Micro heat exchangers, Micro-scale heat exchangers, or microstructured heat exchangers are

heat exchangers A heat exchanger is a system used to transfer heat between a source and a working fluid. Heat exchangers are used in both cooling and heating processes. The fluids may be separated by a solid wall to prevent mixing or they may be in direct contac ...

in which (at least one)

fluid In physics, a fluid is a liquid, gas, or other material that may continuously motion, move and Deformation (physics), deform (''flow'') under an applied shear stress, or external force. They have zero shear modulus, or, in simpler terms, are M ...

flows in lateral confinements with typical dimensions below 1 mm. The most typical such confinement are microchannels, which are channels with a

hydraulic diameter The hydraulic diameter, , is a commonly used term when handling flow in non-circular tubes and channels. Using this term, one can calculate many things in the same way as for a round tube. When the cross-section is uniform along the tube or channe ...

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 A heat pump is a device that uses electricity to transfer heat from a colder place to a warmer place. Specifically, the heat pump transfers thermal energy using a heat pump and refrigeration cycle, cooling the cool space and warming the warm s ...

Microprocessor A microprocessor is a computer processor (computing), processor for which the data processing logic and control is included on a single integrated circuit (IC), or a small number of ICs. The microprocessor contains the arithmetic, logic, a ...

and

microchip An integrated circuit (IC), also known as a microchip or simply chip, is a set of electronic circuits, consisting of various electronic components (such as transistors, resistors, and capacitors) and their interconnections. These components a ...

cooling Cooling is removal of heat, usually resulting in a lower temperature and/or Phase transition, phase change. Temperature lowering achieved by any other means may also be called cooling. The Heat transfer, transfer of Internal energy, thermal energ ...

air conditioning Air conditioning, often abbreviated as A/C (US) or air con (UK), is the process of removing heat from an enclosed space to achieve a more comfortable interior temperature, and in some cases, also controlling the humidity of internal air. Air c ...

Background

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

h=\mathit_c \frac

Where

h

is the

heat transfer coefficient In thermodynamics, the heat transfer coefficient or film coefficient, or film effectiveness, is the Proportional (mathematics), proportionality constant between the heat flux and the thermodynamic driving force for the Heat transfer, flow of heat ...

\mathit_c

is the

Nusselt number In thermal fluid dynamics, the Nusselt number (, after Wilhelm Nusselt) is the ratio of total heat transfer to conductive heat transfer at a boundary in a fluid. Total heat transfer combines conduction and convection. Convection includes both ...

k

is the

thermal conductivity The thermal conductivity of a material is a measure of its ability to heat conduction, conduct heat. It is commonly denoted by k, \lambda, or \kappa and is measured in W·m−1·K−1. Heat transfer occurs at a lower rate in materials of low ...

of the fluid and

d

is the

of the channel or duct. In internal

laminar flow Laminar flow () is the property of fluid particles in fluid dynamics to follow smooth paths in layers, with each layer moving smoothly past the adjacent layers with little or no mixing. At low velocities, the fluid tends to flow without lateral m ...

s, the Nusselt number becomes a constant. This is a result which can be arrived at analytically: For the case of a constant wall temperature,

\mathit_c=3.657

and for the case of constant

heat flux In physics and engineering, heat flux or thermal flux, sometimes also referred to as heat flux density, heat-flow density or heat-flow rate intensity, is a flow of energy per unit area per unit time (physics), time. Its SI units are watts per sq ...

\mathit_c=4.364

for round tubes.Incropera & Dewitt The last value is increased to 140/17 = 8.23 for flat parallel plate

As

Reynolds number In fluid dynamics, the Reynolds number () is a dimensionless quantity that helps predict fluid flow patterns in different situations by measuring the ratio between Inertia, inertial and viscous forces. At low Reynolds numbers, flows tend to ...

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. Their positive results led to further research ranging from classical investigations of single channel heat transferSantiago, Kenny, Goodson, Zhang to more applied investigations in parallel micro-channel and micro scale

plate fin heat exchanger A plate-fin heat exchanger is a type of heat exchanger design that uses plates and finned chambers to transfer heat between fluids, most commonly gases. It is often categorized as a compact heat exchanger to emphasize its relatively high heat trans ...

s. Recent work in the field has focused on the potential of two-phase flows at the micro-scale.Mudawar

Classification

Just like "conventional" or "macro scale"

heat exchanger A heat exchanger is a system used to transfer heat between a source and a working fluid. Heat exchangers are used in both cooling and heating processes. The fluids may be separated by a solid wall to prevent mixing or they may be in direct contac ...

s, micro heat exchangers have one, two or even three
Noel C. Willis, Jr. "Analysis Of Three-Fluid, Crossflow Heat Exchangers." NASA Technical Report, National Aeronautics and Space Administration, Washington, D. C. May 1968, p. 53. fluidic flows. In the case of one fluidic flow,

heat In thermodynamics, heat is energy in transfer between a thermodynamic system and its surroundings by such mechanisms as thermal conduction, electromagnetic radiation, and friction, which are microscopic in nature, involving sub-atomic, ato ...

can be transferred to the fluid (each of the fluids can be a

gas Gas is a state of matter that has neither a fixed volume nor a fixed shape and is a compressible fluid. A ''pure gas'' is made up of individual atoms (e.g. a noble gas like neon) or molecules of either a single type of atom ( elements such as ...

, a

liquid Liquid is a state of matter with a definite volume but no fixed shape. Liquids adapt to the shape of their container and are nearly incompressible, maintaining their volume even under pressure. The density of a liquid is usually close to th ...

, or a

multiphase flow In fluid mechanics, multiphase flow is the simultaneous Fluid dynamics, flow of materials with two or more thermodynamic Phase (matter), phases. Virtually all processing technologies from Cavitation, cavitating pumps and turbines to paper-making ...

) from electrically powered heater cartridges, or removed from the fluid by electrically powered elements like Peltier chillers. In the case of two fluidic flows, micro heat exchangers are usually classified by the orientation of the fluidic flows to another as "cross flow" or " counter flow" devices. If a chemical reaction is conducted inside a micro heat exchanger, the latter is also called a

microreactor A microreactor or microstructured reactor or microchannel reactor is a device in which chemical reactions take place in a confinement with typical lateral dimensions below 1 mm; the most typical form of such confinement are microchannels. M ...

References

{{reflist Microtechnology Microfluidics Heat exchangers Heat transfer

Background

Classification

See also

References