Experimental research on blowing-rain intensity and uniformity of rain tunnel for aircraft rain removal testing

The objective of the present study is to characterize a rain tunnel that is used for performance testing of aircraft windshield rain removal systems. The rain tunnel in the present study is an open jet wind tunnel with six full cone pressure swirl nozzles arranged at its outlet. Experiments are conducted with water at room temperature as the working medium. The influence of total water supply flow rate and air velocity on the blowing-rain intensity and uniformity is studied. The total water supply flow rate varies from 0.57 m3/h to 1.03 m3/h, while the pressure at the inlet of the nozzle ranges from 1.44 bar to 6.52 bar. The air velocity of 30 m/s, 40 m/s, and 50 m/s is studied. A special test setup has been designed for measurements of the blowing-rain intensity and uniformity. Experimental results indicate that the blowing-rain intensity and uniformity both increase with the increasing total water supply flow rate and decrease with the increasing air velocity.


Introduction
The clarity of cockpit visibility in heavy rain is an important factor affecting the safety of aircraft takeoff and landing because the droplets will form a water film on the windshield surface of the aircraft, which will change the light transmittance of the windshield, interfere with the pilot's vision, and affect the flight safety of the aircraft.Therefore, the Federal Aviation Regulations (FAR) of the Federal Aviation Administration (FAA), the Certification Specifications (CS) of the European Aviation Safety Agency (EASA), and the Civil Aviation Regulations (CCAR) of the Civil Aviation Administration of China (CAAC) all require that the airplane must have a means to maintain a clear portion of the windshield, during precipitation conditions, sufficient for both pilots to have a sufficiently extensive view along the flight path in normal flight attitudes of the airplane in heavy rain at speeds up to 1.5 V SR1 .
In order to ensure flight safety and meet the above airworthiness regulatory requirements, modern aircraft are generally equipped with windshield rain removal systems, such as jet rain removal systems, windshield wiper systems, and windshield rain diversion systems, to remove rain from the windshield during aircraft takeoff, approach, and landing processes, and prevent rainwater from obstructing the driver's sight.The performance of windshield rain removal systems is usually verified by flight tests, but the dependence on natural weather conditions leads to low testing efficiency.Therefore, blowingrain test in the laboratory, which simulates the rain and airflow at flight speed, has become the most effective development tool to verify the rain removal performance of windshield systems.It is also one of the methods to verify the compliance of aircraft windshield rain removal systems with airworthiness requirements.
The rain test chamber specified in GJB150A-2009 and wind-driven rain experimental cabin are only applicable to outdoor facilities or low-speed vehicles.The rain intensity and air velocity of these pieces of equipment are usually below 200 mm/h and 40 m/s, and the initial speed of raindrops is perpendicular to the air velocity [1][2][3][4].These pieces of equipment are not suitable for the performance verification test of the aircraft windshield rain removal system for the following reasons:  In the performance verification test of the aircraft windshield rain removal system, the blowing-rain simulates the relative movement of the aircraft with the raindrops and the free flow, so the raindrop speed should be consistent with the direction of the air velocity.
 According to the equivalent rainfall principle proposed by Wang and Liu [5], the rain intensity received by the windshield (i.e., the rain intensity simulated in the laboratory) in heavy rain weather (rainfall intensity 15 mm/h -25 mm/h) and flight speed (40~120) m/s should be between 200 mm/h-800 mm/h. The aircraft windshield rain removal system is mainly used in the take-off and landing phases of the aircraft, so the wind speed of the blowing-rain simulation should not be less than 40 m/s.
At present, the studies of the nozzle are mostly concentrated in a single nozzle, and no studies on the interference of multi-nozzle outlet fields in high-velocity airflow environments have been retrieved [6][7].In this paper, a rain tunnel with six full cone pressure swirl nozzles arranged at its outlet is used to simulate high-speed blowing-rain, and the difficulties lie in how to obtain the required blowing-rain intensity and a uniform blowing-rain intensity field.The main factors affecting the blowing-rain intensity are the water supply flow rate and the distance between the test section (referring to the vertical position of the windshield during the test) and the nozzle.In contrast, the main factors affecting the uniformity of the blowing-rain intensity field are the nozzle spacing, water supply pressure, wind speed, and the distance between the test section and the nozzle.In this paper, the blowing-rain intensity and uniformity under different water supply flow rates and air velocity are tested, and the variation trend of the blowing-rain intensity and uniformity with the water supply flow rate and air velocity is obtained.

