Flame Height (2) Associate Professor Chen-Wei Chiu 副教授邱晨瑋 /9/28 1

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1 Flame Height (2) Associate Professor Chen-Wei Chiu 副教授邱晨瑋 /9/28 1

2 Flame Height at specific wavelength bands a mixture of oxygen (air) and another gas, such as hydrogen, carbon monoxide, or a hydrocarbon (CH). 2016/9/28 2

3 Temperature Distribution in the Flame of a Burning Candle The innermost, nonluminous zone a gas/air mixture at a comparatively low temperature. In the second luminous zone H 2 and CO react with oxygen to form combustion products. 2016/9/28 3

4 Temperature Distribution in the Flame of a Burning Candle Outside the luminous zone invisible zone in which the remaining CO and H2 are finally consumed. This zone is not visible to the human eye. 2016/9/28 4

5 Premixed Flame Flame Categories oxygen is mixed with the combustible gas by some mechanical device prior to combustion Diffusion Flame the fuel and oxygen are transported (diffused) from opposite sides of the reaction zone (flame). 2016/9/28 5

6 Turbulent Premixed Flames Most turbulent premixed flames occur in engineered combustion systems, such as a boiler, furnace, process heater, gas burner, oxyacetylene 乙炔 torch, gasoline engine, or home gas cooking range. 2016/9/28 6

7 Laminar Diffusion Flame Most natural flaming processes produce diffusion flames, since no burner or other mechanical device exists to mix fuel and air. Common examples include a candle flame, a trash can fire, a hydrocarbon pool fire, or a forest fire A laminar diffusion flame produced by a burning candle Laminar means that the flow streamlines are smooth and do not bounce around significantly. 7

8 Flame Extensions with a Free-Burning Flame and Under a Smooth Ceiling Flame Impingement 2016/9/28 8

9 McCaffrey Flame Theory The continuous flame: Luminous region. The intermittent flame:: the temperatures drop as a function of distance up the plume. The thermal(buyant) plume: No more flames are visible and the temperature continues to drop as height increases away from the flame 320 C 900 C 2016/9/28 9

10 McCaffrey Flame Theory McCaffrey correlation Continuous flame INTERMITTENT REGION H fc 0.08Q 2 / 5 CONTINUOUS REGION Intermittent flame H fi 0.20Q 2 / /9/28 10

11 Mean Flame Height Characteristics of Flame Height Fluctuations luminosity H f H f H f max H f min H f min mean flame height H f H f max 2016/9/28 11

12 Definition of mean flame height H f H f /D H f 2016/9/28 12

13 HESKESTAD Flame Height Calculations 2016/9/28 13

14 Example 2.1 A pump breakdown where 20 l of transformer oil were spilled into a sump of 2 m 2 and ignited. Assuming the combustion efficiency to be ~70%, Calculate the resulting flame height. =0.039 kg/m 2 s ΔH c = 46.4 MJ/kg, Kβ = 0.7 (m 1 ). 2016/9/28 14

15 Solution 2.1 H f = (1 exp ( ) ) = 1.69 MW L = (1690) 2/ = 2.96 m H f 2016/9/28 15

16 Flame height correlation Heskestad correlation Z H f 0. 23Q 2 / f H f = Flame Height D ZH f D 3.7Q * 2 / /9/28 16

17 THOMAS Flame Height Calculations 2016/9/28 17

18 The Thomas plume Plume entrainment m pl PH f / 2 Valid up to flame tip; seems to work above P - fire perimeter length (m) H f - height above base of fire (m) H f 2016/9/28 18

19 Estimating the HRR 2016/9/28 19

20 Large-Pool Fire Burning Rate Data 2016/9/28 20

21 Effective Diameter For non-circular pools, the effective diameter is defined as the diameter of a circular pool with an area equal to the actual pool area given by the following equation: Where: A f is the surface area of the non-circular pool

22 HRR of Pool Fire The HRR of the fire can be determined by laboratory or field testing. In the absence of experimental data, the maximum HRR for the fire is given by the following equation: 2016/9/28 22

23 Burning Duration of a Pool Fire For a fixed mass or volume of flammable/combustible liquid, the burning duration (t b ) for the pool fire is estimated using the following expression: Where: V = volume of liquid (gallons or m 3 ) D = pool diameter (m) ν= regression rate (m 3 /m 2 -sec) 2016/9/28 23

24 Burning Duration of a Pool Fire As a pool of liquid combusts and the fuel is consumed, its depth decreases. The rate of burning, also called the regression rate (ν), is defined as a volumetric loss of liquid per unit surface area of the pool per unit time, as illustrated by the following expression: Where: = mass burning rate of fuel per unit area (kg/m 2 -sec) ρ = liquid fuel density (kg/m 3 ) ρ 2016/9/28 24

25 Assumptions and Limitations Assumptions and limitations apply to HRR: (1) The pool fire is burning in the open (2) There is no fire growth period (3) the pool fire instantaneously reaches its maximum HRR Assumptions and limitations apply to burning duration: a fixed mass or volume of flammable/combustible liquid. approximated as a circular measurement. 2016/9/28 25

26 Assumptions and Limitations Assumptions and limitations apply to flame height: flame height correlation horizontal pool fire sources in the center or away from the center of the compartment. The size of the fire (flame height) the diameter of the fuel and the HRR attributable to the combustion. two-dimensional sources (primarily pool fires) the pool is circular or nearly circular. 2016/9/28 26

