Week 10 - Simulating Combustion of Natural Gas.

AIM

To perform the heat transfer analysis on the 4x2 cylindrical lithium ion battery with heat generation boundary conditions for different cases and compare the results.

OBJECTIVES

• To simulate a air flow through a battery pack and analysing the heat transfer between the lithium ion battery with heat generation and the cooling air system.
• Perform Mesh independent study for any one case(For velocity) and use the more suitable model for further possibilities.
• Simulation is performed by varying the velocity of air to 3, 5, and 10m/sec and discussing the results.
• Determining the maximum temperature and heat transfer coefficient obtained by the
• Based on the results and your judgment, select one case (3, 5, or 10 m/s) and vary the space between the batteries as 2, 4, and 6 mm, then discuss the results.
• Create the animation to observe the temperature distribution and velocity vector.

INTRODUCTION

In this project we will be simulating a flow of cooling air through a battery pack of 4x2 lithium ion battery. Heat transfer analysis between the lithium ion battery with heat generation and the cooling air is carried out for different velocities and cases. Mesh independence study is carried out for a optimum mesh size. Contour plots are studied to understand the temperature distribution and flow patterns. Different inlet and outlet combinations are studied for a better understanding. Based on the temperature and heat transfer coefficient between the battery and the air, best cooling configuration is obtained.

THEORY

Electric vehicles:

Conventional vehicles use fossil fuel and pollution due to combustion is a serious concern on the environment. The scope of Electric vehicles (EV) has been essential due to the adverse impact of fossil fuels on the environment.

An electric vehicle does not use any fossil fuel for power generation and has zero emission. Many initiatives have been taken to reduce air pollution using non-conventional energy sources. Electric vehicles use the electric battery and help to reduce pollution. External electric supply charges the battery which supplies electric power to the motor. The electric motors transfer power to the front and back wheels. Many automobile industries are shifting from internal combustion engine cars to electric vehicle.

Automotive industries have invested hugely in the research and development of electric vehicles at an affordable rate. Electric vehicles have several advantages like improved energy efficiency, performance, environmentally friendly and are free from combustion generated pollution. Technologies in electric vehicles have been in development for thermal management for battery systems, power controllers and electric motor.

Lithium-ion Battery:

Lithium-ion cell performance depends on both the temperature and the operating voltage. Lithium-Ion cells work well when cells operate within limited voltage and temperature. Otherwise, the damage will occur to the cells and will be irreversible. In over-voltage situations the charging voltage exceeds the bearable cell voltage, resulting in excessive current flows and at the same time, it causes two problems. At excessive currents the Lithium-ions are deposited more rapidly than intercalation to the anode layers, Lithium ions are then deposited on the surface of the anode as metallic Lithium. This is Lithium plating. It gives rise to the reduction in the free Lithium ions and an irreversible capacity loss. There are two types of metallic lithium plating, namely homogeneous lithium plating and heterogeneous lithium plating, but the lithium plating is dendritic in form. Eventually, it can result in a shortcircuit between the electrodes.

As with over-voltage, under-voltage also brings about problems that give rise to the breakdown of the electrode materials. For the anode, the copper current collector breaks down. It causes the increase of battery discharge rate and battery voltage, however, the copper ions are precipitated as metal copper which is irreversible. The situation is dangerous for it can result in a short-circuit between anode and cathode. For the cathode, the cobalt oxide or manganese oxide will be decomposed after many cycles under low voltage. Meanwhile, oxygen will be released and the battery suffers from capacity loss.
The battery temperature should be controlled carefully. Both excess heat and lack of heat will bring about problems. Chemical reaction rates have a linear relation to temperature. The decrease of the operating temperature will reduce the reaction rate and the capacity of carrying current during charging or discharging. In other words, the battery power capacity is decreased. Moreover, the reduction of the reaction rate makes it harder to insert lithium ions into intercalation spaces. The result is the reduction of power and lithium plating causing capacity loss. High temperature increases the reaction rate with higher power output, however, it also increases the heat dissipation and generates even higher temperatures. Unless the heat is dissipated quicker than heat is generated, the temperature will be higher and finally, a thermal runaway will result.

