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    Polymer Testing 250142-9418/$ - see front matter q 2005 Elsevier Ltd. All rights reservKeywords: Functional analysis; Superposition; Bench-scale test; Heat release rate

    1. Introduction

    The heat release rate in a room fire has to be

    understood [1,2] in hazard assessment. This will give

    key information on the size of the fire; the rate of fire

    growth and, consequently, the release of smoke and

    toxic gases; the time available for escape or fire

    suppression and the type of suppressive action that is

    likely to be effective. Other attributes that define the fire

    hazard, such as the possibility of having a flashover fire,

    can be estimated.

    Different combustibles, including both movable and

    fixed fuel load [3], are stored in a building. The rate of heat

    release in burning these materials together should be

    estimated. Mostly likely, only the heat release rates of

    individual materials or single items are available. How the

    curves can be combined to estimate the resultant heat

    release rate curve [4,5] should be understood.

    The heat release rate per burning surface area of a

    material can be measured by a cone calorimeter [4,6].

    Models based on the cone calorimeter results have beenAbstract: In fire hazard assessment, the resultant heat release rate of burning different combustibles has to be known. The principle

    of superposition is commonly applied to estimate the total heat release rate from the individual curves measured for single items.

    Accuracy of such an approach will be studied with bench-scale tests in this paper.

    The heat release rate curves of burning each sample cube of polymethylmethacrylate (PMMA), polyvinyl chloride (PVC),

    polycarbonate (PC) and wood were first measured individually by a cone calorimeter. Radiative heat fluxes of 50 and 70 kW mK2

    were applied. After that, a PMMA cube was burnt with a cube of another material under the same heat flux. The resultant heat

    release rate curves of burning these two cubes were measured. Heat release rate curves of burning the single cube were used to

    estimate the resultant curves. The technique of fundamental analysis will be applied for comparing the predicted curves with the

    experiments. Importance of the parameter s for estimating the secant inner product cosine specifying the data points intervals will

    also be discussed.

    For the samples tested, it is observed that superposition gives good estimations of the total heat released curve if those for

    individual items were measured under the same radiative heat fluxes. However, the results will not be so good where the curves

    for each combustible were measured at different heat fluxes. This point is very important in estimating the possible heat release rate

    for a design fire.

    q 2005 Elsevier Ltd. All rights reserved.Property

    Superposition of he

    for combustibles w

    W.K. Chow

    Areas of Strength: Fire Safety Engineering, Research Centre for Fire

    Received 29 June 2005;delling

    release rate curves

    bench-scale tests

    , S.S. Han

    neering, The Hong Kong Polytechnic University, Hong Kong, China

    pted 2 September 2005

    (2006) 7582

    www.elsevier.com/locate/polytestdifferent applications such as for train compartments

    [79].

    ed.

    doi:10.1016/j.polymertesting.2005.09.016

    * Corresponding author. Tel.: C86 852 2765 7198.E-mail address: [email protected] (W.K. Chow).developed to predict the heat release rate for burning

    those materials in bigger rooms [5], and applied for

  • THR Z

    0

    Q0ctdt (2)

    Test A: Two sample cubes at two sides of the tray

    with arrangements:

    A1: PMMA and PC at 70 kW mK2, 25 mmunder the cone

    A2: PMMA and PVC at 70 kW mK2, 25 mmunder the cone

    A3: PMMA and PVC at 70 kW mK2, 50 mmunder the cone

    A4: PMMA and PVC at 50 kW mK2, 50 mmunder the cone

    A5: PMMA and wood at 50 kW mK2, 50 mmunder the cone

    Test B: PMMA cube only at one side of the tray with

    arrangements:

    B1: PMMA at 70 kW mK2, 25 mm under thecone

    B2: PMMA at 70 kW mK2, 50 mm under thecone

    B3: PMMA at 50 kW mK2, 50 mm under thecone

    Test C: PC cube only at one side of the tray with only

    Schematic view of heat fluxes

    Rfaff

    Fig. 1. Cone calorimeter tests.

