Download Application of Phase Doppler Particle Analyser in Agricultural Sprays

Application of Phase Doppler Particle Analyser in Agricultural Sprays
JIA Weidong, LI Pingping
Modern Agricultural Equipment and Technology Key Lab of Jiangsu province,
Jiangsu University, P.R.China, 212013
[email protected]
Abstract The basic structure and measurement principle of Phase Doppler Particle Analyzer (PDPA)
were introduced. Relative measurement errors were analyzed. And the system was utilized to test
agricultural sprays with applied voltage and not. It is indicated that PDPA measurement system has great
advantages in the agricultural sprays test compared to other non-contact measurement instruments. The
main movement patterns of spray field at different test conditions were analyzed and reviewed
conveniently by the system. Spray angle can be measured directly with high precision. Visual
distribution of droplet numbers is helpful to design of related equipments and performance predictions.
Key words PDPA, Measurement principle, Spray test
1 Introduction
It is very important to analyze and control the velocity and diameter of droplets quantificationally,
which impact on the relationship between spatial motion track and plant target interface directly[1].
Pattern information of 3D spray field can not be obtained by traditional measurement of droplet
diameter. Malvern Particle Meter, Particle Image Velocimetry (PIV) and Laser Doppler Anemometer
(LDA), which are all laser measurement technology, are widely used in spray researches[2-4]. The former
can only test the distribution of volume median diameter of droplets without velocity information, but
the last two the opposite. Phase Doppler Particle Analyzer (PDPA), developed in recent ten years, has
become an accepted standard facility for diameter and velocity measurement of spray droplets and other
spherical particles[5]. It overcomes the limitations in above means completely and offers a well test
method to study on dynamic process of agricultural spray.
For years, Thompson, Miller, Smith et al. have successively set up models of spatial motion track
of droplets to forecast spray drift[6-8]. But those models have shortages due to limitation of experimental
means. More meticulous researches must depend on the accurate measurement of droplets movement
parameters on line. Therefore, measurement principles of spray droplets velocity, diameter, fan angle
and numbers by PDPA were introduced in this paper. And the system was utilized to test agricultural
sprays with applied voltage and not in order to illustrate the characteristics and advantages in
agricultural spray test.
2 PDPA Synopsis
2.1 Basic stucture
The PDPA system used in this paper is produced by Dantec. It consists of hardware and software
systems. Hardware system includes laser generator, optics system with transmitting and receiving units,
3D traverse and signal processor. BSA software is used as data processing system. The basic structure is
showed in Fig 1.
383
Fig1 Basic Structure of PDPA
2.2 Measurement principle
2.2.1 Velocity measurement
Information of particle velocity is gained from Doppler frequency received from moving particles,
which is the same to LDA. The relation between Doppler frequency and moving velocity is[9]:
（1）
v = f ⋅δ
δ=
（2）
λ
2 sin γ 2
In formulas, f is Doppler frequency, λ is wavelength of incident light, γ is incident angle.
2.2.2 Droplets diameter measurement
Droplets diameter is calculated from phase difference of scattering light between various detectors.
The formula is[9]:
Φi = α ⋅ βi
3
α=π
（）
（4）
n1
D
λ
Here, Φi is phase of one Doppler signal received by detectors.
βi
is geometrical factor fixed by
scattering model and optics parameters setup. n1 is refractive index of scattering medium. λ is laser
wavelength in vacuum. D is particle diameter.
2.2.3 Spray angle measurement
Spray angle and atomization pattern are influenced greatly by external factors, such as jet pressure,
wind speed and so on. Spray image is not regular. Edge is often not a line. Therefore, there’s not a
uniform standard for fan angle calculation. There’re several determining methods for them at present,
for example, outlet spray angle, condition spray angle, obvious line jet flow edge. Agricultural spray
angle can be measured conveniently by PDPA according to condition spray angle. Spray edge can be
determined by received samples in a sampling time[10].
2.2.4 Droplet number density measurement
Besides size and velocity measurements, PDPA can also be utilized to measure droplet number
density. There’s no need to test droplet diameter accurately, but to determine the control volume size and
droplet counts to get the density. It can be showed as:
∑j ti,j (d i )
1
N= ∑
T i Ωi (d i )
Ωi (d i ) = Ai (d i )Wi (d i )
384
（5）
（6）
Ai (d i ) =
π
[rm (d i )] 2
4
（）
7
Here, N is droplet number density. T is the total time all droplets passing through the control
volume spent. tij ( d i ) is the time drops with d i transitting the control volume spent. Ωi ( d i ) is the
volume drops with d i passing through the control volume. Ai ( d i ) is the projection area of control
volume on the plane perpendicular to velocity direction of drops with d i . Wi ( d i ) is the length drops
with d i going through the volume. rm ( d i ) is the effective volume diameter.
Adopting the method above, droplet number density is obtained depending on the accumulating
passing time (or resident time) droplets through the volume relative to total time, but not the droplet
counts.
