micro heat exchanger
File:Scambiatori_di_calore_a_microcanali.png 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.Kee, Robert J., et al. "The design, fabrication, and evaluation of a ceramic counter-flow microchannel heat exchanger." Applied Thermal Engineering 31.11 (2011): 2004-2012.
Microchannel heat exchangers can be used for many applications including:
- high-performance aircraft gas turbine enginesNorthcutt, B., & Mudawar, I. (2012). Enhanced design of cross-flow microchannel heat exchanger module for high-performance aircraft gas turbine engines. Journal of Heat Transfer, 134(6), 061801.
- heat pumpsMoallem, E., Padhmanabhan, S., Cremaschi, L., & Fisher, D. E. (2012). Experimental investigation of the surface temperature and water retention effects on the frosting performance of a compact microchannel heat exchanger for heat pump systems. international journal of refrigeration, 35(1), 171-186.
- Microprocessor and microchip coolingSarvar-Ardeh, S., Rafee, R., Rashidi, S. (2021). Hybrid nanofluids with temperature-dependent properties for use in double-layered microchannel heat sink; hydrothermal investigation. Journal of the Taiwan Institute of Chemical Engineers. cite journal https://doi.org/10.1016/j.jtice.2021.05.007
- air conditioningXu, B., Shi, J., Wang, Y., Chen, J., Li, F., & Li, D. (2014). Experimental Study of Fouling Performance of Air Conditioning System with Microchannel Heat Exchanger.
Background
Investigation of microscale thermal devices is motivated by the single phase internal flow correlation for convective heat transfer:
:
Where is the heat transfer coefficient, is the Nusselt number, is the thermal conductivity of the fluid and 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, and for the case of constant heat flux for round tubes.Incropera & Dewitt{{full citation needed|date=November 2012}} The last value is increased to 140/17 = 8.23 for flat parallel plates.[http://www.calpoly.edu/~kshollen/ME350/Examples/Example_14.pdf] 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.{{cite journal |doi=10.1109/EDL.1981.25367 |title=High-performance heat sinking for VLSI |year=1981 |last1=Tuckerman |first1=D.B. |last2=Pease |first2=R.F.W. |journal=IEEE Electron Device Letters |volume=2 |issue=5 |pages=126–9|bibcode=1981IEDL....2..126T |s2cid=40590765 }}{{psc|date=May 2012}} Their positive results led to further research ranging from classical investigations of single channel heat transferSantiago, Kenny, Goodson, Zhang{{full citation needed|date=November 2012}} 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.{{cite journal |doi=10.1016/j.ijmultiphaseflow.2003.09.004 |title=Forced convective boiling heat transfer in microtubes at low mass and heat fluxes |year=2003 |last1=Yen |first1=Tzu-Hsiang |last2=Kasagi |first2=Nobuhide |last3=Suzuki |first3=Yuji |journal=International Journal of Multiphase Flow |volume=29 |issue=12 |pages=1771–92|bibcode=2003IJMF...29.1771Y }}{{cite journal |doi=10.1115/1.1778187 |title=An Experimental Investigation of Flow Boiling Characteristics of Water in Parallel Microchannels |year=2004 |last1=Steinke |first1=Mark E. |last2=Kandlikar |first2=Satish G. |journal=Journal of Heat Transfer |volume=126 |issue=4 |pages=518}}Mudawar{{full citation needed|date=November 2012}}
Classification
Just like "conventional" or "macro scale" heat exchangers, micro heat exchangers have one, two or even three[https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19680014044.pdf] 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 can be transferred to the fluid (each of the fluids can be a gas, a liquid, or a multiphase flow) 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.