Application of field modelling technique to simulate

Combustion Science and Technology

ISSN: 0010-2202 (Print) 1563-521X (Online) Journal homepage: http://www.tandfonline.com/loi/gcst20

Application of field modelling technique to simulate interaction of sprinkler and fire-induced smoke layer W.K CHOW & N.K. FONG To cite this article: W.K CHOW & N.K. FONG (1993) Application of field modelling technique to simulate interaction of sprinkler and fire-induced smoke layer, Combustion Science and Technology, 89:1-4, 101-151, DOI: 10.1080/00102209308924105 To link to this article: http://dx.doi.org/10.1080/00102209308924105

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Combust. Sci. and Tech., 1993. Vol. 89, pp. 101-151

@Gordon and Breach Science Publishers S.A. Printed in United States of America

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Application of field modelling technique to simulate interaction of sprinkler and fire-induced smoke layer W.K CHOW & N.K. FaNG

Department of Building Services Engineering Hong Kong Polytechnic Hong Kong

Downloaded by [Hong Kong Polytechnic University] at 23:31 10 November 2015

(Received May 30, 1991; in final form May 6, 1992)

Abstract-The interaction between the sprinkler water spray and the fire induced convective air flow is studied using the field modelling technique. A system of equations describing conservation of momentum, enthalpy and mass is used to simulate the physical picture. Solution of the problem. is divided into two parts: gas phase and liquid phase. In the gas phase, a two-equation k - e model is used to account for the turbulent effect with the solid wall boundary described by the traditional wall functions. Numerical finite difference method is employed to solve the system of coupled non-linear partial differential equations. The equations are firstly discretized by the Power Law scheme and then solved using the Pressure Implicit with Splitting of Operators (PISO) algorithm. For the liquid phase, the sprinkler water spray is described by a collection of water droplets with different values of initial velocity components and diameter calculated from the Rossin-Rammler distribution function. The motion of each droplet is described by the Newton's Second Law with air drag and convective heat transfer from the fire induced smoke layer. This set of ordinary differential equations is solved by the fourth order Runge-Kutta method for predicting the droplet trajectories. To simplify the physical picture and bearing in mind that evaporative heat loss measured experimentally is small, coupling of the momentum and heat Iransfer between the smoke layer and water droplets is described by the Particle-Source-In-Cell method. In this way, two-phase flow analysis is avoided by taking the sprinkler water spray as a system of 'hard-spheres'. Neither combustion nor water suppression effect on the burning object is included. However, a 'microscopic' view on the resultant sprinklered fire air-flow pattern, temperature and droplet properties can be visualized. Macroscopic parameters such as the drag to buoyancy ratio and the amount of convective heat transfer are predicted.

Section 1 : Introduction In order to protect life and property from fires, buildings are installed with automatic sprinkler system (e.g. Nash and Young 1974, Vining 1985, Cote 1986). This is because water can be available easily and unlike other extinguishing agent such as BTM (e.g. Cote 1986, FOC rules) or carbon dioxide (e.g. Cote 1986, FOC rules), it is non-toxic and will not produce irritating products to cause environmental problems. It can control the fire spread and extinguishing it effectively. Past record (e.g, Smith 1981) illustrated that a sprinklered building usually has a lower risk of getting uncontrolled fire and so the insurance premium for such a building is reduced significantly. However, to the best of the present knowledge, there is no theory which is capable of explaining how the sprinkler water spray interacts with a fire realistically. But people (e.g. Rasbash 1962, Rasbash et al. 1960, Kung 1977) believed that there are four mechanisms that a sprinkler water spray interacts with a fire: • direct cooling of the burning materials; • cooling of the smoke layer; • prewetting the combustible materials to prevent further fire spread; and • displacement of combustible vapour and oxygen by steam. Among these four mechanisms, the most interesting aspect is the interaction between the sprinkler water spray and the smoke layer induced by a fire (e.g. Bullen 1974, Morgan 1979, Heselden 1984, Hinkley 1986, 1989, Chow and Fong 1990a, 1990b). Qualitatively, the fire-induced buoyant smoke layer would be cooled down by the water spray and hence lost its buoyancy. Also the air drag effect produced by the water droplets would pull the stratified smoke layer downward. The combining effects would disturb the 101

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102

W. K. CHOW AND N. K. FONG

stability of the smoke layer and make it fall to a much lower level, i.e. 'smoke logging' (e.g. Morgan 1979). This would reduce the efficiency of the smoke extraction systems such as a natural vent (e.g. Heselden 1984); and would also cause both psychological and physiological ill-effect to the occupants trapped inside the enclosure. The situation becomes very obvious for high headroom atrium buildings (e.g. Chow 1988, 1989a). This adverse effect depends upon the physical properties of the sprinkler water spray which is a function of the water pressure, flow rate, droplet size distribution, spray angle, separation distance of sprinkler heads and the geometrical feature of the sprinkler head itself etc. (e.g. Kung 1977, You 1986, Gardiner 1989). With good understanding on these features, better design guides on sprinkler systems can be developed. Not much research works in sprinkler have been. reported in the literature. There were even fewer works on studying its interaction with the smoke layer. The earliest related works were perhaps due to Rasbash et at. (1960) and Rasbash (1962) in studying the heat transfer from a flame to the water spray with droplets moving under terminal vertical velocities. The penetration of the water droplets to the flame was calculated and other extinguishing factors were proposed. The effect of a sprinkler on the stability of smoke layer beneath a ceiling was studied by Bullen (1974). There, smoke layer of constant thickness was assumed and the sprinkler spray was taken as water droplets of constant diameter which was calculated from the water pressure. An empirical relation on the mass of water flow rate through the sprinkler nozzle was used. By considering only the thermal convective effect, a macroscopic parameter known as the drag to buoyancy D/B ratio was calculated from the ballistic motion of droplets. This is very useful for assessing the stability of the smoke layer. Evaporative heat transfer was assumed to be unimportant in this study. The concept was extended further by considering the water droplets having a size distribution by Morgan (1977) and Morgan and Baines (1979). The heat extraction by convective cooling was also calculated. A computer program (Morgan and Baines 1979) was developed and validated by experiments. Works along this line have been applied to study the effect of smoke vents on sprinklers (e.g. Heselden 1984, Hinkley 1986, 1989). This was extended further by Gardiner (1989) to include three-dimensional water spray in studying the interaction of a sprinkler water spray on a stratified smoke flowing in a corridor. The corridor was modelled by a system of three-dimensional matrix of discrete control volumes where both the smoke layer and the sprinkler water droplet trajectories were described. Water evaporation effect was included. Actual experimental data on the water droplet trajectories measured for different sprinkler heads was used. The transient nature of the input droplet velocities and size distribution were considered. The interaction of fire and sprinklers had also been studied by Beyler (1977). This included the thermal actuation of sprinkler heads; droplet formation and dynamics; and review on the fire suppression mechanism. The ceiling jet expression of Alpert (1972) was used together with the lumped mass analysis to study the thermal responses of sprinkler heads. With it, preliminary sprinkler design guides were achieved. The droplet size distribution was calculated by the experimental results due to Dundas (1974). The water droplets were assumed to be evaporating with their trajectories in a fire calculated from empirical expressions on plume velocity and temperature fields. However, it was found that evaporation did not play an important role. From the review on the extinguishing water on fire, no theory was found to be good enough to model the transient water suppression behaviour on the burning object. Another approach on studying the sprinkler problem (e.g. Alpert 1979, 1984, 1985, Hoffmann, Galea and Markatos 1989) had been reported by using field modelling technique. The pioneer work was the sprinkler-induced air flow due to Alpert (1979, 1984, 1985). The technique of computational fluid dynamics (e.g. Abbot and Basco

