Efficient Heat Removal Research Articles

Deciphering the molecular mechanism of water boiling at heterogeneous interfaces

Water boiling control evolution of natural geothermal systems is widely exploited in industrial processes due to the unique non-linear thermophysical behavior. Even though the properties of water both in the liquid and gas state have been extensively studied experimentally and by numerical simulations, there is still a fundamental knowledge gap in understanding the mechanism of the heterogeneous nucleate boiling controlling evaporation and condensation. In this study, the molecular mechanism of bubble nucleation at the hydrophilic and hydrophobic solid–water interface was determined by performing unbiased molecular dynamics simulations using the transition path sampling scheme. Analyzing the liquid to vapor transition path, the initiation of small void cavities (vapor bubbles nuclei) and their subsequent merging mechanism, leading to successively growing vacuum domains (vapor phase), has been elucidated. The molecular mechanism and the boiling nucleation sites’ location are strongly dependent on the solid surface hydrophobicity and hydrophilicity. Then simulations reveal the impact of the surface functionality on the adsorbed thin water molecules film structuring and the location of high probability nucleation sites. Our findings provide molecular-scale insights into the computational aided design of new novel materials for more efficient heat removal and rationalizing the damage mechanisms.

Intensification of Gas Hydrate Formation Processes by Renewal of Interfacial Area between Phases

This paper presents the analysis of the main reasons for a significant decrease in the intensity of diffusion processes during the formation of gas hydrates; solutions to this problem are proposed in a new process flow diagram for the continuous synthesis of gas hydrates. The physical processes, occurring at the corresponding stages of the process flow, have been described in detail. In the proposed device, gas hydrate is formed at the boundary of gas bubbles immersed in cooled water. The dynamic effects arising at the bubble boundary contribute to the destruction of a forming gas hydrate structure, making it possible to renew the contact surface and ensure efficient heat removal from the reaction zone. The article proposes an assessment technique for the main process parameters in the synthesis of hydrates based on the criterion of thermodynamic parameters optimization. The optimization criterion determines the relationship of intensity of heat and mass transfer processes at the phase contact interface of reacting phases, correlating with the maximum GH synthesis rate, and makes it possible to determine optimum thermodynamic parameters in the reactor zone.

Novel Ideas for Thermal Management of Filament LED Light Bulbs

The thermal conditions of a filament LED retro light bulb has been investigated. In such light bulbs, LEDs are mounted on filaments which have glass or sapphire substrate and are surrounded by yttrium-aluminium-garnet (YAG). Heat management is challenging as a heatsink cannot normally be used. Instead, heat removal is enabled by using helium as the filling gas. As this provides less efficient heat removal than using a heatsink attached directly to an LED module, LEDs operate in filament LED bulbs typically at rather high temperatures. In this paper thermal performance of a typical filament LED bulb has been studied. LED temperatures and gas flows inside the bulb has been resolved for cases of different orientations of the bulb. Thermal conditions for different filling gases have been resolved. All this has been done using finite element method (FEM) simulations. The main target of this study has been to find out novel solutions for thermal management of filament LED bulbs. Two heat removal solutions have been studied, namely the use of an internal heatsink and an internal piezoelectric oscillating cantilever fan. The results show that solutions can indeed reduce temperature of LED chips and increase the lifespan of the LED bulb.

Vertically Aligned Al2O3 Fiber Framework Leading to Anisotropically Enhanced Thermal Conductivity of Epoxy Composites

With the development of the modern electronic devices, efficient heat removal via thermal management materials in a low filling load has become one of the most critical challenges. Herein, a biomass template method is used to produce 3D alumina skeleton with one‐directionally arranged alumina fibers as the thermal conductive filler of epoxy composites. Thermal conductive composites with anisotropically enhanced thermal conductivity are prepared by impregnating epoxy resin into the 3D alumina skeleton. At an alumina load of 17.7 wt%, the composite exhibits a high out‐of‐plane thermal conductivity of 0.72 W m−1 K−1 (3.27 times of pure epoxy) and an in‐plane thermal conductivity of 0.38 W m−1 K−1, respectively. When compared with literatures in terms of the thermal conductivity enhancement per unit mass of filler, the samples are on the top. By using the biomass template method to construct 3D anisotropic structure, it is effective to improve the out‐of‐plane thermal conductivity of the composites, which exhibits strong potential for thermal management applications.

