Evaluations of emitter clogging in drip irrigation by two-phase flow simulations and laboratory experiments

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Abstract

Emitter clogging will affect greatly the irrigation efficiency and the running cost of a drip irrigation system. If there is an effective method to predict the emitter clogging, the lost will be reduced to a minimum. A solid–liquid two-phase turbulent model describing the flow within drip emitters was studied. Then, the moving trace and depositing feature of suspending solids in emitter channels were obtained by computational fluid dynamics (CFD) based on the turbulent model established, which provided some visual and direct evidences for predicting the clogging performance of drip emitters. The three types of emitters with novel channel form, including eddy drip-arrows, pre-depositing drippers and round-flow drip-tapes, developed by us were used in this study. The simulation results showed that the solids moved along a helical path in the eddy drip-arrow, but no obvious deposition existed in its interior channel. In the pre-depositing dripper, some solids concentrated in the parts of “depositing pones”. In the round-flow drip-tape, a small number of solids adhered to the out-edges of every channel corner, which was a potential factor for the occurrence of emitter clogging. To verify the predictions from the CFD simulations, series of “short-cycle” clogging tests for the three emitters were conducted in laboratory. The statistical data of discharge variation caused by emitter clogging were in good agreement with the two-phase flow CFD simulations.

Introduction

Emitter clogging has become the key obstacle to the development of drip irrigation technology. Once a drip emitter is clogged, its discharge will reduce greatly (Boman, 1995). Meanwhile, the discharges of non-clogged emitters in the same lateral will increase contrarily (Bralts et al., 1981). Thus, the irrigation uniformity of the whole drip irrigation system will decrease greatly (Nakayama and Bucks, 1981, Chieng and Ghaemi, 2003). Therefore, emitter clogging will increase the maintenance cost of drip irrigation systems (Oron et al., 1991), and reduce the working life and the use efficiency (Zhai et al., 1999). As a result, emitter clogging is one of the key factors that determine whether drip irrigation systems can success (Zhu and Cui, 2005).

For a low discharge rate and high irrigation uniformity, most current commercial drip emitters adopt labyrinth channels with a small flow section (usually less than 1.0 mm2). However, the flow in the small and close channels of drip emitters is difficult to be observed and measured by using any traditional experimental methods, such as particle image velocimetry (PIV), acoustic Doppler velocimetry (ADV), and laser Doppler velocimetry (LDV). Without the flow characteristics of fluids passing through channels, emitter clogging cannot be predicted accurately and timely. Therefore, some researchers used some macro-hydraulic parameters, including emitter discharge, operating pressure, irrigating frequency and so on, to establish statistical models to analyze emitter clogging. Talozi and Hills (2001) developed a mathematical model, consisting of a set of equations based on fundamental hydraulic principles and the discharge relationships of inlet laterals, to simulate the effects of emitter clogging on subunit hydraulics. Based on a statistical model, Pedras and Pereira (2002) developed a computer program AVALOC, to simulate emitter clogging in practical applications of drip irrigation. After inputting a group of field data, the program will analyze automatically clogging probability, and output some suggestions for clogging controlling and recovery. Subramanian and Senthilvel (2003) applied Weibull theory to fit temporal variations of emitter clogging in a 20-week drip irrigation application, and used a Monte-Carlo method to predict when specific treatments should be done to recover the emitter clogging. All these statistical simulations can predict the degree and the time of emitter clogging, but cannot reveal the relationship between emitter clogging and the channel structure of drip emitters, as well as the flow characteristics of fluids in emitter channels. To make clear the reason of emitter clogging, some researchers used computational fluid dynamics (CFD) to visualize the “black box” of the inner flow field within drip emitters.

Wang et al. (2000) used the commercial software FLUENT to analyze the flow characteristics of cylindrical labyrinth emitters, and pointed out that the anti-clogging performance of this kind of emitters mainly depended on the movement of swirls existing in the channels. Supposing a laminar flow, Palau-Salvador et al. (2004) simulated the relationship between the pressure and the discharge rate of in-line labyrinth emitters by use of FLUENT, and indicated that the simulation results had a good agreement with experimental measurements. Meng et al. (2004) and Wei et al. (2005) both used FLUENT to simulate the inner flow various types of drip emitters, and drew a conclusion that the stagnant flowing areas in emitter channels were the main reason for emitter clogging. Based on these inferences, they put forward a “main-route anti-clogging design” method to aid the design of new-form drip emitters. All the CFD simulations mentioned above can obtain some micro-hydraulic results, such as the distributions of water pressure and velocity, which provides a visual analysis method for emitter clogging, and makes clogging predictions more direct. However, these CFD simulations just considered the flow in emitters as single-phase water flow, but neglected the hydraulic performances of various suspended solids. Therefore, it is difficult to reveal the location and the reason of emitter clogging accurately. In this study, we try to evaluate emitter clogging by solid–fluid two-phase flow CFD simulations, and to verify the simulations through “short-cycle” clogging tests in laboratory.

Section snippets

Tested emitters

Three new types of emitters, including orifice eddy drip-arrows, long-path pre-depositing drippers and long-path round-flow drip-tape, developed by us were used for this study. The products and their parameters of the three emitters are shown in Fig. 1 and Table 1. To make use of the discharge-regulating and anti-clogging functions of “hydro-cyclone”, “depositing-pond” and “pier” in hydraulic engineering, we designed novel channels (Fig. 2) that differ with the labyrinth case of most current

Simulating results of the eddy channel

The simulating results of suspending solids in the eddy channel are shown in Fig. 5. The moving trace shows that the suspending solids pass through the eddy channel along a helical path, and revolve more times at the big-circle end of the inlet cone because of a high rotating velocity. The force analysis of suspending solids in the cones is shown in Fig. 6. A solid particle, P, is driven to revolve under three forces, including gravity G, wall-holding force FN, and resistance of the channel

Conclusions

The solid–liquid two-phase CFD simulations can obtain some micro-hydraulic characteristics of water and solids passing through drip emitters. Too many stagnant areas of water flow will not be helpful to the movement of the solids; meanwhile, the roundabout movement and local high concentration of the solids mean the occurrence of emitter clogging. From the simulation results, it can be found that suspending solids move along a helical path in the eddy channel, under the strong flushing of some

Acknowledgements

This study is supported by National Natural Science Foundation of China (No. 50709011), the State “863” Project of China (No. 2006AA100214-8), Postdoctoral Science Foundations of China and Huazhong University of Science and Technology (No. 20060400248; No. 20070001) and the Transferring of Agricultural Achievements of Science & Technology Foundation of China (No. 2006GB23600448).

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