Thermal Sink Research Articles

Microstructure and thermophysical properties of graphite foam/Sn-Bi alloy composites for use as a thermal sink for electronics

Two mesophase pitch-based graphite foams with densities of 0.62±0.01 (GF1) and 0.84±0.01 g/cm3 (GF2) were prepared by foaming the pitch in an autoclave at 723 K, 6.0 MPa and 763 K, 13.4 MPa, respectively, followed by carbonization at 1273 K for 2 h and graphitization at 2973 K for 0.5 h. The GFs were infiltrated by a Sn-Bi liquid to prepare GF/Sn-Bi alloy composites for use as thermal sinks for electronics. The microstructures and thermophysical properties of the composites were investigated. Results indicated that GF1 had a larger cells and thinner cell walls than GF2. The Sn-Bi liquid was well infiltrated into cells of the GFs, resulting in composites with densities of 5.60±0.01 and 3.83±0.01 g/cm3 for GF1 and GF2, respectively. The thermal diffusivity and coefficient of thermal expansion (CTE) of the GF1/Sn-Bi composite were 51.6±2 mm2/s and 16.6±0.02 ppm/K, respectively. The corresponding values for the GF2/Sn-Bi were 163.1±3 mm2/s and 8.08±0.02 ppm/K. The GF2/Sn-Bi composite had a high thermal diffusivity and a low CTE value matching that of semiconductor chips and packaging materials.

Exergoeconomic optimization of a novel cascade Kalina/Kalina cycle using geothermal heat source and LNG cold energy recovery

Abstract In recent studies, cascade power generation systems are introduced as promising methodologies in power plants for recovering the waste heat of topping cycle, resulting in low waste heat rejection from the heat exchangers of the single-stage power plant. Due to the importance of this topic, a cascade Kalina cycle (CKC) power plant is proposed using geothermal energy as heat source and liquefied natural gas (LNG) as thermal heat sink. The proposed system is integrated from a high pressure KC (HPKC) as topping cycle and a low pressure KC (LPKC) as bottoming cycle. Energy, exergy and exergoeconomic analysis of the proposed system are conducted to give more detail information for designers. In addition, single- and multi-objective optimizations of the proposed system are performed, using genetic algorithm (GA) method. It is demonstrated that the optimum net power, thermal efficiency, exergy efficiency and total sum unit cost of product of the proposed system can be obtained 9044 kW, 29.87%, 43.19% and 127.8 $/GJ, respectively. Exergy analysis results demonstrated that among all components, the second heat exchanger accounts for the biggest exergy destruction rate. To obtain a better understanding from characteristics of the proposed system, a comprehensive parametric study is conducted showing that the thermal efficiency of system can be optimized based on the geothermal inlet temperature as well as the basic ammonia concentrations of HPKC and LPKC. In addition, it is found that a higher thermal efficiency can be obtained by increasing vapor generator pinch point temperature difference and turbines expansion ratio or by decreasing the second heat exchanger's temperature. Moreover, it is concluded that the exergy efficiency can be increased by increasing the first and second turbines' expansion ratios, geothermal inlet temperature, and basic ammonia concentration of HPKC or by decreasing the second heat exchanger's temperature, third turbine's expansion ratio and vapor generator pinch point temperature difference. In addition, the total sum unit cost of product of system can be decreased by increasing of the geothermal inlet temperature, second heat exchanger temperature, basic ammonia concentration of HPKC and basic ammonia concentration of LPKC or by decreasing of the vapor generator pinch point temperature difference and turbines' expansion ratios.

Thermal modeling of wide bandgap semiconductor devices for high frequency power converters

The emergence of wide bandgap semiconductors has led to development of new generation semiconductor switches that are highly efficient and scalable. To exploit the advantages of GaNFETs in power converters, in terms of reduction in the size of heat sinks and filters, a thorough understanding of the thermal behavior of the device is essential. This paper aims to establish a thermal model for wideband gap semiconductor GaNFETs commercially available, which will enable power electronic designers to obtain the thermal characteristics of the device more effectively. The model parameters is obtained from the manufacturer’s data sheet by adopting an exponential curve fitting technique and the thermal model is validated using PSPICE simulations. The model was developed based on the parametric equivalence that exists between the thermal and electrical components, such that it responds for transient thermal stresses. A suitable power profile has been generated to evaluate the GaNFET model under different power dissipation scenarios. The results were compared with a Silicon MOSFETs to further highlight the advantages of the GaN devices. The proposed modeling approach can be extended for other GaN devices and can provide a platform for the thermal study and heat sink optimization.

