# Experimental estimation of the heat release rate of burning luggage

P Carlotti, M Suzanne

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## To cite this version:

P Carlotti, M Suzanne. Experimental estimation of the heat release rate of burning luggage. Fire Technology, In press, 10.1007/s10694-023-01499-0. hal-04265683

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# Experimental estimation of the heat release rate of burning luggage

P Carlotti*, M Suzanne

Laboratoire central de la Préfecture de police, 39bis rue de Dantzig, Paris, France

(*) corresponding author, Pierre.Carlotti@arteliagroup.com; now at Artelia, 47 avenue de Lugo, 94600 Choisy le Roi, France

Distributed under CC-BY 4.0 licence: https://creativecommons.org/licenses/by/4.0/

Also Published in Fire Technology, 2023 as DOI 10.1007/s10694-023-01499-0

## ABSTRACT

This paper considers the fire risk associated with luggage in trains from an experimental point of view. After a review of existing data, it was decided to test medium isolated suitcases, large isolated suitcases, two medium and one large suitcases in a rack, and two medium and one large suitcases in a confined rack, representative of luggage stacked next to the entrance doors of coaches in high-speed trains. Heat release rates were estimated by three methods: mass loss measurement, dilution calorimetry and oxygen consumption calorimetry. Measured peak heat release rates are of order 250 kW for medium size suitcases and 400 kW for large suitcase. The sum of two medium and one large suitcase would therefore be in the order of 900 kW. However, rack tests with two medium and one large suitcase gave values in the order of 1300 kW for an open rack and 2100 kW for a confined rack. This shows a strong effect of rack geometry and confinement. The concentrations of carbon monoxide and carbon dioxide are also measured, as is the total amount of hydrogen cyanide released in each test. An empirical fire curve is proposed taking into account the effect of confinement.

## KEYWORDS

Luggage fire; heat release rate; full scale test; fire source

## 1 INTRODUCTION

For fire safety design or post-fire investigation, the main physical quantity to be considered that will determine the outcome of the fire scenario is the heat release rate as a function of time (see the discussion in [1]). When considering a fire in a train or rail transit, the regulations on rolling stock ensure that the heat release rate from the train itself is generally very low (see [2, 3, 4]). Any further increase in fire heat
release rate can only be the result of an additional fire load that has not been considered in the train fire risk analysis, such as the fire load carried by passengers that is neither controlled nor standardised. This is consistent with the post-fire analysis of two dramatic accidents that claimed a total of 444 lives (Gletscherbahn Kaprun 2 funicular train [5] and Baku Metro Fire [6]). For the Kaprun fire, skiing equipment of passengers alone would represent a fire load of 10.5 GJ [7].

This has also been confirmed by experiments. As shown in small-scale tests [8], no flashover was observed in cases where the initial fire, consisting of luggage and wood cribs, did not exceed a range of 400 to 600 kW, whereas flashover was observed when the fire grew up to 700 to 900 kW. In a series of ten reduced-scale fire tests, the fuel load representing seats, wall and ceiling linings and floor coverings was supplemented by wood cribs simulating luggage and other combustible materials [9]. It was observed that, without these wood cribs, the fire did not spread in the compartment, highlighting the role of personal items inside the railcar in accentuating the spread of fire along its length, in addition to increasing the total fuel load available to the fire. It is further pointed out that it is not only the extra fuel load in itself that is important, but also the distribution of the fuel.

The importance of luggage in the fire development was confirmed by Lönnermark et al. in [10] in their full-scale experiment. The additional fire load of the carriages was 17 % of the total fire load including the materials that compose the train compartment (ceiling, walls, floor and seats). A total of 79 pieces of luggage were distributed in the train compartment: large duffel bag or suitcase (14 kg), middle size bag (10 kg), cabin bag (5.3 kg), sports bag/shoulder bag (3kg) and backpack (3 kg). These pieces of luggage were found to increase the fire spread significantly and the authors concluded that it is necessary to consider this transient load when conducting risk assessments and designing response tactics. This was confirmed by Hjohlman et al [11] (citing [12]), who concluded from a series of tests that the presence of luggage is one of the parameters with the greatest influence on the development of a fire in a railway or metro car. Li et al [13] also pointed out that the critical fire spread in a train carriage is strongly dependent on the combustible materials in the vicinity of the ignition source, such as luggage and combustible wall linings. Ordinary luggage is mainly made of highly flammable plastics and plays a much more important role in the initial stage of fire spread than in the fully developed stage [13]. In [14],
several possible locations for an interior ignition source in a passenger train (in addition to electrical-related fires) are listed, including in particular a fire on a luggage rack.

