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T his paper analyses the existing literature, and considered the new problems caused by the burning accident of thermal insulation materials attached to the steel structure in recent years . In the process of combustion, thermal insulation materials will drip and melt, producing a large amount of liquid fuel. The combustion of thermal insulation materials attached to steel (diameter long er than 0.2 m) can be regarded as large pool fire combustion, and a mathematical model is established to study the reverse thermal conduction behavior of the combustion of combustible materials attached to steel.

Nowadays, with the rapid development of urban construction, the demand for various kinds of steel and other resources is also rising. Steel has the advantages of good structural performance, low pollution and convenient construction, so steel structure is widely used in modern building construction, the main material for liquefied gas transportation storage tank is mostly steel. However, due to the relatively low fireproof performance of steel structure itself, and the fact that it is easy to reduce its own stiffness and strength at high temperature, the exposure of steel structure to potential fire source may lead to the occurrence of safety accidents. In order to prevent the steel structure from being exposed to the dangerous source, people would like to add some insulation materials to cover it to prevent the damage of high temperature to the steel structure. In fact, this is only exposed to the source of the steel structure to replace the insulation material.

On August 17, 2014, the external insulation material of LNG storage tank under construction in changxing shipyard of China shipbuilding group caught fire, which led to new thinking on the fire caused by combustible materials attached to steel structure. In short, the neglected insulation material will also burn under the constant high temperature, and the thermal feedback will be transmitted down to the surface of the steel structure and affect it. Absolutely, it is necessary to test the influence law of thermal feedback on tank structure for the influence of fire caused by the burning of insulation materials attached to the tank outside. To solve these problems, it is necessary to accurately predict the thermal feedback from the flame to the steel. Wang and Zhou [

Based on the analysis of the existing data, a semi-empirical model of radiative heat feedback is proposed to predict the reverse heat conduction behavior of the flame on the steel surface by the flame to the steel surface.

In the era of highly developed building industry and energy industry, the insulation materials are frequently used. Some studies have been conducted on polyurethane foam materials [

related to the size of the pool. Blinov and Khudyakov [

According to the introduction in the previous section, for the flame of steel attached combustible material (diameter longer than 0.2 m), there will be a radiation-dominated thermal feedback to the steel structure. To explore the influence of flame radiation on steel structure, a suitable flame radiation model should be established.

At present, the applicable semi-empirical models for theoretical calculation can be basically divided into two types: point source model and solid flame model. The point-source model is simple to calculate but not accurate enough [

In this study, we need to make the following assumptions about the flame: 1) the combustion reaction of combustibles belongs to the applied stoichiometric reaction; 2) the flame shape is roughly cylindrical; 3) the flame is equivalent and uniform, with symmetry; 4) radiant heat is evenly distributed in the flame body. To establish a flame model that superimposes the thermal radiation of each layer of flame on the steel plate (the diameter of the combustible material attached to the steel plate is longer than 0.2 m, and the flame radiation feedback is the dominant thermal feedback mode), it is necessary to first determine the average height of the flame and layer the flame on this basis. Reasonable number of layers and height can improve the calculation accuracy [

H / D = 3.7 Q 2 / 5 − 1.02 (1)

where H is the average flame height; D = 4 A / π is the flame equivalent diameter; A is the burn area; Q is heat release rate.

The more layers there are, the more accurate the calculation results will be, and the accuracy assigned to the 20th layer is about 4% [

Q r a d = A σ ( T f 4 − T 0 4 ) [ 1 − exp ( − k l m ) ] (2)

where A is the burning area; σ = 5.67 × 10 − 8 is the boltzmann constant; T f and T 0 are respectively the flame temperature and the initial temperature of the steel surface 20˚C; K is the effective absorption emission coefficient; l m is the average beam length; Q r a d is radiant heat feedback.

We can get the average beam length according to Equation (3) [

l m = 3.6 ( A v ) / A s (3)

where l m is the average beam length, A v and A s are the volume and surface area of each layer.

The effective absorption-emission coefficient k can be determined by the following Equation (4) [

k = 3.72 ( C 0 / C 2 ) f v T f (4)

where C_{2} is Planck’s second constant 1.4388 × 10^{−2} mK; f_{v} is the volume fraction of carbon black, usually 10^{−6}. T_{f} is the flame temperature; C_{0} is a constant between 2 and 6, depending on the complex refractive index (m = n − ki, where the real part n is the refractive index of the absorbent medium, and the imaginary part k is the absorption coefficient, which is determined by the attenuation of light as it travels through the absorbent medium).

For the semi-empirical prediction model, there is still an error between it and the real fire data, which is because factors such as ventilation condition, flame shape and space distance will have a great influence on the difference of Angle coefficient, thus affecting the accuracy of the model [

Q = ∑ Q r a d (5)

In summary, the volume and surface area of each layer of flame need to be quantified accurately to obtain a more accurate calculation result. Using digital image analysis technology to analyze the flame is a relatively effective means. The literature [

In this paper, the new problems caused by the burning accident of the thermal insulation material attached to the steel structure in recent years are considered, and the reverse heat conduction behavior of the combustible material attached to the steel structure is studied. The main conclusions are as follows:

In order to facilitate the study, according to the existing literature, the combustion of steel-attached combustible material is defined as the combustion reaction that occurs when the steel-attached combustible material (diameter longer than 0.2 m) burns, which is the pool fire dominated by radiation feedback.

In the still air of an open area, the combustible material attached to the steel burns and emits a lot of heat to the outside world, compared with a small amount of heat into the steel structure. In this paper, a mathematical model is developed to quantify the radiant heat feedback into the steel structure.

The authors declare no conflicts of interest regarding the publication of this paper.

The author declares no conflicts of interest regarding the publication of this paper.

Xiong, S. (2020) Study on the Reverse Heat Conduction Behavior of Steel. Open Journal of Safety Science and Technology, 10, 24-31. https://doi.org/10.4236/ojsst.2020.101002