Reduction Of Hot Spot Temperature Research Articles

Enhancing Heat Transfer Efficiency in Permanent Magnet Machines through Innovative Thermal Design of Stator Windings

This paper investigates innovative methods for enhancing heat transfer efficiency in high-power permanent magnet electrical machines. The objectives are to quantify the effects of increasing the air speed, increasing the turbulence intensity, and introducing the spacing between windings on cooling performance. The cooling of stator windings is studied through experimental wind tunnel testing and Computational Fluid Dynamics (CFD) modelling. The CFD model is validated against wind tunnel measurements to within 4 Kelvin (K). The results demonstrate that each enhancement method significantly improves the cooling capability. Increasing the air speed from 10 m/s to 40 m/s reduces the winding hotspot temperature by 34%. Introducing a high turbulence intensity of 40% leads to a 21% lower hotspot temperature compared to 0.5% turbulence intensity. Creating a 1.5 mm spacing between coils also substantially improves convection and conduction heat transfer. Overall, combining these optimised design parameters yields over a 40% reduction in hotspot temperature compared to the original design. This research provides practical guidance for maximising heat transfer efficiency in high-power permanent magnet machines, without increasing complexity. The findings will lead to higher machine efficiency, reliability, and longevity for aerospace and other applications.

Modeling the impact of high thermal conductivity paper on the performance and life of power transformers

Degradation of insulation paper is a key contributor to the failure of power transformers. Insulation degradation accelerates at elevated temperatures, which highlights the potential for better thermal management to prolong life. While several studies have analyzed the benefits of high thermal conductivity oil for reducing temperatures inside a transformer, this study is an initial assessment of the benefits of high thermal conductivity paper on transformer life. Blending particulates with cellulosic fibers offers a pathway for high thermal conductivity paper (with good dielectric properties), which can reduce internal temperatures. Presently, life extensions that can be achieved by the use of such thermally conducting papers were estimated, with the thermal conductivity of the paper being the key parameter under study. The analytical-numerical thermal model used in this study was validated against experimental measurements in a distribution transformer, adding confidence to the utility of the model. This model was then used to provide estimates of hot-spot temperature reduction resulting from the use of papers with higher thermal conductivity than baseline. Transformer life was predicted conventionally by tracking the degree of polymerization of paper over time, based on an Arrhenius model. Results indicate that increasing the thermal conductivity of paper from 0.2 W/mK (baseline) to 1 W/mK reduces the hot spot temperature by 10 °C. While degradation significantly depends on the moisture and oxygen content, the model shows that such a temperature reduction can increase life for all conditions, by as much as a factor of three.

Effect of Current Sharing by Metal Core on Reducing Hot-Spot Temperature of Spiral Coated Conductors

We experimentally studied the quench characteristics of a coated conductor wound spirally on a round metal core, which is similar to CORC <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">®</sup> cables. We used the simplest sample configuration, in which one coated conductor was wound sparsely in one layer and which enabled us to attach the voltage taps to the core as well as the coated conductor. We measured the voltages along the coated conductor, those along the core, and those between the coated conductor and the core. The hot-spot temperature and the current shared by the core were calculated by using the measured voltages. The impact of the current sharing on the reduction of hot-spot temperature is discussed.

Precise thermo-fluid dynamics analysis of corrugated wall distribution transformer cooled with mineral oil-based nanofluids: Experimental verification

Corrugated wall distribution transformers (CWDTs) are one of the momentous and expensive equipment for power systems, and their failure has a negative impact on the security of the grid. The continuous operation of the transformer strongly depends on its insulation status and then on the hotspot temperature (HST). In this paper, a comprehensive and precise 3D computational fluid dynamic (CFD) based modeling is proposed for HST prediction. In this modeling, the conservator, which has a remarkable impact on the HST, is thoroughly modeled. Optical fiber sensors (OFSs) in the studied 500 kVA CWDT are employed to validate the precision of the proposed HST prediction method via a 3D CFD-based modeling during the temperature rise test (TRT). The experimental results indicate that the proposed 3D CFD-based model for HST prediction is very precise and there is suitable proximity with the practical values. The error percentage of the proposed 3D CFD-based model is 0.76 % (0.7 °C) compared to the OFSs results, which indicates the accuracy and efficiency of the proposed modeling. Also, a thermal camera is employed to verify the results of 3D CFD-based modeling in top-oil temperature (TOT), bottom-oil temperature (BOT) and conservator oil temperature (COT) during the TRT. According to the obtained results, temperatures of 3D CFD-based simulation and thermal camera in the aforementioned three points are in good agreement with each other. In this case, the error percentage is less than 2.7 %, which indicates the precision and proficiency of the proposed 3D modeling. Finally, the utilization effect of multi-walled carbon nanotubes (MWCNTs) and diamond nanoparticles on CWDT' HST reduction and loading capacity increment (LCI) has been studied and verified via the proposed 3D CFD-based modeling. The results show that the lowest effect is for MWCNT with 0.005 % concentration, where HST reduction and LCI are 3.3 °C and 5.6 %, respectively, and the highest effect is for diamond nanoparticle with 0.01 % concentration, where HST reduction and LCI are 5.1 °C and 7.8 %, respectively.

