Yogendra Joshi

Abstract Outdoor digital displays deployed in harsh environments must withstand wide-ranging stresses driven by internal heat generation and fluctuating ambient conditions. The transient internal and external thermal conditions in the environment of a deployed display cause internal pressure and temperature fluctuations (termed “heartbeats”). This paper presents a transient computational fluid dynamics/heat transfer (CFD/HT) framework for simulating these fluctuations of pressure and temperature in gasketed outdoor digital displays and electronics enclosures. Using a pressure-based solver with the k–ω SST turbulence model and ideal-gas density, a new leakage model (based on experimental decay curves) is used to represent air losses from gasket and seams by using pressure and temperature driven source terms. Variable electronic heat loads and fan PWM duty cycles are imposed through user-defined named expressions. The developed framework is validated against experimental data from a production testing protocol designed to determine the quality of a gasket seal in an outdoor digital display prior to deployment, while under controlled ambient conditions. The MRI BoldVu Gen15 55″ double-sided outdoor digital display was used for all experimental testing. The results reveal that gasket leakage dominates pressure decay during cooling, while the air thermal expansion drives the pressure increase during heating phase. This validated model offers insights into design optimization of enclosure sealing and thermal management strategies.

Abstract Additive manufacturing has transformed thermal management by enabling the production of complex, optimized geometries that conventional manufacturing methods cannot achieve. This study investigates the single-phase convective heat transfer performance of gyroid triply periodic minimal surface (TPMS) lattice structures with functional porosity. TPMS structures provide high surface area to volume ratios and are amenable to 3D printing. A gyroid numerical model was created and validated against an existing experimental study with a similar feature size to the investigated geometries. The TPMS structure has a periodic width of 1.6 mm, a length of 10 mm, and a height of 4 mm, with a functional porosity ranging from 0.5 to 0.8, decreasing with distance from the heated surface. Three different flow configurations were examined for an inlet fluid temperature of 70 °C. The inlet velocities range from 0.01 to 1.2 m/s, corresponding to a Reynolds number range of 10–900 with a heat flux of 50 W/cm2 applied at the base. AmpCool® AC-110 dielectric fluid (Prandtl number 59.5) was used as the coolant. Thermal performance and friction characteristics were studied for the three flow orientations. The parallel flow configuration was identified as the most efficient for heat removal. A detailed analysis of the numerical results highlights the underlying physics behind the thermal performance differences among the flow configurations.

Abstract Since its founding in 1992, Microsystems Technology Office at DARPA has invested in transformative advances in the field of semiconductor devices. These devices, pervasive in all microsystems, are operated within an economic life, dependent on both the material life of the semiconductor and the mechanical life of the electronics package housing the devices. Thermal management solutions directly impact the performance and reliable operational lifetime of microsystems. Multiscale and multiphysics codesign of microsystems can maximize their performance and reliability. The various DARPA programs in the area of thermal management over three decades are summarized here, along with the demonstrated advancements in each program. Current efforts on 3D heterogeneous integration, multiscale modeling, and high power GaN power amplifiers are also discussed.

Effective cooling strategy is critical to achieve improved performance and efficiency in electric-drive vehicle motors. Among approaches, direct winding heat exchangers (DWHXs), positioned inside the motor component slots, have demonstrated superior potential for cooling compared to conventional methods such as forced convection air cooling and liquid jacket cooling. In this work, an in-slot heat exchanger (HEx) based on the DWHX concept is developed for an outer-rotor motor with a 100 kW peak and 55 kW continuous power output, and 50 $\mathrm{kW} / \mathrm{L}$ power density. Initial work developed a baseline additively manufactured aluminum oxide heat exchanger to cool concentrated stator windings in an 18 -slot, 16 -pole outer-rotor motor; however, it lacked performance in cooling stator endwindings. A new design was envisioned to address this issue. The present study introduces a novel in-slot HEx design, which also incorporates a cooling solution for end-windings at both sides of the motor. The thermal performance of this new design is assessed and compared with the baseline concept. The results from the new design indicate an over 50% reduction in thermal resistance and more than 30% reduction in hot-spot temperature. The new design reduces end-winding temperature while maintaining improved thermal uniformity across the winding. The increase in pressure drop in the new design adds only 0.013 W to the pumping power. Furthermore, the results indicate that a potting material used as an interface material - with a thermal conductivity of 3$4 \mathrm{~W} / \mathrm{m} \cdot \mathrm{K}$ - to attach these heat exchangers to motor components is found optimal to ensure effective thermal performance. The achieved performance is realized without altering critical electromagnetic parameters, facilitating seamless integration with the current motor design. These results underscore the potential of ceramic-based in-slot HExs in improving the thermal performance and efficiency of modern electric-drive vehicle motors, representing a substantial advancement in the development of high-power-density electric motors.

