Effects of high-frequency pressure pulse generated by a jet tee on the clogging of labyrinth emitter

Jin Binbin, Wang Xinkun※, Huang Song, Fan Erdong, Yan Hailan, Yao Jicheng, Yu Jianjun

(1. Research Center of Fluid Machinery Engineering and Technology, Jiangsu University, Zhenjiang 212013, China;2. Nanjing Automation Institute of Water Conservancy and Hydrology, Ministry of Water Resources, Nanjing 210012, China; 3.Hydrology and Water Resources Engineering Research Center for Monitoring, Ministry of Water Resources, Nanjing 210012, China)

Abstract: Emitter clogging is the key obstacle to the development of drip irrigation technology.For this reason, a jet tee was developed in this study.Taking three pressures (constant pressure, fluctuating pressure on the left and right sides of the jet tee.)and three turbid concentrations as factors, the experiment was carried out by using a complete combination of the test factors.A study of labyrinth emitter clogging was conducted to determine the effects of high-frequency pressure pulse generated by a jet tee versus constant pressure on the relative discharge of Drip irrigation emitters (Dra) and the coefficient of uniformity (CU).The results showed that the values of Dra and CU of the jet tee group were 10.8%-14.8% and 21.1%-44.9% higher than the ordinary tee group, respectively.At different muddy sediment concentrations (0.5, 1.0 and 1.5 g/L), for constant pressure groups, Dra decreased to 74% after the 16th, 11th and 7th irrigation run respectively and was considered as a severely blocked.However, for the left side of the jet tee, Dra was still higher than 75% after the 18th, 14th, and 11th run respectively.For the right side of the jet tee, Dra was still higher than 75% after the 16th, 13th, and 10th run respectively.It can be seen that high-frequency pressure pulse generated by a jet tee has stable anti-clogging performance.At the low (0.5 g/L) and the medium (1.0 g/L) concentrations of muddy water, the emitters did not easily clog.The Dra at the high muddy (1.5 g/L) sediment concentrations remained to decrease drastically.However, the Dra at the low muddy sediment concentrations remarkably decreased after the 12 th run.The Dra at the medium muddy sediment concentrations remarkably decreased after the 10th run.For pulse pressure group with high concentration, the Dra decreased to 76% after the 11th run.However, for pulse pressure group with the low and the medium concentration, the Dra was still above 92% after the 11th run.For pulse pressure group with the low and the medium concentration, the CU remained above 84% after the 12th run.However, for pulse pressure group with high concentration, the CU decreased below 80% after the 7th run.The Dra and CU of the high muddy sediment concentrations (1.5 g/L) differed significantly (P＜0.01) from those of the low and medium muddy sediment concentrations (0.5 and 1.0 g/L).Under three muddy sediment concentrations, the possibility of clogging quickly increases at high muddy sediment concentrations (1.5 g/L).However,at this concentration, the jet tee was still more resistant to clogging than the ordinary tee.It is recommended that a jet tee be used in place of the ordinary tee in the drip irrigation system to prevent emitter clogging.

Keywords: pulse generator; high sediment water; high-frequency pressure pulse; clogging of labyrinth emitter; drip irrigation

0 Introduction

Agricultural production is considered as the major consumer of water supplies[1].Due to limited water resources, drip irrigation technology has received more attention and been a promising way to solve the problem of water shortage[2-3].Drip irrigation is one of the most effective agricultural irrigation methods to spray water into the root area of plants slowly and directly.However, the clogging of drip irrigation emitters has become one of the obstacles that limit the application and promotion of drip irrigation technology[4].Nakayama et al[5].found that the clogging level of emitters were directly related to the water quality.The water used for irrigation in the Northwest China arid region mainly is from Yellow River water with high sediment concentration[6], and in recent years the Yellow River irrigation area has rapidly adopted efficient drip irrigation technology to save water.The sediment particles with less than 0.1 mm particle size can still enter the emitter channel with filtered water[7], therefore, drip irrigation with muddy water led to the sedimentation of various minerals in the clogging materials and accelerated the risk of emitter clogging[8].The clogging of the emitter has a direct impact on the emission uniformity[6,9], which can causeinsufficiency of water volumes needed for plant growing.Therefore, many researchers had thoroughly studied the problem of emitter clogging.Moreover, these clogging can be divided into physical clogging, chemical clogging, and biological clogging[10].Physical clogging are mainly caused by solid particle[11].Ford-Harry[12]found the cause of chemical clogging of emitters, and proposed corresponding treatment methods.Zhou et al[13].found the exact quantitative relationship between biofilms components and emitter clogging degree.However, the clogging problem of emitters is still not perfectly solved.

