Experimental study on the water film thickness under spray impingement based on planar LIF

https://doi.org/10.1016/j.ijmultiphaseflow.2020.103329Get rights and content

Highlights

  • The experimental setup of the spray impingement with an air-atomizing nozzle was established.

  • The planar LIF technology was applied to measure the water film thickness.

  • The image post-processing method was developed to extract the water film boundaries.

  • Spray heights and water flow rates were 10–40 mm and 60–120 ml/min.

Abstract

Spray impingement is widely applied in many fields, and the liquid film formed on the impact surface is closely related to the heat and mass transfer. In present paper, an experimental setup of the spray impingement with an air-atomizing nozzle was established to study the water film formed on a flat surface. And the planar laser-induced fluorescence technology was applied to measure the film thickness at different spray heights (10–40 mm) and water flow rates (60–120 ml/min). Based on the film topography, the water film was divided into the impact zone and the free flow zone. The film thickness in the impact zone was thinner than that in the free flow zone. The water film at the center of the impact surface intensely fluctuated, and the steady layer thickness was 0.33–0.73 mm. The film thickness in the free flow zone presented a camelback topography of first increasing and then decreasing with the increase of the radial distance. Besides, the film thickness in the free flow zone decreased with increasing the spray heights and increased with increasing the water flow rates. The maximum film thickness was 2.27 mm and the average film thickness in the free flow zone was 0.85–1.50 mm. The study provides much more experimental data with the study on the liquid film thickness under spray impingement.

Introduction

Spray impingement is a typical phenomenon with intense heat and mass transfer, and widely applied in the fields of engineering, medicine and nature, such as metal processing, electronics cooling, laser medicine and aircraft icing (Zhao et al., 2017; Leng et al., 2018). Plenty of fine droplets atomized by the nozzle impact the surface during spray impingement (Labergue et al., 2015), causing a series of behaviors, such as adhesion, splashing and rebound, which depends on the Weber number (Wang et al., 2003). The impact physics of droplets was systematically reviewed by Breitenbach et al. (Breitenbach et al., 2018), and they proposed that the study on the impact characteristics of a single droplet was insufficient to explain the mechanism of spray impingement. When the spray of atomized droplets continuously impinges against the surface, a liquid film is formed, and subsequent droplets mainly impact the liquid film instead of the surface. The film thickness, possible rupture of continuous liquid layers on the impact surface, momentum-driven film flow and wave formation influenced by the spray impingement all contribute to the heat removal efficiency from the surface, which is important for practical applications. Although the studies on the liquid film under spray impingement have been conducted for several decades, the mechanism of the mass transfer and the flow characteristics of the liquid film still lack knowledge. Therefore, investigation on the morphology of the liquid film under spray impingement is of crucial importance and still attracting significant attentions.

In the research of Chen et al. (Chen et al., 1995), the film flow under spray impingement was assumed to be laminar, driven by the momentum of the droplets entering the liquid film (Xie et al., 2012) and balanced by the viscous force within the liquid layer (Qi et al., 2018). The momentum of the droplets entering the liquid film is decomposed into the axial and the radial momentum under spray impingement. The axial momentum is converted into the local pressure inside the liquid film. Meanwhile, the radial momentum drives the upper liquid film to flow and carries the lower liquid film by the viscous force. Since the diameter and the velocity of the droplets entering the liquid film are not uniform (Liu, 2017), the momentum of the droplets is not constant and the pressure difference at different positions generated by the axial momentum also offers driving force for the radial flow of the liquid film. As the droplets continuously impact the surface, the liquid film develops outward and flows out of the actual coverage of the spray impingement. Thereafter, the driving force of the radial flow is only the pressure difference within the liquid film. Since the existence and evolution of the liquid film inevitably affect the heat and mass transfer at the interface of the liquid film and the impact surface, the study on the thickness characteristics of the liquid film is necessary and helps to fundamentally understand the film formation under spray impingement.

The study on the film thickness formed on the impact surface under spray impingement has been attracting the attention of many researchers, and the chosen methods are various. For example, the point gage method was used by Chen et al. (Chen et al., 1995) to measure thickness of the water film produced by the spray impingement with the pressure-atomizing nozzle, and a needle probe was employed in their experiment. In their study, the water film thickness was about 0.05 mm and increased with the increase of the water flow rate, however, the influence of the spray height on the film thickness was insignificant. The thickness of the water film on a flat metal surface under the spray impingement with a full-cone nozzle was investigated by An et al. (An et al., 2004) by measuring the voltage variation between a metal probe and the surface. They proposed that the water film was consisted of the wavy layer and the steady layer. The water film thickness first increased and then decreased with the increase of the spray height, and the influence of the water pressure in the pipeline was limited. However, for the above invasive measurement methods, the probes were placed in the liquid film, which might interfere with the stability of the liquid film and affect the accuracy of measurements. Therefore, the fast, contact-free and spatially resolved methods are widely applied in measuring the film thickness under spray impingement.