A cooling system for an electric motor that includes a stator having a plurality of slot windings and a rotor, coaxial with the stator, having a plurality of magnets, includes a coolant inlet to the motor and a coolant outlet from the motor. A coolant pathway is in fluid communication with the inlet and the outlet. Heat is transferable from the slot windings to the coolant pathway. A coolant flows through the coolant pathway and is in a liquid phase as it enters the coolant inlet, changing into a gaseous phase as heat is transferred to the coolant from the slot windings. A cooling loop is in fluid communication with the coolant inlet and the coolant outlet. The cooling loop cools the coolant so that substantially all of the coolant is in the liquid phase when it enters the coolant inlet.

This chapter presents the application of machine learning/artificial intelligence (ML/AI) approaches for characterization of single-phase fluid flow and heat transfer characteristics of metal foams. Metal foams were scanned using high-resolution microcomputed tomography. The scanned images were numerically investigated using OpenFOAM for combined heat conduction and convection characteristics. The overall pressure drop and heat transfer results agreed well with experimental data and the empirical correlations in the literature. Longitudinal flow mixing across pores due to the blockage of nodes has been analyzed and depicted for different porosities. The temperature distribution, local heat transfer coefficient, and heat flux on the metal foam fluid interface are characterized to reveal the underlying physical reasons for different metal foams exhibiting distinct thermal characteristics. In addition, the variation of these parameters perpendicular to the heated surface has been examined for different velocities. The results showed higher local heat transfer coefficients for thinner filaments. However, the temperature difference between the fluid and solid portions is marginal due to the lower effective thermal conductivity for higher porosity (low-density) metal foams. The study also showed that the effective interfacial area used for heat transfer decreases with inlet velocity and porosity. The local heat transfer coefficients and local heat fluxes on the interface are analyzed in detail. The regions of strong heat transfer are reported. A comprehensive review of ML/AI approaches has been provided in thermal management and, in particular, metal foams. The integrated values from the computational fluid dynamics/heat transfer (CFD/HT) solutions were used to train a deep neural network (DNN) algorithm. Three-dimensional (3D) surface plots were generated to show the agreement of the numerical data and the predictions obtained by using the DNN algorithm. The computational time can be reduced from several hours to a few minutes by using the DNN model. This study can provide guidance in improved channel design and metal foam selection for high-performance heat exchangers.

Cooling presents a significant challenge for high-performance 3-D integrated circuits (3D ICs). To this end, this research explores through-silicon via (TSV)-compatible micropin-fin heat sink (MPFHS) for high-power 3-D chip stacks. Copper TSVs with a diameter of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$5.2~\mu $ </tex-math></inline-formula>m and a high aspect ratio (HAR) of 29:1 are developed. An extensive experimental and computational investigation of the MPFHS under varying flow rates and power conditions was conducted, showing that the MPFHS maintains an average chip temperature below <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$72~^{\circ }$ </tex-math></inline-formula>C, even with a total power dissipation of 500 W and a power density of 312 W/cm2 at a flow rate of 117 mL/min. The minimum total thermal resistance achieved was <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$0.286~^{\circ }$ </tex-math></inline-formula>C<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\cdot $ </tex-math></inline-formula>cm2/W.

