1. Introduction
The performance of an electric vehicle (EV) depends strongly on its battery pack performance. Due to its better characteristics in terms of capacity, energy densities, charge retention, life cycles, and competitive cost when compared with other rechargeable batteries, lithium-ion (Li-ion) battery technology has been considered to be one of the most promising long-term advanced battery technologies and hence is widely used in modern EVs [
1]. However, the battery pack is encountering more and more thermal problems with the demands of long-drive and huge-power EVs. Presently, researchers have come to a consensus that the best range of work temperature for a Li-ion battery is 10–40 °C, and once its temperature is above 50 °C, battery efficiency and life will deteriorate quickly. Especially when a battery module works at 80–119 °C but without in-time heat dissipation, solid electrolyte interfaces will dissolve, which can lead to batter over-charging and internal short circuiting, or even pose a safety hazard [
2,
3].
Mathematical simulation of heat transfer within large batteries is an effective tool to obtain knowledge about whether excessive heat generated during the battery charge or discharge process can be removed, and how operating temperature can be controlled. Chen et al. [
4] built a two-dimensional model to study the effect of various cell components, stack size, and cooling conditions on the performance of Li-polymer electrolyte batteries under different discharge C-rates (C-rate means charge and discharge current with respect to its nominal capacity). Karimi [
5] particularly analyzed the performance of four main types of thermal management systems (TMS): free convection, forced convection, liquid cooling, and phase-change-material cooling. Panchal et al. [
6,
7] developed a numerical model to simulate a mini-channel cold plate and found that increased discharge rates and increased operating temperatures resulted in increased temperatures at cold plates, as experimentally measured. Meanwhile, many experiments were carried out on TMS to validate simulation results, such as serial and parallel air cooling [
8], integrated liquid TMS [
9], and expanded graphite based composite phase change materials [
10]. These studies have produced many findings on the highest temperature control of battery packs.
In theoretical and experimental studies on thermal problems, researchers gradually discovered that temperature uniformity can also greatly influence battery performance as well as maximum temperature. That is because temperature variations cause electrochemical reactions to proceed at different C-rates in different regions of the cell, which thereby lead to incomplete energy utilization, and inefficient management of battery life. As a result, an outstanding TMS for EV should not only maintain the battery pack to work below maximum temperature limits, but should also maintain the battery temperature as uniform as possible. The temperature uniformity can be affected by the C-rate [
11], cooling strategy [
12], battery module arrangement [
13], and so on. With the wide application of liquid cooling systems in modern EVs, coolant mass maldistribution in flowing channels will play a key role on temperature uniformity.
For liquid cooling, the coolant is used to absorb approximately 80–95% of the heat from a battery pack so that the performance of TMS strongly depends on coolant flowing characteristics. In order to maximally elevate its heat dissipation capacity, the cooling plate used in the system is usually manufactured with different internal flowing channels, where the coolant flows through and takes heat away from the battery pack. Regardless of flowing parameters and status, mass maldistribution in flowing channels will become an important force on temperature uniformity and system performance, as the maldistribution phenomenon can greatly induce an uneven liquid convection in the channels and consequently lead to a non-uniform heat transfer in the plate surface. However, for the problem of thermal uniformity caused by mass maldistribution in cooling plates used in EV batteries, there are still few papers focusing on it.
The significance of mass maldistribution on temperature uniformity can be shown in a micro-channel heat sink used in electric devices and other systems. Jin et al. [
14] presented a multi-pass serpentine flow-field that could maximize under-rib convection in a given cell area and was expected to enhance under-rib convection and transport, thereby improving the performance and temperature uniformity of polymer electrolyte membrane fuel cells. Chein et al. [
15] conducted a detailed numerical study on the effect of five different inlet/outlet configurations on flow distribution through comparison of temperature contours. Gui et al. [
16] developed different parallel channels using a manifold with baffles and found the design with one to six bends could improve heat transfer and temperature uniformity significantly, but temperature non-uniformity was not improved by using more than four bends. Lu et al. [
17] investigated the comprehensive performance of pressure drop and heat transfer based on rectangular parallel mini-channels with three different section surface areas and found that large ratios of height to width manifested a better comprehensive performance. Zhao et al. [
18] employed numerical simulation to study the uniformity of velocity and temperature fields in parallel channel heat sinks and found the channels with derangement presented better performance when considering pressure drop and mean temperature of heat surfaces. Manoj et al. [
19] carried out extensive experiments to study the parameters affecting pressure drop and temperature distribution across parallel channels, such as channel hydraulic diameter, flow configurations (U, Z, I type), and chip power, and found the configuration of inlet and outlet was important for heat performance. Javier et al. [
20] analyzed the temperature distribution inside a Lytron CP20 microchannel cold plate under constant heat flux. The pressure drop across the first microchannel was very low due to the resistance to the flow caused mainly by the impact of the fluid to the wall of the outlet tube. Conversely, the pressure drop across the last channel was high because, due to the wall effect, the fluid coming from the previous channels pushed the fluid from the last one to the exit. Trevizoli et al. [
21] carried out an experimental evaluation of the combined effect of void volume and inlet flow maldistribution to the thermal performance of active magnetic regenerators and found that the negative impact of inlet flow maldistribution on the regenerator effectiveness was more significant than that of void volume.
According to literature above, few have researched the effect of mass maldistribution on temperature uniformity of cooling plates, although many have researched micro-channels; however, their results cannot be applied directly to battery cooling as they are working at a much smaller scale dimension. The mass maldistribution can be induced by the variations of the amount of flowing mass, inlet and outlet arrangement, coolant flowing status (laminar or turbulent), and flowing channel configuration, which is considered to be the most important factor according to the studies of micro-channel and other systems. Therefore, the objective of this paper is to apply numerical analyses on different flowing channels in order to improve thermal uniformity (in terms of mass uniformity). The effect of channel configuration on mass maldistribution and pressure drop is compared, and serpentine channel is selected for further optimization by orthogonal experiment and range analysis methods due to its comprehensive performance. The study aims to reveal design principles about how channel configuration acts on mass maldistribution, and to demonstrate that significant performance gains can be realized with optimization techniques that can be utilized in battery cooling plates.