Thermal runaway consists of several stages and each stage will give rise to more irreversible damage to cells. First, the SEI layer is dissolved to electrolyte at around 80ºC. The primary overheating may result from excessive current or high ambient temperature. After the breakdown of the SEI layer, the electrolyte begins to react with the anode. This reaction is exo-thermal which drives the temperature higher. Secondly, the higher temperature causes the organic solvents to break down with the release of hydrocarbon gases. Normally this starts at around 110 ºC. The pressure inside cells is built up by the gas and the temperature is beyond the flashpoint. However, the gas does not burn due to the lack of oxygen. A vent is needed to release the gas in order to keep cells under proper pressure and avoid a possible rupture. Then, the separator is melted and short-circuits occur between the anode and cathode at 135 ºC. Finally, the metal-oxide cathode breaks down at 200 ºC and releases oxygen which allows the electrolyte and hydrogen gas to burn. This reaction is also exo-thermal and drives temperature and pressure still further. In addition, the uneven temperature distribution is another problem of batteries. Typically, it is caused by the excessive local temperature, variable current in a cell, and the thermal conductivity of the case, as well as the placement of positive and negative terminals, and so on. It results in local deterioration and even thermal runaway with reducing the battery life.

Principle of Lithium-Ion Battery

The construction of Lithium-ion batteries consists of the positive(cathode) and negative(anode) electrodes. Here the cathode consists of lithium-ion-based (LiCoO2) and the anode consists of graphite (carbon). A thin plastic membrane called a separator separates both the cathode and anode. The above entire setup is put into the electrolyte called lithium salt-based electrolyte. The working principle is that ions are exchanged between the cathode and anode during the charging or discharging electrons move in the external circuit. During charging elections move from cathode to anode similarly during discharging electrons move from anode to cathode through the external circuits. The battery comes in different shapes such as a cylinder, circle, etc.

Operating Requirements:

The battery temperature should be controlled within temperature limits to avoid thermal issues and improve performance. The temperature range affects battery power and battery cycle life. At the same time, the temperature distribution should be even to guarantee the battery performance and lifetime. That is also the reason why the battery thermal management system is necessary for the battery system. When temperature ranges from 20°C to 40°C, battery power reaches a maximum.

The cycle life goes down slowly below 10°C because of anode plating and drops off quickly above 60°C due to the breakdown of electrode materials. Generally, the temperature must be controlled between 20°C and 40°C to ensure performance and cycle life. Moreover, the temperature distribution is controlled under 5K to keep the safety and lifetime of the battery. In addition, ventilation is also essential to the battery system and should be taken into account.

Difference between the cell, module, and pack

CELL: A single-cell unit device that converts chemical energy to electrical energy.

MODULE: A module is a collection of cells connected in series or parallel. The application of modules is in two-wheeler vehicles.

PACK: A pack is a series of individual modules and protection systems organized in a shape that will be installed in the vehicles. For heavy vehicle applications, we use pack systems.

Types of connections in battery pack/module system

SERIES: Each battery has milliamp hours and volts as specifications, Here each battery has 4.2Volts and 3200mAh so most applications have 12V. Hence 4.2V is not sufficient in order to increase the voltage we connect the batteries in series(we can multiply the voltage hence we get 12V but I can discharge 3400mampH for an hour) which is a positive side to another end and a negative side to another end of the battery as shown in the above figure.

Similarly in the PARALLEL connection if we want to increase the amp rating that is 3400mAh is a very small current rating in order to increase the current flow then we have to connect the batteries in parallel which is 3400mAh multiplied by 6 is 20400mAH with a max voltage of 4.2V.

So here I am considering a 4 by 2 Lithium-Ion battery where 2 batteries are connected in series and 2 batteries are connected in parallel.

C-rating is a unit to declares a current value that is used for estimating or designating the expected sufficient time of the battery under variable charge or discharge conditions. As the C-rating increases, the heat generation of the battery increases due to the flow of electrons from the charging or discharging of the battery.

Heat Sources and Sinks

1. Electrical Heating (Joule Heating)
   The operation of any battery generates heat due to the I2R losses as current flows through the internal resistance of the battery whether it is being charged or discharged. This is also known as Joule heating. In the case of discharging, the total energy within the system is fixed and the temperature rise will be limited by the available energy. However, this can still cause very high localised temperatures even in low power batteries. No such automatic limit applies while charging as there is nothing to stop the user continuing to pump electrical energy into the battery after it has become fully charged. This can be a very risky situation.

Battery designers strive to keep the internal resistance of the cells as low as possible to minimise the heat losses or heat generation within the battery but even with cell resistances as low as 1milli Ohm, the heating can be substantial.