Numerical analysis of forced convection heat transfer in a nuclear fuel storage vault

Forced convection heat transfer from fuel and blanket subassemblies to air in a nuclear fuel storage vault has been investigated in this work. Three dimensional simulations were performed for an actual fuel storage vault to improve the decay heat removal, air exchange rate, and thermal effectiveness. Forced airflow was used to remove the decay heat and an efficient ventilation arrangement was rigorously studied for long˗term storage of the nuclear fuel. The height of the interconnecting ducts between the blanket and fuel storage enclosures was systematically investigated to understand the ventilation performance. For this purpose, different temperature and air distributions inside the vault were analyzed. The mixing of air was enhanced, and the thermal stratification was reduced by connecting the ducts above the heat generating region. However, the fuel magazine temperature was reduced when the ducts were connected below the heat generating region. Furthermore, the effect of different shapes of interconnecting ducts on heat removal and air exchange rate through the vault was investigated. It was found that incorporating nozzles in the interconnecting ducts has led to an enhancement in air jet penetration and consequently a reduction in hot spot temperature. The validated computational model will be helpful in designing different aspects of nuclear fuel storage systems.

Machine Learning-Based Predictions of Benefits of High Thermal Conductivity Encapsulation Materials for Power Electronics Packaging

Abstract Machine learning (ML)-based predictive techniques are used in conjunction with a game-theoretic approach to predict the thermal behavior of a power electronics package, and study the relative influence of encapsulation material properties and thermal management techniques in influencing hotspot temperatures. Parametric steady-state and transient thermal simulations are conducted for a commercially available 1.2 kV/444 A silicon carbide (SiC) half-bridge module. An extensive databank of 2592 (steady-state) and 1200 (transient) data points generated via numerical simulations is used to train and evaluate the performance of three ML algorithms (random forest, support vector regression, and neural network) in modeling the thermal behavior. The parameter space includes the thermal conductivities of the encapsulant, baseplate, heat sink, and cooling conditions deployed at the sink and covers a variety of materials and cooling scenarios. Excellent prediction accuracies with R2 values &amp;gt; 99.5% are obtained for the algorithms. Shapley additive explanations (SHAP) dependence plots are used to quantify the relative impact of device and heat sink parameters on junction temperatures. We observe that while heatsink cooling conditions significantly influence the steady-state junction temperature, their contribution in determining the junction temperature in dynamic mode is diminished. Using ML-SHAP models, we quantify the impact of emerging polymeric nanocomposites (with high conductivities and diffusivities) on hotspot temperature reduction, with the device operating in steady-state and transient modes. Overall, this study highlights the attractiveness of ML-based approaches for thermal design, and provides a framework for setting targets for future encapsulation materials.

Thermal Behavior of Power Transformers Filled With Waste Vegetable Oil-Based Biodiesel Under Dynamic Load

Abstract Petroleum-based oils are widely used as electrically insulating materials in high voltage power transformers for dissipating high generated heat flux and maintaining the temperature below critical values. The operating temperature of a transformer dominantly governs its aging rate. In the present research, a renewable coolant as a versatile substitution for the petroleum-based oils was investigated to be employed in the cooling of transformers. The studied coolant is a vegetable-based oil extracted from the waste cooking oils. A numerical model was developed to follow the instantaneous changes in the load profile and ambient temperature and predict the instantaneous hotspot temperature values in the transformer under dynamic load. Then, this thermal model was used to explore the capability of the studied vegetable oil in the cooling of transformers compared with conventional transformer oil. The realistic ambient temperature and loading profile, as well as thermal properties of oils and characteristics of a transformer, were applied as the model’s inputs. The aging rate of the transformer in the presence of vegetable oil was also compared with the conventional transformer oil. The results indicate a better cooling performance for the vegetable-based oil, where a hotspot temperature reduction of 3 °C was observed in comparison to the petroleum-based oil. Also, the model predicts a significantly longer life for the insulating system of the transformer when the proposed vegetable-based oil is employed. The results of this research suggest a sustainable way of reusing the waste of a renewable resource as an alternative insulating liquid for the cooling of high heat flux electric/electronic devices.