However, the additional fire load from baggage is not considered in most fire studies [15-25]. The heat release rate for design studies is generally evaluated using very basic arguments based on mass and energy contents, see for example [5, 23, 26-28]. The evolution of heat release over time is often based on expert judgement only. In [14, 29], the authors even mention that the greatest unknown in material characterisation for the definition of the fire scenario is baggage brought onto the train as it can contain many materials that vary dramatically in composition, density, quantity, and flammability. Therefore, due to the high degree of variability and the lack of information to characterise the burning behaviour of baggage, it was not considered in their analysis.

The main reason for not utilising the heat released by luggage fires may be the relative scarcity of available experimental data. For such a study, it is necessary first to consider the typical baggage items that might be encountered in a given situation, and then to evaluate the heat release rate of that baggage. Such studies have been conducted in [30, 7]. The work of [30] focused on baggage found in airport concourses and showed that the average baggage trolley is loaded with a total weight of 22.6 kg of baggage. However, little information is available on the type and content of the baggage and the ignition method. The peak heat release rate was found to be 1.2 MW reached in 625 s, followed by an immediate decrease, with the heat release rate being less than 300 kW after 1000 s and less than 100 kW after 1500 s. The work of [7] is described in more details. It focuses on luggage transported in suburban trains in the Stockholm area. The first step of the study was a field observation of luggage in the trains. More than three hundred real pieces of luggage were considered, which led the author of the study to design twelve typical bags to be analysed, ranging from a paper bag containing a small amount of food to several types of backpacks. Due to the suburban nature of the train network considered, the average mass of 4.2 to 4.6 kg was much lower than in the previous study. It should be noted that 28 % of the passengers had pressurised aerosol cans in their luggage. To measure the heat release rate, the pieces of baggage were placed on a grid raised of the ground to eliminate any possible effect of liquefied plastic contributing to the fire source. The ignition was performed with a 25 kW liquefied petroleum gas flame applied for 90 s.
In conclusion, luggage is seldom considered in fire safety studies, even though the heat it releases when burning can be of primary importance to the development of the fire and the occurrence of flashover, as demonstrated by experiments or post-fire analyses. However, experimental data on luggage fires are scarce and lack of repeatability. Therefore, the present study was undertaken to provide information, to give some indications on the time dynamics of the heat release rate and to evaluate the possible effects of heat confinement in luggage racks inside trains, which has been identified as a critical fire scenario. The experimental procedure is first explained and then the results are presented and discussed.

# 2 EXPERIMENTAL PROCEDURE

## 2.1 Selection of luggage to be tested

The first step is to determine which type of luggage is characteristic of a given real-life situation. Since the aim of the study was train transportation, it was decided to focus on suitcases and luggage racks. To this end, luggage lost by travellers in Paris was analysed with the help of the *Service des Objets Trouvés*, a department of the Paris Prefecture of Police responsible in particular for lost property at all main train stations in Paris. On the basis of this analysis, it was decided to focus on two types of item:

Type 1. medium-size suitcase, 46 x 33 x 24 cm³ composed mainly of polycarbonate, with an empty mass of 2.5 kg;

Type 2. large size suitcase, 75 x 47 x 28 cm³, composed mainly of acrylonitrile butadiene styrene (ABS), with an empty mass of 3.6 kg.

Two series of identical suitcases with the above characteristics were purchased (see Figure 1).

In terms of content, an analysis was carried out on a panel of 35 unclaimed suitcases (after the legal claim period). On average, the luggage was composed of 60 % of cellulosic materials (mainly cotton textiles and books) and 40 % synthetic polymers (suitcases, fleece jackets, sport clothes, hairdryers...). These two categories are not directly comparable with those defined in [9] (i.e. 59% textile, 35% plastic, 3% of metal and 1% electronics for the suitcase and 49% textile, 49% plastic and 2% paper for the cabin bag), but the overall distribution is comparable. A similar mix of clothes and other goods were loaded into the suitcases. It should be noted that none of the suitcases analysed contained a computer or similar
item with large batteries (see Figure 2). The total mass of type 1 suitcases is 9 kg, and the total mass of type 2 suitcases is 18 kg.

