| With the increase in the number of tunnels constructed in China,people’s travel is becoming more convenient,but the number and frequency of tunnel fire accidents are also increasing.The investment of reduced scale fire model test is relatively small,and it is easy to control boundary conditions and achieve high-precision measurement of fire parameters,which provides possible conditions for a more comprehensive study of fire laws.The reflection degree to which the test results of the reduced scale model test reflect the full-scale test is closely related to the similarity theory adopted in the reduced scale model test.The theoretical basis of the reduced scale fire model test of tunnel fire is the similarity criterion.It is necessary to carry out research on the similarity criterion of tunnel fire.Therefore,according to the characteristics of tunnel fire,the similarity theory model of tunnel fire was analyzed.The applicability and accuracy of the the similarity relationship of tunnel fire field based on the Froude model were studied by Fluent.The influence of fuel on the fire field similarity was analyzed,and the similarity of fire temperature field under various scale ratios of polyurethane and ethanol as fuel was simulated and analyzed;Then,the influence of lining materials on the similarity of tunnel fire temperature field was analyzed,and the tunnel fire temperature field in various thermal thick lining materials and lining thicknesses were simulated and analyzed by Fluent;Finally,the reasons for the inconsistency between the lining heat conduction time and the fire duration time were analyzed,and the error of the hysteresis of the temperature rise inside the lining caused by the incongruity of the two times was simulated and analyzed by Fluent.The main research contents and conclusions are as follows:(1)The similarity relation of tunnel fire similarity theory model could be divided into fire field similarity relation and structural similarity relation.According to the three theorems of similarity theory,based on the similarity criterions which have essential influence on the development of tunnel fire,the similarity control criterions of tunnel fire field similarity relation and structural similarity relation were analyzed through the derivation of similarity criterions.The similarity relation of fire field was realized by ensuring geometric similarity,velocity similarity,temperature similarity,fire source power similarity and convective heat transfer similarity.The structural similarity relation was realized by the structural temperature and time similarity relationship and the structural load and deformation similarity relationship.(2)The numerical simulation of the similarity relationship of tunnel fire temperature field was carried out.The similarity of tunnel fire temperature field used polyurethane and ethanol as fuel under various scales and fire load was simulated and analyzed.Under the fire load of 3MW,4MW,and 5MW,the simulation results show that error of the similar fire field based on the Froude model,used polyurethane and ethanol as fuel,between the measuring points of the 1/2 and 1/4 reduced scale model and the measuring points of the prototype measuring points,under the prototyping time is within 10%.The similarity of the tunnel fire temperature field in the fluid region is good,indicating that the tunnel fire field similarity relationship based on the Froude model has good applicability and accuracy.The larger the fire load is,the greater the temperature rise.The radiation heat transfer accounts for a large proportion when the smoke temperature is high,resulting in a larger error between the reduced scale model and the prototype caused by the larger fire load.The reduced scale fire model has a size effect.The smaller the scale ratio,the greater the error between the reduced scale model and the prototype.When the tunnel fire load is small,different fire load and different fire site fuels have a small impact on the error between the reduced scale model and the prototype,because the smoke temperature in the tunnel is low,and the proportion of radiation heat transfer is small.(3)When using the Froude similarity model to design the reduced scale fire model experiment,wall materials should be selected according to the similarity criteria.The purpose of the similarity of the wall material is to ensure that the fire field and external heat exchange between the reduced scale model and the prototype are similar.The tunnel fire temperature field in various thermal thick lining materials and lining thicknesses were simulated and analyzed by Fluent.The simulation results show that when thermal thick materials are used as materials for tunnel lining structures,the error caused by ignoring the similarity of wall materials is very small,and has a slight effect on the similarity of the tunnel fire temperature field;The reasons for the inconsistency between the heat conduction time of the lining and the fire duration time were analyzed theoretically,and the error of the hysteresis of the temperature rise inside the lining caused by the incongruity of the two times was simulated and analyzed by Fluent.Due to the lack of theoretical coordination between the heat conduction time of the lining structure and the fire duration time,in order to enable the reduced scale tunnel fire model to more accurately reflect the full scale tunnel fire model,it is recommended that the scale ratio should not be less than 1/2.Using multiple nonlinear regression theory,the variables that have an impact on the similarity error of the reduced scale model,such as the scale ratioλ_l,the coordinate Z of the lining thickness direction,etc,were fitted and analyzed.The R~2 of the equation obtained by multiple nonlinear regression was 0.962>0.8,indicating that the similarity error formula had a good fitting effect.The formula was validated using the relative error values of multiple reduced scale fire models,and the predicted values of the fitting formula were in good agreement with the actual numerical simulation error values. |