High Thermal Dissipation of Normally off p-GaN Gate AlGaN/GaN HEMTs on 6-Inch N-Doped Low-Resistivity SiC Substrate.

Efficient heat removal through the substrate is required in high-power operation of AlGaN/GaN high-electron-mobility transistors (HEMTs). Thus, a SiC substrate was used due to its popularity. This article reports the electrical characteristics of normally off p-GaN gate AlGaN/GaN high-electron-mobility transistors (HEMTs) on a low-resistivity SiC substrate compared with the traditional Si substrate. The p-GaN HEMTs on the SiC substrate possess several advantages, including electrical characteristics and good qualities of epitaxial crystals, especially on temperature performance. Additionally, the price of the low-resistivity SiC substrate is three times lower than the ordinary SiC substrate.

49‐3: High Power Laser Phosphor Light Source Using a Scanning Mirror for Projectors

With the advancement of blue laser developments at low cost, laser diodes and laser diode arrays are getting popular in various light system from low power to very high power. This paper presents a stationary phosphor system in which the phosphor plate is not in motion and can be attached to heat sinks for efficient removal of heat for high power operations, which cannot be achieve easily when the phosphor material is coated onto a rotating wheel. The highest effective power density, limited by the available laser source and the current heat sinking, of over 301 W per sq. mm has been achieved and is expected to increase in the near future with higher power laser sources and improved heat sinking.

Sample Preparation and Transfer Protocol for In-Vacuum Long-Wavelength Crystallography on Beamline I23 at Diamond Light Source

Long-wavelength macromolecular crystallography (MX) exploits the anomalous scattering properties of elements, such as sulfur, phosphorus, potassium, chlorine, or calcium, that are often natively present in macromolecules. This enables the direct structure solution of proteins and nucleic acids via experimental phasing without the need of additional labelling. To eliminate the significant air absorption of X-rays in this wavelength regime, these experiments are performed in a vacuum environment. Beamline I23 at Diamond Light Source, UK, is the first synchrotron instrument of its kind, designed and optimized for MX experiments in the long wavelength range towards 5 Å. To make this possible, a large vacuum vessel encloses all endstation components of the sample environment. The necessity to maintain samples at cryogenic temperatures during storage and data collection in vacuum requires the use of thermally conductive sample holders. This facilitates efficient heat removal to ensure sample cooling to approximately 50 K. The current protocol describes the procedures used for sample preparation and transfer of samples into vacuum on beamline I23. Ensuring uniformity in practices and methods already established within the macromolecular crystallography community, sample cooling to liquid nitrogen temperature can be performed in any laboratory setting equipped with standard MX tools. Cryogenic storage and transport of samples only require standard commercially available equipment. Specialized equipment is required for the transfer of cryogenically cooled crystals from liquid nitrogen into the vacuum endstation. Bespoke sample handling tools and a dedicated Cryogenic Transfer System (CTS) have been developed in house. Diffraction data collected on samples prepared using this protocol show excellent merging statistics, indicating that the quality of samples is unaltered during the procedure. This opens unique opportunities for in-vacuum MX in a wavelength range beyond standard synchrotron beamlines.

