Investigation Of Melt Flow Dropping And Combustion Characteristics Of Flammable Materials In Ship Cabin Fires

Polymers such as flexible polyurethane foam(FPUF),polyethylene,plexiglass,etc.are present in the passenger compartment of the ship.When these materials combust,their melting products exhibit flow and dropping behaviour that can accelerate the fire spread and ignite other nearby flammable materials.This study focuses on investigating the dropping combustion behaviour of typical solid materials in ship cabin fires,with FPUF as the representative fuel.The study takes into account the complex marine air environment and the structural characteristics of ship passenger cabins,utilizing small-scale experiments,mesoscale experiments,and numerical simulations to analyze the combustion characteristics of fire sources involved in ship passenger cabin fires.The main work and conclusions of this work are as follows.(1)Study of pyrolysis mechanism and kinetic parameters of FPUF and its molten liquidA thermogravimetric analysis experiment(TG)was conducted under a nitrogen atmosphere to investigate the pyrolysis process of FPUF and its molten liquid.To obtain the pyrolysis kinetic parameters,a mathematical model was constructed using the global search algorithm(GS).The results demonstrate the efficiency and accuracy of the model in predicting single and multi-step pyrolysis kinetic parameters.The analysis revealed that FPUF undergoes pyrolysis at around200°C,producing a molten stream comprising polyol and carbodiimide(CD)in proportions of approximately 83% and 17%,respectively.The polyol begins to pyrolyze at around 300°C,whereas the CD begins to pyrolyze at approximately 310°C.(2)Study of the dropping behaviour of FPUF molten fluidAn experimental rig was developed to investigate the dropping behaviour of FPUF molten fluids during combustion,under both conventional air and saline humid air environments.The study reveals that melt droplets with a diameter of less than 0.7mm float during the dropping process.Moreover,there is a critical dropping diameter for droplet dropping behaviour,with droplets between 0.7mm and the critical dropping diameter dropping onto the pallet without a flame,and droplets larger than the critical dropping diameter dropping onto the pallet with a flame.In the foggy environment,the increase in relative humidity reduces the flow rate of the molten fluids,the number of drops,and the probability of droplets in the tray being flamed,while increasing the acceleration of the droplets.However,the saline environment provided by the brine of 0 ～ 2% salinity does not significantly affect these parameters.The study also provides a theoretical analysis of the critical drop diameter of the droplets based on the physical parameters of the molten fluids,taking into account the influence of dropping height and humidity.The results indicate that the critical dropping diameter is positively correlated with the dropping height.When the relative humidity is high(i.e.,in a foggy environment with relative humidity > 90%),the critical dropping diameter of the droplet increases with increasing relative humidity.On the other hand,when the relative humidity is low(relative humidity < 80%),the effect of relative humidity on the critical dropping diameter of the droplet is negligible.(3)Study of FPUF dropping combustion characteristics in cabin firesAn experimental rig was developed to investigate the dropping behaviour of FPUF molten fluids during combustion,under both conventional air and saline humid air environments.The results demonstrate that FPUF dropping combustion behavior significantly increases the mass loss rate of FPUF combustion.Raising the fire source height can reduce the temperature of the lower flue gas layer.When the fire source height exceeds 0.50 m,the flame topping phenomenon leads to an increase in the mass loss rate of the fuel,total mass and mass flow rate of the droplets,and upper flue gas temperature parameters.On the other hand,fire height has no significant effect on these parameters when it is ≤ 0.50 m.Moreover,in a mist environment with a relative humidity of90%,the peak mass loss rate,the total mass of the droplet population,and the mass flow rate of FPUF combustion are all reduced.Furthermore,in a salt mist environment with a humidifier solution salinity of 0 ～ 2%,a 1% increase in salinity results in an approximate 3%,5%,and 5%decrease in peak mass loss rate,the total mass of the dropping particle population,and peak mass flow rate,respectively.(4)Numerical modeling of molten flow droplet combustion of FPUFBased on the existing two-layer model of FPUF no-drop combustion and incorporating the experimental findings of TG and cone calorimeter(CONE),a revised model for FPUF no-drop combustion in both piloted ignition and non-piloted ignition modes was formulated.Furthermore,based on an investigation of droplet-dropping behavior and a droplet evaporation combustion model,a droplet-dropping model was developed.The modified and droplet-dropping models were then combined to establish a numerical model for FPUF melt droplet combustion.The results show that the no-drop combustion modified model agrees well with the experimental results in piloted ignition mode,while the droplet-dropping model better predicts droplet behavior.The maximum discrepancy of the predicted heat release rate(HRR),flue gas temperature,CO concentration,and HCN concentration from the experimental results obtained by the numerical model for FPUF melt droplet combustion does not exceed 12%,8%,30%,and 30% respectively.(5)Study of the effect of FPUF dropping combustion on fire spread in the cabinThe numerical model for FPUF melt droplet combustion was utilized to investigate the effects of no-dropping,dropping,and dropping ignition modes,as well as the height of the upper fire source on HRR,airflow,temperature distribution,CO concentration distribution,HCN concentration distribution,and safe escape height for the aisle.The results indicate that the HRR curves of FPUF combustion exhibit a bimodal pattern in the dropping ignition mode,whereas they are single-peaked in the other modes.Moreover,the second peak HRR is significantly greater in the dropping ignition mode than in the dropping and no-dropping modes due to the flame of the lower FPUF combustion touching down with the grid.The average temperature,CO concentration,and HCN concentration at the centre sections of the fire source and aisle are found to be highest in the dropping ignition mode and lowest in the no-dropping mode,in line with HRR.The influence of upper-layer FPUF height on the peak values of the HRR curve in both no-dropping and dropping modes,as well as the first peak value of the HRR curve in the dropping ignition mode,was found to be insignificant.This leads to a decrease in the average temperature of the central section of the fire source and the aisle with the increasing upper height of FPUF in both the no-dropping and dropping modes.The HRR of the second peak of the FPUF and the average temperature of the central section of the fire source and aisle increases with the height of the upper ignition source in the drop ignition mode.Furthermore,in the no-dropping and dropping modes,an increase in the height of the upper FPUF results in two distinct,reversed flow loops in the central section of the fire source,whereas,in the dropping ignition mode,there is a shift from two flow loops to a single loop in the central section of the fire source.The average CO concentration and HCN concentration in the central section of the fire source and aisle decrease as the height of the upper fire source increased.In addition,three dimensionless heights are determined based on the aisle temperature,CO concentration,and HCN concentration to human tolerance values,which are the temperature dimensionless safe height,the CO dimensionless safe height,and the HCN dimensionless safe height.The results indicate that person has the lowest level of safety in the aisle when the upper FPUF is positioned low and the FPUF is in the dropping ignition mode.As the height of the upper fire source decreases,the temperature dimensionless safe height in the aisle decreases,and the dimensionless safety heights of CO and HCN decrease faster.