Combustion Behavior Characteristics of Transformer

Jet Fire with Multiple Factors

Authors: Ruibang Sun, Chang Liu, Peng Li, Xinwei Zhang, Fengju Shang

Abstract

Transformer jet fire is one of the most serious types of substation fire accidents, which often causes heavy casualties and property losses and even causes other secondary disasters, further expanding the impact of thermal disasters. Compared with the jet fire formed by the leakage of gaseous combustibles during production, transportation, and storage, the research on the combustion behavior of transformer jet fire under multiple factors, such as flame shape and flame height, is relatively scarce. This study used the research method of combining experimental research with theoretical analysis. Flame shape and flame height of transformer jet fire under different vessel diameters (5, 8, and 10 cm), opening diameters (5, 10, and 15 mm), and filling rate (60, 80, and 100%) were measured and analyzed. The results show that jet momentum and flame buoyancy are the main reasons for the change in flame shape characteristics. The transformer jet fire development process can be divided into three typical stages of full development, stable jet, decay, and extinguish. The flame height increases with the increase of the opening diameter and container diameter, and increases first and then decreases with the increase of filling rate. The functional relationship between the flame height and the 0.2 power of the characteristic Froude number is established. The research results provide reliable theoretical guidance and data support for transformer fire prevention and control.

Introduction

Background

and Literature Review

With the increasing use of global power energy, the safety problems caused by transformer jet fire have attracted more and more scholars’ attention in recent years. Due to partial discharge, partial overheating, and other faults, accidents such as penetrating breakdown, interturn short circuit, insulation damage, burning, and explosion occur inside the transformer, which leads to the rupture of the oil-filled equipment body. The transformer oil leaks and accumulates from the rupture and forms an external heat source after encountering the ignition source, which causes the transformer jet fire. Post-disaster accident investigations found that jet fire is a typical phenomenon of transformer fire, and it is also one of the key reasons for the increase in fire risk. In these major transformer fire accidents, the occurrence of jet fire directly leads to the escalation of the accident, resulting in catastrophic consequences. According to the statistics of the Electric Power Research Institute, the probability of transformer fires in China is about 0.01 to 0.03% per year, and the number of transformers currently operating in the network is as high as 17 million. It can be seen that the transformer fire prevention and control work in the substation is still facing a severe situation.

Scholars have carried out many studies on the jet fire formed by the leakage of gas combustibles in the process of production, transportation, and storage, mainly focusing on the dynamic characteristics of gas jet fire under the conditions of opening diameter, fire source power, and fuel type. In the aspect of opening diameter, Wang et al. studied the width characteristics of propane jet flame with opening diameters of 4, 5, 6, 8 and 10 mm, respectively. The results show that the width of the jet flame becomes wider with the increase of the opening diameter. Mogi and Horiguchi reported the variation of hydrogen jet flame length with opening diameters of 0.0000004, 0.0000008, 0.000002 and 0.000004 m, respectively, and constructed a 0.43 power function relationship between flame length and release pressure. Later, Mogi et al. revealed that the flame length of dimethyl ether (DME) jet was positively correlated with the 0.39 power of mass flow rate under different opening diameters. Imamura et al. studied the variation characteristics of hydrogen jet flame length with opening diameters of 1, 2, 3, and 4 mm, and established a functional relationship between dimensionless flame length and the 0.43 power of pressure. Wang et al. studied the effects of opening diameters of 5, 10 and 15 mm on the shape of propane jet flame. It is found that there is a 0.39 power relationship between the flame width and the flame Froude number of the jet flame. Palacios and Rengel studied the dynamic characteristics of propane jet fire under different opening diameters, and the results showed that the jet flame height increased with the increase of Reynolds number. In terms of fire source power, Zhang et al. studied the dynamic characteristics of methane jet fire under different fire source powers, and revealed that the flame height was positively correlated with the 0.248 power of mass flow. Guo et al. divided the combustion process of kerosene spray fire into three growth stage, quasi-steady state stage, and self-extinguishing stage, and found that the flame height increased with the increase of fire source power. Laboureur et al. found that the flame length of liquefied petroleum gas jet increases with the increase of fire source power, and the flame width increases first and then remains flat with the increase of fire source power. Gopalaswami et al. studied the influence of fire source power on the flame height of a propane jet and revealed the correlation between flame height and the 0.2 power of the Froude number. Schefer et al. found that the flame height of the hydrogen jet has a turning point. The critical value of the Froude number is 5. Liu and Hu studied the influence of different fire source powers on the morphological characteristics of propane jet flame, and found that the normalized flame width and flame falling distance of the nozzle diameter have a good correlation with the dimensionless heat release rate of 1/2 power and 3/5 power, respectively. In terms of fuel types, Palacios et al. used the experimental data of jet flames of various hydrocarbon fuels to reveal that the dimensionless flame length increases with the increase of dimensionless fire source power. Hankinson et al. studied the dynamic characteristics of crude oil and water-injected crude oil jet fire, respectively. The results showed that the flame height of the water-injected crude oil jet was greater than that of the crude oil jet, and the flame height decreased significantly when the water volume exceeded about 50%. Based on the Richardson number, Mashhadimoslem established a flame height prediction model for the jet diffusion combustion of combustibles such as hydrogen, propane, methane, and ethylene in static air.