Sample Preparation and Transfer Protocol for In-Vacuum Long-Wavelength Crystallography on Beamline I23 at Diamond Light Source

Long-wavelength macromolecular crystallography (MX) exploits the anomalous scattering properties of elements, such as sulfur, phosphorus, potassium, chlorine, or calcium, that are often natively present in macromolecules. This enables the direct structure solution of proteins and nucleic acids via experimental phasing without the need of additional labelling. To eliminate the significant air absorption of X-rays in this wavelength regime, these experiments are performed in a vacuum environment. Beamline I23 at Diamond Light Source, UK, is the first synchrotron instrument of its kind, designed and optimized for MX experiments in the long wavelength range towards 5 Å. To make this possible, a large vacuum vessel encloses all endstation components of the sample environment. The necessity to maintain samples at cryogenic temperatures during storage and data collection in vacuum requires the use of thermally conductive sample holders. This facilitates efficient heat removal to ensure sample cooling to approximately 50 K. The current protocol describes the procedures used for sample preparation and transfer of samples into vacuum on beamline I23. Ensuring uniformity in practices and methods already established within the macromolecular crystallography community, sample cooling to liquid nitrogen temperature can be performed in any laboratory setting equipped with standard MX tools. Cryogenic storage and transport of samples only require standard commercially available equipment. Specialized equipment is required for the transfer of cryogenically cooled crystals from liquid nitrogen into the vacuum endstation. Bespoke sample handling tools and a dedicated Cryogenic Transfer System (CTS) have been developed in house. Diffraction data collected on samples prepared using this protocol show excellent merging statistics, indicating that the quality of samples is unaltered during the procedure. This opens unique opportunities for in-vacuum MX in a wavelength range beyond standard synchrotron beamlines.

Progress and perspectives on phononic crystals

Phononic crystals (PnCs) control the transport of sound and heat similar to the control of electric currents by semiconductors and metals or light by photonic crystals. Basic and applied research on PnCs spans the entire phononic spectrum, from seismic waves and audible sound to gigahertz phononics for telecommunications and thermal transport in the terahertz range. Here, we review the progress and applications of PnCs across their spectrum, and we offer some perspectives in view of the growing demand for vibrational isolation, fast signal processing, and miniaturization of devices. Current research on macroscopic low-frequency PnCs offers complete solutions from design and optimization to construction and characterization, e.g., sound insulators, seismic shields, and ultrasonic imaging devices. Hypersonic PnCs made of novel low-dimensional nanomaterials can be used to develop smaller microelectromechanical systems and faster wireless networks. The operational frequency, compactness, and efficiency of wireless communications can also increase using principles of optomechanics. In the terahertz range, PnCs can be used for efficient heat removal from electronic devices and for novel thermoelectrics. Finally, the introduction of topology in condensed matter physics has provided revolutionary designs of macroscopic sub-gigahertz PnCs, which can now be transferred to the gigahertz range with advanced nanofabrication techniques and momentum-resolved spectroscopy of acoustic phonons.

Effect of the corrugated bipolar plate design on the self‐humidification of a high power density PEMFC stack for UAVs

AbstractOpen‐cathode PEMFCs for unmanned aerial vehicles operate at a near‐ambient temperature, which requires high super‐stoichiometric air flow rates to remove heat. This, in turn, makes the stack vulnerable to drying out. Using a combination of experimental measurements with a 1.2 kW stack and 3D multiphysics modelling, we show that the design of the corrugated metal bipolar plate with a height above the commonly used 1.05 mm and with cathode cooling channels wider than air‐suppling channels allows for a more efficient heat removal at lower air flow rates, thus assuring a wider range of operating parameters, while maintaining adequate hydration. The self‐humidifying stack attains 1000 W kg−1 power, and the full system with a nominal runtime of 4 h attains specific energy of 700 Wh kg−1.

Heat transfer evaluation for conventional and extractive ethanol fermentations: Saving cooling water