## 2.2 Experimental set-up

The experiments were carried out in the LCPP test facilities at Fort de la Briche in the Paris suburbs. This site is equipped with a calorimetric hood able to test fires up to 4 MW, a scale capable of recording mass evolution over time, and video recording. The design of the hood used to collect the combustion products and its exhaust system is based on the specifications of the ISO 9705 standard [31]. However, the hood is placed directly above the burning specimen to avoid any modification of the heat release rate due to a confinement effect of the fire test room. Particular attention was paid to the size and position of the natural vents to avoid any unbalanced ventilation of the fire.

Four configurations were considered:

a. a medium size suitcase (type 1) positioned vertically;

b. a large size suitcase (type 2) positioned vertically;

c. a two-level open rack, with two medium suitcases on the bottom level and one large suitcase on the top level (see Figure 3a);

d. a two-level confined rack, with two medium size suitcases on the bottom level, and one large suitcase on the top level (see Figure 3b).

The confined rack is designed to be similar to those found in high-speed trains It has total height of 2 m, a width of 0.9 m and a depth of 0.5 m with an intermediate shelf consisting of a grid at a height of 0.75 cm. It is closed on three sides and on top.

The open rack has the same dimensions but is open on all sides and at the top. It was chosen to provide a basis for comparison with, on the one hand, the sum of three isolated suitcases and, on the other hand, the confined rack.

For the isolated suitcases, two possible ignitions were considered: (i) spillage of 200 ml of gasoline on the top of the suitcase, ignited by a small flame; (ii) flame of a 5 kW gas burner applied to the bottom of
the suitcase for 20 seconds. For the rack configurations, the same gas burner was applied to one of the medium size suitcases at the bottom level for the same duration. Table 1 provides a summary of the configurations analysed and the tags used to identify these scenarios. Only one fire test was performed for each configuration investigated. This represents a total of six full-scale experiments.

<table><caption>Table 1: summary of the configurations used in the fire tests.</caption><thead><tr><td>Label</td><td>Configuration</td><td>Types of suitcases</td></tr></thead><tbody><tr><td>a1i</td><td>a. isolated suitcase</td><td>1</td></tr><tr><td>a1ii</td><td>a. isolated suitcase</td><td>1</td></tr><tr><td>b2i</td><td>b. isolated suitcase</td><td>2</td></tr><tr><td>b2ii</td><td>b. isolated suitcase</td><td>2</td></tr><tr><td>c112ii</td><td>c. open rack</td><td>1;1;2</td></tr><tr><td>d112ii</td><td>a. confined rack</td><td>1;1;2</td></tr></tbody></table>

## 2.3 Gas concentration measurements

Combustion gases are collected by the hood. They are then filtered to remove soot and water and then cooled before being sent to the analysers. Infrared cells are used to measure carbon monoxide and carbon dioxide concentrations. A paramagnetic cell is used to measure the oxygen concentration. All these data are collected at a sampling rate of 1 Hz.

The determination of hydrogen cyanide is obtained by sparging a fixed amount of gas flow (1 l/min) into a 70 ml water solution of sodium hydroxide at concentration 0.1N throughout the fire test. The resulting solution is analysed by a method based on distillation and colourimetry, very similar to that described in [32]. The result is thus a mass of hydrogen cyanide for the whole duration of the test, expressed in mg per unit volume of gas. Knowing the total amount of gas and smoke extracted by the hood during the fire test, the total mass of hydrogen cyanide released is determined. This is then divided by the total mass loss of combustible material to obtain the average yield of hydrogen cyanide per unit mass of combustible material.
## 2.4 Heat release rate measurement

Heat release rate is an essential parameter in assessing the intensity of a given fire. It can be determined in a number of ways, including mass loss rate measurements, dilution temperature or oxygen consumption calorimetry.

### 2.4.1 Determination from mass loss rate

When a solid or a liquid burns, its mass $M(t)$ decreases as components evaporate or pyrolyse and then burn. The associated heat release rate is therefore

$$ \dot{Q} = -\Delta H \frac{dM}{dt} $$

where $\Delta H$ is the average effective heat of combustion of the material. The effective heat of combustion is estimated to be $\Delta H \approx 19$ MJ/kg, assuming an effective heat of combustion of 28 MJ/kg for synthetic polymers (for PA in [33]) and 13 MJ/kg for cellulosic materials (average value determined from equivalent materials in [33]), with the 40%/60% distribution between these two categories mentioned previously (cf. section 2.1). This effective heat of combustion is close to the 20 MJ/kg used in [10] to assess the fire load of the added baggage, but lower than 25 MJ/kg mentioned in [13] for luggage composed 50% Nylon and 50% polystyrene.