Historically, constant water pressure has been always used in the irrigation systems[14].Recently, the pulse pressures has been used in irrigation systems, which provides a new way to solve the problem with clogging emitter.Alnoeem[15]found that pulsing discharges generated by pulse generator might decrease the emitter clogging.Pulsating pressure improves the ability of the flow to transport particles, and significantly improves the anti-clogging performance of the emitter[14,16-18].However,these pulse generators have the characteristics of high cost,complicated installation and maintenance.Scholars mainly studied the anti-clogging performance of low-frequency (the period is 10, 30 and 60 s) fluctuating water pressure on emitters, while the anti-clogging performance of the emitter by high-frequency (period 0.27 s) pulses generated by the jet tee had been rarely studied.

The jet tee that applies the technology[19]of jet wall-attachment and periodic switching to drive water to form a pulse water flow in the drip irrigation emitter has the characteristics of simple structure, low cost, reliable operation, high pulse frequency and high irrigation uniformity.Xu et al[20].studied the hydraulic performance of drip irrigation system with jet tee, and found that jet tee could effectively improve the irrigation uniformity of drip irrigation system.When the pressure wave generated by the jet tee propagates well in drip irrigation tape at 60 m long,the inlet flow of the jet tee is designed to range from 0.5 to 0.8 m3/h.The head loss in drip irrigation tape connected with jet tee is 62.5%-83.3% lower than that in drip irrigation tape connected with ordinary tee; the irrigation uniformity coefficient of drip irrigation tape connected with jet tee is 0.6%-0.9% higher than that of drip irrigation tape connected with ordinary tee; the deviation of emitter flow rate in the drip irrigation connected with the jet tee are 1.2%-4.1% lower than that in the drip irrigation connected with ordinary tee[21].

In this paper, jet pulse generator (jet tee) with new structural parameters was developed, which only depended on the hydrodynamic jets to generate pulses without the use of elastic materials and electronic components.The jet tee was used in the experiment of labyrinth emitters’anti-clogging performance.The objectives of this study were to test the anti-clogging ability of jet tee when the total inlet water pressure of the test system was 75 kPa.Based on this,a new method of applying high-frequency pulses to drip irrigation was proposed in order to solve emitter clogging.

1 Materials and methods

1.1 Jet tee and drip irrigation tape

The three-dimensional model and two-dimensional model of jet tee are shown in Fig.1.The structural size of the jet tee is the same as the size of the jet tee in Ref.[22].Its structure includes the jet element, the inlet and outlet section.

Fig.1 Physical and structural drawings of the jet tee

The drip tape with labyrinth emitter manufactured by Shanghai Irrist in Shanghai city, China, which is commonly used in agricultural irrigation, was installed on the test platform.It has an external diameter of 16 mm.The labyrinth emitter is presented Fig.2 and Table 1.The outer diameter of ordinary tee is about 16 mm.

Fig.2 Structural drawing of the labyrinth emitter

Table 1 Characteristic parameters of the emitter.

1.2 Experimental setup

The experiment was conducted in the Micro Irrigation Laboratory of the Water Saving Irrigation Technology Research Center of Jiangsu University.The flow of the outlet of the emitter was determined by the weighing method[10].The dynamic pressure in the drip irrigation tape on both sides of the jet tee was captured by a high-speed camera (JT-HD61E model; the frame rate is 10 000 frame/s;the focal length is 50 mm.) and recorded by i-SPEED,finally the curve was made through Origin 2019.The testdesign is shown in Fig.3.The anti-clogging test platform was built with ref[11].The frame of the test platform was built by angle steel shelf.The drip irrigation tape was installed on the platform as shown in the Fig.3.Each of the 15 tapes with length of 1.8 m and 5 emitters.In the test,there were 75 emitters in each group.Beneath each emitter,there was a plastic cup collecting water from the emitter.The weight of the water was measured by an electronic balance (with an accuracy of 0.1 g) to calculate the emitter flow rate.

