1. Introduction
The performance enhancement of battery thermal management systems (BTMSs) has been a vital research focus in recent times because of their roles in advancing the design and development of novel highly efficient electric vehicles (EVs). Over the years, EVs have proven to be reliable in the bid to minimize environmental pollution and fossil fuel consumption [
1] because of the choice of batteries as an energy source as an alternative to conventional fossil fuels. Different types of batteries have been used to power EVs; however, the most common type is the lithium-ion battery. The power consumption, mileage, and cost of electric vehicles generally depend on the health and capacity of batteries [
2]. Interestingly, despite their wide acceptance, lithium-ion batteries generate too much heat during charging and discharging, which poses severe threats to the health and service life of the batteries. In order to overcome this challenge, novel BTMSs that are highly efficient and effective must be developed. Generally, BTMSs are required to maintain the working temperature of battery packs between
and
, while the maximum temperature difference (
) between any two batteries in a pack remains below
[
3,
4]. This is necessary because a
value higher than the recommended value and temperatures outside the recommended range will negatively impact the performance of batteries [
5]. Several cooling methods for BTMSs have been developed over the years and can be categorized as passive and active cooling methods. For active cooling, batteries are cooled by passing air across the batteries [
6,
7], by the direct contact of liquid on the batteries, by passing the liquid through cooling plates or mini channels [
8,
9], or by placing heat pipes between and around batteries [
10,
11]. Passive cooling utilizes phase-change materials (PCMs), which will be in direct contact with batteries to maintain the batteries’ operational temperature [
12,
13]. The combination of two methods has also been explored, such as passive/passive, active/active, or passive/active methods [
14,
15,
16]. Despite the progress recorded in the design of BTMSs, the air-cooling BTMS still remains one of the most preferred choices because of its numerous benefits of design affordability, simplicity, stability, light weight, and longevity [
17]. It has been employed by many companies, such as Nissan, Toyota, Volkswagen, and Chevrolet [
17,
18].
The research on the air-cooling technique is being explored, with new designs supplementing the air-cooling techniques by adding other enhancement techniques, to form hybrid thermal management systems (HTMSs). Focusing HTMSs, Mo et al. [
1] combined phase-change materials (PCMs) and air-cooling techniques for managing the performance of batteries under all climate conditions. In a study by Qin et al. [
19], PCMs and air-cooling techniques were integrated together to form a hybrid BTMS, which was investigated experimentally and numerically. Behi et al. [
20] proposed a novel concept that combines heat pipes and air-cooling for managing battery temperatures in electric vehicles. Zhang et al. [
21] carried out a performance analysis of another hybrid BTMS, which combines PCMs and air-cooling, by leveraging the potential of machine-learning algorithms to optimize the choice of the operational and structural parameters of the design. Zhou et al. [
22] utilized liquid cooling while designing the channel for the liquid flow in the form of steps. A new concept has also emerged for PCMs, as reported by Song et al. [
23], where a dual-layer PCM was designed, while air provided additional cooling. A recent concept proposed by Zhu et al. [
24] combined two different outlet positions employed in the Z-type BTMS. The PCM-based hybrid design has been further investigated using nano-PCM, fins, and metal foam [
25,
26]. The modification of the structural orientations of battery packs to optimize their performance has also been widely employed. Argade and De [
27] optimized the design of the Z-type BTMS by adopting a tapered air inlet and secondary vents. Oyewola and Idowu [
28] investigated the waved and straight structures of the divergence plenum of the Z-type BTMS to alter the flow of the air to improve the thermal uniformity. Suo et al. [
29] also investigated the effects of different plenum shapes, such as slanted, convex, concave, and stepped, and different plenum positions on the heat dissipation performances of batteries. Chen et al. [
30] explored a control strategy based on the temperature difference among batteries for a J-type-structured BTMS. Alzwayi and Paul [
31] considered the control of flow patterns while introducing vertical and spiral fins to enhance the cooling performance of the system. The installation of baffles, spoilers, and fins along the airflow path has been widely adopted. For instance, Qin et al. [
32] carried out experimental and numerical simulation studies for a design with a combination of internal fin structures and air cooling to enhance the thermal performance of a system. Wang et al. [
33] installed spoilers along the airflow path to enhance the thermal performance of the Z-type BTMS. Wang et al. [
34] introduced parallel plates to the airflow of the same Z-type BTMS to enhance the performance of the system. Oyewola et al. [
35] investigated the use of straight and inclined baffles in enhancing the performance of the Z-type BTMS. Alzwayi and Paul [
36] examined the heat transfer enhancement of batteries using vertical and spiral cooling fins. Each of these studies provides distinct performances and demonstrates huge potential in enhancing the performance of Z-type BTMSs.