It can be seen that flame height is the most important characteristic scale parameter of jet flame, which has been widely concerned and studied by predecessors. Palacios et al. established a functional expression of 0.2 power between flame height and Froude number by using methane and propane jet fire experiments. It is revealed that when Fr is less than the critical value, the buoyancy effect is the main control mechanism of diffusion jet flame, and when Fr exceeds the critical value, the momentum effect is the main control mechanism of diffusion jet flame. Kiran and Mishra quantified the transition height of LPG jet flame from buoyancy-driven to momentum-driven based on the Froude number, and proposed a semi-empirical formula of 0.2 power to characterize the jet flame height. Table lists the jet flame height prediction models established by predecessors based on the Froude number.

Aim of This Work

The above research mainly uses factors such as changing the opening diameter, fire source power, and fuel type to study the combustion behavior characteristics of gas jet fire under the action of multiple factors, and reveals the influence mechanism of different factors on the height of gas jet flame. Little attention has been paid to the influence of law and mechanisms of different factors on the dynamic characteristics of the transformer jet fire. Therefore, it is urgent to carry out research on the combustion behavior of a transformer jet fire under the action of multiple factors, which has positive practical significance for improving the fire prevention and control ability of the power industry and developing major fire monitoring and early warning technology.

Experiment Setup

Figure shows the experimental setup of the transformer jet fire combustion behavior. The transformer models with diameters of 5, 8 and 10 cm were used in the experiment, and the transformer models were placed in the center of the oil pool. The electronic balance (AND GP-61) was placed directly below the oil pool to measure the mass loss rate of combustibles in the oil-filled equipment during the development of the fire. In order to ensure the normal operation of the electronic balance in the experiment, it was separated from the oil pool with a fire board. Three stainless-steel opening devices with diameters of 5, 10, and 15 mm, were used, respectively. The opening device was 2 cm long, and the wall thickness was 0.5 mm. Different opening devices were combined by built-in threads and external threads at the top opening of the oil-filled equipment. A high-speed camera (SONY NEX-FS700) was used to record flame images at a recording frequency of 25 fps to obtain flame morphological features; 10 mL of n-heptane was poured evenly into the external oil pool as the ignition source, the measurement system was opened, and the spark igniter was used to ignite the oil pool.

The brightness of the background was reduced by cutting off the background of the flame to improve the accuracy of the image. In the process of flame shape analysis, it is necessary to pay attention to the contour information on the flame, but not to the color information on the flame. The Matlab software was used to convert the color flame image recorded in the experiment into a grayscale image to extract the brightness information on the flame. After converting the grayscale image, the binary processing was performed to distinguish the combustion zone and the noncombustion zone in the flame image. The binarization process must determine the image threshold. In this study, the OTSU method was used to dynamically select the image threshold, and the threshold value of 0.5 was chosen to segment the combustion and noncombustion zones. After the flame image was transformed from the gray image to the binary image, the flame distribution probability cloud map can be obtained by a summation operation and imported into Tecplot software to obtain the color flame distribution probability cloud map. Using the flame distribution probability cloud diagram, flame shape parameters such as the flame height and flame width can be obtained intuitively. The flame image processing flow is shown in Figure .

