Analysis of heat transfer of spilling fire spread over steady flow of n-butanol fuel

https://doi.org/10.1016/j.icheatmasstransfer.2020.104685Get rights and content

Abstract

The continuously spilling n-butanol fire is systematically investigated in steady flow state. The influence of thermal conductivity of substrate on spilling fire is revealed. According to the temperature distribution and spreading characteristics, the spilling fire over concurrent steady flow n-butanol is separated into the heat transfer dominant stage in low-discharge rate and the forced flow dominant stage in high-discharge rate with the critical discharge rate Q = 141.5 ml/min. The low-discharge rate spilling fire spread depends on the coupled effects of subsurface flow preheating and intrinsic dragging of flowing fuel, whereas the high-discharge rate spilling fire depends mainly on the dragging effect of non-slip boundary condition on the liquid surface. The temperature distribution of liquid surface proves that the behavior of thermal fluid diffusion occurs in high-discharge rate. The velocity of spilling fire decreases with an increase in thermal conductivity of substrate. The magnitudes of heat transfer involving spilling fire spread over n-butanol inside the subsurface flow namely gas conduction, liquid conduction, liquid convection, sensible heat and heat dissipation from substrate and walls of spilling trench are quantitatively calculated. The sum of sensible heat and convective heat loss accounts for more than 80% of total heat flux.

Introduction

The spilling fire is a subsequent phenomenon accompanying by accidental leakage of oil. Owing to the flowing characteristics, the damage and contamination areas caused by the spilling fire are relatively large, and the spilling fire puts a great threat to the surrounding facilities. In addition, according to the location and environment of fuel leakage, the development of spilling fire may proceed under the varied discharge rates and substrates. Therefore, it is of great significance to carry out experimental investigations on spilling fires under different discharge rates and spilling substrates.

There are complicated fluid flow and combustion processes involved in the fuel spilling fire propagation, such as the coupled effect of oil flow, combustion and heat transfer [1]. According to the amount and time of leakage fuels, the spilling fire is divided into instantaneously flowing spilling fire and continuously flowing spilling fire [2]. The influence of environmental factors on the spilling fire has been widely investigated by previous researchers, including the discharge rate, spilling tray size, spilling on solid substrate or water surfaces, slope or horizontal surfaces [3]. For unburned fuel flowing on water, according to the force balances inside the fuel layer, the fuel spilling could be divided into three regimes: “gravity-inertia” spread, “gravity-viscosity” spread and “surface tension-viscosity” spread [4]. It was found that the force balances of fuel spilling on the land were similar to those on the water surface [5]. The previous scientists conducted some experiments on large-scale leakage of liquefied natural gas (LNG) on the surfaces of sea, metal or land. These experiments mainly focused on flame shape, radiation flux and burning rate of pool fire after the fuel spilling was fully developed. However, few researchers concentrated on the dynamic process of flowing pool. Mealy and Benfer [[6], [7], [8], [9]] endeavoured to reveal the instantaneously spilling fires at laboratory size, namely the gasoline, heptane, kerosene and alcohol spilling fires on the concrete, wood board, fireproof mat and other materials. The influences of ignition delay time, material and temperature of substrate on the steady and maximum burning areas of the fuel spilling were revealed. For the spilling fire on the concrete surface, the average burning rate increased linearly with a decrease in leakage volume, whereas the flame height decreased significantly for the spilling of gasoline [6]. For both 1 and 5 mm deep pools, the burning rate on steel was slightly smaller than that on water. This was possibly due to the higher thermal conductivity of the steel, so the larger amount of heat was dissipated from the fuel layer to the steel substrate, thereby reducing the rates of fuel burning and vaporization [7]. It was also found that the burning rate of a spilling fire on concrete surface was always lower than that on steel surface. This difference might be attributed to the larger reflectivity of the steel. The greater heat flux was transferred to the steel substrate. The reflected heat was transferred to the combustion fuel layer to promote the development of the spilling fire in turn [8]. Then, it was concluded that the maximum heat release rate of spilling fire occurring on high-thermal conductivity substrate (such as concrete) was lower than that on low-thermal conductivity substrate (such as wood) [9].

