Elsevier

Biosystems Engineering

Volume 119, March 2014, Pages 13-24
Biosystems Engineering

Research Paper
Characterising droplets and precipitation profiles of a fixed spray-plate sprinkler

https://doi.org/10.1016/j.biosystemseng.2013.12.011Get rights and content

Highlights

  • Droplet size and velocity for a fixed spray plate sprinkler were measured.

  • Equation developed to predict spray wetted diameter.

  • The deflection plate type has a great influence on the drop characteristics.

  • Equations were presented to estimate mean diameter and velocity of drops.

Droplet characteristics and precipitation profiles of a fixed spray plate sprinkler (FSPS) were characterised in some indoor experiments were conducted using various deflection plates (36-grooved blue, 30-grooved green and 36-grooved black plates). Four nozzle diameters (2.78, 3.97, 4.76 and 7.14 mm) were mounted on the sprinkler and operated at three pressures (69, 138 and 241 kPa) and at a nozzle height of 1.5 m. Drop characteristics were determined using a low speed digital photography method. The results showed that by increasing the nozzle size at a fixed operating pressure the resulted wetted diameter, peak application rate, droplet sizes and velocities were increased. With smaller nozzle diameters (2.78 and 3.97 mm) drop diameter increased as working pressure increased, while with larger diameters (4.76 and 7.14 mm) a reverse trend between drop size and working pressure was observed. Empirical equations were developed to estimate the wetted diameter and also volumetric mean diameter (VMD) and volume median diameters (D50) of droplets at different distances from the sprinkler as functions of sprinkler configurations.

Introduction

Size distribution of the drops discharged by the water jet of an agricultural sprinkler has major influence on evaporation losses, modifying the infiltration capacity of the soil, and distortion of water distribution pattern applied by sprinkler (Kincaid et al., 1996, King et al., 2010, Montero et al., 2003, Salvador et al., 2009). The drop break-up process is quite complex. The relative speed of the water jet is sufficient to cause disintegration into drops in air with inertia, viscosity and capillary forces involved in this process. However, the complexity of the breakage process makes a rigorous theoretical analysis more difficult. Nevertheless, it appears that the drop formation begins at the surface of the jet and continues towards the centre (Kohl, 1974, Montero et al., 2003, Seginer et al., 1991, Von Bernuth and Gilley, 1984). The development of drop characterisation techniques such as stains, flour pellets, oil immersion, momentum, photographic and optical methods have been reported by a number of researchers (Bautista-Capetillo et al., 2012, King et al., 2010, Montero et al., 2003, Salles et al., 1999, Salvador et al., 2009, Sudheer and Panda, 2000).

Centre pivot irrigation is a popular system due to its remarkable advantages such as its large area of coverage, ease of use, and degree of automation. Currently, a wide range of sprinkler devices is available for the centre pivot irrigation from conventional single or double nozzle impact sprinkler with different types of nozzles to various types of low-pressure spray plate sprinklers. Low-pressure spray plate sprinklers can be classified as fixed spray-plate sprinkler (FSPS) and rotating spray-plate sprinkler (RSPS). These sprinklers produce very different droplet size distributions and water application patterns (DeBoer, 2002, Faci et al., 2001, HaiJun et al., 2010, Kincaid, 2005, Kincaid et al., 1996, Sourell et al., 2003).

FSPSs can be equipped with different plates having various numbers or sizes of grooves to break up the nozzle jet into discrete water drops. Several researchers have shown that the shape of water application pattern of a single FSPS is similar to a wetted circular “crown” with drop sizes concentrated in a narrow range of diameters (Clark et al., 2003, DeBoer, 2002, Faci et al., 2001, HaiJun et al., 2010, Kincaid et al., 1996, King and Bjorneberg, 2010). The deflection plate reduces the velocity of the water, and the initial trajectory velocity across the plate is less than a nozzle jet velocity (Kincaid, 1996).

Many researchers have found that the spray droplet size at any distance from the sprinkler is related to the nozzle size (Dadiao and Wallender, 1985, Kincaid, 1996, Kincaid et al., 1996, Kohl, 1974). Kincaid et al. (1996) reported that at a fixed working pressure, the average droplet diameters of a grooved-plate sprinkler nozzle increased with increasing nozzle sizes. Also they stated that with spray heads, drop sizes were more influenced by the size of nozzle than the working pressure. Kohl and DeBoer (1984) observed that for low pressure spray type agricultural sprinklers, the geometry of the spray plate surface, rather than the nozzle size and operating pressure, was the dominant variable that influenced drop size distribution.

Many research papers reported that the uniformity coefficients ranging from 70% to 90% for the FSPS were lower than those for the RSPS in terms of the same nozzle diameter, nozzle height, sprinkler spacing and working pressure (Clark et al., 2003, Delirhasannia et al., 2010, Faci et al., 2001, Playán et al., 2004). Kohl, Kohl, and DeBoer (1987) reported that the total droplet evaporation losses from the FSPS would be expected to range from 0.4% to 0.6% (Kohl et al., 1987). Faci et al. (2001) presented that the wind drift and evaporation losses were lesser for the FSPS than those for the RSPS.

The objective of the current study was to determine the distribution of the water drops size, velocity and angle formed by an FSPS. It also aimed to discover if these characteristics are affected by different factors such as working pressure, nozzle size and deflection plate specifications.

Section snippets

Sprinkler

An FSPS (Nelson D3000, Nelson Irrigation Co., Walla Walla, WA 99362-2271, USA) spray head with four nozzle diameters of 2.78, 3.97, 4.76 and 7.14 mm and three operating pressures of 69, 138 and 241 kPa were used in the experiments. Because of pump discharge limitations, the combination of a 7.14 mm diameter nozzle with the working pressure of 241 kPa could not be implemented. Three deflection plates (blue, green and black) were used in the current study. Therefore, 33 different combinations of

Characterising radial application pattern

For the first step in sprinkler characterisation, water distribution (at the observed sector) and radial application patterns were obtained using catch-can data. Figure 4 represents the distribution of water precipitation rate versus the distance from the sprinkler for the three analysed deflection plates with nozzle size of 4.76 mm and working pressure of 138 kPa. As shown in Fig. 4, the distribution of water resulted from one distinct jet of the sprinkler has very large variability in both

Conclusions

The size distribution of the droplets discharged by the water jet of a sprinkler is very important, as this can explain several processes related to water distribution. An indoor experiment was conducted to obtain droplet size distribution and radial application patterns of a sprinkler fitted with various deflection plates, nozzle sizes and working pressures. The results showed that various configurations of the sprinkler could greatly affect droplet and water distribution characteristics. An

Acknowledgement

This research was supported by the Department of Research and Technology, University of Tabriz. The use of certain products in this study does not imply any endorsement of them.

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