2. Thermochemical Heating and Cooling

The chemical reactions which take place in the cells may be exothermic, adding to the heat generated or they may be endothermic, absorbing heat during the process of the chemical action. Overheating is, therefore, more likely to be a problem with exothermic reactions in which the chemical reaction reinforces the heat generated by the current flow rather than with endothermic reactions where the chemical action counteracts it. In secondary batteries, because the chemical reactions are reversible, chemistries which are exothermic during charging will be endothermic during discharging and vice versa. So there's no escaping the problem. In most situations, the Joule heating will exceed the endothermic cooling effect so precautions still need to be taken.

Lead-acid batteries are exothermic during charging and VRLA batteries are prone to thermal runaway (See below). NiMH cells are also exothermic during charging and as they approach full charge, the cell temperature can rise dramatically. Consequently, chargers for NiMH cells must be designed to sense this temperature rise and cut off the charger to prevent damage to the cells. By contrast, Nickel based batteries with alkaline electrolytes (NiCads) and Lithium batteries are endothermic during charging. Nevertheless, thermal runaway is still possible during charging with these batteries if they are subject to overcharging.

The thermochemistry of Lithium cells is slightly more complex, depending on the state of intercalation of the Lithium ions into the crystal lattice. During charging the reaction is initially endothermic then moving to slightly exothermic during most of the charging cycle. During discharge, the reaction is the reverse, initially exothermic then moving to slightly endothermic for most of the discharge cycle. In common with the other chemistries, the Joule heating effect is greater than the thermochemical effect so long as the cells remain within their design limits.

3. External Thermal Effects
   The thermal condition of the battery is also dependent on its environment. If its temperature is above the ambient temperature it will lose heat through conduction, convection and radiation. If the ambient temperature is higher, the battery will gain heat from its surroundings. When the ambient temperature is very high the thermal management system has to work very hard to keep the temperature under control. A single cell may work very well at room temperature on its own, but if it is part of a battery pack surrounded by similar cells all generating heat, even if it is carrying the same load, it could well exceed its temperature limits.

Temperature - The Accelerator

The net result of the thermo-electrical and thermo-chemical effects possibly augmented by the environmental conditions is usually a rise in temperature and as we noted above this will cause an exponential increase in the rate at which a chemical reaction proceeds. We also know that if the temperature rise is excessive a lot of nasty things can happen

• The active chemicals expand causing the cell to swell
• Mechanical distortion of the cell components may result in short circuits or open circuits
• Irreversible chemical reactions can occur which cause a permanent reduction in the active chemicals and hence the capacity of the cell
• Prolonged operation at a high temperature can cause cracking in plastic parts of the cell
• The temperature rise causes the chemical reaction to speed up increasing the temperature even more and could lead to thermal runaway
• Gases may be given off
• Pressure builds up inside the cell
• The cell may eventually rupture or explode
• Toxic or inflammable chemicals may be released
• Law suits will follow

Thermal Consideration with Vehicle Application

The EV battery is large with good heat dissipation possibilities by convection and conduction and subject to a low-temperature rise due to its high thermal capacity. On the other hand, the HEV battery with fewer cells, but each carrying higher currents, must handle the same power as the EV battery in less than one-tenth of the size. With a lower thermal capacity and lower heat dissipation properties, this means that the HEV battery will be subject to a much higher temperature rise.

Taking into account the need to keep the cells operating within their allowable temperature range the EV battery is more likely to encounter problems to keep it warm at the low end of the temperature range while the HEV battery is more likely to have overheating problems in high-temperature environments even though they both dissipate the same amount of heat.

In the case of the EV, at very low ambient temperatures, self-heating (I²R heating) by the current flow during operation will most likely be insufficient to raise the temperature to the desired operating levels because of the battery's bulk and external heaters may be required to raise the temperature. This could be provided by diverting some of the battery capacity for heating purposes. On the other hand, the same I²R heat generation in the HEV battery working in high-temperature environments could send it into thermal runaway and forced cooling must be provided.

Thermal Runaway

The operating temperature which is reached in a battery is the result of the ambient temperature augmented by the heat generated by the battery. If a battery is subject to excessive currents the possibility of thermal runaway arises resulting in the catastrophic destruction of the battery. This occurs when the rate of heat generation within the battery exceeds its heat dissipation capacity.
There are several conditions which can bring this about:

• Initially, the thermal I2R losses of the charging current flowing through the cell heat up the electrolyte, but the resistance of the electrolyte decreases with temperature, so this will, in turn, result in a higher current driving the temperature still higher, reinforcing the reaction till a runaway condition is reached.
• During charging the charging current induces an exothermic chemical reaction of the chemicals in the cell which reinforces the heat generated by the charging current.
• During discharging the heat produced by the exothermic chemical action generating the current reinforces the resistive heating due to the current flow within the cell.
• The ambient temperature is excessive.
• Inadequate cooling

Unless some protective measures are in place the consequences of the thermal runaway could be the meltdown of the cell or a build-up of pressure resulting in an explosion or fire depending on the cell chemistry and construction. The thermal management system must keep all of these factors under control.