Rain tunnel and nozzles
The high-speed blowing-rain simulation test bench is mainly composed of an open jet wind tunnel and water supply system, as shown in Figure 1.The outlet of the wind tunnel is provided with an airfoil sprinkler rack, on which six nozzles are installed.The open jet wind tunnel generates a high-velocity airflow, and the droplets ejected from the nozzles are carried by the high-velocity air flow and diffuse and develop forward, forming a blowing-rain environment.Both the air velocity and the flow rate of water supplied to the nozzles can be adjusted.The water flow rate is measured by an electromagnetic flowmeter (kewill brand, DN15, range 0~6.4 m 3 /h, error ±1%), and the pressure at the inlet of the nozzles is measured by a pressure transmitter (Siemens, 7MF4033-1DA10-2ACO, range 0~1.6 MPa, error ±0.5%).Wind speed is measured by a pitot tube anemometer arranged at the outlet of the wind tunnel.The full cone pressure swirl nozzles are Spaying brand, model B1/8G-SS3, and the specific parameters are shown in Figure 2 and Table 1.The water flowing through the nozzle rotates and crushes under the action of the X-shape deflector inside the nozzle to form droplets, which are ejected from the nozzle outlet to form a cone angle of about 60°, as shown in Figure 3.In order to obtain blowing-rain intensity between 200-800 mm/h, a total of about 5-20 L/min water supply is required under the assumptions of 1 m 2 effective blowing-rain area and 50% water loss.The total amount of water supplied divided by the flow rate of each nozzle is the number of nozzles.In the choice of nozzle working pressure, on the one hand, considering the drag effect of the high-speed airflow on the raindrop particles, the spray angle will be "compressed".In order to ensure that the droplets sprayed by the nozzle can still have a certain degree of diffusion under the action of the high-speed airflow, working conditions with high water pressure should be selected.On the other hand, if the water pressure is high, the flow rate of a single nozzle is also large, so the number of nozzles required decreases.However, the number of nozzles is too small to be conducive to the uniform distribution of rain intensity, and if the water pressure is too high, the raindrop particle size will become smaller, and the difference between the raindrop size and the actual raindrop size will be too large.Considering comprehensively, the range of nozzle working pressure of 1.5-6 bar is selected, so six nozzles are used in this study.The size of the wind tunnel outlet is 0.9 m*1.1 m, and the six nozzles are arranged in a 2 (horizontal) *3 (longitudinal) manner, while the transverse and longitudinal spacing are both 30 cm, as shown in Figure 4.The size of the test space is 14825 mm (length) * 9125 mm (width) * 7100 mm (height), and the length of the wind tunnel in the inner part of the test space is 5125 mm.

Experimental setup
A test setup was designed to measure the blowing-rain intensity and uniformity.As shown in Figure 5, five collection elbows with a diameter of about 191 mm are supported on vertical beams.In turn, the center of the circle of the three rows of water collectors is aligned with the height of the three rows of nozzles.The water collection elbows and the rain gauges are connected by rubber hoses.and the rain gauge has a built-in 2000 mL measuring cylinder.The spray water is blown horizontally into the water collection elbows by the strong wind and introduced into the graduated cylinder through the rubber hoses.The blowing-rain intensity H i at each water collection inlet and the average blowing-rain intensity H is calculated by the volume of the collected water and the collection time, as shown in Equations ( 1) - (2).
2400    ⁄ ( 1 ) Where H i is the blowing-rain intensity (mm/h), S is the water volume collected by the rain gauge (mL), D is the diameter of the water collection elbow inlet (cm), and T is the collection duration (min).

Test procedure
The test setup is placed directly in front of the wind tunnel outlet and the distance between the test section (the plane where the water collection port is located).The nozzle is 1.8 m.The steps are followed for testing: a) The graduated cylinders are put into the rain gauge, and the cover plate is covered.It is made sure that the rubber hose is located outside the graduated cylinder.
b) The wind tunnel system is turned on, and the wind speed is set.c) The pump frequency is set, and the pump is turned on.d) When the wind speed and water flow are stable, the rubber hose is placed inside the graduated cylinder and the timing is started with a stopwatch.e) After a few minutes, the stopwatch is pressed, and the pump and wind tunnel are turned off.
f) The lid of the cylinder is removed, and the reading and position are recorded together, as shown in Figure 6.

Test results
The test results are shown in Table 2.

