  Nanofiber surface coating is a promising method for the enhancement of heat
transfer during spray cooling. In the present work, the drop dynamics as well
as local and overall heat transfer during single drop impact onto a heated wall
covered with a nanofiber mat are investigated to obtain insight into the
mechanisms governing the heat transport enhancement. The influence of wall
superheat, drop impact velocity, and mat thickness on the hydrodynamics and
heat transfer from the heated wall to the fluid is studied. Polyacrylonitrile
(PAN) was electrospun on a heater surface to manufacture the nanofiber mat
coatings. The experiments were conducted inside a temperature-controlled test
cell with a pure vapor atmosphere maintained with refrigerant FC-72. The
temperature field at the solid-fluid interface was observed with a high-speed
infrared camera, and the heat flux field was derived by solving a
three-dimensional transient heat conduction equation within the substrate. The
presence of the nanofiber mat on the heater surface suppresses the drop
receding phase due to the pinning of the contact line at the end of the
spreading phase. Two different scenarios are observed depending on the wall
superheat and drop impact velocity: scenario (I), in which the liquid drop
penetrated the porous nanofiber mat and touches the heater surface; and
scenario (II) in which the vapor produced inside the pores of the nanofiber mat
prevented the liquid drop from touching the heater surface. At a certain point
in time after impact, the energy transferred from the nanofiber-coated surface
to the liquid exceeds that of the uncovered heater owing to the larger drop
footprint. If scenario (I) occurs, then the total transported heat increases
significantly compared with the drop impact on a bare substrate.