Ethanol production in Brazil has two major drawbacks, related to the maximum product concentration and control of the process temperature. During the fermentation, yeast metabolism completely ceases when the ethanol concentration reaches 95 g.L−1, while high temperatures increase both the risk of bacterial contamination and the sensitivity of the yeast to ethanol toxicity. In order to address these problems, extractive ethanol fermentation with carbon dioxide stripping offers a way to overcome the toxic effect of ethanol on yeast cell growth, while also avoiding increase of the broth temperature during the fermentation. Although ethanol removal by gas stripping has a positive effect on ethanol volumetric productivity, the application of this technique for economization of cooling water during the ethanol fermentation process requires a technical analysis. Therefore, in the present work, evaluation was made of heat removal from the culture broth during extractive fermentation with ethanol stripping by CO2. First, ethanol fermentations were carried out using a 10-L bench-scale bubble column bioreactor. The mass and energy balances were used to obtain kinetic parameters, with the heat generated during fermentation being close to theoretical values. Extractive batch fermentations were performed using two carbon dioxide specific flow rates (0.5 and 1.0 vvm), resulting in efficient removal of heat from the fermentation broth, as well as reducing cooling water consumption by up to 62.7%. These results were then validated in a pilot-scale (100-L) extractive fed-batch fermentation performed at 1.0 vvm, resulting in a 63.1% reduction of cooling water consumption. In an industrial distillery, this lower consumption of cooling water would allow substantial savings of chemicals needed for cooling water treatment, making the ethanol production process in Brazil more competitive in the near future.

Thermal performance analysis of naturally ventilated and perforated sheet based double skin facade system for hot summer conditions

A new skin of perforated sheets in the cavity of double skin facades (DSF) system is introduced for efficient heat removal from the cavity in hot summer conditions. In this paper, the analytical solution is developed to estimate the thermal performance of the double-skin façade system with and without perforated sheet operating under natural ventilation mode. This analytical model is designed to predict the solar heat gain coefficient (SHGC) for hot summer conditions. An experimental investigation has also been conducted to validate the proposed analytical model. Results showed good agreement between the measured and calculated value of SHGC for DSF system with and without perforated sheet. It is observed that the addition of a perforated sheet reduces the SHGC by nearly 51.5% as compared to DSF system. Perforated sheets enable a better stack effect within the air cavity zone and thereby improve heat transfer characteristics. Therefore, the benefit of the perforated sheet within the air cavity of DSF system is profound over conventional DSF system.

Exploring the advantages of photo-voltaic triple skin façade in hot summer conditions

A new skin of perforated metallic sheet is introduced in the cavity of a photo-voltaic double skin façade (PV-DSF) system for an efficient heat removal from the cavity without losing substantial light into the built space. Quantitative experimental analysis is conducted to evaluate its thermal and electrical performance for various combinations of ventilation modes, air cavities, and perforation ratios. The advantages of Photo-Voltaic Triple Skin Facade system (PV-TSF) over commonly used PV-DSF are also evaluated experimentally. The results showed that naturally ventilated PV-TSF provides lower temperatures and Solar Heat Gain Coefficient (SHGC) with slightly higher power when compared to no ventilation mode in PV-DSF system. The SHGC of the PV-DSF system can be reduced by nearly 4.7% with variation in air cavity (Thickness 50–200 mm); after which no noticeable improvement is observed. In the case of PV-TSF system, SHGC can be reduced by almost 14.7% compared to the PV-DSF system without significant loss of natural daylight into built space. Experimental results also showed that the reduction in perforation ratio from 60% to 20% reduced the SHGC by 3.8% with an average illuminance of 334 lux (minimum perforation, 20%). It was also observed that the addition of a perforated sheet can offset the large air cavity of a PV-DSF system. Therefore, the benefit of the PV-TSF system is profound as compared to PV-DSF.

Effect of spray modes on electrospray cooling heat transfer of ethanol

Abstract Spray cooling is a promising approach to efficient removal of heat and has attracted increasing interest with its excellent characteristics, such as strong heat exchange capacity, low flow rate, and uniform temperature distribution. Recently, high voltages have been successfully employed in electrospray (ES) cooling, which has improved cooling performance but has poor heat transfer control. We experimentally investigate heat transfer performance of ES cooling with different spray modes, and we study spray characteristics and droplet impinging behavior of the different modes. The results show that droplet size decreases and spray velocity increases with increasing applied voltage, and droplet formation and subsequent impinging behavior are gradually dominated by electric forces. In addition, the nucleate boiling region is significantly extended, and the critical heat flux (CHF) increases to a higher surface temperature when the spray mode is transformed to cone-jet/multi-jet modes. Under certain conditions, ES cooling achieves a 2.8 times enhancement rate over neutral (no voltage) dropwise cooling. Moreover, the visualization results of the droplet impinging behavior on the hot surface for different spray modes are consistent with their cooling performance. Overall, ES cooling is found to greatly improve the cooling capacity, and the specific mechanisms for enhancement vary with spray mode. Our results contribute to precise control of ES cooling and give a better understanding of the effect of electric fields on spray cooling.

