design and Development of RF Window for Gyro-TWT

In this paper, the electrical design of an RF quartz window for a Ka-band Gyro-TWT amplifier has been carried out using CST microwave studio. The return loss (S11) and insertion loss (S12) of the RF quartz window have been −35.3 dB and −0.02 dB, respectively. The thermal design of the RF quartz window for Gyro-TWT during extreme cases of operation, i.e. at saturation has been carried out using ANSYS software and as discussed in the paper. The temperature 368 K on the disc of the RF window for Gyro-TWT has been obtained. The RF quartz window for the Ka-band Gyro-TWT amplifier has been developed. The cold characterization of the RF quartz window for the Ka-band Gyro-TWT amplifier has also been carried out using a Vector Network Analyzer. A good agreement between simulation and measurement has been obtained.

Pulsating heat pipes filled with acetone and water under radial rotation conditions: Heat transfer performance and semi-empirical correlation

A significant amount of heat can be generated during the operation of rotary machinery, which degrades its working performance and lifetime. It has been proven that, under radial rotation conditions, pulsating heat pipes (PHPs) can operate with good thermal performance. PHPs have excellent potential for application in rotary machinery, owing to their enhanced heat transfer and material duration. Experimental studies on radially rotating PHPs remain scarce, and further in-depth physical understanding is required to achieve a better thermal design for practical configurations. In this study, the effects of centrifugal acceleration and heat flux on the heat transport in radially rotating PHPs were investigated in detail. The results demonstrate that centrifugal acceleration can promote the circular motion of chains of liquid plugs and vapor slugs with an improvement in thermal performance, particularly above a given heat flux threshold and a critical value of the rotation velocity. In addition, by considering the key influencing parameters, a semi-empirical correlation of dimensionless numbers was built to estimate the heat transport. Additionally, dimensionless numbers, i.e., Weber number, temperature number (ΔT/Tev), and a newly-defined number (Ψ1, represents the influence of heat flux, surface tension and centrifugal acceleration), significantly impact the thermal performance.

Experimental investigation on photovoltaic thermal system performance with different attachment techniques

Photovoltaic thermal (PVT) systems promise a more optimal solar energy utilization by producing electricity and heat simultaneously. However, the thermal collector design and the way it is connected to the PV panels are still a problem for the PVT system. This study aims to answer the use of thermal paste and insulation in thermal collectors are some of the issues in PVT system manufacturing. An experimental study was conducted by making three scenarios of thermal collectors combined with PV panels and then tested in an indoor solar simulator. Each design was tested with five different cooling water flow rates. This study found that thermal paste and insulation can improve the electrical power of PVT systems by up to 17.59% and its thermal performance increase by 10.13%. Thus, the use of thermal paste and insulation should be done in the manufacturing of each thermal collector for PVT systems.

Thermal Consideration and Design for a 200-kW SiC-Based High-Density Three-Phase Inverter in More Electric Aircraft

Thermal Consideration and Design for a 200-kW SiC-Based High-Density Three-Phase Inverter in More Electric Aircraft

Numerical study on the mixed convection with nanofluids in vertical channel asymmetrically heated

Mixed convection heat transfer in cavities is of great interest in many industrial applications, such as the development of crystals, the thermal control of electronic components, the nuclear reactors and various manufacturing processes. Components are generally located on the wall of a channel, that can be also tilted and their thermal control is carried out by the fluid flow. They can be schematized as thermal sources on vertical boards and the heat flux is removed by natural or mixed convection. As to the mixed convection, the interaction between the buoyancy forces and the external flow plays a significant role in the thermal design of systems. When natural convection is not strong enough to remove heat flux, mixed convection is adoptedIn particular, mixed convection due to moving surfaces is very important is these applications. Mixed convection because of buoyancy and motion of one of the channel walls has received little research attention and few guidelines are available for choosing the best performing channel configuration. Moreover, to optimize the transfer through natural convection, it is possible to modify the geometric parameters of the component, thus maximizing the convective heat transfer coefficient. If this is not possible or if one chooses to keep the geometry of the component unchanged for design choices, a possible alternative is the use of nanofluids. The properties of nanofluids exhibit a behaviour ranging between the properties of the base fluid and the properties of the nanoparticles. However, the addition of nanoparticles in the working fluid is considered to enhance the thermal behaviour of the base fluid. In this study a numerical investigation of mixed convection in nanofluids due to the interaction between a buoyancy flow and a moving plate induced flow in a vertical channel is carried out. The moving plate has a constant velocity and moves in the same direction with respect to the buoyancy force. The channel principal walls are heated with uniform heat flux. The numerical analysis is obtained by means of the commercial code Fluent. The effects of the different wall heat fluxes and moving plate velocities are investigated and results in terms of the channel wall and moving plate temperatures and Nusselt numbers are given.

Porous Material for Gas Thermal Compression in Space Conditions: Thermal Design Aspects

Abstract It is almost impossible to name a field of human activity where porous materials are not used. Filters, membranes, thermal insulators, fuel cells, catalysts, wicks of heat transfer devices, sorbents, textiles, and other examples of porous materials explain such an increased interest in the study of their properties, design of new materials, and development of technologies for their manufacture. The recent need for in-orbit spacecraft refuelling has led to the development of gas thermal compression technology based on the use of porous material as both a fuel retention mechanism and thermal interface. The present paper reveals the thermal aspects of porous material design for a device intended for pumping xenon, fuel for ion thrusters, in zero-gravity conditions – Xenon Refuelling Compressor.