INTERACfION OF SPRINKLER AND SMOKE LAYER

103

or

, - - - - - - - - INTERACTION SPRfNICl.&R WATER SPRAY 1rmf THE:

rnu:

SWOICI u.YER

---f7f=====~il?===t71

INDUCED SWOKE UYER

CAUSE 'sllon

LOCC~C'

REllUctn TKE £mCQl'CY or SIIlOKt D:nU.CTfOM SYSTEW lIfUl(CS DOTH PSYCKOLOC1CAL

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AND PHYSIOLOC1CAL lll.-PTECTS TO ntr. OCCUPANTS 11W'ppm

FIGURE I: Graphical Description of the Problem to be Solved

1989) had been applied to study large-scale flow fields induced by water droplet sprays. The axisymmetric spray pattern could be described by a two-dimensional cylindrical coordinate system. The water droplets were assumed to interact with th~ air through the particle-source-in-cell concept of Crowe et al. (1977). The turbulent viscosity was fitted by an effective value calculated from the k - E: model. Evaporation effect had been included and being coupled in the heat and mass source terms of the air flow equations. The computer program developed was known as 13SPRY3. It was basically derived from the TEACH code due to Gosman and Pun (1973). Later, this technique was applied to study the interaction of spray with buoyant opposed flows induced by a point source (i.e. Alpert 1982). The work was further extended to study the interaction between the sprinkler water spray with a fire located under it. Again, an axisymmetric arrangement could be achieved. An upgraded version of the computer program TEACH-T by Gosman and 1deriah (1976) was used. Macroscopic parameters such as the water penetration ratio and air entrainment had been calculated. However, the work is only good for simulating axisymmetric flow problems due to the two dimensional nature of the TEACH-series of programs. Similar technique had also been applied to study a sprinklered fire and a model CLYTIE (e.g. Hoffmann et al. 1989) was developed. There, the water evaporation effect was strongly emphasized and the 'volume fraction' approach was applied in solving both the liquid and gas phases. In this way, a set of non-linear partial differential equations describing the flow, heat, mass and the volume fractions of the two phases were solved with the aid of PHOENICS (e.g. Rosten and Spalding 1986). Water droplets were treated as a continuous phase with the gaseous field. The work is still under development and the model will be modified and validated. All the above works have not yet give a clear picture of visualizing the interaction between a fire-induced smoke layer and sprinkler water spray. Field modelling has been applied to study this problem (e.g. Chow and Fong 1990a, 1990b). The fire is assumed to be at the preflashover stage and there is only a single fire source inside the compartment. A sprinkler is located at a distance away from the burning object and a parabolic water envelope is discharged. To avoid studying the suppression effect on the fire, the water spray does not act on the burning object. In this way, both combustion and direct cooling effect on the fuel can be omitted. It becomes a problem of analyzing how a collection of water droplets would interact with the fire-induced air flow as illustrated in Fig. 1. Since water evaporation effect is found to be of very small magnitude in comparing with convection (e.g. Beyler 1977, Kung 1977, Morgan 1979, Chow 1989b), the ParticleSource-In-Cell method (Crowe et al. 1977) can then be employed. Here, both the air flow

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104

W. K. CHOW AND N. K. FONG

and the motion of water droplets can be treated separately with their thermal and mass coupling effects appearing only at the source term in the set of conservation equations. In this way, the heat extraction and air dragging effect can be studied. The whole physical problem is therefore a single phase gas flow problem with the presence of free falling non-evaporative water droplets which are treated as 'hard spheres'. However, the water trajectories reported previously were taken to be the ones calculated when there was no fire. In other words, the path of droplets would not be influenced by the hot smoke layer. This is not too realistic and the air drag computed might be too large. The model is extended in this paper to include simulation of water droplet in the hot smoke layer. After predicting the fire environment, the liquid phase of the problem is considered. The Lagrangian approach (e.g, Dukowicz 1980) is used to solve the droplet phase problem. Location of the water droplets and their velocities are computed from the Newlon's law of motion and the Rossin-Rammler distribution function (e.g. Alpert 1985). Other parameters of sprinkler head such as initial velocity, projectile angles, water flow rate and water temperature are considered as input parameters. Solution of this problem can allow fire engineers to visualize the cooling effect on a smoke layer. Combustion and radiation in the fire have not yet been included. The governing equations and the numerical methods concerned are introduced in Section 2 and 3. Application of the model to study a sprinklered fire is reported in Section 4. Finally, the paper ends with a conclusion in Section 5. Section 2 : Theoretical Analysis A 'gas-droplet' model is developed to study how a sprinkler water spray would interact with the fire induced hot air flow. Physically,when water is discharged from the sprinkler head, a collection of water droplets is formed. The droplets experience aerodynamic drag and thermal convection while travelling through the air (e.g, Bullen 1974). This aerodynamic drag changes both the momentum of the water droplets and that of the air. Heat will also be transferred from the hot air to the droplets. All these depend on the relative velocity between the droplets and the air. There. are two sets of equations describing the physical picture: one for the fire induced air flow field and the other for the motion and cooling effect of water droplets. These two sets of equations are then coupled through the source terms of the momentum and enthalpy equations for the air flow. The buoyancy driven flow is treated as a continuum as the fire induced flow is taken to be an incompressible Newtonian fluid (e.g. Spalding 1980, Kumar 1983). Further, volume of the droplet is negligibly small in comparing with the air volume. In this way, the Navier-Stokes equation can be applied to describe the gas flow field. Only the turbulence phenomena of the gas flow is considered. The evaporation, radiation and the effect of turbulence on the droplets are not taken into account. In other words, it becomes a single phase flow problem under the action of non-evaporating 'hard spheres' of prescribed ballistic trajectories. (2.1) The Air Flow Field Model

