Forced convective heat transfer in vegetation by measuring liquid water evaporation

Abstract

A series of experiments was conducted to develop a method to estimate the convective heat transfer in vegetative fuels with a complex geometry through the measurement of liquid water evaporation. A water mist was sprayed onto vegetative test specimens, coating their entire surface with a thin water layer. The water evaporation rate was measured using a load cell in a wind tunnel under controlled conditions while an infrared camera tracked the surface temperatures. Convective heat transfer was calculated by the difference between the free stream and surface temperatures and the measured evaporation rate, considering the energy balance of the water layer at steady state. The method was verified through evaporation tests using a wood cylinder array. Experiments were conducted using nominally 30 cm branches of a typical conifer, Norway Spruce ( Picea abies), yielding the conventional form of the Nusselt number–Reynolds number power–law relationship: Nu=C Ren Pr1/3 with coefficients C = 0.69 ± 0.25 and n = 0.34 ± 0.06.