| Jet fires often account for a large proportion of fire accidents that occur during industrial production or transportation of hazardous materials.In serious acc idents such as fire and explosions,about half of the accidents are caused by jet fire as the first stage of the fire.Generally,the fuel jet velocity of gas pipeline leakage is relatively fast,and the intense jet fire column will be formed,which will cause serious harm to surrounding buildings and person.In the case of leakage,the direction of the initial fuel velocity is arbitrary,which will lead to the emergence of jet flames released at any tilt angle.Gas leakage has a high exit velocity.Once a gas leakage fire occurs in a pipeline,the high-speed jet flame usually impinge on the ceiling and walls of the surrounding buildings,forming a more complex form of combustion and flame propagation,and causing different degrees of thermal damage to the ceiling and walls.In the domino effect,jet flame impingement and thermal radiation as the escalation vector of jet fire should be paid more attention to.Previous studies mostly focused on the ceiling jet temperature distribution when the vertical jet flame impinges on the ceiling,but there are relatively few studies on the working conditions of the inclined jet flame impinging on the vertical wall.In this paper,experimental studies are carried out on the wall impinging jet generated by the turbulent jet flame impinging on the ceiling or wall with different jetting directions to reveal the behavioral characteristics of the impinging jet,which is expected to provide theoretical basis and experimental data support for the best choice for building structural materials,installation of fire protection facilities,and domino effect analysis of jet fire.The main work includes the following aspects:1.The combustion experiment of vertical turbulent jet flame impinging on the horizontal and inclined ceiling was carried out,and the change of flame extension length under the ceiling and ceiling temperature distribution were revealed.First,based on the flame extension length model under the horizontal ceiling proposed by You and Faeth,the nozzle size as the dimensionless parameter characteristic length in the formula is corrected to the vertical distance between the fire source and the ceiling,and the new formula to predict the flame extension length under the horizontal ceiling is obtained.On this basis,considering the asymmetric distribution characteristics of unburned fuel in the upward and downward directions along the inclined ceiling,a prediction model that can uniformly predict the flame expansion length in the upward and downward directions along the inclined ceiling at different inclination angles is established.Based on the correlation between the two sets of non-dimensional parameters in the classical prediction model of the temperature distribution of ceiling jet,the applicability of the jet fire source conditions in this experiment is analyzed respectively.It is obtained that the important parameters in the temperature distribution prediction model is related to the Froude Number of the fire source,and a temperature attenuation prediction model based on the Froude Number is proposed.By introducing the influence factor of the ceiling inclination angles to correct the dimensionless temperature rise,a prediction model that can uniformly predict the temperature distribution of the ceiling jet at different inclination angles is obtained.And the prediction accuracy of the model is verified by comparing with the experimental values.2.A set of experiments in which a horizontally jetted turbulent jet flame impinges on a vertical plate was conducted to reveal the evolution behaviors of flame morphology and the temperature distribution of wall impingement jet.Firstly,it is verified that the total length of horizontal jet flame with different inclined angles in open space conforms to the relationship between the dimensionless flame height and dimensionless heat release rate proposed by Heskestad.According to the evolution of flame morphology of the horizontal jet flame under the restriction of the vertical plate,the flame morphology is divided into three evolution stages:unimpinging flame,flame spreading upward along the wall,and flame spreading bidirectionally along the wall.Based on dimensional analysis,a segmented prediction model of the total flame length is proposed.The critical condition discriminant formula is established for the critical conditions of each evolution stage,which revealed the correlation between the dimensionless critical horizontal distance and the dimensionless heat release rate of fire source.Based on the classical prediction model of the temperature distribution of the ceiling jet,the applicability of the horizontal jet flame impinging on the vertical wall in this experiment is analyzed.By introducing the ratio of the horizontal distance between the nozzle and the vertical wall and the horizontal projection distance of the free horizontal jet flame to correct the plume radius,a new model to predict temperature distribution along the wall is established for the horizontal jet flame impinging on the vertical wall.Comparing the model with the experimental results,the maximum error line isą31%,which verifies the accuracy and rationality of the prediction model.3.The combustion experiment in which an upwardly inclined turbulent jet flame impinges on a vertical plate was carried out,and the evolution of flame morphology and temperature distribution along the wall are revealed.Firstly,the applicability of the classical prediction model of vertical jet flame height proposed by Heskestad for the total length of the jet flame with different inclined angles in open space was verified.And the formula to predict the total flame length and the horizontal projection distance of inclined jet flame under different inclined angles is obtained.Based on dimensional analysis and experimental research,the influence of the nozzle inclination angle,the power of the fire source,and the horizontal distance between the nozzle and the vertical plate on the total flame length and temperature distribution along the wall are revealed.The segmented prediction model and critical condition judgment formula of the total flame length at different inclination angles are proposed.The ratio of the horizontal distance between the nozzle and the vertical wall to the flame horizontal projection distance is introduced to modify the characteristic length,and the prediction model is developed to uniformly describe the wall temperature distribution of the inclined jet flame impinging on the vertical wall in different injection directions. |
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