Enhanced Heat Exchange Using Microchannel Array Architectures

School of Chemical, Biological, and Environmental EngineeringTed Carter and Paula Pérez Rodríguez

 

Metrics and Goals• Maximize preheated water temperature• Hold water at 65˚C for two seconds• Maintain a pressure drop below 7 PSI• Allow for a throughput of 20 mL/min

Cu

T preheatTc in

Th inTh out

Pressure Drop as a Function of Channel Length and Diameter

Heat Transfer Fundamentals ResultsMotivationOf all the water on Earth only 0.1% is both potable and accessible to humans. Fresh water shortage has become increasingly evident in recent years as developing countries struggle to keep booming populations hydrated. Water pasteurization is commonly used treat contaminated water sources. Solar energy is not always available, and most microbes harmful to humans can be killed at 65˚C. Since microchannels provide dramatically improved heat transfer rates as well as smaller equipment, such technology can be used to treat water continually and efficiently.

Objectives• Design a small, low-cost microchannel water pasteurization system• Model heat transfer within heat exchanger portion of device• Measure energy recovery efficiency in device and compare to model predictions

Q: Energy transfer rate (Watts)

m: Mass flow rate of liquid (kg/s)

Cp: Liquid heat capacity (J/kg K)

U: Overall heat transfer coefficient

ΔT: Log mean temperature difference

for parallel plates

Heat recovery zone of device uses copper metal to conduct energy from the hot to cold water stream. Thickness of copper plate is 5 to 8 times greater

than channel height.

h: Convective heat transfer coefficient

d: Characteristic channel dimension

k: Thermal conductivity of fluid

μ: Viscosity V: Volumetric flow

L: Length D: Channel height

References: Introduction to Fluid Mechanics by Fox et al, Fundamentals of Momentum, Heat, and Mass Transfer by Welty et al.

This project was made possible by Todd Miller and Steve Leith, our sponsors, and Dr. Philip Harding for project guidance.

Think BIG, build SMALLThink BIG, build SMALL

Issues•The pump does not have enough volume capacity to reach steady state.•Tc1 thermocouple does not work.

•The heater does not have enough capacity to reach 65°C at high flow rates•Thermocouple joints leak.

Testing deviceThe pump forces water into the system, which heats the water using a potentiometer controlled by a feedback loop. The inlet and outlet temperatures for hot and cold water are recorded.

• A car battery has 12V and 40Ah This device can run for 32h

• 10 gal (38 L) per battery• Device can be used in a stationary outlet or

for emergency purposes.

Economic analysis

For 8 and 5 copper plates, the heat transfer was worse than predicted, but for 1 copper plate the heat transfer was better than predicted,. This suggests that the copper plate temperature is not uniform.The model assumed the system to be adiabatic, but the heat loss was found to be ~28%, which may explain the differences between the model and the experiment for 5 and 8 copper plates.

Looking at the heat transfer properties, the system has its best result at higher flow rates and smaller channel heights, but the number of copper plates is not statistically relevant.

The maximum pressure drop allowed for this system will be 7 psi in order to minimize the size of the pump needed. Since the system has a total length of 8 inches, the minimum diameter allowed will be 250 µm for the target flow rate of 20 mL/min.