Battery thermal management system :

Since electric vehicles have become so widely used, there is a high demand for longer battery life and higher power output. To achieve this, the battery thermal management systems will need to be able to transfer heat away from the battery pack as they are charged and discharged at higher rates. The heat generated as the battery is used can pose safety threats to the passengers. Due to the high stress and temperatures generated by the batteries, there is even higher importance on having the correct coolant and additive package. While companies such as Tesla, BMW, and LG Chem can use a traditional liquid coolant for their indirect cooling systems, continued research and development will need to be done on battery packs and coolants to advance electric vehicle safety.

The battery thermal management system should be equipped with four essential functions:

• Cooling:Due to inefficiency, battery cells will not only generate electricity but also heat. This heat should be rejected form the battery pack to ensure the safety of the battery pack, thus, a cooling function is required.
• Heating:As set up that the battery does not perform up to its full potential in very harsh cold weather, hence a heater such as a PTC heater, is needed to help the battery pack to maintain the optimum temperature conditions.
• Insulation: To protect the battery from the influence of the ambient conditions, a proper insulation of the battery from the external conditions is needed, else the battery will be affected by the extreme temperature conditions. To prevent this a good insulation is needed.
• Ventilation:Ventilation is required to remove the hazardous gases from the battery pack.

Different types of Battery cooling methods :

• To adapt to EVs, BTMS must ensure features such as high performance, simplicity, low weight, reduced cost, low use of parasitic power and fast packaging, and easy maintenance
• Two types of battery cooling systems (BCS) are common which are external or internal. Many researchers and industries have been developing BCSs
• External Battery cooling systems (EBCS) are classified into several different ways
• Battery cooling can be categorized based on the method or technique.
  • Liquid or gas cooling: plate type or use of mini-channel
  • Heat pipe
  • Phase Change Material (PCM)

1. Air cooling :

• Air is easily available for cooling purposes and it is widely used in many industries. However, the air is not an effective coolant because of its lower heat capacity and low thermal conductivity.
• Due to low maintenance of costs, the following type of air cooling system is popular in some automobile industries likeToyota Prius and Nissan Leaf.
• The cooling is possible either by forced convection (active cooling) mode and natural convection (passive cooling) mode.
• Natural convection cooling is suitable only for low-density batteries, and typically blowers/fans can be used to enhance the convection heat transfer rate.
• When air is used for cooling of battery modules arranged in series, the middle and rear portion of batteries are at high temperature to the low heat capacity of air. The temperature of the battery pack near the outlet is very high and the temperature distribution is highly non-uniform. To cool effectively, we need to increase the flow rate and turbulence of airflow.
• CFD study of different battery modules can be carried out for different configurations of airflow rates and electric vehicles.

Air cooling Battery Pack in EVs :

The following are popular battery pacts with air cooling in electrical vehicles

• Honda Insight
• Honda FitEV
• Hyundai IONIQ
• Nissan e-NV 200
• Nissan Leaf
• Renault Zoe
• Toyota Prius Prime

2. Liquid cooling :

• Liquid coolants have a high convective heat removal rate due to higher density and heat capacity compared to air.
• A liquid cooling system is more compact than an air system. We can save up to 40% of separate power compared to the fans required for air cooling.
• Moreover, liquid cooling can reduce the noise level. However, the complexity of the liquid cooling system and its leakage potential can be defined into direct and indirect cooling.

Indirect liquid cooling :

• Water has been an effective coolant for several industrial applications But the main challenge is the short-circuited potential for direct cooling of batteries
• To avoid the short-circuit issue, indirect methods are effective to avoid electrical conduction with cells while maintaining high thermal diffusion
• The addition of electrical resistance may resist thermal diffusion. But its effects on cooling is not significant.
• Some EV manufacturers such as General Motors, and Tesla are using indirect cooling in their cars. General Motor used cold plates between each prismatic cell. The cold plates have several micro-channels for convective heat removal from batteries.
• Tesla has used wavy tubes placed between cylindrical cells. Thermally conductive and electrically insulated material is used to fill the gaps between the battery cells and cooling ducts.
• The wavy tubes may not be effective for cooling because of the small heat transfer contact area, it is safer from the mechanical and electrical point of view. All the connections of coolants are kept outside of the enclosure to avoid the leakage of coolant.