In order to investigate the characteristics of transformer jet fire combustion behavior under multifactorial effects, a total of three series of experiments were set up in this study based on factors such as vessel diameter, opening diameter, and filling level. In the first series of experiments, container diameters of 5, 8, and 10 cm were used, in the second series of experiments, opening diameters of 5, 10, and 15 mm were used, and in the third series of experiments, filling rates of 60, 80, and 100% were used. A total of 21 sets of experiments were carried out to investigate the combustion behavior of oil-filled equipment jet fires under different conditions. The power of the external fire source was set at 33 and 73 kW, respectively. The specific experimental conditions are shown in Table .

All experimental conditions were carried out three times, and the arithmetic mean and arithmetic mean error δ of the three groups of experimental data were calculated; if the error between the three groups of experimental data and the mean was within ±2δ, the arithmetic mean of the three groups was taken as the experimental data under the working conditions for subsequent quantification and analysis. The experimental fuel selected for the experiment is KI25X transformer oil. An environmental monitoring station measured an ambient temperature fluctuation range of 25 °C ± 1.5 °C, a relative humidity fluctuation range of 56% ± 4.7%, and the ambient pressure is 101 kPa.

Result and Discussion

Typical Phenomenon

Figure shows the general development of a fire in a transformer with a container diameter of 5 cm. The white dashed line in the figure marks the location of the opening of the transformer. In the initial combustion stage (0∼216 s), n-heptane vapor mixed with air near the liquid surface of the oil pool was ignited by an electric spark, and a combustion reaction occurred and gradually spread to the entire liquid surface from the ignition point to develop into an oil pool fire. At this stage, the transformer as a whole is in the heating stage, and the transformer oil is not sprayed out to form a jet fire. The height of the oil pool flame is relatively small, maintained at about 0.5 m. The height of the oil pool flame is relatively small. With the heat accumulation inside the transformer (216∼235 s), the gas–liquid two-phase transformer oil is ejected from the opening of the transformer under the action of overpressure and is ignited by the external oil pool fire to form a jet fire. In the 216 to 224 s combustion stage, the flame height rises significantly, the flame color becomes white, the flame radiant heat flux increases, and the flame height at 224 s is about 2.08 m. In the 224 to 235 s combustion stage, the combustion intensity gradually reduced, and the jet flame height begins to decrease. With the transformer oil all burned out (235∼255 s), the combustion intensity decreases, the jet flame height will be near the opening until it disappears, and the jet fire is extinguished. 255 s later, the oil pool fire continues to burn until it is extinguished. It can be seen that once a jet fire occurs, the transformer fire pattern undergoes an abrupt change with significant changes in flame pattern and flame height produced by combustion reactions.

Figure shows the basic combustion phenomenon of a transformer jet fire with a vessel diameter of 5 cm, with t 0 being the moment of onset of the jet fire. At the moment of t0, transformer oil was ejected from the opening. Continuous ejection of combustible material to form an obvious jet trajectory, and by the external oil pool fire ignited to form a jet fire, the flame height of about 1.0 m. From t 0 to t 0 + 13 s combustion stage, with the gradual strengthening of the combustion intensity, the flame height increased significantly. At t 0 + 13 s or so, the flame height reached the maximum, about 2.08 m. From t 0 + 13 s to t 0 + 15 s, the combustion intensity is relatively stable, and the flame height does not change much. From t 0 + 15 to t 0 + 21 s, the flame height starts to decrease gradually over time as the combustion intensity weakens. At t 0 + 21 s, the jet fire is extinguished. It can be seen that the flame height increased significantly at the beginning of the jet fire development and then tended to be relatively stable. As the fire continues to develop, the flame height decreases until the flame is extinguished.

Figure shows the variation rule of the mass loss rate of transformer oil with time during the development of a transformer jet fire for a transformer with a diameter of 10 cm. It can be found that the transformer jet fire development process can be divided into three typical stages of development, stable jet and decay extinguishing. Different fire development stages, oil combustion produces different thermal feedback mechanisms, which are manifested as differences in the typical characteristic parameters of the fire. Therefore, based on the law of energy conservation, the heat balance equation of combustible microelements is constructed to qualitatively analyze the three typical stages of the fire development process.

