Abstract:
During boiling of a liquid at film boiling region, the surface temperature at where heat flux is
the minimum and liquid takes the maximum time to evaporate completely is known as
Leidenfrost temperature. An experimental investigation for the evaporation time in film
boiling region of a sessile drop of liquid on a hot metallic surface has been conducted in the
present study. The droplet is assumed to have a stable vapor film formed beneath the droplet
and is supported by the excess vapor pressure of the film. Heat is assumed to be transferred to
the liquid droplet by conduction and radiation through vapor film. Sessile drop of five
different liquids such as Distilled Water, Saturated NaCl solution, Methanol, Ethanol and
Emulsion (20% cutting Oil by volume and 80% pure Water) were used to conduct the
experiment for a wide range of solid surface temperature (50-400 oC). Four solid surfaces;
Copper, Aluminum, Brass and Mild Steel were used to conduct the experiment. The
Leidenfrost time (complete evaporation time corresponding to Leidenfrost temperature) has
been estimated in the present study from the evaporation time verses solid surface
temperature plot. The dominating parameters for the sessile drop evaporation time have been
explored in the present study. The Leidenfrost points are well defined for organic liquids
which are approximately between 150 oC to 200 oC, while for water it is about between 200
°C and 250 °C plate temperature. For emulsion, the temperature is around 250 oC for all the
experimental conditions. Sessile drop evaporation time is the maximum for water, then
decreases gradually for NaCl solution, methanol and is the minimum for ethanol for a
particular solid material, this sequence follows the sequence of boiling temperature of
different liquids. It is difficult to match the emulsion’s evaporation time with the sequence of
other liquids, because emulsion’s evaporation pattern is different from others. Leidenfrost
time is the highest for copper and the lowest for mild steel for a specific liquid which follows
the sequence of conductivity of the materials, but there is an exception (might be due to the
experimental uncertainty); brass shows the maximum evaporation time for all liquids.