Abstract As power densities increase in heterogeneously integrated systems, with the introduction of new 3D architectures and the increasing number of transistors on chips, there exists a continued bottleneck for thermal management. High temperatures have a drastic impact on memory performances and refresh cycles. Moreover, thermal coupling between neighboring chiplets on a package is increasing as the types of chips on a heterogeneously integrated package diversify, and this, in turn, creates different heat flux densities within a heterogeneously integrated package. Thus, there arises a need for the implementation of efficient thermal design and solutions that cater to high heat fluxes within a package as well as different heights for different chip stacks within a package. In this paper, we present a parametric thermal design of heterogeneously integrated packages for high-performance computing. We focus on a 2.5D packaging structure, which includes components including artificial intelligence (AI) accelerators and high bandwidth memory (HBM) on a silicon interposer. Analytically and numerically, we investigate the thermal challenges stemming from high power density in stacked dies, variations in die heights, and cooling limitations at the package surface. To mitigate temperature gradients within the package, we propose a thermal-aware package structure, emphasizing the inside architecture. Also, the thermal coupling effect is studied for multiple cooling technologies on the outer surface using a thermal violation region graph. This research has shown that not only the internal structure of the package but also its ability to transfer heat to the outer surface has a significant impact on the thermal coupling effect. Using our approach, we can design package architecture systematically considering the external cooling environment in the early design stage.

Abstract As the capacity of electric vehicle (EV) battery packs increases and recharging times are reduced, effective battery thermal management becomes a key challenge. When exposed to elevated temperatures and non-uniform thermal conditions, insufficient heat dissipation within the battery module can detrimentally impact both its operational lifespan and performance, posing a potential risk of thermal runaway. Therefore, by reducing maximum temperatures and enhancing temperature uniformity through the batteries in order to improve lifespan and performance. Furthermore, depending on the specific battery cell technologies employed in EVs, battery swelling also emerges as a significant challenge as it can increase heat generation and drops in battery life. Thus, this paper develops a two-phase flexible battery thermal management system, enhanced with a copper microstructure, using a dielectric coolant (HFE-7000) to mitigate battery surface temperatures. The heat transfer performances were experimentally studied in a mass flow rate range of 4.5 g.s−1 to 11 g.s−1 with representitive heat dissipation values between 10 W.m−2 and 1.4 W.m−2 and shows the ability of the system to maintain average surface temperatures in a the battery optimal operational range between 25°C and 35°C.

This project, initially part of OPEN 2018, and subsequently the ASCEND effort, targeted demonstration of significant enhancements in internal permanent magnet (IPM) motor torque and power density for current and future ground and air electric transportation applications. These were achieved through: (1) embedded two-phase system thermal management, (2) coupled, multi-scale electrical-electromagnetic-thermal-mechanical co-design and optimization, (3) size and weight reduction of motor and drive electronics through elimination of redundant cooling and coupling hardware, and (4) higher efficiency operation of SiC wide bandgap power electronics packaging through high temperature operation (200 oC). The proposed approach utilizes a single dielectric coolant for closed loop two-phase thermal management, and a combined heat rejection unit for the IPM and drive. Wick assisted liquid delivery for evaporative thermal management is utilized for the motor, and the drive electronics utilize the same coolant in flow boiling within the cold plate structures. Through the use of three-dimensional packaging for SiC, and novel drive topologies with reduced switching losses, significant increases in power density and compactness were targeted. In the first phase of the effort, to demonstrate the effectiveness of wick assisted two-phase thermal management of the IPM, a 2 kW motorette that included eight slots, with the same dimensions as the BMW i3 motor was designed and tested to dissipate a total of 2 kW of heat (at 3X higher power density compared to the BMW i3 in each slot). Wick structures were integrated within stator slots to demonstrate the heat removal capabilities of the evaporative cooling system while keeping winding temperature below 150°C. No local hot spots were detected under steady conditions. Subsequently, a 25-kW 9-slot motor was adapted to the two-phase in-stator slot cooling method. While the design originally sourced was not optimized for power or torque density, it did demonstrate the concept of a closed-loop cooling system using 3M’s Novec 7200 dielectric fluid when spun on a dynamometer, and further, validated the modeling framework. A novel soft-switching inverter topology was integrated with a custom package design to maximize power density. Modular half-bridge power cards were built to be implemented into two paralleled 40 kW motor drives with DC/DC and DC/AC converters, and tested for limited functionality through double pulse testing, though the SSCSI control strategy was demonstrated at 25kW using discretely packaged components. The second phase of the project, through a one year extension, focused on the implementation of the two-phase wick assisted thermal management on a 60 kW motor built for a low acoustic signature aerospace application. The re-design achieved up to 80 kW, continuous power (with 220 to 240 kW, peak), with maximum winding temperatures below 125 oC.