A major difficulty with this method is calculating the derivative of $M(t)$ when the experimental signal is noisy. We used Bernstein polynomials of the same degree as the number of measured points to smooth the data. The uniform convergence property of Bernstein polynomials (cf. [34]) guarantees a very good damping of high frequency noise while maintaining a good resolution of the time evolution of the underlying smooth function. See also [35] for application examples of the method.

### 2.4.2 Estimation from dilution calorimetry

Neglecting heat losses through solid boundaries, when a mass of air is heated, its temperature rise is proportional to the heat release rate and inversely proportional to the air volumetric flow rate. This flow rate can be estimated by simultaneously measuring the temperature and pressure drop across a constriction section (e.g., a Venturi device). After some algebra, this results in

$$ \dot{Q} = C_1 \sqrt{\frac{\Delta P}{T}} (T - T_0) $$

Where the value of $C_1$ depends on:
* the geometry of the extraction hood and pipework (including the Venturi device), and

* the heat losses through solid boundaries.

This quantity can be estimated using calibration fires with heat release rates as close as possible to the tested fire to minimise uncertainties associated with heat losses through solid boundaries. However, this method is very robust and was sometimes used in the context of large fires (e.g., the EUREKA fire test programme [36] and many tunnel fire tests which are not published in scientific literature).

### 2.4.3 Estimation from oxygen consumption calorimetry

Oxygen consumption calorimetry is based on the principle that the amount of energy released by complete combustion of hydrocarbon-based material is $E = 13.1$ MJ per kg of oxygen consumed for a well-ventilated fire [33, 37]. According to reference [37], this value is valid to within 5 % for all common combustible materials. In case of high carbon monoxide production, a correction is necessary, but this is not the case in our fire tests (cf. section 3). After calculations, this leads to

$$ \dot{Q} = C_2 \sqrt{\frac{\Delta P}{T}} \delta X $$

Where $\delta X$ is the oxygen concentration reduction from standard atmosphere and the value of $C_2$ depends only on the geometry of the extraction hood and pipework (since the Venturi device is used to estimate volumetric flow rate). $C_2$ is estimated by calibration fires of known heat release rates.

Oxygen consumption calorimetry is considered the method of choice for estimating heat release rates, as it does not require an estimate of the effective heat combustion, nor does it neglect the effect of the heat release rate on heat losses through boundaries.

## 3 RESULTS

### 3.1 Description of the fire development

As previously stated, two ignition methods were tested, with and without gasoline spillage. Both ignition modes are regularly encountered in fire investigations, and it is interesting to assess the difference in fire development with the addition of gasoline over the suitcases.

When the spilled gasoline is ignited, the flame size increases rapidly from half the height of the suitcase in 1 s to three times the height in 4 s (see Figures 4a and 4b). As the gasoline is consumed quickly, the flame size decreases accordingly. Approximately 45 s after ignition (see Fig. 4c), flames are visible on
the floor around the wheels, on the top of the suitcase around the handles, and from the centre of the
suitcase to the top on the zip fastening. The flames then spread progressively and cover the side of the
suitcase 1 min 20 s after ignition (see Fig. 4d). Approximately 3 min 30 s after ignition, the suitcase falls
down and burns from a horizontal position. The volume of the flames continues to increase, reaching a
peak about 5 min after ignition. The fire intensity decreases then steadily. At this point, most of the
external envelope of the luggage is consumed, and the heat is now released mainly from the combustion
of the clothes. Visually, although the volume of the flames has clearly decreased, the production of grey
smoke remains important until the end of the test.

Concerning the second ignition method, without gasoline, the flame is applied at the bottom of one side
of the suitcase (see Fig. 5a). When the burner is removed, the flame is approximately 10 cm high. This
flame spreads progressively and its tips becomes higher than the top of the suitcase about 3 min after
ignition. One minute later, the side of the luggage where the burner was applied is covered by flames (see
Fig. 5b). Four minutes after ignition, the tip of the flames reaches approximately 2 m (see Fig. 5c). Soon
after, the suitcase falls as the side of the luggage melts, releasing its contents. The intensity of the fire
decreases slightly after this fall, but the size of the flames continues to increase, and the suitcase is fully
engulfed in flames 6 min after the ignition. The external casing of the suitcase gradually melts, forming a
pool fire whose surface area increases. It appears that, when all the materials of the suitcase have burned,
the intensity of the fire decreases (see Fig. 5d), with clusters of flames unevenly distributed over the pile
of blackened clothes (22 minutes from ignition).