Fig.3 Diagram of the test set

1.3 Experimental method

The possibility of emitter clogging was dramatically increased when the particle diameters were larger than 0.05 mm[9,11,23].The majority of researches are focused on clogging problems induced by solid particles with diameters around 0.1-0.25 mm[11].Therefore, all the particle sizes of sand used in the experiment were less than 0.1 mm.The actual average maximum sediment concentration of irrigation water was about 0.8 g/L in practice[11].To accelerate emitter clogging and study the effect of concentration on the clogging of emitters, the sediment concentration of irrigation water were 0.5, 1.0 and 1.5 g/L in this experiment.Using the above sandy water, the drip irrigation tape was tested under the ordinary tee group and jet tee’ left and right exit group respectively.The experimental treatment was shown in Table 2.According to the test method for short time plugging of emitters[24], the operation cycle and interval time of each test drip irrigation belt were 30 min.For convenient description, the irrigation process of a 30 min operation and a 30 min intermission was referred as one irrigation event.The nominal pressure of drip irrigation is 100 kPa and low pressure drip irrigation is a current trend in the irrigation.However, when the constant pressure was 50 kPa, the emitter was easily blocked.Therefore, the total inlet water pressure of the test device was determined to be 75 kPa by adjusting the first valve.At the end of irrigation event, the flow rate of each emitter was measured to calculate the relative discharge of drip irrigation emitters (Dra).When theDrais 75%, the irrigation was stopped and frequency of irrigation events and the number of completely blocked drippers were recorded.

1.4 Evaluation method

The relative discharge of drip irrigation emitters (Dra)was calculated by Eq.(1)[25]for each irrigation event:

Table 2 Experimental treatments

In this experiment, according to the head loss of drip tape and dripper manufacturing variation were very small,the flow difference among emitters is mainly due to emitter clogging[11].Therefore, the coefficient of uniformity (CU)could be used to evaluate the emitter clogging level.The CU was calculated by Eqs.(2) and (3) as follows[26]:

whereis average flow of each emitter along the lateral at timet, L/h.

If theDrawas less than 75%, then the emitter was considered to be clogged[23].The outflow uniformity of the drip irrigation emitters could be represented by CU, which reflected the randomness during emitter clogging.The irrigation capacity of emitters was considered to be excellent when CU was greater than 89%.The medium condition was achieved when CU ranges between 71% and 89%; however,when CU was lower than 71%, the emitter performance was considered to be poor[27-28].

2 Results and analysis

2.1 Pressure pulse generated by a new jet tee

The pulsating pressure and constant pressure in the experiment were measured by a pressure gauge at the head of the drip irrigation tape (Fig.4).The pulse amplitude and oscillation frequency of the pulsed flow generated by the jet tee were 17 kPa and 223 cycles per minute, respectively.The data acquisition method has been used in previous studies[29].Two pressure gauge were present before and after each set of drippers.By noticing any variation of the pressure differential value during the experiments, wefound that the fluctuation frequency of the pulsating pressure behind each group of droppers was the same as that of the pulsating pressure before the dropper.In short,the pressure wave generated by the jet tee propagates well in drip irrigation tape.

Fig.4 The pulsating pressure versus time

2.2 Changes in the relative discharge

The relationships between the relative discharge of drip irrigation emitters (Dra) tested system and irrigation times under the condition of connected common tee or jet tee were shown in Fig.5.TheDraincreased by 10.8%-14.8%, compared to the ordinary tee group.For the test system connected jet tee and common tee, the relative discharge of drip irrigation emitters decreased as number of irrigation increased at three concentrations of muddy water(0.5, 1.0 and 1.5 g/L).However, theDrareduced more remarkably in ordinary tee group than that in jet tee along the increase of irrigation times.At the low concentrations of the muddy water (0.5 g/L), theDraof the ordinary tee group decreased below 75% after the 16th irrigation.It was showed that the emitters had reached the criterion for severe emitter clogging[24].But theDraon the left and right sides of the jet tee just respectively decreased to 83.7% and 83.8%after the 16th irrigation.When the sediment concentration was 1.0 g/L, for the ordinary tee group, theDradecreased to 74% after the 11th run.However, for the jet tee group, theDrawas still higher than 75% after the 13th run.When the concentrations of the muddy water was 1.5g/L, for the ordinary tee group, theDradecreased to 73% after the 7th run.However, for the jet tee group, theDrawas still higher than 84% after the 7th run.It showed that the emitters of the ordinary tee group were clogged extremely, whereas the emitters of the jet tee group were just slightly clogged.Therefore, the emitters of the jet tee group had displayed an excellent anti-clogging performance.

