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Abstract of "HEAT TRANSFER AND GAS FLOW IN PLANAR MICRO HEAT EXCHANGER CHANNELS"

DOI: 10.1615/ICHMT.2000.TherSieProcVol2.270
95 pages

Yury A. Shevich
Department of Refrigeration, Cryogenics, Air Conditioning and Life Support Systems, Bauman Moscow State Technical University, 5, 2nd Baumanskaya, 107005, Moscow, Russia

Tatiana K. Danilenko
Department of Refrigeration, Cryogenics, Air Conditioning and Life Support Systems, Bauman Moscow State Technical University, 5, 2nd Baumanskaya, 107005, Moscow, Russia

Abstract

Complex research of heat transfer and gas flow in the microchannels of planar heat exchangers (PHE) is carried out. Such PHE are generally designed for microrefrigerators operating on Joule-Thomson cycle (with throttle expansion), and can be used in the cryogenic thermostatting systems of small required refrigerating capacity for different purposes.
It is a specificity of J-T microrefrigerator that the PHE forms basically the whole refrigerator construction, and defines generally its performance. Planar refrigerator constructions are manufactured with applying the novel technologies to form microchannels, and subsequent diffusion welding of the PHE thin plate elements. Various shapes and dimensions of the microchannels configured and manufactured in the thin metallic plates or disks composing the PHE, are considered.
The plain PHE channels under investigation were of different form and configuration, namely, straight extending channels, bent with angles 90° and 180°, those like Archimedean spiral, channels filled with porous microstructure. The channel depth varied over a range of 0.1...0.5 mm, width 0.5...2.0 mm and up, the spiral channel full length 25...250 mm. Air, nitrogen, argon or gas mixtures serve as working substances (refrigerants, heat carriers).
The microchannel configuration and size influence significantly the main gas flow process parameters such as heat transfer intensity and hydraulic loss, and, respectively, the general PHE and refrigerator as a whole performance and efficiency. Heat transfer coefficient h and hydraulic friction coefficient ξ serve to estimate the processes numerically, and these very parameters are necessary to be known for designing the PHE and J-T refrigerator. Present research deals with theoretical and some cases of experimental investigation of the main PHE characteristics h and ξ.
The first method is applied for all the channels except those filled with porous microstructure. The program complex is used to compute three-dimensional parabolic flows in rectangular and cylindrical co-ordinate systems. The cross-section-averaged values h and ξ were determined, as well as length-averaged hm and ξm. The problems were solved for given inlet velocity and temperature, and different pressures of gaseous heat transfer medium (6...20 MPa); channel wall temperature varied along the PHE from 280 K to 80 K in response to linear and exponential laws.
Thermal and hydraulic characteristics of the channels with more complicated (filling with porous structure) internal structure are determined experimentally and presented graphically in the traditional dimensionless coordinates.
In subsequent experiments carried out on the microrefrigerator models with PHE operating with different heat transfer media, their serviceability, and accuracy of the developed thermo- hydraulic simulation methods were confirmed.

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