The external heat source, under the action of oil-filled equipment as a system, that is, the jet fire and oil pool fire feedback to the combustible heat and combustible consumption of heat, is equal to the heat balance equation of the transformer oil jet fire.Q˙f=Q˙out1

The heat fed back to the combustible material by the flame is transferred to the interior of the transformer primarily through thermal conduction, including heat transferred to the walls by radiation and convection from the oil pool flame and jet flame (Figure ):Q˙f=Q˙cond,f=Q˙conv,f+Q˙rad,f+Q˙conv,ff+Q˙rad,ff2

The total heat consumed by combustibles consists mainly of heat used for the evaporation of combustibles, heat used to heat combustibles, and heat lost to the Q˙out=Q˙evap+Q˙heat+Q˙loss3

As the heat lost to the outside world is negligible, the heat used to heat combustibles continues to diminish as the fire develops, and the heat loss term for heating combustibles is therefore ignored. Based on the heat production term and the heat consumption term, the heat balance relationship for microelements on the surface of combustibles can be expressed asQ˙evap=Q˙conv,f+Q˙rad,f+Q˙conv,ff+Q˙rad,ff4

The heat required to evaporate a combustible substance can be expressed asQ˙evap=m˙[cpl(Ts−Tc)+Δhv]5

Equation is substituted into eq to obtain an expression for the rate of mass loss of combustible m˙=Q˙conv,f+Q˙rad,f+Q˙conv,ff+Q˙rad,ff[cpl(Ts−Tc)+Δhv]6

According to eq , the thermal feedback that maintains the rate of mass loss of combustibles comes mainly from the convective and radiative terms of the oil pool flame and jet flame.1.Developed stageAfter the formation of the transformer jet fire, the jet flame transfers heat to the interior of the oil-filled equipment by convection and radiation, sharing some of the heat required for evaporation of the combustible material. With the development of the fire, the heat transferred by the jet flame to the transformer gradually increases, resulting in an increase in the rate of mass loss of combustible material. The burning intensity of the flame under this stage increases, and the typical characteristic parameters of fire, such as flame height, flame temperature, and external radiant heat flux from the flame, increase.2.Stable jet stageWith the amount of combustibles generated inside the transformer and the amount of combustibles involved in the combustion reaction to reach equilibrium, the jet flame through convection, radiation to the transformer to maintain a relatively stable internal transfer, and the rate of mass loss of combustibles to achieve dynamic equilibrium. Under this stage, the burning intensity of the flame is relatively stable, and the typical characteristics of the fire, such as flame height, flame temperature, and external radiation heat flux of the flame, have reached their maximum, and the magnitude of change is small.3.Decay extinguish stageWith the reduction of combustible materials involved in the combustion reaction, the jet flame is reduced by convection and radiation to the internal transmission of oil-filled equipment. Coupled with the cooling effect of the air volume suction on the flame, the thermal feedback of the jet fire on the transformer is further reduced, resulting in a gradual decrease in the rate of mass loss of combustibles. The intensity of combustion under this stage is gradually weakened, and the typical characteristic parameters of fire, such as flame height, flame temperature, and external radiation heat flux, are gradually reduced.

Flame Shape

According to the experimental observation, the transformer jet flame morphology under different container diameters, opening diameters, and filling rates showed similar characteristics. Figure shows the typical time series flame morphology of a transformer jet fire with different filling rates. In the fully developed stage, as the initial momentum of the jet increases, the inertial force on the combustible material increases, the flame height gradually increases, the amount of carbon black generated within the flame increases with the flame height, and the color of the flame takes on an orange-yellow color. Since most of the carbon black particles are formed in the fuel-rich zone and oxidized in the high-temperature oxidation zone, the color of the flame center is brighter than that of the flame boundary. The unoxidized carbon black particles break through the flame boundary to form carbon soot, resulting in smoke around the jet flame of the oil charging equipment. With the enhancement of the combustion reaction, the flame is subjected to the buoyancy effect to strengthen the flame boundary to form a negative pressure region, resulting in unstable pulsating changes in the flame. With the enhancement of the lateral diffusion of combustible materials to the surrounding environment, the flame surface gradually increased. The development of the fire into a stable jet stage, the combustible material is maintained by the inertial force to maintain dynamic stability, the flame height in a certain combustion time to reach a relatively stable state, the flame surface increases to the maximum, and the flame center color changes from orange to white. The carbon black particles escaping around the flame increase, resulting in an increase in the smoke area. At the same time, the flame is subjected to increased buoyancy, and the flame pulsation frequency is significantly accelerated. After the fire develops into the attenuation and extinguishing stage, with the combustible material by the inertia force decreases, the intensity of the combustion reaction is weakened, the flame by the buoyancy gradually reduced, the flame height gradually reduced, and the flame surface is reduced. The flame color changes from white to orange-yellow, and the flame pulsation frequency decreases.