It is interesting to compare the two ignition methods. The gasoline burns intensely, but for a short
duration: in less than 45 seconds the fuel is completely consumed, leaving only few patches of flames.
The fire develops more quickly when the additional liquid fuel is spilled, but not as much as expected
because the gasoline eventually fails to ignite the outer casing of the case.

In the open rack configuration, the ignition flame is applied to one of the medium size suitcases standing
vertically. Three minutes after ignition, the tip of the flame reaches the large size suitcase located on the
upper level of the rack. One minute later, the flames have spread over the entire surface of the ignited
luggage and towards the upper suitcase. At this point, the second medium size suitcase has not yet caught
fire (see Fig. 6a). Soon after, the ignited suitcase falls out of the rack, but flames still on the wheels of the luggage spread to the second medium size suitcase. At the same time, the intensity of the large size suitcase increases; part of it and its contents fall to the floor and, 5 min 30 s after ignition, the fire is fully developed on both levels of the rack (see Fig. 6b).

In the confined rack configuration, it takes 6 min for the tip of the flames on the medium size luggage to reach the large size suitcase located on the upper level of the rack (see Fig. 7a). When the ignited luggage falls out of the rack 6 min 30 from the start, neither the second medium nor the large size suitcase is ignited. It takes 1 min 30 for the fire to spread to the second medium size suitcase and another 30 seconds for the flames to penetrate the large suitcase and ignite its contents. Approximately 8 min 30 s after ignition, the fire is fully developed on both levels of the rack (see Fig. 7b).

## 3.2 Oxygen, carbon dioxide and carbon monoxide concentrations

Figures 8 and 9 show the oxygen concentration reduction $δX = 20.95 - [O_2]$ for all tests, with the time origin shifted to allow the curves to be compared. From Fig. 8, the measurements for the isolated suitcases are quite repeatable, especially for the decay of the fire. The discrepancies observed during the growth phase are mainly due to the different ignition methods. From Fig. 9, which compares the open and confined rack configurations, the growth of the fire is perfectly repeatable until $δX$ reaches 5 % vol.; after this moment, differences become visible, mainly due to the burning behaviour of the ignited medium suitcase and later the confinement effect of the rack.

Value of $δX$ is highly correlated with the carbon dioxide concentration, since a stoichiometric reaction would produce one mole of carbon dioxide for each mole of oxygen consumed. In reality, the chemistry is much more complex as other reactions are involved, in particular the production of water by combining oxygen atoms with hydrogen from hydrocarbon chains. Therefore, the quantity

$$ \gamma = \frac{[CO_2] - [CO_2]_0}{\delta X} $$

should be less than 1 but not too far. Measurements show that $γ$ is between 0.7 and 0.85 for active fires stages, and less for incipient fires and residual combustion (see Figs. 1 and 11).
The ratio of carbon monoxide to carbon dioxide was also measured. It has a peak value in the range of 0.04 to 0.06 for active fire phases, then decreases to a plateau in the range of 0.02 to 0.04 for residual combustion, except when the fire is almost extinguished, when carbon the dioxide concentration is close to ambient, while carbon monoxide may remain at a residual concentration of about 0.02 % by volume (200 ppm). Figure 12 presents the ratio carbon monoxide concentration to carbon dioxide concentration $[CO]/[CO_2]$ versus carbon dioxide concentration for all tests (except for test ali where the carbon monoxide cell was not functional). This figure show that, as expected, the ration is not constant. For use in fire risk assessment calculations, the solid line gives an empirical upper limit:

$$ \frac{[CO]}{[CO_2]} < 0.06 [CO_2]^{-0.3} $$

### 3.3 Heat release rate

The values of C₁ and C₂ were estimated from two calibration fires of small pans of alcohol with a plateau heat release rate of 200 kW. This resulted in:

$$ C_1 = 3600 \text{ kJ kg s}^{-1} \text{ Pa}^{-1/2} \text{ K}^{-1/2} $$

$$ C_2 = 3.875 \times 10^7 \text{ kJ kg s}^{-1} \text{ K}^{1/2} \text{ Pa}^{-1/2} $$

Uncertainties arise from four factors:

1. measurement uncertainties in the calibration fire;

2. unreducible uncertainties in the calibration fires due to simplifications in the models;

3. measurement uncertainties in the suitcase fires;

4. uncertainties due to the choice of the value of ΔH and due to heat losses through solid boundaries in the suitcases fires.

