Thermal Management of Electronics to Avoid Fire Using Different Air Flow Strategies

Abstract

Due to high heat generation within closely packed smart electronic devices, some efficient thermal management systems are required for their reliable performance, avoid overheating, long lifecycle and safety. In this study, a novel thermal management system based on forced air cooling having three airflow configurations is developed to explore the thermal characteristics of each configuration. A customized cavity is designed to have provision for three airflow configurations (axial, cross, and reverse flow) and temperature profiles are investigated within the cavity for each configuration. The experiments are performed at three heat generation rates, i.e., 10 W, 20 W, and 30 W to analyze the cooling effectiveness at a variable heat generation rate. It was observed that the maximum temperature within the setup increases with the increase in heat generation rate. In axial flow air configuration, cavity temperature has been reduced remarkably by 69 and 82.4% at 10 W and 30 W, respectively. Second to axial flow, cross flow configuration performs better than reverse flow and an overall 65.7~78.6% temperature drop is obtained compared with enclosed cavity from 10 W to 30 W, respectively. Furthermore, a similar cooling rate trend in the cavity is obtained for an increased heat generation rate in the cavity.

1. Introduction

In recent decades, with the technological advancements, the size of electronic devices is becoming more and more compact and is shifting towards portable and modular designs. To fulfill the high operating power requirements of these devices, lithium-ion (Li-ion) batteries have become a promising energy source due to their long-life cycle, high-energy density, and low self-discharge rate [1,2]. On the other hand, due to the compactness and portability of these devices, there is an inherent problem of overheating these devices due to the high charge/discharge rate of lithium-ion batteries (LIB). This temperature increase can adversely affect LIB performance and life cycle [3]. According to the study conducted by Panchal et al., there is a reduction in the life of smart batteries by two months for every degree rise in temperature in the range of 30~40 °C [4]. Therefore, different cooling techniques are developed to control the temperature of smart electronics and their associated battery packs. Based on cooling media, these cooling systems are classified as active cooling systems, such as air-cooled [5,6,7,8,9,10,11,12,13,14], liquid-cooled [15,16,17,18,19], and passive cooling systems, such as phase change materials (PCM)-based systems.

Active cooling systems require additional utilization of energy, i.e., fluid flow, by a fan or pump. Air-cooled systems have been widely used for thermal management systems due to their low manufacturing and maintenance cost, thermal safety, and lightweight-ness. Most of these investigations focused on intake configuration, airflow patterns, and electronics/battery configuration with the enclosure. Shahid et al. [10] studied the 32-cell battery pack and concluded that the uneven temperature distribution in the battery cell could be reduced by optimizing the cell arrangements in the pack, such as the inlet and outlet positions. Tong et al. [11] concluded that increasing the air inlet velocity could achieve a more uniform temperature distribution within the battery pack. Yang et al. [12] studied the heat distribution by varying battery configurations from staggered to aligned, and observed that the battery temperature increases with the increase in battery interval.

Air-cooled thermal management systems also include cooling with tubes and microchannels. In this regard, Zhou et al. [13] developed a unique tube-based thermal management method. They concluded that the required cooling for cylindrical battery cells could be achieved by increasing the air pressure at the inlet. Park et al. [14] developed enclosures with five different airflow patterns to study the thermal management of a 72-cell battery pack and concluded that the tapered configuration is best for the thermal management of the battery pack. M. Al-Zareer et al. [15] varied the battery pack spacing and measured the thermal performance in terms of maximum temperature for the 600 s charging/discharging cycle and proposed that with the decrease in the battery spacing, temperature of battery pack increases.

The liquid cooling method is another way of effective thermal management. Liquid cooling is effective for batteries with a high charge and discharge rate. Y. Zheng et al. [16] designed the hybrid thermal management system consisting of liquid cooling and phase change material and assessed the system’s effectiveness at an 8C charge rate. The maximum temperature of 38.69 °C was obtained in the battery at an 8C charge rate, which is well within limits. Zhao et al. [17] developed a mini-channel liquid cooling cylinder for battery pack thermal management and concluded that the maximum temperature could be controlled under 40 °C using four mini channels having a mass flow rate of 1 × 10⁻³ kg/s. C. Zhao et al. [18] proposed two methods to reduce the temperature nonuniformity in battery packs: shortening the flow path and increasing the contact area. The numerical simulations concluded that for the 5C discharge rate, nonuniformity could be reduced to 2.2 K and 0.7 K for each method, respectively. Panchal et al. [19] numerically and experimentally studied the temperature and velocity profile of cooling plates. It was observed that the temperature nonuniformity increases with the increase in C-rate; hence, the heat flux also increases.