Analysis and Modeling of Thermal-Electric Coupling Effect of High-Power Monolithically Integrated Light-Emitting Diode

A 2-D thermal-electric coupling model has been developed, which enables us to evaluate the current distribution of monolithically integrated light-emitting diode (MI-LED). Based on the experimental IR images, the current distributions have been evaluated for the MI-LEDs on two types’ heat sinks. For the MI-LED on poor thermal conductive chip-on-board heat sink, although there is a large junction temperature variation of 40 °C all over the chip, the current only varies from −16% to 11% due to the high parasitic series resistance and the topological design of MI-LED. The experiments show that the impact of the heat-crowding-induced uneven current distribution on the wall-plug efficiency is negligible at a power density of 1 W/mm2. The average junction temperature is still the dominant issue. With a high thermal conductive copper heat sink, the average junction temperature could be suppressed below 50 °C, and there is no obvious heat crowding and uneven current distribution observed.

Eye Opener to EtOH Ablation for Juxta-Cardiac Hepatocellular Carcinoma

Background: Hepatocellular carcinoma (HCC) is notoriously refractory to systemic chemotherapy, mandating an interventional approach. Mortality may be avoided by neutralizing rapidly growing tumors that approach the heart and major vessels. When the risk/benefit ratio of surgery is unacceptable, percutaneous ablation can achieve remarkable results. High volumes of flowing blood adjacent to the treatment area may impact the ability to reliably achieve an adequate ablation margin for modalities that rely on extreme temperatures to destroy malignant cells. Ethanol ablation is safe, efficacious, and unaffected by this “thermal sink” effect. This report describes a juxta-cardiac (JC) HCC in segment 4a measuring 35 × 26 mm, which exhibited rapid growth until it was abutting the pericardium and 7.5 mm from the chamber of the right ventricle (RV). Methods: One 21-gauge needle was inserted using direct CT fluoroscopy into the center of the hepatic mass. In order to confirm the position of the needle, 0.5 mL of diluted Visipaque was injected. Then, under CT fluoroscopy guidance, a mixture of 1 mL of Ethiodol and 10 mL of 98% dehydrated alcohol was slowly injected into the mass. Results: Repeat CT scan 1 month post-ablation demonstrated decreased arterial enhancement and dense Ethiodol throughout the tumor consistent with ablation. Tumor size decreased to 30 × 23 mm with a distance of 12.4 mm from the chamber of the RV. Conclusion: Pericardial involvement or large vessels near the treatment area may limit the use of thermal ablation techniques for JC HCC. Percutaneous, intratumoral ethanol injection provides safe and effective alternative that is not subject to the “thermal sink” effect.

Simulation and Performance Comparison of Si and SiC Based on a Proposed H6 Inverter for PV Grid-tied Applications

Multilevel transformerless inverters are widely employed in PV grid-tied applications because of their significant benefits in reducing switching losses and enhancing system efficiency. Wide bandgap (WBG) semiconductor devices such as SiC MOSFETs can increase the system performance and reduce the total losses due to their superior features over Si switching devices. This paper investigates a three level transformerless inverter based on H6 topology. The proposed topology is implemented with SiC MOSFETs and a modulation approach is adopted to reduce the number of conducting switches. Also, a detailed comparison of the use of SiC and Si switching devices in terms of switching and conduction losses, high switching frequency operation, filter size reduction, and thermal analysis and heat sink volume is presented. Simulation results show that total conduction and switching losses are reduced by about 50%, which allow a significant increase in either the switching frequency or the inverter power rating level for the same switching device losses. Additionally, operating at higher switching frequencies using SiC MOSFET leads to a significant reduction in volume and weight of the inductor filter. Furthermore, a thermal analysis is performed using COMSOL software to investigate the heat sink requirement.

