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• W608566767 abstract "The measurement of the centerline temperature rise and velocity above a line heat source was made. The entrainment of the line thermal plume was calculated based on the centerline data. The mass flux of the hot gas above the line fire was meaSured and compared with the calculation results. The comparison between the experimental results and literature values gives an appropriate equation to calculate the entrainment of the line thermal plume. 1. INTRODUCfION The entrainment of thermal plumes from a circular or a square heat source have been ·investigated by many researchers[I,2]. Nowadays more interests are developing in the entrainment of the thermal plumes above a line heat source because of some practical applications in smoke control design in the building [3,4]. Some works have been carried out in the line thermal plumes. Rouse.II,Yih.C.S. and Humphreys [5] made an elementary analysis ofthe mean patterns offree convection from a line source. Velocity and temperature measurements were made in the thermal plume. Their velocity measurement was thought with a large uncertainty. Yokoi [6] made a more complicated and more precise analysis ofthe upward current from an infinite line heat source. He also measured temperature and velocity distribution over the line heat source with a very low heat release rate.. Lee and Emmons [7] investigated natural convection above a line fire. A similar theoretical analysis was also made and just temperature distnoution over the line fire was measured. Thomas [3] derived an equation to calculate the entrainment ofthe line thermal plume from Lee and Emmons' analysis and experimental results. Zukoski [8] also did some works about the thermal line plume. In the environmental research area similar works were also done without fire. Kotsovinos et al [9] made an extensive study on plane buoyant jets and plumes. Recently Ramaprian and Chandrasehora [10] made a similar research. Hasemi et al [11] studied the fuel shape effect on the deterministic properties of turbulent fire plume. Flame height and maximum excess temperature in the thermal plume were measured over a line burner. In this paper two different aspect ratio line burners were used to produced line heat sources. The centerline temperature and velocity above line fires was measured with the heat release rate ranging from 2kw-I04kw. A simple model was developed to calculate the mass flux of the hot gas above the line fire based on experiment data and real mass flux ofthe hot gas were also measured in a similar way described in [2]. All experimental results in this paper can help to fully understand the behavior of the thermal line plumes and improve the smoke control design. 2. THEORETICAL CONSIDERATIONS 200 Copyright © International Association for Fire Safety Science From Rouse et aI's derivation [7], for a conventional thermal plume above a line heat source, the width of the plume b, the centerline velocity urn and centerline temperature rise !l Tm have the following relationships with the height z respectively: b oc Z, U m oc zo, f:J.T, oc Z -1 ••.•••.•...••.•.••••••••....•••..•• ( 1) Assuming the uniform profiles for the temperature and velocity distribution, the mean motion is then governed by the following three conselVation equations for continuity, momentum and buoyancy: d(2b/u) --=2Iua (2) dz d(2b/u ) ~-~= 2bg/(po -p)/Po··································(3) dz d[2b/ug(po p) / Po] --~-~=O (4) dz Here I is the length of the line burner, a is the entrainment of the line thermal plumes, p is the density, Po is the constant ambient density and g is the acceleration due to gravity. The buoyancy equation can be related to the convective heat Q in the plume, Here Cp is the specific heat at constant pressure and TO is the ambient temperature. Equation (4) can be integrated to 2blug(po p) / Po = constant =gQ / PoCpTo (5) Combining equations (1),(2), (3) and (5) gives to b ocza u ex:; ( gg )1/3 •••••••••••••••••••••••••••••••••••••••••••••••••••••••••••(6) 2apoCp To /t,.T ex:; ( g )213 Z-I To 2a!ipoCp To Here Q1 =QIl. Now we can assume further: 11 =C (~)1/3 m u C ' Po pTo 11 2 2 I1T fi1. 2 2 -=exI>(-Y Ib), -=ex-p(-/-y Ib) 11m L T,n 111ree consclvation equations still hold with following fonns: 201 ~ Jl/udy = 2/au, (8) !!-r2/U2dy =g fi/dy~T / To (9) dz 0 Jo~ ~V/ugdyLlT / To = 0 (10) Equation 10 can be changed to: l2/ugdy~T / To = constant = Qg / (PoC ~) (11) Jo P Substituting equations (7) into equations(8),(9) and (11), yields C1=2a/~ J'jjC 1I 2 =J2cr (12) C.CrCI =(1 +fJ)1I2 /~ In these three equations if Cu and CT are determined by experimental results,CI, B and a can be calculated. From above analysis, the mass flux ofthe thermal plume can be calculated: Ifwe still assume P = Po' m =r2/upofly = ,J;c,C.!