Under the action of overpressure, transformer oil with a certain inertia force from the opening jet, lateral diffusion, and oxidant mixing combustion reaction. It can be seen that the combustible material, by the inertia force, and the flame, by the buoyancy force, play the joint roles in the jet flame morphology characteristics of change. The structure of the jet flame is shown in Figure . When the fuel medium is a gas-phase flow, the fuel combustion efficiency is higher due to the sufficient mixing of air and fuel. The color of the flame is almost transparent. The heat generated by combustion is transferred to the outside world, mainly in the form of convection. When the fuel medium is a gas–liquid two-phase flow, the combustion situation is significantly different. Since the gas–liquid two-phase flow seriously affects the degree of mixing of fuel and air, it leads to a low combustion efficiency. Insufficient combustion favors the generation of carbon black particles, resulting in a yellow or even white flame color. Meanwhile, the generated carbon black particles significantly enhanced the radiative heat flux from the flame, and the radiative heat flux emitted from the gas–liquid two-phase jet flame was higher than that from the gas-phase jet flame. In addition, the increase in the fuel mass flux due to the presence of droplets promotes a significant increase in the flame height.

Flame Height

Once the jet fire occurs, the fire mode of the oil-filled equipment changes abruptly, and the flame shape, the temperature generated by the combustion reaction, and the thermal radiation distribution around the flame all change significantly, which in turn affects the risk. Therefore, it is necessary to further study the basic combustion phenomenon of jet fire in oil-filled equipment and determine the key research stage of jet fire. Through the analysis of the whole development process of the jet fire of the oil-filled equipment, in the development stage and the attenuation and extinction stage, the flame height, the temperature generated by the flame, and the radiation heat flux of the flame to the surroundings are all in a state of change and do not reach the maximum value. In the stable jet stage, the combustion reaction is sufficient, and the fire development enters a relatively stable stage. Therefore, in the research process of jet fire dynamics of oil-filled equipment, the stable jet stage of a fire should be considered emphatically.

Figure shows the transformer jet flame height change law under the role of multiple factors. From Figure a, it can be seen that the jet flame height under different vessel diameters shows a trend of rapid growth to the maximum value, then remains relatively stable, and then gradually decreases to the initial height with the development of the fire. The jet flame height increases slowly when the transformer diameter increases from 5 to 8 cm, and increases significantly when the transformer diameter increases to 10 cm. The combustible material and flame develop upward motion under the initial inertial force. The thermal buoyancy force on the flame is in the same direction as the inertial force on the combustible material, which promotes the upward development of the flame. As the diameter of the transformer increases, the power of the fire source increases, implying an increase in the amount of combustible material involved in the combustion reaction per unit time. On the one hand, the combustible material by the inertia force increases, on the other hand, the flame temperature and the ambient temperature difference lead to the flame by the thermal buoyancy increases, the two together lead to the transformer jet flame height increases.

Figure b–d shows the effect of the opening diameter on the flame height of the transformer jet. The flame height increases with increasing opening diameter for a given transformer diameter. It can be seen that the transformer jet flame is in a turbulent state for different opening diameters, and the power of the fire source increases as the opening diameter increases. The inertia force of the combustible material is increased by the increase of the initial momentum of the jet and the buoyancy force on the flame is increased by the increase of the intensity of the combustion reaction. The combined effect of combustible inertial force and flame buoyancy promotes upward flame movement, resulting in an increase in jet flame height.

