P15441: Mini-Air
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Sub-Systems Design

Table of Contents

All documents listed on this page can be found in the Sub-Systems Design Documents directory.

Sub-System Overview and Spec Distribution

Our system consists of 4 sub-systems: The thermal, electrical, airflow, and user sub-systems. The user sub-system is simply defined with the interface being the on-off switch. The other systems are detailed in the later sections. Overall, the sub-systems map should map as shown in the table below.
Sub-Systems Thermal Airflow Electrical
Weight 55% 35% 10%
Cost 45% 45% 10%
Power 65% 35% 5%

Thermal Sub-System

The thermal sub-system consists of the thermoelectric and heatsinks on either side to increase the surface area and heat transfer. An overview of the sub-system is shown below.
Thermal Sub-System

Thermal Sub-System


Currently, the heat sinks in our mock-ups are the best fit for our design. More analysis is going to be performed to see if a machined heatsink that we design will be better. With the number of fins and thin air slots, a significant pressure drop is being seen in the mock-up. With a machined heatsink, we want to see if this can be avoided and the effect that it has. An image of the heatsinks we bought on the thermoelectric for our initial mock-up is shown below.
Heatsinks on Thermoelectric inside our initial mock-up

Heatsinks on Thermoelectric inside our initial mock-up



Airflow Sub-System

For the airflow sub-system consists mostly of the fan but also the nozzle to distribute the cool air to the user. For the fan, we have been looking into DC brushless fans. The fan used in our mock-up is a 40x40x10mm 7CFM fan. The nozzle we plan on making small and wide to gain a wide distribution on the user's neck but also getting a greater velocity at the user. The following pictures show the fan in our initial mock-up and the nozzle in our second mock-up.
DC fan in initial mock-up

DC fan in initial mock-up



Nozzle outlet in the bottom left corner of photo

Nozzle outlet in the bottom left corner of photo

Electrical Sub-System

The electrical sub-system consists of a number of different components, mainly being the battery and power regulation or boosting for the fan and thermoelectric. Another part of the electrical sub-system is the switch that needs to be interfaced for the user. An overview of the electrical sub-system is shown below along with switch options.
Image:Insert Electrical sub-system diagram

Switch Options for User Interface

Switch Options for User Interface


Another big component of the electrical sub-system is the battery selection. Below is a table of power densities for different types of batteries. It shows that the best case currently is a Li-ion battery with 3.7V. This would require a booster to reach the fan and thermoelectric voltages however. Depending on how much power is needed in total, one cell or two cells may be chosen for the battery. This also comes with multiple trade-offs in size, weight, and cost.

Mock-Ups

Our initial mock-up consisted of foam that we cut to house all the components. It didn't perform well and with the porous foam, we wanted to recreate it with modifications. Our initial mock-up didn't dissipate heat well enough, causing the thermoelectric to overheat entirely. With the second mock-up, we were able to get a cooling effect and better flow at the exit. The following link goes to the pictures for our mock-ups. Some can be seen above in the previous sections. To obtain readings for the mock-ups, we used an anemometer, thermocouples, and a thermocouple reader. These allowed us to obtain ambient temperature, fan velocity, exit velocity, and hot and cold side temperature readings at the heat sinks. The readings are as show below.
Mock-Up Fan Velocity Exit Velocity Ambient Temperature Cold Side Temperature Hot Side Temperature
Initial 2.5 m/s 0.24 m/s 71oF 74oF 121oF
Second 2.5 m/s 0.45 m/s 75oF 64oF 108oF

Test Plans

Risk Assessment

Our updated risk assessment is as shown below. The file can be found here.
Updated Risk Assessment

Updated Risk Assessment

Sub-Systems Design Review


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