This is a buoyancy driven flow problem with the fire induced flow field described by a set of non-linear partial differential equations derived from the laws of conservation (e.g. Tennekes and Lumley 1972, Hinze 1975). Variables of the air flow include momentum, mass, thermal energy and species concentration etc. Since most buildings are of rectangular shape, a Cartesian co-ordinate system is used. As the equations are strongly coupled, there is no analytical solution. Numerical techniques (e.g. Roache 1976, Shih 1988, Fletcher 1988) are required to solve the equations. However, this three

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INTERACfION OF SPRINKLER AND SMOKE LAYER

105

dimensional turbulent flow contains micro-scale motion whose magnitudes (of order 1 mm) are much smaller than the computational domain (of order 1 m). Very large computational effort is needed which cannot be handled by the storage capacity and speed of a super-computer for this kind of direct simulation. Fortunately, detailed fluctuating motion of the turbulent flow is not so important in solving practical engineering problems (e.g. Spalding 1980 and 1983). A statistical approach can be adopted so that the equations are averaged over a time scale which is long compared with that the turbulent motion. Mean flow equations for different physical properties are derived. The two-equation k - E model is used to close the set of conservation equations. A closed set of equations describing the transport of mass, momentum, enthalpy and the turbulent parameters k , E is obtained. For simplicity, the overbars for mean values will be discarded and the set of equations for the mean motion in Cartesian coordinate system (with y being the vertical direction) becomes: Averaged continuity equation: op ot

0

0

0

+ ox (pu) + oy (pv) + oz (pw)

(2.1)

= 0

Averaged x-momentum equation: OU p( ot

ou

ou

ou

op f.)x

+ Uox + v oy + w OZ) = o

+ ox (I',

f.)u ox )

0

ou

OV

f.)

f.)

OV

0

ou

f.)

OU

+ ox (Pell Oy) + oy (Peft Oy) + OZ (Pell OZ)

f.)

ow ox ) - D x

+ Oy (IL, oX ) + OZ (I',

(2.2)

Average y-momentum equation: OV p(ot

ov

ov

ov

+ uf.)x- + vf.)y- + w-) OZ f.)

+ f.)x (I',

Bu oy)

op oy

= -0

+ oy (I',

0

0

ov

f.)v

+ f.)x -(ILell-;-) + -(Pell-) + -(Pell-) ox Oy oy oz f.)z f.)v 0 Dw f.)y) + OZ (I', f.)y )

+ Fg

-

(2.3)

Dy

Averaged z-mornentum equation: ow p(ot

ow

f.)w

ow

+ uox- + voy- + w-) Dz o

+ f.)z (Pel I

ow f.)z )

op

= -f.)z

0

+ ox (I',

o

+ OZ (I',

0

ow

0

ow

+ ox -(Pell-) + -(Pell-) ox f.)y oy f.)u f.)z )

0

Dv

+ oy (I', f.)z )

ow f.)z ) - D',

(2.4)

Averaged enthalpy equations: Dh p(f.)t

f.)h

f.)h

oh

0

ox

oy

Dz

ox

I' oh

f.)

f.)x

Oy

I' oh

0

Oy

OZ

I' oh

+ u - + v - + w-) = -(--) + -(--) + -(--) o 1', 8h

f.)

1', oh

0"\

0 1', oh

0"\

.

+-(--;-) + -;-(--) + -(--) + q fu 0", ox Oy O",Oy & 0",&

Qcool

0"1

OZ

(2.5)

W. K. CHOW AND N. K. FaNG

106

TABLE I Temperature dependent quantities for air From: '"I~'"I1 +'"IZTp+'"I3Tt+'"I4Tft I'

I.7E-5 1.14E-5

5.34E-8 I.S6E-7

-5.4E-II -1.9E-1O

3.9E-14 2.7E-13

k

2.37E-2

9.22E-5

-1.05E-7

8.4E-II

c;

1.006

-4.9E-4

8.46E-6

-3.5E-8

(kglno,,) v (m 2/s)

(WlmOC)

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(1I kg OC)

Averaged turbulence kinetic energy equation: 1 a 1'1 8k 1 8 1'1 8k = -[-(+ 1')-] + - [ - ( - + 1')-] p 8x Uk 8x P 8y o« ay

ak ak ak ak - , + u-, + v - + w8t 8x 8y az

1 8 IL, 8k 1'1 [ 8u z av 2 Bw 2 ] +-[ - ( - + IL)-] + - 2[(-) + (-) + (-) ] p az a« 8z p 8x 8y 8z

1'1 [au Dv 2 8u Dv 8w 2] +[-+] +[ - +8w - ] 2 +[-+-] P ay 8x 8z 8x 8z 8y 1 18p

-e+l'l-g-p

(2.6)

pay

Averaged turbulence energy dissipation equation:

ae

8e

8e

ae = -[-(1 a 1'1 ae 1 8 1'1 De + 1')-] + -[-(- + 1')-] p ax 8x p 8y 8y

-;- + u - + v - + wat 8x 8y 8z

Ue

Ue

1 1'1 ae e 1'1 [ 2[(-) 8u 2 +(-) 8v 2 +(-)] Bw 2 ] +-[(-+IL)-;-]+C el - P a; 8z k p 8x 8y 8z

e 1'1 [ Bu Dv 2 Bu 8w 2 8v 8w 2] [-+-] +[-+-] +[-+-] k p 8y 8x 8z 8x aZ 8y

+C el -

(2.7) In order to predict smoke movement, another conservation equation for the smoke concentration is required. If the fire is taken as a mass emission source, the derivation for the smoke transport equation is similar to the enthalpy equation. It is used to describe how the smoke concentration f is affected by the convective transport and the diffusion due to the concentration gradient. Again no chemical reaction is considered and a constant is assigned to denote the rate of smoke generation which appears as a source term S

af 8f 8f 8f p( at + u 8x + v ay + w 8z )