    W.K. Chow, S.S. Han / Polymer Testing 25 (2006) 758276In this paper, how the resultant heat release rate

    of two different polymeric materials can be

    combined is explored. Polymethylmethacrylate

    (PMMA), unplasticized polyvinyl chloride (PVC),

    polycarbonate (PC) and oak wood widely used in

    the market as consumer products and construction

    materials are taken as examples. Samples of those

    materials were selected and cut into small cubes of

    size 20 mm. The samples were exposed to the same

    conditions in a cone calorimeter for measuring the

    heat release rates under heat fluxes of 50 and

    70 kW mK2. The samples were tested individually

    by themselves first. A sample PMMA cube was

    then burnt with another sample for comparing with

    the calculated heat release rate using Eq. (1).

    2. Samples tested

    Experimental measurements on one or two sample

    cubes of PMMA, PVC, PC and wood were conducted

    in a cone calorimeter. The samples were placed at the

    side of the cone tray as shown in Fig. 1. The radiative

    heat flux of the cone was set at 70 or 50 kW mK2. In

    following the procedures in the standard tests [13],

    samples were placed at 25 mm below the cone. In this

    paper, some samples were also tested by moving

    down to 50 mm below the cone [14]. This will give

    different ventilation conditions as in real fire

    scenarios. Different heat release rate curves can then

    be achieved.

    The testing arrangements as shown in Table 1 areThe combined heat release rate Q0T for burning twosamples A and B with heat release rates Q0A and Q0B issuggested to be [10]:

    Q0T Z Q0A CQ

    0B (1)

    This was tested before with a cone calorimeter

    [11,12].

    The heat release rate per unit area Q0ct curves ofdifferent materials measured with bench-scale tests in a

    cone calorimeter, time to ignition (TTI, in s), peak heat

    release rate per unit area (pkRHR, in kW mK2) and

    total heat released per unit area (THR, in MJ mK2) are

    estimated. Note that THR is given by:

    Nsummarized in the following:Test A

    Tests B, C, D, E

    100 mm20 mm cube

    100 mm

    Cone tray

    Thermal radiation

    R

    Rfc

    (a)

    (b)

    (c)one test:

  • Table 1

    Functional analysis on the superposition results

    Curves

    tested

    Para-

    meters

    A1: PMMACPC A2: PMMACPVC A3: PMMACPVC A4: PMMACPVC A5: PMMACwood

    Same

    heat flux

    Under different

    heat fluxes

    Same

    heat flux

    Under different

    heat fluxes

    Different

    heat flux

    Same

    heat flux

    Different

    heat flux

    Under different

    heat fluxes

    Same

    heat flux

    Under different

    heat fluxes

    Same

    heat flux

    B1CC1 B2CC1 B3CC1 B1CD1 B2CD1 B3CD1 B1CD2 B2CD2 B3CD2 B1CD3 B2CD3 B3CD3 B1CE1 B2CE1 B3CE1

    Q0ct Norm 0.13 0.21 0.46 0.17 0.31 0.55 0.19 0.23 0.51 0.70 0.5 0.22 0.82 0.52 0.15Cosine

    (sZ1)0.86 0.83 0.56 0.83 0.53 0.16 0.44 0.72 0.14 K0.07 K0.02 0.16 0.11 0.16 0.60

    Cosine

    (sZ2)0.90 0.88 0.61 0.89 0.59 0.18 0.49 0.78 0.16 K0.07 K0.01 0.18 0.12 0.19 0.65

    Cosine

    (sZ3)0.91 0.91 0.63 0.91 0.62 0.20 0.54 0.80 0.17 K0.07 K0.01 0.20 0.12 0.21 0.70

    Cosine

    (sZ4)0.92 0.93 0.65 0.93 0.65 0.21 0.59 0.82 0.19 K0.07 0.01 0.22 0.13 0.23 0.75

    Cosine

    (sZ5)0.93 0.93 0.66 0.94 0.68 0.23 0.63 0.83 0.20 K0.06 0.02 0.25 0.15 0.25 0.80

    THR Norm 0.08 0.16 0.30 0.16 0.25 0.39 0.06 0.14 0.31 0.05 0.51 0.04 0.61 0.41 0.11