Thermal Peak Management Using Organic Phase Change Materials for Latent Heat Storage in Electronic Applications

Modern high power electronics devices consists of a large amount of integrated circuits for switching and supply applications. Beside the benefits, the technology exhibits the problem of an ever increasing power density. Nowadays, heat sinks that are directly mounted on a device, are used to reduce the on-chip temperature and dissipate the thermal energy to the environment. This paper presents a concept of a composite coating for electronic components on printed circuit boards or electronic assemblies that is able to buffer a certain amount of thermal energy, dissipated from a device. The idea is to suppress temperature peaks in electronic components during load peaks or electronic shorts, which otherwise could damage or destroy the device, by using a phase change material to buffer the thermal energy. The phase change material coating could be directly applied on the chip package or the PCB using different mechanical retaining jigs.

A novel trigeneration system using geothermal heat source and liquefied natural gas cold energy recovery: Energy, exergy and exergoeconomic analysis

This paper deals with the energy, exergy, and exergoeconomic analysis of a novel trigeneration system working with geothermal heat source and liquefied natural gas (LNG) cold energy recovery as thermal sink. The proposed trigeneration system is constructed based on the absorption refrigeration cycle (ARC). Due to the high ammonia concentration at condenser outlet of ARC, a heat pump system is employed for heating production, while LNG cold energy is used for recovering the latent heat of the basic solution from the condenser as heat source as well as for producing power output. A comprehensive thermodynamic modeling of the proposed system is presented and the performance of the system is investigated based on the following performance criteria: net power output, cooling output, heating output, thermal efficiency, exergy efficiency and sum unit cost of the product (SUCP) of the system. In this respect, the simulation revealed that the net power output, cooling output, heating capacity, thermal efficiency, exergy efficiency and total SUCP of the system can be calculated 405.1 kW, 1109 kW, 35.3 kW, 85.92%, 18.52%, and 68.76 $/GJ, respectively, under the proposed constrain variables. In addition, the irreversibility of each component and overall system are presented showing that condenser accounts for the highest exergy destruction among all components which constitutes around 55.51% of the overall exergy destruction rate of the system. Moreover, a comprehensive parametric study is conducted to investigate the effects of some key parameters on the main performance criteria. It is observed that one can obtain a higher thermal efficiency at high evaporator temperature or at low condenser temperature, heating unit temperature and absorber temperature; while a higher exergy efficiency can be obtained at high evaporator temperature or at low absorber and condenser temperatures. In addition, a lower total cost of product of the system can be obtained at high heating unit and evaporator temperatures or low absorber and condenser temperatures. Moreover, it is found that the overall performance of system can be optimized based on the generator hot pinch point temperature difference as well as geothermal inlet temperature.

Heat and Mass Transfer of Unsteady Hydromagnetic Free Convection Flow Through Porous Medium Past a Vertical Plate with Uniform Surface Heat Flux

AbstractSimultaneous effects of thermal and concentration diffusions in unsteady magnetohydrodynamic free convection flow past a moving plate maintained at constant heat flux and embedded in a viscous fluid saturated porous medium is presented. The transport model employed includes the effects of thermal radiation, heat sink, Soret and chemical reaction. The fluid is considered as a gray absorbing-emitting but non-scattering medium and the Rosseland approximation in the energy equations is used to describe the radiative heat flux for optically thick fluid. The dimensionless coupled linear partial differential equations are solved by using Laplace transform technique. Numerical results for the velocity, temperature, concentration as well as the skin friction coefficient and the rates of heat and mass transfer are shown graphically for different values of physical parameters involved.

Energy, exergy and thermoeconomic analysis of a novel combined cooling and power system using low-temperature heat source and LNG cold energy recovery

