Metrology for Fire Experiments in Outdoor Conditions

49 Pages·2013·1.823 MB·English

by  Xavier Silvani (auth.)

SPRINGER BRIEFS IN FIRE
Xavier Silvani
Metrology for Fire
Experiments in
Outdoor Conditions
SpringerBriefs in Fire
Series Editor
James A. Milke
For further volumes:
http://www.springer.com/series/10476
Xavier Silvani
Metrology for Fire
Experiments in Outdoor
Conditions
1 3
Xavier Silvani
Laboratory Sciences for the Environment
University of Corsica
Corte
France
ISSN 2193-6595 ISSN 2193-6609 (electronic)
ISBN 978-1-4614-7961-1 ISBN 978-1-4614-7962-8 (eBook)
DOI 10.1007/978-1-4614-7962-8
Springer New York Heidelberg Dordrecht London
Library of Congress Control Number: 2013939829
© The Author(s) 2013
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Contents
1 Introduction ................................................ 1
References .................................................. 6
2 Measurement Systems ....................................... 9
2.1 Heat Measurements: The Temperature ........................ 10
2.1.1 Temperature Measurement ........................... 10
2.1.2 Devices for Temperature Measurement ................. 11
2.1.3 Errors in Temperature Measurements .................. 16
2.2 Heat Measurements: The Heat Flux .......................... 17
2.2.1 Heat Flux Measurement ............................. 18
2.2.2 Devices .......................................... 19
2.2.3 Measurement Errors ................................ 21
2.3 Measuring Heat Convection and Source Term:
Outlooks and Prospects ................................... 23
2.3.1 Measuring the Velocity Field ......................... 23
2.3.2 Measuring Scalars: Mass Loss Rate, Density Field,
and Reaction Rate .................................. 27
References .................................................. 30
3 Beyond Measurement Devices ................................. 33
3.1 Post-Processing Field-Scale Data: The Stochastic
Nature of the Data ....................................... 33
3.1.1 The Unsteady Nature of Fire Data ..................... 33
3.1.2 Setting the Sampling Rate ........................... 34
3.1.3 Investigating Fluctuations. . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
3.2 New Trends in Fire Sensing ................................ 35
3.2.1 Wireless Intrusive Sensors ........................... 35
3.2.2 Optical Non-Intrusive Diagnostics ..................... 36
3.2.3 Remote Sensing ................................... 37
v
vi Contents
3.3 Managing the Fire Experiments ............................. 38
3.3.1 Meteorological Conditions ........................... 38
3.3.2 Cost ............................................. 39
References .................................................. 40
4 Conclusion ................................................. 43
Reference ................................................... 45
Symbols
d Diameter (m)
h Convective heat transfer (W/m2K)
I Modified Bessel’s function of the first kind
0
k Thermal conductivity (W/mK)
Nu Nusselt number
r Radius (m)
Re Reynolds number
Pr Prandtl number
Gr Grassh of number
T Temperature (°C or K)
u Gas velocity (m/s)
ΔT Thermocouple error (K)
Greek Symbols
β Thermal expansion coefficient (K 1)
−
ε Emissivity
σ Stefan–Boltzmann constant (5.67 10 8 W/m2K4)
−
×
ρ Density (kg/m3)
δ Thickness of the gauge constantan foil
κ Conductive heat transfer coefficient
Subscripts
g Gas
GG Gardon gauge
TC Thermocouple
f Film
F Flame
vii
Chapter 1
Introduction
Fire is a physical phenomenon in continuum mechanics involving the transport of
mass, momentum, and heat in a complex medium formed by at least two phases,
solid and gaseous; the fuel is usually in the solid phase and chemical reactions
occur in both the solid and gaseous phases, with the flame burning in the latter.
Fire creates a thermally and chemically aggressive environment with strong radia-
tive properties. The standard diagnostics for continuum mechanics must be set up
and adapted to the fire intensity. Fire research evaluates the fire hazard, ideally by
predicting its consequences using numerical simulations. Within this framework,
experiments have to provide a large set of data from which empirical laws can be
derived. These laws will help validate numerical simulations in a wide range of
parameters.
During the last two decades, a large number of laboratory-scale studies of rfi es
spreading across fuel beds have been performed. The following references are pro-
vided as a non-exhaustive list for information purposes (Dupuy et al. 2006; Mendes-
Lopes et al. 2003; Viegas 2004). Because it is difcfi ult to design, set-up, use, and
interpret results from a metrology dedicated to real-scale rfi es, the experimental
research on the subject has used the results from rfi e studies at the laboratory scale.