    Cosine

    (sZ1)0.99 0.99 0.90 0.99 0.97 0.84 0.98 0.97 0.86 0.06 0.07 0.97 0.77 0.89 0.99

    W.K

    .C

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    .H

    an

    /P

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    esting

    25

    (20

    06

    )7

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    77

  • C1: PC at 70 kW mK2, 25 mm under the coneTest D: PVC cube only at one side of the tray with

    arrangements:

    D1: PVC at 70 kW mK2, 25 mm under thecone

    D2: PVC at 70 kW mK2, 50 mm under thecone

    D3: PVC at 50 kW mK2, 50 mm under thecone

    Test E: Wood cube only at one side of the tray with

    only one test:

    E1: Wood at 50 kW mK2, 50 mm under thecone.

    Each testing arrangement was tested several times to

    check the repeatability [15]. Only one typical set of

    results was used to study the superposition. Results of

    the heat release rate per unit area Q0ct measured in thecone and the total heat released per unit area THR for

    each test are shown in Figs. 26.

    In a real fire, combustibles placed together are

    exposed to different radiative heat fluxes. Therefore,

    different external radiative heat fluxes and separation

    distances among them should be assessed. For the two

    samples as tested in this paper, there was a constant heat

    flux emitted from the cone Rfc, a heat flux from the

    adjacent burning sample Rfa and a heat flux feedback

    from the flames Rff acting on the burning surface of the

    sample, as shown in Fig. 1c. The distances between the

    two samples might become shorter than 6 cm (even

    2 cm for PVC) due to melting and swelling upon

    burning. There might be stronger interaction between

    the two combustibles due to the shorter distance.

    Although Rfa might be higher than Rfc, both heat fluxes

    are likely to be less than Rff . Except at very high values,

    external heat fluxes such as Rfc would only be important

    in ignition. Effects of these couplings should be further

    studied quantitatively but are not included in this paper.

    PVC samples were difficult to ignite under lower

    1000

    1500

    2000

    2500 A1 PMMA+PCB1 PMMA at 70 kWm2C1 PC at 70 kWm2S Calculated

    B1

    C1

    S

    ase

    rate

    per

    uni

    t are

    a Q'

    c / k

    Wm

    2

    A1

    (a)

    0 100 200 300 4000

    100

    200

    300To

    tal h

    eat r

    elea

    sed

    per u

    nit

    Time / s

    D1

    Total heat released per unit area

    Fig. 3. Test A2.

    W.K. Chow, S.S. Han / Polymer Testing 25 (2006) 7582780 100 200 300 4000

    500

    Hea

    t rel

    e

    Time / sHeat release rate per unit area

    0 100 200 300 4000

    100

    200

    300

    400

    500A1

    B1

    C1

    S

    Tota

    l hea

    t rel

    ease

    d pe

    r uni

    t are

    a TH

    R /

    MJm

    2

    Time / sTotal heat released per unit area

    (b)Fig. 2. Test A1.0 100 200 300 4000

    500

    1000

    1500

    2000

    2500 A2 PMMA+PVCB1 PMMA at 70 kWm2D1 PVC at 70 kWm2S Calculated

    Hea

    t rel

    ease

    rate

    per

    uni

    t are

    a Q c

    ' / k

    Wm

    2

    Time / s

    D1

    B1

    S

    A2

    Heat release rate per unit area

    400

    500

    are

    a TH

    R /

    MJm

    2

    B1

    S

    A2

    (a)

    (b)heat fluxes. Therefore, higher heat fluxes of 50 and

  • W.K. Chow, S.S. Han / Polymer Testing 25 (2006) 7582 790 100 200 300 4000

    500

    1000

    1500

    2000 A3 PMMA+PVCB2 PMMA at 70 kWm2D2 PVC at 70 kWm2S Calculated

    B2

    A3

    S

    Hea

    t rel

    ease

    rate

    per

    uni

    t are

    a Q'

    c / k

    Wm

    2

    Time / s

    D2

    Heat release rate per unit area

    400

    500

    TH

    R /

    MJm

    2

    B2

    A3

    (a)

    (b)70 kW mK2 were used. As observed in the tests, PVC

    samples melted quickly and evaporated into fuel

    vapour. A large quantity of smoke with irritating

    smell was liberated upon exposure to the heat fluxes.

