EFFECT OF FORCED CONVECTION ON HEAT LOSS IN BASALTIC VS. RHYOLITIC LAVA FLOWS

Lava flows lose heat through thermal radiation, free or forced atmospheric convection, and conduction to the substrate, while gaining latent heat from crystallization and advection. Current thermodynamic models of basaltic lava flows indicate that wind-driven forced convection overtakes radiative heat loss as the dominant cooling mechanism below lava surface temperatures of 700°C. However, there have been few previous attempts to assess the heat transfer coefficient [h] within the general equation for convective heat loss [Q_conv = h(T_surf - T_air)] and its variation with wind speed.

In this study, we analyzed the cooling rates of samples from three basaltic lava flows (McCarty’s, Paxton Springs, Carrizozo, all from New Mexico) and four rhyolitic samples (Bishop Tuff, California; Cerro Chao dacite, Argentina; Los Chocoyos pumice, Guatemala; Medicine Lake obsidian, California). Samples were heated to 500 °C, then placed in front of a fan (wind speed ≤ 1 m/s) and monitored by a FLIR Infrared Camera. Values for h were calculated by fitting the data to a 1-D cooling model for heat loss by radiation, conduction, and convection. In our preliminary results, the samples' h_free values ranged from 2.8 Wm^-2K^-1 (pumice) to 7.9 Wm^-2K^-1 (Carrizozo) and the h_forced values ranged from 12.2 Wm^-2K^-1 (pumice) to 26.3 Wm^-2K^-1 (Carrizozo). No systematic variation in h was observed as a function of wind speed. The most significant variable appears to be the sample density; in a plot of density vs. h, the datasets had linear slopes of dh_free/dρ=5.4 WmK^-1kg^-1 (R²=0.74) and dh_forced/dρ=14.5 WmK^-1kg^-1 (R²=0.83). Despite the average sample density only varying 1.75% between the basaltic and rhyolitic groups, the h_free value for basaltic samples was 3-19% greater than the h_free of rhyolitic samples, while h_forced for basalts was 9-19% greater than for rhyolites. The difference in h between free and forced convection also increases alongside density (dh_diff/dρ=9.1 WmK^-1kg^-1, R²=0.64).

Better understanding the effect of forced convection on lava flow cooling will allow for improved models for volcanic hazard mitigation and extraterrestrial volcanism. Further experiments will be conducted to more reliably characterize the influence of sample dimensions, stronger wind speed, crystallinity, porosity, and bulk composition on lava flow cooling.