Possible venting of smoke in a room fire while discharging a solid -cone water spray was studied . The water spray would act at the smoke layer at position s away from the burning object. In this way, water droplets would not reach the fire plume and affect the combustion directly . The spray dimension was assumed to be relatively small in comparing with the compartment . It is observed that operating the solid -cone water spray w ould give smoke venting effect . The compartment can be divided into three quasi -steady regions : an upper smoke layer, a lower air layer and a one-dimensional spray. Fire environment under different spray characteristics was investigated by apply ing a two -layer zone model developed earlier with some modifications . Results indicate d that the fire environment would be affected significantly by the soli d-cone water spray through hot gas entrainment and water evaporation. The smoke layer interface height w ould move up with the smoke layer temperature and the flow rate to adja cent space decrease d. Predicted results agree with experimental observation s in the small burning test. Keywords: fire engineering, computational mechanics, mathematical modeling , water spray 3 1. Introduction A good understanding of the effect o f fire protection systems on a compartment fire is important for providing adequate s afety . Poorly designed system s would cause damage s instead of controlling the fire. One of the important areas is the effect of water spray on the smoke layer. There are con cerns on whether discharging water into a hot smoke layer would disturb its stability. Smoke might then be pulled down to lower level to give adverse effects to the occupants [1-4]. Variables includ ing smoke layer interface height, smoke temperature and outgoing flow rate through the opening to adjacent space should be studied caref ully. Works were reported on the effect of hollow -cone spray with large droplet s such as those discharged from a sprinkler system [3,5-9] on a fire . Other s were reported on solid -cone water sprays with small droplet s discharged from water mist fire sup pression systems [10-18]. Surrounding gas would have more influence on the motion and heat transfer for smaller droplets than f or larger droplets . Water spray in hot gas shou ld be considered carefully to investigate the entrainment effect on the smoke. Upon discharging water spray, the smoke layer would not be stable [e.g. 1-4] if the drag force due to the spray is greater than the buoyancy of the smoke layer . Action of a solid-cone water spray of relatively small droplet s in a compartment fire is more complicated. The interaction area would be relative ly small for spray dimension smaller than that of the room geometry . Smoke layer under the spray nozzle is affected by the discharged water spray . However, smoke layer outside the spray envelope might not be dragged by the droplet directly , and so remains 4 stable . This small solid -cone spray would perform like a ‘vent’ in entrain ing the hot gas inside the spray envelope withou t affect ing the stability of the smoke layer significantly. Therefore, two-layer zone models [e.g. 19,20] can be applied for the part of smoke layer other than the spray envelope to predict the fire environment . This case is different from that with the entire compartment covered by sprinkler spray s, water mist s or gas agents from total flooding systems [e.g. 13] . In this paper, a small -scale experiment was carried out first to demonstrate the ‘vent’ effect of the solid -cone spray on the smoke layer. The change of smoke layer height was obviously observed . Then, physics of the vent effect for real -scale scenario would be investigated by modifying a one-dimensional model developed earlier [10,18] for study ing the interaction of a water spray with smoke l ayer. In this model [18], the hot smoke and cool air are taken as two layers under quasi -steady state . Water droplets of the solid cone spray can be described by four distribution function s for four sizes . The hot gas would be entrained inside the spray envelope. The spray pattern is modeled using a Lagrangian approach on balancin g heat, mass and momentum of single droplets. The system of equation s is solved using a space marching Runge - Kutta scheme of the fourth order. Spray characteristics such as d roplet diameter and velocity, shape of the spray envelope, gas temperature and f low rate inside the spray envelope can be calculated for various smoke layer conditions . This modified model w ould be applied to study the vent effect of a solid -cone water sp ray. The results would help to better understand the effect of fire protection s ystems on a compartment fire, and select and design suitable water spray for safety purpose . 5 2. Smoke Venting Effect The small -scale e xperiments on a chamber of size 1.0 m by 0.6 m and 0.6 m high as in Figure 1 a were carried out. There was an opening of height 0.25 m and width 0.1 m at one end with a sill. A small diesel pool fire of di ameter 0.05 m was placed on the floor at a distance 0.25 m away from the other end . The water mist nozzle was placed 0.25 m away from the opening, and 0.02 m below the ce iling. The solid -cone spray discharged is of maximum dimension 0.2 m. Water was discharged manually from the nozzle after getting a stable smoke layer. A digital video camer a was used to record the transient changes of the smoke layer . Before dischargi ng the water spray, a rather stable smoke layer was formed in the upper part of the test chamber as shown in Figure 1 b. Hot smoke flowed out through the opening . When water w as discharg ed, the smoke near the spray was brought down, while the other part o f the smoke layer away from the spray rose up as shown in Figure 1c. The smoke layer was observed to be stable . The smoke layer thickness was thinner than that before dischar ging water spray , if the spray velocity was not too large. For this small -scale test demonstration , the effect of discharging water spray is similar to operating a ‘vent’ to reduce the smoke layer thickness . Note that for a vent, the smoke w ould be rem oved out of the compartment . But for such a ‘vent’ induced by the solid spray , smoke w ould be brought down to the low er air layer, then mixed with air in the low er layer. The low er layer characteristics w ould be chang ed significantly and w ater vapor prod uced would affect the intermediate combustion chemical reaction s. In general, heat release rate under some spray characteristics would be decreased d ue to physical, thermal and 6 chemical effects . But the combustion might be enhanced if the reactions are a ccelerated by radicals such as H· and ·OH produced from water vapor ; or when the soot formed within the flame is reduced by water vapor to give incomplete combustion [21,22] . Under the condition s in this experiment , the flame size was observed to become l arger when water mist s were discharg ed as shown in Figure 1c. Further detailed investigation should be carried out to describe the complicated phenomena. 