8

= 8x (I'e f f

af 8 8f 8 8f . ax ) + 8y (I'eff 8y) + 8z (I'eff 8z) + s (2.8)

INTERACfION OF SPRINKLER AND SMOKE LAYER

107

Further, the equation of state is used to relate pressure p, density p and temperature T through the gas constant R: P

= pRT

(2.9)

The temperature T is calculated from the predicted values of air enthalpy h by: (2.10)

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where

Cp

is the constant pressure specific heat for air as shown in Table 1.

(2.2) The Droplet Model

After obtaining the fire induced flow field, the next stage is to calculate the effect due to the sprinkler water sprays. The droplet model employing the Particle-Source-in-Cell method (e.g. Crowe et at. 1977) is used to study the gas-droplet interaction. The sprinkler water spray is described by a finite and discrete distribution of water droplets with different diameters and initial velocity components. The Eulerian approach is employed to solve the gas flow field but the Lagrangian formulation (e.g. Dukowicz 1980) is used for describing the droplet motion. In this way, calculation of the discharged water droplets can be tracked for determining the water droplet trajectories. After calculating the drag force and the amount of convective heat transfer along each trajectory, the effect of droplets on the gas phase can be evaluated by introducing appropriate source terms in the air flow equations. The water droplets are assumed to be spherical and non-evaporating. Attempt is not made to model the flow field around the individual droplet. At the same time, no droplet-droplet interaction is considered so that there are no collisions between droplets nor droplet breaking-up. The discharged water envelope (e.g. Wraight and Morgan 1986) is predicted by solving the trajectories of water droplets with defined initial velocities. Suppose the velocity components of a water droplet d w along the x, y, z directions are Upi, Vpi (taken to be the vertical direction) and Wpi respectively. From Newton's second law, the equations of motion are:

dw.;

tt

2

mdt - - = - -gwW d p Cdlw pi·-wl(w pi·-w)

(2.11)

where m is the droplet mass (i.e. ~Pwd~), Fg is the body force due to gravity, Cd is the drag coefficient and Pw is the density of water droplet. The droplet size of each trajectory is assumed to have a Rossin-Rammler distribution (e.g. Alpert 1982) as: F(d)

d2

= 1- exp[-(ln2)dT l

(2.12)

m

where F(d) is the mass fraction of droplets having diameters less than d, d.; is the mean droplet size which can be calculated using the following equations: (2.13)

108

W. K. CHOW AND N. K. FaNG

where the Weber number We is expressed in terms of the nozzle diameter d; and the surface tension of water droplet a (e.g. Wendt and Prandt 1981): We

zd n = pwvra

(2.14)

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The initial droplet properties such as the position, velocity components, water temperature and angle of projectile are all taken as input parameters. The air properties such as 1', v, k , C p etc. are assumed to be temperature dependent and calculated from: (2.15) where "ii, i = 1, ... ,4 are empirical coefficients and "i being the values of either 1', v, k , C p as shown in Thble 1. The Reynold number R; is determined from the relative velocity between air and water droplet vre , ; (2.17) where d.; is the diameter of the water droplet and v is the kinematic viscosity of air. The air drag components D x , D y , D., experienced by the water droplets are given by:

D,

= p(Cd)AxIUa -

vplx(Ua - vph 2

D D

y z

= p(Cd)AyIU. -

vply(U. - vp)y 2

= p(Cd)AzIU. -

vplz(Ua - vp)z 2

(2.18)

where Cd is the drag coefficient which varies with Reynolds number; Ax> A y , A" are the projected areas of the water droplet, Ua is the velocity vector of hot air and vp is the velocity vector of the water droplets. Values of Cd are given by: 24 Re l / Z Cd = 0.6 0.47 0.2

Re < 1 1 < Re < 1 x 103 1 ;; 103 < Re < 3 x 105 3 X 105 ~ Re

(2.19)

The choice of this equation for Cd is reasonable and is consistent to the expressions used by other sprinkler researchers, e.g. Hesketed et at. (1976), Beyler (1978), Alpert (1986), Gardiner (1989) and Cooper (1991). All these expressions give values of Cd very closed to the experimental results of Yuen and Chen (1986) for Reynolds number of the droplet greater than 100. But it is unlikely to have sprinkler droplets moving with a Reynold number less than 100. Using the droplet size distribution (2.12), and the expression for the Reynold number given by equation (2.17), Reynold number smaller than 100 is found only when the droplet diameter is very small and the relative velocity with air is low. For example, when the water droplet diameter is less than 1 mm together with relative velocity below 0.5 ms- I , the Reynold number will be less than 33. This does not occur in the present simulation as the initial water droplet velocity magnitude used is greater than 4 ms :' and the mean droplet diameter being 3 mm.

INTERACfION OF SPRINKLER AND SMOKE LAYER

109

Surface Temperature Tw

Air Velocity ~

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diameter dw

FIGURE 2: Hot Air Stream Toward Water Droplet

(2.3) Microscopic Quantities

An important factor known as the Drag-to-Buoyancy, i.e. D/B ratio (e.g. Bullen 1974) which accounts for the stability of the smoke layer, can be calculated with the field modelling technique. This is achieved by summing up the total downward air drag experienced by the water droplets and the buoyancy of air in every control volume containing water. (2.20)

DlOtal

all cells containing water Btolat

9.81(p - Po)~V

=

(2.21)

all cells containing water

where Cd is the drag coefficient, A f is the frontal area of droplet, Vrel is the relative velocity in y direction, N D is the number of droplets, p is density, Po is the initial density and ~ V is the volume of the cell. The amount of convective heat transfer can also be determined. The Prandti number Pr is taken to be 0.7. The heat extracted by the sprinkler water, Qcool, is a sum of all the convective heat transfer for a water droplet travelling through the hot air (as shown in Fig. 2). It can be calculated from Nusselt number N; (e.g. Whitaker 1972).

