Performance analysis of parallel plate and rectangular micro channel counter flow heat exchangers

In this paper, a comprehensive study on the performance of parallel plate and rectangular micro channel counter flow heat exchangers is presented. The geometrical parameters of the two configurations are optimized for a given heat transfer rate, effectiveness and pressure drop. In case of rectangular micro channel counter flow heat exchanger, a reduced order model is developed first in which rectangular micro channel is transformed into a hydro-dynamically and thermally equivalent parallel plate heat exchanger. The reduced order model of rectangular channel counter flow micro heat exchanger is developed by equating flow rate, number of transfer units (NTU) and axial conduction (M) with an equivalent parallel plate counter flow micro heat exchanger. The accuracy of the model is improved by introducing correction factors obtained from detailed CFD model. The ratio of plate thickness and channel height and the aspect ratio of channel affect the heat transfer, axial conduction and dimensionless power density. It is found that the axial conduction plays an important role on the performance of rectangular channel counter flow micro heat exchanger. At the limiting case where the channel aspect ratio tends to zero, the dimensionless power density of rectangular channel approaches that of a parallel plate counter flow micro heat exchanger. NOMENCLATURE A Area (m2) Across Cross section area normal to the flow (m2) cp Specific heat (J/Kg.K) D Channel height (m) Dh Hydraulic diameter (m) f Fanning friction factor H Total height (m) h Mean convective heat transfer coefficient (W/m2.K) k Thermal conductivity of fluid (W/m.K) kw Thermal conductivity of plate (W/m.K) L Channel length (m) M Axial conduction parameter ṁ Mass flow rate (kg/s) NTU Number of transfer unit Nu Nusselt number n Number of hot or number of cold channels nf Number of fins Δp Pressure drop (Pa) Q Non-dimensional power density t Plate thickness (m) U Overall heat transfer coefficient (W/m2K) W Total width (m) w Channel width (m) x, y ,z Cartesian coordinates GREEK SYMBOLS ξ Aspect ratio ρ Density (kg/m3) μ Dynamic viscosity (Pa.s) ε Effectiveness ηf Fin efficiency