Some other cooling techniques are also cited in the literature such as natural convection, heat pipes, nanoparticles, and hybrid thermal management systems. Hybrid systems use the combined effect of two cooling methods simultaneously, i.e., active cooling integrated with the passive cooling technique. Panchal et al. [20] studied the 20Ah LiFePO₄ battery at different discharge rates for natural convection (NC) and found the direct relation between discharge rate and battery temperature, i.e., with the increase in discharge rate, the battery temperature rises. Wiriyasart et al. [21] studied a nanofluid-based cooling system’s temperature and pressure distribution within a battery pack comprising 444 cylindrical lithium-ion cells. The opposite flow configuration was observed to be the most efficient cooling pattern. Akbarzadeh et al. [22] developed a novel hybrid cooling plate capable of both liquid-phase cooling and PCM-based cooling under two scenarios, i.e., cold start performance and operational thermal management. It is concluded that the hybrid cooling plate significantly reduces battery temperature and consumes 30% less energy to maintain battery temperature. Li et al. [23] experimentally studied the effect of the crossflow of air through a horizontally aligned battery and developed a numerical model to forecast the thermal distribution within the battery pack. Liu et al. [24] studied the effect of heat capacitance, charge–discharge ratio, and heat source characteristics and indicated that during the charging or discharging of batteries, there is more uniform temperature distribution at 45 °C than at 25 °C. Juhyeon Lee et al. [25] studied the effect of combined air and PCM thermal management systems with the air velocity range 0.005–0.2 m/s for an aspect ratio of 0.2–0.35. They concluded that the 0.2 aspect ratio corresponds to the lowest average temperature in the PCM chamber; moreover, the temperature inside the chamber decreases with the increase in air velocity.

Nanoparticles drastically improve the heat-carrying capacity of fluids. Selimefendigil et al. [26] studied the effect of air, hydrogen, water, and alumina particles as nanofluids and concluded that the nanofluid comprising alumina particles performs better. Omri et al. [27] used a numerical method to investigate the thermal management system of the photovoltaic (PV) panels having porous deflectors (PDs) and nanofluids. The research assessed the impact of the Reynolds number (Re), Darcy number, PD number and aspect ratio of PDs. It was concluded that the most efficient cooling was achieved when five PDs having an aspect ratio of 1 were installed in the channel. Maatoug et al. [28] designed a PV cooling system using pulsating flow with multiple jet impingements using various fluids. The research concluded that pulsating amplitude plays a major role in cooling performance enhancement. A temperature drop of 2.16 °C was achieved by increasing the pulsating amplitude from 0 to 1. Similarly, alumina–water nanofluid at the highest pulsating amplitude yields a temperature drop of 37.30 °C.

Chen et al. [29] investigated the effect of air and PCM cooling by studying the two-dimensional thermal models for a 16-cell lithium-ion battery and concluded that the active air cooling is more efficient at higher ambient temperatures. Hamidreza Behi et al. [30] developed a mathematical model to measure the effectiveness of air cooling, heat pipe cooling, and a heat pipe covered with a copper sheet. The results indicate a 34.5%, 42.1%, and 42.7% reduction in temperature for forced air cooling, heat pipe, and heat pipe with copper sheet, respectively. S. Khalid et al. [31] studied the heat transfer characteristics of the sintered copper wicked heat pipe and grooved heat pipe. Heat pipes filled with distilled water were tested in a gravity-assisted position. The sintered copper heat pipe showed high capillary pressure, low thermal resistance, and a doubled heat transfer coefficient as compared to the grooved heat pipe. Sinan Gocmen et al. [32] numerically and experimentally investigated the effect of battery positioning in the air-cooled chamber for fast and ultra-fast charge/discharge conditions. Wang et al. [33] developed a novel modular liquid cooling thermal management system. The effect of the cooling mode, i.e., parallel and serial, and the coolant flow rate was studied. It was concluded that the parallel flow cooling mode is more efficient, which results in a lower and uniform peak temperature within the battery pack.

Jawed et al. [34] investigated the effect of air path size, battery position, and air inlet and outlet positions in the cooling system using COMSOL Multiphysics software. It was concluded that the heat transfer rate increases with the increase in airflow. Wang et al. [35] investigated the 18-cell battery pack experimentally using the reciprocating airflow configuration and concluded that the thermal management system could reduce the battery pack’s temperature by 15%.

