Experimental Study on Spray Uniformity

This study involves simulating rainfall conditions to analyze rainfall intensity and spray uniformity, conducting environmental experimental research. Using numerical analysis methods, water mass fraction cloud maps under different flow conditions were obtained to determine the relationship between uniformity, water flow rate, and water mass fraction. Based on the simulation results in accordance with relevant standards, two methods for measuring rainfall uniformity are selected based on water pressure, nozzle type, and spray height as basic parameters for setting measurement points, and rainfall environmental simulation experiments are conducted. The theoretical analysis provides a clearer representation of water quality distribution than steady-state analysis; water mass fraction is proportional to water flow rate, while uniformity is independent of water flow rate. Through comprehensive comparison of uniformity data for different nozzle models under varying water pressure and height conditions, recommendations are made for the placement of rain gauges in rainfall environmental experiments. Adjusting various test environment parameters can provide references for the test lab to accurately select test equipment, parameter measurement, and evaluation; the water mist environment generated during rainfall is simulated through spray towers, and the numerical uniformity test results of the water mist environment are consistent with the requirements, providing theoretical data support for developing more authoritative test methods for equipment sprinkler tests and ultra-fine water mist environment simulation in the future.


Introduction
In extreme weather conditions, rainfall affects equipment communication and causes restricted equipment operation.A large amount of domestic and international researches [1,2] has shown that if the sealing performance of the equipment is poor, rainwater seepage into the internal electronic devices can cause some functional damage and failures [3] or accidents [4], weakening the working ability of the equipment.Water mist is generated during rainfall and enters the internal electronic devices.The internal circuits and other precision components are affected by air humidity, the working environment temperature changes, and liquid condensation may occur, affecting the normal operation of electronic devices.Relevant standards [5][6][7][8] require that if the equipment needs to operate in a rain environment, rain tests must be conducted to verify its water resistance [9][10][11][12].In this study, rainfall environments were simulated using nozzles [13].Under the influence of different water pressures and sprinkling frame heights, different nozzle models were used for environmental simulation tests, resulting in different levels of rainfall intensity and uniformity.Rainfall sprinkling test is a necessary means to examine the adaptability of equipment to withstand rain environment.The degree to which it simulates real-world conditions directly impacts the confidence level of the assessment results.However, the natural rain environment is complex and variable, and a large amount of data accumulation is required to determine the test conditions.To meet the test requirements, determine the nozzle model, water pressure, and sprinkling height used in the test, and complete the equipment commissioning before the test in a short time, it is necessary to study the nozzle selection, rain intensity, rain uniformity and its influencing factors during the test [14].The water mist rainfall environment is simulated by spray towers.The mist particle diameter sprayed by the spray tower is between 1-5μm, allowing for water mist settling experiments to be conducted.

Numerical simulation
According to Reynolds' transfer theory [15], the continuity equation for 3D incompressible unsteady flow can be simplified as: When the volumetric force is only gravity, the momentum equation can be simplified as: Considering that the flow state of water in the spray pipeline is turbulent, the spray array model uses a k-flow field model containing turbulent kinetic energy (k) and turbulent dissipation rate () for calculation.Assuming that the flow field is completely turbulent, ignoring inter-molecular viscosity, the equations [16,17] have the following form: Gk is the turbulent kinetic energy caused by the average velocity gradient, and the turbulent viscosity coefficient is: According to Lauder's recommended values, the relevant parameters use software default values: C 1ε =1.44, 2ε =1.92, k =1.0, =1.3.To establish a sprinkler array model, a block-structured grid format and a turbulent flow model were used.Component selection and discrete phase settings were applied.A fragmented model was employed for the positions of the four nozzles.Since the nozzle length has no effect on the sprinkling [18], individual nozzles were not independently depicted in the model.Instead, the nozzle's performance characteristics were accurately simulated by setting the nozzle diameter and cone angle.The injection angle of the nozzle model was set to 60°, the nozzle spacing was 0.7m, and the nozzle diameter was set according to the pore size of three centrifugal atomizing nozzles: 1.5mm, 1.6mm, 3.2mm and 4.2mm.Four different nozzle combinations were used in the spray field CFD simulations, namely 1.5mm×4, 1.6mm×4, 1.5mm×2+3.2mm×2,and 1.6mm×2+3.2mm×2.These combinations were analyzed to observe the distribution of water quality for different nozzle array configurations.Taking the example of 1.5mm×4 nozzle combination, the steady-state analysis diagrams of water mass fraction under different flow conditions are shown in Figure 1.The results show that transient analysis provides a clearer representation of water quality distribution compared to steady-state analysis.When selecting an initial condition with a water flow rate of 20L/h, nozzle spacing of 0.7m, and sprinkler array height of 2m from the bottom are selected as initial conditions, the water mass fraction distributions after spraying for 10s, 15s, 20s, and 30s have the most uniformity compared with other flow rates.The instantaneous analysis mass fraction cloud map of water after 600s is shown in Figure 4.The distribution of water at the bottom is relatively uniform, indicating that the uniformity is independent of the water flow rate.Water mass fraction is proportional to the water flow rate, and the water flow rate correlates with the ejection velocity of water droplets.In subsequent research, to obtain complete trajectory of water droplets and clear water quality analysis cloud maps of various nozzle array models, it may be necessary to increase the time increment steps in the calculations appropriately.

