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Cooling characteristics of cooking oil using water mist during fire extinguishment
Applied Thermal Engineering 107 (2016) 863–869
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Applied Thermal Engineering journal homepage: www.elsevier.com/locate/apthermeng
Research Paper
Cooling characteristics of cooking oil using water mist during fire extinguishment Zhang Tian-wei a,b, Han Zhi-yue a,⇑, Du Zhi-ming a, Liu Kai a, Zhang Ze-lin a a b
State Key Laboratory of Explosion Science and Technology, Beijing Institute of Technology, Beijing 100081, China Department of Fire Commanding, The Chinese People’s Armed Police Forces Academy, Langfang, Hebei 065000, China
h i g h l i g h t s The cooling characteristics of a home scale of cooking oil fire was first successfully studied. The influence factors of the boiling over layer was proved by temperature analysis. The suppression efficiency of the boiling over layer using additives are higher than pure water. The mechanism of suppression expansion of the boiling over layer by the six additives are different.
a r t i c l e
i n f o
Article history: Received 28 March 2016 Revised 6 July 2016 Accepted 6 July 2016 Available online 7 July 2016 Keywords: Water mist Additive Cooking oil Boiling layer Cooling
a b s t r a c t This study analyzed the effects of both water mist and water mist with different additives on cooling of cooking oils during fire extinguishment. The sizes of water mist drops with different additives were measured using a split-type laser particle analyzer. Full-scale extinguishment–cooling experiments using different fire extinguishing agents and oil temperatures were also conducted. The boiling over layer formed during the cooling of cooking oil using water mist could significantly enhance the cooling rate. Moreover, the expanding rate of this boiling over layer is related to oil temperature and water mass in oils. Although additives reduce water evaporation and hinder oil cooling, such additives also inhibit the expansion of the boiling over layer, thereby reducing the risk of secondary damage caused by abundant oil overflow after the fire extinguishment. Ó 2016 Elsevier Ltd. All rights reserved.
1. Introduction The rapid development of kitchen equipment and production technology of cooking oils has resulted in the extensive use of highly efficient and high-energy cookers and cooking oils with high spontaneous ignition points. However, using these innovations also increases the risk of kitchen fires. Cooking oil, small pan, and large-scale oil tank fire accidents in food processing industries are all caused by spontaneous ignition of oils at several hundred degrees Celsius. Oil temperature in combustion is substantially higher than the boiling point of water [1]. Although dry powder and gaseous extinguishing agents could extinguish the surface fires of cooking oils, these substances cannot cool fuels effectively and can easily cause a recrudescence phenomenon [2]. Water could cool both flame and fuel effectively because of its high heat capacity. However, water will cause splash and boiling over upon contact with the hot oil surface; these situations are dangerous to ⇑ Corresponding author. E-mail address: [email protected] (Z.-y. Han). http://dx.doi.org/10.1016/j.applthermaleng.2016.07.043 1359-4311/Ó 2016 Elsevier Ltd. All rights reserved.
people, including firefighters, near the fire. Therefore, hot oil fire accidents should not be extinguished using water [3]. Splash refers to the rapid evaporation that occurs when water surpasses its boiling temperature and reaches a superheated steam temperature [4–6]. As water evaporates and absorbs heat, oil temperature decreases while liquid drops remaining on the oil surface are heated and develop into bubbles. Hot oil will be extruded from the container with the expansion and breakage of bubbles, thereby causing boiling over. The overflow of oil will ignite surrounding combustibles rapidly, which intensifies the fire immediately. In previous boiling over experiments of oil products [7,8], water was placed under the fuel bottom and oil burned on the water layer. Only a few studies have been conducted on the boiling over phenomenon caused by water mist during extinguishment; hence, the corresponding influencing factors remain unknown. Nam [9] performed a full-scale experiment using water spray to extinguish large-scale industrial oil tank fires. Water spraying could extinguish oil tank fires effectively without causing splashing. However, abundant oil boiling over was observed in every set of experiments conducted, thereby causing extensive flowing fire. A similar result
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was reported in another study of Nam [10], although he did not conduct further theoretical analysis on the boiling over phenomenon. Liu [11–13] conducted a full-scale experiment on extinguishing cooking oil fire using water mist, in which only small boiling over was observed. Subsequently, he studied the cooling characteristics of oil products during this extinguishment and proposed boiling over and splash as influencing factors. Nevertheless, he selected a fire model for large-scale food processing enterprises and did not consider the effects of environment-friendly additives on the cooling process. Water mist is extensively used in restaurants or hotels with high risks for kitchen fire. The uniqueness of these cooking sites entails that fire extinguishing agents must be non-toxic, harmless, and should prevent the after-combustion of oils. Therefore, performing small-scale extinguishment–cooling experiment for kitchen fire accidents is necessary. Using a small-scale fire model, this study conducted experimental and theoretical analyses of splash and boiling over during oil cooling under different oil temperatures and additives. The results could provide several references in designing water mist fire extinguishing systems for places with potential cooking oil fire risks to decrease the number of firefighter casualties while extinguishing hot oil fires. 2. Experimental apparatus and methods The experiment was conducted in a 3 m 2.1 m 2.8 m confined space. The water mist fire extinguishing system comprised a water storage tank, nitrogen gas cylinder, pressure regulator, water mist spray nozzle, and connecting lines, a split-type laser particle analyzer was placed in the middle of the nozzle and oil pan, and connected with the computer which measured the droplets size in real-time, as shown in Fig. 1. The drip pan was 20 cm in diameter and 12 cm high, and was placed underneath the water mist spray nozzle. The upper edge of the drip pan was 1 m away from the water mist spray nozzle. Three thermocouples (labeled 1#, 2#, and 3#) were placed vertically; the vertical distances between them and the drip pan bottom were 3, 6, and 9 cm, respectively. The tested cooking oil was peanut oil, which a total of 1500 ml was placed in the oil pan. The height of the cooking oil in the oil pan was 4.7 cm, thereby immersing thermocouple 1#. A resistance wire pan was placed under the oil pan with a steel sleeve which diameter was 20 cm. On one hand, the steel sleeve could avoid the hot resistance wire from contacting water mist, it could also played the uniform heating of oil pan. The cooking oil was heated
Pressure regulator
at a temperature rising rate of 10.8 °C/min by controlling the heat resistance wire until the spontaneous combustion point was reached. Thereafter, the reducing valve was adjusted to 0.4 MPa. Water mist was applied when the temperature of thermocouple 1# was 390 °C. After the fire was extinguished, water mist was applied continuously until the temperature of thermocouple 1# measured 300 °C. Six additives for water mist were selected: K2C2O4, KNO3, CH3COOK, KCl, KH2PO4, and NH4H2PO4. All additives were manufactured by Beijing Chemical Works and were analytically pure. The additives were prepared into 5% (mass fraction) solutions and used as fire extinguishing agents.