Nu

= 2

+ [0.4(R e )t/2 + 0.06(R e )2/3]p r 2.4( 1l00 )t/4 /1",

(2.22)

where /100 is the viscosity of the free air stream and Ilw is the viscosity on water surface. The heat transfer coefficient h for each droplet of diameter d ; can then be computed by (e.g. Holman 1986): (2.23)

With it, the convective heat transferred to one water droplet can be calculated: (2.24)

W. K. CHOW AND N. K. FONG

110

where h is the convective heat transfer coefficient, A w is droplet surface area, T w is the droplet temperature and Too is the gas temperature. By summing all the cooling terms for the droplets, the total heat removed by the sprinkler water spray can be calculated: (2.25)

Qeool all cells containing droplets

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Section 3 : The Numerical Method A gas-droplet model for studying the interaction of the sprinkler water spray with a fire induced flow is reported in Section (2). The fire induced air flow field is described by a set of of non-linear elliptical partial differential equations. On the other hand, a set of ordinary differential equations is used to describe the motion of water droplets. These two set of equations are coupled and have to be solved numerically.

(3. J) Basic Equations The set of equations describing the fire induced air flow field is solved by the finite difference method (e.g. Roache 1976, Shih 1988, Fletcher 1988). Since the equations concerned are derived from the laws of conservation, there exists a balance between the unsteady term, the convection term, the diffusion term and the production or destruction term for each mean flow variable e which can be expressed in a general form (e.g. Patankar 1980): 8(p.p) + 8(puj.p) = ~(r~ 84» ot OXj OXj OXj

+ S~

(3.1)

The flow variables .p can be the velocity components (u, v, w), enthalpy (h), smoke concentration (f), turbulence kinetic energy (k) and its energy dissipation rate (0). The term r~ represents the diffusion coefficient and S~ represents the source term for the appropriate variables e, Integrating the continuity equation (3.1) over the control volume (as shown in Fig. 3) gives:

where aE aw aN

as aF aB

= = =

DeA (IPel) + max (-Fe, 0) DwA (lPwl) + max (Fw,O) DnA (IPnl) + max (-Fn,O) D,A (IPsl) + max (F"O) DfA (Wfl) + max (-Ff'O) DbA (IP bl) + max (Fb,O) ap = aE

(3.3) O~V

+ aw + aN + as + aF + aB + p b = Sc~V

- - - Sp~V ~t

(3.4)

0",0 ~V

+ Pp'l'P

~t

(3.5)

The power law scheme (e.g. Patankar 1980) is used: (3.6)

INTERACTION OF SPRINKLER AND SMOKE LAYER

111

N

F

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w

E B

s FIGURE 3: Control Volume

The discretization equation relates the global value of q, at the node P to its immediate neighbouring nodes E, W, N, S, F and B with local production terms. The coefficients a p, a E, a w, a «, as, a F, and a B reflect the contributions from distinct nodes due to the convective and diffusive transport of q, along the direction joining with the node P the neighbours E, W, N, S, F, B. The equation are solved using the Pressure Implicit with Splitting of Operators (lssa et al. 1985, 1986) with flowcharts shown in Figs. 4a and b.

(3.2) Numerical Solution lor the Droplet Equations The droplet equations are solved simultaneously with the air flow equations. The fourthorder Runge-Kutta technique is used for the system of ordinary differential equations under different sets of initial conditions. There are two reasons for choosing the RungeKutta method. First, it is a one-step method with the values to be calculated at the point (Xm+l, Im+d related only to the value of preceding point (xm ,1 m). Second, it is not required to evaluate any derivatives of the function I (Xm , 1m ) . Satisfactory results have been reported in studies concerning a cooling tower (e.g. Weinacht and Buchlin 1982) and a spray combustor (e.g. Boysan et al. 1982). From section (2), the equation describing the motion of the droplet is: dvp 1r d 2 Pdv-vv-v+F C I~ -1(- -) m--=-dt 8 p p p s

This can be expressed, in turn, as a function of droplet velocity and time dv p

(3.7)

I

(v p , I):

_

d/=/(v p , I)

(3.8)

112

W. K. CHOW AND N. K. FaNG

(

start

)

I

First Predictor for velocity

Solve for u' ,v' ,w (implicit)

First Predictor for Pressure

Use u' ,v' ,we ,plio Solve for t)' (implicit

.

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-

First corrector for velocity

Use u' ,v' ,w· ,p' Solve for u' ,v' ,w.. (explicit)

First Predictor for scalar variables

Solve for scalar variables h ,k' .e (implicit)

First corrector for pressure

.

Solve for p " (implicit)

... .

I " Solve 'Ior u tV... Second correc tor fo r W velocity (explicit)

I First corrector for scalar variables

Solve for h" ,k" (explicit) I

Second corrector fo r pressure

Solve for) (implicit

I Third corrector for velocity

u

vD.=

-

pm= p.'

.. .. ..

Solve for u-,v",w" (explicit)

Yes

(

No Stop )

FIGURE 4a: Flow Chart for Piso Scheme

..

v··

w m= WOO"

.. ,e

...

March to next lime step

m= u··"

t

INTERACfION OF SPRINKLER AND SMOKE LAYER

113

First Predictor step for velocity (implicit)

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Substitute the following eqns. into continuity eqn,

.. = u.+

Vn+ d rI. P~- P~). W, + d,( P~ - ~) ~

First Predlc lor step for pressure (Implicit) First Corrector step for velocity (explicit)

d.( P~- ~)

U.

a.U~·=

P: +

at P; + 6b

i\ +

U~b+

A.(

I;anb

••

P: +

a ... P~ + an

•

n

P~- P~)

.

.

•

First Corrector step for pressure (Implicit)

ap ~p = E

anb~"b + b

+ b:

••

n

••

n

aIW,= I;anb WDb+ A,( P p

First PredIctor step for. scalar variables (implicit)

+

b

anVrF I;aDbVDb+ A,,( P p ...

