P17280: Hot Wheelz Thermal Management System
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Detailed Design

Table of Contents

Project Objective Statement

Quantify and optimize heat flow through the vehicle's electrical system.

Engineering Requirements (Specifications)

Customer Requirements

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The full revision history can be viewed here.

Engineering Requirements

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The full revision history can be viewed here.

Major Changes: Provided values for the battery operating temperature and the amount of heat transfered. New engineering requirement pertaining to the motor controller high voltage wire strain relief.

Note: For full mapping of specs and customer requirements, please see the House of Quality document.

Controller

Finalized Concept Selection

Mounting Location

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Note: Heat sink is exposed to air on all sides except for area covered by controller.

Heat Sink Design

As of last phase, the controller was going to be mounted to a single plate that would as a heat sink. As per Formula Hybrid Rule EV 3.3.1:

"Cable or conduit exiting or entering a tractive system enclosure or the drive motor must use a liquid-tight fitting proving strain relief to the cable or conduit such that it will withstand a force of 200N without straining the cable."

Our design was modified to comply with this rule and alleviate strain on the conduit. 4 metal posts are now being used to hold up a lid where the conduit fittings will be placed.

This rule is tested at competition using a spring scale. The Hot Wheelz team is currently looking into purchasing one a spring scale to use for testing prior to the competition.

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Important Design Parameters

Proof of Concept

Justification of Design Parameters

Theoretical Calculations The PDF for the current motor controller heat sink analysis can be found here.

Note: The following analysis was reviewed by Dr. Rob Stevens, a professor in Mechanical Engineering.

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The largest assumption made in this analysis was the power dissipated. This parameter is still unknown for the actual system, and the power is based off a proprietary spec sheet from Curtis. Further testing is needed to determine whether this number is accurate. Another number that changed from the last phase is the convection coefficient. An error was found in the previous calculations and has been fixed here.

The worst case scenario is based off the endurance event during competition. In this event, the car is traveling around a track for as long as it can go. There is a significant hill on this course which we determined would cause the biggest load on the controller. The car will be traveling about 9 m/s on this track and the convective coefficient was based off of this speed.

Based on the current calculations, fins will not be required to keep the controller below 85 degrees C, the point at which performance degradation begins. We will validate this assumption in later phases.

Simulation

Note: The following analysis was reviewed by Dr. Rob Stevens, a professor in Mechanical Engineering.

Thermal simulation was done in Solidworks Simulation using the results of the above calculations. Simulation was done to confirm the surface temperature results from above, and to get a better idea of the temperature gradient over the plate's surface.

Parameters:

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The temperature will be highest in the center of the plate, where the controller sits. The lowest temperature is 120F at the edges farthest away from the controller, and 152F in the center.

The steady state temperature range shown by this model implies that the controller’s temperature will never be within 20 degrees of the temperature at which performance begins to degrade (185F).

These results also imply that we don’t need to utilize fins in our design. Although fins will make heat transfer more efficient, they are not necessary to keep our controller within our desired temperature range.

Testing

Wiring Testbench - full pdf here.

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Testbench on Dyno

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The test plan for running the Hot Wheelz test bench on the RIT FSAE shop dyno can be found here

Several problems were encountered when trying to execute the test plan. The Hot Wheelz program designed specifically for the test bench did not work. To work around this, our team decided to run the dyno manually. During testing, the batteries were measured and found to be gaining voltage, indicating that the electric motor might be acting as a generator. The dyno might have been over-torquing the motor to cause this to happen. Since we were unable to find a SME on the manual mode of the dyno, we could not determine if this was the problem or not. With the phase coming to an end, and the dyno being requested for other teams' projects, testing could not be completed on time.

Testing to be Done

Testing to determine whether the heat sink and airflow are effective are important to for validating our design. Designing off of the team specs is good enough for now, but how the heat sink actually performs is what really matters.

Heat sink testing will be done by mounting an external heater to the heat sink and have a fan blowing air across it. Temperature sensors will be placed at various locations on the heat sink. This test will be done during Intersession.

Theoretical Models

The heat sink motor controller resistance model (shown above) developed for this phase will be used for testing the heat sink. The power will change based on how much heat the external heater is producing and the h value if we decide to run different speeds of air across it. We will calculate surface temperature based on these parameters to validate that the heat sink works like we predict it will.

If it is found during testing that the heat sink alone is inadequate, theoretical models for the resistance needed to remove the heat will be needed. This in turn would be used to purchase fins that would add the right amount of resistance. Another option would be to use this data to design fins, and a model would need to be developed to explore this option. However, this can not be determined until more testing is complete.

Benchmarking

Originally benchmarked using a fan in addition to the heatsink but deemed this as unnecessary. Benchmarking information can be viewed on the PDDR page.