Fig.5 Influences of jet tee on the relative discharge of drip irrigation emitters for different concentrations of muddy water

2.3 Changes in the irrigation uniformity of drip irrigation.

The coefficient of uniformity (CU)of drip irrigation systemwere calculated at the end of the 18th run, 14th run and 11th run, respectively, for three constant concentrations of 0.5, 1.0, and 1.5 g/L, to evaluate the clogging(Fig.6).The CU increased by 21.1%-44.9%, compared to the ordinary tee group.A decrease in CU with increased irrigation frequency means that the emitters began to clog.For the ordinary tee group with the constant concentration of 0.5 g/L, theCU began to decrease after the 6th run.However, for the jet tee group with the constant concentration of 0.5 g/L, theCU began to decrease after the 9th or 10th run.For the ordinary tee group with the constant concentration of 1.0 g/L, theCU began to decrease after the 5th run.However, for the jet tee group with the constant concentration of 1.0 g/L, theCU began to decrease after the 8th run.At the end of the 14th irrigation event, the CU values of HLN1, HRN1, and CN1 were 76.0%, 83.9%, and 68.0%, respectively.CN1 declined to poor grade, and the other treatments (HLN1, HRN1) all obtained medium grade.At the end of the 9th irrigation event, the CU values of HLN2, HRN2, and CN2 were 94.3%,91.7%, and 68.7%, respectively.CN2 declined to poor grade,and the other treatments (HLN2, HRN2) all obtained good grade.At the end of the 5th irrigation event, the CU values of HLN3, HRN3, and CN3 were 82.1%, 92%, and 68.0%,respectively.HRN3 obtained good grade, HLN3 declined to medium grade, and poor grade was obtained by CN3.These results indicated that the sand particles in the muddy water could not easily deposit in the emitter channel of the jet tee group.Additionally, theCU was under 80% after the 9th RUN for CN1 group, while it was above 80% after the 13th RUN for HLN1 and HRN1 group.For CN2 group, theCU remained above after the 7th run.However, for HLN2 and HRN2 group, theCU was above after the 12th run.TheCU remained above 80% after the 3th run N for CN3 group, while it was under 80% after the 6th run for HLN3 and HRN3 group.The CU was higher than that of the ordinary tee under different muddy water concentrations.The CU values of jet tee for three concentrations were 21.1% (Fig.6a), 44.9%(Fig.6b), and 42.1% (Fig.6c) which were higher than ordinarytee respectively.These results further proved that the rate of the emitter clogging could be greatly reduced by the jet tee.

Fig.6 Influences of jet tee on the coefficient of uniformity for different concentrations of muddy water

2.4 Impact of muddy sediment concentrations on emitter clogging

Fig.5 and 6 showed that at the low (0.5 g/L) and the medium (1.0 g/L) concentrations of muddy water, the emitters did not easily clog.TheDraat the high muddy(1.5 g/L) sediment concentrations remained to decrease drastically.However, theDraat the low muddy sediment concentrations remarkably decreased after the 12th run.TheDraat the medium muddy sediment concentrations remarkably decreased after the 10th run.For pulse pressure group with high concentration, theDradecreased to 76%after the 11th run.However, for pulse pressure group with the low and the medium concentration, theDrawas still above 92% after the 11th run.For pulse pressure group with the low and the medium concentration, the CU remained above 84% after the 12th run.However, for pulse pressure group with high concentration, the CU decreased below 80%after the 7th run.These results proved that the possibility of clogging quickly increases at high muddy sediment concentrations.Because, as concentration increasing, the possibility of collision and interaction among parcels enhanced and the particle sedimentation got thicker as well.In order to analyze the influence of muddy water concentration on emitter clogging, one-way ANOVA was conducted by Excel2013 software in this paper.Table 3 showed that theDraand CU of the high muddy sediment concentrations (N3) differed significantly from those of the low and medium muddy sediment concentrations (N1 and N2).In contrast, theDraand CU values of the low muddy sediment concentrations (N1) were closed to those of the medium muddy sediment concentrations (N2).

Table 3 Significance analysis of Dra and CU under different particle concentrations.

