Study Of Flame Instability And Explosion Characteristics Of Premixed Gas In Confined Spaces

Fully utilizing hydrogen energy is a key step towards the"double carbon"goal in the14th Five-Year Plan.The future of hydrogen energy as a clean and renewable energy source is of great significance for promoting the development of China’s shipbuilding industry towards green ships and low-carbon shipping.The hydrogen production and refueling integrated stations have the advantages of low hydrogen production cost,high equipment integration,and high hydrogen supply efficiency,and are considered an effective way to solve the problems of urban hydrogen production economy and hydrogen storage and transportation safety.Bioethanol to hydrogen is a highly advantageous option for hydrogen production due to its renewable nature,sustainability,and net zero emissions,meanwhile,hydrogen blending with natural gas technology has the advantages of low cost and low carbon emission to accelerate the development and utilization of hydrogen.However,when hydrogen production and refueling are carried out in the station,there is a risk of equipment leakage in the confined space,which may lead to fire and explosion accidents after encountering ignition sources.The interaction with obstacles during flame propagation will lead to a more complex and unpredictable explosion behavior,so studying the combustion and explosion characteristics of premixed gas in confined spaces is a prerequisite for the full-scale application of hydrogen production and refueling integrated stations.This thesis investigated the flame instability and explosion characteristics of hydrogen/methane/ethanol premixed gas under different initial operating conditions.The combustion and explosion characteristics of the premixed gas were investigated using a constant volume combustion bomb experimental system at initial pressures of 0.1 MPa,0.2 MPa,0.3 MPa,and 0.4 MPa,initial temperatures of 400 K,equivalent ratios of 0.7-1.4,and ethanol ratios of 20%,50%,and 80%,respectively,and simulated to study the explosion process of premixed gas in confined spaces.The main elements are as follows:Firstly,the flame instability and cellular structure properties inherent to hydrogen/methane/ethanol premixed gases were experimentally investigated.The flame morphology at different stages showed that the flame became unstable when large cracks appeared on the front surface of the flame,creating small cracks and new cell structures.The flame became more unstable as the initial pressure and equivalent ratio increased.Furthermore,as judged jointly by the effective Lewis number and the critical Lewis number,the thermal diffusion effect was found to stabilize the flame surface under all the studied operating conditions.Therefore,the main cause of flame instability was hydrodynamic effects.This conclusion was further validated at the same time based on the growth rate trends of various flame instability parameters.Secondly,the linear stability theory model with variable transport properties was used to analyze premixed flame instability.The critical conditions for the change of flame state from stable to unstable under different initial operating conditions were investigated,proposed the simultaneous use of critical flame radius and critical Peclet number to jointly determine the critical conditions for the transition.The results showed that the theoretically calculated critical flame radius was in general agreement with the experimental results,with some discrepancies only at the equivalent ratio of 1.4.This was probably mainly due to the uncertainty in the global activation energy assessment in the theoretical analysis and the fact that the effect of thermal radiation was not taken into account in the theory.More importantly,an empirical correlation formula between the critical Peclet number（Pe_c）and Markstein number（M_b）was constructed.In addition,the critical Karlovitz number（Ka_c）was used to determine the flame instability of premixed gas,and the best fit empirical correlation was obtained.Again,the basic explosion characteristics parameters of the premixed gas were experimentally investigated.The results showed that the peak explosion pressure(P_max),the maximum pressure rise rate(（dp/dt）_max),and the maximum value of the deflagration index（K_G）,as well as the minimum value of the explosion duration（t_c）all occur at the equivalent ratio of 1.2-1.3.The P_max,（dp/dt）_max,K_G,and t_c all increased significantly with increasing initial pressure.In addition,an increase in the ethanol ratio decreased the（dp/dt）_max and K_Gbut increased the P_max and t_c.Finally,the influence of obstacles in confined space on the explosion characteristics of premixed gas was investigated by numerical simulation.The results showed that the blockage ratios and the number of obstructions perturbed the flame propagation structure significantly.Variations in the blockage ratio and the number of obstructions had essentially no effect on the final P_max.The（dp/dt）_max increased with increasing blockage ratio and the number of obstructions increased in the same trend.As the initial pressure increased,both the P_max and the（dp/dt）_max gradually increased.