3. Results and discussion 3.1. Cooling process of cooking oils Oil surface cooling while extinguishing cooking oil fires with pure water mist can be divided into two stages according to temperature changes. The first stage is from the application of water mist to the extinguishment of the naked flame. The second stage is from the extinguishment of the naked flame to the end of the water mist application. The extinguishing effect is shown in Fig. 2. Fig. 3 is the temperature-time curve during cooking oil fires cooling process which shows that the oil surface temperature in the first stage is high and retains its temperature for a short period after water mist is applied. The naked flame has not been completely controlled by the water mist and only a few water mist drops reach the oil surface because of the fire plume. Cooking oils increase volume during the heating process. The temperature of thermocouple 2# was the same as that of thermocouple 3# during the heating process, thereby indicating that the volume expansion of the cooking oil did not immerse the former. The temperature measured on thermocouple 2# was equivalent to the flame temperature, which dropped significantly under the effect of the water mist. No splash was observed during this stage. Reid [14] explained that the critical condition of splash is oil 1 6 TTsup 6 1.1, where T is the temperature in K and the subscripts
oil and sup represent the oil and superheated steam of water, respectively. In the current experiment, the spontaneous ignition temperature range of the cooking oil ranged between 357.3 °C and 362.7 °C. The superheated steam temperature of pure water in Ref. [15] was 279–302 °C. The ratio between the spontaneous ignition temperature range of the cooking oil and superheated steam temperature of pure water was over 1.1, thereby causing
Connecting lines
Data collection Nozzle
Laser emission
Laser particle analyzer Nitrogen
Liquid
Laser receiver
Oil pan
Resistance
Stainless steel frame
Fig. 1. Schematic of the experimental set-up.
Thermocouple
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Heating
Near ignition point
Normal combustion
Discharge water mist
Ignition
Extinguishing
Fig. 2. The extinguishing effect of cooking oil fire by water mist.
Discharge water
600
1# thermocouple 2# thermocouple 3# thermocouple
500
Temperature (ºC)
Spontaneous combustion 400
Water mist stop
300
perature data in Nam [10] substantially decreased because the three thermocouples have different sensitivities to temperature, which was caused by the transformation between oil and water temperatures. This finding is side evidence of the existence of the boiling over layer. After a considerable decrease, the temperature of the boiling over layer decreases gradually when the application of water mist is stopped. 3.2. Influencing factors of the boiling over layer
Extinguishing 200
The expression of bubble diameter in the boiling over layer is as follows [14]:
100
Du ¼ 900
1000
1100
1200
1300
1400
1500
1600
Time (s) Fig. 3. Effect of temperature using water mist during fire extinguishment.
no splash. At this moment, a layer of steam film will develop between the liquid drops and hot oil surface when they come into contact with each other [13], thereby preventing their direct contact and avoiding splash. With the continuous application of water mist, the oil surface cooled to the superheated steam temperature of water, thereby making the steam film disappear. Consequently, liquid drop comes directly in contact with the hot oil surface, thereby forming bubbles and causing boiling. During the second stage of cooling when no disturbance is conducted by flames, air, oil steam, and water steam mix intensively at the oil surface and form a boiling over layer. The formation of bubbles intensifies heat convection in oil. The heat steam released during bubble breakage could accelerate cooling, thereby explaining the rapid decrease of oil temperature. Frequency formation and bubble breaks in oil occur because of the existence of the boiling over layer. Most recorded temperatures of thermocouple 1# were temperatures of the water steam in bubbles rather than the oil surface temperature. The measured tem-
4rðT oil Þ ; PB ðT oil Þ Px
ð1Þ
where Du is the bubble diameter in m, r is the surface tension in N/ m, PB is the bubble pressure in Pa, and Px is the pressure of the mixture in Pa. Surface tension and bubble pressure are directly related with oil temperature. A high oil temperature is accompanied by low surface tension and high bubble pressure. Consequently, bubbles are small or no bubbles are formed during the first stage of cooling. No boiling and bubbles were observed in the experiment. Instead, bubbles and boiling occur in the second stage. Eq. (1) demonstrates that bubbles will expand rapidly as bubble pressure reduces and surface tension increases. However, if the oil temperature is considerably low to maintain adequate bubble pressure for bubble growth, then the bubbles will disappear. Theoretically, an optimum temperature range for rapid bubble growth exists [14]. The boiling over layer is initially formed above the oil surface. This layer’s thickness (HB) increases with the application of water mist. Fig. 4 shows the simple structure of the boiling over layer. The boiling over layer thickens toward two directions. When water mist drops in the oil have not evaporated completely, the residual drops continue to mix with the oil and the boiling over layer extends toward the drop pan bottom (oil depth). The extension rate dHBD/dt is determined by the energy balance between the oil and the water [14].
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Nozzle
1# thermocouple 2# thermocouple 3# thermocouple
300
Discharge first time
Oil
Temperature (ºC)
Boiling layer with bubble
250
Discharge second time 200
150
Thermocouple Fig. 4. Boiling over layer.
100 1200
dHBD mw ðC PW DT w þ Lv w Þ qoil Aoil C Poil ðT oil T bl Þ P 0 dt
ð2Þ
1400
1600
1800
2000
2200
2400
Time (s) Fig. 5. Temperature of the thermocouples without fire.
or
dHBD mw ðC pw DT w þ Lv w Þ ¼ ; dt qoil Aoil C poil ðT oil T bl Þ
ð3Þ
where m is the mass in kg, Cp is the heat capacity in J/kgK, Lv is the latent heat of evaporation in kJ/kg, q is the density in kg/m3, A is the area in m2, HBD is the increased thickness of the boiling-over layer toward the oil depth in m, the subscript w is water, the subscript oil represents the cooking oil, and the subscript bl is the boiling over layer. The upward extension rate of the boiling over layer (dHBU/dt) is mainly determined by the growth rate of the bubbles and the mass fractions of water and oil in the boiling over layer. Its expression is as follows [14]:
dHBU dr b / ðmwbl þ moilbl Þ ; dt dt
ð4Þ
where HBU is the increased thickness of the boiling-over layer toward the external direction of the drop pan in m. Water mass in the boiling-over layer (mwbl) can be expressed as follows:
mwbl ¼ mwt mwe ;
ð5Þ
where mwt is the total water discharged from water mist in kg and mwe is the evaporated water in oil in kg. In Eq. (4), the amount of cooking oil in the boiling over layer (moilbl) is determined by the extension of the boiling over layer to the drop pan bottom, which can be calculated from Eq. (3). The growth rate of bubbles is as follows [14]:
dr b kðT oil T b Þ / ; dt DHv qv ðT b ÞðatÞ0:5
ð6Þ
where k is the thermal conductivity in W/mK, DH is the enthalpy of vaporization, a is the thermal diffusion coefficient; t is the time in s, the subscript v means water steam, and the subscript b means boiling. From Eqs. (2)–(6), the development of the boiling over layer during oil surface cooling by water mist is mainly determined by oil temperature and water volume. To explore the effects of oil temperature on the boiling over layer, water mist was applied for a second and third time without flame after the first application. Each application lasted for 5 s. The second application started at 230 °C of thermocouple 1# and the third application started at 200 °C of the same thermocouple. The pressure of the water mist was maintained at 0.4 MPa. The smoking point of the cooking oil is 230 °C [3]; 200 °C is slightly higher than the oil temperature (190 °C) in normal cooking. Fig. 5 shows the variations of oil temperature with time.