This paper presents a 20,000 rpm 72-slot/12-pole heavy rare-earth-free interior permanent magnet synchronous motor (IPMSM) design integrated with novel in-slot evaporative cooling and single-phase shaft cooling techniques. The in-slot cooling design replaces traditional slot-liner paper with a micropin fin-enhanced liner to facilitate coolant flow between the active winding and slot-liner paper, allowing for direct heat extraction from the winding via thin-film evaporation. Additionally, single-phase shaft cooling has been implemented to regulate magnet temperatures below the threshold limit at high speeds. The motor topology has been systematically optimized to enhance power density and prevent premature demagnetization of heavy rare- earth-free magnets. An extensive electro-thermal analysis was conducted to evaluate the performance of the optimized motor configuration. Results showed that the proposed motor design can provide a 10 sec. peak power density of ~86.59 kW/L and a continuous power of ~78 kW without demagnetization, which are ~73% and 41.82% higher than the Department of Energy’s 2025 target for electric motor for electric vehicle (EV) applications.

Modeling flow boiling in finned micro-gaps commonly employed in cold plates for thermal management of microelectronics remains a complex challenge due to the lack of a model that can accurately represent interfacial heat and mass transfer, across a range of conditions. Among the phase change models used in computational fluid dynamics and heat transfer (CFD/HT) modeling of flow boiling, the Lee model has gained popularity. The Lee model assumes that phase change occurs at a constant saturation temperature and does not account for the incipience superheat required for the onset of nucleate boiling (ONB). In this paper, we present an improved Lee model that considers both the change in saturation temperature with pressure and the required surface superheat for the ONB. We assess the improved model’s performance against the original Lee model for a fin-enhanced micro-gap with nonuniform heating. The current CFD/HT study uses the dielectric fluid HFE-7200, which allows for near junction, or direct contact cooling of electrically active components. We also investigate the impact of variation in the contact angle on the improved Lee model predictions of the two-phase flow regimes.

Abstract Cities account for over 66% of global energy use and with over 68% of the population expected to live in urbanized areas by 2050, anthropogenic urban heat release is likely to become one of the most significant contributors to the creation of urban microclimates. In the present work, an open-source framework for one-way upstream coupled multiscale urban thermal environment simulations is examined and validated and can provide valuable insights about the flow behavior and energy transport between spatial scales. In this study, a city-wide multiscale model with over 500,000 building, road, and tree canopy data points parameterizing Atlanta, GA as a digital twin is developed and validated with a spatial scale of 5 m. The validated model is used to perform a parametric study on the implications bulk surface albedo (SA) has on the city's anthropogenic heat (AH) release in terms of heat flux. The study demonstrates that anthropogenic heat flux for building waste energy accounts for a small part of the total surface heat flux, and a detailed understanding of the components of urban heat (particularly with respect to total surface heat flux) is required to predict and simulate an urban thermal environment.

This article reviews and summarizes research conducted during and related to ASHRAE RP-1675, Guidance for Data Center CFD. The ultimate objective is to provide users and vendors of computational fluid dynamics (CFD) tools with best-practice modeling advice to support greater adoption of the technology to design and operate reliable and sustainable facilities. RP-1675 identified and developed good CFD modeling practices by benchmarking CFD simulations against experimental measurements for a “laboratory data center,” considering past and concurrent research, and performing additional sensitivity studies in CFD. This article provides a detailed review of the experimental and CFD work conducted during RP-1675, which focused on the modeling of computer room air handler units (CRAHs) with vertical-axis blowers located in the raised-floor plenum, and modeling perforated floor tiles, and raised-floor stanchions. This article ultimately summarizes current guidance for data-center CFD.