27 Summary 1 pool fires dependent on pool diameter. spilled liquids, fires in diked or curbed areas, fires in open areas. Circular Estimating the burning duration (1) Determine the regression rate of the pool fire. (2) Calculate the equivalent diameter of the pool fire. (3) Calculate the burning duration of the pool fire. 2016/9/28 27

28 Flame height Summary 2 the flame is observed at least 50-percent of the time. Visual observations & slight overestimations of flame height Estimating flame height from a pool fire : (1) Determine the HRR of the pool fire. (2) Calculate the equivalent diameter of the pool fire. (3) Determine the height of the pool fire flame. 2016/9/28 28

29 Example Problem 2.2 A pool fire scenario arises from a breach (leak or rupture) in an auxiliary cooling water pump oil tank. This event allows the fuel contents of the pump to spill and spread over the compartment floor. A m 3 ( 5-gallon), 0.836m 2 (9.0-ft 2 ) surface area spill of flammable liquid (lubricating 潤滑 oil) leads to consideration of a pool fire in a compartment with a concrete floor. The fuel is ignited and spreads rapidly over the surface, reaching steady burning almost instantly. Compute the HRR, burning duration, and flame height of the pool fire. 2016/9/28 29

30 Example Problem 2.2 The dimensions of the compartment are 15 ft wide 15 ft deep 10 ft heigh. The cable tray is located 8 ft above the pool fire. Determine whether the flame will impinge upon the cable tray. Assume instantaneous and complete involvement of the liquid pool with no fire growth and no intervention by the plant fire department or automatic suppression systems. 2016/9/28 30

31 Example Problem /9/28 31

32 Example Problem 2.2 Purpose: (1) Determine the Heat Release Rate (HRR) of the fire source. (2) Determine the burning duration of the pool fire. (3) Determine the flame height of the pool fire. (THOMAS, Heskestad) (4) Determine whether the flame will impinge upon the cable tray. 2016/9/28 32

33 Assumptions 2.2 (1) Instantaneous and complete involvement of the liquid in the pool fire (2) The pool fire is burning in the open (3) No fire growth period (instantaneous HRRmax) (4) The pool is circular or nearly circular and contains a fixed mass of liquid volume (5) The fire is located at the center of the compartment or away from the walls 2016/9/28 33

34 Burning Rate Data for Liquid Hydrocarbon Fuels 2016/9/28 34

35 Input Parameters 2016/9/28 35

36 Estimating Fire Pool Heat Release Rate 36

37 Estimating Fire Pool Burning Duration 2016/9/28 37

38 Estimating Pool Fire Flame Height of HESKESTAD 2016/9/28 38

39 Estimating Pool Fire Flame Height of THOMAS 2016/9/28 39

40 Flame Height ( 防止延燒手冊 ) L F : 火燄高度 (m) D: 火源直徑 (m) Q: 火源熱釋放率 (kw) n: 實驗乘冪 r: 實驗常數 Q*: 火源熱釋放率無次元數 ρ : 捲入空氣之密度 C p : 捲入空氣之比熱 T o : 捲入空氣之溫度 g: 重力加速度 40

41 Example 4.3 發生火災的火源為內填泡棉的沙發座椅, 長度及寬度各為 1 公尺, 高度為 0.5 公尺, 燃燒過程中座椅最大的熱釋放率 Q max = 2MW, 火焰可以達到的居室最高高度為何? Zf D 41

42 Example 4.3 < 解 > 為計算火源最高火焰高度, 取最大熱釋放率 Q max = 2MW, 計算火源熱釋放率無次元數 Q* Q* = 2000/ ( /2 ) =1.79 因為 Q* =1.79>1.0, 依據上表取 n=2/5 為計算火焰可達最大高度, 取間歇火焰實驗常數,r=3.5 L F = 3.5 (1.79) 2/5 (4/π) 1/2 = 4.98m 由於火源面的高度為 0.5m, 故火焰可達居室最大高度為 L F =5.49 m 42

43 Homework 2.1 A standby diesel generator (SBDG) room in a power plant has a 3-gallon spill of diesel fuel over a 1-ft 2 diked area. This event allows the diesel fuel to form a pool. The diesel is ignited and fire spreads rapidly over the surface, reaching steady burning almost instantly. Compute the HRR, burning duration, and flame height of the pool fire. The dimensions of the compartment are 10 ft wide 12 ft deep 12 ft high. 2016/9/28 43

44 Homework 2.1 The cable tray is located 10 ft above the pool fire. Determine whether flame will impinge upon the cable tray. Also, determine the minimum area required of the pool fire for the flame to impinge upon the cable tray. Assume instantaneous, complete involvement of the liquid pool with no fire growth and no intervention by plant fire department or automatic suppression. 2016/9/28 44

45 Homework /9/28 45

46 Homework 2.1 Solution (1) Determine the Heat Release Rate (HRR) of the fire source. (2) Determine the burning duration of the pool fire. (3) Determine the flame height of the pool fire. (4) Determine whether the flame will impinge upon the cable tray. (5) Determine the minimum dike area required for the flame to impinge upon the cable tray. 2016/9/28 46

47 Homework 2.1 Assumptions: (1) Instantaneous and complete involvement of the liquid in the pool fire (2) The pool fire is burning in the open (3) No fire growth period (instantaneous HRRmax) (4) The pool is circular or nearly circular and contains a fixed mass of liquid volume (5) The fire is located at the center of the compartment or away from the walls 2016/9/28 47