• Liquid cooling of batteries is shown below and heat transfer takes place from the cells to cooling channels

• The design strategy of thermal management system for battery modules, controller, and electric motor.

Direct liquid cooling (Immersion) :

• Direct cooling is also called immersion cooling, covers the entire surface of the cell and cools it uniformly
• Immersion cooling removes hot or cold spots in the cell and significantly improves the performance of the cell.
• Ideal coolant mush be dielectric with minimum viscosity and high thermal conductivity and thermal capacity
• Immersion cooling is widely used for data center servers and high-power electronics devices
• BTMS for this method is not suitable for most mass-produced EV due to high cost and safety concerns
• Immersion cooling is useful for batteries of high-performing EVs and EV racing. The fluid has a boiling point between 60 – 80 °C to prevent overheating of the cells and avoid thermal runaway. Modular type containers are used for the cells where they can be submerged in the liquid

Liquid cooling Battery pack in EVs :

The following are popular battery pacts with liquid cooling in electrical vehicles

• Audi R8 e-Tron
• Ford Focus
• GM Chevrolet Bolt and Chevrolet Volt
• Tesla Model X, Model S, Model 3
• Toyota –iQ
• Volvo XC90 T8

3. Phase change material (PCM) Cooling :

• The phase change material absorbs heat significantly due to high latent heat during battery discharge
• The PCMs have been used for thermal management have a melting point in the optimum performing range of lithium cells. The cell temperature needs to remain for a long time.

• PCM as paraffin has a melting point in the range of 32-38°C which is mixed with graphite flakes to improve thermal conductivity
• The graphite flakes will also create a semi-matrix block which will contain the paraffin particles. When the paraffin melts, it stays within this matrix and the whole composition remains as the solid form
• The basic equation for phase change material (PCM): V*ρ*L = P*t, where

V = Volume of PCM in cell (m3)

ρ = Density of PCM in cell (kg/m3)

L = Latent heat of fusion of PCM (per kg)

P = Power absorbed (Watt)

t = Time of power absorption (seconds)

• The advantage of solid PCM is that they also act as shields, in case one cell enters a thermal runaway
• Classification of PCM materials for battery cooling is given below

• Multiphase modelling needs to be considered for phase change materials during the cooling of batteries

4. Heat pipe :

• Heat pipes have been considered versatile in many industrial applications for their efficient cooling and
  thermal management
• Similar to the passive cooling method of PCM, applying the heat pipes to cool or remove heat energy from the battery provides efficient
  heat transfer t low power consumption
• In this mechanism, heat is transferred through latent heat of vaporization (phase change) from the evaporator to the condense
• The working fluid can be passively transported back to the evaporator by capillary pressures it is developed within a porous wick lining. Operating in this situation the heat is continuously absorbed and released.

PTC Heater

PTC heaters utilize Positive Temperature Coefficient materials. PTC thermistors have many self-heated applications by utilizing their own voltage-current or current-time characteristics. One of the applications is a self-regulating heater known as a PTC heater. The temperature of a PTC heater can be kept at a fixed point by adjusting the resistance of the PTC heater automatically.

Comparison of cooling methods :

• Commonly used cooling methods are compared in the following table

SOLVING & MODELLING APPROACH

1. Construct a battery pack of 4x2 with an enclosure around it..
2. Mesh independence study is carried out for different mesh length of 8,6,4,2mm
3. Steady state turbulent flow is used. K-Ω SST & K-ε Std models are used.
4. The simulation is carried out till the convergence is obtained
5. Animations are created to analyse the flow patterns
6. The following studies are carried out to determine the better cooling condition for the given battery pack.
   1. Mesh Independence Study
   2. Flow model and velocity inlet study
   3. Inlet and outlet study
   4. Battery spacing study
   5. Enclosure study

PRE PROCESSING AND SOLVER SETTING

In our challenge we will create a flow simulation of air through the battery pack. In general I will be explaining only one case and posting the screenshots of the other cases.

Case 1

Mesh size - 8mm

Fluid - air, 3 m/s

Flow model - K-Ω SST

• Create the circle of diameter 18mm diameter in spaceclaim.

• Extrude the circle to create a cylinder with length of 65mm using pull tool.

