Refrigerant distribution effects on the performance of microchannel evaporators.
The performance of air to refrigerant heat exchangers can be affected greatly by both refrigerant and air flow distribution. Maldistribution of either fluid stream can lead to reduced heat transfer effectiveness of the heat exchanger and thus lower system efficiency and capacity. Microchannel evaporators are especially susceptible to refrigerant maldistribution as they typically have many parallel channels fed by a common header or manifold. This paper outlines general trends in refrigerant flow distribution in such evaporators taking into account various heat exchanger orientations. In addition, links between heat exchanger performance and refrigerant flow distribution characteristics in microchannel evaporators are made using both analytical and experimental methods. These results show that unification of the liquid refrigerant flow distribution in the evaporator can lead to quantifiable improvements in evaporator performance.
Oscillating flow in a heat sink with parallel micro channels
The performance of two phase heat sinks can be greatly affected by the coolant flow instabilities. For the applications in electronic cooling system, the micro-channel heat sink usually consists of some parallel channels and common header or manifold, the flow instabilities can be an especially noticeable problem. Flow instabilities will cause a severe reverse flow and subsequent maldistribution of cooling medium in the header. In addition, the instabilities degrade the total heat transfer performance of the heat sink and lead to a remarkable variation of pressure drop. As the cooling medium coming from the upstream pipe is always in two-phase flow state, the common header filled with this coming vapor will serve as a buffer tank. This tank provides significant compressible volume to the downstream heated channels and may lead to sustained pressure and temperature oscillations in the system. In this paper, the periodic reversed flow and maldistribution of fluid in a heat sink with parallel micro channels are studied experimentally. Visualization results are presented simultaneously. The effects of channel size, surface tension and fluid viscosity are considered. The results show that the micro-channel heat sinks are especially susceptible to flow instabilities and fluid maldistribution. The channel size and the surface tension effect on the reverse flow as well. And the heat transfer performance deteriorates when the ammonia reverse flow occurs.
Chemosymbiotic bivalves and stable carbon isotopes indicate hydrocarbon seepage at four unusual Cenozoic fossil localities
Four unusual Cenozoic fossil localities are identified here as ancient hydrocarbon seep sites using palaeontological, petrological, and stable carbon isotope data. Late Eocene carbonate-cemented sandstone bodies in the Wagonwheel Mountains in California were previously suspected to represent ancient hydrocarbon seep sites, but the relatively high δ13Ccarbonate values (−12.2 to −5.0‰) suggest oil rather than methane seepage. The Oligocene fauna of the Elmira asphalt mine, Cuba, was previously interpreted as a mix of freshwater and marine taxa, but all species are here identified as belonging to marine groups, including the bivalve families Lucinidae and Vesicomyidae, whose extant members live largely in symbiosis with chemoautotrophic endosymbionts. A carbonate concretion from this site showed δ13Ccarbonate values as low as −32.2‰, which most likely indicates methane seepage. A previously unpublished Oligocene fossil locality in Atlantico, northern Colombia, is dominated by large solemyid, mytilid, lucinid, and vesicomyid bivalves, which most likely lived with chemotrophic endosymbionts. Seepage of biogenic methane without a significant contribution of thermogenic methane is indicated by δ13Ccarbonate values as low as −51.3‰. We confirm that the Pleurophopsis-dominated sites of the Heath Shale and Lomitos Chert in northern Peru are ancient seep sites, although the previous identification of Pleurophopsis peruviana as a vesicomyid is doubtful.