Research On The Combustion Characteristics Of Rectangular Fires With Different Aspect Ratios

There are many different fuel source shapes in practical fire scenarios,such as the fire line in forest fire spread is always seen as the rectangular or line fire plume,the fire spills from a rectangular opening in a building fire is a typical square or rectangular fire plume.Compared with the previously studied round fire plumes,the flame combustion and flow behaviors with different fuel source shapes or aspect ratios will undergo complex changes.What’s more,the existence of a nearby fire source may also trigger the combustion of double fires.The interaction of two adjacent fires results in significant differences of the combustion characteristics compared with the single fire source.At present,the study on the flame combustion characteristics is mostly focused on axisymmetric round fires,while the study of the flame combustion characteristics of square,rectangular and double rectangular fire plume is relatively rare.Research on combustion characteristics of single rectangular and square fire sources and double rectangular fire sources can enrich existing combustion theories and provide basic data for fire simulation and even fire prevention and control.Therefore,an experimental and theoretical investigation of the combustion behaviors of square buoyant diffusion flame and jet flame is conducted.The different combustion and flow behaviors between square fire and round fire is analyzed.At the same time,by expanding the scale of the long and short sides of the fire source,the flame combustion characteristics of a single rectangular fire plume with different aspect ratios are studied,and the influence of aspect ratio on axial and radial distributions of velocity and temperature as well as the flame height is discussed.Based on the study of single fire source,we conduct two rectangular fire experiments to extend the fire scenarios,and analyze the changes of flame combustion and entrainment flow caused by the fire source shape,aspect ratio and the adjacent fire source.Finally,the flame height,temperature and velocity distributions are predicted and characterized.The detailed work and results are summarized as follows:(1)The radial distributions of the temperature and axial velocity and the variation of the half-width indicate that,as the square fire progresses downstream from the flame base,its cross-sectional shape undergoes a square-to-round transition.In this process,the self-induced axis-switching phenomenon intensifies the square fire’s turbulence,manifested as more significant axial and radial temperature fluctuations than the round fire.This enhanced turbulent mixing promotes flame burning,resulting in a greater axial temperature and velocity of the square fire than the round fire.At different fire scales,the critical height at which a square fire completes the axis-switching first increases and then slows down with increasing heat release rate,which can be well correlated by the dimensionless heat release rate normalized by the side length.Based on the fire plume theory and experimental data analysis,the applicability of the classic flame height model established by Quintiere and Grove in square fires with different burner scales and heat release rates is verified.It is also found that the near-field entrainment constant of square fire is significantly increased compared with that of round fire,which is closely related to the increased turbulent mixing in square fire.The geometric characteristics of flame show that,with the increase of exit velocity,the square fire will experience the evolution from laminar to turbulent flow and finally fulldeveloped turbulent flow from laminar flow to turbulent flow.By introducing the entrainment correction factor,the prediction model of flame height of square jet fire is established,which is consistent with the experimental data.Besides,it is found that the lift-off height of square jet fire still varies linearly with exit velocity,but depends on burner scales and fuel types.Then a unified linear expression derived from the mixedness-reactedness flamelet theory is proposed to predict the lift-off height of square jet fire.(2)The axial and radial distributions of velocity and temperature as well as the half-width all indicate that the flame flow in rectangular fire plume evolves from threeto two-dimensional as the aspect ratio increases from unity to higher values.In this process,the asymmetric entrainment in the length and width directions of the rectangular fire is verified to play an important role in the flow development.Based on the air entrainment theory and flame height data,the perimeter diameter of the fire source is verified to be a reasonable characteristic length scale to normalize the flame length,a new flame length model is thus derived,which reasonably well correlates with the flame height data under different aspect ratios,initial heat release rates,and scales.A shape factor extracted from the model can be used as a measure of the aspect ratio effect separately.A dimensionless height coupled with the shape factor well correlates the axial temperature and velocity data under different aspect ratios and initial heat release rates.(3)For the interaction experiments of two rectangular fire source,the flame merging stage is divided based on the flame combustion phenomenon and the analysis of flame merging probability,and then the flame merging probability model is established with the introduction of dimensionless burner distances.The radial temperature evolution law shows that the flame structure of the double rectangular fires presents typical three-regions variation in the axial height direction.On this basis,the changes of flame air entrainment mechanism in different interaction regions are revealed by the variation law of the critical transition height and the temperature and velocity distribution.At the same time,a uniform flame height prediction and characterization model under different merging stages is established by introducing the equal-perimeter diameter as the characteristic length scale,which can characterize the effects of aspect ratio,burner distances and heat release rates on flame height.By analyzing the axial temperature in the fully-merged region of the two rectangular fires,an empirical virtual point source expression is given.Then the unified expression of axial excess temperature in the fully-merged region is established based on the virtual point source modification.