In recent several years, some researchers performed tests on the continuously spilling fire. Based upon the large-scale spilling fires of gasoline and heptane leakages, Li et al. [10,11] found that as the discharge flow rate increased, the radiant heat flux increased, however, the flame height initially increased but decreased afterwards. The first augment of flame height was due to the increase of burning area, while the subsequent diminution was due to the sheltering effect of dense smoke generated by the incomplete combustion. Li et al. [12] further studied the characteristics of spilling fire on water and metal surfaces, and concluded that the burning rate of spilling fire on water surface was higher than that of steel substrate, owing to the thicker fuel layer on the surface of steel substrate. Zhao et al. [13] conducted thermal analysis of spilling fire, finding that the burning rate of spilling fire accounted for approximately 0.57 times smaller than that of pool fire. The heat conduction and transmitted thermal radiation to the underlying water were the main reasons for the smaller burning rate of spilling fire. Ingason et al. [14] found that the heat release rate of gasoline spilling fire in the tunnel was about 1/3 to 2/5 times smaller than that of pool fire.

In summary, the behavioural illustration and thermal exchange of continuously spilling fire have not been fully understood, especially on the precise measurement of temperature distribution and quantitative calculation of heat transfer. First, the previous investigations mainly focused on characteristic parameters of pool fire, such as the combustion area, flame height, burning rate and radiation flux. The spilling fire is a highly transient process, however, less work has been designed to reveal the dynamic development process of spilling fire, such as the flow speed, subsurface flow length and velocity of spilling fire. Second, although the temperature distribution in the thin fuel layer is essential for the analysis of heat transfer of spilling fire, no relative work has considered this point. Third, no work quantitatively calculated the heat transfer through liquid convection and liquid conduction which are the main heat transfer models involving laboratory-sized spilling fires. Finally, most of the previous studies on the spilling fire were conducted under unsteady flow conditions, so the behaviors and heat transfer mechanism of spilling fire are very difficult to characterize due to the numerous uncertain factors. In current work, the continuously spilling n-butanol fire is systematically investigated in steady flow state. The influence of thermal conductivity of substrate is revealed. Based upon the energy conservation theorem, a theoretical model is established to quantitatively calculate the heat transfer of spilling fire spread over steady flow liquid.

Section snippets

Experimental setup

The experimental system is shown in Fig. 1. All experiments were conducted in a closed experimental lab with length × width × height = 5 × 3 × 3 m3 to isolate the external air flow. The ambient temperature kept at room temperature (~20 °C). The whole experimental system included the oil supply device, the oil spilling trench, the oil collecting apparatus and the data collector [15]. Using the peristaltic pump, the n-butanol fuel was pumped from the fuel inlet tank (tank 1) into the spilling

Temperature distribution and scale characteristic of subsurface flow

Fig. 3 shows the longitudinal temperature distributions near the oil surface at different discharge rates. For the still or low-discharge rate (Q < 141.5 ml/min), the temperatures on the liquid surface and the substrate surface simultaneously increase, regardless of spilling directions. However, the temperature rise on substrate is significantly smaller than that on fuel surface. This phenomenon indicates that the local depth of fuel layer is shallower than the thermal boundary layer thickness,

Conclusions

A series of laboratory-scale tests is conducted on the continuously spilling n-butanol fire under the steady flow state. Based upon the energy conservation theorem, the thermal analysis was conducted to reveal the heat transfer mechanisms of the subsurface flow layer. The main conclusions are summarized.

  • (1)

    The spilling fire over concurrent steady flow n-butanol is separated into the heat transfer dominant stage in low-discharge rate and the forced flow dominant stage in high-discharge rate with

Credit author statement

Yang Pan plays the role of Investigation and Methodology. Manhou Li plays the role of Conceptualization, Writing - original draft, and Funding acquisition. Xinjiao Luo plays the role of Data curation. Changjian Wang plays the role of Project administration, Supervision and Resources. Qiuting Luo plays the role of Software and Methodology. Jingchuan Li plays the role of Writing - review & editing.

Declaration of Competing Interest

The authors wish to confirm that there are no known conflicts of interest associated with the publication named as “Analysis of heat transfer of spilling fire spread over steady flow of n-butanol fuel” and they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

The authors confirm that the manuscript has been read and approved by all named authors and that there are no other persons who satisfied the

Acknowledgements

This work is supported by National Natural Science Foundation of China (No. 51806054), Anhui Provincial Natural Science Foundation (No. 1808085QE151) and China Postdoctoral Science Foundation (No. 2018 T110614).

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