When cooking oils are cooled to 230 °C, no flame is present on the oil surface. The continuous application of water mist resulted in the temperature of thermocouple 3# dropping the most rapidly, followed successively by thermocouples 2# and 1#. When the application water mist stops, the temperatures of all three thermocouples decrease gradually. When the oil temperature decreases to 200 °C, no flame is present on the oil surface. With the continuous application of water mist, violent boiling occurs upon contact of the water and hot oil. Almost all oils boiled. The temperatures of all three thermocouples are consistent with one another. The boiling over layer rapidly increases to the boundary of the drop pan, thereby making considerable cooking oil spill over. Such spilling continues when the application of water mist is stopped. It is difficult to record the process of spill over due to the large amounts of water vapor caused by the water mist which contacts with the hot oil. However, it could be clearly that the oil experienced a process of rising from the oil descent process, as shown in Fig. 6. From (a) to (f), the oil surface gradually dropped to the bottom of the pan with the temperature decreasing, and the produced bubble also disappeared with the oil surface decreased. Eq. (3) reveals that the heat capacity of oil decreases with the continuous application of water mist. When the growth rate of the water mass in the oil layer is constant, more water will remain in the oil because of the weakening water evaporation. A low oil temperature corresponds to a rapid formation of the boiling over layer. From Eqs. (4)–(6), bubbles expand rapidly under high oil temperature; however, the boiling over layer could not expand upward or expand gradually because substantial water evaporates and limited water is left in the boiling over layer. Under low oil temperature, bubbles expand gradually, but the boiling-over layer expands upward rapidly because water evaporation decreases and the water volume in the boiling over layer increases. In the present experiment, the boiling over layer did not expand upward to thermocouple 2# at 230 °C. Even though a small upward expansion was observed, such expansion disappeared immediately when the application of water mist stops. By contrast, the boiling over layer expanded upward rapidly and immersed thermocouples 2# and 3# under 200 °C. The layer declined gradually even after the application of water mist was stopped; a huge spillover of cooking oils occurred thereafter. 3.3. Effect of water mist additives on cooking oil cooling Additives can change the efficiency of water mist in extinguishing cooking oil fires. Table 1 presents the extinguishing times of water mist using seven different additives. All fire extinguishing
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(a)
(b)
(d)
(c)
(e)
(f)
Fig. 6. The descent process of oil surface.
Table 1 Extinguishing times of cooking oil fire by different extinguishing agents. Agents
Water
K2C2O4 solution
CH3COOK solution
KNO3 solution
KCl solution
NH4H2PO4 solution
KH2PO4 solution
Extinguishing time (s)
34.47
1.6
4.4
3.1
4.8
25.7
18.4
water 5% potassium oxalate 5% potassium acetate 5% potassium nitrate 5% potassium chloride 5% ammonium dihydrogen phosphate 5% potassium hihydrogen phosphate
400 380 360
Temperature (ºC)
agents, except for the pure water mist, are 5% (mass fraction) solutions. Table 1 shows that different additives contribute varied flame extinguishing efficiencies because of varying extinguishing mechanisms [16]. Alkali metal additives could significantly increase the extinguishing efficiency of water mist, whereas the NH4H2PO4 solution slightly improves the extinguishing efficiency. The efficiencies of the seven fire extinguishing agents are as follows: K2C2O4 > KNO3 > CH3COOK > KCl > KH2PO4 > NH4H2PO4 > water. Fig. 7 shows the variations of oil surface temperature during the extinguishment using pure water mist and water mist with additives. Liu [17] reported that for cooking oil fires, an increase in oil temperature is mainly caused by the thermal radiation of the flame to the fuel, and cooling the fuel surface is the main extinguishing mechanism of water mist. Therefore, the abilities of the seven fire extinguishing agents to cool the oil surface have the same order as their extinguishing efficiencies. The water mist with alkali metal additives could extinguish flame rapidly, thereby reducing the thermal irradiation of the flame to the fuel and exhibiting an evident oil cooling effect. The NH4H2PO4 solution and pure water mist take a long time to extinguish the flame, thereby resulting in a long feedback time of thermal irradiation and poor cooling effect. Figs. 8–10 show the temperature curves of the three thermocouples under seven fire extinguishing agents. When the oil temperature decreases to 230 °C, the temperature of thermocouple 3# declines the fastest with a continuous application of water mist, followed successively by thermocouples 2# and 1#. Fig. 7 shows that when the oil temperature is 200 °C, the boiling over layer under all six water mists, except for the 5% (mass fraction) CH3-
340 320 300 280 260 240 1200
1220
1240
1260
1280
1300
1320
1340
Time (s) Fig. 7. Temperatures of thermocouple 1# under different extinguishing agents during fire extinguishment.
COOK solution, expands upward after the application of water mist was stopped. Fig. 8 illustrates that the boiling over layer immerses thermocouple 3# and oil spillover is observed only under pure water mist. The boiling over layer did not expand upward under high oil temperature (230 °C). This result can be explained from two aspects. First, extensive water evaporation occurs and only limited water is left in the oil under high oil temperature. Second, water
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water 5% potassium oxalate 5% potassium acetate 5% potassium nitrate 5% potassium chloride 5% ammonium dihydrogen phosphate 5% potassium hihydrogen phosphate
300
400 350 300
Solubility (g)
Temperature (ºC)
250
200
150
potassium oxalate potassium acetate potassium nitrate potassium chloride ammonium dihydrogen phosphate potassium dihydrogen phosphate
250 200 150 100
100
50 1200
1400
1600
1800
2000
2200
2400
0
Time (s) 0 Fig. 8. Temperature of thermocouple 1# under different extinguishing agents without fire.
20
40
60
80
100
Temperature ( Fig. 11. Solubility curves of the different additives.
water 5% potassium oxalate 5% potassium acetate 5% potassium nitrate 5% potassium chloride 5% ammonium dihydrogen phosphate 5% potassium hihydrogen phosphate
300
Temperature (ºC)
250
200
150
100
50 1200
1400
1600
1800
2000
2200
2400
Time (s) Fig. 9. Temperature of thermocouple 2# under different extinguishing agents without fire.