• Now, to create multiple number of cylinders in order of 4x2 which is to be the Lithium-ion batteries, use pattern option under design menu tab. Give the X count and Y count as 4 and 2 respectively, also specify the X pitch and Y pitch length as 20mm which creates a 2mm gap between the cylinders.

Now, create an enclosure for the 4x2 cylindrical batteries with required dimensions

• Now, to share the information between the enclosure and the cylinders, perform share topology. This is used to create the conformal mesh on the domain.

• Use measure tool to check if the gap between any two consecutive cylinders are 2mm. Then, save the model and close the space claim.

• Open the mesh window and give name selections to every regions of the Battery pack model.

• Now, generate mesh by keeping the default mesh element size for the model.

• In this, we provide fundamental physics, types of solvers, boundary conditions, and the kind of turbulent model used.
• Initially, we go for the SOLVER option here we are the type of solver used is PRESSURE BASED SOLVER (M<0.3), and since the variable is not varying with respect to time select the STEADY STATE option.

• In the following step for the PHYSICS, the window selects the VISCOUS option we get the window opened up. Here we are selecting the type of viscous model, a TURBULENT model (k-omega standard). Since we have to calculate the temperature energy is turned on Here, we use k-omega SST turbulence model which is more effective turbulence model for this simulation since it gives more accurate results compared to other models because of the use of k-ω formulation in the inner parts of the boundary layer makes the model directly usable all the way down to the wall through the viscous sub-layer, and the SST formulation also switches to a k-ε behaviour in the free-stream and thereby avoids the common k-ω problem that the model is too sensitive to the inlet free-stream turbulence properties.

• Here for solids materials Lithium and PVC this assigned under the solids sections there are properties are stated below.

• Provide the cell zone condition of battery with heat generating source term to be 48750 W/m^3 .

• Give the necessary boundary conditions for the simulation. For inlet, it is taken as velocity inlet with velocity of air as 3m/s and temperature as 300K, and for outlet, it is taken as pressure outlet with atmospheric pressure. Since the outer wall of the enclosure will also have convective heat transfer to the surrounding air, Give the boundary condition of the convection wall to be convection thermal condition with heat transfer coefficient as 5W/m^2-K and material is taken as pvc. Parameter set is enabled for a parametric study.

• Create report definitions for average temperature of the battery, maximum temperature of the battery and heat transfer coefficient over the battery. Output parameter is enabled for parametric study.

Initialize the simulation using hybrid initialization

• Then run the calculation for a given number of iterations till the convergence is obtained. Depending upon the number of elements and model selected the time required for convergence varies. In the CFD module itself we can compute, measure, plot, animate, etc., if needed.
• After the solutions and calculations move to the result module to get different graphs, plots, contours, animations etc.,. Sectional views can be created if required.

