Aspects Regarding of a UGV Fire Fighting Thermal Shield ^†

Abstract

This article presents aspects related to the protection (with a double shield made of stainless steel) of a robot for emergency situations against the effect of flames due to a fire. The ground robot is semi-autonomous/autonomous, with a wheeled propeller (6 × 6). The robot, designed and built at the TRL 2 level, is intended for fire investigation, monitoring, and intervention (and, in particular, for petrochemical plants). The role of the shield is to protect the equipment that is part of the robot including its controllers, sensors, communications, power supply, etc. The need to mount a thermal protection shield on the intervention robot was given by the fact that fires at petrochemical plants generate very large thermal fields and gradients which are responsible for creating blind spots. These blind spots do not allow intervention crews to see what is happening in that area. These blind spots are characterized by very high temperatures. The dynamics of these fires can be unpredictable. Therefore, to analyze the performance of the heat shield in this study we perform a numerical-experimental analysis.

1. Introduction

Using robotic systems for risk situations intervention is extremely useful [1,2,3], and using them on a large scale helps to protect endangered personnel and increase the efficiency with which intervention missions are performed [4,5,6]. However, it is recommended that robotic intervention systems are equipped with radiation protection equipment [7,8,9,10] and equipment to protect against the high temperatures generated by fires [11,12,13,14], since it is possible that the moving path of the fire intervention robot might pass through areas which are contaminated with radiation, chemical substances, etc. Excessively equipping the robot with sensors isn’t the best solution. This is why additional risk area monitoring systems are used for areas where robotic system must intervene [15,16,17,18,19,20,21].

Designing complex and robust robotic intervention systems has led to prohibitive prices. In this sense, it is necessary to take into consideration low-cost solutions so that an efficiency from the material point of view can be maintained [22,23,24].

The method of declaring the issue involves the following question: which is the optimal solution so that intervention robots are operational while simultaneously ensuring a degree of protection against external destructive actions?

By virtue of the antagonism of the two requirements (i.e., optimal intervention [25,26,27,28] versus protecting the robot [29,30,31]), it has been considered that the task, in a manner which ensures feasible compromises, can be reduced to the following: identifying conditions imposed by the types of propulsion (e.g., on wheels or on tracks); identifying the payload [32,33,34,35,36]; identifying the dynamical characteristics depending on the terrain and artificial obstacles [37,38,39,40,41,42,43]; determining limitations due to the low level of predictability of the evolution of the fire [44,45,46,47]; determining limitations due to the level of thermal gradients of the heat sources [48,49,50,51,52,53]; and identifying the material characteristics specific to the structure of the robot and to the components destined to the protection of the robot and the equipment (sensors, batteries, controllers, etc.) against high temperatures [54,55,56].

From our research point of view, the task involves to the possibility of robot design imperfections occurrence, since it will be TRL2, in the research laboratory; the lack of specialized utilities at the level of the laboratory for mechanical processing of high temperatures resistant; establishing the geometrical shape of the protection system so that, from the thermodynamic point of view, refraction and reflexion coefficients corresponding to reducing the thermal energy of the robot can be obtained [57,58,59,60]; establishing a numerical-analytical model for calculating the thermal transfer processes

The actual state of the research domain represented in the specialized literature [61,62,63,64,65,66], and testing and evaluating the protection system against high temperatures; allows us to analyze an important part of the robot fire intervention.

From studying the specialty references [61,62,63,64,65,66], the various requirements of the beneficiaries, and the diverse requests of the beneficiaries, of the requirements imposed by the DOR (Documents with Operational Requirements), MND (Mission Needs Documents) operational environment, the following requirements specific for a fire intervention robot have been generated: it should be accessible from the costs point of view; it should operate both from inside and from outside; it should acquire data from the environment; it should be able to communicate from distances as big as possible; it should not be influenced by gamma radiation; it should be able to operate in both a remote control and an autonomous regimen; it should transmit large data quantities; and it should evaluate and map the interior of the buildings so that the data is transited to an operator and the intervention team can know which are the dangers.

