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With the growing interest on electric vehicles comes the question of the thermal management of their battery pack. In this work, we propose a thermally efficient solution consisting in inserting between the cells a liquid cooling system based on constructal canopy-to-canopy architectures. In such systems, the cooling fluid is driven from a trunk channel to perpendicular branches that make the tree canopy. An opposite tree collects the liquid in such a way that the two trees match canopy-to-canopy. The configuration of the cooling solution is predicted following the constructal methodology, leading to the choice of the hydraulic diameter ratios. We show that such configurations allow extracting most of the non-uniformly generated heat by the battery cell during the discharging phase, while using a small mass flow rate. (c) 2021 Elsevier Ltd. All rights reserved.

The effects of global warming on the climate becomes more evident each year. Approximately 20% of the emitted CO2 is due to transportation. Therefore, many countries took action to minimize their effect on the environment by accelerating transition to electric vehicles via tax deduction and bans of internal combustion engines. Most of the components in electric vehicles such as electric motor, inverter and control system are well known and satisfy the requirements for advanced vehicles. However, battery technology limits the advancement. Literature focus on the battery chemistry to maximize energy and power density. Publications show that battery life and performance can be enhanced if battery cell temperature is kept at the desired operation temperature which is around 30oC. In addition, if the temperature exceeds a limit temperature short circuit is triggered due to melted separator, Lithium dendrite growth and/or decreased electric resistance in the cell. Thermal management of battery cells is essential due to these reasons. Furthermore, heat generation rate in cells is not easily predictable and constant as in electronic components. Depending on the charging (or discharging) rate and state of charge (or depth of discharge) heat generation rate varies and it is non-homogeneous. Furthermore, battery cells required to be heated in some ambient conditions. There are many distinct strategies for thermal management of electric vehicle battery packs such as air cooling, liquid cooling, phase changing materials (PCMs) and immersion in dielectric liquid bath in the literature. This paper compares the distinct strategies for various use cases and documents their advantages as well as disadvantages. In addition to the removal of the generated heat, required weight and space allocations also considered as these are important parameters is commercial electric vehicles in the market. The results show that there is no strategy perfect for all the use cases, and that the strategy should be decided based on battery chemistry, ambient conditions, operation conditions and constraint related with the vehicle design such as weight and volume.

In this paper, we show how the vascular channel configuration and its shape affect the mechanical strength which is simultaneously subjected to heating and mechanical load. The effect of channel cross-section on the coolant mass flow rate, peak temperature and peak stresses are documented. The material properties were defined as functions of temperature in simulations. The results show that the flow of stresses and fluid is minimum with the circular channels and the resistance to the heat flow is the smallest with semi-circular channels. In addition, morphing the vascular design provides almost the smallest resistance to the heat flow with circular channels (0.3% difference in the peak temperature). This shows that even the convective resistances are the smallest with circular-cross section, overall thermal resistance is smaller in semi-circular design for the fixed fluid volume. The peak stress is smaller with semi-circular channels than the circular ones if the pressure drop is less than 500 Pa for the radial design. However, the peak stress is smaller with hybrid design than the semi-circular radial designs for the entire pressure drop range. In addition, the effects of mechanical load, heating rate and reference temperature on the stress distribution are also documented. Furthermore, the thermal and mechanical stresses are also documented separately, and then compared with the coupled solution cases. The chief result of this paper is that for a coupled system minimizing only one of the resistance terms is not sufficient, all the resistances considered simultaneously in order to uncover the best performing design. In addition, the results show that the designs should be free to vary, the unexpected designs can be the best performing designs for the given parameters and constraints. Therefore, the design parameters based on the experience does not always yield the best performing designs as the objectives and constraints vary. This result is in accord with the constructal law.

Cetkin, Erdal/0000-0003-3686-0208, This paper shows that a conductive domain which is subjected to heating from its bottom can be cooled with embedded microvascular cooling channels in it. The volume of the domain and the coolant are fixed. The actively cooled domain is mimicked from the human skin (which regulates temperature with microvascular blood vessels). The effect of the shape of cooling channels (sinusoidal or straight) and their locations in the direction perpendicular to the bottom surface on the peak and average temperatures are studied. In addition, the effect of pressure difference in between the inlet and outlet is varied. The pressure drop in the sinusoidal channel configurations is greater than the straight channel configurations for a fixed cooling channel volume. The peak and average temperatures are the smallest with straight cooling channels located at y = 0.7 mm. Furthermore, how the cooling channel configuration should change when the heat is generated throughout the volume is studied. The peak and average temperatures are smaller with straight channels than the sinusoidal ones when the pressure drop is less than 420 Pa, and they become smaller with sinusoidal channel configurations when the pressure drop is greater than 420 Pa. In addition, the peak and average temperatures are the smallest with sinusoidal channels for a fixed flow rate. Furthermore, the peak temperatures for multiple cooling channels is documented, and the multiple channel configurations promise to the smallest peak temperature for a fixed pressure drop value. This paper uncovers that there is no optimal cooling channel design for any condition, but there is one for specific objectives and conditions., Scientific and Technological Research Council of Turkey (TUBITAK)Turkiye Bilimsel ve Teknolojik Arastirma Kurumu (TUBITAK) [114M592], This work was supported by The Scientific and Technological Research Council of Turkey (TUBITAK) under project number 114M592.

Cetkin, Erdal/0000-0003-3686-0208, This paper shows that the peak temperature of a non-uniformly heated region can be decreased by embedding high-conductivity tree-shaped inserts which is in contact with a heat sink from its stem. The volume fraction of the high-conductivity material is fixed, and so is the volume of the solid region. The length scale of the solid domain is L. Inside there is a cube-shaped region with length scale of 0.1L and heat production 100 times greater than the rest of the domain. The location of this hot spot was varied to uncover how its location affects the peak temperature and the design of inverted fins, i.e. highconductivity tree-shaped inserts. The volume fraction of the high-conductivity tree was varied for number of bifurcation levels of 0, 1 and 2. This showed that increasing the number of the bifurcation levels decreases the peak temperature when the volume fraction decreases. The optimal diameter ratios and optimal bifurcation angles at the each junction level are also documented. Y-shaped trees promise smaller peak temperatures than T-shaped trees. The location of the vascular tree in the z direction also affects the peak temperature when the heat generation is non-uniform. In addition, the peak temperature is minimum when z = 0.65L even though the hot spot is located on z = 0.75L., Republic of Turkey, This work was supported by the Republic of Turkey. Almogbel and Bejan's [19] data was used in this paper for the validation process. The author wishes to thank him for his contribution to the subject of conductive cooling.