Research On The Behavior Characteristics Of Rectangular Fire Under Different Restricted Conditions

Predecessors conducted a series of studies on the combustion behavior of rectangular fires under open space conditions and found that some models established for square and circular fire sources are not suitable for rectangular fires.In addition,the fire environment is complex,and the development process of the fire will be restricted by certain environmental factors in many cases.For example,the existence of buildings such as walls will restrict air entrainment into the fire plume,and the flame shape and combustion heat feedback mechanism will be different from those in an open space.The heat generated by a fire in a confined space is not easy to dissipate,which may further accelerate the development of the fire.Therefore,it is necessary to systematically study the combustion behavior and smoke movement characteristics of rectangular fires in different restricted environments.In this paper,a combination of experimental research,numerical simulation,and theoretical analysis is used to systematically study rectangular fires in open spaces,along the sidewall,in the corner,and in long confined spaces.The related laws of flame pulsation frequency,flame height,radiant heat feedback,plume temperature,and velocity distribution,and smoke movement characteristics are studied.It mainly includes the following four aspects:This paper conducts experimental research on rectangular gas fires with different aspect ratios(n)in open spaces.The temperature and velocity distributions of rectangular fire plumes on horizontal cross-sections at different heights are obtained through experimental measurements.The temperature on the center line of the rectangular fire source in the continuous flame area is a certain value,and the velocity increases with the increase of the height(Z).In the intermittent flame zone,the plume velocity is not affected by Z,and the temperature decreases with the increase of Z.The temperature and velocity distribution model of the center line of the rectangular fire source is established.An empirical formula for calculating the Gaussian half-width of the plume of square and rectangular fire sources is proposed.Finally,according to the proposed temperature and velocity distribution model,the plume mass flow prediction formulas for continuous and intermittent flame regions are obtained through integral calculation.It is obtained that when the heat release rate of the fire source is the same,the plume mass flow rate at the same height increases with the increase of n.The experiment studied rectangular pool fires(ethanol and n-heptane)in open space and with sidewall.The law of flame height and mass loss rate under different restricted conditions with the aspect ratio of the fire source is obtained.The flame height of ethanol fuel in open space and with sidewall decreases with the increase of n.The flame height of n-heptane fuel in an open space decreases with the increase of n,but it shows the opposite law when the fire is attached to the wall.Therefore,the dimensionless flame height expressions of the two fuels under different restricted conditions are established respectively.Through theoretical analysis and experimental data,the change law of conduction heat feedback and radiant heat feedback of rectangular pool fires under different restricted conditions is revealed.The conductive heat feedback fraction of ethanol and n-heptane pool fires increases with aspect ratios.The radiant heat feedback fraction of an n-heptane pool fire is greater than that of an ethanol pool fire.Carry out rectangular propane gas fire experiments in the case of against the wall and the corner.The changes of the flame pulsation frequency and flame height of the rectangular fire with the aspect ratio of the fire source under two restricted conditions are studied.It is found that when the fire source is against the wall:the flame pulsation frequency and the effective entrainment perimeter of the fire source first increase and then decrease with the increase of n,and the flame height first decreases and then increases with the increase of n.When the fire source is against the corner of the wall:the flame pulsation frequency and the effective entrainment perimeter of the fire source increase with the increase of n,and the flame height decreases with the increase of n.The effective entrainment perimeter of the fire source was further introduced to establish a flame pulsation frequency model and a flame height prediction model suitable for in an open space,with sidewall,and in the corner.The reliability of the model is verified by comparison with previous experimental data.Through a combination of numerical simulation,theoretical analysis and experimental comparison,the entrainment behavior of rectangular fire sources under the condition of with sidewall,and in the corner is quantitatively studied,and a prediction formula for rectangular fire plume entrainment is established.The shapes of the fire source may affect the characteristics of smoke movement in tunnels.In the current study,a numerical investigation is conducted to explore the effect of fire source shape on the smoke movement and temperature distribution in a full-scale road tunnel.Results show that with increasing heat release rate and decreasing n,the impinging flow on the tunnel ceiling transits from buoyancy plume to continuous flame and the smoke becomes more difficultly to be uniform along the longitudinal direction,resulting in the further starting position of the one-dimensional spread stage.For a certain HRR,the smoke mass flow rate decreases with decreasing n,which is proportional to the perimeter of the fire source.As n decreases from 11 to 1,the smoke mass flow rate decreases for 23%.Using the dimensional analysis method,the relationship between the starting position of the one-dimensional spread stage and the aspect ratio of the fire source,and the relationship between the smoke mass flow rate and the aspect ratio of the fire source are obtained.Moreover,a correlation of the longitudinal smoke temperature distribution under the tunnel ceiling is developed,with the temperature at the initial position of the one-dimension spread stage as the reference value.