2.3 Errors analysis
PDPA system itself has definited errors. Doppler frequency received in the test is not a single
frequency but with a width, which is called Spectrum Frequency Widen. This phenomenon will result in
errors. There’re many reasons for it. For example, the finite transit time scattering droplets passing
through the volume, velocity gradient and Brownian motion of drops in the test field, and so on. The
errors caused by Frequency Widen can be controlled by system parameters setup suited to the flow field.
Location of measurement volume has certain influence on the results. If it was done depending on
human eyes, it will bring big errors. Signal-watch method, determining the start point by observing
Doppler frequency received, is taken here. Then 3D traverse system is used to precisely locate.
Lock-in phase average method is utilized in data processing. Considering every error above,
velocity measuring precision is up to 0.2%, diameter to 0.5%, number density and fan angle to 1% in
agricultural spray test by PDPA.
3 Application of PDPA Measurement
3.1 Test set and scheme
In order to show the advantages of PDPA using under different conditions, two statuses, with
applied voltage and without that, were adopted to analyze the differences in spray angle, droplet
diameter, velocity distribution and drops number. Test set is showed in Fig 2. Electric charge device is a
combination electrode consisted of ring and pin electrodes. Common spray nozzle is chosen to compare
in the two conditions. Nozzle diameter is 1.0mm.
℃
Fig 2 Diagram of PDPA Test Set in Spray Measurement
Test scheme is as following. Environment temperature is 25 . Relative humidity is 68%. Testing
medium is water. Nozzle pressure is 0. 35 MPa, and its flux is 750 mL/min. Testing is under three
operating conditions, with applied voltage of 0kV, 20kV and 40kV. Reference frame is set. Vertical
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direction (axis Z) is along center line of the spray. Horizontal direction (axis Y) is along transmitting
detector orientation. The intersection point of two directions on the horizontal is zero. Distance of
measuring line along horizontal direction from the vertical are 80mm, 130mm, 180mm, 230mm,
280mm.
3.2 Test reasults
3.2.1 Spray angle
Change of spray fan angle under three conditions is showed in Tab 1. The calculated datum line is
280mm away from nozzle. The edge where sampling number is less than 30 in 30sec is considered as
spray edge. Agricultural spray angle under different conditions can be measured directly by PDPA.
Applied voltage(kV)
Fan angle(0)
Tab 1 Spray Angles Under Various Applied Voltages
0
20
49.5
51.5
40
53.9
3.2.2 Droplets velocity and diameter distribution
Fig 3 shows the drops velocity and VMD distributions without applied voltage and with applied
voltage (40KV). Rulers of velocity and diameter in the figures are average value under two conditions.
The VMD is getting smaller and velocity of them increasing with applied voltage, compared to that
without applied voltage. Homogeneity and symmetry of two parameters also improved. It is indicated
that diameter spectrum of spray drops with applied voltage gets narrow under repulsion function of the
same charge. Meanwhile, spray area is increasing with applied voltage at the same spray quantity which
leads to decrease the chance of collision and agglomeration in droplets. This also decreases the kinetic
energy wasting of a single drop. Combined with the distributions, it can be known that atomization
performance enhanced largely with applied voltage.
Test results in Fig 3 show great advantages of PDPA applying on agricultural spray measurements.
It can measure the drops velocity, diameter distributions simultaneously. Therefore, this system can be
used to analyze the main motion structures of spray field under different conditions conveniently.
（a） Results Without Applied Voltage
（）
b Results With Applied Voltage 40KV
Fig 3 Drops Velocity and Diameter Distributions With and Without Applied Voltage
3.2.3 Droplet number density distribution
Isoline diagram of droplet number with and without applied voltage is showed in Fig 4. It increases
obviously and distributes uniformly with applied voltage which related to spray characteristics with
electric charge.
In agricultural spray field, droplet number distribution always determines the atomization quality.
Test results by PDPA are visual, which is helpful to design of related equipments and performance
predictions.
386
（a） Results Without Applied Voltage
（）
b Results With Applied Voltage 40KV
Fig 4 Isoline Diagram of Droplet Number Distributions With and Without Applied Voltage
4 Conclusions
PDPA measurement system has great advantages in the agricultural sprays test compared to other
non-contact measurement instruments. It can express kinds of important messages such as drops velocity,
diameter distributions, spray angle, drops number and so on. All the information is not only helpful for
realizing the motion structures of spray, but also to design of related equipments and performance
predictions.
(1) PDPA system can measure the drops velocity and diameter distributions simultaneously. It can be
used to analyze the main motion structures of spray field under different conditions conveniently.
(2) Spray angle can be measured directly by PDPA using condition spray angle concept. It is much more
precise than indirectly test.
(3) In agricultural spray field, droplet number distribution always determines the atomization quality.
Test results by PDPA are visual, which is helpful to design of related equipments and performance
predictions.
Acknowledgments
The authors would like to express their gratitude to the financial support by the National Science
Foundation of China (50476068), China Postdoctoral Science Foundation (20070411023) and Jiangsu
Planned Projects for Postdoctoral Research Funds (0602011B).
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