•

as Ps

PlI) + b"

Pr) + b,

-

"

where 4> n . h, k, e b : source term

.. =

••

••

r•

a" p... + a" P" + as p. +

8 p'Pp

8b

F\,' +

n

b

Second Corrector :'Itep for velocity (explicit)

Firsl Corrector step for scalar variables (explicit) Second Corrector step for pressure (Implicit) Third Corrector slep for velocitr (explicit

•••

lip Pp "

...

... ... •••

•••

•••

llo p. 1- a... p.. + a" P" t- a. p. fl llf P, "t llbl\+ b

11

•••

...

II

...

.....~

a"V,ra I;a."bV"b+ A,,( P p-

a,W',. Ea."b W"b+ A,( Pp -

FIGURE 4b: Equation Sets for Piso Scheme

'"

PIIJ T

Prj

+

b: D

+ bf

114

W. K. CHOW AND N. K. FONG

where

(3.9) By solving the equations with initial condition (vp)o at at t = II = to + 6t can be obtained from the formulae: _

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(v p)'

I

= to, the

new velocity (Vp)1

1

= (vp)o + 6(k 1 + 2k 2 + 2k 3 + k 4 )

(3.10)

where k , =

6t f«vp)o, to) k z = 6tf«v p)0 + t k" to + t61) k 3 = 6tf«vp)0 + t k2,to + 6 1) k 4 = 6If«vp)0+tk3,t,) (3.11)

t

This is a 'marching forward' approach (e.g. Dorn and McCracken 1972) and by repeating the process, values of (Vp)l, (vph, (vph, ... etc. can be evaluated as shown in Fig. 5. This time interval 61 must be kept sufficiently small in order to obtain satisfactory results. Once this set of equations is solved, the predicted droplet position and velocity components along the trajectories are applied to calculate the source terms for the momentum and enthalpy equations for the fire-induced air flow to describe the heat and momentum transfer between air and the water droplets. The initial droplet velocities, angles of projectile and water temperature are taken as input parameters. The total water flow rate is divided into five equal portions with each portion water having their own droplet sizes as shown in Fig. 6 and 7. These droplet sizes are calculated using the Rossin Ramler distribution function which is a function of mean droplet diameter. The mean diameter of droplet d m is obtained from (Prahl and Wendt 1988): (3.12) where We is the Weber number which depends on the water sheet velocity vr s the nozzle diameter d., the water surface tension a and the water density Pw. We

_ Pw v~ d. a

-

(3.13)

A projection angle is assigned to each droplet class. For each angle, five droplet trajectories are evaluated. The trajectories are assumed to be equally distributed about the axisymmetric axis. The droplet properties are calculated using the values predicted from the previous time step. This will be repeated until the water droplet reaches the ground. A detailed description of the algorithm for the droplet calculation is shown in the flow charts from Figs. 8 to 11. After calculating the droplet positions, a line segment is defined by joining the old and new droplet positions. The coupling of the droplet momentum to source term of the momentum equation for air flow is evaluated along each trajectory segment. This is repeated in regions from the sprinkler head to the position where the droplets hit the ground. Since the segments might overlap with some of the control volumes, the intersection point of the droplet segment with control volume surface has to be calculated (e.g. Bowyer and Woodwark 1983) for determining the sub-segments inside each control

INTERACTION OF SPRINKLER AND SMOKE LAYER

_0_

. ...... ....... ... . ... '>

~

~

o~o

I

I

~

:l

-

~

II II II

l:l.

o l:l .... 0

....

Q ....

~'Ol

-l 0

-e

a

"

. I

~

N

~

~ I FIGURE II: Flow Chart for Each Droplet Segment

INTERACfION OF SPRINKLER AND SMOKE LAYER

121

Droplet segments

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Control Volume

Droplet segment intersect with control volume face

Droplet Segments pass through control volume FIGURE 12: Droplet Segments Pass Through Control Volume

The corrected shear stress term will be added to the source terms in the air flow equation calculating the near-wall velocity components. (3.3.2) Boundary Conditions for the Enthalpy Equations

For the enthalpy equation, an adiabatic wall condition is assumed. The convective heat flux through the interior wall (h l t ux) is equal to zero and the free boundary condition is determined by the expression:

8h 8x

=0

(3.19)

as shown in Fig. 14. (3.3.3) Boundary Conditions for k and

to

Equations

Special treatment of wall boundary conditions for k is necessary. The local convection and diffusion of turbulence energy at a wall are assumed to be negligible and there is no flux contributed from the wall. The flux expression for k is suppressed by setting the coefficient as (shown in Fig. 14) equal to zero. The generation term G k for k p on the solid wall can be evaluated using the following expression (Gosman and Ideriah 1983):

Gk

= TsU p YP

where G k is shown in Table (2) and

Ts

is given by:

(3.20)

W. K. CHOW AND N. K. FONG

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122

o

o

II

II

I

, I

~+

-

I

",

e-

lL

--

-, I

§

L

§

~ -/ 0

o

rJ:l

~~ / /

~ >


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Variable 4>

r. o

Source of 4> ;

s.

0

':!JL

Rf (In fire plume area)

af

rs

0.42y+ =

I n(Ey+)

(3.21)

The modified generation term is then incorporated into the source term of the k equation to solve for k p at the wall boundary. For the e-equation, the following fixed equilibrium relation is used to describe e at the solid wall boundary: cp

=

C J / 4k 3/ 2 d ; ky

(3.22)

The treatment of k and e at the free boundary are similar to the enthalpy equation. The boundary conditions can be expressed as:

ok

ox

= 0

& -=0

ox

(3.23)

(3.3.4) Boundary Conditions for the Droplet Phase

No special treatment for the droplet equations is necessary. The velocity components of the water droplet Ud, Vd, Wd are set to zero when it hits the solid boundary such as the floor or the surrounding walls. The boundary conditions of the water droplets are shown in Fig.15. (3.4) Convergence

The accuracy of the final solution depends on the specified convergence criterion. This is very important and for obtaining converged results, several guidelines for handling the coefficients and source terms in the discretization equations are used (e.g. Patankar 1980). The following conditions borrowed from the convergence of the Gauss-Seidel iteration method is used:

W. K. CHOW AND N. K. FONG

126

TABLE 2b

"IF + Uj

General form of the equation: D""

Dp¢ D D. 7F. = 1£ (r.1£) J

J

I

+ S.

