Browsing by Author "de Dios Rivera, Juan"

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    Aerodynamic collection efficiency of fog water collectors
    (2011) de Dios Rivera, Juan
    Fog water collectors (EWC) can provide water to arid zones with persistent advection and orographic fog. A key feature of any FWC is the mesh used to capture fog droplets. Two relevant mesh characteristics are its shade coefficient and the characteristics of the fibers used to weave or knit the mesh. This paper develops a simple superposition model to analyze the effect of these factors on the Aerodynamic Collection Efficiency (ACE) of FWCs. Due to the simplicity of the model it cannot be directly applied to actual FWC meshes, and serve only for guidance on the order of magnitude of the optimum shade coefficient and the corresponding ACE. The model shows that there is a maximum ACE of the order of 20-24.5% for shade coefficients between 0.5 and 0.6, for the particular mesh simulated. Aerodynamic collection efficiency can be increased by making the FWC concave and improving the aerodynamics of the mesh fibers. (C) 2011 Elsevier B.V. All rights reserved.
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    Effect of Wind on Smoldering Combustion Limits of Moist Pine Needle Beds
    (2014) Pablo Valdivieso, Juan; de Dios Rivera, Juan
    We studied moist pine needle beds burning under the effect of wind, in order to determine the upper moisture limit for which there is fire propagation for different wind velocities. For this purpose we built a wind tunnel that allowed us to burn a 600 mm by 150 mm by 40 mm bed under wind velocities between 0.5 m/s and 5.0 m/s and controlled air temperature. Results show an increase in moisture limit from 54% to approximately 140%, for the velocity range indicated. Combustion at limiting conditions proceeds mainly by smoldering with some periods of flaming combustion. It was observed that, for conditions close to extinction, the smoldering front is not quenched at the surface. Additionally, it was also observed that a strong flow of hot gases exit from the fuel bed at the free surface. These two observations lead to the conclusion that the main heat sink is moisture evaporation and that heat losses to the surroundings is reduced by the blowing effect of the hot gases coming off the bed. A dimensional analysis suggests a correlation between moisture limit and wind velocity of the form M = A - B/ (w) (2) , where M is moisture limit for fire propagation, A and B are constants, and (w) is wind speed. Two dimensionless numbers helped to plot the smoldering temperature and fire propagation velocity in a more meaningful way. They are and , where T (sml) is smoldering temperature, c (p,g) is the gas specific heat, M is fuel moisture content and h (fg) is the latent heat of water evaporation. A relatively high moisture limit at 5 m/s wind velocity is possible due to the relatively high air flow into the smoldering front and the efficient heat feedback produced in forward smoldering.
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    Influence of Permeability on the Rate of Fire Spread over Natural and Artificial Pinus radiata Forest Litter
    (2019) Figueroa, Sofia; de Dios Rivera, Juan; Jahn, Wolfram
    The rate of fire spread is of paramount interest in order to prevent and control wildland fires. It has been extensively studied under laboratory conditions, but little research has been made regarding the differences between artificially constructed and natural litter. By using a specially designed sampling method, in this study differences in the rate of spread between natural and constructed pine needle litter samples are assessed using a combustion tunnel with controlled wind speed. A total of 68 experiments were conducted with four levels of wind speed and six levels of permeability (five for constructed samples and one for the natural sample). It was found that permeability has a strong influence on the rate of spread when wind speeds are high (2.1-3.6ms-1). It was also found that while wind speed and permeability have a strong influence on the rate of spread, their combined effect is not significant. Thus, both variables may be treated independently from one another. Lastly, it was found that constructed samples with permeability of 1x10-7m2 behave similarly to natural samples in terms of the rate of spread. This suggests that it is possible to use constructed samples in the laboratory by matching their permeability, and, thus, obtain realistic values for the rate of fire spread. This could be of use for medium and large scale laboratory tests, where it is not possible to use undisturbed litter samples.