Study On The Two-Dimensional Estimation Of Ceiling Jet Temperature Field In An Urban Utility Tunnel Fire

Nowadays,utility tunnel has been widely regarded as an important infrastructure for the promotion of city's sustainable development.Utility tunnels,or utilidors are those new underground tunnels built to accommodate multiple utility pipelines such as water,sewerage,gas,electrical power,telephone,and heat supply.Building utility tunnels as a sustainable solution can facilitate the adjustment of pipelines,take full advantage of urban underground space and guarantee the durability of utilities.However,the potential fire problems need to be carefully treated during their rapid construction.In such a highly confined narrow structure,bundling high-voltage urban cables with limited ventilation may finally lead to severe fire accidents due to thermal conservation and inappropriate operations.On the other hand,catastrophic fire accidents have motivated abundant researches with a focus on traffic tunnels.Compared with these common tunnels,a utility tunnel normally has a relatively small cross-section,transversely separated compartments,longitudinally isolated fire zones,a mass of layered high-voltage cables and different ventilation design,which brings about the unique utility tunnel fire problems.During such a fire scenario,the strong fire plume driven ceiling jet flow would propagate to the two portals,causing considerable hazards to the safety of tunnel structures and the facilities and cables installed.Undoubtedly,the ability to predict temperature distribution in ceiling jets can thus be helpful for the assessment of fire detection time,firefighting and reducing the risks for fire spread.In the current work,the theoretical analysis and multi-scale experiments are carried out to comprehensively investigate the burning characteristics of cables,enhanced fuel burning and the temperature field of the ceiling jets driven by strong fire plumes in a utility tunnel.The main researches are as follows:Bench-scale fire tests are conducted to examine the burning behaviors of a common type of 10 kV flame retardant cable in utility tunnels,i.e.the ZRC-YJV-8.7/10 3 × 95 mm2.The critical exposed radiative heat flux for the cable to achieve complete combustion is identified.The whole burning process can be divided into five phases,i.e.the sheath fire,slow spread,rapid spread,full developed fire and decay.The behaviours of main components are discussed.A series of model scale tests are also performed.The results indicate that the phenomenon of enhanced fuel burning in a utility tunnel is affected by WF/WT,HF/HT and LF/LT simultaneously.Carvel et al.'s model is further modified to integrate the effects of these factors and estimate the enhancement.Full-scale fire tests are also conducted in a utility tunnel under construction,and the effects of fire size,horizontal aImpinging nd vertical locations of fire source and the condition of portals are considered.conditions of strong plumes under different ceiling clearances above fuel are divided into three states.A theoretical correlation is derived by incorporating the approximate boundary layer thickness,with the purpose to estimate the longitudinal maximum gas temperature attenuation.It is seen that the decay is influenced by impinging conditions,and thos'e overestimated Stanton numbers by Delichatsios are reassessed under these states.In the one-dead-end cases,it is also found the end wall has a great impact on the flow pattern and temperature decay rate.The applicability of four existing models for the estimation of longitudinal temperature decay in current situations is analysed.The vertical temperature distribution are then studied in the next step.The self similarity of the vertical temperature distributions has been confirmed to exist and be maintained.Three types of similarity profiles are formed in the one-dead-end and fully-enclosed cases.In order to characterize them,the expected apex is determined,and a series of piecewise correlations are then developed by combining a power function and Gortler's error function with transformations.The practicality of all the three existing models in current situations is discussed and summarized.Furthermore,the ceiling jet thickness is found to vary as a power function with the longitudinal distance from fire.Finally,with the integration of the vertical distribution model,the ceiling jet thickness model and the longitudinal temperature decay model that we proposed earlier,the framework is established to empirically estimate the two-dimensional thermal environment generated by strong fire plumes in a utility tunnel.This methodology has the capability to provide fast and precise prediction of smoke temperature field in practices of fire safety engineering.