Effective diffusivity

for 1>

r

•

Variable 1>

Source of 1> :

q

~)

(1t,- +

h

III

s.

(in fire plume area)

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k

1.82 x 10- 5 kg.m.-ls- 1

G8

ILl

g Iil!!t tTy

GkiLl ~ (~ iJXj iJx/ C1

1.44; C z =

««

1.0 ;

"t

I "2 3 + 5.).. {)Xj I.J = 1.92; CD = 0.09

= 1.0;

fTr

= 1.3

R f is the flux Richardson number C 3 is given by Gn =

q ",

- G k (I -

C3 )

is a sink term in regions containing water droplets

D x ; DJ' ; D z are the drag force terms due to water droplets

L1a

bl

n =;-'-.,.--'- ~

lapj

~~:Ibl

1 for all equations

< 1 for at least one equation

(3.24) (3.25)

In addition, the sum of absolute residual errors in the solution of any variable ¢ is checked. The value has to be small, e.g. less than 10- 6 . A residual source term is defined as; (3.26) Convergence is assumed if the following condition is achieved: (3.27) where Rref. = 10- 6 and R¢ represents the sum of absolute residual errors for any variable ¢. This condition must be satisfied before marching to the next time step. Besides the

127

INTERACfION OF SPRINKLER AND SMOKE LAYER

MWr-

----.

.....

:3 0-

.s ~ I-.

2

f-

1.832 f - - - - - - 7 r - - - - - - - - - - - - - - - - - -

I I I I I I

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:3

~

1-

o

I

o

:

I

100

I

300

I

I

I

600

Time (sec) FIGURE 16: Heal Source Release Rate

above criterion, the requirements of having positive coefficients and negative values for the slope of the source term Spare followed throughout the computation (e.g. Chow and Leung 1990) described in the next section. Moreover, the mass conservation over the whole computational domain is tested using the following expression: ImaSSinl-lmassoUlI '----,---'------'-----,------' < O. I Imass ou! I

(3.5) Treatment in the k and

E

(3.28)

Equation

Since there is a strong heat source term in the enthalpy equation, large temperature gradients will be found in the region close to the fire source. This might give negative values of k and e, leading to error propagation and diverging results. The problem is found to be in the equations for k and e and can be eliminated by a checking scheme reported by Chow and Leung (1990). Section 4 : Numerical Experiments The full-scale burning facility at the FRS with geometrical configuration described by Young and Rogers (1977) was used. A wood crib fire source of size 2m x 1m x 0.9m was located 3m from left wall. The heat release curve was arbitrarily fitted to denote burning of wood as shown in Fig. 16. The whole building was divided into 51 x 9 x 19 control volumes as shown in Fig. 17. The computer used was a VAX6420 mini-computer. The time step was 0.1 sec and total CPU time being 40 hours for simulating the case listed below. The water spray was represented by a discrete and finite distribution of droplets of varying sizes and initial projectile angles projected into the fire field from a chosen starting point. The droplet size distribution was described by the Rossin-Rammler distribution function (e.g. Alpert 1985). Each class of droplet sizes was further discretized

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1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1

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1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1

W. K, CHOW AND N, K FaNG

130

TABLE 3 Simulation Conditions for Cases I to 7

outer angle (degree)

velocity

flow rate

(degree)

(rn/s)

(dm 3/min)

CASE I

40 to 44

CASE 2 CASE 3

40 to 44 90 to 130

89 to 129

4

CASE 4

90 10 130 90 to 130

89 to 129 89 to 129

4 4

651

90 to 130 40 to 44

89 to 129 39 to 44

4

50 50

CASE 5 CASE 6

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Inner angle

CASE 7

39 39

10 10

44 44

4 4

4

217 651 217 100

by considering a distribution of initial projectile angles in order to represent a full spray cone. The droplet equations were then solved using Runge-Kutta method. The solution for the droplet segments and the coupling of the sources were mentioned in section (3). (4.1) Part 1

In the first part, only the fire environment was simulated. The predicted velocity vectors, temperature and smoke contours at the central plane of the water spray are shown in Fig. 18. For the second part, the sprinkler was actuated to discharge a water spray when the air temperature rose to 185°C (i.e, about 150 sec after igniting the fire). The flow rate assigned to the water spray was 217 drrr'rmin. After solving the droplet equations, the resultant air flow and temperature fields due to fire and sprinkler were predicted. Fig. 19 shows the steady state air flow, temperature and smoke field together with the droplet trajectories. From this figure, the influence of fire induced flow by sprinkler water spray can be illustrated. In the last part of the numerical experiment, the water flow rate was increased to 651 dm 3/min to strengthen the water effect. Results on air flow, temperature, smoke, and water trajectories are shown in Fig. 20. Smoke logging was found in positions close to the sprinkler as demonstrated by the velocity vectors and smoke concentration contours. (4.2) Part 2: Varying Water Flow Rates and Projectile Angles

Another series of seven tests were performed by changing the initial droplet conditions such as water flow rates and projectile angles. The conditions for them are listed in Table (3). Results of the simulation are illustrated from Fig. 21 to Fig. 27 with the DIB ratio computed from each trial run shown in Fig. 28. The results on the flow field indicated that both the water trajectories and the air flow pattern were affected by the water flow rate and the droplet projectile angles. If the water flow rate was increased, the drag force would increase. Besides depending on the water flow rate, the drag force would also be affected by the projectile angles. For example, same values of water flow rate and initial velocity were applied in cases I and 3 but the dragging effects were much higher for case I. Similar results were obtained for the other four cases in view of Figs. 21 to 27. It is shown that the values of predicted D/B ratio did not exceed one, even though smoke logging was found in some regions. This is because the DIB ratio was calculated by summing up all the gas control volumes appearing in equations (2.20) and (2.21). The value of Dtatal would not be big enough since the air dragging effect was not so large in some regions. Therefore, the macroscopic parameter DIB ratio might be inadequate for specifying the stability of the smoke layer. This was not found when the sprinkler water

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1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1

1

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1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1

1

INTERACfION OF SPRINKLER AND SMOKE LAYER

133

spray was assumed not to be affected by the fire (e.g. Chow and Fong 1990a,b) in the earlier studies. Therefore, calculating the water droplet trajectories is very important.

