Water spray

Water deluge modelling with the Static Water Spray Model (SWSM) addresses two key physical aspects of water spray mitigation effects:

1. Affecting the reaction rate during explosions, and

2. Providing radiation shielding

While the first aspect is effective only for explosion simulations, radiation shielding is relevant in both explosion and fire scenarios. Therefore, in fire simulations, the current water deluge model should be positioned at a distance from the fire. This is because it does not include fire extinguishing effects but functions as a water curtain or sprinkler system to shield equipment from the fire's radiation. The water spray model effects are explained in the Water spray section of the Technical Reference chapter. The scenario parameters that define a water spray region are described in the diagram and an example of a scenario setup for non-uniform conic shape spray is given below.

Attention

The deluge model in FLACS only models the effect of water spray on deflagrations and radiation shielding. It cannot model the effect of deluge on dispersion or fire extinguishing scenarios

Insert

This integer field identifies the specific water spray region. Note, regions are automatically numbered when created in CASD. In the current version of FLACS, there is no limit on the maximum number of water spray regions.

Position

Cartesian coordinates [m] of the corner of the box-shaped water spray region (the lowest coordinate value in each axis). FLACS will assign the nearest grid line to your given position as the actual position of the water spray region i.e., snap the water spray region to grid lines. Spray regions are also allowed to overlap with each other.

This region acts as a bounding box for the water spray. Note that the positions of nozzles are defined within the water spray region by defining NOZZLE_POSITION.

Size

The extent [m] in each of the axis directions is given for the box-shaped water spray region. All three dimensions should be positive. FLACS will assign the nearest grid line to your given (position + size) as the actual maximum of the water spray region.

Note

FLACS will issue a warning message if any water spray regions are defined partially or completely outside the computational domain.

Volume fraction

The volume fraction refers to the liquid water volume fraction, which is defined as:

The liquid water volume fraction can be estimated using the water formula found in the Water spray section 8.18 in the FLACS User’s manual.

Typical values for the liquid water volume fraction for water spray applications, are expected to be in the range 0.1-0.4 [litre/m³] (Dale, E. K., 2004).

If the volume fraction is less than 0.01 [litre/m³] (unlikely for industrial water deluge applications), FLACS will provide a warning that the water spray might not be effective in explosion scenarios.

Mean droplet diameter

The mean diameter MEAN_DROPLET_DIAMETER [micrometer] of the water droplets before break-up due to acceleration of the gas flow, is defined by the user. In the uniform water spray model, it is assumed that all droplets have the same size, and that the droplets are uniformly distributed in space inside the water spray region. This is an approximation as in most real situations there will be a droplet size distribution, which in most cases is non-uniform in space, i.e., the distribution may change from one region to another region. In the water spray model, the mean droplet diameter is defined to be the so-called Sauter Mean Diameter (SMD or D32). This diameter is defined as the mean of the diameter cubed, divided by the mean of the diameter squared. The SMD depends on the operating water pressure forcing water out of the nozzle. In some cases, SMD values are specified in the technical documentation of the nozzle, an empirical relation to calculate the SMD from operating pressure and nozzle properties is provided in chapter Technical Reference.

When SHAPE is defined as a "spray", there is an option to apply a non-uniform droplet size distribution. Activation of that option is described in the following section, while the algorithm is described in section Non-uniform water distribution of the Technical Reference chapter of the FLACS User’s Manual.

Water spray shape

The entry SHAPE is a string containing information about water spray shape. There are three available water spray shapes: "box", "curtain", and "spray".

1. Box: a box shape is equivalent to the water spray region described by POSITION and SIZE entries and fills it uniformly with the water volume fraction and droplet size provided by the user. The model is activated when the SHAPE entry is defined as "box". It is also the default shape when no SHAPE entry is provided by the user.

2. Curtain: a half cylinder shape water curtain with the convex side facing the specified direction (+X, -X, +Y, -Y, +Z, -Z). For SIZE (Length, Width, Height): the radius of the curtain is determined by the height (H) along the specified direction. The thickness of the water curtain is determined by the shorter edge of the box perpendicular to the specified direction i.e., width, W. In curtain water sprays only uniform water droplets distribution is available.

3. Spray: a conical water spray requiring the following parameters:

   1. DIRECTION, dir: water spray cone direction (+X, -X, +Y, -Y, +Z, -Z)

   2. ANGLE, : water spray cone angle, between 0-180 degrees

   3. NOZZLE_DIAMETER, D_n: water spray sprinkler nozzle diameter [mm]

   4. NOZZLE_POSITION: water spray nozzle position defined in Cartesian coordinates [metres] within the water spray box

   Note that if the cone is broader than the spray region, it will be clipped by the bounding box. The spray cone axis is located at the center of the cell closest to NOZZLE_POSITION. By default, the water volume fraction and water droplets uniformly fill the specified region.

Non-uniform distribution

To include non-uniform distribution of water volume fraction and droplet sizes to more realistically represent FLOW a conic water spray properties, the user must set NON-UNIFORM to 1 (default is 0) within the CS file. In CASD this can be set in the water spray tab when a spray shape water spray is selected.

Upon selecting non-uniform distribution, additional inputs are required:

• The water volume rate, VOLUME_FLOW_RATE, [litre/min]

• Initial velocity of water droplets INITIAL_VELOCITY, U0  [m/s]

The volume flow rate can be estimated using relations provided in the Water spray Technical Reference section of the FLACS User’s Manual.

The extent of the water spray, unless clipped by the box, is limited by the so-called wetted diameter, which is calculated using the droplets' trajectory based on the momentum equation. This calculation depends on the droplets' size, cone angle, and initial velocity.

More information about the non-uniform distribution fields algorithm is provided in Non-uniform water distribution section of the FLACS User’s Manual

Attention

The non-uniform distribution algorithm is based on empirical relations calibrated from a limited number of experiments with downward conic water spray systems. Therefore, it can be used only in the "-Z" direction. If V is not specified or specified incorrectly, the solver will run with uniform distributions.

Water spray start time

By default, the water spray is activated at the start of the simulation. This can be modified in CASD by changing Time. Conversely, TIME can be included as a parameter in the CS file.

In explosion simulations, the effect of the water spray is immediately reflected in the combustion model.

In fire simulations, the effect of radiation shielding dominates. Therefore, the water spray TIME value must be synchronized with the interval between calls to the radiation solver DTM_MOD_TIME (the default value of DTM_MOD_TIME is 0.25 in FLACS-Fire and 0.01 otherwise). Consequently, it is recommended to use integer multiples of DTM_TIME to set the water spray TIME.

Attention:

There is an associated entry called DROPLET_START_TIME in the radiation model section. By default CASD automatically sets its value to zero. This means the radiation model will account for the absorption and scattering effects of water at the earliest start TIME of water sprays. For most applications, it is recommended to use only the water spray start TIME. This avoids situations where the water spray is active, but its effects are not yet accounted for in the radiation model due to a later DROPLET_START_TIME.

DTM domain

Radiation absorption and scattering by a water spray are only accounted for within the DTM domain. Therefore, it is important to ensure that monitored surfaces or points located behind the water spray, positioned between the radiation source and the monitor points are within the DTM domain. To optimize the simulation setup, DTM_AUTO_MIN should be used to ensure minimal coverage in the region of interest.

Alternatively, the FULL DTM domain can be applied for complete coverage, although this may increase CPU time.

Water spray example

Scenario files are included in the FLACS example folder. A demonstration of the water spray setup is provided here.