Study On Combustion Characteristics Of Pool Fires Under Vibration Environment Of Ship Engine Room

As the power core of the ship,the engine room has a large number of electrical facilities,various types of oil pipelines,and high-temperature hot walls,making the engine room the main area of ship fires.The occurrence of fires seriously affects shipping and personal and property safety.During the voyage of the ship,the engine room is always in a vibration environment due to the influence of the work of the main engine and the auxiliary engine.This will change the external conditions of the engine room oil pool fire,affect the shape of the fuel liquid level,the flame heat transfer of the oil pool and the distribution of the fuel temperature,and then affect the development,and change characteristics of the engine room fire.However,in the existing research,the oil pool is generally placed under stable conditions to carry out related fire research,ignoring the influence of the engine room vibration environment on oil pool fire combustion.Therefore,based on the complex vibration environment of the engine room,this study built an oil pool fire combustion experimental platform under a simulated vibration environment.With n-heptane fuel as the fire source,the fire experiments of oil pool fires with diameters of 6 cm,10 cm,15 cm,and 20 cm under different vibration frequencies were carried out.Combined with numerical simulation,the fuel mass loss rate,flame shape,flame temperature,fuel layer temperature and heat transfer process were analyzed,and the combustion characteristics of oil pool fire under the influence of vibration environment were studied.The specific work content of this thesis is as follows:（1）The variation characteristics of pool fire burning rate under different vibration environments were studied.In the non-vibration environment,the combustion process of pool fires with different diameters can be divided into four typical stages:pre-combustion stage,stable combustion stage,full combustion stage,and attenuation extinction stage.However,the second stable combustion stage appeared after the full combustion stage in the 6 cm and10 cm oil pools.At the same time,the mass loss rate in the stable combustion stage changes nonlinearly with the increase of the pool diameter.In the vibration environment,although the fuel mass change of the smaller diameter pool（6 cm,10 cm）is similar to that in the non-vibration environment,it changes the combustion process of the larger diameter pool fire（15cm,20 cm）.The stable combustion stage gradually disappears with the increase of vibration frequency,and the second stable combustion stage appears in the 15 cm diameter pool with the increase of vibration frequency.In addition,the combustion rate in the pre-combustion stage is inversely proportional to the vibration frequency,while the combustion rate in the full combustion stage is proportional to the vibration frequency.At the same time,based on Blinov’s combustion rate change theory,a combustion rate prediction model for a stable combustion stage under a vibration environment is further established.It is pointed out that vibration changes the combustion rate of pool fire by affecting the heat transfer process of a pool fire.（2）Based on the image theory,the flame shape of the oil pool under different vibration frequencies was studied.The results show that the flame height and width increase with the increase in pool diameter and vibration frequency.By introducing the dimensionless Strouhal number（st）describing the vibration frequency,a dimensionless flame height model under vibration environment is established,which has a power function relationship with the dimensionless heat release rate and st.At the same time,under the action of thermal buoyancy,the flame height under different vibration conditions shows the periodic oscillation characteristics of“lift-drop”.The oscillation frequency f₀ is inversely proportional to the diameter D of the oil pool:f₀²∝1/D,the results show that the fire plume density increases with the vibration frequency.In addition,according to the temperature change of the flame centerline,the flame region is divided into four regions:flame development region,continuous region,intermittent region and flame plume region.The location of the highest temperature of the flame centerline changes with the process of combustion development.At the same time,based on Mc Caffrey’s traditional flame centerline temperature rise model,a flame centerline temperature rise model suitable for vibration environment is developed,which shows that there is a power function coupling relationship between dimensionless temperature rise in different flame regions and flame centerline height and dimensionless heat release rate.（3）The characteristics of the temperature change inside the fuel layer under the vibration environment were studied.It was found that the applied vertical vibration enhanced the rate of thermal wave diffusion in the fuel layer.The higher the vibration frequency,the faster the temperature rise of the fuel layer.Based on this,a dimensionless temperature rise model of the internal heat dissipation zone was established.The results show that the vibration has a strong influence on the thermal wave diffusion of the small-diameter pool fire.At the same time,based on the Fluent software platform,the numerical simulation analysis of the temperature change of the fuel layer under the vibration condition is carried out by using the dynamic grid and the sinusoidal periodic vibration boundary condition.It is pointed out that the temperature of the fuel layer near the wall on both sides of the center line is first disturbed and increased.At the same time,the transverse temperature comparison in the fuel layer shows that the vertical vibration not only disturbs the fuel layer but also enhances the heat transfer process between the fuel and the wall.In addition,the heat balance analysis of the combustion process of the pool fire is carried out,and it is pointed out that the vertical vibration increases the heat transfer area of the fuel,which is the main reason for the increase of the fuel mass loss rate in the full combustion stage.Combined with the comparison between the theoretical value and the experimental value of the fuel surface area change and the mass loss rate,the heat transfer model coefficient affecting the mass loss rate is further modified,and the heat transfer model in the vertical vibration environment is improved,and the coupling relationship between the vibration frequency and the liquid surface correction coefficient on the Boltzmann model is clarified.