The Fresnel diffraction was adopted by Yang et al. (Yang et al., 1992) to study the thickness of the water film produced by the spray impingement with an air-atomizing nozzle. It was found that the time-average maximum film thickness was 0.09–0.24 mm and increased with the increase of the water flow rate. However, they ignored the influence of the optical disturbance caused by the airborne droplets on the experimental measurements. The total internal reflection (TIR) was employed by Pautsch et al. (Pautsch et al., 2004; Pautsch and Shedd, 2006) to perform the spray impingement experiment with ethyl alcohol and FC-72 as the working liquid, and the film thickness was in the range of 0 to 0.08 mm. In their study, it was found that the heat load did not affect the film thickness of the single-nozzle spray. However, only the film thickness at limited locations was obtained. In addition, it was difficult to determine the critical incident angle of the TIR due to the violent fluctuation of the film surface under spray impingement. The refractive index matching (RIM) (Drake et al., 2003; Yang and Ghandhi, 2007; Wang, 2014; Henkel et al., 2016) was widely used to measure the film thickness under fuel spray impingement. However, the measuring accuracy greatly depended on the calibration between the scattered light intensity and the film thickness. Besides, the fuel film thickness was less than 0.03 mm and the scattered light became insensitive with the increase of the liquid film thickness. The high-speed photography was selected by Martinez-Galvan et al. (Martinez-Galvan et al., 2011; Martínez-Galván et al., 2013; Hsieh and Luo, 2016; Hou, 2014), and it was found that the film thickness was less than 1.75 mm within a wide range of the spray height and flow rate. However, the internal characteristics of the liquid film were not captured since the camera only recorded the peripheral topography of the liquid film.

The laser-induced fluorescence method (LIF) is that, under the irradiation of laser, the fluorescent dye mixed into liquid emits fluorescence, and then the excited fluorescent signal is collected by the high speed CCD camera with a filter lens (R Volko and Strizhak, 2020; R Volko and Strizhak, 2020). In general, the LIF for measuring film thickness is classified into the intensity-based LIF and the planar LIF according to the film thickness restoring methods (Chang et al., 2019). For the intensity-based LIF, the intensity values of the fluorescent signal are converted to the film thickness based on the Lambert-Beer law (Senda et al., 1999; Y Cheng et al., 2010; Y Cheng et al., 2010; Alonso et al., 2010; Alonso et al., 2012). For the planar LIF, the film thickness is recovered based on the shape of glowing section in the images of the liquid film (Alekseenko et al., 2012; Cherdantsev et al., 2014; Dupont et al., 2015; Xue et al., 2017). Since the fluorescent interference introduced by the airborne droplets might result in inaccurate results (Senda et al., 1999) when the intensity-based LIF method was applied to measure the film thickness, the experimental data of the film thickness after spray impingement stopped were obtained by Senda et al. (Senda et al., 1999) and Cheng et al. (Y Cheng et al., 2010; Y Cheng et al., 2010). To measure the transient film thickness under spray impingement, the technology of the intensity-based LIF combined with the TIR was proposed by Alonso et al. (Alonso et al., 2010; Alonso et al., 2012). However, the experimental system was more complex, and still could not overcome the limitations of the TIR method. The planar LIF method was used by Hsieh et al. (Hsieh et al., 2015) to discuss the relation between the film thickness at a location beneath a single-cone spray and the wall heat flux. It was found that the film thickness of the DI water was 1.75–2.75 mm and greatly related to the wall heat flux. However, in their study, the method of extracting the liquid film thickness from images was not described in detail.

The liquid film flow under spray impingement is a dynamical process, so that a fast and non-invasive measurement method is required to observe the film thickness. By comparison, the liquid film characteristics in a certain plane could be obtained by using the planar LIF technology, and the film thickness could be recovered based on the shape of glowing section in the images of liquid film. Therefore, the planar LIF technology was applied to measure the liquid film thickness under spray impingement with an air-atomizing nozzle in the present study, and the image post-processing method was developed to obtain the film thickness. A series of experiments were performed. Based on the flow mechanism of the liquid film under spray impingement, the liquid film topography on the impact surface was analyzed in detail, and then the experimental results on the film thickness were interpreted using the statistical methods. This study is conductive to offer much more experimental data and understand the liquid film dynamics under spray impingement.

Section snippets

Experimental setup

The experimental setup of spray impingement is shown in Fig. 1. As depicted in Fig. 1(a), the water mixed with fluorescent dye sequentially passed through the filter, needle valve and flowmeter under the action of the micropump. The water entered the nozzle and was atomized by the compressed air in the nozzle. The air was provided by the air compressor and purified by the oil-water separator. And then the purified air passed through the buffer tank to stabilize the pressure. Thereafter, the

Film topography under spray impingement

The water film topography under spray impingement was investigated before discussing the film thickness. Fig. 4 shows the film topography in Case 5. A crater was formed in the center, as shown in Fig. 4(a). Fig. 4(b) was the cross-sectional image of the water film after binary processing, showing that the fluorescent intensity was stronger in the periphery. In addition, there was still some emitted fluorescence in the crater. According to Guo (Guo, 2009), the crater was called as the impact

Conclusions

The planar laser-induced fluorescence technology was applied to study the water film under steady-state spray impingement. And the thickness of the water film produced by the spray impingement with an air-atomizing nozzle was analyzed by extracting the film boundaries. It was found that the water film in the free flow zone was obviously thicker than that in the impact zone. Moreover, for the impact zone, the film fluctuation was more intense, and the film thickness of the steady layer at the

CRediT authorship contribution statement

Dongyun MA: Methodology, Software, Validation, Formal analysis, Investigation, Data curation, Writing - original draft, Writing - review & editing, Visualization. Shinan CHANG: Conceptualization, Methodology, Software, Resources, Writing - review & editing, Supervision, Project administration, Funding acquisition. Ke WU: Formal analysis, Writing - review & editing. Mengjie SONG: Writing - review & editing.

Declaration of Competing Interest

None.

Acknowledgement

This work was financially supported by the National Natural Science Foundation of China (Nos. 11372026 and 11672024).

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