Micro heat exchanger

Micro heat exchangers, Micro-scale heat exchangers, or microstructured heat exchangers are heat exchangers in which (at least one) fluid flows in lateral confinements with typical dimensions below 1 mm. The most typical such confinement are microchannels, which are channels with a hydraulic diameter below 1 mm. Microchannel heat exchangers can be made from metal or ceramic.

Microchannel heat exchangers can be used for many applications including:

high-performance aircraft gas turbine engines

heat pumps

Microprocessor and microchip cooling

air conditioning

Background

Investigation of microscale thermal devices is motivated by the single phase internal flow correlation for convective heat transfer:

h

=

N

u

c

k

d

{\displaystyle h={\mathit {Nu}}_{c}{\frac {k}{d}}}

Where

h

{\displaystyle h}

is the heat transfer coefficient,

N

u

c

{\displaystyle {\mathit {Nu}}_{c}}

is the Nusselt number,

k

{\displaystyle k}

is the thermal conductivity of the fluid and

d

{\displaystyle d}

is the hydraulic diameter of the channel or duct. In internal laminar flows, the Nusselt number becomes a constant. This is a result which can be arrived at analytically: For the case of a constant wall temperature,

N

u

c

=

3.657

{\displaystyle {\mathit {Nu}}_{c}=3.657}

and for the case of constant heat flux

N

u

c

=

4.364

{\displaystyle {\mathit {Nu}}_{c}=4.364}

for round tubes. The last value is increased to 140/17 = 8.23 for flat parallel plates.[2] As Reynolds number is proportional to hydraulic diameter, fluid flow in channels of small hydraulic diameter will predominantly be laminar in character. This correlation therefore indicates that the heat transfer coefficient increases as channel diameter decreases. Should the hydraulic diameter in forced convection be on the order of tens or hundreds of micrometres, an extremely high heat transfer coefficient should result.

This hypothesis was initially investigated by Tuckerman and Pease.