    Among the four samples tested, PVC was the most

    difficult to burn with the longest ignition time. Further,

    PVC cubes did not burn steadily with charring. Taking

    test A4 as an example, the heat release rate oscillated

    with several peaks.

    There was smouldering at first in burning the wood

    samples. Char was formed later at the burning stage.

    PMMA was ignited easily and burnt completely with a

    steady rate. The PC samples were also difficult to burn.

    However, once ignited, they burnt vigorously with

    smoke liberated. Except for PMMA, some residues

    were left after burning.

    Rate of heat transfer inside the sample from the

    external heat flux depends on the effective exposure

    area and the distance away from the conical heater.

    0 100 200 300 4000

    100

    200

    300

    Tota

    l hea

    t rel

    ease

    d pe

    r uni

    t are

    a

    Time / s

    S

    D2

    Total heat released per unit area

    Fig. 4. Test A3.0 200 400 6000

    500

    1000

    1500

    2000B3

    S

    A4 PMMA+PVCB3 PMMA at 50 kWm2D3 PVC at 50 kWm2S Calculated

    D3

    A4

    Hea

    t rel

    ease

    rate

    per

    uni

    t are

    a Q'

    c / k

    Wm

    2

    Time / sHeat release rate per unit area

    300

    400

    500

    nit a

    rea T

    HR

    / M

    Jm2

    B3

    S

    A4

    (b)

    (a)The heat flux might be reduced by 515 kW mK2 when

    the distance of the sample from the heater was moved

    from 25 to 50 mm when set at 50 and 70 kW mK2.

    There were differences in the exposure areas for the

    different samples upon burning. The melted PVC is an

    obvious example. Results of heat release rate per unit

    area deduced from the cone would be affected. The

    exposure areas for all sample cubes were taken to be the

    same upper surface area of 4 cm2. The accuracy of the

    heat release rate would be affected by the above factors.

    However, those effects should be the same to all

    samples, giving very little deviations in judging the

    superposition principle.

    3. Superposition of heat release rate curves

    Whether the individual heat release rate curves of

    two different materials can be added together (i.e.

    superposition) to give the resultant heat release rate

    while burning both of them will be assessed by the

    measured results [2,1012,16,17].

    0 200 400 6000

    100

    200

    Tota

    l hea

    t rel

    ease

    d pe

    r u

    Time / s

    D3

    Total heat released per unit area

    Fig. 5. Test A4.

  • W.K. Chow, S.S. Han / Polymer Testing 25 (2006) 7582800 200 400 6000

    500

    1000

    1500

    2000 A5 PMMA+WoodB3 PMMA at 50 kWm2E1 Wood at 50 kWm2S Calculated

    B3

    S

    E1

    A5

    Hea

    t rel

    ease

    rate

    Q' c /

    kW

    m2

    Time / sHeat release rate per unit area

    400

    500

    area

    TH

    R /

    MJm

    2

    B3

    (a)

    (b)From the heat release rate per unit area curves Q0CAand Q0CB of two samples A and B, the transient heatrelease rate per unit area estimated Q0CTE is:

    Q0CTEt Z 12

    Q0CAt CQ0CBt

    (3)

    Experimental curve on the heat release rate per unit

    area for burning the samples A and B Q0CTt under thesame heat flux and the calculated curve Q0CTEt fromEq. (3) are compared in Figs. 26.

    Adding the heat release rate curves of two

    combustibles by superposition is useful in predicting

    real fire scenarios when the combustibles are not placed

    too close to each other. The results can at least be taken

    as a minimum estimation.