Personalized Cooling System Using Phase Change Materials

The most widely used personal protective system against heat stress is cooling vest that contains phase change material (PCM) for thermal energy storage. PCMs have the property of absorbing/releasing heat when they change their phase at their melting point. If the PCM has greater heat of fusion, more heat is absorbed; furthermore, good thermal conductivity assists in efficient removal of heat. In this work different PCMs are explored for use in personalized cooling vest. Hexadecane is finally selected to be used as a PCM having a melting point of 18-20 °C (which lies in the human thermal comfort) and heat of fusion of 241 kJ/kg. Carbon nanotubes have excellent capability of increasing thermal conductivity of a material. Carbon nanotubes were added in hexadecane, and latent heat of fusion of the mixture increased up to 262.6 kJ/kg.

Effect of vibration treatment during welding on technological strength of chromium martensitic steel P91

The paper shows the possibility of using the technology of vibration treatment in the process of welding of high-temperature chromium steel of martensitic class P91 applied to the product to increase the technological strength of welded joints and the effect of this technology on mechanical properties before and after thermal treatment. Application of vibration action on welding bath and weld seam in process of manufacturing of welded structure may allow to reduce inhomogeneity of weld metal structure in liquid phase, as well as increase its microchemical inhomogeneity within crystallites during crystallization, reduce grain size due to more efficient heat removal and increase of number of crystallization centers. On the example of manufacturing the turbine support housing of the gas turbine drive, it is shown that it is necessary to replace part of the heat treatment operations with other more energy - efficient and less labor-intensive operations. Results of studies of influence of vibration treatment of welded joint on tendency of welded joint metal to form cold cracks, strength, hardness and impact toughness are given. Based on the obtained results, diagrams of average values of parameters are constructed and conclusions are drawn on the possibility of using the technology of vibration treatment of the turbine casing in the process of welding in order to reduce the number of heat treatment operations during its manufacture.

Temperature characteristics of high repetition rate water-cooled Nd:YAG active mirror amplifier

In order to deal with the thermal management problem of high-energy high-repetition rate laser amplifiers, the efficient heat removal in water-cooled Nd:YAG active mirror amplifiers is investigated in detail through numerical modeling and experimental analysis. According to the low Reynolds number <i>k-ε</i> turbulence model, a full fluid-solid conjugate heat transfer model is established to give a comprehensive model of flow and thermal characteristics in three dimensions. The thermal distributions obtained from the model are then used to calculate all mechanical stresses in the laser medium and thermally-induced wavefront distortions. In comparison with the standard <i>k-ε</i> turbulence model, the influences of the near-wall treatments of the above model on the process of fluid flow, convection diffusion and heat conduction, and temperature distributions are analyzed. Meanwhile, the effects of coolant flow rate and pump parameter on the flow field characteristics, temperature and wavefront distributions of the YAG disk are also studied. Numerical simulation results reveal that the temperature distribution of the laser medium is closely related to the viscous effect in the solid-liquid boundary layer. Although the heat deposition distribution of the laser medium is symmetrical, the temperature profile is asymmetrical as a result of the increasing water temperature along the water flow. The maximum temperature rise of the disk is at the outlet end, and the position remains almost unchanged. The front-surface temperature distributions and wavefront profiles of Nd:YAG vary nonlinearly with the coolant flow rates, but linearly with the pump parameter. Model predictions show that when the laser amplifier operates at a repetition rate of 50 Hz, the thermal diffusion of the coolant mainly occurs in a range of 100 μm, and the maximum temperature difference of the coolant reaches up to 10.85 ℃. Correspondingly, the maximum temperature variation over the front-surface active region is less than 4 ℃, with an average temperature of 49.62 ℃, which leads to a total peak-to-valley wave front distortion of 7.27<i>λ</i>. The experimentally measured temperature distributions are in reasonable agreement with numerical simulations. The research results are beneficial to designing and optimizing the high-energy, high-repetition rate water-cooled Nd:YAG active mirror amplifiers.