The first two factors can be estimated from the results of the calibration fires. It shows that the main inaccuracies factors are modulations in the heat release rate of the alcohol pools and noise in the weighting system. From a detailed analysis of calibration curves, the accuracy is of order 20 % for C₁ and of the order of 10 % for C₂. The third factor is relatively small because of the care taken in the measurement process, except for the mass loss, which provides a noisy signal. The last factor is non-existent for oxygen consumption calorimetry but is expected to be higher for the mass loss rate method and very large for dilution calorimetry if the heat release rate differs substantially from the heat release
rate of the calibration fires, due to dissimilar heat losses. In addition, since the calibrations were
performed with alcohol, the radiative fraction of the flames is likely to be much larger for the luggage
fire than for the alcohol fire, leading to a further underestimation of the heat release rate from dilution
calorimetry. Figures 13 to 18 show the heat release rates for the six fire tests. Mass loss and oxygen
consumption calorimetry (curves labelled a and c) are consistent, but dilution calorimetry (curves
labelled b) gives much lower values, except for small values of heat release rates. Therefore, dilution
calorimetry is not used in further analysis.

**3.4 Hydrogen cyanide yield**

Due to the method of measurement, it was only possible to obtain directly but the total amount of
cyanide produced for each fire test. The results are given terms of yield Y<sub>HCN</sub>, i.e. in grams of cyanide
production per kg of combustible in Table 2. The values are relatively scattered, with yields ranging from
0.2 to 0.7 g/kg, and due to the high complexity of combustion mechanisms at stake in these real fires.

<table><caption>**Table 2: hydrogen cyanide production per kg of combustible**</caption>
  <tr>
    <td></td>
    <td>a1i</td>
    <td>a1ii</td>
    <td>b2i</td>
    <td>b2ii</td>
    <td>c112ii</td>
    <td>d112ii</td>
  </tr>
  <tr>
    <td>HCN yield (g/kg)</td>
    <td>0.226</td>
    <td>0.744</td>
    <td>0.179</td>
    <td>0.199</td>
    <td>0.368</td>
    <td>0.569</td>
  </tr>
</table>

Defining r as the mass production of hydrogen cyanide per unit time and Y<sub>HCN</sub> the mass production of
hydrogen cyanide per unit mass of combustible, this allows, as an order of magnitude estimate of r as a
function of instantaneous heat release rate:

$r = k \dot{Q}$

where k = Y<sub>HCN</sub>/ΔH is in the range of 0.01 to 0.035 g/MJ.

There is a large variation in the value of k coming from the uncertainties in yield values. As already stated, these uncertainties are due to the complexity of chemical reactions during combustion process which have the consequence that there cannot be one to one correspondence between value of instantaneous HRR and hydrogen cyanide production. On the other hand, measurements in the DIN 53 436 tube furnace show that the production rate of hydrogen cyanide follows the same trend as carbon monoxide [38]. This is also verified in [39] for other experimental set-up such as the ISO 9705 room or the room corridor.
4 DISCUSSION

Heat release rates measured for isolated luggage give typical peak values of 250 kW for medium size suitcases (see Figs. 13 and 14) and 400 kW for large suitcase (see Figs.15 and 16), and a fire duration of the order of 10 minutes. The sum of two medium and a large suitcase would therefore be in the order of 900 kW. However, the rack tests gave values in the order of 1300 kW for the open rack and 2100 kW for the confined rack (see Figs. 17 and 18), in spite of the fact that residual mass in proportion to initial mass was less than 10 % of initial mass for all tests (except for test a1ii). Thus total energy of the three configurations is approximately the same, and the more intense fire leads to the shorter duration. Assuming a linear increase followed by a power law decrease, which is the simplest formulation consistent with the observed shape of HRR versus time curve, the results can be summarised with the following empirical equation

$$
\dot{Q} = c \dot{Q}_{max} \times \begin{cases} \frac{t}{t_p} & \text{for } (t < t_p) \\ \left(\frac{t_p}{t}\right)^\alpha & \text{for } (t > t_p) \end{cases}
$$

With t<sub>p</sub>= 5 minutes, Q<sub>max</sub> = 28 x M expressed in kW where M is the total luggage mass expressed in kg, and c is a dimensionless constant with a value of 1 for an isolated suitcase, up to 1.5 for suitcases in a rack and 2.3 for suitcases in a confined rack. The exponent α is chosen so that the total energy is conserved:

$$
\alpha = \frac{2 \frac{\Delta HM}{\dot{Q}t_p} + c}{2 \frac{\Delta HM}{\dot{Q}t_p} - c} \approx \frac{4.4 + c}{4.4 - c}
$$

Figure 19 show HRR values given by the above equation for the four configurations considered in the present paper.

