[S-VC27] Mitigation of volcanic disasters - basic and applied researches

[SVC27-02] On the Effects of the Inertial Forces of the Falling Volcano Ash on the Ash absorption by an External Unit of Building Air Conditioning

*Kiyotoshi Otsuka¹, Arihide Nobata¹, Hitoshi Suwa¹, Tomohiro Kubo², Yosuke Miyagi³, Masamitsu Miyamura³ (1.Obayashi Corporation, 2.Mount Fuji Research Institute, Yamanashi Prefectural Government , 3.National Research Institute for Earth Science and Disaster Resilience)

Keywords:Ashfall, Functional Damage, Concentration, Inertial Force, Building Air Conditioning

Explosive eruption of volcanos can affect infrastructures of the cities in many ways, even if they are situated far from the volcanos, say, more than 100km. Degradation of performances of the building air conditioning due to ashfall events is among such volcano impacts. In this context, we carried out experiments on ashfall influences on the operation of an open type cooling tower¹⁾, where the amount of the ashfall leading to the abnormal operation of it was investigated, showing the deviation from the normal operation of the cooling tower begins to be recognized when the cumulative ashfall depth exceeded around 20 to 30mm, mainly due to the intrusion of ash into the piping of the cooling water circulation. In their study, the amount of the ash absorbed into the cooling tower was related to the ashfall depth by assuming that the rate of the ash absorption at the air intake is expressed by the product of the air intake velocity and the airborne ash concentration which is directly related to ash fall rate when it is multiplied by bulk terminal fall velocities of ash particles. It is noted that ash particles falling in the vertical direction are assumed to quickly respond to the, usually horizontal, suctioning force of the air intake of the air conditioning system, which is usually set on the side walls of the devices or building walls. However, in reality, the inertia of the ash particles will more or less retard the response of the particle motion to the suctioning force near the air intakes, so it is possible that the relationships between the absorbed ash amount and the ash fall depth derived in our previous study will become somewhat different when the above mentioned inertia effects are taken into account.

In the present study, the effects of the inertia forces of the falling ash particles on the ash absorption rate at the air intake are investigated numerically. In reference to the method of the multi-phase flow simulations, the equations of motion of the ash concentration, which takes into account particle sizes and non-sphericities, are numerically integrated and the deviation of the ash flow from the air flow near the air intake due to inertia force are investigated for the simple cases where a rectangular exterior unit of the air conditioning device is placed at the bottom of the rectangular computational domain with ashfall from the ceiling of the domain. The air flow obtained from mass-consistent flow simulations with an incompressible continuity restriction condition is used to represent the air flow on which ash particles are laden. The results of the numerical calculation show that the inclusion of inertia effect tends, in general, to lessen the ash absorption rate at the air intake, and this tendency is more apparent for heavy particles than the light ones, and, in addition, for the particles with the same weight, the tendency is more of the case for the particles with small non-sphericity than with low non-sphericity. These results suggest the possibility of the ashfall depth at which the air conditioning begins to deviate from normal operation shift to somewhat larger side than the one indicated in our previous study.