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Section 5 : Conclusions The effect of sprinkler water spray on the fire-induced hot air layer was studied using the field modelling technique (e.g. Spalding 1980). The fire-induced air flow and temperature fields were simulated by solving a system of conservation equations of mass, momentum, enthalpy and turbulent quantities. The sprinkler water was described by a number of water droplets with different initial sizes and velocities discharged from the sprinkler head. Effect of fire on the droplet trajectories were then calculated by taking the air drag and convective cooling experienced while travelling through the smoke layer. The Particle Source-In-Cell method (e.g. Crowe 1977) was used to model the momentum and heat coupling between hot air and water. Neither combustion nor water suppression of the burning objects has been included. The numerical discretization scheme used was the first order Power Law. The set of pressure-linked equation were solved by the Pressure Implicit with Splitting Operators (PISO) scheme (e.g. Issa 1982, 1986). The water flow rate, droplet projectile angles and initial droplet velocities of the sprinkler were taken as input parameters for calculating the water trajectories. A microscopic view of the resultant fire-sprinkler air flow pattern, temperature contour and smoke concentration can then be predicted. By changing these input parameters, different cases of hot air flow patterns, water trajectories, temperature profiles and O/B ratio were evaluated. The advantage of this approach is that the resultant air flow, temperature and smoke pattern as well as the sprinkler water droplets trajectories can be computed. By observing the aerodynamic pattern, the positions where 'smoke-logging' occurs can be identified. Further, two important macroscopic parameters (e.g. Bullen 1974, Morgan 1979), i.e. the drag to buoyancy ratio O/B for the smoke layer and the amount of convective cooling due to the sprinkler water spray Qcool can be computed. Including the motion of water droplets is important since the air drag effect experienced by the water droplets affects the gas flow field. On the other hand, the heating effect of the hot gas and air dragging forces experienced by the water droplets would also influence their own velocities. The air drag effect experienced by the hot gas might be reduced and therefore calculating the water trajectories with fire effect is necessary. This is the reason why the results were different from these works obtained by neglecting fire effect on the water trajectories (e.g. Chow & Fong 1990a, 1990b) Obviously much work can be further developed along this line. Including the combustion process inside the plume will give a more realistic physical picture. But progress depends on the availability of a suitable combustion model. Perhaps the coherent flamelet model (Candel et al. 1990, Yang 1991) is a good choice. Fire suppression effect on the burning object might also be studied (Takahashi 1986). Thermal radiation effect from the fire (e.g. de Ris 1978, Modak 1978179) is another important issue. The effect has to be included in the fire model (e.g. Raycraft et al. 1988, Adiga et al. 1990) as well as on the water droplets (e.g. Ravigurajan and Beltran 1989). The water evaporation effects can also be investigated although it has been illustrated by Beyler (1977), Kung (1977, 1986) and Chow (1989) that the evaporative heat loss rate might be small. If the evaporation effect of water droplets are included, the turbulent structure concerned will be changed. Therefore a different turbulence model is needed for simulating the two-phase flow. A possible candidate is the two-equation turbulent model of Elghobashi and Abou-Arab (1983) which is derived from Reynolds decomposition and time averaging the instantaneous transport equations. Closure of the time-mean equation is achieved by modelling the correlation terms up to the third order. The turbulence parameter k and e can also be

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I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I

1 1

1 1 1

1 1

1 1 1

1 1 1

1 1 1

1 1

1 1 1

1 1 1

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1 1 1

1 1

1 1 1

1 1 1

1 1 1

1 1

1 1 1

1 1 1

1 1 1

1 1 1

1 1

1 1 1

1 1 1

1 1 1

1 1

1 1 1

1 1 1

1

1

1

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1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 I

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1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1I

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I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I

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148

W. K. CHOW AND N. K. FONG

C£D •

0.5 •

o

150

155

160

165

170

175 Time (sec)

FIGURE 28: DIB ratio for numerical tests (from case I to case 7)

used but a ,Lagrangian frequency function is now added. Also, the evaporating water (e.g. Mostafa and Mongia 1981) will either be handled by the Lagrangian (e.g. Dukowicz 1980) or the Eulerian approach (e.g. Mostafa and Elghobashi 1984). Theories available for droplet combustion (e.g. Abramzou and Siriganano 1989) might be helpful for this development. Lastly, detailed experimental studies have to be carried out as unlike for combustor (e.g. Boysan and Binark 1979, Boysan et al. 1982, 1986, Weber et al. 1990), most of the experimental results on sprinkle red fire reported in the literature (e.g. Yao and Kalelkar 1970, Heskestad 1976 and 1979, Heskestad et al. 1981, Lee 1986, Sako and Hasemi 1986, You et al. 1988, Vincent et al. 1988) were not quite suitable for validating field models. There are two routes to consider. The first one lies in measuring properties of the sprinkler water spray itself precisely. This includes the initial droplet size distribution and velocity components which are believed to be transient properties (e.g. Gardiner 1988); the spray pattern (e.g. Austin 1981, Wraight and Morgan 1986) for verifying the droplet trajectories to be computed from the theory of ballistic motion; variation of the droplet diameters as it travels through the air; the amount of water collected at the lower level, air entrainment rates of the water spray, thermal responses of the sprinkler, etc. The second one lies in the air flow and temperature field resulting from the interaction of the fire and sprinkler. Obviously, the objectives are to validate the predicted results and to provide empirical parameters for tuning the model. Studies on some of the above are in progress and will be reported later. REFERENCES Abbott. M.B. and Basco. D.R. (1989). Computational fluid dynamics. an introduction for engineers. Longman. Abrornzon, B. and Sirignano, W.A. (1989). Droplet vaporization model for spray combustion calculation, International Journal of Hea/ and Mass Transfer, 32, 1605-1618. Adiga, K.C.. Ramaker. D.E.. Tatem, P.A. and Williams, F.W (1990). Numerical predictions for a simulated methane fire, Fire Safety Journal, 16, 443-458. Alpert, R.L. (1972). Calculation of response time of ceiling-mounted fire detectors, Fire Technology, 8,181. Alpert, R.L. and Mathews, M.K. (1979). Calculation of large-scale flow fields induced by droplet sprays, Technical Report FMRC 1.1. OEOJ4.BU, RC79-BT-14, Factory Mutual Research, Norwood Ma, U.S.A.

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