Figure e–g shows the effect of charging rate on the transformer jet flame height. For a given transformer diameter, the flame height shows a tendency to increase and then decrease with the increase in charging rate. Specifically, it is shown that the filling rate increases from 60 to 80%, the flame height shows an increasing trend, and the filling rate increases from 80 to 100%, the flame height shows a decreasing trend. For the transformer, jet flame height is mainly affected by the combustible material, by the inertia force, and the flame by the interaction of buoyancy and air volume suction efficiency. With the change of charging rate, the inertia force of combustible material, flame buoyancy, and air suction are significant changes, thus affecting the change of flame height. As the filling rate increases from 60 to 80%, the inertial force on the combustible material increases, the flame jet distance increases, which promotes the flame's upward development, and the flame height increases. The buoyancy acceleration is mainly induced by the difference between the flame temperature and the ambient temperature. As the filling rate increases from 60 to 80%, the mass of the combustible material increases, which promotes the combustion reaction, leading to an increase in the flame buoyancy force, which promotes the upward extension of the top of the flame. At the same time, the increase in the intensity of the combustion reaction reduces the density of the surrounding environment, which enhances the efficiency of the air coil suction and leads to an increase in the height of the flame. The combined effect of the three promotes the height of the transformer jet flame, so as the filling rate increases from 60 to 80%, the height of the transformer jet flame increases. With the charging rate increased from 80 to 100%, the combustible material by the inertia force is reduced, the amount of combustible material involved in the combustion reaction is reduced, resulting in a reduction in the heat generated by combustion, which leads to the flame by the buoyancy of the weakening of the suppression of the air volume suction efficiency, the combined effect of the three transformer jet flame height is gradually reduced, therefore, with the charging rate increased from 80 to 100%, the height of transformer jet flame decrease.

Height Model

The height of the jet flame is affected by the initial jet velocity of the combustible material, and the flame is subjected to the joint effect of thermal buoyancy; according to the equation of conservation of momentum, the flame buoyancy force expression can be established as:∫0Fρω2dF~=ρ1ω12dF~−∫0l∫0F(ρ−ρ∞)gdFx7

The combustion reaction can be expressed in terms of the components of the mixture, yielding a component conservation ∫0F(x−x∞)ρωdF1=ρ1ω1dF1(x1−x∞)8

Substituting the jet flame flow parameters into eqs and and applying the median theorem to the integrals on the right-hand side of eq yieldsFρmωm2I1=F1ρ1ω12−(ρ®−ρ∞)gV®f9 (xm−x∞)FρmωmI2=ρ1ω1F1(x1−x∞)10where ρ® is the flame density, V®f is the flame volume.I1=∫0lρρmw2wm2dFF11 I2=∫0lρρmwwmΔxΔxmdFF12 Δx=x−x∞13 Δxm=xm−x∞14

Equations – can be obtained by substitution into eqs and :ΔxmΔx1=xm−x∞xm−x115

Substituting the Froude number into eq yields(ΔxmΔx1)2ρ1ρm+(ΔxmΔx1)2Δρ®V®fρmF1D11Fr=I1I22ρ12D12ρm2b216

Based on eq , Suris et al. established a dimensionless expression for the flame height of a gas jet as HfD=BFrn17

Among the variables, the main ones include the dimensionless flame height on the left side of eq and the Froude number on the right side. The dimensionless flame height is the ratio of the flame height to the equivalent diameter. In this case, the flame height is affected by the diameter of the oil-filling equipment, the diameter of the opening, and the filling rate, and the equivalent diameter is affected by the diameter of the oil-filling equipment, the diameter of the opening, and other factors.Hf=f(Fl,Fr,d1,d2)18

In order to consider the influence of oil filling diameter, opening diameter, and other factors, this article proposes the concept of equivalent diameter. It is shown as De=0.02d12/16d23319

By dimensionless processing of eq , the dimensionless flame height is obtained asHf*=HfDe20

The Froude number is the ratio of flame inertia force to buoyancy force, which is affected by the equivalent diameter, jet rate, and other factors. Considering the charging rate as a dimensionless factor, the characteristic Froude number is constructed using magnitude analysis, as shown in eq :Fr*=Flu2gDe21

Based on this, eqs and were used to jointly construct a functional relationship between the dimensionless flame height and the characteristic Froude number, as shown in eq :HfDe=f(Flu2gDe)22

Figure demonstrates the correlation between the normalized transformer jet flame height and the characteristic Froude number under the effect of multiple factors. Based on the characteristic Froude number, the expression for the normalized jet flame height obtained by fitting the experimental data using the transformer jet flame height under the effect of multifactors follows the form of eq as HfDe=1.17(Flu2gDe)0.223

As can be seen from Figure , the dimensionless flame height of the transformer jet fire with the increase of the characteristic Froude number shows rapid growth first and then tends to flatten out the trend. F r* < 16,515, the jet flame height increases with the increase of the characteristic Froude number, F r* = 16,515, the growth trend of the jet flame height begins to slow down, F r* > 16,515, with the increase of the characteristic Froude number, the jet Flame height flattens out with the increase of characteristic Froude number, which is consistent with the previous study.

