  The cooling process associated with the flow induced by a spark plasma
discharge generated between a pair of electrodes is measured using stereoscopic
particle image velocimetry (S-PIV) and background oriented schlieren (BOS).
Density measurements show that the hot gas kernel initially cools fast by
convective cooling, followed by a slower cooling process. The cooling rates
during the fast regime range from being 2 to 10 times those in the slower
regime. An analytical model is developed to relate the cooling observed in the
fast regime from BOS, to the total entrainment of cold ambient fluid per unit
volume of the hot gas kernel, measured from S-PIV. The model calculates the
cooling ratio to characterize the cooling process and shows that the cooling
ratio estimated from the density measurements are in close agreement with those
calculated from the entrainment. These measurements represent the first ever
quantitative density and velocity measurements of the flow induced by a spark
discharge and reveal the role of entrainment on the cooling of the hot gas
kernel. These results underscore that convective cooling of the hot gas kernel,
in the fast regime, leads to approximately 50% of the cooling and occurs within
the first millisecond of the induced flow.
