Experimental Verification of Experimental Verification of a Radiant Heat Exposure Model a Radiant Heat Exposure Model Miss Hui, P. S. and Hui, P. S. and Dr. Dr. Wong, L. T. Wong, L. T. Miss Research Centre for Fire Engineering Research Centre for Fire Engineering Department of Building Services Engineering, Department of Building Services Engineering, The Hong Kong Polytechnic University, Hong Kong, China. The Hong Kong Polytechnic University, Hong Kong, China.
I ntroduction I ntroduction Thermal hazard and heat exposure Atmospheres at dry air Thermal radiation temperature 250 o F (121 o C) •Pain •Skin burns •Blistering •Hyperthermia (Heat •Skin burns stroke): also at a T < 121oC
I ntroduction I ntroduction Thermal tolerance for humans at rest, naked with low air movement. (Blockley 1973) e.g. Skin Pain 250 o F 121 o C Heat stroke Dry air Humid air Exposure time (min)
I ntroduction I ntroduction Average time t (min) to incapacitation for exposures to humid air and dry air at an elevated temperature T ( o C) (Purser 2002) t = exp(5.185-0.0273 T)
I ntroduction I ntroduction Exposure to Thermal Radiation Only addressed in some fire scenarios (“Limited” temperature)
I ntroduction I ntroduction Why Thermal Radiation is studied? •Direct hazard – case dependent; Skin pain, skin burns •Psychological effects – would affect occupant’s decision Example: A large high-rise building fire in 1996 Some building occupants would decide not to use an escape corridor with hot smoke; stayed in the rooms adjacent to the corridor for hours before they used the building windows for evacuation
Skin damage Skin damage Exposure Time: t at the Skin Temperature: T > 44 o C Sensation ∝ t (T > 44 o C) Pain � Burns � Injury
Classifying skin burns by Degree Classifying skin burns by Degree First Degree burns: minor and result only in a mild inflammation of the skin. E.g. Sunburn Second Degree burns: � blisters on the skin. Superficial: heal with little/no scarring Deeper 2 nd burn: forming thin layer of coagulated and dead cells, feel leathery to the touch Third Degree burns: penetrates through both epidermis and the dermis; or body tissue (deep burn)
Exposure models Exposure models Some examples: Radiant heat flux (kWm -2 ) Stoll and Chianata (1969) Exposure time to pain (s): t r,p = 85 q r -1.35 Exposure time to blister (s): t r,b = 223 q r -1.35 100 Blister Destruction zone 10 Pain Injury zone Exposure Time (s) 1 Safety zone Radiant heat flux (kWm -2 ) 0.1 1 10 100
Exposure models Exposure models Some examples: SFPE Model (Wieczorek and Dembsey 2001) Exposure time to pain (s): t r,p = 125 q r -1.9 Exposure time to burns (s): t r,b = 260 q r -1.56 100 Burns Destruction zone 10 Exposure Injury zone Pain Time (s) 1 Safety zone 0.1 Radiant heat flux 1 10 100 (kWm -2 )
A simple model: Heat balance at A simple model: Heat balance at the skin surface the skin surface Incident radiant heat flux Air temperature T o q r Convective heat transfer coefficient h c Skin temperature T Emissivity ε Skin surface, x=0 Conductance K Skin: x a single layer an opaque semi-infinite solid Radiant heat gain ∂ T ( ) k + ε q = h T − T Temperature Convective heat loss b R c 0 ∂ x
One- -dimensional heat conduction dimensional heat conduction One equation equation (Modest, 1993; Siegel and Howell, 2002; Tien Tien et al et al ., 2002 ., 2002 ) ) (Modest, 1993; Siegel and Howell, 2002; temperature temperature 2 ∂ T ∂ T = α ∂ 2 t ∂ x time time skin depth skin depth thermal diffusivity thermal diffusivity of the skin of the skin
One- -dimensional heat conduction dimensional heat conduction One equation: solution equation: solution Solved for the temperature at the depth of x (m) below skin surf Solved for the temperature at the depth of x (m) below skin surface ace (Tien Tien et al. et al. 2002, 2002, Wieczorek Wieczorek and and Dembsey Dembsey, 2001) , 2001) ( ⎡ ⎤ ⎛ ⎞ ⎛ ⎞ 2 x x 2 α (st) q ⎜ ⎟ ⎜ ⎟ = + − − ⎢ ⎥ T T R exp x erfc ⎜ ⎟ ⎜ ⎟ 0 α k 4 (st) π 2 α (st) ⎢ ⎥ ⎝ ⎠ ⎝ ⎠ ⎣ ⎦ Safety factor (S = 2) for Safety factor (S = 2) for Human variability variability Human (SFPE, 2000; Wieczorek Wieczorek and and Dembsey Dembsey, 2001 , 2001 ). ). (SFPE, 2000;