RESULTS

Case 1

Mesh size - 8mm

Fluid - air, 3 m/s

Flow model - K-Ω SST

Inlet outlet view

Residual

Temperature and vector Plot1

Temperature and vector Plot2

Average Temperature at battery surface

Surface heat transfer coefficient at battery surface

Maximum temperature at battery surface

Animation

Case 2

Mesh size - 6mm

Fluid - air, 3 m/s

Flow model - K-Ω SST

Inlet outlet view

Residual

Temperature and vector Plot1

Temperature and vector Plot2

Average Temperature at battery surface

Surface heat transfer coefficient at battery surface

Maximum temperature at battery surface

Animation

Case 3

Mesh size - 4mm

Fluid - air, 3 m/s

Flow model - K-Ω SST

Inlet outlet view

Residual

Temperature and vector Plot1

Temperature and vector Plot2

Average Temperature at battery surface

Surface heat transfer coefficient at battery surface

Maximum temperature at battery surface

Animation

Case 4

Mesh size - 2mm

Fluid - air, 3 m/s

Flow model - K-Ω SST

Inlet outlet view

Residual

Temperature and vector Plot1

Temperature and vector Plot2

Average Temperature at battery surface

Surface heat transfer coefficient at battery surface

Maximum temperature at battery surface

Animation

Case 5

Mesh size - 4mm

Fluid - air, 3 m/s

Flow model - K-ε Std

Inlet outlet view

Residual

Temperature and vector Plot1

Temperature and vector Plot2

Average Temperature at battery surface

Surface heat transfer coefficient at battery surface

Maximum temperature at battery surface

Animation

Case 6

Mesh size - 4mm

Fluid - air, 5 m/s

Flow model - K-Ω SST

Inlet outlet view

Residual

Temperature and vector Plot1

Temperature and vector Plot2

Average Temperature at battery surface

Surface heat transfer coefficient at battery surface

Maximum temperature at battery surface

Animation

Case 7

Mesh size - 4mm

Fluid - air, 5 m/s

Flow model - K-ε Std

Inlet outlet view

Residual

Temperature and vector Plot1

Temperature and vector Plot2

Average Temperature at battery surface

Surface heat transfer coefficient at battery surface

Maximum temperature at battery surface

Animation

Case 8

Mesh size - 4mm

Fluid - air, 10 m/s

Flow model - K-Ω SST

Inlet outlet view

Residual

Temperature and vector Plot1

Temperature and vector Plot2

Average Temperature at battery surface

Surface heat transfer coefficient at battery surface

Maximum temperature at battery surface

Animation

Case 9

Mesh size - 4mm

Fluid - air, 10 m/s

Flow model - K-ε Std

Inlet outlet view

Residual

Temperature and vector Plot1

Temperature and vector Plot2

Average Temperature at battery surface

Surface heat transfer coefficient at battery surface

Maximum temperature at battery surface

Animation

Case 10

Mesh size - 4mm

Fluid - air, 3 m/s

Flow model - K-ε Std

Inlet & Outlet – Side ways

Side way Inlet outlet view

Residual

Temperature and vector Plot1

Temperature and vector Plot2

Average Temperature at battery surface

Surface heat transfer coefficient at battery surface

Maximum temperature at battery surface

Animation

Case 11

Mesh size - 4mm

Fluid - air, 3 m/s

Flow model - K-ε Std

Inlet & Outlet – Top and bottom

Top bottom Inlet outlet view

Residual

Temperature and vector Plot1

Temperature and vector Plot2

Average Temperature at battery surface

Surface heat transfer coefficient at battery surface

Maximum temperature at battery surface

Animation

Case 12

Mesh size - 4mm

Fluid - air, 3 m/s

Flow model - K-ε Std

Inlet & Outlet – 2 Inlet (front and back), 1 outlet (top)

Inlet outlet view

Residual

Temperature and vector Plot1

Temperature and vector Plot2

Average Temperature at battery surface

Surface heat transfer coefficient at battery surface

Maximum temperature at battery surface

Animation

Case 13

Mesh size - 4mm

Fluid - air, 3 m/s

Flow model - K-ε Std

Inlet & Outlet – 2 Inlet (Side ways), 1 outlet (front)

Inlet outlet view

Residual

Temperature and vector Plot1

Temperature and vector Plot2

Average Temperature at battery surface

Surface heat transfer coefficient at battery surface

Maximum temperature at battery surface

Animation

Case 14

Mesh size - 4mm

Fluid - air, 3 m/s

Flow model - K-ε Std

Inlet & Outlet – 2 Inlet (Top and bottom), 1 outlet (front)

Inlet outlet view

Residual

Temperature and vector Plot1

Temperature and vector Plot2

Average Temperature at battery surface

Surface heat transfer coefficient at battery surface

Maximum temperature at battery surface

Animation

Case 15

Mesh size - 4mm

Fluid - air, 3 m/s

Flow model - K-ε Std

Battery Spacing – 4mm

Inlet outlet view

Residual

Temperature and vector Plot1

Temperature and vector Plot2

Average Temperature at battery surface

Surface heat transfer coefficient at battery surface

Maximum temperature at battery surface

Animation

Case 16

Mesh size - 4mm

Fluid - air, 3 m/s

Flow model - K-ε Std

Battery Spacing – 6mm

Inlet outlet view

Residual

Temperature and vector Plot1

Temperature and vector Plot2

Average Temperature at battery surface

Surface heat transfer coefficient at battery surface

Maximum temperature at battery surface

Animation

Case 17

Mesh size - 4mm

Fluid - air, 3 m/s

Flow model - K-ε Std

Enclosure - 10x10x10

Inlet outlet view

Residual

Temperature and vector Plot1

Temperature and vector Plot2

Average Temperature at battery surface

Surface heat transfer coefficient at battery surface

Maximum temperature at battery surface

Animation

Case 18

Mesh size - 4mm

Fluid - air, 3 m/s

Flow model - K-ε Std

Enclosure - 20x20x20

Inlet outlet view

Residual

Temperature and vector Plot1

Temperature and vector Plot2

Average Temperature at battery surface

Surface heat transfer coefficient at battery surface

Maximum temperature at battery surface

Animation

Average temperature, maximum temperature and heat transfer coefficient of the battery wall is found out for each cases and are tabulated as above. Each of the study is discussed separately for better understanding.