Peak HRR must be compared with typical values for technical installations and rolling stock [2, 3, 26, 28]. In these references, the maximum heat release rate is less than 1000 kW, except for the rubber tyred metro, for which the peak is estimated to be 2000 kW. This would argue in favour of systematically consider baggage fires in safety studies and quantitative risk assessments, unless there is good reason to believe that the probability of occurrence is very low.
# 5 CONCLUSION

This paper focussed on fire risk associated with luggage in trains. Literature review has shown that this is rarely considered in fire safety studies, although the heat it releases when burning can be important with respect to the development of the fire. Literature review also made clear that experimental data on luggage fires remains scarce, which is why the present study was undertaken.

The main findings are the following. Igniting an isolated suitcase for 20 seconds with the flame of a 5 kW gas burner applied to the bottom of the luggage or after spilling 200 ml of gasoline on the top of the luggage does not drastically change the time when the peak of heat release rate is reached. This is probably due to the fact that most of the materials constituting the suitcase are waterproof.

For the single burning suitcase standing vertically, the peak of heat release rate is observed approximately 5 minutes after ignition, regardless of the size of the baggage. It reaches 250 kW for the medium-size suitcase, and 400 kW for the large suitcase. When considering two medium-size suitcases and a large-size suitcase placed in an open rack, a peak heat release rate of 1 400 kW is reached approximately 5 min 30 s after ignition. When considering two medium-size suitcases and a large-size suitcase placed in a confined rack, a peak heat release rate of 2 000 kW is reached approximately 8 min 30 s after ignition. In this configuration, the confinement effect is clearly visible on the fire development and intensity.

The measured CO and CO2 production rates are reported, as well as the modelled production rate of hydrogen cyanide. An empirical equation is presented for estimating the heat release rate of luggage in fire risk studies. The equation is kept as basic as possible to guarantee robustness in risk assessments.

## ETHICAL APPROVAL

(non applicable)

## COMPETING INTERESTS

No identified competing interests.

## FUNDING

This work was partly funded by ANR through Re(H)sTrain project.

## AVAILABILITY OF DATA AND MATERIALS

Data available on request to the authors.
## REFERENCES

[1] V. Babrauskas and D. Peacock. Heat release rate: The single most important variable in fire hazard. Fire Safety Journal, 19:255–272, 1992.

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LIST OF FIGURES

**Figure 1:** suitcases in front of the experimental structure.

**Figure 2:** large size suitcase filled with a mix of clothes and other goods for a total mass of 18 kg.

**Figure 3:** suitcases tested in a) open rack and b) confined rack configurations, with two medium suitcases of type 1 (pink) and one large suitcase of type 2 (black).

**Figure 4:** Ignition of a medium size suitcase with gasoline.

**Figure 5:** Ignition of a medium size suitcase without gasoline.

**Figure 6:** Development of the fire in the open rack configuration.

**Figure 7:** Development of the fire in the confined rack configuration.

**Figure 2 8:** Oxygen concentration reduction $\delta X=20.95-\text{[O}_2\text{]}$ expressed in % vol. for isolated suitcases tests.

**Figure 9:** Oxygen concentration reduction $\delta X=20.95-\text{[O}_2\text{]}$ expressed in % vol. for rack tests.

**Figure 10:** ratio $\gamma$ for isolated suitcases tests.

**Figure 11:** Ratio $\gamma$ for rack tests.

**Figure 12:** ratio [CO]/[CO2] as function of [CO2] expressed in % vol; solid curve is given by equation
$$ [CO]/[CO_2] = 0.06 [CO_2]^{-0.3} $$

**Figure 13:** heat release rate as function of time for test a1i (medium isolated luggage); (a): mass loss rate method; (b): dilution calorimetry; (c) oxygen consumption calorimetry.

**Figure 14:** heat release rate as function of time for test a1ii (medium isolated luggage); (a): mass loss rate method; (b): dilution calorimetry; (c) oxygen consumption calorimetry.

**Figure 15:** heat release rate as function of time for test b2i (large isolated luggage); (a): mass loss rate method; (b): dilution calorimetry; (c) oxygen consumption calorimetry.