Spray uniformity analysis
The nozzle [19] vary in model types, each having different injection angles.Water droplets sprayed out are affected by gravity during their descent, resulting in a smaller actual coverage area.When multiple nozzles simultaneously spray, they interact with each other.Therefore, simulating the distribution of rainfall unit water volume based on this interaction will inevitably yield significant differences between simulated results and actual experimental results.To validate the simulations, the rainfall unit test benches were constructed based on the simulation results, following relevant standards.Solid conical spray nozzles with three different models (1.5mm,1.6mmand 3.2mm) were installed on the sprinkling frame.Different combinations and water pressure adjustments were used to cover a certain range of rainfall intensity values required in practical applications.The arrangement of rainfall intensity measurement points for uniformity is illustrated in Figure 5.In Figure 5(a), the placement method is as follows: Rain gauges 1 to 9 are placed directly below the test nozzle of a specific model, and rain gauges 10 to 13 are placed at the center point of the square.In Figure 5(b), the arrangement is as follows: Gauge 5 is located directly below the center nozzle, and the other measurement points are centered around gauge 5, with 40cm spacing between each rain gauge [17] .The waterproof sprinkler array in the rainfall test lab mainly uses 4.2mm nozzle model.As weapon equipment often carries out waterproof tests with rain intensity requirements, an analysis of rainfall intensity and spray uniformity for this nozzle under different water pressure and sprinkling height conditions was conducted.During the test analysis, the data in the table covers water pressure ranging from 0.02MPa to 0.4MPa, with rain gauges placed according to Figure 5(a).The last set of data pertains to a water pressure of 0.3MPa, with rain gauges arranged as shown in Figure 5 According to the test data records of the nozzle in Table 1 and Figure 7,the following conclusions can be drawn: (1) As the water pressure increases, the rain intensity also increases, but the overall uniformity decreases.When the rain gauges are placed according to Figure 5(a), the sprinkling uniformity is poor.There is no clear correlation between water pressure and rain intensity.(2) Rainfall intensity varies at different measurement points, depending on their positions, leading to differences in uniformity.To achieve relatively uniform rainfall intensity, it is advisable to place test specimens closer to the center.Overlapping areas experience higher rainfall intensity compared to non-overlapping regions.(3) The degree of uniformity improves as rain gauges are positioned in closer proximity to each other.The spatial arrangement of rain gauges plays a crucial role in determining uniformity, significantly affecting the calculation and measurement of rainfall intensity.(4) When placed as shown in Figure 5(b), with a spacing of 40cm and a water pressure of 0.3MPa, the average rain intensity ranges from 1.68cm/min to 1.75cm/min, equivalent to 100.8-105cm/h.Different heights (95cm, 112cm, 152cm, 222cm, and 272cm) were chosen.Calculations revealed that as the height increased, the sprinkling uniformity increased, with uniformity values of 0.75, 0.79, 0.723, 0.798, and 0.806, respectively.Based on these findings, it is suggested to avoid using this nozzle model for waterproof tests with rainfall intensity requirements.For rainfall intensity testing, it is advisable to use a large-area spray rack.Additionally, the rain gauges should be placed closer to the center of the spray rack to ensure uniformity and achieve relatively consistent rainfall intensity.Drawing upon the above experimental data and analysis conclusions, this study focuses on 1.5mm nozzle array as an example, conducting measurement of rainfall intensity and spray uniformity.To achieve the required rain intensity for testing, the selection of water pressure, sprinkling height, and sprinkling uniformity can be guided by the data in Table 3.This approach allows for the rapid and accurate enhancement of test responsiveness when measuring rainfall intensity before testing.The relationship between water pressure, height, rainfall intensity, and spray uniformity is illustrated in Figure 7. Based on the nozzle test data recorded and the analysis presented in Figure 7, it can be summarized that when rain gauges are arranged in the manner shown in Figure 6(b) and appropriate nozzles are selected for sprinkling, the rainfall intensity increases proportionally with water pressure.Under the same water pressure, an increase in height leads to improved sprinkling uniformity.Several studies [20-24] suggested that the water mist environment generated during the rain test could be simulated using a spray tower stand.Conducting experimental research in this context can yield data on rainfall intensity and sprinkling uniformity.For water mist environment test, the test ambient temperature can be controlled.Assuming the test is carried out at room temperature, the ambient temperature can be set at 22℃±2℃, with a saturator temperature of 22℃ and a saturator gas flow rate of 0.12MPa±0.02MPa,maintaining a humidity level of 28.7%.12 rain gauges are placed on the ground according to the arrangement shown in Figure 6 Amount, and Sedimentation Uniformity.From the aforementioned test data, it can be observed that without the placement of collection funnels on the rain gauges, it takes at least 3h for manual reading of the gauge scales.The average mist precipitation is 1.09mm/h, meeting the minimum rain intensity requirement of 1mm/h in the lab.However, when collection funnel is installed on the rain gauge, the corresponding value can be manually read in 1h, with an average mist precipitation of 2.78mm/h, meeting the test requirement of 2.5mm/h in the lab.The placement of collection funnels not only expedites data collection but also leads to higher water mist deposition rates and better uniformity during mist deposition.

