Experimental study on splash phenomena of liquid jet impinging on a vertical wall
Graphical abstract
Introduction
The corrosion of various components in a coal-fired power plant can be diminished significantly by deaeration, which reduces the amount of oxygen dissolved in the water–steam cycle. For this purpose, a power plant with multiple feedwater heaters has at least one open-type feedwater heater (also called a direct-contact deaeration feedwater heater), which combines the functions of deaeration, direct-contact condensate preheater, and water storage in a single vessel [1], [2]. Fig. 1 shows a spray-type deaerator that is widely used in power plants. Here, the feedwater is ejected into a steam-filled space through nozzles and splashed onto a surrounding cylindrical baffle, thus producing fine droplets in the spray zone. The splashed fine droplets, as well as the draining film on the cylindrical baffle, fall down and are accumulated in a vessel. The steam extracted from a low-pressure turbine is injected through a steam sparger; thus, the resulting steam bubbles rise in the accumulated water, which provides effective contact between water and steam to complete the deaeration process. The deaerated feedwater is supplied to the next feedwater heater through a pump, while the steam, with an increased air concentration, leaves the water surface and moves toward the spraying zone. There, the steam condenses further on the droplet surface, and thus, high-concentration air with a small amount of steam can be vented out [1].
Therefore, in the spray zone shown in Fig. 1, it is important to generate the small droplets to maximize the surface-to-volume ratio and thus promote the droplet-heating process, as well as steam condensation. In addition, this promotes the deaeration process, even though complete deaeration cannot be achieved owing to the short residence time of the falling droplets. One exemplary operating condition of the deaerator is a steam temperature ( of 177 °C, spray temperature () of 145 °C, and pressure of 9 bar (the corresponding saturation temperature is 175 °C). Assuming a falling distance of 1 m ( the distance between the nozzles and the water level), the residence time of free-falling droplets can be estimated to be approximately 0.45 s (, where z and represent distance and gravitational acceleration, respectively), neglecting drag force. Using a series of assumptions (see the appendix), the temporal change in droplet temperature () can be approximated as follows:
Here, , , and are the liquid density, specific heat, and steam thermal conductivity, respectively, and represents the droplet diameter. According to Eq. (1), the temperature of a droplet with a diameter of 100 can increase by approximately 25 °C within 0.45 s, while that of a droplet of 1 increases less than 1 °C. Thus, this estimation confirms the significance of the small droplet size produced in a spray zone.
While a liquid jet ejected from a nozzle is disintegrated into drops with sizes comparable to the size of the nozzle, smaller droplets can be obtained through splashing of the drops onto the cylindrical baffle. As an example, Yarin and Weiss [3] reported that the size of droplets generated by splashing was primarily between 0.03 and 0.2 times the size of an impinging drop.
Considering this, the general splash phenomena of a plain orifice jet impinging on a vertical wall were studied experimentally in this work. Experiments were performed by changing the jet speed (), impact distance (i.e., the distance between the nozzle and the vertical wall, ), and geometric configurations of the nozzles (e.g., nozzle diameter (), length-to-diameter ratio (), and entrance roundness ()); the splashed fraction was measured by weighing the mass drained on the vertical wall. In addition, the breakup of the liquid jet and the flow patterns of the draining film resulting from the jet impinging on the vertical wall were visualized.
Fig. 2(a) shows an exemplary photograph of the flow pattern resulting from impinging a liquid jet onto a vertical wall (Video 1). Here, the column of liquid ejected horizontally from the nozzle disintegrates into consecutive drops with a diameter ( larger than the nozzle diameter () [4], [5]. These drops impinge continuously on the vertical wall, and thus, each drop impacts onto a liquid film produced by the previous drop. The impacted drop spreads out radially; therefore, each drop produces a radially expanding circular rim upon impact. These circular rims are crown-shaped liquid sheets that emerge in the normal direction of the liquid film, as seen in Fig. 2(b) (Video 2). These liquid sheets become unstable and generate numerous small secondary droplets (i.e., splashed droplets) [6], [7]. Eventually, the expanding liquid film loses its momentum; thus, its height increases abruptly at a specific radial distance, owing to hydraulic jump, which can be observed frequently in kitchen sinks [8], [9], [10], [11], [12], [13]. On the vertical surface above the droplet impingement point, the liquid falls circumferentially in a rope formed by hydraulic jump, and the draining film falls downward [14]. Based on the aforementioned observation, splashing occurs through a series of processes of jet ejection through the nozzle, liquid-jet breakup, and drop impact and spreading; these processes again affect the circular hydraulic jump and flow pattern of the draining liquid film.
A review of the aforementioned phenomena is provided first in the next section. Details of the experimental method are given in the third section. The fourth section presents the experimental results, and some of the results are compared with theoretical estimations. The final section lists the concluding remarks.
It is noteworthy that the experimental results in this study were obtained at room temperature and under atmospheric conditions, but actual deaeration processes generally occur under higher temperatures and pressures. Therefore, this study focused on providing an overall understanding of spray phenomena in order to identify the key factors that need to be considered to design a deaerator spray zone. However, this study did not attempt to discover specific optimum operating conditions or designs.
Section snippets
Review
In this section, an overview of the internal flow through the nozzle, breakup of the liquid jet, drop impact and splashing, and circular hydraulic jump are provided in order. Some of these topics have been studied for more than a century, and a comprehensive review is also available in the existing literature [4], [5], [6], [7], [15], [16]. Herein, the results closely related to this work are summarized concisely to facilitate an overall understating of each phenomenological observation and to
Experimental method
Fig. 9 shows a schematic diagram of the experimental setup used in this work. Filtered tap water (19 (2) °C) was supplied to a 100-L pressure tank. After the pressure tank was filled, it was pressurized to the desired pressure (0.2–10 bar) using a nitrogen cylinder (100 bar) through a pressure regulator. This pressure was controlled to set the mass flow rate. Nozzles of different diameters ( 1, 1.5, and 2 (30 )) were machined at the center using a 1-cm-thick circular aluminum (AL6061)
Non-cavitating nozzle
Fig. 10 shows the pressure drop across the nozzle ( with the different geometric configurations listed in Table 3 and the splashed fraction () at the impact distance of 75 cm. The x-axis of each graph represents the volume flow rate divided by the nozzle area (), which corresponds to the jet velocity for the non-cavitating nozzles (but not for the cavitating nozzles). The results of = 1, 1.5, and 2 mm are shown in order from left to right, and the results for the sharp-edged
Summary and conclusions
In this work, the splash phenomena of a plain orifice jet impinging on a vertical wall were studied. Experiments were performed by changing the jet speed (i.e., inlet pressure) and the impact distance, with rounded and sharp-edged nozzles of different diameters, and the splashed fraction was measured by weighing the mass drained on the vertical wall. In addition, to understand the experimental results, the breakup of the liquid jet and the flow patterns resulting from the jet impinging on a
CRediT authorship contribution statement
Hyunsuh Kim: Writing - original draft, Investigation. Hyunhun Choi: Formal analysis, Data curation. Daegil Kim: Visualization, Methodology. Jaewon Chung: Writing - review & editing. Hyojun Kim: Project administration, Resources, Software. Kihyun Lee: Funding acquisition.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgment
This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF), which is funded by the Ministry of Education, Science and Technology (NRF-2012R1A1A2042447), South Korea and by Doosan Heavy Industries and Construction Co., Ltd., South Korea
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These authors contributed equally to this work.