**Figure 16:** heat release rate as function of time for test b2ii (large isolated luggage); (a): mass loss rate method; (b): dilution calorimetry; (c) oxygen consumption calorimetry.

**Figure 17:** heat release rate as function of time for test c112ii (open rack); (a): mass loss rate method; (b): dilution calorimetry; (c) oxygen consumption calorimetry.

**Figure 18:** heat release rate as function of time for test d112ii (confined rack); (a): mass loss rate method; (b): dilution calorimetry; (c) oxygen consumption calorimetry.

**Figure 19:** HRR given by empirical equation of section 4; (a) single medium isolated suitcase; (b) single large isolated suitcase; (c) two medium and one large suitcases in an open rack; (d) two medium and one large suitcases in a closed rack.
<figure><img src="image_3.png" /><figcaption>Figure 1: suitcases in front of the experimental structure.</figcaption></figure>

<figure><img src="image_4.png" /><figcaption>Figure 2: large size suitcase filled with a mix of clothes and other goods for a total mass of 18 kg.</figcaption></figure>

<figure><img src="image_5.png" /><figcaption>Figure 3: suitcases tested in a) open rack and b) confined rack configurations, with two medium suitcases of type 1 (pink) and one large suitcase of type 2 (black).</figcaption></figure>
<figure><img src="image_6.png" /><figcaption>Figure 4: Ignition of a medium size suitcase with gasoline.</figcaption></figure>
<figure><img src="image_7.png" /><figcaption>Figure 5: Ignition of a medium size suitcase without gasoline.</figcaption></figure>

<figure><img src="image_8.png" /><figcaption>Figure 6: Development of the fire in the open rack configuration.</figcaption></figure>
<figure><img src="image_9.png" /><figcaption>Figure 7: Development of the fire in the confined rack configuration.</figcaption></figure>

<figure><img src="image_10.png" /><figcaption>Figure 8: Oxygen concentration reduction $δX = 20.95 - [O_2]$ expressed in % vol. for isolated suitcases tests.</figcaption></figure>
<figure><img src="image_11.png" /><figcaption>Figure 9: Oxygen concentration reduction $\delta X = 20.95 - [O_2]$ expressed in % vol. for rack tests.</figcaption></figure>

<figure><img src="image_12.png" /><figcaption>Figure 10: ratio $\gamma$ for isolated suitcases tests.</figcaption></figure>
<figure><img src="image_13.png" /><figcaption>Figure 11: Ratio γ for rack tests.</figcaption></figure>

<figure><img src="image_14.png" /><figcaption>Figure 12: ratio [CO]/[CO2] as function of [CO2] expressed in % vol; solid curve is given by
equation [CO]/[CO_2]=0.06 [CO_2]^{(-0.3)}.</figcaption></figure>
<figure><img src="image_15.png" /><figcaption>Figure 13: heat release rate as function of time for test a1i (medium isolated luggage); (a): mass loss rate method; (b): dilution calorimetry; (c) oxygen consumption calorimetry.</figcaption></figure>

<figure><img src="image_16.png" /><figcaption>Figure 14: heat release rate as function of time for test a1ii (medium isolated luggage); (a): mass loss rate method; (b): dilution calorimetry; (c) oxygen consumption calorimetry.</figcaption></figure>
<figure><img src="image_17.png" /><figcaption>Figure 15: heat release rate as function of time for test b2i (large isolated luggage); (a): mass loss rate method; (b): dilution calorimetry; (c) oxygen consumption calorimetry.</figcaption></figure>

<figure><img src="image_18.png" /><figcaption>Figure 16: heat release rate as function of time for test b2ii (large isolated luggage); (a): mass loss rate method; (b): dilution calorimetry; (c) oxygen consumption calorimetry.</figcaption></figure>
<figure><img src="image_19.png" /><figcaption>Figure 17: heat release rate as function of time for test c112ii (open rack); (a): mass loss rate method; (b): dilution calorimetry; (c) oxygen consumption calorimetry.</figcaption></figure>

<figure><img src="image_20.png" /><figcaption>Figure 18: heat release rate as function of time for test d112ii (confined rack); (a): mass loss rate method; (b): dilution calorimetry; (c) oxygen consumption calorimetry.</figcaption></figure>
<figure><img src="image_21.png" /><figcaption>**Figure 19:** HRR given by empirical equation of section 4; (a) single medium isolated suitcase; (b) single large isolated suitcase; (c) two medium and one large suitcases in an open rack; (d) two medium and one large suitcases in a closed rack.</figcaption></figure>