Thermal management of smart and compact battery packs and electronic systems is critical to their life cycle, charge/discharge rate, energy storage capacity, safety, etc. Various techniques, such as air, water, nanoparticles, phase-change materials, heat pipe, etc., have been used to keep the Li-ion batteries’ temperature in control. However, there has not been significant research focusing on air cooling system using various flow arrangements on modular levels. In this research work, a novel cooling method was developed to understand the forced air cooling of smart and compact electronics. Three different airflow configurations (cross flow, axial flow, and reverse flow) were built into a specialized cavity. The current research investigates the comparison of different airflow configurations, the effect of an increase in heat generation rate, i.e., an increase in the charge/discharge rate of the battery and temperature uniformity within the cavity.

2. Materials and Methods

2.1. Experimental Setup

Experimentation is performed in a research laboratory at UET, Taxila. The experimental setup pictorial view is shown in Figure 1, and the schematic diagram is shown in Figure 2.

The specifications of hardware components are as follows:

2.2. Heater and Power Supply

An experimental setup consisting of a cavity was developed to simulate the real-time working of smart electronics generating a large amount of heat. The archetypal heat generation within the cavity was achieved by mimicry of electronics using a silicon rubber heater having the dimensions of 100 × 100 mm². The other specifications of the silicon rubber heater are given in

Table 1. Technical details of silicone pad heater.

Item	Power (W)	Voltage (V)	Current (A)	Resistance (Ω)	Temperature Range (°C)	Tolerance
Specifications	50	12	4.1	2.88	155–160	−/+10%

.

A DC power supply by Keysight Technologies^® (Keysight Technologies, Colorado Springs, CO, USA) Colorado Springs, CO, USA (Model: Agilent 6675A) was used to power the electric heater, as shown in Figure 3.

Table 2. Technical specification of power supply.

Parameter	Voltage Range (V)	Current Range (A)	Programming Accuracy
			Voltage	Current
Agilent 6675A	0~120	0~18	0.04% + 120 mV	0.1% + 12 mA

shows the technical specification of the power supply, and

Table 3. Power values and their heat fluxes.

Power (W)	10 W	20 W	30 W
q (kW/m2)	1	2	3

shows the power input and their respective heat flux input in the cavity.

2.3. Air Flow Configurations

An experimental setup was developed to simulate the real-time conditions of closely packed electronics. The acrylic glass (methyl methacrylate), commonly known as plexiglass, was used to make the cavity as it is economical, lightweight, strong, and has low heat transfer capacity. The cavity was insulated with a thermopore to minimize the heat loss to the surroundings. This heat loss prevention represents the 1 D condition of heat transfer in the cavity. The cavity was designed using the modular design concept. The side walls were replaceable to generate three different airflow configurations, i.e., axial, cross, and reverse flow. The cavity dimensions were 140 × 140 × 80 mm and the silicon pad heater was installed precisely in the center of the cavity. The air velocity was kept constant at 3 m/s during the entire experimentation as this velocity corresponds to the base velocity range used in numerical and simulation studies [5,7,8,9,10]. The airflow configurations are shown in Figure 4.

A Delta Electronic fans (model number AUB0512HHB CA, USA) generates forced airflow within the cavity. The dimensions of the fan are 50 × 50 × 15 mm³. Other technical specifications are given in

Table 4. Technical details of the fan.

Item	Power (W)	Voltage (V)	Current (A)	Speed (R.P.M.)	Max Air Flow (m3/min)	Noise (db)
Specifications	2.16	12	0.18	7400	0.572	38.5

.

2.4. Thermocouples Positions

The temperature is measured using OMEGA^TM thermocouples, which are precisely calibrated on ASTM standards [36]. The temperature in the cavity is measured in three directions (x, y, z), using four K-type thermocouples, having wide temperature range, accuracy, and precision. The maximum discrepancy of the used thermocouples has been found to be ±0.1 °C within a temperature range of 0–100 °C. The technical specifications of thermocouples are mentioned in

Table 5. Technical details of thermocouples.

Item	Make	Type	Temperature Range (°C)	% Accuracy
Specification	OMEGATM	K-Type	−200~1250	0.75

.