The findings from a few of the aforementioned studies are further presented to demonstrate how they impact the thermal performances of batteries. A study by Wang et al. [
34] added parallel plates to the Z-type BTMS to enhance its performance. It was revealed that this technique reduced
and
by 3.37 K (6.17%) and 5.5 K (71.9%), respectively, in comparison with those of the original Z-type design. In a study by Oyewola et al. [
35], where straight and inclined baffles were installed to understand how they influence the performance of the system, the results showed that the baffle thickness can either decrease or increase the
values of the batteries, while inclined baffles generally produce better
and
performances. In a study by Alzwayi and Paul [
36], the heat transfer enhancement in a Z-type BTMS with cylindrical lithium-ion batteries having vertical and spiral fins installed on them was investigated. It was observed that spiral fins reduced the battery temperature by 3.2% when compared with longitudinal fins, which also resulted in a 65.6% reduction in the amount of material used. Other studies that explore the hybrid approach have also reported enhancements in performance. For instance, Moaveni et al. [
26] investigated passive and hybrid BTMSs by employing fins, nano-PCMs, and metal foam, which served as flow control mechanisms. From the study, using 9% nanoparticles and four fins in the hybrid system,
reduced by 5.18 K and 10.36 K, when compared with that of the system without fins and nanoparticles, respectively. Song et al. [
23] considered a hybrid BTMS having a double-layer PCM and air-cooling techniques. The
value of the hybrid BTMS achieved a reduction of 21.5% in comparison with that of the PCM BTMS at an ambient temperature of
. The aforementioned studies reported improvements in the thermal performance of the BTMS, with most of the techniques requiring the addition of materials, which, consequently, implies an increase in the overall weight of the system. This is not favorable in applications where weight reduction is crucial to achieve the optimal operation. Hence, more studies have been carried out to improve thermal performance without necessarily adding more materials. One such approach can be seen in a study by Oyewola et al. [
28], where different wave-like and straight configurations were investigated. The study reported that
and
reduced by 8 K and 4.61 K, respectively, using a wave-inclined plenum configuration. Targeting improvement in thermal performance without adding materials, Argade and De [
27] created outlet vents on the convergence plenum while further redesigning the divergence plenum to have an inclined configuration. The study revealed that
and
were reduced by 2.18% and 72.84% when compared to those of the system without vents.
From the literature presented in this study, it is evident that Z-type BTMSs have been extensively investigated through different structural modifications. One of these designs that has shown huge thermal enhancement potential is the Step-like plenum BTMS, with a significant reduction in
. A lot of investigations have also been conducted on the Z-type BTMS with a Step-like plenum design to gain detailed insight into the performance of the design. Zhou et al. [
22] proposed stepped channels to achieve a lightweight design, using liquid as a cooling fluid. Zhu et al. [
24] combined the Step-like design with two outlet positions. Oyewola et al. [
37] also investigated the Step-like design, focusing on four different numbers of steps, with each design having the same step height. In order to better understand the Step-like design, Oyewola et al. [
38] investigated each number of steps with variable step heights. A further study by Oyewola et al. [
39] leveraged machine learning’s benefits to investigate additional numbers of steps and various inlet air velocities and temperatures. Each of these studies demonstrates different performances in terms of
,
, and
. By installing steps on the divergent plenum and changing the positions of the outlets, the performances of the designs varied. However, increases in the pressure drop were recorded for all the Step-like plenum designs. This work differs from the previous works by investigating the performance of Step-like plenum BTMSs through the introduction of two outlets, positioned at different locations on the convergent plenum, with the aim of maintaining a reduction in
while reasonably reducing
. Considering the potential of increasing the number of steps while adopting and repositioning two outlet channels, more investigations can be carried out for the Step-like plenum configuration. In view of these, the current study explored additional possibilities not covered by Oyewola et al. [
37] and Oyewola et al. [
39] by investigating seven different numbers of steps, combined with one or two outlets, placed at different flow pattern positions. The numerical simulation method was employed to investigate different structural designs, thereby proposing the best structure in terms of reductions in
,
, and
. It should be noted that the performances of these designs will be presented based on each of the aforementioned parameters and not based on a combined effect of the three parameters. A future study is underway to optimize the designs and select the performance metric that balances the tradeoffs between the parameters.