As one of the most active volcanoes, Mount Merapi poses an open hazard to Yogyakarta with
its frequent pyroclastic eruptions. Despite of the hazard, Merapi also offers wealth of real
cases for chemical engineering courses as described by this paper. Pyroclastic flow comprised
of mixture of hot particles and gas which resulting from the collapse of the magma dome.
Pyroclastic flow may move down along the slope with a speed up to hundreds of meters per
second. The pyroclastic current is comprised of dense block-and-ash flow (BAF) overlain by
a lean ash-cloud surge (ACS). While the dense BAF has solid concentration up to tens of
percent, the lean ACS has solid concentration only on the order of one percent. ACS has been
shown to tend to move faster than BAF and often able to overpass a hill which is able to stop
BAF. From the disaster mitigation point of few, it is important to have a theoretical prediction
of ACS temperature change while moving fast along the slope of the mountain. The heat
transfer simulation, shows that under turbulent lean ACS flow, temperature gradient within
ash particle is not significant due to the low interphase heat transfer coefficient between gas
and particle. This phenomena can also be well represented by a small Biot number (<< 0.1),
hence the use of lumped model is justified. The simulation also demonstrates that particle
diameter is the most important factor in determining particle rate of cooling. Smaller particle
with average diameter in the order of 1 mm cool very quickly to reach final equilibrium
temperature within 0.5 seconds, while larger particle with diameter on the order more than
10 mm will retain hazardously high temperature even after traveling within the lean ACS for
more than 3 seconds. This paper shows that temperature changes in ACS can be described by
a pseudo homogeneous fluid for particle size commonly found in ACS which range between
30 up to 1000 ????m.