    There might be some other effects which are more

    obvious for bigger fires. A correction factor might be

    required in using superposition. Assuming these effects

    can be neglected under a specified standard external

    heat flux higher than normal heat fluxes, the net heat

    calculated:

    0 200 400 6000

    100

    200

    300

    Tota

    l hea

    t rel

    ease

    d pe

    r uni

    t

    Time / s

    S

    E1

    A5

    Total heat released per unit area

    Fig. 6. Test A5. The parameter norm is a measure of the relativedifference in magnitude of the two curves.

    The parameter inner product or cosine describes theangular difference between the resultant vectors to

    provide a quantitative measure on the similarity of

    the curve shape.

    For good agreement between the experimental and

    predicted curves, the value of the norm is expected to be

    close to zero, and the value of the cosine is expected to

    be close to one.

    Following the recommendation by Peacock et al.

    [18], the Euclidean norm is calculated by the ith

    experimental and model values Ei and mi at the ith timerelease rate for one sample by burning it alone will be

    the same as that from burning it together with other

    combustible under the same conditions. Therefore, the

    upper limits of predicted results can be determined by

    applying the results tested under the same standard

    external heat fluxes. Test results of each sample

    exposed to the same conditions could be added, though

    more tests should be carried out to confirm this.

    Another method is to calculate the increase in heat

    flux by the adjacent burning item. The geometries of the

    two combustibles and the relevant flames, the variable

    heat flux and other potential factors should be

    considered. These aspects will be further reported

    separately later.

    4. Functional analysis

    Instead of comparing the estimated results from

    superposition with the experiments in qualitative terms

    such as good, satisfactory or bad, functional

    analysis proposed for evaluating fire models by

    Peacock et al. [18] is used. Fire model predictions

    have been compared with test data by Friday et al. [19]

    with such approach.

    As both experimental and predicted data can be

    described by transient curves, functional analysis

    would quantify the difference between two curves in

    terms of magnitude and shape. The data points within

    each curve are described by vectors, summing them up

    would give a resultant single vector for each curve. The

    distance between the resultant vectors for the predicted

    and measured curves is the error. This error can be

    normalized to provide a relative difference, or norm,

    between the curves. The following parameters will beincrement ti with s data points to be considered at each

  • Norms and inner product cosines were calculated for

    the curves of Q0 t and THR for each case under the

    indicating very good predictions on curve magnitude.

    The shapes of the curves are in fact very close, as shown

    in the figures.

    For s equal to 1, the computed values of cosine at

    higher heat fluxes are higher than 0.72. However, the

    values of cosine for Q0ct are 0.16 for test A4 and 0.60

    experiment.

    7. Conclusions

    Samples of PMMA, PVC, PC and wood in different

    From the above study, the heat release rate of

    burning two material samples together can be estimated

    tiK1miKs2=s2tiKtiK1

    W.K. Chow, S.S. Han / Polymer Testing 25 (2006) 7582 81(with wood at 50 kW mK2) for test A5. A possible

    reason might be because wood and PVC samples did

    not burn steadily under the lower heat flux of

    50 kW mK2.

    Values of cosine would be higher for higher values

    of s. For example, the values of cosine for tests A4 and

    A5 would be changed to 0.25 and 0.80, respectively, by

    taking s as 5.

    Results on comparing the curves with functionalc

    same radiative heat fluxes. The values were computed

    over the burning duration period of 400 and 600 s for

    radiative heat fluxes of 70 and 50 kW mK2,

    respectively.

    Functional analysis results of the point-to-point

    comparison are presented in Table 1. It is observed

    that the values of the norm are lower than 0.23,increment inside as:

    jEKmjjEj Z

    PniZ1

    EiKmi2s

    PniZ1

    Ei2s (4)

    The secant inner product cosine is:

    The parameter s R1 would smoothe the results togive better estimates of large-scale differences. Higher

    value of s might not overcome the effects of small-scale

    noise between dense data, depending on the shapes of

    the curves. Values of s will be varied as 1, 2, 3, 4 and 5

    in this paper to investigate its effect on the secant inner

    product cosine.