240
water 5% potassium oxalate 5% potassium acetate 5% potassium nitrate 5% potassium chloride 5% ammonium dihydrogen phosphate 5% potassium hihydrogen phosphate
220
Temperature (ºC)
200 180 160
over layer in oil. High water mass in oil will have substantial water involved in boiling, and the boiling over layer will expand rapidly. Additives also influence water evaporation. Under the same flow rate of water mist, water mass in oil decreases. Fig. 6 also shows that after completing the secondary water mist application, additives affect water evaporation and the oil temperature decreases more gradually than that under pure water mist. The effect of additives on water evaporation is determined by the hydratability of salt, that is, solubility. Fig. 11 presents the solubility curves [18] of the six additives. CH3COOK has a considerably higher solubility than the remaining five additives. A 200 °C oil temperature is lower than the decomposition temperature of CH3COOK. Undecomposed CH3COOK will be abundant in the boiling over layer because oil temperature decreases with the application of water mist. The surrounding ions as hydrated ions will adsorb water in the solution because of the strong hydratability of CH3COOK and limited amount of water in oil. Consequently, the boiling over layer would not expand to immerse thermocouple 2#. The hydratabilities of the remaining five additives are weaker than that of CH3COOK but can still influence the water mass in oil more than the pure water mist does. Hence, the boiling over layer expands to immerse thermocouple 3#, and oil spillover is only observed under pure water mist. The typical expression of drop evaporation time is as follows [19]:
140
te ¼ ln
120 100 80 60 1200
1400
1600
1800
2000
2200
2400
Time (s) Fig. 10. Temperature of thermocouple 3# under different extinguishing agents without fire.
mist was applied for only 5 s, which supplied limited water to the oil. Eqs. (3) and (4) reflect that water mass in oil is an important factor to the formation and upward expansion rates of the boiling
T 0 T max qw C pw0 D2 qw C pw D2 h i: þ C T b T max 12k 8kln 1 þ kkb Lvpw ðT max T b Þ
ð7Þ
This expression comprises two parts. The first part is the period from the initial heating to the boiling point, whereas the second part is the time for drop evaporating into the water steam. The subscript e means evaporation, 0 means the initial state, and max means the maximum state. The drop size of the water mist was measured using a split-type laser particle analyzer (Table 2). Eq. (7) shows that the evaporation time of the drops is significantly influenced by drop size. Adding additives into the water mist will increase drop size, thereby increasing drop evaporation time and reducing water mass in oil. This addition goes against the development of the boiling over layer.
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T.-w. Zhang et al. / Applied Thermal Engineering 107 (2016) 863–869 Table 2 Diameter of the droplets with and without different additives. Agents
Water
K2C2O4 solution
CH3COOK solution
KNO3 solution
KCl solution
NH4H2PO4 solution
KH2PO4 solution
D50 (lm) D90 (lm)
220.71 394.21
325.88 464.45
266.54 432.77
355.90 497.37
315.74 498.21
293.44 504.94
305.10 523.15
4. Conclusions
References
1. When the oil temperature is higher than the superheated steam temperature of water, a steam film will form when the water mist drop makes contact with the hot oil surface, thereby preventing direct contact between them, as well as water splash. When the oil temperature cools considerably close to the superheated steam temperature of water, this steam film disappears and water splash occurs. 2. During the cooling process of cooking oil using water mist, a boiling over layer will form on the surface of the hot oil, which will expand toward the bottom and upper edge of the drop pan. Bubbles in the boiling over layer intensify heat convection in the oil and accelerate oil cooling. The expansion rate calculated from theoretical analysis is determined using the water mass in oil and oil temperature. Oil temperature directly influences water evaporation. Therefore, water mass in oil is the main influencing factor in the development of the boiling over layer. 3. Under same flow rate of water mist, the existence of additives will increase the hydratability of ions and drop size, thereby reducing water evaporation, and is adverse to oil cooling. However, water mist with additives could effectively inhibit the upward expansion of the boiling over layer and reduce the risk of secondary damage caused by abundant oil spill over after extinguishment.
[1] M.S. Wijayasinghe, T.B. Makey, Cooking oil: a home fire hazard in Alberta, Canada, Fire Technol. 33 (2) (1997) 140–166. [2] X. Ni, W.K. Chow, Q. Li, et al., Experimental study of new gas-solid composite particles in extinguishing cooking oil fires, J. Fire Sci. 29 (2) (2011) 152–176. [3] H. John, Asphalt fume exposures during the application of hot asphalt to roofs, National Inst. Occup. Saf. Health 6 (2003) 35–38. [4] P. Zhang, X. Gao, X. Zhang, Study on the suppressing effectiveness of water mist spray parameters in cooking oil fire, Shenyang Jianzhu Daxue Xuebao 26 (5) (2010) 951–956. [5] J. Chen, The experimental study on extinguishing cooking oil fires by water mist, Appl. Mech. Mater. 166–169 (2012) 2804–2809. [6] W. Chow, X. Ni, Experimental evaluation on performance of open kitchen fire suppression systems, Fire Saf. Sci. 11 (2014) 1298–1311. [7] F. Ferrero, M. Munoz, B. Kozanoglu, et al., Experimental study of thin-layer boilover in large-scale pool fires, J. Hazard. Mater. 137 (3) (2006) 1293–1302. [8] H. Koseki, Y. Natsume, Y. Iwata, et al., Large-scale boilover experiments using crude oil, Fire Saf. J. 41 (2006) 529–535. [9] S. Nam, Application of water sprays to industrial oil cooker fire, Fire Saf. J. 7 (2003) 469–480. [10] S. Nam, Boilover upon suppression of cooking oil fire by water spray, Fire Saf. J. 39 (5) (2004) 429–432. [11] Z. Liu, A.K. Kim, D. Carpenter, Extinguishment of cooking oil fires by water mist fire suppression systems, Fire Technol. 40 (4) (2004) 309–333. [12] Z. Liu, D. Carpenter, A.K. Kim, Characteristics of large cooking oil pool fires and their extinguishment by water mist, J. Loss Prev. Process Ind. 19 (6) (2006) 516–526. [13] Z. Liu, D. Carpenter, A.K. Kim, Cooling characteristics of hot oil pool by water mist during fire suppression, Fire Saf. J. 43 (4) (2008) 269–281. [14] R.C. Reid, Rapid phase transitions from liquid to vapor, Adv. Chem. Eng. (1983) 105–208. [15] C.T. Avedisian, The homogeneous nucleation limits of liquids, J. Phys. Chem. Ref. Data 14 (3) (1985) 695–729. [16] Z.L. Zhang, Experimental Study on Oil Pool Fire Suppression Effectiveness of Water Mist with Additives, Beijing Institute of Technology, Beijing, 2015. 28. [17] Z. Liu, A.K. Kim, D. Carpenter, A study of portable water mist fire extinguishers used for extinguishment of multiple fire types, Fire Saf. J. 42 (1) (2007) 25–42. [18] J.G. Speight, Lange’s Handbook of Chemistry, 15th ed., McGRAW-HILL, New York, 1999, pp. 8–20. Section 5. [19] B. Lewis, R.N. Pease, H.S. Taylor, High speed aerodynamics and jet propulsion. volume II. combustion processes, High Speed Aerodynam. Jet Propul. (1956) 431.