INTENSIFICATION OF HEAT EXCHANGE IN DIABATIC RECTIFICATION COLUMNS

The heat exchange in a diabatic column was investigated during the rectification of an ethanol-water mixture, in which partial condensation of rising vapors on the surface of vertical heat exchange tubes installed vertically along the height of the installation was carried out, as well as the evaporation of intermediate condensate on the surface of horizontal plates. Based on the review of diabatic columns, it is shown that they can reduce the cost of conducting the rectification process. Heat-exchange devices placed on trays of rectification units are considered and ways to intensify heat transfer in them are proposed. It has been established that the most efficient heat removal in heat exchangers of diabatic columns is achieved when using a film flow of a coolant on a heat transfer surface. Heat transfer in a diabatic column is investigated during gravitational flow of surfaces of heat exchange tubes, as well as when organizing an ascending and descending co-current film flow, both in the case of heating and boiling of the coolant. To intensify heat transfer in the coolant film, a helical artificial roughness was installed on the surface of the pipes, made in the form of a wire spiral tightly mounted on the heat transfer surface. The geometric parameters of the helical roughness, such as the distance between the turns of the spiral and the height of the wire, which have the greatest influence on the intensity of heat transfer, have been established. Dependences for determining the value of the heat transfer coefficient are presented and an estimate of the value of the specific heat flux in the diabatic column is given.

Numerical Development of Heat Transfer Coefficient Correlation for Spray Cooling in Continuous Casting

The desire to remain competitive and continuously produce high quality and high strength steel at the maximum production rate in continuous casting requires dynamic control over the spray cooling rate. Efficient and uniform heat removal without cracking or deforming the slab during spray cooling is critical. The challenge is to obtain accurate Heat Transfer Coefficient (HTC) on the slab surface as boundary condition for solidification calculations. Experiment based HTC correlations are limited to handful operating conditions and they might fail when changes occur. The current study presents a numerical model for spray cooling featuring the simulation of atomization and droplet impingement heat transfer in continuous casting. With the aid of high-performance computer, parametric studies were performed and the results were converted into mathematically simple HTC correlations as a function of essential operating parameters. Finally, a Graphic User Interface (GUI) was developed to facilitate future applications of the correlations. The HTC prediction is stored in the versatile comma-separated values (csv) format and it can be directly applied to solidification calculations. The proposed numerical methodology should benefit the steel industry by expediting the development process of HTC correlations and can further improve the accuracy of the existing casting control systems.

Experimental investigation on cooling the photovoltaic panel using hybrid nanofluids

This work presents an experimental investigation on the use of CNT/Al $$_{2}$$ O $$_{3}$$ hybrid nanoparticles in a Photovoltaic/ Thermal (PV/T) system to enhance the photovoltaic electrical efficiency by reducing the temperature of PV cell. An experimental comparison on thermal and electrical efficiency of PV panel with and without cooling is experimentally analyzed. Furthermore, instead of using a serpentine tube collector, a spiral tube collector is used to enhance the rate of heat transfer from the photovoltaic panel. From the experimental results it is found that the enhancement is observed in the average electrical efficiency with water and nanofluid in the spiral tube collector and found as 7.15 and 8.2% respectively, whereas, the standalone photo voltaic panel it is found as 6.2%. The efficient removal of heat from the collector increased the power production by 11.7 and 21.4% using water and hybrid nanofluid in the PV/T system respectively, while compared to standalone PV system. Similarly, the overall PV/T efficiency using hybrid nanofluids in the spiral tube collector enhances by 27.3% than using water medium.