This paper deals with the energy, exergy, and economic analysis of a novel ammonia-water combined cooling and power cycle using waste heat as low-temperature heat source and LNG cold energy as thermal sink. For this purpose, thermodynamic modeling of the proposed system is presented and the performance of the cycle is investigated based on the following performance criteria: net power output, cooling output, first-law efficiency, second-law efficiency, and sum unit cost of the products (SUCPs) of the system. In this respect, the simulation revealed that the net power output, cooling output, first-law efficiency, second-law efficiency, and total SUCP of the system can be calculated 1379kW, 1736kW, 43.25%, 22.51%, and 133.7 $/GJ, respectively, showing a considerable enhancement through this integration. In addition, the irreversibility of each component and overall system are presented showing that heat exchanger 2 accounts for the highest exergy destruction among all components which is followed by the throttling valve 1. Moreover, a comprehensive parametric study is conducted to investigate the effects of considered key parameters, namely, vapor generator pressure, LNG turbine inlet pressure, evaporator temperature, condenser temperature, heat source temperature, and ammonia concentration on the performance criteria. It is observed that one can obtain a higher first-law efficiency at higher ammonia concentrations, heat source temperatures and LNG turbine inlet pressures or at lower condenser temperatures and vapor generator pressures, while a higher second-law efficiency can be obtained at lower ammonia concentrations, heat source temperatures and condenser temperatures or at higher vapor generator pressures, evaporator temperatures as well as LNG turbine inlet pressures.

Experimental investigation into an ORC-based low-grade energy recovery system equipped with sliding-vane expander using hot oil from an air compressor as thermal source

Compressed air production is an energy–intensive sector, thus compressor manufacturers are constantly looking for enhancing the efficiency, by acting on several technological aspects. In an air compressor, about 80-90% of the input electric power used is wasted into the environment through the oil circuit, continuously cooled by ambient air blown via a fan. An interesting way to optimize the overall system efficiency is to exploit this waste heat to produce electrical power. Organic Rankine Cycles (ORCs) are a suitable solution for recovering energy from low-grade heat source. In this paper, an experimental analysis of two low-grade ORC-based recovery systems is presented. The thermal source is the hot lubricant of a mid-size air compressor, while the thermal sink is tap water. The first system is tested in a simple cycle configuration while the second in a recuperative one. An extensive experimental campaign is carried out on a test bench composed by sliding-vane expander, pump and plate heat exchangers. The expander differs in terms of geometry and aspect ratio between the two cycles. R236fa is used as working fluid in both the systems. The expander operating conditions are deeply investigated by using piezoelectric pressure transducers to determine the expansion indicated diagram and the expander mechanical efficiency. Experimental results show that the recuperative cycle has a better performance, in terms of cycle efficiency and expander mechanical efficiency, compared with the simple cycle. For this configuration, two off-design conditions are investigated, acting on the pump rotational speed. Finally, an exergy analysis is conducted, in order to evaluate the irreversible losses produced by each component.

Climate-potential of earth-to-air heat exchangers

Amid several natural cooling sources that refer to thermal sinks (water, air, ground and night sky), a possible system is represented by the horizontal earth-to-air heat exchangers (EAHX). This technology, even if less diffused if compared to other geothermal systems, has been demonstrated to be effective alone and coupled with mechanical air-handling systems to reduce the energy consumption for both cooling and heating.After a short review of existing EAHX simulation software, the paper will introduce a new simplified method to assess the potential of earth-to-air heat exchangers according to local soil composition and climate data.

A computational theoretical model for radiofrequency ablation of tumor with complex vascularization

Radiofrequency ablation (RFA) for liver tumors is a minimally invasive procedure that uses electrical energy and heat to destroy cancer cells. One of the critical factors that impedes its successful outcome is the thermal heat sink effects from complex vascular systems that give rise to incomplete destruction of the target tumor tissue, resulting in therapy failure. To better understand the thermal influence of the complex vascular system during RFA, this work proposes the employment of two 3D fractal tree-like branched networks to investigate which key factors of the tree-like vascular system impact heating process. A three-dimensional finite difference analysis is employed to simulate the RFA treatment. Based on the data acquired from the measured experiments, the simulated results derived from combining the Pennes bioheat model and the boundary condition-enforced immersed boundary method (IBM) have demonstrated close agreement with experimental data with a maximum discrepancy of ±8.3%. We employed the orthogonal design approach to analyze 3 factors, namely, the blood vessel's volume, the average distance between probe center and the blood vessel system and the number of the selected part's branches at three different levels. Results have revealed that the distance between RFA probe and blood vessel plays a major role during the heating process compared with the other two factors. In addition, both the ablating rates and the volume of damaged tissue are slightly reduced with increasing number of blood vessel branches.