Effect of total water supply flow rate on blowing-rain intensity and uniformity
Figure 7 presents the blowing-rain intensity at different water supply flow rates and the polynomial fitting curve.It is observed that the blowing-rain intensity increases with the increase of water supply flow.The fitting polynomial trend curve with R 2 =0.9992 shows that the change law is nonlinear because under the same air velocity and at the same test cross-section, the main influencing factor of the blowingrain intensity is the water supply flow rate.The secondary influencing factors are the raindrops particle size distribution and velocity distribution.For a certain nozzle, the larger the water supply flow rate and water supply pressure are, the smaller the raindrop particle size and the greater the particle's initial velocity will be.The influence of raindrop particle size and the particle's initial velocity on the diffusivity of the spray is opposite.The smaller the size of the raindrop is, the better the particle's followup in the air will be, and the smaller the diffusion will be.However, the large initial velocity of the particle leads to the small influence of the drag force of the air flow on the particle trajectory, so a large diffusivity is obtained.Under the combined effect of the above factors, the blowing-rain intensity increases nonlinearly with the water supply flow.
Figure 7. Variation of blowing-rain intensity with water supply flow rates.Figure 8 shows the uniformity of blowing-rain intensity under different water supply flow rates.The CV value represents the dispersion degree of blowing-rain intensity at five water catchments, which is negatively correlated with rain uniformity.Under the determined nozzle arrangement and air velocity, the main influencing factor of blowing-rain uniformity is the degree of diffusion A good diffusivity results in good dispersion of raindrop particles from a single nozzle over the test cross-section and better overall uniformity.As can be seen from Figure 8, the higher the degree of spray diffusion as the water supply flow increases, the more it can be inferred that the initial velocity of particles has a greater effect on the degree of diffusion than the size of the raindrops.

Effect of air velocity on blowing-rain intensity and uniformity
Figure 9 shows the blowing-rain intensity and uniformity at different air velocity (30 m/s, 40 m/s, and 50 m/s).It can be seen that the higher the wind speed and the lower the blowing-rain intensity are, the worse the uniformity at a water supply flow rate of 0.61 m 3 /h and test cross-section of 1.8 m from the nozzles will be.This is because high wind speeds can cause spray particles to aggregate and, therefore, become less uniform.At the same time, the position of the water collection is in a place where the spray particles are sparse, so the calculated blowing-rain intensity decreases.However, the amount of water received by the actual test section still increases with the increase of air velocity.

Conclusion
In this paper, the blowing-rain intensity and uniformity of a rain tunnel that is composed of an open jet wind tunnel with six full cone pressure swirl nozzles arranged at its outlet are tested.The blowing-rain intensity varies from 249.3 mm/h to 712.5 mm/h, while the uniformity ranges from 23.24% to 49.85%.The influence of total water supply flow rate and air velocity on the blowing-rain intensity and uniformity is studied.Experimental results indicate that the blowing-rain intensity and uniformity both increase with the increasing total water supply flow rate and decrease with the increasing air velocity.

Figure 2 .
Figure 2. External dimensions of a solid conical nozzle.

Figure 3 .
Figure 3. Internal structure of solid cone nozzle (Spaying Product Manual).In order to obtain blowing-rain intensity between 200-800 mm/h, a total of about 5-20 L/min water supply is required under the assumptions of 1 m 2 effective blowing-rain area and 50% water loss.The total amount of water supplied divided by the flow rate of each nozzle is the number of nozzles.In the choice of nozzle working pressure, on the one hand, considering the drag effect of the high-speed airflow on the raindrop particles, the spray angle will be "compressed".In order to ensure that the droplets sprayed by the nozzle can still have a certain degree of diffusion under the action of the high-speed airflow, working conditions with high water pressure should be selected.On the other hand, if the water pressure is high, the flow rate of a single nozzle is also large, so the number of nozzles required decreases.However, the number of nozzles is too small to be conducive to the uniform distribution of rain intensity, and if the water pressure is too high, the raindrop particle size will become smaller, and the difference between the raindrop size and the actual raindrop size will be too large.Considering comprehensively, the range of nozzle working pressure of 1.5-6 bar is selected, so six nozzles are used in this study.The size of the wind tunnel outlet is 0.9 m*1.1 m, and the six nozzles are arranged in a 2 (horizontal) *3 (longitudinal) manner, while the transverse and longitudinal spacing are both 30 cm, as shown in Figure4.The size of the test space is 14825 mm (length) * 9125 mm (width) * 7100 mm (height), and the length of the wind tunnel in the inner part of the test space is 5125 mm.

Figure 8 .
Figure 8. Variation of coefficient of variation with water supply flow rates.

Figure 9 .
Figure 9. Variation of blowing-rain intensity and coefficient of variation with air velocity.

Table 2 .
Result of test.