Scholarly article on topic 'Experimental Study on the Dynamic Process of a Water Drop with Additives Impact Upon Hot Liquid Fuel Surfaces'

Experimental Study on the Dynamic Process of a Water Drop with Additives Impact Upon Hot Liquid Fuel Surfaces Academic research paper on "Materials engineering"

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Energy Procedia
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{"Liquid pool fire" / "Fire suppression" / "Water mist/spray" / "Drop impact" / "Surface cooling" / "Vapour explosion"}

Abstract of research paper on Materials engineering, author of scientific article — Meijuan Lan, Xishi Wang, Pei Zhu, Pingping Chen

Abstract In order to deepen the knowledge of surface cooling mechanism on liquid pool fire suppression with multi-component water spray/mist, preliminary experimental studies on water drop with or without additives impacting upon hot liquid fuel surfaces, such as alcohol and molten-ghee were conducted. NaCl and AFFF (Aqueous Film-Forming Foam) were considered as additives. Each liquid fuel was heated to simulate the temperature conditions as it burns or boils. The impacting processes were recorded using a high-speed digital camera with 2,000 fps. The results show that the maximum column height of the jet was enlarged and more daughter drops were splashed in the cases with additives comparing to a pure water drop as it impacted upon alcohol surface. In addition, to the cases with NaCl additive and a 493K molten-ghee surface, a tiny vapor explosion occurred at 36ms and a second vapor explosion occurred at 2563ms, while there was no any vapor explosion occurred to the case with AFFF additive.

Academic research paper on topic "Experimental Study on the Dynamic Process of a Water Drop with Additives Impact Upon Hot Liquid Fuel Surfaces"

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Energy Procedía 66 (2015) 173 - 176

The 12th International Conference on Combustion & Energy Utilisation - 12ICCEU

Experimental Study on the Dynamic Process of a Water Drop with Additives Impact upon Hot Liquid Fuel Surfaces

Meijuan Lan, Xishi Wang*, Pei Zhu, Pingping Chen

State Key Laboratory of Fire Science, University of Science and Technology of China, Hefei, 230026, China

Abstract

In order to deepen the knowledge of surface cooling mechanism on liquid pool fire suppression with multi-component water spray/mist, preliminary experimental studies on water drop with or without additives impacting upon hot liquid fuel surfaces, such as alcohol and molten-ghee were conducted. NaCl and AFFF (Aqueous Film-Forming Foam) were considered as additives. Each liquid fuel was heated to simulate the temperature conditions as it burns or boils. The impacting processes were recorded using a high-speed digital camera with 2,000 fps. The results show that the maximum column height of the jet was enlarged and more daughter drops were splashed in the cases with additives comparing to a pure water drop as it impacted upon alcohol surface. In addition, to the cases with NaCl additive and a 493 K molten-ghee surface, a tiny vapor explosion occurred at 36 ms and a second vapor explosion occurred at 2563 ms, while there was no any vapor explosion occurred to the case with AFFF additive.

© 2015The Authors.PublishedbyElsevierLtd. Thisis an open access article under the CC BY-NC-ND license (http://creativecommons.Org/licenses/by-nc-nd/4.0/).

Peer-review under responsibility of the Engineering Department, Lancaster University

Keywords: Liquid pool fire; Fire suppression; Water mist/spray; Drop impact; Surface cooling; Vapour explosion

1. Introduction

Dynamic process of a liquid drop impact upon different surfaces are important to several technical applications, such as combustion engine, oil extraction, surface cooling, fire suppression, thermal spray coating, etc. [1, 2]. The behaviour of a drop after collision with the hot surface depends on the physicochemical characteristics of the drop as well as those of the surfaces themselves. Manzello and Yang [3] pointed out that the different initial impact velocities and surface temperatures have important effects on forming jet height, the critical Weber number of splash, etc. Bernardin et al. [4] found the surface roughness and the thermal conductivity have important effects on drop spreading. The effects of surface density, drop diameter, liquid viscosity and surface tension were also considered and discussed elsewhere [5, 6].

Some studies addressed the impingement of water droplets on liquid surfaces [7-9], which stated that if

* Corresponding author. Tel.: 86-551-63606437

E-mail address: wxs@ustc.edu.cn

1876-6102 © 2015 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Peer-review under responsibility of the Engineering Department, Lancaster University doi: 10.1016/j.egypro.2015.02.008

the additive concentration was relatively high, some differences would be observed, especially at low Weber numbers. However, there is few study focused on drop impact with additives upon hot liquid surfaces.

Water mist have been regarded as the better substitute of conventional means for liquid pool fire suppression [10-11]. But how does a drop cool the fuel surface, especially what is the behaviour when a drop with additives impact upon a hot liquid fuel surface, are still not well known. Chen et al. [8] and Manzello et al. [12] found that vapour explosion occurred during the pure water drop impact upon hot ghee and peanut oil surfaces which would lead to combustion enhancement. Additives usually were mixed into water to improve the efficiency of water based fire suppression technologies [13]. So the dynamical process of a water drop with additives impact upon hot alcohol and molten-ghee surfaces were investigated.

2. Experimental apparatus

The experimental apparatus of this study is similar as that described elsewhere [8, 14], which consists of a drop generator, an illumination system, a heater and a high-speed video camera. The initial diameter of the pure water drop and the water drop with 5% NaCl additive was about 2.4 ±0.1 mm, while the water drop with 2.2% AFFF was about 1.8 ±0.1 mm. The initial drop diameter was determined with Do = 2^Np • ain .