Thermoeconomic analysis of a new combination of ammonia/water power generation cycle with GT-MHR cycle and LNG cryogenic exergy

This paper deals with a thermoeconomic analysis of a new combination of ammonia/water power generation cycle and Turbine-Modular Helium Reactor (GT-MHR) cycle and liquefied natural gas (LNG) cryogenic exergy. In the proposed power generation system, the GT-MHR waste heat is considered as heat source as well as cryogenic exergy of LNG as thermal sink. Parametric study is performed to investigate the effects of key parameters. Also, a comparative study is conducted for proposed system with the same combined cycle without LNG cryogenic exergy. The results show that minimum temperature difference in the evaporator, ΔTEV, has the greatest effect on the energy and exergy efficiencies as well as total cost rate in comparison to other key parameters. For the proposed system, an increase in ΔTEV from 20K to 30K, leads to a reduction of 2.8% and 3.4%in both energy and exergy efficiencies and total cost rate, respectively. The results also show that the temperature and pressure of ammonia/water solution exiting the evaporator have the negligible effects on energy and exergy efficiencies. Also, ammonia concentration of solution entering the separator, has the greatest effect on the product cost. Moreover, a multi-objective optimization is performed to obtain optimal values of design parameters.

Demonstration of a Pixel-Scale Superconducting Heat Switch for an Ideal Integrating Bolometer

The sensitivity of a bolometer is in principle limited by the phonon noise, which is a function of the conductance between its absorber and the thermal sink. However, practical issues limit the degree to which conductance can be minimized. For example, in a high-radiation environment, extremely low conductance, or, equivalently, long thermal recovery time, can lead to high data loss. To circumvent such limitations, we are developing a technique of modulating the conductance through the use of pixel-scale superconducting heat switches. We have fabricated bolometer-like test devices consisting of isolated regions connected to a thermal ground plane through narrow aluminum strips. Niobium coils are patterned around the aluminum strips. Flowing current through a coil allows the aluminum to be driven into the normal or high thermal conductance state. We have measured the conductance of these devices as a function of temperature and drive coil current.

Integration of Solar Thermal Systems into District Heating – DH System Simulation

This paper presents latest results and outcomes of a research project about the decentralized feed-in of solar thermal energy into district heating networks. While solar heat exemplarily stands for any other heat source (as waste heat) the focus is on smaller distributed solar thermal heating plants rather than large central systems. The idea is to use the district-heating system as thermal source and sink to ensure the supply of the local heat consumers as well as the transmission of solar heat to other customers. This approach is supposed to minimize investment costs and increase the solar share, thus substituting fossil fuels.Analyses and investigations were done based on computational DH system simulations. A special version of the TRNSYS-simulation program was applied to calculate transient operation of a 3rd generation DH network (following IEA-DHC Annex X classification, see [1]). Several feed-in substations have been integrated into the model. Heat consumption and load profiles of the consumers have been estimated using the Typical-Day Method. This approach allows to adapt any measured data (daily load profiles) to a specific weather data set and to scale heat load profiles of the same type according to a given peak load or daily heat consumption. The simulation offers the opportunity to have a look to any internal state of the system as local pressure, fluid temperature or mass flow. With this information component stress can be analyzed very well.The paper presents findings of the recently finnished project “Dezentrale Einspeisung in Nah- und Fernwärmesysteme unter besonderer Berücksichtigung der Solarthermie”11Founded by Federal Ministry for Economic Affairs and Energy FKZ 03ET1039B. and the ongoing research project “Kostenreduktionspotential beim Ausbau der Solarisierung von Fernwärmenetzen durch Standardisierung”22Founded by Federal Ministry for Economic Affairs and Energy FKZ 0325553A. It is based on the German publications [2] and [3].

Energy, exergy and economic assessments of a novel integrated biomass based multigeneration energy system with hydrogen production and LNG regasification cycle

In this work, a novel integrated biomass based multigeneration energy system is presented and investigated for power, cooling and hydrogen production. The proposed system consists of a combination of biomass integrated gasifier-gas turbine cycle, a Rankine cycle, a cascade organic Rankine cycle, an absorption refrigeration system and a PEM to produce hydrogen. This system uses cold energy of LNG as a thermal sink. Comprehensive thermodynamic and economic analyses as well as an optimization are performed. The effects of operating parameters on thermodynamic performance and total cost rate are investigated for overall system and subsystems. The results show that the fuel mass flow rate is the dominant factor affecting the variation of energy efficiency and total cost rate. An increase in fuel mass flow rate from 4 kg s−1 to 10 kg s−1 leads to a decrease of 8.5% and an increase of 122.8% overall energy efficiency and total cost rate, respectively. Also, the largest increase in exergy efficiency occurs when gas turbine inlet temperature increases. The results of optimization showed that the highest net power output, mass flow rate of natural gas delivered to city and the flue gas temperature discharged to the environment are obtained for the exergy efficiency optimal design.