Thermocouples provide analog data, which are then converted to digital form using a national instrumentations (NI-USB-9162) data acquisition system (DAQ). DAQ is further connected to a computer that logs the temperature at an interval of 5 s using LabView. Thermocouples’ placement with their dimension is schematically shown in Figure 5. The designation T1y, T2y, and T4y are at the center of the cavity, i.e., 70 mm along the base of the cavity. These thermocouples are placed at different heights of 20 mm, 40 mm, and 60 mm from the base, and are used to measure the temperature variation in the y direction of the cavity. Similarly, T3x is located 20 mm along the base of the cavity and at a height of 40 mm from the base. T3x measures the x direction temperature variation in the cavity. Araldite is used to properly fix these thermocouples and ensure the cavity’s proper sealing.

3. Results and Discussion

The primary focus of this work is to find the most efficient airflow configuration to reduce the cavity temperature of smart electronics, i.e., increasing the heat dissipation from the cavity, hence lowering the IC’s temperature. So, the data for cavity temperature are obtained and presented graphically. Considering the uniform heating from the electrical heat source and cavity symmetry along the x- and z-axis, the trend obtained in the x-axis can be replicated for the z-axis. The same temperature variation in the x and z directions is recorded using a T3x thermocouple.

3.1. Temperature Profile of Enclosed Cavity

The temperatures measured by various thermocouples within the cavity are plotted against time at 10 W and 30 W power levels. In this configuration, the cavity is thermally isolated from the surroundings using insulation and the thermocouples are fixed in the cavity along respective x, y, and z axis, as shown schematically in Figure 5. This experimentation is carried out to estimate the max temperature achieved in the cavity and to gauge the cooling efficiency of different airflow configurations against it.

The room temperature of the lab is 24 °C, and is maintained constant during experimentation. Initially, when power is supplied to the heater, the temperature at time zero is recorded as 24 °C. As time passes, the heat flux from the silicon pad heater having a surface of 100 × 100 mm², exchanges heat with air in the cavity through convection, increasing cavity temperature until a steady-state equilibrium is achieved. The max temperature achieved in the cavity is 82 °C and 156 °C at 10 W and 30 W, respectively. For the second case, a steady state is achieved faster due to an increased heat flux input at a higher power level of 30 W for the same cavity. The results are shown in Figure 6 at 10 W and 30 W.

3.2. Temperature Variation in the Cavity with Fan along X, Y, and Z-Axis

Thermocouples are set up in the cavity to record small temperature variations along the X-, Y-, and Z-axis, as shown in Figure 5. Keeping in view the geometrical symmetry, the trends for the x-axis can be replicated for the z-axis. As the temperature varies along the y-axis, thermocouples are positioned vertically with a pitch distance of 20 mm from the base of the cavity. The obtained data describes the temperature variation within the cavity for power ranges of 10 W, 20 W, and 30 W for all three configurations. This temperature variation within the cavity provides the overall view of heat content in the cavity, i.e., more heat is dissipated when the cavity’s temperature distribution is more consistent. Additionally, the regions with higher temperatures are a sign of a poorly ventilated area in a cavity arrangement, which must be considered when constructing an enclosure for commercial use.

3.2.1. For Cross Flow Configuration

The setup is run at a rated input power of 10 W, 20 W, and 30 W, and after each experiment, we let the setup cool back to the ambient temperature. The heat from the heater is transmitted to the air in the cavity when the input power is provided, causing the cavity temperature to rise over time. Temperature rise trends at 10 W, 20 W, and 30 W are shown in Figure 7, Figure 8 and Figure 9, respectively. There is a steep rise in cavity temperature in the beginning due to a greater temperature difference with the ambient temperature. However, the rate of temperature that increase with the passing of time is substantially reduced when the cavity temperature approaches the equilibrium condition. In the case of crossflow, the forced air entering the cavity also exchanges heat within the cavity; thus, lowering the cavity temperature. The average temperatures obtained within the cavity are 26.1 °C, 27.2 °C, and 29.2 °C for the 10 W, 20 W, and 30 W heat generation rates, respectively. It is observed that at 10 W of heat generation rate, the cavity temperature is more uniform than at 20 W and 30 W. This trend is caused by the fact that as the cavity temperature rises, the sections inside the cavity having no proper ventilation start to accumulate heat, which results in an overall temperature rise in the cavity.

3.2.2. For Reverse Flow Configuration

The experiment is repeated for reverse flow at 10 W, 20 W, and 30 W power levels. The average temperatures obtained within the cavity are 28.1 °C, 30.4 °C and 33.4 °C at 10 W, 20 W, and 30 W of heat generation rate, respectively. The peak temperature (T_max) values plotted against time are shown in Figure 10, Figure 11 and Figure 12. T_max values are higher in the case of reverse flow configuration due to the low heat transfer rate in the cavity, i.e., the indicator of low heat dissipation. The lower heat dissipation in reverse flow configuration is due to the more erratic airflow in the cavity owing to no specific air path, which causes cavity air to form swirl/circulate within the cavity. On the other hand, forced air entering the cavity does not properly mix with cavity air and leaves the cavity without sufficient heat transfer, forming poorly ventilated areas in the cavity.