(gpo / CpTa )113 {2113Z m//=.../nC,C.(gpo2/C p 1;,)113{2113z (13) Ifthe mass flux ofthe fuel is neglected the entrainment of the line thermal plumes will be equal to the mass flux ofthe hot gas. 3. EXPERIMENTAL DETAILS The experiments were conducted using a 0.018 x 0.5m and a 0.05 x 0.5m line burners. These burners were constructed of porous refractory material. The fuel is natural gas (35kJll) and the rate was controlled through a flow meter. The heat release rate was calculated from flow rate and changed from approximately 2-110KW. The burner sat O.71m above the floor and under a passive hood in a large laboratory. The instruments was located over the centerline ofthe burner using a plum bob. Thereafter, vertical and side to side movement ofthe instrument cluster was accomplished with a micrometer lathe-type movement device which held the pressure probe and thermocouples. Temperature measurements were made using approximately O.2mm diameter chromel-alumel thermocouples. The time constant of the temperature measurement 202 is approximately 10 seconds. No corrections have been made for the radiation and conduction losses from the junction of each thermocouple. For 0.2mm chromol-alumel thermocouples with an emissivity of around 0.9, the error due to radiation loss will range from 2 to 20% over the temperature range 300-1000 °C. To eliminate the influence of accidentally sway ofthe flame, temperature was monitored at each height not only just above the burner center but also at two different points 5 cm apart from the center in the direction of shorter side. The reported values oftemperature are the average ofthe temperature at each height over more than 3 minutes during which the temperature above the burner center was higher than the other two. Velocity was measured using bidrectional pressure probes which responds like a pito-tube static probe except the measuring area is quite large which spatially averages the signals in order to obtain the gross structure of the flame. The output of the microman-ometer was time-averaged over more than 3 minutes. The pressure signal is very sensitive to any disturbance caused by draught. A mesh screen was hung up from the hood bottom to the floor at one side parallel to the burner center(another side was a experiment rig). The disturbance couldn't be prevented completely. Some disturbance still could be found on the data record and data recorded during these disturbance was not used. The density was calculated by the temperature from the attached thermocou-ples on the probe according to ideal gas law. Each probe was calibrated in a standard wind tunnel before use. The calculated velocity was then inverted to real velocity acc-ording to calibration. The mass flux above the flame was measured in an apparatus similar to that described in [2]. The eX1Jerimental set-up was shown in figure 1. The hood with the dimensions of l.Om x 0.6m x 0.6m was made of fireboard and the duct with a diameter ofO.4m was made ofsteel. The bidirectional probe was used to measure the velocity distnoution over the duct cross-section. Because of short distance (O.6m) above the hood, the velocity distribution isn't homogeneous. A typical velocity distribution was shown in Fig.2. It can be found an approximate axiysimmetry holds well. The mass flux was calculated by integrating over the section. It was estimated that this method could cause the maximum error within 15%. Because of limited dimension of the hood, the interface layer between the cool room air and the hotter hood gas was kept as close to the bottom ofthe hood as possible by changing the height ofthe burner which was fixed on a jack according to different heat release rate. TIle location of the interface layer was detenllined from the temperature distribution measured with the vertical arrays of themlocouples vhich span the interface. TIle 0.2mm thermocouples was used at inteIVals of 0.04m for the distance near the bottom and O.lm for the other distance. A typical temperature profile is shown in Fig.3. In the regions both above and below the interface the gas temperature is nearly constant. In the interface layer there exist a large temperature gradients and fluctuations. Typical interface thickness were 1020cm TIle interface height was defined as the height at which the temperature is nearest to 0.5(Tu +11 )+11 , where Tu is the upper layer temperature and 11 is the room temperature." @default.
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• W608566767 date "1995-01-01" @default.
• W608566767 modified "2023-09-23" @default.
• W608566767 title "Entrainment Of Line Thermal Plumes" @default.
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