Enhacement Mechanism in Convective Heat Tarnsfer by an Effective Utilization of Droplet Inertia

Flow characteristics and heat transfer of a water droplets laden flow behind a backward-facing step have been experimentally and numerically investigated for the condition of isothermally heated dry wall. Two sizes of step heights were employed corresponding to ReH = 5.3x103, 1.1x104. Water droplets of about 60 μm in mean diameter and small fine mist of less than 1 mm as a tracer of air flow were added at the section of the spraying system which was located far upstream of the test section. Humidity in this flow system was almost saturated for all experimental condition.The Phase Doppler Anemometer (PDA) system was employed for the measurements of the droplet size, droplet mass flux, and X and Y components of velocity. Numerical simulations were performed by a standard k-ε model for the gas phase and a Lagrangian Stochastic model for the trajectories of droplets. The equation of particle motion Figure A-1 Experimental Setup which included drag, gravity and drift forces was soluved in the flow field f single phase.Two-dimensional probability density model was applied to introduce the instantaneous velocity field where the Reynolds shear stress existed. The experimental and numerical results showed that the heat transfer in an air-water droplet flow increased considerably over that of a single-phase flow downstream of the reattachment region for both step heights examined. Heat transfer coefficients were enhanced more than twice of single phase flow in the redeveloping region even for the very low droplet mass loading ration of 4.1 %. The influence of the turbulent motion of the droplets to the heat transfer enhancement appeared downstream of the reattachment region. This enhancement was due to the wider dispersion of the droplets in the separated shear layer which leaded to higher droplet mass fluxes towards the heated wall, and was well correlated to the turbulent Stokes number which expressed the feature of the energy containing eddies. It was concluded from these results that there was a high potential to control the large heat transfer enhancement by varying actively the Stokes numbers. Experimental Conditions Step Hights H=10, 20 mm Free Stream Velocity : 10.0m/s Droplet diameter : 60 μm Mass loading ration : up to 4.1% dry heated wall Wall Temerature : up to 400K Figure A-2 Heat Transfer Characteristics SA M PL E