Phonon-assisted reduction of hot spot temperature in AlInN ternaries

A longstanding challenge is the reduction of temperature in hot spots occurring in AlN-based ternary device structures. In this paper, we develop a uniaxial dielectric model and present theoretical analysis of the Frohlich interaction between electrons and interface and confined optical phonons, and their properties. This model is used to introduce new ways of achieving temperature reduction in hot spots in regions of elevated temperature in AlN- and InN-based electronic and optoelectronic devices.

Heat transfer enhancement of microchannel heat sink with longitudinal vortex generators and bypass jet flow

Aiming to enhance the heat transfer characteristics of microchannel heat sink, the present study firstly proposes a method to construct 45° ribs on lower surface of the channel wall. The ribs could induce longitudinal vortices, and cause drastic mixing of the coolant fluid. Furthermore, a bypass channel with an opening located at downstream region is set up on upper walls of the channels. The bypass jet flow would augment furtherly the mixing of coolant fluid. The effects of rib number (N), rib height (Rh) and bypass channel configuration on flow structure, thermal field, pressure drop and hot spot temperature of the heat sink are investigated detailed. Results show that the reduction in hot spot temperature of the heat sink is larger for a higher Rh. For a given Rh, there is an optimum N for minimizing hot spot temperature. The influences of bypass channel height on heat transfer characteristics are more significant than the opening width and location. The maximum reduction in hot spot temperature is 30.81% for ribs constructed on channel wall. While it can be up to 49.97% when the ribs and a bypass channel are constructed simultaneously.

Reduction of Hot-Spot Temperature of REBCO Coil by Use of Stabilizing Cu Strips on REBCO-Coated Wire

The most probable cause of quench damage of a rare earth barium copper oxide (REBCO)-coated wire is localized overheating at a hot-spot in the wire of the REBCO coil during the quench protection sequence. An effective method for reducing the hot-spot temperature is to increase the thickness of the Cu layer of the coated wire. However, it is difficult and costly to obtain a thick Cu layer with a uniform thickness and a flat surface by plating. In this paper, the effectiveness of forming a coil by placing bare stabilizing Cu strips on the bare surface of the Cu layer of a REBCO wire as an alternative method to the plating of a thick Cu layer is investigated. Experiments show that the use of stabilizing Cu strips provides quench protection performance that is equivalent to that obtained by increasing the Cu layer thickness of the REBCO wire.

Improved Thermal Management and Analysis for Stator End-Windings of Electrical Machines

In an electrical machine design, thermal management plays a key role in improving the performance and reducing size. End-windings are commonly identified as the machine hot-spot. Hence, lowering and predicting end-windings temperature are crucial tasks in thermal management of electrical machines. This paper proposes and investigates a noninvasive but effective cooling method that aims for a uniform cooling of a machine's winding by implementing direct cooling on its end-windings. Modeling and experimental results show that a 25% hot-spot temperature reduction on a particular application can be achieved. To analyze the proposed technique in detail, an accurate but computationally economic lumped parameter thermal network is developed. Comparison between a “standard” thermal network and its simplified equivalent (with less nodes) is presented where the models are developed and fine-tuned based on experimental data. All the above is used to investigate the potential of the proposed end-winding cooling method with different configurations of the methodology.

Improvement of partial shading resilience of PV array though modified bypass arrangement

Generation of hot spot, due to partial shading and other mismatch conditions, is associated with photovoltaic (PV) systems since its very early applications in satellites, but a simple, economic and effective solution is still unavailable. Conventional hot spot mitigation technique, using a bypass diode (BPD) across each sub-panel, reduces the reverse bias voltage only up to −12 V to −19 V across the shaded cell, which matches exactly with the voltage range at which avalanche breakdown of PV cells occur. Reverse breakdown of an acidic texturized PV cell occurs merely at −13 V and between −15 V and −20 V for alkaline texturized ones. Hence, the standard BPD based circuit is not effective enough in preventing reverse breakdown of PV cells and hot spot generation. In this paper, a modified bypass circuit is proposed which successfully strikes a balance between the increase in reliability through reduction of hot spot temperature and increase in complexity of bypass circuit. The experimental study and simulation reveal that the proposed bypass circuit effectively reduced hotspot temperature of the obscure cell below average temperature of the module, without increasing the power loss, system complexity and cost thereof, substantially.