    5. Superposition of curves under the same radiative

    heat fluxes

    E;mh ijEj,jmj Z

    PniZsC1

    EiKEiKsmi KmiKs=s2ti KPniZsC1

    Ei KEiKs2=s2tiKtiK1,Pn

    iZsC1mi K

    sanalysis suggested that superposition is better for testsby simple addition, i.e. by superposition of the curves

    measured for each sample under the same heat flux.arrangements were tested with a cone calorimeter under

    different heat fluxes and distances from the conical

    heater. Several tests were repeated for each testing

    arrangement to ensure its repeatability. One typical set

    of results was used to study the superposition.with the distance and exposure area. The result of one

    sample tested in other conditions can be taken as a

    reference curve, which might be varied under the same

    initial conditions. Deviation of the calculated results by

    superposition can be estimated by functional analysis.

    If the curves under different heat fluxes are added

    together, say B3 (under 50 kW mK2) with C1 (under

    70 kW mK2) instead of B1 with C1, both the norm and

    cosine compared with test A1 deviated from the

    matching value of 0 and 1.0. The values of norm and

    cosine are 0.46 and 0.56 respectively for B3 with C1

    when s is 1. The value of cosine only increased to 0.66

    when s is 5.

    For curves under the same heat flux but at different

    distances below the cone, say combining B2 of

    70 kW mK2 for 50 mm and C1 for 25 mm, the values

    of norm and cosine are 0.21 and 0.83, respectively. The

    value of cosine increased up to 0.93 when s is 5.

    Therefore, combining the curves measured under the

    same heat flux but at different distances below the cone

    would not give results deviating so much from theunder higher heat fluxes, under which the combustion

    was more complete.

    6. Curves under different radiative heat fluxes

    For real fire scenarios, the heat release rate of burning a

    combustible will be affected by the total heat feedback

    and the total external heat fluxes, which might be varied

    (5)Values of norm and cosine gave better agreement for

  • exposed to higher external heat fluxes, such as a

    W.K. Chow, S.S. Han / Polymer Testing 25 (2006) 758282external thermal radiation might give more complete

    combustion. The results can be taken as a minimum

    estimation. The coupling effects between the combus-

    tibles, external heat and ambient conditions should also

    be considered.

    Many more tests, especially full-scale burning

    tests [16,20,21] should be carried out on those

    samples to support the application of the super-

    position principle.

    Acknowledgements

    This paper is supported by the RGC project

    Determination of the concentration needed for extin-

    guishing fires with clean agent heptafluoropropane

    (FM200) under Grant No. B-Q669.

    References

    [1] C. Huggett, Estimation of rate of heat release by means

    of oxygen-consumption measurements, Fire Mater. 4 (1980)

    6165.

    [2] R.D. Peacock, R.W. Bukowski, W.W. Jones, P.A. Reneke, V.

    Babrauskas, J.E. Brown, Fire safety of passenger trains: a review

    of current approaches and of new concepts, NIST Technical

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    [3] W.K. Chow, C. Cheung, Aspect of fires for factories in Hong

    Kong, J. Appl. Fire Sci. 5 (1) (1996) 1732.

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    they are placed not so closely together with relatively

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    However, the results of superposition might be

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    placed close to each other and with lower external heat

    fluxes from the ceiling, walls and smoke layer. Highthe higher heat flux of 70 kW mK2. Examples of

    burning PMMA with PVC, PC or wood demonstrated

    this. Superposition can further be applied to burning

    multiple combustibles. The ignition time, burning time

    and peak heat release rate are key points to be

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    Functional analysis suggested that the predicted

    curves agreed better with the measured curves for the

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    [8] W.K. Chow, Fire safety in train vehicle: design based on

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    [9] V.P. Dowling, N. White, Fire sizes in railway passenger saloons,

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    Superposition of heat release rate curves for combustibles with bench-scale testsIntroductionSamples testedSuperposition of heat release rate curvesFunctional analysisSuperposition of curves under the same radiative heat fluxesCurves under different radiative heat fluxesConclusionsAcknowledgementsReferences