7 3. Fire Scenarios A scenario with a water spray acting at the fire-induced smok e layer in a compartment as shown in Figure 2 is consider ed. The soli d-cone spr ay is discharged at some distance away from both the burning object and the opening . The gas flow through the opening and the fire plume would not be affected directly by the water spray. Therefore, three regions can be divided :  Region 1: An upper hot smoke layer,  Region 2: A lower cool air layer and ,  Region 3: A solid -cone water spray. The fire environment under various spray characteristics can be studied by a two -layer zone model . Interactions among the three regions can be deduced by c onservatio n of mass, momentum and heat . The one-dimensional model for solid -cone spray developed by Chow and Yao [10] was further developed by Yao and Chow [18] by considering the ‘quasi-steady ’ smoke layer during each time step in calculating the smoke l ayer chara cteristics . Note that the characteristic changing time of the smoke layer is much longer than that of the water spray . Main assumptions made in the model are:  Pre-flashover f ire is assumed with the pressure in the compartment taken as very clos e to the a mbient pressure.  All gases are assumed to obey the ideal gas law. 8  Smoke is entrained into the spray envelope in a perpendicular direction and then mixed with the gas inside the spray thoroughly . If the water spray travels through the interfa ce of two layers , or even reaches the floor , the gas inside the spray would mix well with the lower layer ; or with the upper layer and almost all water droplets would be evaporate d.  The heat release rate of the fire is kept at the design value without affected by reduction in oxygen concentration in the air layer due to water evaporation . Effect of oxygen on the heat release would be discuss ed later. All the key equations in the thr ee regions had been described by Yao and Chow [18]. 9 4. Numerical S imulations A compartment of 8.0 m long, 5.0 m wide and 3.5 m high with an opening of 2.0 m high and 2.0 m wide was considered. The nozzle was located at 10 cm below the ceili ng which was away from both the open ing and the fire . The spray radius was much small er than the compartment dimension s. The fire was taken as an ultra -fast growth t-square fire with peak heat release rates of 0.5 MW or 1.0 MW. The heat loss fraction cL due to radiation and conduction to the compartment was assumed to be 0.3, and the radiation loss fraction of the total heat release rate r was assumed to be 0.35. The initial velocity and spray angle at the noz zle were 20 ms-1 and 35o respectively. The volume mean diameter (VMD) of the wa ter spray was selected as 600 m and 300 m. For the spray of VMD 600 m, the diameters of the four classes were 965, 697, 400 and 132 m respectively . For the spray of VMD 300 m, the four typical classes were 480, 350, 200 and 65 m respectively . The w ater flow rate vari ed from 0.1 ls-1 to 1.0 ls-1. The water spray was activated at 20 s after the ignition of the fire, and the total calculating time is 300 s. 10 5. Results and Discussions The model developed earlier was modifi ed [10,18] to study the spray characteristics such as droplet velocity, diameter, spray envelope radius, and gas flow rate while traveling through the stable hot smoke layer. As the smoke layer i tself changes with time , these spray characteristics would b e affected. Heat and mass transfer among the two layers and spray envelope would then be affected significantly by the gas flow rate inside the spray envelope. Transient smoke interface heigh ts for a water application rate of 0.5 ls-1 were measured [18]. Before dischar ging water spray, the smoke interface height fell down to 1.3 m for a 0.5 MW fire . Upon discharging water spray, the smoke layer interface height up to 1.7 m for VMD of 600 m. Gas flow entrained rate increased by the water spray. This entrainment effec t due to water spray is similar to a vent in remov ing heat and mass . The smoke flow rate through the open ing decreased when water spray wa s appli ed. In contrast to a vent, the lower air layer would be affected by the water spray sig nificantly . The hot gas including water vapor w ould be brought to the lower layer by the spray -induced flow. The water application rate would affect the mass, momentum and heat transfer betw een the water droplet s and hot gas . This would give differe nt spray envelopes a nd smoke layer development. 11 6. Conclusions The compartment fire environment under the action of a solid -cone water spray was studied. There might be vent effect if th e water spray is relatively small and located away from the burning object. A s imple small -scale experiment was performed to demonstrate the vent effect. In fact, the compartment can be divided into three quasi -steady regions , an upper smoke layer, a lower air layer and a one-dimensional spray. Characteristics of t he water spray su ch as drop let size distribution, velocity distribution and water application rate would affect the vent effect . Suitable water spray can be used to reduce the smoke damage to adjacent space through decreasing the smoke temperature and out going flow rate of gases . 12 7. Additional Comments The paper was pub lished as a journal paper [ 23]. With the p ractice of publisher, preprint i s allowed to upload at a website. There are furth er studi es on this topic [ 24-52]. Acknowledgement This project is supported by The Croucher Foundation with title “Active Fire Protection in Supertall Buildings” and account number 5 -ZH46. 13