Detailed Design of System, Subsystem, and Parts

Heat Sink Design

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Heat Sink Detailed Drawing

Lid Detailed Drawing

Post Detailed Drawing

Procurement

Manufacturability

Assembly

Bill of Materials

A full revision history can be found here

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The estimated total cost of all components in the controller assembly is ~$170. before shipping. The components from McMaster will be ordered with other Hot Wheelz parts so the shipping costs will be minimal.

Test Plans

Heat Sink Validation

The link to the motor controller heat sink test plan is here

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This test will be able to gather temperature data of the motor controller heat sink sub system under an induced load independently of any other components. This will be verification that the design works as predicted.

Waterproofing Validation

Will test waterproofing by spraying with water from a certain distance away to simulate the IP rating requirements.

Strain Relief Validation

Will pull on cables (with system off) to see if movement occurs.

Alternatively, can purchase a 200N capacity spring scale as called out in the rules (~$150).

Batteries

Finalized Concept Selection

Battery Box Cooling

Vehicle Location
The battery box will be located in the frame behind the driver as shown below.

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Note: The motor controller "lid" was hidden in these assemblies to better view the location of the battery box.

Battery Box
The complete battery box cooling system will consist of fans and vents to bring air into the box and exhaust air out of the box. A total of 5 fans will be mounted on the battery box and vent slits will be cut into panels of the box. To allow for more control over that air flow that occurs in the battery box, panels will be used to "seal" the battery structures. Therefore, each structure will be independently cooled.

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Important Design Parameters

Battery Structures

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Important Design Parameters

Proof of Concept

Justification of Design Parameters

Fan Circuitry Design
Fan Circuitry Design Justification - File

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A full speed fan system would be simpler than a variable speed system which would help ensure that the team meets CR7: System is easy to install and service. A simpler system would also aid the team in meeting CR12: System is built and tested in time for competition. A MOSFET device has never been used by the team before and will add complexity to the thermal management system which could require more troubleshooting and testing.

The team is also aiming to only utilize the thermal management system when the batteries reach a critical temperature before they start to degrade. At this point in time, it may be necessary to have a fan blowing at full speed to ensure the batteries stay under the degradation temperature.

Testing So Far

Prototype Construction
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Prototype Trial 1
Purpose:

The first trial of this test is to observe whether there was sufficient airflow near the front corner of the prototype. This is an area of concern because there are several batteries clustered in the area and there is the possibility for stagnant airflow in that section. If these batteries were to overheat due to poor air circulation, there is a risk that the vehicle would shut down during operation at competition.

Procedure:

No Barrier Trial - Video

Conclusions:

A patch of stagnant air was observed in the front corner of the prototype. A design is needed to direct the air to that area of the structure to ensure those batteries see sufficient airflow. A barrier will be added inside the structure to attempt to achieve this result.

Prototype Trial 2
Purpose:

The second trial of this test is to observe if a barrier will allow for more airflow in the front corner of the prototype. A paper "barrier" was added into the prototype to observe the results.

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Procedure:

With Barrier Trial - Video

Conclusions:

The barrier provided a channel to direct air to the front corner of the prototype. The fog had sufficient movement in the front corner and exited the prototype through the fan. A barrier in this location will be needed in the final design to allow for the air to reach those corner batteries and ensure sufficient heat transfer.

Testing to be Done

Testing to see if the fans, vents and internal barriers are good enough to keep the batteries under the cut off temperature will be critical for justifying the sub-system. Although testing done so far has been good at showing the direction of the airflow (and that the internal barrier effectively moves it to parts of the battery structure needed) whether or not the flow is enough to remove the heat required is yet to be tested. This will be done in MSD II.

Theoretical Models

Consulted with Robert Stevens, Heat Transfer Professor.

A detailed working document can be viewed here.

The MATLAB script can be viewed here.

The flow over a bank of tubes approach was used to analyze the heat transfer characteristics of the batteries when they are assembled in their structures. All equations, properties, correction factors, and coefficients were directly taken from the Fundamentals of Heat and Mass Transfer, 7th Edition textbook, which can be found here.

Purpose:

  1. Identify the amount of heat a battery structure will generate in an endurance scenario (ER10)
  2. Identify the amount of heat a fan can remove when running at a specified speed
  3. Identify the pressure drop across a battery structure that a fan will have to overcome
  4. Identify the surface temperature of the battery cell that will signal to power on the fans (ER8)

Assumptions:

  1. Continuous current through a battery cell is 30A during the endurance event
  2. The battery cell’s internal resistance is 8 milli-ohms;
  3. Each battery structure will generate 20% of the total heat generated by the entire battery pack (5 structures total)
  4. A battery structure will be represented by a staggered bank of tubes consisting of 11 rows of 4 cells each
  5. Inlet air temperature into the battery structure is 18 degrees celsius
  6. Inlet air velocity is equivalent to the fan blade velocity

Process:

  1. Assume the inlet air temperature and the battery surface temperature (18C and 55C, respectively).
  2. Find air properties at the inlet air temperature (at 18C).
  3. Compute the heat generated for one battery structure based on the battery cell’s internal resistance and the nominal current at a 2C discharge rate.
  4. Assume the air inlet velocity (0.314 m/s or 1.03 ft/s).
  5. Compute the Reynolds number for the air.
  6. Compute the Nusselt number using the Reynolds number.
  7. Compute the convective heat transfer coefficient using the Nusselt number.
  8. Find the outlet air temperature using the convective heat transfer coefficient.
  9. Find the mean temperature between the inlet and calculated outlet temperature
  10. Find air properties at this mean temperature.
  11. Repeat steps 5 through 8 to find the new Reynolds number, Nusselt number, convective heat transfer coefficient, and outlet air temperature. These are now more accurate with the new air properties.
  12. Find the log-mean temperature difference using the new outlet air temperature.
  13. Compute the heat transfer rate using the log-mean temperature difference.
  14. Compute the pressure drop. Find the correction factor and friction factor using the Reynolds number.

Results:

Conclusions:

Because the velocity of the air entering the battery structures is not equivalent to the velocity of the air leaving the battery structures through the fans, this analysis does not give insight to what speed the fans should run at. This is an important parameter when picking a fan.

However, the analysis does give insight on how much pressure drop occurs within the structures. Another parameter used when picking a fan is its static pressure. The fan should be rated so that it can overcome the pressure drop within the structure. This allows the air to flow into and out of the structures. To be safe, we are assuming that the calculated pressure drop within the structures is a low value. All fans will be sourced to have a static pressure of at least 25 Pa to ensure that it can overcome the pressure within the structure.

The analysis assumed that the temperature of the cells was at 55 degrees celsius. We will be using this temperature as the point when the fans should turn on. This is because the amount of heat transferred is much greater when there is a larger temperature difference. Future testing will determine is this is an acceptable tripping temperature. At this moment, we are comfortable with this temperature and have updated engineering requirement ER8 to reflect this.

Based on the heat generation calculations done in this analysis, engineering requirement ER10 has been updated. The thermal management system should be expected to remove at least the amount of heat that the batteries generate, if not more.

Benchmarking

Relays
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Wired in relay that Hot Wheelz had in inventory. Successfully worked with fan! Full test plan can be viewed here.

Fans

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The full document can be viewed here.

Vents

Decided to create our own "vents" by cutting slits in the box to reduce manufacturing time and prevent issues with sizing. It would also be easier to ensure that the "vents" pass the finger probe test.

Thermal Insulation
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The most current revision can be viewed here

Detailed Design of System, Subsystem, and Parts

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Main Battery Pack Panels

Back Panel Drawing

Rear Battery Pack Panels

Front Panel Drawing

Bottom Panel Drawing

Back Panel Drawing

Bracket Drawing

Battery Box Component Placement

Battery Box Top Plate Drawing

Battery Box Back Plate Drawing

Battery Box Bottom Plate Drawing

Electrical Schematics

PDF Electrical Schematic - File

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Note: This was incorporated into Hot Wheelz's template and their current schematic to save time.

Procurement

Tools and Jigs

  1. ShopSabre 4860 CNC Router
  2. CAM software
  3. Sandpaper
  4. Measuring equipment
  5. Drill Press
  6. Power drill
  7. Clamps
  8. Knife

Preliminary Fabrication Procedure

Anticipated Fabrication Issues

Bill of Materials

A full revision history can be found here.

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The battery cooling system will cost about $295 before shipping costs.

Test Plans

Battery System Validation

A latest revision for the battery sub-system test plan can be found here

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Waterproof Validation

Will spray with water from a certain distance. Will observe how much water is collected and if anything turns off.

Safety Validation

Will use pen (or something smaller in diameter) to try to poke through different areas of the box to simulate the finger probe test.

System Design Layout

Functional Decomposition

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System Architecture

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Full revision history here.

Risk Management

Risk Management Update

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A working document can be found here.

Plot of Total Risk Importance

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Risks Already Resolved:

Risks Resolved by Subsystem Build & Test Phase

Risks Resolved by System Build & Integrate Phase

Addressing High Risks

System Changes due to Hot Wheelz Changes

System not Completed before Competition

Thermal System does not Cool Components

Project Plan & Budget

Project Schedule

Full PDF here.

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Intersession Highlights - note that all of these depend on the completion of tasks from Hot Wheelz so MSD team will be jumping in to help as needed!

Week 1 of January

Week 2 of January

Week 3 of January

Work Breakdown Structure

Actual from Phase 4 - full PDF here.

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Projected for MSD II

We are all planning to be heavily involved in all aspects of the build, test, and integration phases. However, we decided to identify "roles" for the next semester so that someone is in charge of thinking about all the little details for that aspect of the project!

Projected Costs

Action Items from Review

See full document of notes and action items here.


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