The purpose of this article is to analyze and validate a numerical-analytical model for the behaviour of an item thermal protection equipment, namely a thermal shield (Figure 1).

2. The Thermal Shield

2.1. Materials Used

The preliminary characteristic situations for choosing a suitable material for the thermal shield are that the robot is stationary; the environment temperature is +20 °C; the atmospherically humidity is 60%; the atmospheric pressure is 1015 mbar; and the wind velocity is 0.5 m/s (Figure 2).

The design guide for the material which has been used (stainless steel) follows the BS 5950-1 international standards, the good practice code, published by BSI in 2000, the Eurocode stainless steel ENV 1993-1-4 recommendations, and from other sources, with adequate changes made in order to implement the recommendation in the BS 5950-1:200 format.

The main characteristics are represented by the high capacity of thermal impact absorption due to the excellent ductility (especially for the austenite classes).

2.2. Geometrical Characteristics

In the case of our study, we have equipped a robot for intervention in emergency situations (fires) with a thermal shield. The purpose is to reduce the total heat transfer between two radiant surfaces.

This thing can be achieved by placing a system of two shields against radiation between surfaces (i.e., emitter and receiver) [67]. The heat transfer through this layered construction is dominated by radiations (Figure 3).

The thermal transfer is produced through finite surfaces (the thermal shields), which have reduced emissivity indices (the Law of Kirchhoff—rel. 3).

Thus, the surface of the shield against radiation will be highly reflective, reducing the net radiative heat transfer through the two shield surfaces when the two of them are placed together in series. In the infinite case, the emissivity factor between the two surfaces is equal to the unit since the surfaces were considered dimensionally infinite.

Since this thing cannot happen [68] (3.107), thus relationship (1) becomes:

$${\dot{q}}_{14}=\frac{\mathsf{\sigma }\left(T_1^4-T_2^4\right)}{\left(\frac{1-{\mathsf{\epsilon }}_1}{{\mathsf{\epsilon }}_1{A}_1}+\frac{1}{A_1{F}_{1-2}}+\frac{1-{\mathsf{\epsilon }}_{2A}}{{\mathsf{\epsilon }}_{2A}{A}_2}+\frac{1-{\mathsf{\epsilon }}_{2B}}{{\mathsf{\epsilon }}_{2B}{A}_2}+\frac{1}{A_3{F}_{2-3}}+\frac{1-{\mathsf{\epsilon }}_{3B}}{{\mathsf{\epsilon }}_{2A}{A}_3}+\frac{1-{\mathsf{\epsilon }}_{3B}}{{\mathsf{\epsilon }}_{3B}}+\frac{1}{A_3{F}_{3-4}}+\frac{1-{\mathsf{\epsilon }}_4}{{\mathsf{\epsilon }}_4{A}_4}\right)}[{\mathrm{W}/\mathrm{m}}^2]$$

(1)

3. The Analytical Model

3.1. The Thermodynamic Laws

The thermodynamic laws which ensure the support for the analytical model are as follows: Prevost’s law; Kirchhoff’s Laws; the Stefan–Boltzmann law; Plank’s Law; Wien’s Laws; the Reyleigh–Jeans law; and Lambert’s law. Taking the previous laws into consideration, we obtain the total power of emission includes the radiant energy of all of the significant wavelengths for the thermal radiation and can be determined in the following manner:

$$E_T={\displaystyle \underset{0}{\overset{\infty }{\int }}{\epsilon }_{\lambda }{E}_{\lambda }d\lambda }={\epsilon }_T{E}_{0T}={\epsilon }_T\sigma T^4$$

(2)

3.2. Analytical Method for Determining the Temperature Variation in the Thermal Shield

The analytical method for determining the temperature variation in the material is as follows:

$$\Delta {\theta }_S=\frac{{\alpha }_c+{\alpha }_r}{c_S\cdot {\rho }_S}\cdot \frac{H_p}{A_g}\cdot \left({\theta }_f-{\theta }_S\right)\cdot \Delta t$$