The impact velocity of the drop was determined and controlled by varying the injector height as described by Chen et al.[8]. The whole process was recorded by a Photorn FASTCAM high-speed video camera at 2,000 fps with 1024x1024 pixels. A 1 kW iodine tungsten filament lamp was used for illumination. The surface tension was previously measured with a SL201 Surface Tensionmeter.

The detail information of the drop, such as its diameter, density and surface tension are listed in Table 1. The temperature of the liquid surface was measured by a mercury thermometer with measurement range of 273K-633K. The alcohol surface temperatures were varied from 293K to 343K. The molten-ghee surface temperature was fixed at 493K. Each test was repeated for three times to insure its reliability.

Table 1. Initial diameter, surface tension, density of different drops

Type of water drop Diameter (mm) Surface tension (mN/m) Density (kg/m3)

Pure water 2.4±0.1 72.0 1.0x103

With 5% NaCl 2.4±0.1 59.4 1.05x103

With 2.2% AFFF 1.8±0.1 21 0.97x103

3. Results and discussions

3.1 Impact upon hot alcohol surface

The dynamical process of a distilled water drop impinging upon hot alcohol surface was investigated for comparison with the one with additives. The results are similar as presented elsewhere [14] and would be omitted due to page limit. Fig. 1 shows the collision dynamics of water drop with 5% NaCl additive impinging upon alcohol surface. A crown was formed at 12 ms after the drop impact, and the volatility became much more strenuous as the liquid surface temperature increased. After that, the liquid began to rise up and formed a jet which was similar to the cases of pure water drop impacting, although more small daughter drops were splashed in these cases.

Fig. 2 shows the time-elapsed images of water drop with 2.2% AFFF impact on hot alcohol surfaces with higher We. The impact dynamics of water drop with 2.2% AFFF additive were different from the cases of pure water drop impact and the drop impact with 5% NaCl additive. There only formed a small crown and a small jet. Fig. 3 gives the non-dimensional maximum height of the liquid column under

different temperatures. To the cases with 2.2% AFFF, h^ obviously decreased as the bulk liquid surface

temperature increased. To the pure water drop and water drop with 5.0% NaCl, H^ firstly decreased

with the temperature increase, and then slightly increased again after the bulk liquid temperature exceeds 313 K.

0 ms (313K) 0 ms (333K) 0 ms (343K) 12ms(313K) 12 ms (333K) 12 ins (343K)

74 ms (313K) 74 ms (333K) 74 ms (343K) 108 ms (313K) 108 ms (333K) 108 ms(343K) Fig. 1. Water drops with 5% NaCl impact on alcohol surfaces with different temperature (We=299)

0 ms (313K) 0 ills (333K) 0 ms (343K) 12 ms (313K) 12 ms (333K) 12 ms (343K)

75 ms (313K) 73 ms (333K) 73 ms (343K) 106 ms (313K) 106 ms (333K) 106 ms (343K) Fig. 2 Water drop with 2.2% AFFF impact on alcohol surfaces with different temperature (We=651) 3.2 Impact upon hot molten-ghee surface

Fig. 4 shows that a weak vapour explosion was observed at 52 ms. At 87 ms many small daughter drops were splashed which rebounded to a big jet and immediately shed to secondary droplets. After that, the drop ultimately sank and remained at the bottom of the oil pool. At 2455 ms, vapour bubbles appeared and then a vapour explosion was caused.

Fig. 5 shows the impact dynamics of water drop with 2.2% AFFF impacting upon molten-ghee surface. An obvious jet formed at 43 ms and then splashed several daughter drops. Only a

S 25 0)

I -,20

■ Water drop

* 5,0% NaCl drop

A * 2.2% AFFF drop

V=2. 8b/s

* « *

-1-.-1— i ■ —i-,-1—,—«—i— ■

J. 290 300 310 320 330 340 350

] Temperature(K)

3 The non-dimensional maximum height of the liquid column under different temperatures

weak vapour explosion occurred at 1436.5 ms. ■

J F | weak J F |

explosion

..... J L..-n J ' -i .......: J

M 1Ê " M J

-_ _1___9___i___J

2455 ms

2533 ms

2563 ms

Fig. 4 The impact process of water drop with 5% Nacl on 493 K molten-ghee surface (We=299)

59.5 ms

910.5 ms 1374.5 ms 1404.5 ms 1436.5 ms 1905.5 ms

Fig. 5 The impact process of water drop with 2.2% AFFF on 493 K molten-ghee surface (We=652) 4. Conclusions

The collision dynamics of a water drop containing 5% NaCl or 2.2% AFFF additive impacting onto hot alcohol and molten-ghee surfaces with high Weber numbers were experimentally investigated. The height of the jet of pure water drop and water drop with 5% NaCl impact increased, while it decreased to the cases of water drop with 2.2% AFFF. The results indicated that the additives would enlarge the maximum liquid column height and splash more daughter drops, especially to the cases of water drop with 2.2% AFFF. Pure water drop or water drop with 5% NaCl additive impacting on 493 K molten-ghee surface would cause vapour explosion and splash more daughter drops, this should be avoided during molten-ghee fire suppression with water based technologies. References

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[4] Bernardin J. D., Stebbins C. J., Mudawar I. Effects of surface roughness on water droplet impact history and heat transfer regimes. International Journal of Heat and Mass Transfer 1997; 40: 73-88.

[5] King M. D., Yang J. C., Chien W. S., Grosshandler W. L. Evaporation of a small water droplet containing an additive. Proc. of the 32nd ASME National Heat Transfer Conference, 1997,ASME, New York.

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