Acknowledgments This work was supported by the basic research fund of Beijing Institute of Technology (20130242017), the project fund of State Key Laboratory of Explosion Science and Technology (Beijing Institute of Technology) (YBKT16-09 and QNKT16-03).
Contents lists available at ScienceDirect
Applied Thermal Engineering journal homepage: www.elsevier.com/locate/apthermeng
Research Paper
Cooling characteristics of cooking oil using water mist during fire extinguishment Zhang Tian-wei a,b, Han Zhi-yue a,⇑, Du Zhi-ming a, Liu Kai a, Zhang Ze-lin a a b
State Key Laboratory of Explosion Science and Technology, Beijing Institute of Technology, Beijing 100081, China Department of Fire Commanding, The Chinese People’s Armed Police Forces Academy, Langfang, Hebei 065000, China
h i g h l i g h t s The cooling characteristics of a home scale of cooking oil fire was first successfully studied. The influence factors of the boiling over layer was proved by temperature analysis. The suppression efficiency of the boiling over layer using additives are higher than pure water. The mechanism of suppression expansion of the boiling over layer by the six additives are different.
a r t i c l e
i n f o
Article history: Received 28 March 2016 Revised 6 July 2016 Accepted 6 July 2016 Available online 7 July 2016 Keywords: Water mist Additive Cooking oil Boiling layer Cooling
a b s t r a c t This study analyzed the effects of both water mist and water mist with different additives on cooling of cooking oils during fire extinguishment. The sizes of water mist drops with different additives were measured using a split-type laser particle analyzer. Full-scale extinguishment–cooling experiments using different fire extinguishing agents and oil temperatures were also conducted. The boiling over layer formed during the cooling of cooking oil using water mist could significantly enhance the cooling rate. Moreover, the expanding rate of this boiling over layer is related to oil temperature and water mass in oils. Although additives reduce water evaporation and hinder oil cooling, such additives also inhibit the expansion of the boiling over layer, thereby reducing the risk of secondary damage caused by abundant oil overflow after the fire extinguishment. Ó 2016 Elsevier Ltd. All rights reserved.
1. Introduction The rapid development of kitchen equipment and production technology of cooking oils has resulted in the extensive use of highly efficient and high-energy cookers and cooking oils with high spontaneous ignition points. However, using these innovations also increases the risk of kitchen fires. Cooking oil, small pan, and large-scale oil tank fire accidents in food processing industries are all caused by spontaneous ignition of oils at several hundred degrees Celsius. Oil temperature in combustion is substantially higher than the boiling point of water [1]. Although dry powder and gaseous extinguishing agents could extinguish the surface fires of cooking oils, these substances cannot cool fuels effectively and can easily cause a recrudescence phenomenon [2]. Water could cool both flame and fuel effectively because of its high heat capacity. However, water will cause splash and boiling over upon contact with the hot oil surface; these situations are dangerous to ⇑ Corresponding author. E-mail address: [email protected] (Z.-y. Han). http://dx.doi.org/10.1016/j.applthermaleng.2016.07.043 1359-4311/Ó 2016 Elsevier Ltd. All rights reserved.
people, including firefighters, near the fire. Therefore, hot oil fire accidents should not be extinguished using water [3]. Splash refers to the rapid evaporation that occurs when water surpasses its boiling temperature and reaches a superheated steam temperature [4–6]. As water evaporates and absorbs heat, oil temperature decreases while liquid drops remaining on the oil surface are heated and develop into bubbles. Hot oil will be extruded from the container with the expansion and breakage of bubbles, thereby causing boiling over. The overflow of oil will ignite surrounding combustibles rapidly, which intensifies the fire immediately. In previous boiling over experiments of oil products [7,8], water was placed under the fuel bottom and oil burned on the water layer. Only a few studies have been conducted on the boiling over phenomenon caused by water mist during extinguishment; hence, the corresponding influencing factors remain unknown. Nam [9] performed a full-scale experiment using water spray to extinguish large-scale industrial oil tank fires. Water spraying could extinguish oil tank fires effectively without causing splashing. However, abundant oil boiling over was observed in every set of experiments conducted, thereby causing extensive flowing fire. A similar result
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was reported in another study of Nam [10], although he did not conduct further theoretical analysis on the boiling over phenomenon. Liu [11–13] conducted a full-scale experiment on extinguishing cooking oil fire using water mist, in which only small boiling over was observed. Subsequently, he studied the cooling characteristics of oil products during this extinguishment and proposed boiling over and splash as influencing factors. Nevertheless, he selected a fire model for large-scale food processing enterprises and did not consider the effects of environment-friendly additives on the cooling process. Water mist is extensively used in restaurants or hotels with high risks for kitchen fire. The uniqueness of these cooking sites entails that fire extinguishing agents must be non-toxic, harmless, and should prevent the after-combustion of oils. Therefore, performing small-scale extinguishment–cooling experiment for kitchen fire accidents is necessary. Using a small-scale fire model, this study conducted experimental and theoretical analyses of splash and boiling over during oil cooling under different oil temperatures and additives. The results could provide several references in designing water mist fire extinguishing systems for places with potential cooking oil fire risks to decrease the number of firefighter casualties while extinguishing hot oil fires. 2. Experimental apparatus and methods The experiment was conducted in a 3 m 2.1 m 2.8 m confined space. The water mist fire extinguishing system comprised a water storage tank, nitrogen gas cylinder, pressure regulator, water mist spray nozzle, and connecting lines, a split-type laser particle analyzer was placed in the middle of the nozzle and oil pan, and connected with the computer which measured the droplets size in real-time, as shown in Fig. 1. The drip pan was 20 cm in diameter and 12 cm high, and was placed underneath the water mist spray nozzle. The upper edge of the drip pan was 1 m away from the water mist spray nozzle. Three thermocouples (labeled 1#, 2#, and 3#) were placed vertically; the vertical distances between them and the drip pan bottom were 3, 6, and 9 cm, respectively. The tested cooking oil was peanut oil, which a total of 1500 ml was placed in the oil pan. The height of the cooking oil in the oil pan was 4.7 cm, thereby immersing thermocouple 1#. A resistance wire pan was placed under the oil pan with a steel sleeve which diameter was 20 cm. On one hand, the steel sleeve could avoid the hot resistance wire from contacting water mist, it could also played the uniform heating of oil pan. The cooking oil was heated
Pressure regulator
at a temperature rising rate of 10.8 °C/min by controlling the heat resistance wire until the spontaneous combustion point was reached. Thereafter, the reducing valve was adjusted to 0.4 MPa. Water mist was applied when the temperature of thermocouple 1# was 390 °C. After the fire was extinguished, water mist was applied continuously until the temperature of thermocouple 1# measured 300 °C. Six additives for water mist were selected: K2C2O4, KNO3, CH3COOK, KCl, KH2PO4, and NH4H2PO4. All additives were manufactured by Beijing Chemical Works and were analytically pure. The additives were prepared into 5% (mass fraction) solutions and used as fire extinguishing agents.