3.2.3. For Axial Flow Configuration

The experiments were repeated for the axial flow configuration at 10 W, 20 W, and 30 W. Temperature is recorded and plotted against time as shown in Figure 13, Figure 14 and Figure 15. The average temperature obtained within the cavity is 25.4 °C, 27.2 °C and 27.4 °C at 10 W, 20 W, and 30 W, respectively. However, it can be observed from the graph that the max temperature (T_max) reached in the cavity is significantly lower than the case of the cross and reverse flow conditions. The reason being the properly ventilated cavity design and low hindrance to the air entering the cavity. This accounts for a large quantity of ambient air entering the cavity, i.e., the temperature delta between ambient entering air and cavity temperature is higher, resulting in more heat dissipation from the cavity.

3.3. Cavity Configuration Comparison

This section compares different cavity configurations to find the most suitable configuration for max heat dissipation from the cavity. The experiments are performed at three power levels to observe the trends with an increase in power level. The results are shown in Figure 16, Figure 17 and Figure 18. It can be observed from the graphs that the maximum temperature drop is obtained in case of axial flow configuration. In comparison, the crossflow cavity performs better than the reverse flow configuration. The airflow characteristics within the cavity can explain these trends. As in the case of axial flow configuration, the flow is streamlined, facing minimum obstruction at the cavity entrance. Hence, the maximum air flow rate passes the cavity compared to the cross or reverse flow configuration. Additionally, the air enters the cavity at ambient temperature, so a large temperature difference allows maximum heat transfer. The same parameters contribute to the crossflow configuration’s better heat transfer capabilities when compared with reverse-flow configuration. Next, experiments are conducted at 10 W, 20 W, and 30 W power levels, and the trends depict similar results as the 10 W power level.

3.4. The Effect of Varying Power Levels

Experiments are performed on each cavity configuration to analyze the effect of increasing power levels on cavity cooling by increasing power levels from 10 W to 30 W. Graphical trends are shown in Figure 19, Figure 20 and Figure 21. T_max is measured after a steady state is achieved in the cavity. T_max is directly proportional to power level, i.e., for each cavity configuration, the T_max increases with an increase in power level. Within the cavity, two processes are taking place simultaneously; heat flux generation in the heater and heat dissipation from the cavity by forced airflow. The maximum temperature achieved within the cavity is determined at the equilibrium state of these two processes.

4. Conclusions

Experimentation is conducted to find the most efficient and optimized forced air cooling technique for closely packed smart electronics. Optimization scenarios include the effect of temperature variation within the cavity, air flow configuration comparison, and the effect of varying power levels. Based on experiments conducted in this research, the following conclusions are made:

i.

The maximum temperature within the cavity without external cooling source reached up to 82 °C and 156 °C at 10 W and 30 W of heater input power, respectively;

ii.

The axial flow configuration is the most efficient airflow configuration, i.e., maximum temperature drop was achieved. About 9.6%, 10.75%, and 17.91% temperature drop was observed at 10 W, 20 W, and 30 W, respectively, compared to reverse flow configuration. However, a large T_max drop of 69~82.5% is obtained compared to an enclosed cavity with no thermal management;

iii.

For crossflow configurations, the temperature recorded in the cavity is 7.29%, 10.68%, and 12.76% lower than the reverse flow at 10 W, 20 W, and 30 W, respectively. However, compared to enclosed cavities without thermal management, a temperature drop of 65.7~78.6% is obtained;

iv.

For a lower heat generation rate, the cavity temperature is more uniform. Therefore, a more sophisticated thermal management method is required for temperature control at elevated temperatures;

v.

The effect of varying power levels suggests the direct relation between input power level and max temperature T_max obtained within the cavity.

5. Future Work

In the current study, the effect of a forced air convection cooling system has been studied experimentally by changing the airflow configurations. Future work will focus on the replication of the same study at different airflow velocities, the optimization of the current thermal management technique using various fin types (pin, square, rectangular) fins with phase change materials, heat pipes, and hybrid configurations of fins with a heat pipe to draw a direct comparison for the most efficient cooling strategy.