Abstract Coldplates are a crucial component in various cooling applications, such as cooling data center servers and power electronics. The unprecedented growth in electronics power density, along with the resulting ultrahigh heat fluxes, demands a transition from single-phase forced convection to two-phase flow boiling heat transfer. The majority of studies in the literature have focused on flow boiling in fin-enhanced silicon microgaps and microchannels, with only a few addressing flow boiling in millimeter-scale heat sinks. In the present study, flow boiling of HFE-7200 dielectric fluid in a millimeter-scale pin-fin coldplate is experimentally investigated under nonuniform heating conditions. Four background heaters represent the low-dissipating-power devices. On the other hand, five hotspot heaters mimic the high-heat-flux devices and generate heat fluxes ranging from 50 W/cm2 to 1000 W/cm2, corresponding to hotspot heat inputs ranging from 62.5 W to 1.25 kW, respectively. The coldplate's thermohydraulic performance is investigated for various flow rates and inlet temperature ranging from 0.5 L/min to 1.5 L/min and from 25 °C to 60 °C, respectively. A high-speed camera is utilized for a narrow field of view (FOV) flow visualization at a frame rate of 2229 fps while a digital camera is used for a wider FOV at 60 fps. Flow visualization demonstrated the transition between bubbly, slug/churn, and stratified two-phase flow regimes.

While data center cooling energy usage optimization studies have been performed through computational fluid dynamics/heat transfer (CFD/HT), and heuristic methods, data driven modeling techniques are now also being used for these applications. This paper investigates the air temperature prediction capabilities of static artificial neural network (ANN), Gaussian progress regression (GPR), support vector regression (SVR), relevance vector machine (RVM), linear regression, and regression trees; and transient long-short term memory (LSTM), and nonlinear autoregressive neural network with external input (NARX)) data driven modeling frameworks. The static study compared various models and found that GPR provided the best results (average error of 0.56 °C), closely followed by the ANN and SVR (average error of 0.60 °C and 0.68 °C respectively) methods. The transient study compared models based on an experimental data set and found that NARX outperforms LSTM for normal operations (0.83 °C and 1.07 °C average error respectively), and that data driven models are able to provide relatively good predictions, even if the input variables are slightly outside the training domain.

Abstract The unique properties of metal foams make them potential candidates for a range of applications, including microsystem thermal management. Using additive manufacturing to create foam-type structures can improve upon prior thermal solutions by eliminating thermal interface materials and allowing for customization/local control of parameters. In the present investigation, flow boiling in additive-manufactured metal foams is investigated both experimentally and numerically. Two test samples, one with uniform structure and the other with pathways for vapor removal, are compared both experimentally and numerically. A conjugate computational fluid dynamics and heat transfer (CFD-HT) model utilizing a three-dimensional volume of fluid (VOF) model with accompanying evaporation/condensation model provided in-depth visualization of the boiling flow phenomena. The experiments generated the thermohydraulic performance over a range of heat fluxes, demonstrating that the sample incorporating dedicated vapor pathways performed better in both pressure and heat transfer performance metrics compared to the uniform foam. Additionally, negative system-level effects (i.e., hydraulic oscillations) were shown to be abated using the vapor removal structures. The numerical model yielded further insight into the factors contributing to the improved performance. Results indicated the pathways functioned as vapor removal channels, allowing the generated vapor to vent from the foam structure into the lanes. Further computational investigations demonstrated changes in flow regimes, where the addition of vapor channels caused the flow to change from churn to annular. Bubble behavior unique to the vapor pathway structure was studied, showing stagnant regions that eject vapor into the channel.

Abstract The next generation of integrated power electronics packages will implement wide-bandgap devices with ultrahigh device heat fluxes. Although jet impingement has received attention for power electronics thermal management, it is not used in commercial electric vehicles (EVs) because of the associated pressure drop and reliability concerns. In this paper, we present a modular thermal management system designed for automotive power electronics. The system achieves superior thermal performance to benchmarked EVs while adhering to reliability standards and with low pumping power. The system utilizes a low-cost and lightweight plastic manifold to generate jets over an optimized heat sink, which is embedded in the direct-bonded-copper (DBC) substrate. The embedded heat sink concept leverages additive manufacturing to add elliptical pin fins to the DBC substrate. The heat sink geometry is optimized for submerged jet impingement using a unit-cell model and an exhaustive search algorithm. The model predictions are validated using unit-cell experiments. A full-scale power module model is then used to compare the DBC-embedded heat sink against direct DBC cooling and baseplate-integrated heat sinks for single-sided (SS) and double-sided (DS) cooling concepts. Using the SS and DS DBC-embedded cooling concepts, the models predict a thermal resistance that represents a reduction of 75% and 85% compared to the 2015 BMW i3, respectively, for the same water-ethylene glycol inverter flowrate. We have shown that an inverter with a 100-kilo-Watt-per-liter power density is achievable with the proposed design.