(3)

where: $c_S\left[J/kgK\right]$ is the specific heat for the stainless steel; ${\rho }_S\left[kg/m^3\right]$ is the stainless steel density; ${\theta }_f\left[°C\right]$ is the flame temperature at a given moment $t\left[s\right]$; ${\theta }_S\left[°C\right]$ is the temperature of the stainless steel section, considered uniform, at the same given moment $t\left[s\right]$; $\raisebox{1ex}{$H_p$}\!\left/ \!\raisebox{-1ex}{$A_g$}\right.\left[m^{-1}\right]$ is the coefficient of the section, the ratio between the heated perimeter $H_p\left[m\right]$ to the gross transversal area $A_g\left[m^2\right]$; and ${\alpha }_{c,r}\left[\raisebox{1ex}{$W$}\!\left/ \!\raisebox{-1ex}{$m^{2}K$}\right.\right]$ is the convection heat transfer coefficient.

The designed shield is a system made from two relatively thin semi-round plates, opaquely placed in the direction perpendicular on the radiated heat propagation. As it has been previously mentioned, it is made from materials with low absorption and high reflectivity. In this case, two sheets made from stainless material have been used for the manufacturing of the shield. We mention that the chassis of the robot is made from stainless steel:

(i)

With no stainless-steel shields, the net heat exchange between the two parallel infinite planes is:

$$\{\begin{array}{l}{Q}_{12}={\left(F_g\right)}_{12}{A}_1{\sigma }_b\left(T_1^4-T_2^4\right)\left[W\right]\hfill \\ F_{12}=1\left[{\left(W/m^{2}K\right)}^{-1}\right]A_1=A_2=A\left[\raisebox{1ex}{$W$}\!\left/ \!\raisebox{-1ex}{$K$}\right.\right]\hfill \\ {\left(F_g\right)}_{12}=1/\frac{1}{{\epsilon }_1}+\frac{1}{{\epsilon }_2}-1\left[{\left(\raisebox{1ex}{$W$}\!\left/ \!\raisebox{-1ex}{$m^{2}K$}\right.\right)}^{-1}\right]\hfill \\ Q_{12}=A{\sigma }_b\left(T_1^4-T_2^4\right)/\left(\frac{1}{{\epsilon }_1}+\frac{1}{{\epsilon }_2}-1\right)\left[W\right]\hfill \end{array}$$

(4)

(ii)

Placing thermal radiation shields does not remove or add heat to the system, but, in equilibrium conditions, the plates of the thermal shields reach the T2 and T3 temperatures, considering that both faces of the shield plates have the same emissivity:

$$\frac{A{\sigma }_b\left(T_1^4-T_3^4\right)}{\left(\frac{1}{{\epsilon }_1}+\frac{1}{{\epsilon }_3}-1\right)}=\frac{A{\sigma }_b\left(T_3^4-T_2^4\right)}{\left(\frac{1}{{\epsilon }_3}+\frac{1}{{\epsilon }_2}-1\right)}$$

(5)

From (4 ÷ 5):

$${\left(Q_{12}\right)}_{net}=\frac{A{\sigma }_b\left(T_1^4-T_2^4\right)}{\left(\frac{1}{{\epsilon }_1}+\frac{1}{{\epsilon }_3}-1\right)+\left(\frac{1}{{\epsilon }_3}+\frac{1}{{\epsilon }_2}-1\right)}\left[W\right]$$

(6)

Computing the ratio between the radiant energy expression with and without shield, we will obtain:

$$\frac{\mathrm{with}\mathrm{shield}}{\mathrm{without}\mathrm{shield}}=\frac{\left(\frac{1}{{\epsilon }_1}+\frac{1}{{\epsilon }_2}-1\right)}{\left(\frac{1}{{\epsilon }_1}+\frac{1}{{\epsilon }_3}-1\right)+\left(\frac{1}{{\epsilon }_3}+\frac{1}{{\epsilon }_2}-1\right)}[-]$$

(7)

If ${\epsilon }_1={\epsilon }_2={\epsilon }_3$, then the ratio from relationship (22) is ½, which makes it obvious that, by introducing a shield, the heat transfer gets substantially reduced.