3. Results and discussion 3.1. Cooling process of cooking oils Oil surface cooling while extinguishing cooking oil fires with pure water mist can be divided into two stages according to temperature changes. The first stage is from the application of water mist to the extinguishment of the naked flame. The second stage is from the extinguishment of the naked flame to the end of the water mist application. The extinguishing effect is shown in Fig. 2. Fig. 3 is the temperature-time curve during cooking oil fires cooling process which shows that the oil surface temperature in the first stage is high and retains its temperature for a short period after water mist is applied. The naked flame has not been completely controlled by the water mist and only a few water mist drops reach the oil surface because of the fire plume. Cooking oils increase volume during the heating process. The temperature of thermocouple 2# was the same as that of thermocouple 3# during the heating process, thereby indicating that the volume expansion of the cooking oil did not immerse the former. The temperature measured on thermocouple 2# was equivalent to the flame temperature, which dropped significantly under the effect of the water mist. No splash was observed during this stage. Reid [14] explained that the critical condition of splash is oil 1 6 TTsup 6 1.1, where T is the temperature in K and the subscripts
oil and sup represent the oil and superheated steam of water, respectively. In the current experiment, the spontaneous ignition temperature range of the cooking oil ranged between 357.3 °C and 362.7 °C. The superheated steam temperature of pure water in Ref. [15] was 279–302 °C. The ratio between the spontaneous ignition temperature range of the cooking oil and superheated steam temperature of pure water was over 1.1, thereby causing
Connecting lines
Data collection Nozzle
Laser emission
Laser particle analyzer Nitrogen
Liquid
Laser receiver
Oil pan
Resistance
Stainless steel frame
Fig. 1. Schematic of the experimental set-up.
Thermocouple
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Heating
Near ignition point
Normal combustion
Discharge water mist
Ignition
Extinguishing
Fig. 2. The extinguishing effect of cooking oil fire by water mist.
Discharge water
600
1# thermocouple 2# thermocouple 3# thermocouple
500
Temperature (ºC)
Spontaneous combustion 400
Water mist stop
300
perature data in Nam [10] substantially decreased because the three thermocouples have different sensitivities to temperature, which was caused by the transformation between oil and water temperatures. This finding is side evidence of the existence of the boiling over layer. After a considerable decrease, the temperature of the boiling over layer decreases gradually when the application of water mist is stopped. 3.2. Influencing factors of the boiling over layer
Extinguishing 200
The expression of bubble diameter in the boiling over layer is as follows [14]:
100
Du ¼ 900
1000
1100
1200
1300
1400
1500
1600
Time (s) Fig. 3. Effect of temperature using water mist during fire extinguishment.
no splash. At this moment, a layer of steam film will develop between the liquid drops and hot oil surface when they come into contact with each other [13], thereby preventing their direct contact and avoiding splash. With the continuous application of water mist, the oil surface cooled to the superheated steam temperature of water, thereby making the steam film disappear. Consequently, liquid drop comes directly in contact with the hot oil surface, thereby forming bubbles and causing boiling. During the second stage of cooling when no disturbance is conducted by flames, air, oil steam, and water steam mix intensively at the oil surface and form a boiling over layer. The formation of bubbles intensifies heat convection in oil. The heat steam released during bubble breakage could accelerate cooling, thereby explaining the rapid decrease of oil temperature. Frequency formation and bubble breaks in oil occur because of the existence of the boiling over layer. Most recorded temperatures of thermocouple 1# were temperatures of the water steam in bubbles rather than the oil surface temperature. The measured tem-
4rðT oil Þ ; PB ðT oil Þ Px
ð1Þ
where Du is the bubble diameter in m, r is the surface tension in N/ m, PB is the bubble pressure in Pa, and Px is the pressure of the mixture in Pa. Surface tension and bubble pressure are directly related with oil temperature. A high oil temperature is accompanied by low surface tension and high bubble pressure. Consequently, bubbles are small or no bubbles are formed during the first stage of cooling. No boiling and bubbles were observed in the experiment. Instead, bubbles and boiling occur in the second stage. Eq. (1) demonstrates that bubbles will expand rapidly as bubble pressure reduces and surface tension increases. However, if the oil temperature is considerably low to maintain adequate bubble pressure for bubble growth, then the bubbles will disappear. Theoretically, an optimum temperature range for rapid bubble growth exists [14]. The boiling over layer is initially formed above the oil surface. This layer’s thickness (HB) increases with the application of water mist. Fig. 4 shows the simple structure of the boiling over layer. The boiling over layer thickens toward two directions. When water mist drops in the oil have not evaporated completely, the residual drops continue to mix with the oil and the boiling over layer extends toward the drop pan bottom (oil depth). The extension rate dHBD/dt is determined by the energy balance between the oil and the water [14].
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Nozzle
1# thermocouple 2# thermocouple 3# thermocouple
300
Discharge first time
Oil
Temperature (ºC)
Boiling layer with bubble
250
Discharge second time 200
150
Thermocouple Fig. 4. Boiling over layer.
100 1200
dHBD mw ðC PW DT w þ Lv w Þ qoil Aoil C Poil ðT oil T bl Þ P 0 dt
ð2Þ
1400
1600
1800
2000
2200
2400
Time (s) Fig. 5. Temperature of the thermocouples without fire.
or
dHBD mw ðC pw DT w þ Lv w Þ ¼ ; dt qoil Aoil C poil ðT oil T bl Þ
ð3Þ
where m is the mass in kg, Cp is the heat capacity in J/kgK, Lv is the latent heat of evaporation in kJ/kg, q is the density in kg/m3, A is the area in m2, HBD is the increased thickness of the boiling-over layer toward the oil depth in m, the subscript w is water, the subscript oil represents the cooking oil, and the subscript bl is the boiling over layer. The upward extension rate of the boiling over layer (dHBU/dt) is mainly determined by the growth rate of the bubbles and the mass fractions of water and oil in the boiling over layer. Its expression is as follows [14]:
dHBU dr b / ðmwbl þ moilbl Þ ; dt dt
ð4Þ
where HBU is the increased thickness of the boiling-over layer toward the external direction of the drop pan in m. Water mass in the boiling-over layer (mwbl) can be expressed as follows:
mwbl ¼ mwt mwe ;
ð5Þ
where mwt is the total water discharged from water mist in kg and mwe is the evaporated water in oil in kg. In Eq. (4), the amount of cooking oil in the boiling over layer (moilbl) is determined by the extension of the boiling over layer to the drop pan bottom, which can be calculated from Eq. (3). The growth rate of bubbles is as follows [14]:
dr b kðT oil T b Þ / ; dt DHv qv ðT b ÞðatÞ0:5
ð6Þ
where k is the thermal conductivity in W/mK, DH is the enthalpy of vaporization, a is the thermal diffusion coefficient; t is the time in s, the subscript v means water steam, and the subscript b means boiling. From Eqs. (2)–(6), the development of the boiling over layer during oil surface cooling by water mist is mainly determined by oil temperature and water volume. To explore the effects of oil temperature on the boiling over layer, water mist was applied for a second and third time without flame after the first application. Each application lasted for 5 s. The second application started at 230 °C of thermocouple 1# and the third application started at 200 °C of the same thermocouple. The pressure of the water mist was maintained at 0.4 MPa. The smoking point of the cooking oil is 230 °C [3]; 200 °C is slightly higher than the oil temperature (190 °C) in normal cooking. Fig. 5 shows the variations of oil temperature with time.
