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 conta ...

in which (at least one) fluid flows in lateral confinements with typical dimensions below 1 mm. The most typical such confinement are

microchannel Micro Channel architecture, or the Micro Channel bus, is a proprietary 16- or 32-bit parallel computer bus introduced by IBM in 1987 which was used on PS/2 and other computers until the mid-1990s. Its name is commonly abbreviated as "MCA", alth ...

s, 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 channel ...

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 A gas turbine, also called a combustion turbine, is a type of continuous flow internal combustion engine. The main parts common to all gas turbine engines form the power-producing part (known as the gas generator or core) and are, in the directi ...

heat pumps A heat pump is a device that can heat a building (or part of a building) by transferring thermal energy from the outside using a refrigeration cycle. Many heat pumps can also operate in the opposite direction, cooling the building by removing h ...

Microprocessor A microprocessor is a computer processor where the data processing logic and control is included on a single integrated circuit, or a small number of integrated circuits. The microprocessor contains the arithmetic, logic, and control circ ...

and

microchip An integrated circuit or monolithic integrated circuit (also referred to as an IC, a chip, or a microchip) is a set of electronic circuits on one small flat piece (or "chip") of semiconductor material, usually silicon. Large numbers of tiny ...

cooling Cooling is removal of heat, usually resulting in a lower temperature and/or phase change. Temperature lowering achieved by any other means may also be called cooling.ASHRAE Terminology, https://www.ashrae.org/technical-resources/free-resources/as ...

air conditioning Air conditioning, often abbreviated as A/C or AC, is the process of removing heat from an enclosed space to achieve a more comfortable interior environment (sometimes referred to as 'comfort cooling') and in some cases also strictly controlling ...

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,

\mathit_c

is the

Nusselt number In thermal fluid dynamics, the Nusselt number (, after Wilhelm Nusselt) is the ratio of convective to conductive heat transfer at a boundary in a fluid. Convection includes both advection (fluid motion) and diffusion (conduction). The conductiv ...

k

is the

thermal conductivity The thermal conductivity of a material is a measure of its ability to conduct heat. It is commonly denoted by k, \lambda, or \kappa. Heat transfer occurs at a lower rate in materials of low thermal conductivity than in materials of high thermal ...

of the fluid and

d

is the

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,

\mathit_c=3.657

and for the case of constant heat flux

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

Classification of micro heat exchangers

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 conta ...

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 defined as the form of energy crossing the boundary of a thermodynamic system by virtue of a temperature difference across the boundary. A thermodynamic system does not ''contain'' heat. Nevertheless, the term is ...

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

gas Gas is one of the four fundamental states of matter (the others being solid, liquid, and plasma). A pure gas may be made up of individual atoms (e.g. a noble gas like neon), elemental molecules made from one type of atom (e.g. oxygen), or ...

, a liquid, or a

multiphase flow In fluid mechanics, multiphase flow is the simultaneous flow of materials with two or more thermodynamic phases. Virtually all processing technologies from cavitating pumps and turbines to paper-making and the construction of plastics involve so ...

) from electrically powered heater cartridges, or removed from the fluid by electrically powered elements like

Peltier Peltier is a French surname. Notable people with the surname include: * Autumn Peltier (born 2004), climate activist * Harvey Peltier, Jr. (1923-1980), American politician *Harvey Peltier, Sr. (1899-1977), American politician *Jean Charles Athanase ...

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.''R ...

References

{{reflist Microtechnology Microfluidics Heat exchangers Heat transfer

Background

Classification of micro heat exchangers

See also

References