Experimental set-up and procedure 74 

4.2.2.1 Determination of flux density distribution

An experimental rig is constructed to measure the distribution of the flux density produced by the sprays. This is achieved by placing a 2 m 2 m 0.1 m water collection tray beneath the nozzles. To spatially resolve the distribution of flux densities, the tray is divided into 400 compartments, each with dimensions of 10 cm

10 cm 10 cm. The single-orifice and the multi-orifice nozzle heads are clamped at heights of 2.3 m and 2.0 m, respectively, above the floor. Water is supplied to the nozzles by means of a pump that could operate up to a pressure of 400 bars. The experimental rig is illustrated in Figure 4.3.

Figure 4.3: Schematic view of the experimental set-up.

The experiments are designed to measure the distribution of flux density produced by the single and the multi-orifice nozzles. It has been found that the key feature of the distribution of flux density produced by the single-orifice nozzle is best obtained by placing the nozzle head above the centre of the water collection tray. This set-up is designated as Case A. However, as might be expected, multi-orifice nozzles produce a maximum intensity of distribution directly beneath the centre of the nozzles and in regions that correspond to their azimuthal orifices. Hence, in this situation, experiments are carried out with the multi-orifice nozzle located above the centre and at one of the corners of the water collection tray. The former set-up is similar to that of the single-orifice nozzle and referred to as Case A, and the latter set-up is referred to as Case B. In both cases, the boundary wall was located at 2 m away from the nozzle head. The locations of the nozzles and boundary wall for these two cases are shown in Figure 4.4.

Chapter 4: Spray distribution – a benchmark experiment and validation of FDS 

Figure 4.4: Location of nozzle (a) Case A; (b) Case B.

To supply the water from the reservoir to the nozzle head, the pump is operated at a pressure of 34.5 bars for the single-orifice nozzle spray and this has produced a volume flow rate of 1.7 L/min. In the case of the multi-orifice nozzle, the pump is operated at a pressure of 70 bars and this has produced a volume flow rate from the central orifice comparable to that produced by the single-orifice nozzle; the total volume flow rate is 8.8 L/min. The nozzles are allowed to operate until their flow regime is stabilised. The sprayed water is collected on the tray that enabled the flux densities (L/m2/min) to be measured to an accuracy exceeding 99%. The angles of the sprays for both of the nozzles are determined from the photographs of sprays. Schematic views of measuring the angle of sprays are illustrated in Figure 4.5. The parameters of the spray i.e. water flow rates, spray angles and spray heights of the experiment were used as input parameters in the FDS model.

Figure 4.5: Schematic view of the measurements of the angle of sprays (a) spray produced by the single-orifice nozzle; (b) spray produced by the multi-orifice nozzle.

In the experiment: the volume flow rates of the sprays produced by the nozzles are measured. In the case of the multi-orifice nozzle spray, the flow rates for the central and the azimuthal orifices and for the whole nozzle are measured separately: they are 2.2, 1.1 and 8.79 L/min, respectively. The measured flow rates from the nozzle heads are also corroborated by means of the following correlation [Tanner and Knasiak

2003]:

√ 4.1

where Q is the discharge rate of water and P is the operating pressure of the water flow. According to the manufacturer’s data, the K-factor of the multi-orifice nozzle is 0.073 L/min/psi1/2, which gives a flow rate of 8.74 L/min. This also validates the accuracy of the experimental measurement of the water flow rate.

4.2.2.2 Determination of median diameter of droplets

The median diameter of the droplets of the spray produced by the multi-orifice nozzle is available from the manufacturer’s data. The median diameter of the droplets of the spray produced by the single-orifice nozzle is determined using the same experimental set-up, and the procedure of determining the median diameter of droplets is described below.

Firstly, the flux density distribution of the nozzle is measured for a certain flow pressure (P1). The water flow rate and spray angle are also recorded. A numerical

tool is required to simulate this experiment. This is followed by carrying out simulations with a range of the median diameters of droplets. The distribution of flux densities is calculated for the spray of each median size of the diameter of the droplets. These are compared with the corresponding experimental data of the distribution of flux densities for the flow pressure of P1. When the numerical data of

distribution matches the experimental measurement, the corresponding size of the droplet can be taken as the hypothetical size of the droplet (dm1) of the spray. Then, a

Chapter 4: Spray distribution – a benchmark experiment and validation of FDS 

mathematical relationship is used to determine the second median size of the droplet (dm2) for a different flow pressure (P2). If the numerical tool can accurately simulate

the second set of distribution of flux densities using dm2 and P2, this will provide

confidence that the numerical tool is well validated. Hence dm1 and dm2 can be

considered the actual median diameters of droplets corresponding to the pressures of P1 and P2, respectively.

According to Fleming [2008], the median diameter of droplets generated by a sprinkler has been empirically found to be inversely proportional to one-third of the power of water pressure and directly proportional to the two-third power of the orifice diameter i.e.

∝ 4.2

where dm is the median size of droplets, D is the orifice diameter and P is the flow

pressure. Therefore, for a specific nozzle, the relationship between two median diameters of droplets corresponding to two different pressures can be expressed as:

4.3

From the first experiment and numerical simulation, is determined for the corresponding . Conducting a second experiment with a different flow pressure ( ), a second set of the distribution of flux densities is collected. Using Eq. 4.3, is calculated for the corresponding P2. Finally, this median diameter of droplets and the corresponding pressure are used as input parameters for the spray in the numerical model. If it is found that the numerical distribution of flux densities matches the experimental measurements, then it can be concluded that the numerical model is capable of predicting the distribution of flux densities, and the

the true median diameter of droplets at pressures P1 and P2, respectively. Therefore,

using this method, the median diameter of droplets of a spray can be estimated corresponding to their flow pressure.