Metal foams are promising materials for electronics cooling applications due to their high surface area-to-volume ratio. Their use in electronics cooling requires excellent contact between the heated surface and the foam, which is typically achieved using a thermal interface material (TIM). However, TIMs are typically designed for metal-to-metal interfaces and are not assessed for improving heat transfer at metal foam contacts. Thus, this study aims to reveal the performance of different TIMs when used with metal foams. An experimental setup is built where a metal foam is placed inside a rectangular channel and exposed to a constant heat flux. The thermal performance of the thermal gap pad, thermal epoxy applied to the heated surface, thermal epoxy applied to the strut ends, flexible graphite, and the condition of no TIM are assessed. A comprehensive investigation of the fluid flow and heat transfer in the metal foam for these configurations is also conducted numerically using local thermal non-equilibrium porous media equations. The numerically predicted thermal characteristics are compared with measurements. It is found that using in-plane thermally conductive TIMs can enhance heat transfer by around 18% for the same flow rate and pumping power.

The adoption of silicon carbide (SiC) devices in the electric vehicle (EV) industry is increasing due to their superior performance over silicon devices. SiC devices enable further miniaturization of EV inverters, increasing their power density, which results in thermal management challenges. In this article, the limits of single-phase jet impingement cooling are explored for an automotive SiC power module. We propose embedding pin fins in the direct-bonded-copper (DBC) substrate of the power module package using laser powder bed fusion (LPBF) additive manufacturing. The thermal-hydraulic performance of the DBC-embedded pin fins is compared against folded fins that are directly soldered to the DBC substrate. A heat conduction analysis was conducted on an SiC package to determine the target heat transfer coefficient (HTC) for the heat sink. A water–ethylene glycol (WEG) jet impingement on the proposed concepts was studied using unit-cell models to achieve the target HTC. The studied designs put emphasis on the reliability and manufacturability requirements of the automotive industry. The thermal performance of DBC-embedded pin fins outperformed the DBC-soldered folded fins. The performance of the DBC-embedded pin fins is benchmarked against WEG-based cooling systems of commercial EVs. With the proposed cooling solution, we have shown a pathway of reducing the specific thermal resistance by 75% compared to the BMW i3 thermal management system without any penalty on pressure drop or parasitic power.

Abstract Structural flexibility has become a common feature in emerging microsystems with increasing heat fluxes. The thermal control of such applications is a significant challenge because of both structural and volumetric requirements, where standard cooling solutions are not applicable. Flexible polymer microlayers are a promising solution for the embedded cooling of such microsystems. In the present investigation, a flexible polydimethylsiloxane (PDMS) microgap is proposed and assessed in an effort to prove its viability for thermal management in the aforementioned applications. The analyzed polymer microgap features a dedicated vapor pathway design which is proven to assist in the efficient removal of vapor from the microsystem. The dielectric refrigerant HFE-7100 is used as the working fluid under flow boiling conditions, reporting on the two-phase flow regime, heat transfer, and pressure drop. In addition to experimental results, the numerical modeling of the relevant features of flow boiling is explored with the use of a mechanistic phase-change model that is proven to accurately predict the flow variables and constitutes a valuable tool in the analysis and design of such microsystems. The results from this study demonstrate that this approach is feasible for the removal of relatively high heat fluxes which are comparable to metallic-based or silicon microchannels, with the added advantage of structural flexibility while also providing a stable two-phase cooling mechanism.

