An introduction to heat transfer

Biomass pyrolysis kinetics: A comparative critical review with relevant agricultural residue case studies

Polymer/layered silicate (clay) nanocomposites: An overview of flame retardancy

Combustion and gasification rates of lignocellulosic chars

Thermal decomposition kinetics of natural fibers: Activation energy with dynamic thermogravimetric analysis

Kinetic modeling of thermal decomposition of natural cellulosic materials in air atmosphere

This paper gives an experimental and methodological investigation on the behaviors of square fire arrays which are composed of 3 × 3 to 7 × 7 n-heptane fires initiated from fuel pans of 5 cm in diameter and 2 cm in height. It is intended to develop a burn-out time (BOT) data analysis method to analyze the interaction effects (which may induce fire merging and fire whirls) among the multiple fires. In 26 fire tests the fire point spacing D varied from 20 to 50 cm for each array size and in several cases shear flow was added from one side of the array. By considering the flame height L a reasonable critical condition for initiation of fire merging was implied to be D/L = 0.29 − 0.34, which is independent of the fire array size and fire point spacing. By burn-out time data the Interaction index I (m) and Interaction link index A (m, n) were defined to characterize the fire interactions. The assumptions essential to solve the equation system of I (m) = ∑n A (n, m) were examined in detail, whereby the equation system was solved. The analysis showed that the burn-out time data analysis realizes a quantitatively reasonable comparison of the fire interaction effects, thus indicating that it is reasonable to regard the burn-out time as a measure for the average burning rate for each specific fire point. An apparent criterion of BOT(m)/BOTR = 0.5 was summarized to identify whether any fire point m will be completely involved in fire merging (where BOTR is the burn-out time of the free burning reference fire point). It was implied that the interaction effect imposed on any fire is mainly ascribed to its adjacent four fires. The effects of shear flow to fire burning and occurrences of fire whirls were also discussed.

Burn-out time data analysis on interaction effects among multiple fires in fire arrays

A quantitative description and interpretation of the global burning rate for square fire arrays consisting of multiple equidistant fires is presented. Altogether 38 experiments of n-heptane fire arrays (3 × 3–7 × 7, and 15 × 15) were performed, with the fire spacing varying within 5–50 cm and the fuel pans being 5 cm in diameter and 2 cm in height. The global average burning rate of fire arrays is extracted from burn-out time data, and is shown to be intimately connected with fire spacing and fire array size in a complex way. Analysis shows that the complexity arises from the competition between two fire interaction mechanisms: heat feedback enhancement and air entrainment restriction. When the fires are close to each other within certain ranges, the effect of heat feedback enhancement is more significant than the air entrainment restriction, inducing more intense burning compared with one single fire with the same fuel area. With increasing fire spacing, the heat feedback decays rapidly than the improvement of air entrainment. The global average burning rate shows complex fluctuations with increase of fire array size, and especially the fire array of 5 × 5 involves remarkable competition between the two mechanisms. Further, it is found that in the cases of fire spacing 0.2–0.5 m, the global average burning rate is linearly correlated with a well-defined dimensionless fire area ratio which characterizes the fuel load. Analysis shows that the linear correlation results from competition between the two fire interaction mechanisms. In such cases, the global burning rate is not apparently affected by the phenomena of fire merging and fire whirls. The deviation from linear correlation in other cases is also discussed. Finally, three regimes, each possessing specific fire interaction mechanisms, are distinguished for the variation of the global average burning rate within the whole range of fire area ratio.

Global burning rate of square fire arrays: Experimental correlation and interpretation

The medium-scale fire whirl was extensively investigated by experimental means, in order to establish correlations of the burning rate, flame height and flame temperature of fire whirl, and to clarify the difference between fire whirls and general pool fires. Experimental observations and data confirmed that a free burning fire whirl is a highly stable burning phenomenon with large quasi-steady periods. Burning rates of fire whirls depend on pool diameter similarly to those of general pool fires; however the transition turbulent burning occurs sooner as the pool diameter increases. The lip height seems to have little effect on the burning rate of fire whirls. The correlation H ∗ = K · ( Q ˙ ∗ · Γ ∗ 2 ) m was proposed to couple the height of fire whirl to the fire release rate and ambient circulation. It correlates the data from both this work and the literature. Radial temperature profiles in the continuous region of the fire whirl were confirmed to be hump-type, implying the existence of fuel-rich inner core. The pool diameter and heat release rate do not significantly affect the radial temperature profiles in non-dimensional radial coordinates. It was found that the fire plume of fire whirl involves three distinct zones just like that of pool fire, but with different normalized ranges. Fire whirls maintain a higher ratio of continuous flame height to the overall flame height, and also higher maximum centerline excess temperature in continuous flame region, as compared to general pool fires. It was further demonstrated that the fire whirl plume at its origin behaves like a turbulent jet with moderate swirling, and then tends to become buoyancy dominated downstream, with slight swirling. With an increase in dimensionless height adjusted by the plume origin, the plume centerline excess temperature decays rapidly and approaches the theoretical value of −5/3 for free buoyancy plume.

Naian Liu

Papers

Kinetic modeling of thermal decomposition of natural cellulosic materials in air atmosphere

Burn-out time data analysis on interaction effects among multiple fires in fire arrays

Global burning rate of square fire arrays: Experimental correlation and interpretation

Experimental research on combustion dynamics of medium-scale fire whirl

Ignition of expandable polystyrene foam by a hot particle: An experimental and numerical study