Characteristics and enhancement of heat transfer from heat-generating blocks mounted on back wall of a 3D cabinet to an ambient natural convective air stream

In this study, the physical system under consideration is a three-dimensional (3D) cabinet with arrays of heat-generating blocks mounted on inner surface of back wall of the cabinet. The heat-generating blocks dissipate heat to the surrounding of cabinet through conjugate conduction and natural convection. Efforts are performed to investigate the cooling performance enhancement of the blocks by constructing circular air vents in walls and installing round fins onto back wall of the cabinet. Results show that the air vents and fins can significantly reduce the hot spot temperature of heat-generating blocks, and improve temperature uniformity of the blocks. The maximum reduction in hot spot temperature can be up to 26.2% for cases under investigation in this study.

Thermal management of a distribution transformer: An optimization study of the cooling system using CFD and response surface methodology

In this paper, a numerical scheme has been developed to examine the effective parameters on thermal management of distribution transformers and subsequently to optimize their cooling systems. In this regard, the response surface methodology (RSM) was used as the optimization method to minimize the hotspot temperature in the transformer as the response factor. A comprehensive three-dimensional computational scheme was employed considering the detailed geometrical specifications of an actual 200kVA distribution transformer to obtain the temperature field and the hotspot temperature. The accuracy of the numerical model was established via comparing the numerical results with the measured temperatures of a running transformer. The thermal variations of the thermo-physical properties of the transformer oil are determined experimentally and incorporated in the numerical modeling. A comprehensive parametric study among seven evidently effective parameters has been performed to identify the most effective parameters on the thermal performance of the transformer. It was found that fin height, length, and spacing are the more influential parameters among the examined parameters, which are also considered as the input variables in the optimization procedure. According to the RSM, the effects of the variations of these input variables in pre-specified ranges on the response, which is the hotspot temperature, are examined through the suggested runs by RSM. The results indicated that the hotspot temperature is more influenced by the fin height as compared to the fin length and spacing. Furthermore, the hotspot temperature decreases with the increase in fin height and length, while decreases as fin spacing increases. In addition, a correlation for the variations of the hotspot temperature as a function of the fin height (H), length (L), and spacing (S) is suggested using RSM. The significant finding is that the proposed optimum transformer configuration (H = 0.9 m, L = 0.08 m and S = 0.036 m) leads to the hotspot temperature reduction of about 16 °C as compared to the actual transformer geometry, which greatly affects the transformer life expectancy and safer performance.

Large Reduction of Hot Spot Temperature in Graphene Electronic Devices with Heat-Spreading Hexagonal Boron Nitride.

Scanning thermal microscopy measurements reveal a significant thermal benefit of including a high thermal conductivity hexagonal boron nitride (h-BN) heat-spreading layer between graphene and either a SiO2/Si substrate or a 100 μm thick Corning flexible Willow glass (WG) substrate. At the same power density, an 80 nm thick h-BN layer on the silicon substrate can yield a factor of 2.2 reduction of the hot spot temperature, whereas a 35 nm thick h-BN layer on the WG substrate is sufficient to obtain a factor of 4.1 reduction. The larger effect of the h-BN heat spreader on WG than on SiO2/Si is attributed to a smaller effective heat transfer coefficient per unit area for three-dimensional heat conduction into the thick, low-thermal conductivity WG substrate than for one-dimensional heat conduction through the thin oxide layer on silicon. Consequently, the h-BN lateral heat-spreading length is much larger on WG than on SiO2/Si, resulting in a larger degree of temperature reduction.

Performance enhancement of the concentrated photovoltaic using different phase change material configurations

Concentrator photovoltaic (CPV) systems are used to generate electrical power with a relatively lower cost per KWh. Because of high solar concentration, the cell temperature increases rabidly, and accordingly an efficient cooling method is needed. Therefore, the use of intensive cooling with PCM can reduce the cell temperature and enhance the system performance. The disadvantage of using this technique is that a hot spot appears at the top of CPV system due to the high-temperature gradient. Therefore, the main objective of the current work is to overcome such problem by first, adding a bulk of PCM at the top of the regular configuration and second changing the PCM tank configuration to a parallelogram shape. The performance of the CPV system with the developed configuration of PCM is investigated numerically. Results indicate that a significant reduction of hot spot temperature of about 9 oC is attained using the new configurations.