If the temperature of the shield plate, the second one from the robot, reaches temperature T3, according to (22), then it will have the value: $T_3^4=\frac{1}{2}\left(T_1^4+T_2^4\right)\left[K^4\right]$, case in which the thermal transfer without the shield will be:

$$Q=\frac{A\left(E_{b_1}-E_{b_2}\right)}{\left(\frac{1-{\epsilon }_1}{{\epsilon }_1}+\frac{1}{F_{12}}+\frac{1-{\epsilon }_2}{{\epsilon }_2}\right)}=\frac{A{\sigma }_b\left(T_1^4-T_2^4\right)}{\left(\frac{1}{{\epsilon }_1}-1\right)+1+\left(\frac{1}{{\epsilon }_2}-1\right)}=\frac{A{\sigma }_b\left(T_1^4-T_2^4\right)}{\frac{2}{\epsilon }-1}$$

(8)

We find that the ratio of the heat flux with a thermal protection shield reduces the thermal flux to half compared to the case in which we could have exposed the robot to thermal radiation without the shield.

Thus, the total resistance of the physical system for n plates that form a protection shield will be:

$$R\left(n-shields\right)=\left(2n+2\right)\frac{1-\epsilon }{\epsilon }+\left(n+1\right)\cdot 1=\left(n+1\right)\cdot \left(\frac{2}{\epsilon }-1\right),$$

(9)

so that the heat transfer is expressed thought the following:

$$Q=\frac{1}{n+1}\cdot \frac{A{\sigma }_b\left(T_1^4-T_2^4\right)}{\left(\frac{2}{\epsilon }-1\right)}$$

(10)

The conclusion which can be reached after the analytical model is that the presence of n component plates for a thermal shield leads to reducing the thermal radiant heat transfer with an (n + 1) coefficient (n + 1).

4. Simulating the Behavior of the Thermal Shield through the Finite Element Method

Let us consider that a flame jet from a burner has been directed towards the shield (Figure 4 and Figure 5).

The general simulation case consists of positioning the burner in the centre of the shield. Furthermore, the three air jet velocities have been taken into consideration: 5, 7.5, and 10 m/s.

5. The Experimental Study Regarding the Behavior of the Thermal Shield under the Action of Fire

For this experiment we have used a flame obtained from burning liquefied petroleum gas, a hydrocarbons mix composed mainly of propane and butane. The jet flame has been oriented on the longitudinal axis of the shield until the moment when the temperature of the steel became constant. Direct contact with the flame occurred at temperatures over 150 °C. The plate width is 4mm (material: Inox 316L), and the exposure time is 4 min. The measurements acquired with the infrared thermometer are represented in the following figure (Figure 6).

6. Conclusions

Although there are substantial uncertainties in the present theoretical and computational models regarding the prediction of thermal shields’ evolution for mobile robots, the simulation from chapter four corresponds well to the real conditions:

• The multi-parameter estimation developed for an inverse problem in which we have multiple constant parameters, such as the material properties, are close to the statistical data corresponding to the stainless-steel open flame exposure;
• After the simulation, we could identify the parameters which need to be measured and which should allow recorrelating the numerical analysis calculations with MEF.
• The evolution of the heating phenomenon in time demonstrates that due to the special properties of the stainless steel, the temperature gradients rise moderately;
• Introducing the second wall to the protection shield demonstrates a decrease in temperature on the exposed side;
• In the case of the simulation, upon a nominal analysis the temperature differences can have uncertainties and sensitivity differences, a fact that must be checked during the experiments;
• In the case of a hot air jet (open flame), as previously mentioned, the problem of an inverse analysis arises in the gas fluid jet modelling (however, this is not the subject of this research);
• Unlike the jet temperature, the surface of the shield gets heated up in time, with different gradients for the same initial simulated (a fact which can be explained due to the molecular structure of the metal from which the shield is made).

Future Developments Directions

One of the future development directions consists in designing and building a multi-layer robot, from the chassis point of view, to reduce its internal temperature.

The second direction of development consistuts in introducing an equipment for detecting dangerous radiation levels in areas which are characterized by high temperatures.