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Numerical Investigation on the Asymmetric Flow Characteristics of Two Propane Fires of Unequal Heat Release Rate in Open Space

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Abstract

Multiple fires of unequal heat release rate (HRR) is a common fire scenario in real fire accidents. Differing from most previous research assumption of identical fire sources, the unequal HRR for those fire sources is regarded as more reasonable in reality. To explore the asymmetric flow characteristics surrounding multiple fires under such circumstances, simulations of two square propane burners with the same side length but different HRRs were carried out. The HRR combination and burner separation distance were varied. The results showed that the asymmetric flow characteristic was found in both the flame and the smoke plume zone. In the flame region, the flame morphology in terms of the tilt angle is a good parameter indicating the asymmetric flow characteristic. In general, the tilt angle of the small fire is larger than that of the big fire. The tilt angle of the small fire decreases while that of the big fire increases until equaling to each other under the HRR ratio reaching unity. In the smoke plume region, the smoke plumes from the small and the big fire will converge at a certain height with some shifting distance, which is another parameter indicating the asymmetric flow characteristic. Besides, the smoke plume merging process driven by asymmetric air entrainment can be divided into three stages, namely the I) Separate stage, II) Converging stage and III) Complete coalescence stage. Correlations of the converging & coalescence height suggesting the starting point of the II and III stages were proposed.

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Abbreviations

A:

The coefficient in Eq. (4)

b:

The plume radius (m)

bsmall :

The plume radius of the small fire (m)

bbig :

The plume radius of the big fire (m)

cp :

Specific heat capacity (kJ/(kgK))

D:

The burner side length (m)

D* :

The characteristic fire diameter (m)

De :

The circular pool fires equivalent diameter (m)

g:

Acceleration of gravity (9.81 m/s2)

Hc/r:

The heat liberated per unit mass of air entering the combustion reactions (kJ/kg)

Lf :

The flame height (m)

Lf, small :

The small flame height (m)

Lf, big :

The big flame height (m)

\(\dot{m}_{ent}\) :

The air entrainment rate below a certain height (kg/s)

\(\dot{m}_{g}\) :

The mass flow rate through a certain height (kg/s)

Qsmall :

The heat release rate of the small fire (kW)

Qbig :

The heat release rate of the big fire (kW)

S:

The separation distance between two burners (m)

T:

Temperature (K)

T0 :

Ambient temperature (K)

V :

The vertical velocity (m/s)

ZI-II :

The converging height (m)

ZII-III :

The coalescence height (m

α:

The entrainment constant

θsmall :

The tilt angle of the small fire (°)

θbig :

The tilt angle of the big fire (°)

λsmall :

The heat release rate per unit volume corresponding to the small flame height (kW/m3)

λbig :

The heat release rate per unit volume corresponding to the big flame height (kW/m3)

ρ0 :

Air density (kg/m3

big:

The big fire

f:

Flame

small:

The small fire

0:

Ambient

I-II:

From stage I to stage II

II-III:

From stage II to stage III

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Acknowledgements

This work was supported by National Natural Science Foundation of China (NSFC, No. 52036009 and Grant No. 51722605) and the China Postdoctoral Science Foundation (Grant No. 2019M652212).

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Authors and Affiliations

  1. State Key Laboratory of Fire Science, University of Science and Technology of China, JinZhai Road 96, Hefei, 230026, Anhui, China

    Zheng Wang, Longxing Yu & Jie Ji

  2. College of Environment and Resources, Fuzhou University, Fuzhou, 350116, China

    Longxing Yu

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Correspondence to Longxing Yu.

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Wang, Z., Yu, L. & Ji, J. Numerical Investigation on the Asymmetric Flow Characteristics of Two Propane Fires of Unequal Heat Release Rate in Open Space. Fire Technol 57, 2181–2203 (2021). https://doi.org/10.1007/s10694-021-01111-3

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