Abstract As the rate of urbanization increases, local vegetation is being replaced with man-made materials, causing increasingly adverse impacts on the surface-atmosphere energy balance. These negative effects can be simulated by modeling the urban landscapes in question; however, the main challenges of modeling urban thermal environments are the scale and resolution at which to perform such tasks. Current modeling of urban thermal environments is typically limited to either mesoscale (1 –2000 km) or microscale (&amp;lt;1 km) phenomena. In the present work, an open-source framework for one-way upstream coupled multiscale urban thermal environment simulations is examined and validated. This coupled simulation can provide valuable insights into the flow behavior and energy transport between mesoscale and microscale interactions. The mesoscale to microscale boundary conditions are coupled together using simulated data from the advanced research weather research and forecasting model (WRF-ARW), a mesoscale numerical weather prediction software, and assimilating it into parallelized large-eddy simulation model (PALM), a computational fluid dynamics style (CFD-style) software designed for microscale atmospheric and oceanic flows. The multiscale urban thermal environment simulations are tested for grid sensitivity to variations in model input and control parameters, and then experimentally validated against distributed sensor measurements at the Georgia Institute of Technology (Georgia Tech) campus in Atlanta, GA. Validated microscale atmospheric models with heterogeneous domains can be used to project the thermal benefits of urban heat mitigation strategies (increase use of high-albedo surfaces, tree and vegetation cover, and smart growth practices) and advise building energy usage modeling and policies.

This Special Issue honors and celebrates the career of Professor Ivan Catton, an internationally acclaimed expert in the field of thermal science and engineering and Distinguished Professor Emeritus of Engineering at the University of California, Los Angeles. He was active in research for over five decades and worked in many different areas. The originality, analytical treatment and physical reasoning presented in his papers were impressive. He contributed extensively to natural convection, instability, porous media transport, nuclear reactor thermal-hydraulics and safety, materials processing, and aerospace heat transfer, among others. In each of these areas, he made important innovative fundamental contributions. His work spans a wide range of problems, from basic to applied, and, consequently, his papers are widely cited around the world. Some of his papers, such as his keynote paper Natural Convection in Enclosures at the 1978 International Heat Transfer Conference and the paper Wavenumber Selection in the 1988 Journal of Heat Transfer Special Bicentennial Issue, have become classic and have been extensively cited. Similarly, his other papers, edited conference proceedings, and review articles on convection in porous media, two-phase flow, natural convection, cooling of electronic devices and nuclear plant safety and design have become landmarks in these areas. He stands out as one of the dominant figures in the field, with a long list of outstanding successful graduate students who have made their mark in academia and industry. His work has influenced researchers in many areas in thermal sciences and has thus provided outstanding leadership to generations of researchers, educators, and engineers in heat transfer.Professor Catton was a member of the Advisory Committee on Reactor Safeguards (ACRS) of the U.S. Nuclear Regulatory Commission (NRC), the top advisory committee in the field. After the NRC, Prof. Catton turned his attention to aerospace engineering's leading-edge cooling problems as well as research on the impact of laser weapons on space power cooling systems. Later, he ventured into the area of information processing using neural nets. His foundational work formed the basis for optimization of heat sinks and heat exchangers. He served as an associate editor of the Journal of Heat Transfer and as a member of other editorial boards. He also served as a member and as chair of various committees in the ASME Heat Transfer Division and in the American Nuclear Society. Prof. Catton was the recipient of numerous awards, including the ASME Heat Transfer Memorial Award and the Max Jakob Memorial Award, considered to be the highest international honor in the field of heat transfer.The papers in this special issue are presented in 2 volumes, containing 33 papers. About half of these papers were invited from former students, colleagues, and friends of Professor Catton, as well as from leading experts in areas of interest to him. The remaining papers were contributed by researchers in heat transfer from around the world. All these papers underwent the rigorous review process of the Journal of Heat and Mass Transfer and were revised and updated to meet the standards of the journal. The papers cover a wide range of topics in natural and forced convection, porous media, thermal management of electronic systems, boiling, nanofluids, microchannel flow, heat transfer in biological systems, and several other fundamental and applied areas. A memorial tribute to Professor Catton is also included.We have been colleagues and friends of Professor Catton for many years. We have admired his fundamental and applied research and have learned a lot from his excellent contributions. We have followed and cited his papers in our own publications. It is certainly an honor and a privilege for us to serve as guest editors for this special issue dedicated to Professor Catton, who passed away on June 12, 2021, leaving behind an impressive legacy of research in heat transfer.We would like to thank Professor P. S. Ayyaswamy, Editor-in-Chief, Journal of Heat and Mass Transfer, for his support, encouragement and help in developing this special issue. We are also very appreciative of the help and guidance provided by Ms. Elizabeth Saas, JHT Editorial Assistant, in processing the papers submitted to the journal. The editorial staff at ASME, particularly Ms. Jennifer Smith, was instrumental in putting the two volumes together and arranging the sequence of the papers. Finally, we would like to thank the authors for their contributions and the reviewers for their insightful and timely review of the papers.