Conclusion
To meet the requirements of equipment rain test, an experimental study on the uniformity of spray fields with different nozzle models and the generation of water mist environments during the rain test process was conducted.Using existing laboratory equipment and experimental verification, suggestions were made for the arrangement of rainfall intensity measurement points.Through an in-depth investigation into the sprinkling uniformity, detailed test data and variation curves were achieved regarding sprinkling height and uniformity.The test values obtained from the completed water mist environment test in the lab demonstrate the feasibility of carrying out water mist environment simulation tests.In accordance with the testing requirements provided by equipment manufacturers, the option of placing or not placing collection funnels can be selected.Furthermore, the control of gas pressure values can be achieved by adjusting the airflow rate, addressing the requirements for mist precipitation.The study on sprinkling uniformity and environmental testing serves as a theoretical foundation, providing essential data support for the development of more instructive equipment rain test methods and water mist environment simulations in the future.

3 Figure 1 .
Figure 1.Cloud Chart of Water Mass Fraction.Steady state analysis: a.10L/h; b. 15L/h; c. 20L/h; d. 35L/h The instantaneous analysis cloud maps of water mass fraction are shown in Figure 2.

Figure 2 .
Figure 2. Cloud Chart of Water Mass Fraction.Transient analysis: a.10L/h; b. 15L/h; c. 20L/h; d. 35L/h The water mass fraction cloud map after 20s of spraying is shown in Figure 3.

Figure 4 . 1 .
Figure 4.1.5mm×4Cloud Chart of Water Mass Fraction after 600s.The distribution of water at the bottom is relatively uniform, indicating that the uniformity is independent of the water flow rate.Water mass fraction is proportional to the water flow rate, and the water flow rate correlates with the ejection velocity of water droplets.In subsequent research, to obtain complete trajectory of water droplets and clear water quality analysis cloud maps of various nozzle array models, it may be necessary to increase the time increment steps in the calculations appropriately.
(b). a.Water pressure height rain intensity relationship curve b.3D diagram of 95cm high -water pressure -rain intensity -uniformity c.Water pressure height uniformity scatter plot d.Waterfall diagram of water pressure height rainfall intensity Figure 6.4.2mm Spray Array Water Pressure-Height-Rain Intensity-Spray Uniformity Relationship.
a.Water pressure height rain intensity vertical line diagram b.Composite curve of height water pressure spray uniformity Figure 7. 1.5mm Spray Array Water Pressure-Height-Rain Intensity-Spray Uniformity Relationship.

8 .
(b), and 2 water mist collection modes are used for experimental data comparison: one involves placing collection funnels on the rain gauges, while the other does not.The test data is depicted in Figure 8. a.Composite curve of 12 rain gauges-test time-water mist settlement b.Composite curve of test time, water mist settlement amount, and settlement uniformity Figure Relationship between Water Mist Environmental Test Time, Water Mist Sedimentation