3 Discussion

Some studies showed that applying the low frequency(the period is 10, 36, 40 and 60 s; fluctuation frequency is 0.02-0.1 Hz) fluctuated water pressure could effectively reduce clogged drip irrigation emitter[30].At present, the fluctuating water pressure is mainly realized by frequency converter and programmable logic controllers (PLC).The investment for installing frequency converters and PLCs in the drip irrigation system in the He Tao irrigation district of northern China was 2 300-2 620$/hm2[31].However, the investment in jet tee was 15$/ hm2.The project cost of using the jet tee as a pulse water flow generator should be reduced.Thus, this study developed a jet tee (pulse generator) to make fluctuated water pressure.To test the anti-clogging ability of jet tee, the experiment on the clogging of labyrinth emitters in ordinary tee and jet tee group was conducted.We found that jet tee weakened the clogging of emitters and significantly increased the CU andDravalues.The CU andDraincreased by 21.1%-44.9% and 10.8%-14.8%,respectively, compared to the ordinary tee group.This result was mainly because the fluctuating water pressure generated by the jet tee, which played a role in flushing the emitter and reduced sediment inside the emitter.As the pressure flow enters the jet tee, it was driven by the negative pressure generated by the feedback control channel to achieve intermittent flow at the outlet of the runner.We found that the jet tee had a pulse frequency of 3.7 Hz.Existing pulsed drip irrigation was also difficult to construct a drip irrigation system with high-frequency pulses because it was limited by the response time and sensitivity of the mechanical movement.Compared with the pulse generator applying frequency conversion speed regulation technology, the jet tee was considered as a high-frequency pulse generator.High-frequency pulse drip irrigation created a stronger turbulent water flow in the drip emitter, which was beneficial for flushing the irrigation runner.Therefore, it exhibited a better anti-clogging ability.The anti-clogging properties of the jet tee were mainly related to the pulsed water flow.

The jet tee is a pulse generator with a simplified structure.It eliminates the electronic pulses, inverter and elastomeric structure in conventional pulse generators.The elastomeric construction includes rubber, plastic membranesand springs that are easily fatigued and damaged.As a result,the jet pulse generator is more reliable and long-lasting in operation.The jet tees are made of the same materials as the commonly used plastic tees.Once a new mold is developed,it can be injection-molded using the existing tee's processing methods and equipment.The processing technology is simple and the manufacturing cost is low.By replacing the common tees or bypasses in conventional drip irrigation with jet tees, a pulse drip irrigation system can be formed easily, and it is easy to use.Therefore, jet pulse drip irrigation is feasible in terms of developing means, economy and practical results.And it has significant advantages in anti-clogging performance, management and maintenance.

For this study, particles less than 0.1 mm in diameter were used, and no filters were used.A certain concentration of sediment was necessary that caused particle collision and flocculation.As the sediment concentration rose, the distribution of particle concentration in the channel increased[9].If the irrigation time was short, the low (0.5 g/L)and medium (1.0 g/L) concentrations of the muddy water do not significantly affect the clogging of emitters.When the sediment concentration reached a certain level (1.5 g/L), the particle concentration had a significant effect on emitter clogging.Finally, when the concentration of particles in the irrigation water is high, it is recommended that a good filter be applied to the jet tee’ pulse drip irrigation system in order to reduce the concentration of irrigation water.

In general, some good results and conclusions were obtained.However, due to the limited test time, this paper also has the following shortcomings: It is necessary to further study the relationship between the anti-clogging characteristics of the jet tee and the inlet pressure; It is recommended to conduct anti-clogging tests in the field at a later stage and further verify the conclusions of this article.

4 Conclusions

1) The experimental values of the relative discharge of drip irrigation emitters (Dra)and the coefficient of uniformity(CU)in jet tee group were higher than that in ordinary tee group by 10.8%-14.8% and 21.1%-44.9%, respectively.Therefore,the emitters of the jet tee group had displayed a very good anti-clogging performance.

2) TheDraand CU of the high muddy sediment concentrations (1.5 g/L) differed significantly (P＜0.01) from those of the low and medium muddy sediment concentrations (0.5 g/L and 1.0 g/L).In contrast, theDraand CU values of the low muddy sediment concentrations(0.5 g/L) were closed to those of the medium muddy sediment concentrations (1.0 g/L).

3) The possibility of clogging increases quickly at the high muddy sediment concentrations (1.5 g/L).However, the jet tee group was still more resistant to clogging than the ordinary tee group.When the irrigation water had a high sediment concentration, the filter needed to be installed in a pulse drip irrigation system with a jet tee in order to achieve a better anti-clogging effect.

In conclusion, the jet tee has stable anti-clogging properties compared to ordinary tee.Therefore, it is recommended to use new jet tee instead of ordinary tees to prevent clogging of the emitter.