Figure shows the effect of each factor on the characteristic Froude number. Factors such as opening diameter, charging rate, and transformer diameter all have an effect on the jet flame height by affecting the characteristic Froude number. From Figure a, it can be seen that the characteristic Froude number increases with the increase of the opening diameter because the initial momentum of the combustible material increases with the increase of the opening diameter, and the characteristic Froude number increases with the increase of the initial momentum of the combustible material, which leads to the increase of the flame height. From Figure b, it can be seen that the characteristic Froude number tends to increase and then decrease with the filling rate, which is because the initial momentum of the combustible material increases and then decreases with the increase in the filling rate, and thus the flame height varies nonlinearly with the filling rate. From Figure c, it can be seen that the characteristic Froude number increases monotonically with the transformer diameter, because the initial momentum of the combustible material increases with the diameter of the transformer, which leads to an increase in the flame height.

Figure shows the evolution of the mass loss rate under the influence of various factors. It can be seen that these factors can affect the mass loss rate of combustible materials and then affect the size of the jet flame height. There is a positive correlation between the flame height and the mass loss rate of combustible materials. The change in the jet flame height of oil-filling equipment involves the interaction of the gas phase and liquid phase, combustion reaction, fire dynamics, and other theories. On the one hand, the combustible phase form of an oil-filled equipment jet flame is a gas–liquid two-phase flow, which involves a complex interaction of gas and liquid droplets. At the beginning of the jet, the combustible material is ejected from the opening in the form of a gas–liquid two-phase flow, while the liquid droplets in the combustible material undergo evaporation and gasification processes in the air. The heat of the phase change released by the liquid fuel during the phase change process can increase the rate of mass loss and promote the height of the flame. On the other hand, the presence of liquid droplets leads to an increase in the amount of combustible material involved in the combustion reaction per unit time. The increase in the amount of combustibles is an important factor in increasing the rate of mass loss. A larger amount of combustibles means that more fuel is involved in the combustion process, generating more heat and products of combustion, which, in turn, pushes up the flame height. The stability of the gas–liquid phase flame is also a key factor in the difference in mass loss rates. In oil-filled equipment jets, the jet of liquid droplets and the flow of gas form a relatively stable flame structure, which makes it easier to maintain the flame at a certain height and reduces the influence of external disturbances on it. The mass loss rate increases with an increasing opening diameter, which in turn leads to an increase in flame height. The mass loss rate tends to increase and then decrease with increasing charging rate, so the flame height increases and then decreases. The rate of mass loss increases with the diameter of the filling device, which leads to an increase in the flame height.

Conclusions

In this study, the basic combustion phenomena, typical stages, and key behavior of transformer jet fire are clarified by changing the factors of vessel diameter, opening diameter, and charging rate, and the influences of different factors on the typical characteristic parameters, such as mass loss rate, flame morphology, flame height, flame temperature, and flame radiation, etc., of the transformer jet fire and the mechanism of their effects are revealed. The major findings include the 1.Based on the basic combustion behavior of transformer jet fire, the external heat source under the action of transformer jet fire development process can be divided into three typical stages of full development, stable jet and attenuation extinguishing, etc. The main reason for the change in flame morphology is that combustibles are subjected to inertial force and the flame is subjected to buoyancy force together to affect combustion reaction, carbon black concentration, and the air rolled up to absorb the cause.2.The flame height is positively correlated with the transformer diameter, and the fire source power increases with the increase of the transformer diameter, resulting in a higher flame height. The flame height and opening diameter are positively correlated and the fire source power increases with the increase of opening diameter, resulting in higher flame height. The flame height with the increase in the filling rate shows a trend of increasing and then decreasing.3.The dimensionless flame height and the characteristic Froude number of 0.2 times the function of the transformer jet fire dimensionless flame height, with the increase in the characteristic Froude number, showed rapid growth first, after a tendency to flatten out. The impact of various factors on the characteristic Froude number is analyzed.