Many studies have used the cone calorimeter and the rfi e-propagation apparatus to
derive experimental laws for rfi es (Kuang-Chung and Drysdale 2002; Consalvi et al.
(2011); Luche et al. 2011; Bartoli et al. 2011). However, current results from recent
studies (Boulet et al. 2012) also illustrate that experimental conditions using these
laboratory devices deviate signicfi antly from real rfi es. Thus, their results remain
limited to academic considerations. These lab methods also are limited by the rfi e
size and are not really capable of reproducing, for instance, the upwind ofl w effects
of the wind at actual scale. In addition, they are diagnostically sensitive to ufl ctua-
tions when the rfi e size increases. At the laboratory scale, analytical chemistry and
thermal chemistry are process-engineering disciplines that also endeavour to con-
tribute signicfi antly to rfi e science: they study rfi e as the result of a thermal-degra-
dation process (Leroy et al. 2010). However, this point of view presents rfi e physics
without considering large-scale turbulent mixing of the reactive gases produced by
X. Silvani, Metrology for Fire Experiments in Outdoor Conditions, 1
SpringerBriefs in Fire, DOI: 10.1007/978-1-4614-7962-8_1,
© The Author(s) 2013
2 1 Introduction
pyrolysis and their combustion. In this sense, if the measurement technologies from
these disciplines are important and efcfi ient at the laboratory scale, they must be
adapted to outdoor conditions to evaluate real-scale rfi es. Moreover, even in this
case, they cannot produce a complete overview of the rfi e physics, because they do
not describe the hydrodynamics of the rfi e.
More generally, attempts to extrapolate experiments from the lab to the field
scale may be doomed to failure because of the difficulty in predicting, a priori,
how fire is a scale-dependent process (Pitts 1991). This is mainly due to the hydro-
dynamic nature of fire, which can be considered a turbulent reactive and strongly
radiative flow. Inherent in this characteristic is the stochastic behaviour of gov-
erning physical quantities such as temperature, mass flow rate, and heat fluxes.
These properties vary strongly in time and space. Furthermore, if fire is considered
a turbulent reactive and radiative flow, its dynamics cover a range of spatial and
temporal scales. This range should be extended as the size of the fire increases.
Measurements of real-scale fires, therefore, are simultaneously necessary and
extremely difficult to perform, because experimental devices must be able to cap-
ture both long-scale phenomena and rapid fluctuations. They consider turbulent
reactive flows which are coupled strongly with other complex non-linear processes
such as radiation and soot formation; fine measurements in such flows usually use
optical methods to prevent flow from local perturbations. Such methods are dif-
ficult at the laboratory scale (Nathan et al. 2012). Furthermore, as we will discuss,
the most important processes are often poorly understood and are blurred into a
complex thermal environment. This is why it is extremely hard to develop devices
that are simultaneously non-intrusive, not influenced by the fire, and sufficiently
accurate in field-scale experiments.
These represent the main reasons why experiments under outdoor conditions have
been neglected for a long time, despite some extraordinary and rare scenarios, such
as the International Crown Fire Modeling Experiment (Butler et al. 2004), and why
studies have focused instead on repeatable, short-duration, and easy-to-perform labo-
ratory-scale experiments.
This brief will review experimental techniques for fire experiments in the field.
It presents solutions, and their advantages and limitations, for studying fire in the
open, beyond academic and unrealistic laboratory-scale configurations. It deals
mainly with natural fires, in which fuel is formed by vegetation, but measurement
techniques also can be used for confined fires or liquid fuels. As previously noted,
the main problem for experimentalists working with large-scale fires is that the
fire environment is thermally and chemically aggressive. However, measurement
devices must be accurate because of the strong spatial and temporal variability in
fire properties, especially those related to heat transfer.
Let us observe the temperature signal obtained from a wildfire experiment in
southern France in spring 2006 (Silvani and Morandini 2009).
Figure 1.1 shows both the instantaneous and time-filtered curves of the temper -
ature signal obtained with a sensor immersed in a real-scale fire of Mediterranean
shrubs. The flame front spread over 3000 m² and measured 6 m high and 20 m
long. The sensor was fire-resistant and therefore measurements were possible
1 Introduction 3
Fig. 1.1 Temperature record during a field-scale fire experiment