Multi-variable driving thermal energy control model of dry hobbing machine tool

Dry hobbing is a new gear machining process with high efficiency and environmental friendliness, replacing traditional wet hobbing process. However, to get high reliable precision, it is a critical work for the dry hobbing system to reduce and control the thermal impact. Even new hobbing machine tool structure with high thermal stability such as double stand columns was designed to substitute the traditional structure; some new structures, such as air extraction and filtration device, air-cooling component, and magnetic chip conveyor, would affect the thermal energy accumulation of dry hobbing machine tool. In this paper, a multi-variable thermal energy control model was developed to describe the thermal energy accumulation characteristic of dry hobbing machine tool. Variables which would affect thermal energy generation and sink of dry hobbing machine tool are analyzed. A multi-objective optimization algorithm is proposed for variable optimization, combined with the multi-variable thermal energy control model. An application method is presented to show the thermal energy control procedure of dry hobbing machine tool. As a result, the temperature variation of interior space air, workpiece column, and anterior end cover is fluctuated in an acceptable range with optimization. Furthermore, thermal deformation errors, which range from −7 to 3 μm, could meet production requirements. It illustrates that the multi-variable driving thermal energy control model of dry hobbing machine tool is available.

Development of a Superconducting Heat Switch for an Ideal Integrating Bolometer

Improving the sensitivity of current state-of-the-art bolometers is essential for future cosmology endeavors. Having a bolometer that is thermally isolated from its thermal sink is ideal in order to reduce phonon noise: the factor limiting bolometer sensitivity. To thermally isolate a bolometer as much as possible a pixel-scale superconducting heat switch is being developed to control the conductance between a bolometer and its thermal sink. The heat switch is comprised of a superconducting thin aluminum film surrounded by a niobium coil. Current is passed through the coil and the resulting magnetic field drives the aluminum into its high conductance normal state. Experimental data agrees well with the results of a thermal model of the device. Â

Influence of non-linear thermal radiation on 2D unsteady flow of a Williamson fluid with heat source/sink

Abstract The present article reports a new mathematical formulation for a time dependent flow of a non-Newtonian Williamson fluid model by taking into account the impacts of infinite shear rate viscosity. By incorporating the constitutive relation of Williamson fluid model the basic conservation equations are obtained for two dimensional boundary layer flow. In addition, the heat transfer characteristics for flow filed over a stretching surface has been explored within the sight of thermal radiation and heat sink or source. The Rosseland approximation is being employed for non-linear thermal radiation impacts in the presence of convective heat transfer mode. The current work aims at revealing the solution of equations describing the flow of Williamson fluid by mean of employing the dimensionless approach. Therefore, the leading nonlinear momentum and energy equations are rendered into a set of simultaneously ordinary differential equations via non-dimensional variables with associated physical boundary conditions. Numerical treatment of these reduced conservation equations has been conducted by utilizing the Runge-Kutta Fehlberg integration scheme. We have examined the influence of various physical variables, like, the unsteadiness parameter, Weissenberg number, viscosity ratio parameter, Biot number, radiation parameter, temperature ratio parameter, Prandtl number and heat source/sink parameter on momentum and thermal boundary layers, which is illustrated by means of graphs and tables. The results suggest that the impact of larger viscosity ratio parameter lead to higher fluid velocity while the converse is true for the temperature field. It is noted that the greater unsteadiness parameter results in a significant enhancement in the friction factor. In addition, an increase in thermal radiation as well as temperature ratio parameters improves the heat transfer performance in fluid flow. The work of previous researchers is correlated with the findings of this paper in some special cases in the absence of Weissenberg number and viscosity ratio parameter for different values of unsteadiness parameter. A reasonably accurate predictions are achieved in this comparison.