Experimental investigation of direct contact baseplate cooling for electric vehicle power electronics

An experimental setup has been built to investigate the thermo-hydraulic performance of the direct contact baseplate cooling technique for power electronics in electric vehicles, to improve the design and to validate the modelling of this technique. The setup consists of an electrical heater to emulate the heat dissipation of the power electronics and which is cooled by a 60/40% mixture by mass of water-glycol. It is equipped with a flow rate sensor, absolute and differential pressure sensors and temperature measurements at the inlet, outlet and baseplate over the channel length, to determine the performance parameters used in the comparison: thermal resistance and pumping power. Three fluid inlet temperatures, four power levels and four flow rates have been tested for three channel heights (1.5mm, 3mm and 7.6mm). Increasing the fluid temperature and/or heating power, results in a lower thermal resistance and pumping power, due to a lower viscosity of the fluid. The performance of the 1.5mm and 7.6mm channel was found to be quite similar, while the 3mm channel results on average in a 5.8% lower thermal resistance compared to the other two channel heights. The heat transfer in terms of the Nusselt number was also evaluated in function of the Reynolds number. By analyzing the hydraulic and thermal entrance lengths it could be concluded that the flow in all measurements is simultaneously developing. A comparison with two correlations from scientific literature for simultaneously developing flow did not show a good agreement, possibly due to the specific inlet and outlet effect, which is more pronounced for a bigger channel height than a smaller channel height

Modelling and validation of a switched reluctance motor stator tooth with direct coil cooling

This paper presents the modelling and validation of an advanced thermal lumped parameter (LP) model for a stator tooth of a switched reluctance motor (SRM) with a dry lateral slot cooling method. Standard and simple lumped parameter models for electric motors can insufficiently predict the temperature distribution within the components of the motor. In standard LP models, only several nodes are used to model each component, while more accurate models are needed to predict the effect of different cooling methods on the thermal performance of the motor without the need for experiments. A fully 3D thermal finite element (FE) model could be used but this would increase effort, complexity and computing time unnecessarily. Therefore, an advanced 3D LP model including the dry lateral slot cooling method was developed and validated based on experiments on a real stator tooth cooled with the modelled cooling method. The 3D LP model is extracted from a 2D FE radial simulation of the stator tooth and extended axially in 3D to include axial heat transfer. Experiments were performed with a setup consisting of one tooth of a SRM without rotor, but including stator iron, one winding and two triangular stainless steel tubes in the slots at both sides of the winding cooled by a 60/40% mixture by mass of water-glycol. The setup is equipped with several thermocouples integrated within the components to determine the component temperatures. Three inlet temperatures (20, 35 and 50°C) and four flow rates (2, 6, 9 and 13 l/min) of the coolant were tested at three different heat losses in the winding (10, 30 and 50 W). A comparison between the simulated and measured temperatures showed generally higher temperatures in the experiment. The presence of imperfections in the manufacturing of the experimental setup was determined as the cause of this offset. These imperfections result in lower material thermal conductivities and higher contact resistances than expected from scientific literature. After fitting those thermal properties on the measurements, similar simulated temperatures could be obtained as in the experiments

Experimental study of a switched reluctance motor stator tooth with slot and end winding cooling

This paper presents an experimental study of direct coil cooling applied to a stator tooth of a switched reluctance motor where a direct contact is realized between the winding and fluid. Experiments were performed with a setup consisting of one tooth of a SRM without rotor, but including stator iron and one preformed winding. Three configurations of the cooling method were investigated: slot cooling, end winding cooling and a combination of the two by pumping an Automatic Transmission Fluid (ATF) over the designated sides of the winding. The setup is equipped with 17 thermocouples integrated within the components to determine the temperatures. Three inlet temperatures (21, 33 and 44°C) and four flow rates (1.5, 2, 3.5 and 5 l/min) of the coolant were tested at four different heat losses in the winding (10, 30, 50 and 70W). The results show that the maximum temperature is always located in the centre of the winding and is the lowest for the combined cooling (73.0°C), followed by the slot cooling (79.7°C) and then by the end winding cooling (91.6°C) for the lowest ATF inlet temperature and the highest heat losses and flow rate. With a determined current density in the range 13.8A/mm² to 19.5 A/mm², all three direct coil cooling methods show a great potential in increasing the power density of electric motors