Thermal radiant heat flux Galbraith G. H., McLean R. C. and Stewart D. (1989). Occupational hot exposures: a review of heat and mass transfer theory. Journal of Engineering in Medicine 203:3, 123-131. ( ) A 4 4 s = σ ζ − q T T Hot surface, s R s b A b q R − 1 ⎡ ⎤ ⎛ ⎞ ⎛ ⎞ 1 1 A 1 ⎜ ⎟ ⎜ ⎟ ζ = − + + s − 1 1 ⎢ ⎥ ⎜ ⎟ ⎜ ⎟ ε ε F A ⎢ ⎥ ⎝ ⎠ ⎝ ⎠ ⎣ ⎦ s sb b b Human body, b A = × = b A W H F F s R R sb bs A s
The Standard Man standard object (H b = 1.73 m and w = 72.7 kg) with an orientation of α = ϕ = 0 Ref: Dunkle R. V. (1963). Configuration 1 / 3 factors for radiant heat transfer H w 2 2 = b b calculations involving people. R R b Journal of Heat Transfer 85:1, 71- 7 . 21 76. [ ] ( ) R 2 = + α + ϕ 0 . 0929 0 . 65 cos 7 . 15 0 . 52 cos
view factor 1 ⎛ ⎞ 1 1 2 − ⎜ ⎟ 1 = F tan ⎜ ⎟ 12 π 2 2 2 2 4 + + D D D D ⎝ ⎠ 1 2 1 2 D1 = d/ l 1 and D2 = d/ l 2
Assumptions Assumptions Human shape and variability, Human shape and variability, values of emissivity emissivity, diffusivity, conductivity , diffusivity, conductivity values of depth of basal layer depth of basal layer single layer skin, 1D heat conduction … single layer skin, 1D heat conduction … Verification: accidental skin burn statistics or sample tests Verification: accidental skin burn statistics or sample tests
This study This study Measure the temperature T on the thermal manikin Measure the temperature T on the thermal manikin skin layer when being exposed to different radiation skin layer when being exposed to different radiation fluxes fluxes Compare with the calculations for the estimated Compare with the calculations for the estimated temperature rise at the head portion of the thermal temperature rise at the head portion manikin
Skin temperature measurement Skin temperature measurement Insulated removable barrier Thermal camera Thermal camera Heated iron oxide sheet 1 m Heater Thermal manikin Head of the thermal manikin Iron oxide sheet Partition (a) Schematic (b) Photo (a) Schematic (b) Photo Experimental setup
Thermal manikin Thermal manikin (Bjorn, E., Nielsen, P. V. 2002) Shape: •accurate geometrical likeness to a real person •1.7 m tall average-sized woman •body surface area of 1.47 m 2 Construction of for skin temperature measurements: •4 mm glass fibre-armed polyester shell wounded round with 0.3 mm diameter nickel wire at a spacing of 2 mm •The wiring covered by a protective coating of about 0.1 mm in thickness •maintain a body temperature (individual control of temperature of 16 body parts of the manikin)
Experimental set- -up up Experimental set •The thermal manikin in BSE Fire chambers (PolyU) •Heated iron oxide plate (180 mm × 180 mm thickness 3 mm) •1.7 kW electric heater; steady state surface temperature = 426 ° C (699K) •Thermocouples and thermal cameras were used to monitor the surface temperatures.
Measured results: Skin surface Measured results: Skin surface temperature temperature By thermal camera
Measured results: Skin temperature Measured results: Skin temperature 34 By thermal manikin Modelled Skin temperature T ( ° C) 33.5 r = 0.9969 p ≤ 0.0001 33 Measured 32.5 Safety factor, s = 1 32 0 100 200 300 400 Exposure time t (s) Skin temperature of a head
An example application Corridor with a hot smoke layer = × A W H s R R H s 2.1 m min. Journal of Fire Science 23(2), Wong 2005
Smoke layer properties: considered as gas- soot mixtures The total emissivities for homogeneous gas-soot mixtures ε s Gas Soot ( ) ε = ε + ε 1 ε − s soot g soot Ref: Modest M. F. (1993). Radiative heat transfer , McGraw-Hill, New York, USA
Emissivity: Soot κ soot L ε = − L s (m) is physical path length 1 e s soot C κ soot is the Planck mean absorption 0 κ soot = 3 . 72 f T v coefficient of the soot for entire range C 2 of optical thickness f v is the soot volume fraction, C 2 is Planck’s second constant (1.4388 × 10 − 2 mK) and C 0 , is a constant between 2 and 6 dependent on the complex index of refraction m = n − ik, π 36 nk = C ( ) 0 2 2 2 2 2 − + + n k 2 4 n k Ref: Modest M. F. (1993). Radiative heat transfer , McGraw-Hill, New York, USA
Emissivity: Gas mixture ε g is the total emissivity of the gas mixture of CO 2 ε CO2 and water vapour ε H2O 1 ε ε = ε + g H 2 O CO 2 2 The emittance of CO 2 and water vapour can be found from emissivity charts or by exponential wide-band model Ref: Modest M. F. (1993). Radiative heat transfer , McGraw-Hill, New York, USA
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