BACKGROUND: Hepatic encephalopathy, which is a serious complication, and sarcopenia are undesirable consequences in cirrhosis. Transjugular intrahepatic portosystemic shunt increases the risk of hepatic encephalopathy. We investigated the effect of sarcopenia on the incidence of post-transjugular intrahepatic portosystemic shunt hepatic encephalopathy. METHODS: Clinical data of patients who underwent transjugular intrahepatic portosystemic shunt were extracted retrospectively. Computed tomography images at L3 level of scans performed prior to transjugular intrahepatic portosystemic shunt were analyzed to assess skeletal muscle index-expressed as skeletal muscle area (cm2)/ height (m2). RESULTS: Of 210 patients who underwent transjugular intrahepatic portosystemic shunt, complete information was available in 79 [male: 68 (86%); age: 50.5 ± 11.2 years; Child-Turcotte-Pugh score: 8.81 ± 1.23; etiology-alcohol: 44 (56%), non-alcoholic steatohepatitis: 16 (20%), others: 19 (24%); transjugular intrahepatic portosystemic shunt indication-ascites: 56 (71%); bleed: 23 (29%); sarcopenics: 42 (53%)]. Post-transjugular intrahepatic portosystemic shunt hepatic encephalopathy developed in 29 (37%) patients. In patients who developed hepatic encephalopathy, both serum ammonia [177.6 ± 82.5 vs. 115.5 ± 40.5 µg/dL, P =.008] and prevalence of sarcopenia [69% vs. 44%; P =.02; odds ratio (95% CI): 2.8 (1.08-7.4), P =.02] were higher, with sarcopenics having 3 times higher risk of hepatic encephalopathy and 8 times higher risk of multiple episode of hepatic encephalopathy [31% vs. 5.4%; odds ratio (95% CI): 8.2 (1.68- 40.5), P =.009]. In multivariate analysis, age [odds ratio (95% CI): 1.05 (1.001-1.11), P =.047], serum albumin [odds ratio (95% CI): 0.162 (0.05-0.56), P =.004], and skeletal muscle index [odds ratio (95% CI): 0.925 (0.89-0.99), P =.017] were independently associated with post-transjugular intrahepatic portosystemic shunt hepatic encephalopathy. CONCLUSIONS: Sarcopenia is present in nearly half of the cirrhotic patients undergoing transjugular intrahepatic portosystemic shunt, which increases the risk of a single episode of hepatic encephalopathy by 3-fold and that of multiple episodes of hepatic encephalopathy by 8-fold after transjugular intrahepatic portosystemic shunt procedure. Increased skeletal muscle index is associated with decreased risk of hepatic encephalopathy.

This study investigates the issues involved in the design of compact two-phase thermosyphon systems. In such systems the locations of the evaporator and condenser need be given a high degree of freedom. Anticipating situations where gravity does not provide sufficient potential to drive the condensate, a pump-assisted circulation loop was studied. Also, enhancement of boiling heat transfer in compact space was achieved by at enhancement structure having mutually connected microchannels. The key components used in the experiment are; a simulated chip (0.907 cm²) with an enhancement structure (6.8 mm high), a dielectric coolant (PF-5060, boiling point 56 &#176;C), a naturally cooled condenser occupying a 6.5 &#215; 6.5 &#215; 17.8 cm³ volume, and a displacement pump (2 &minus; 40 ml/min). The relative height between the evaporator and the condenser, the pumping rate, and the heat input were systematically varied. Theseparameters affect the overall thermal resistance from the chip to the ambient in a complex way. However, close examination of the data suggests that there could be an optimum point in the parametric domain where the thermal resistance is minimized with a least assistance from the pump.