Shock and Vibration

Volume 2017 (2017), Article ID 5427485, 11 pages

https://doi.org/10.1155/2017/5427485

## Thermal Environment inside a Tunnel after Thermobaric Explosion

College of Defense Engineering, PLA University of Science & Technology, Nanjing 210007, China

Correspondence should be addressed to Jinfeng Mao

Received 3 February 2017; Revised 8 May 2017; Accepted 11 June 2017; Published 19 July 2017

Academic Editor: Isabelle Sochet

Copyright © 2017 Fei Chen et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

#### Abstract

The outstanding thermal damage effect of thermobaric explosive (TBX) is enhanced in closed or semiclosed spaces, which may pose a serious threat to the security of people sheltered in tunnels or other protective engineering. In order to investigate the thermal environment inside a tunnel after thermobaric explosion, we developed a damage evaluation method for the thermal radiation of explosion fireballs in tunnels; secondly, the air temperature distribution inside a tunnel shortly after explosion was theoretically analyzed; finally, the dynamic thermal environment after the explosion and the influences of TBXs mass and initial ground temperature on it in cases of open and blocked tunnels were numerically simulated with the FLUENT software. The results show that the fireball thermal radiation damage occurs mainly in the vicinity of the explosion source. The air temperature inside a tunnel shortly after the explosion decreases continuously with increasing distance from the explosion source and finally reaches the initial air temperature. The decay rate of air temperature inside a tunnel is slower in the blocked case, which increases the probability of causing a secondary fire disaster. The increase of explosive mass and the initial ground temperature favor the high-temperature performance of TBX, especially for the blocked tunnel.

#### 1. Introduction

Thermobaric explosive (TBX), a new subcomponent of volumetric explosives presents a number of advantages with respect to traditional high explosives, including longer duration, higher impulse shock wave, and diversified damage effects [1, 2]. The weapons filled with TBX depend mainly on the pressure and thermal effects instead of armour-penetrating or fragmentation damage effects to achieve destruction, which overcomes the shortcomings of typical blast/fragmentation munitions for specific targets, such as buildings, field fortifications, and tunnels [3–5]. Therefore, TBX is a serious threat to the survival of existing protective engineering in wartime.

Due to its outstanding features, the damage effects of TBX have been the focus of much research in recent years. Many studies have been conducted on the overpressure field and pressure damage effects [6–10], while research on thermal effects is less because of the inherent view that the damage severity of shock wave is far greater than that of thermal radiation. This view is always correct for traditional high explosives, but not for TBX, since the addition of high energy metal powder makes the reaction time and space scale of TBX higher than those of high explosives. In some closed, semiclosed, and other specific circumstances, the explosion thermal damage severity even exceeds the shock wave [11]. Therefore, it is of great significance to study the thermal effects of TBX, including explosion fireball radiation and dynamic thermal environment after explosion in adjacent areas of explosion source.

Mohamed et al. [12] recorded the explosion events in an open field for a novel thermobaric formulation with 2 kg using the high speed camera recorder; the results showed that the mushroom shape and extended action of fireball are two main TBX characteristics, and the effective lethal fireball duration can be up to 50 ms. The work of Yan et al. [13] has come to similar conclusions. Li and Hui [14] investigated the detonation temperature of TBX using infrared thermal image instrument and found that the duration of high temperature and the volume of the high-temperature cloud were 2~5 and 2~10 times as much as those of TNT, respectively, which implies that TBX is superior to the traditional high explosive on the temperature field.

Combined with certain fireball models and thermal damage criterion, the damage range of thermal radiation can be evaluated. Guo et al. [15] measured the blasting fireball’s temperature of TNT and TBX with different mass and then calculated the thermal damage radius of them combined with Baker’s fireball model; the results showed that TBX possesses much better thermal damage effects than TNT. On the basis of their work, Li et al. [16] studied the thermal sustaining damage ability of TBX by using the thermal radiation damage equation of Pietersen, which takes into account the influence of explosion duration, as the criterion of heat damage effect and the results showed that TBX has a stronger high-temperature sustaining damage capacity. Zhong et al. [17] analyzed the thermal damage of TBX and TNT fireballs based on the Martinsen dynamic model, which can describe the dynamic changes of fireball; the results showed that the heat dose of TBX was 3.6~4.8 times as much as that of TNT.

The above studies were conducted in the open field and cannot well reflect the thermal damage effects of TBX because of the shorter duration of fireballs. In terms of closed or semiclosed environment explosion, there are several studies. Using an enclosed explosion container to simulate the limited space, Yan et al. [18] measured the explosion field temperature of TBX to investigate the influence of postburn effect and the results showed that the effect could obviously increase the temperature of explosive products and make it keep at high level for a long time. Yan et al. [19] studied the thermal effect of TBX under semiclosed condition by testing the thermal response in tunnel after explosion using thermocouples and the results showed that the temperature field of TBX has wider range, higher temperature, and longer duration in tunnel condition, compared with that of TNT with the same mass.

However, few studies have been done on the thermal radiation evaluation method of thermobaric explosion in closed or semiclosed spaces, and even fewer works are available on the dynamic thermal environment after the explosion in such spaces, due to the difficulty in conducting effective measurements in such poor test conditions and with limited testing methods. In this paper, we first establish a damage evaluation method for the thermal radiation of thermobaric explosion fireball in a tunnel and then theoretically deduce the air temperature distribution inside a tunnel shortly after the explosion. Taking this distribution as an initial condition, we analyze the dynamic thermal environment after the explosion and the influences of TBXs mass and initial ground temperature on it by simulating the temporal and spatial variation of air temperature in a tunnel with the FLUENT software. The purpose of this paper is to investigate the thermal environment inside a tunnel after thermobaric explosion and provide prerequisites for thermoprotection design of protective engineering.

#### 2. Thermal Radiation Damage Evaluation

In order to evaluate the thermal radiation damage effects of a thermobaric explosion fireball, it is first necessary to determine the heat flux or heat dose received by the targets surface. However, it is often difficult to achieve an accurate experimental measurement of these two parameters. To solve this problem, various types of thermal radiation models for fireballs, suitable for different fuels or explosives, have been proposed, including the Dorofeev model [20], the Baker model [21], and the Martinsen model [22]. The Baker model is a kind of semiempirical, universal, and static model, whose applicability to TBX has been demonstrated [15]. Therefore, this model is chosen for the present study.

In the Baker model, the heat flux and heat dose received by the targets surface can be, respectively, expressed according to the following two equations [21]:where is the heat flux; is the heat dose; is the temperature of the fireball; is its diameter; is the distance from the center of the fireball; is the mass of consumed fuel, which in this case can be taken as the mass of thermobaric charge here;* F* is a constant whose value is 161.7; is another constant whose value is .

The heat dose damage criterion is chosen in this paper to evaluate the thermal radiation damage effects of a thermobaric explosion fireball due to its applicability to a variety of transient combustion or explosion processes [15]. The corresponding damage threshold values are shown in Table 1.