When cooking oils are cooled to 230 °C, no flame is present on the oil surface. The continuous application of water mist resulted in the temperature of thermocouple 3# dropping the most rapidly, followed successively by thermocouples 2# and 1#. When the application water mist stops, the temperatures of all three thermocouples decrease gradually. When the oil temperature decreases to 200 °C, no flame is present on the oil surface. With the continuous application of water mist, violent boiling occurs upon contact of the water and hot oil. Almost all oils boiled. The temperatures of all three thermocouples are consistent with one another. The boiling over layer rapidly increases to the boundary of the drop pan, thereby making considerable cooking oil spill over. Such spilling continues when the application of water mist is stopped. It is difficult to record the process of spill over due to the large amounts of water vapor caused by the water mist which contacts with the hot oil. However, it could be clearly that the oil experienced a process of rising from the oil descent process, as shown in Fig. 6. From (a) to (f), the oil surface gradually dropped to the bottom of the pan with the temperature decreasing, and the produced bubble also disappeared with the oil surface decreased. Eq. (3) reveals that the heat capacity of oil decreases with the continuous application of water mist. When the growth rate of the water mass in the oil layer is constant, more water will remain in the oil because of the weakening water evaporation. A low oil temperature corresponds to a rapid formation of the boiling over layer. From Eqs. (4)–(6), bubbles expand rapidly under high oil temperature; however, the boiling over layer could not expand upward or expand gradually because substantial water evaporates and limited water is left in the boiling over layer. Under low oil temperature, bubbles expand gradually, but the boiling-over layer expands upward rapidly because water evaporation decreases and the water volume in the boiling over layer increases. In the present experiment, the boiling over layer did not expand upward to thermocouple 2# at 230 °C. Even though a small upward expansion was observed, such expansion disappeared immediately when the application of water mist stops. By contrast, the boiling over layer expanded upward rapidly and immersed thermocouples 2# and 3# under 200 °C. The layer declined gradually even after the application of water mist was stopped; a huge spillover of cooking oils occurred thereafter. 3.3. Effect of water mist additives on cooking oil cooling Additives can change the efficiency of water mist in extinguishing cooking oil fires. Table 1 presents the extinguishing times of water mist using seven different additives. All fire extinguishing
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(a)
(b)
(d)
(c)
(e)
(f)
Fig. 6. The descent process of oil surface.
Table 1 Extinguishing times of cooking oil fire by different extinguishing agents. Agents
Water
K2C2O4 solution
CH3COOK solution
KNO3 solution
KCl solution
NH4H2PO4 solution
KH2PO4 solution
Extinguishing time (s)
34.47
1.6
4.4
3.1
4.8
25.7
18.4
water 5% potassium oxalate 5% potassium acetate 5% potassium nitrate 5% potassium chloride 5% ammonium dihydrogen phosphate 5% potassium hihydrogen phosphate
400 380 360
Temperature (ºC)
agents, except for the pure water mist, are 5% (mass fraction) solutions. Table 1 shows that different additives contribute varied flame extinguishing efficiencies because of varying extinguishing mechanisms [16]. Alkali metal additives could significantly increase the extinguishing efficiency of water mist, whereas the NH4H2PO4 solution slightly improves the extinguishing efficiency. The efficiencies of the seven fire extinguishing agents are as follows: K2C2O4 > KNO3 > CH3COOK > KCl > KH2PO4 > NH4H2PO4 > water. Fig. 7 shows the variations of oil surface temperature during the extinguishment using pure water mist and water mist with additives. Liu [17] reported that for cooking oil fires, an increase in oil temperature is mainly caused by the thermal radiation of the flame to the fuel, and cooling the fuel surface is the main extinguishing mechanism of water mist. Therefore, the abilities of the seven fire extinguishing agents to cool the oil surface have the same order as their extinguishing efficiencies. The water mist with alkali metal additives could extinguish flame rapidly, thereby reducing the thermal irradiation of the flame to the fuel and exhibiting an evident oil cooling effect. The NH4H2PO4 solution and pure water mist take a long time to extinguish the flame, thereby resulting in a long feedback time of thermal irradiation and poor cooling effect. Figs. 8–10 show the temperature curves of the three thermocouples under seven fire extinguishing agents. When the oil temperature decreases to 230 °C, the temperature of thermocouple 3# declines the fastest with a continuous application of water mist, followed successively by thermocouples 2# and 1#. Fig. 7 shows that when the oil temperature is 200 °C, the boiling over layer under all six water mists, except for the 5% (mass fraction) CH3-
340 320 300 280 260 240 1200
1220
1240
1260
1280
1300
1320
1340
Time (s) Fig. 7. Temperatures of thermocouple 1# under different extinguishing agents during fire extinguishment.
COOK solution, expands upward after the application of water mist was stopped. Fig. 8 illustrates that the boiling over layer immerses thermocouple 3# and oil spillover is observed only under pure water mist. The boiling over layer did not expand upward under high oil temperature (230 °C). This result can be explained from two aspects. First, extensive water evaporation occurs and only limited water is left in the oil under high oil temperature. Second, water
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water 5% potassium oxalate 5% potassium acetate 5% potassium nitrate 5% potassium chloride 5% ammonium dihydrogen phosphate 5% potassium hihydrogen phosphate
300
400 350 300
Solubility (g)
Temperature (ºC)
250
200
150
potassium oxalate potassium acetate potassium nitrate potassium chloride ammonium dihydrogen phosphate potassium dihydrogen phosphate
250 200 150 100
100
50 1200
1400
1600
1800
2000
2200
2400
0
Time (s) 0 Fig. 8. Temperature of thermocouple 1# under different extinguishing agents without fire.
20
40
60
80
100
Temperature ( Fig. 11. Solubility curves of the different additives.
water 5% potassium oxalate 5% potassium acetate 5% potassium nitrate 5% potassium chloride 5% ammonium dihydrogen phosphate 5% potassium hihydrogen phosphate
300
Temperature (ºC)
250
200
150
100
50 1200
1400
1600
1800
2000
2200
2400
Time (s) Fig. 9. Temperature of thermocouple 2# under different extinguishing agents without fire.
