Research On Flow Characteristics Of Thermal Buoyancy Driven Fluid In Long Channels

Fluid flow, heat transfer in long channel widely exist in the construction engineering, highway tunnel engineering, thermal energy and power engineering and many other fields. Hot and cold fluid in different structure of the long channel will show different flow characteristics due to buoyancy and other driving forces and it will make the turbulent flow more complex.Channel fire is a very important research subject in fire safety field. In this paper, scaled experiment, numerical simulation and theoretical analysis were used to analyze and research for thermally driven flow in long channel under different conditions. The channel fires in this paper mainly refer to horizontal and inclined tunnel fires and vertical shaft fires.Firstly, control equations suitable for weakly compressible, low Mach number thermal buoyancy driven flow were established. Combustion model and thermal radiation model were also provided. Then the filtered control equations were obtained after Favre filtration. On the analysis of the phenomenon of fire in a long channel, dimensionless equations were derived by similarity transformation, so as to get the similarity criterion.The small-scaled inclined tunnel (120cm×16cm×20cm) fire tests were carried out, and smoke temperature distribution under the roof was measured. Also the smoke spread was transcribed. Gas temperature along the inclined tunnel was power exponential attenuation. The dimensionless temperature of upstream, downstream versus dimensionless distance was analyzed, and then the general rules of temperature attenuation coefficient K along with the angle were obtained. Smoke velocity along the inclined tunnel longitudinal attenuation law was deduced, and the smoke front spread along the tunnel was studied subsequently. CFD-FDS method was used to simulate the inclined tunnel fire tests, and temperature field and velocity field varies with the angle in tunnels were also analyzed.The medium-scaled inclined tunnel (8.0m×1.5m×1.0m) fire tests were carried out, and the vertical and longitudinal temperatures were measured under natural ventilation conditions when a fire occured in the tunnel. Then the change rule of buoyancy force difference between smoke layer and the cold air layer with angle was gained by experimental research and numerical simulaton. U-velocity of smoke under the roof and U-velocity at the two ends for different angles was studied by numerical simulation. Meanwhile, when the inclination angle was increased to a certain degree, the neutral plane of the lower end would disappear. Experimental and numerical study carried out on tunnel multiple fires. It was found that fire would move closer to each other under the pressure gradient, and non-continuous fires showed a large area of the fire burning characteristics. Subsequently, critical velocity for smoke controlling in inclined tunnel was studied mostly. Combination of theoretical analysis and empirical formulas, prediction model of back laying of smoke in horizontal tunnel was established. It was found that dimensionless critical velocity was1/3power of the dimensionless heat release rate, which was agree with Wu and Baker’s opinion. But the dimensionless critical velocity in this work was slightly higher than before, and the differences may be derived from different fuels. Finally, the dimensionless critical velocity correction formula was obtained.Combined with FDS simulation, scale shaft model (5cm x5cm x200cm) with winding electrical wires over the outside shaft wall was used to study the vertical driving force under different vents, while temperature in the shaft was kept constant value. Hydrostatic equations of pressure difference distribution were obtained. And then the pressure and velocity field distribution under the condition of different openings within the vertical shaft were descriped detailedly. Finally, the effect of openings on air pressure was discussed.A small-scaled fire shaft bench (20cm x16cm x120cm) was established and experiments were conducted to investigate the rise time of plume in two-end-open shaft and bottom-end-closed shaft. Semi-empirical equations for rise time of plume in two-end-open shaft and bottom-end-closed shaft were gained. Subsequently, driving force under different vents (buoyancy and chimney effect) were quantitatively calculated and analyzed.In order to investigate the flow characteristics of thermally driven fluid in shaft further, a medium-scaled fire shaft bench (1.5m×1.0m×8.0m) was established and multiple experiments were carried out. Combined with CFD-FDS method, temperature and velocity field distribution under the condition of different side openings within the vertical shaft were investigated detailedly. Finally, theoretical analysis and experiments were carried out to verify two-zone model for predicting neutral plane in two-end-open shaft. Compared with traditional Klote model, all the predictive values of two-zone model were lower than the predictive values of Klote model. The study concludes that two-zone model can be more comprehensive and reasonable to predict the position of neutral plane in shaft.