BSI PD CEN/TR 16988:2016

1.1 General

The measuring technique of the SBI (single burning item) test instrument is based on the observation that, in general, the heats of combustion per unit mass of oxygen consumed are approximately the same for most fuels commonly encountered in fires (Huggett [12]). The mass flow, together with the oxygen concentration in the extraction system, suffices to continuously calculate the amount of heat released. Some corrections can be introduced if CO ₂, CO and/or H ₂O are additionally measured.

1.2 Calculation procedure

1.2.1 Introduction

The main calculation procedures for obtaining the HRR and its derived parameters are summarized here for convenience. The formulas will be used in the following clauses and especially in the clause on uncertainty.

The calculations and procedures can be found in full detail in the SBI standard [1].

1.2.2 Synchronization of data

The measured data are synchronized making use of the dips and peaks that occur in the data due to the switch from ‘primary’ to ‘main’ burner around t = 300 s, i.e. at the start of the thermal attack to the test specimen. Synchronization is necessary due to the delayed response of the oxygen and carbon dioxide analysers. The filters, long transport lines, the cooler, etc. in between the gas sample probe and the analyser unit, cause this shift in time.

After synchronization, all data are shifted so that the ‘main’ burner ignites - by definition - at time t = 300 s.

1.2.3 Heat output

1.2.3.1 Average heat release rate of the specimen (HRR _30s)

A first step in the calculation of the HRR contribution of the specimen is the calculation of the global HRR. The global HRR is constituted of the HRR contribution of both the specimen and the burner and is defined as

[Formula removed.]

where

• HRR_total(t): is the total heat release rate of the specimen and burner (kW);
• E′: is the heat release per unit volume of oxygen consumed at 298 K, = 17 200 (kJ/m3);
• V_D298(t): is the volume flow rate of the exhaust system, normalized at 298 K (m3/s);
• x_{a_O2}: is the mole fraction of oxygen in the ambient air including water vapour;
• ϕ(t): is the oxygen depletion factor.

ϕ (t) The last two terms x_{a_O2} and [Formula removed.] express the amount of moles of oxygen, per unit volume, that have chemically reacted into some combustion gases. Multiplication with the volume flow gives the amount of moles of oxygen that have reacted away. Finally this value is multiplied with the ‘Huggett’ factor. Huggett stated that regardless of the fuel burnt roughly a same amount of heat is released.

The volume flow of the exhaust system, normalized at 298 K, V_D298(t) is given by

[Formula removed.]

where

• c: [Formula removed.]
• A: is the area of the exhaust duct at the general measurement section (m2);
• k_t: is the flow profile correction factor; converts the velocity at the height of the bi-directional probe in the axis of the duct to the mean velocity over the cross section of the duct;
• k_?: is the Reynolds number correction for the bidirectional probe, taken as 1,08;
• Δp(t): is the pressure difference over the bi-directional probe (Pa);
• T_ms(t): is the temperature in the measurement section (K).

The oxygen depletion factor ϕ(t) is defined as

[Formula removed.]

where

• xO₂(t): is the oxygen concentration in mole fraction;
• xCO₂(t): is the carbon dioxide concentration in mole fraction;
• Ys...Zs: mean taken over interval Y s to Z s.

The mole fraction of oxygen in ambient air, taking into account the moisture content, is given by

[Formula removed.]

where

• xO₂(t): is the oxygen concentration in mole fraction;
• H: is the relative humidity (%);
• p: is the ambient pressure (Pa);
• Tms(t): is the temperature in the general measurement section (K).

Since we are interested in the HRR contribution of the specimen only, the HRR contribution of the burner should be subtracted. An estimate of the burner contribution HRR_burner(t) is taken as the HRR_total(t) during the base line period preceding the thermal attack to the specimen. A mass flow controller ensures an identical HRR through the burners before and after switching from primary to the main burner. The average HRR of the burner is calculated as the average HRR_total(t) during the base line period with the primary burner on (210 s ≤ t ≤ 270 s):

[Formula removed.]

where

• HRRav_burner: is the average heat release rate of the burner (kW);
• HRRtotal(t): is the total heat release rate of specimen and burner (kW).

HRR of the specimen

In general, the HRR of the specimen is taken as the global HRR, HRR_total(t), minus the average HRR of the burner, HRR_{av_burner}:

For t > 312 s:

[Formula removed.]

where:

• HRR(t): is the heat release rate of the specimen (kW);
• HRRtotal(t): is the global heat release rate of specimen and burner (kW);
• HRRav_burner: is the average heat release rate of the burner (kW).

During the switch from the primary to the main burner at the start of the exposure period, the total heat output of the two burners is less than HRR_{av_burner} (it takes some time for the gas to be directed from one burner to the other). Formula (24) gives negative values for HRR(t) for at most 12 s (burner switch response time). Such negative values and the value for t = 300 s are set to zero, as follows:

For t = 300 s:

[Formula removed.]