Microchannel Condensation Heat Transfer

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Around 50% of the electricity consumption of large food retailers in the UK is for cooling chilled and frozen food cabinets. Their total annual electrical energy consumption is almost 10 TWh. As early as 1994, 10% of floor space in the UK was serviced by air-conditioning. The figure is now more than 25% for buildings constructed during the past 10 years. The condenser is a key component of vapour compression refrigeration and air conditioning plant. Wide implementation of well-designed microchannel condensers would lead to a reduction of around 20% in power requirement. In the UK this represents a significant decrease in fossil fuel consumption and carbon dioxide emissions. Improved designs would also be significantly smaller, have smaller fluid inventories and possibly lower capital cost. Established design methods for larger channels fail for channel dimensions around 1 mm owing to surface tension effects. A wholly-theoretical model, valid for condensation of any fluid in microchannels, has been developed at Queen Mary (QM) under research programmes supported by EPSRC. Available experimental data are insufficient in number and reliability, and do not cover a sufficiently wide range of fluid properties, to validate and extend the theory. In most investigations vapour-side, heat-transfer coefficients are deduced from overall coolant-to-vapour measurements and are consequently of relatively low accuracy. Four correlations representing measurements for R134a agree to within about 30% for this fluid but differ by a factor of around 4 for ammonia. Existing correlations evidently do not correctly capture fluid property effects.The QM theoretical model has no recourse to experimental data. It includes transverse flow due to surface tension as well as shear stress and gravity effects and has been used to generate results for various fluids, channel shapes and dimensions, vapour flow rates and channel inclinations. The predictions fall within the ranges of the results given by the correlations. Theoretical results for seven fluids have been widely disseminated in conference proceedings (9 papers) and in archival journals (5 papers). A simple algebraic relation between two dimensionless parameters has recently been developed which predicts, very accurately, numerically-obtained results for the surface tension dominated flow regime for different channel cross section, fluids, vapour flow rates and temperature differences. Similar simplified results will be obtained for the flow regimes upstream and downstream of the surface tension dominated region so as to provide readily usable formulae for complete condenser design.An innovative technique will be used to measure local temperatures and heat fluxes in microchannels. A new apparatus has been designed in which parallel microchannels (1.5 mm x 1.0 mm) pass through the centre of a copper block (30 mm x 40 mm x 500 mm long) in which temperatures are very accurately measured (to within 0.05 K) at 98 precisely-known (to within 0.3 mm) locations. The observations will be used in inverse solutions of the conduction equation to determine local vapour-side surface temperatures (to within 0.1 K) and heat fluxes (to within 5%). Flow visualization studies will also be performed in which the upper part of the test block is replaced by glass so that the flow in the channels may be observed using high-speed photography.Two fluids (steam and FC72) with widely different thermophysical properties, notably surface tension, will be used. These data will provide a stringent test of the present theory and facilitate its extension to cover other flow regimes. The final model should be valid for any fluid, channel geometry, vapour flow rate and vapour-to-wall temperature difference. This will facilitate design and optimisation of a new generation of large scale refrigeration and air conditioning equipment.