240
water 5% potassium oxalate 5% potassium acetate 5% potassium nitrate 5% potassium chloride 5% ammonium dihydrogen phosphate 5% potassium hihydrogen phosphate
220
Temperature (ºC)
200 180 160
over layer in oil. High water mass in oil will have substantial water involved in boiling, and the boiling over layer will expand rapidly. Additives also influence water evaporation. Under the same flow rate of water mist, water mass in oil decreases. Fig. 6 also shows that after completing the secondary water mist application, additives affect water evaporation and the oil temperature decreases more gradually than that under pure water mist. The effect of additives on water evaporation is determined by the hydratability of salt, that is, solubility. Fig. 11 presents the solubility curves [18] of the six additives. CH3COOK has a considerably higher solubility than the remaining five additives. A 200 °C oil temperature is lower than the decomposition temperature of CH3COOK. Undecomposed CH3COOK will be abundant in the boiling over layer because oil temperature decreases with the application of water mist. The surrounding ions as hydrated ions will adsorb water in the solution because of the strong hydratability of CH3COOK and limited amount of water in oil. Consequently, the boiling over layer would not expand to immerse thermocouple 2#. The hydratabilities of the remaining five additives are weaker than that of CH3COOK but can still influence the water mass in oil more than the pure water mist does. Hence, the boiling over layer expands to immerse thermocouple 3#, and oil spillover is only observed under pure water mist. The typical expression of drop evaporation time is as follows [19]:
140
te ¼ ln
120 100 80 60 1200
1400
1600
1800
2000
2200
2400
Time (s) Fig. 10. Temperature of thermocouple 3# under different extinguishing agents without fire.
mist was applied for only 5 s, which supplied limited water to the oil. Eqs. (3) and (4) reflect that water mass in oil is an important factor to the formation and upward expansion rates of the boiling
T 0 T max qw C pw0 D2 qw C pw D2 h i: þ C T b T max 12k 8kln 1 þ kkb Lvpw ðT max T b Þ
ð7Þ
This expression comprises two parts. The first part is the period from the initial heating to the boiling point, whereas the second part is the time for drop evaporating into the water steam. The subscript e means evaporation, 0 means the initial state, and max means the maximum state. The drop size of the water mist was measured using a split-type laser particle analyzer (Table 2). Eq. (7) shows that the evaporation time of the drops is significantly influenced by drop size. Adding additives into the water mist will increase drop size, thereby increasing drop evaporation time and reducing water mass in oil. This addition goes against the development of the boiling over layer.
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Water
K2C2O4 solution
CH3COOK solution
KNO3 solution
KCl solution
NH4H2PO4 solution
KH2PO4 solution
D50 (lm) D90 (lm)
220.71 394.21
325.88 464.45
266.54 432.77
355.90 497.37
315.74 498.21
293.44 504.94
305.10 523.15
4. Conclusions
References
1. When the oil temperature is higher than the superheated steam temperature of water, a steam film will form when the water mist drop makes contact with the hot oil surface, thereby preventing direct contact between them, as well as water splash. When the oil temperature cools considerably close to the superheated steam temperature of water, this steam film disappears and water splash occurs. 2. During the cooling process of cooking oil using water mist, a boiling over layer will form on the surface of the hot oil, which will expand toward the bottom and upper edge of the drop pan. Bubbles in the boiling over layer intensify heat convection in the oil and accelerate oil cooling. The expansion rate calculated from theoretical analysis is determined using the water mass in oil and oil temperature. Oil temperature directly influences water evaporation. Therefore, water mass in oil is the main influencing factor in the development of the boiling over layer. 3. Under same flow rate of water mist, the existence of additives will increase the hydratability of ions and drop size, thereby reducing water evaporation, and is adverse to oil cooling. However, water mist with additives could effectively inhibit the upward expansion of the boiling over layer and reduce the risk of secondary damage caused by abundant oil spill over after extinguishment.
[1] M.S. Wijayasinghe, T.B. Makey, Cooking oil: a home fire hazard in Alberta, Canada, Fire Technol. 33 (2) (1997) 140–166. [2] X. Ni, W.K. Chow, Q. Li, et al., Experimental study of new gas-solid composite particles in extinguishing cooking oil fires, J. Fire Sci. 29 (2) (2011) 152–176. [3] H. John, Asphalt fume exposures during the application of hot asphalt to roofs, National Inst. Occup. Saf. Health 6 (2003) 35–38. [4] P. Zhang, X. Gao, X. Zhang, Study on the suppressing effectiveness of water mist spray parameters in cooking oil fire, Shenyang Jianzhu Daxue Xuebao 26 (5) (2010) 951–956. [5] J. Chen, The experimental study on extinguishing cooking oil fires by water mist, Appl. Mech. Mater. 166–169 (2012) 2804–2809. [6] W. Chow, X. Ni, Experimental evaluation on performance of open kitchen fire suppression systems, Fire Saf. Sci. 11 (2014) 1298–1311. [7] F. Ferrero, M. Munoz, B. Kozanoglu, et al., Experimental study of thin-layer boilover in large-scale pool fires, J. Hazard. Mater. 137 (3) (2006) 1293–1302. [8] H. Koseki, Y. Natsume, Y. Iwata, et al., Large-scale boilover experiments using crude oil, Fire Saf. J. 41 (2006) 529–535. [9] S. Nam, Application of water sprays to industrial oil cooker fire, Fire Saf. J. 7 (2003) 469–480. [10] S. Nam, Boilover upon suppression of cooking oil fire by water spray, Fire Saf. J. 39 (5) (2004) 429–432. [11] Z. Liu, A.K. Kim, D. Carpenter, Extinguishment of cooking oil fires by water mist fire suppression systems, Fire Technol. 40 (4) (2004) 309–333. [12] Z. Liu, D. Carpenter, A.K. Kim, Characteristics of large cooking oil pool fires and their extinguishment by water mist, J. Loss Prev. Process Ind. 19 (6) (2006) 516–526. [13] Z. Liu, D. Carpenter, A.K. Kim, Cooling characteristics of hot oil pool by water mist during fire suppression, Fire Saf. J. 43 (4) (2008) 269–281. [14] R.C. Reid, Rapid phase transitions from liquid to vapor, Adv. Chem. Eng. (1983) 105–208. [15] C.T. Avedisian, The homogeneous nucleation limits of liquids, J. Phys. Chem. Ref. Data 14 (3) (1985) 695–729. [16] Z.L. Zhang, Experimental Study on Oil Pool Fire Suppression Effectiveness of Water Mist with Additives, Beijing Institute of Technology, Beijing, 2015. 28. [17] Z. Liu, A.K. Kim, D. Carpenter, A study of portable water mist fire extinguishers used for extinguishment of multiple fire types, Fire Saf. J. 42 (1) (2007) 25–42. [18] J.G. Speight, Lange’s Handbook of Chemistry, 15th ed., McGRAW-HILL, New York, 1999, pp. 8–20. Section 5. [19] B. Lewis, R.N. Pease, H.S. Taylor, High speed aerodynamics and jet propulsion. volume II. combustion processes, High Speed Aerodynam. Jet Propul. (1956) 431.
Acknowledgments This work was supported by the basic research fund of Beijing Institute of Technology (20130242017), the project fund of State Key Laboratory of Explosion Science and Technology (Beijing Institute of Technology) (YBKT16-09 and QNKT16-03).