A modified bypass circuit for improved hot spot reliability of solar panels subject to partial shading

A new bypass strategy for monocrystalline and polycrystalline solar panels allowing significant hot spot temperature reduction in both partial and full shading conditions is presented. The approach relies on a series-connected power MOSFET that subtracts part of the reverse voltage from the shaded solar cell, thereby acting as a voltage divider. Differently from other active bypass circuits, the proposed solution does not require either a control logic or power supply and can be easily substituted to the standard bypass diode. The operation of the new circuit is described with reference to the shading condition prescribed by the EN 61215 qualification procedure. Experiments performed on two commercial solar panels have shown that the shaded cell can be cooled up to 24°C with respect to the case in which the standard bypass diode is adopted.

Thermal characterization of carbon nanotube foam using MEMS microhotplates and thermographic analysis

Thermal material properties play a fundamental role in the thermal management of microelectronic systems. The porous nature of carbon nanotube (CNT) arrays results in a very high surface area to volume ratio, which makes the material attractive for surface driven heat transfer mechanisms. Here, we report on the heat transfer performance of lithographically defined micropins made of carbon nanotube (CNT) nanofoam, directly grown on microhotplates (MHPs). The MHP is used as an in situ characterization platform with controllable hot-spot and integrated temperature sensor. Under natural convection, and equivalent power supplied, we measured a significant reduction in hot-spot temperature when augmenting the MHP surface with CNT micropins. In particular, a strong enhancement of convective and radiative heat transfer towards the surrounding environment is recorded, due to the high aspect ratio and the foam-like morphology of the patterned CNTs. By combining electrical characterizations with high-resolution thermographic microscopy analysis, we quantified the heat losses induced by the integrated CNT nanofoams and we found a unique temperature dependency of the equivalent convective heat transfer coefficient, Hc. The obtained results with the proposed non-destructive characterization method demonstrate that significant improvements can be achieved in microelectronic thermal management and hierarchical structured porous material characterization.

Pulsed Thermoelectric Cooling for Improved Suppression of a Germanium Hotspot

As hotspots become an increasingly important factor in the design of electronic devices, it has become essential to develop novel near-junction cooling methods. Steady-state thermoelectric cooling has previously been considered for the removal of localized hotspots on various substrates. In this paper, the transient behavior of a germanium thermoelectric self-cooler, in which the chip substrate is used as a leg of the thermoelectric circuit, is described. A 3-D thermoelectric numerical model was created in the commercial FEA package ANSYS and is used to explore the effects of various initial conditions, current pulse durations, current pulse magnitudes, pulse shapes, and die thicknesses. The results suggest that pulsed transient thermoelectric cooling has the potential to improve hotspot temperature reduction by approximately 30% relative to what is achievable in steady state. In addition, it was found that larger currents generally cause more rapid thermoelectric cooling, but also result in large overshoot temperatures and that the applied current profile has a strong effect on the transient behavior of the cooler.

Intensification of highly exothermic fast reaction by multi-injection microstructured reactor

Microstructured reactors (MSR) with characteristic dimensions below 100μm are warranted to maintain close to isothermal conditions when carrying out quasi-instantaneous highly exothermic reactions. Unfortunately, such small dimensions increase the risk of clogging, create high pressure drop and are costly to number-up. The multi-injection (MI) MSR, where one of the reactants is added stepwise along the reactor length, allows working with larger dimensions (diameter >500μm) while maintaining good temperature control. Herein presented MI-MSR is made of low temperature co-fired ceramics (LTCC) with herringbone mixing structure inside the reactor channels and is shown to mix efficiently in a large range of Reynolds numbers Re=20–130. The cyclization of pseudoionone is studied as a model of a highly exothermic fast reaction. The temperature profiles are characterized by a quantitative infrared thermography. The developed LTCC MI-MSR allows ∼ 8-fold reduction of hot spot temperature as compared to the adiabatic temperature rise. Moreover, ∼500-fold intensification is achieved as compared to the conventional semi-batch process with reduced solvent mass by a factor of 2 while attaining a yield of target product above 98%.