Inscrição na biblioteca: Guest

ISSN Online: 2379-1748

ISBN Flash Drive: 978-1-56700-483-0

ISBN Online: 978-1-56700-482-3

4th Thermal and Fluids Engineering Conference
April, 14–17, 2019 , Las Vegas, NV, USA

EFFECT OF HUMIDITY AND AIRFLOW VELOCITY ON DROPLETS ELAPSED TIME AND RADIUS AT THE ONSET OF FREEZING AND FROST NUCLEATION FOR SUPER-HYDROPHILIC AND SUPERHYDROPHOBIC SURFACES

Get access (open in a dialog) pages 1131-1142
DOI: 10.1615/TFEC2019.phc.026880

Resumo

Frost accretion decreases the energy efficiency of refrigeration and air-source heat pump systems by increasing thermal resistance and obstructing the air flow in a heat exchanger. It worsens the performance of aircrafts, unmanned aerial vehicles, and of wind turbines. Recently, we developed a new experimental methodology to measure droplets growth and freezing time as well as frost thickness during frost nucleation and subsequent frost growth processes during air forced convection channel flows. This paper presents new data of frost nucleation and subsequent frost growth for a fine finished Aluminum surface, a super-hydrophobic-coated, and a super-hydrophilic-coated surface. Tests were conducted by varying the airflow velocity and humidity of the air in a parametric fashion, in order to quantify the effects of surface wettability characteristics on droplets elapsed time and radius at the onset of freezing.
Frost thickness patterns followed "S" shaped-like profiles with respect to the elapsed time due to transitions of droplets to ice beads, to crystals growth, and to initial frost accretion on the top of the ice beads. The wettability characteristics of the surface coatings affected the elapsed time at which these transitions occurred and determined the thresholds of the frost thicknesses during the switch to the frost growth phase. Super-hydrophobic surface had late phase changeovers but high thresholds of the frost thickness before switching to the frost growth phase. These results were due to the presence of large droplets on the surfaces before they froze into ice beads.
During water vapor condensation and droplets growth, the heat transfer coefficients were similar to those of steady state non-freezing wet tests all three surfaces. Since the air flow in our experiments was set constant, as droplets began to turn into ice beads and as crystals started to grow on the top of the ice beads, the heat transfer coefficients gradually increased because of the increased local air velocity at the top of the surface and augmented surface roughness created by the frost accumulated on the surfaces.