Advanced lumped parameter model for switched reluctance motors with high performance cooling

In this paper an advanced thermal lumped parameter model for a switched reluctance electric motor (SRM) is constructed, based on a 2D thermal finite element simulation of a radial cross section of the motor. When applying and combining advanced cooling methods such as direct coil cooling, end winding cooling (radial stretched) and spray cooling on an SRM, the conventional lumped parameter models can no longer be used due to the 3D and complex temperature gradients in the motor. In standard LP models, mostly one simple cooling method is implemented by which the thermal gradients are also quite simple (1D or 2D). When combining different cooling methods, the gradients become highly 3D and these LPM are no longer valid. To improve the accuracy of this problem, a fully 3D thermal finite element simulation could be performed, but this would unnecessarily increase effort, complexity and computational time. To avoid this an advanced lumped parameter model is constructed in this paper, such that the high thermal gradients are modeled in more detail. The results from one 2D finite element simulation of a radial cross section of half of a stator tooth are reduced to a simpler lumped parameter model with more nodes in the most crucial parts, i.e., where the highest thermal gradients are expected. The 2D thermal model is then expanded to a 3D lumped parameter model, including the gradients in axial direction. Using this model, various cooling configurations and geometry parameters can be varied easily such that the design of an SRM with advanced cooling can be optimized efficiently

Forschungskompetenz als Lehrziel: Langzeitevaluation eines Blended-Learning-Moduls

Combustion instabilities can cause serious problems which limit the operating envelope of low-emission lean premixed combustion systems. Predicting the onset of combustion instability requires a description of the unsteady heat release driving the instability, i.e., the heat release response transfer function of the system. This study focuses on the analysis of fully coupled two-way interactions between a disturbance field and a laminar premixed flame that incorporates gas expansion effects by solving the conservation equations of a compressible fluid. Results of the minimum and maximum flame front deflections are presented to underline the impact of the hydrodynamic instability on the flame and the shear layer effect on the initial flame front wrinkling which is increased at decreasing gas expansion. These phenomena influence the magnitude of the burning area and burning area rate response of the flame at lower frequency excitation more drastically than reduced-order model (ROM) predictions even for low temperature ratios. It is shown that the general trend of the flame response magnitudes can be well captured at higher frequency excitation, where stretch effects are dominant. The phase response is influenced by the DL mechanism, which cannot be captured by the ROM, and by the resulting discrepancy in the flame pocket formation and annihilation process at the flame tip. (C) 2014 The Combustion Institute. Published by Elsevier Inc. All rights reserved

Experimental comparison of two-phase and single-phase cooling methods of a power module

Abstract: Power modules of an electric drivetrain generate some of the highest heat fluxes in the system and are therefore challenging to cool. State-of-the-art cooling methods use single-phase water-glycol as coolant. Two-phase cooling with refrigerants can be a way to improve the heat transfer rate as high heat transfer coefficients are expected during boiling. A downside of two-phase cooling is the possibility of dryout when the heat flux goes above the critical heat flux, which is therefore important to be able to determine. Traditional refrigerants used in two-phase cooling (e.g. hydrofluorocarbons and perfluorocarbons) are being replaced by newer environmentally friendly refrigerants (e.g. hydrofluoroethers and fluoroketons). One such fluid is studied experimentally in this work, FK-649. Both heat transfer rates and critical heat flux are determined for varying refrigerant saturation temperature. The experimentally achievable heat flux of the two-phase cooled power module is compared to that of alternative methods (cold plate, direct contact and pin fin water-glycol cooling) on the same power module. It is also compared to experimental data of two-phase cooling with other refrigerants from literature. The results show that boiling with FK-649 is comparable in performance to cold plate cooling, but performs worse than direct contact baseplate and pin fin water-glycol cooling. Also, boiling heat transfer with R134a and HFE-7100 is superior to that of FK-649. Using correlations for boiling heat transfer and critical heat flux, other new refrigerants are assessed. This analysis shows that other refrigerants have more potential than FK-649 but these refrigerants require higher operating pressures