MESH INDEPENDENCE STUDY

For the mesh independence study we have taken 8mm as the starting mesh size and decreased the mesh size in a step of two. So there are a total of 4 cases 8,6,4,2mm respectively. It can be noticed that the mesh element count increases drastically as the mesh size decreases. More element means more computational time.

In order to get accurate results we have to reduce the mesh element size as we see from the above graph study cases from 8mm to 2mm. Here the mesh size of 2mm has the desirable results as the temperatures are minimum and the heat transfer coefficient is high. But the element size is around 6 lakhs. Computational time required was also very high compare to others. Since there is not much difference in the results hence we can go with the optimum mesh size of 4mm.

FLOW MODEL AND INLET VELOCITY STUDY

In this study we are changing the velocity as well as the flow model used in the computation. Three different inlet velocity (3,5,10m/s) conditions are used in the study with two different flow models (K-Ω SST & K-ε Std). So there are a total of 6 cases for the comparative study. All the parameters are kept the same and the mesh size is taken as 4mm as discussed in the mesh independence study.

K-Ω SST & K-ε Std models does not give much difference in the results. Hence both the models can be used in designing. Now coming to inlet velocity it is noted that as the velocity increases the heat transfer coefficient also increases. Which means more cooling of the battery pack giving us a much lower maximum and average temperature at the battery wall. So we can come to a conclusion that the efficiency of cooling increases as the inlet velocity increases and in this case 10m/s has the highest heat transfer coefficient.

FLOW MODEL AND INLET VELOCITY STUDY

In this study we are changing the positions on the inlet and outlet and also the number of inlets. 1 Inlet and 1 outlet is varied for three different locations. Also 2 Inlets and 1 Outlet is varied for three different locations. Hence there are a total of 6 cases for this comparative study.

From the table and graph it is evident that the battery pack with 2 inlet sideways and 1 outlet in the front or back give the best cooling. It has the highest coefficient of heat transfer. It is because the area of contact increases as the air enters from both the sides and interact with all the battery walls. More the area of contact more heat transfer. So we can say that the case 13 is the most efficient cooling method out of these cases.

BATTERY SPACING STUDY

In the battery spacing study we are varying the horizontal and vertical spacing of the battery placed in the battery pack. Distance between the battery are varied for 2,4,6mm respectively. Hence there are a total of 3 cases in this study. All other parameters are kept constant.

From the table and graph it can be understand that the heat transfer increases as the battery spacing increases. But comparing 4mm and 6mm battery spacing there is not much change in the results. 2mmbattery spacing doesn’t have enough space between them to have a good flow of air. So in this case 4mm battery spacing can be used for our design.

ENCLOSURE STUDY

In this study we are changing the dimensions of the enclosure. Means the air volume is varying around\d the batteries. Here we are using a total of 3 cases with enclosure dimensions of 15x18x18, 10x10x10 & 20x20x20.

From the table and graph it can be noted that the enclosure with 10x10x10 has the highest heat transfer coefficient. Which means that this case has the highest efficiency of cooling. If the air volume around the battery is less, there is less chance for creating turbulence. Turbulence determines the flow vector of the air around the battery. Less turbulence results in even flow of air around the battery. In comparing the three cases enclosure with 10x10x10 has the least volume. So in real life application one must design the enclosure with minimum volume as possible considering the other parameters that are relevant.

CONCLUSION

1. In mesh independent study we can infer that in order to get accurate results we have to reduce the mesh element size as we see from the above study case from 8mm to 2mm. But here there is not much difference in the results hence we can go with the optimum mesh size (4mm- more the mesh elements computational time is more).
2. The temperature of the battery facing the inlet have less temperature compared to others since the heat transfer coefficient is higher over those regions.
3. The temperature of the battery near the outlet of the battery pack has the higher temperature compared to others since the air flow near to that surface is less which results in less convective heat transfer over that region. For the same gap between batteries, we can observe that with higher velocity, the average temperature decreases.
4. The average temperature values are less when the gap is 6mm compared to 2mm.
5. as the inlet velocity of air increases, the temperature of the battery decreases since the heat transfer coefficient over the battery is increased. Also for the same inlet velocity of 3 m/s, as the gap is increased between the batteries, the temperature decreases but the temperature difference between 4mm gap and 6mm gap is almost same. So, we can conclude 4mm gap and 3 m/s velocity is considered as the effective cooling criteria.