1 Suppression effectiveness of water sprays on accelerated wood-crib fires Paolo E. Santangelo a,1 , Bryson C. Jacobs a , Ning Ren a , Joshua A. Sheffel b , May L. Corn b , André W. Marshall a a Department of Fire Protection Engineering, University of Maryland, 3106 J.M. Patterson Bldg., College Park, MD 20742, USA b United Technologies Research Center, 411 Silver Ln., East Hartford, CT 06108, USA Introduction Solid-fire extinguishment by water sprays has received remarkable attention over the decades. Though several approaches have been taken to understand the important physical phenomena, empirical correlations remain very common in discussions of these fires. Historically, wood-cribs have offered an important means by which to study the extinguishment of combustible solids, as their simple and well- defined structure lends a degree of repeatability to experimental investigations. Therefore, several classical experiments in this area focus on wood-crib fuel packages [1-3]. The primary objective of this mainly experimental study is to determine the relationship between the water flux applied to a burning wood crib and the total damage sustained by the crib, as assessed by the crib’s mass loss. A thermodynamic discussion of the potential cooling mechanisms also provides physical insight into the suppression results. As an additional feature of the present work, an experimental characterization of the water mist spray was performed in terms of flux and drop-size distribution, following previous spray studies [4]. Moreover, additional parameters were investigated to better understand the wood-crib fire dynamics (e.g., air entrainment at the lateral surface and heat release rate). Experimental facility and procedure Pine wood cribs were used as the combustible test article for these experiments. Each crib comprised twelve layers, with six individual wood members (sticks) per layer. Each member was square in cross section (3.2 cm x 3.2 cm) and 51 cm in length. The cribs had outside dimensions of 51 cm x 51 cm x 38 cm (L x W x H), total volume (including fuel and voids) of 0.099 m 3 , and total exposed wood surface are of 4.0 m 2 . The average crib mass was 20.8 kg. The cribs were conditioned in a climate-controlled room to a mass moisture content of 5% or less before each suppression test. The experimental setup (Fig. 1a) was inspired by standard UL 2167 [5], with specific reference to the Light Hazard Area Fire Test section. The wood cribs were supported inside a circular pan by four incombustible bricks. The cribs were ignited by a layer of n-heptane (950 mL), which was floated on top of 2 L of water in the circular pan. The whole structure (pan, bricks, and crib) was placed onto a load cell to measure mass variation throughout the tests. Each crib was instrumented with a set of five K-type thermocouples, which recorded both surface and gas temperatures at various locations within the crib. Each experiment was recorded by a standard video camera. A bi-directional gas velocity probe was placed at the center of one of the crib faces to evaluate air entrainment. The nozzles employed in this study belong to the P class manufactured by BETE Fog Nozzle, Inc.. They feature a single orifice and an impingement arm (Fig. 1b). The orifice diameter of the nozzles used here varied from 1.37 mm to 3.05 mm. The nozzles were installed 1.8 m above the base of the circular pan. The entire assembly was placed beneath a large ventilation hood into which a set of thermocouples was placed in order to evaluate heat release rate through temperature-rise calorimetry (free burns only). To obtain a range of water fluxes, the nozzles were operated at both 150 and 200 psi. The procedure for suppression tests was as follows: after the heptane was ignited, the fire was allowed to burn unhindered for 45 s, after which the water-mist discharge was begun and sustained for 10 min. At 10 min and 45 s past ignition, the experiment was halted, and the fire, if not already extinguished 1 Corresponding author. Tel.: +1 301 405 9332; Fax: +1 301 405 9383; E-mail: psantang@umd.edu