ISO/PAS 24499:2024
(Main)Method of test for burning velocity measurement of A2L flammable gases
Method of test for burning velocity measurement of A2L flammable gases
This document specifies a method of measuring the burning velocity (BV) of slowing burning refrigerants (
Méthode d'essai pour mesurer la vitesse d'inflammabilité des gaz inflammables A2L
General Information
Standards Content (Sample)
Publicly
Available
Specification
ISO/PAS 24499
First edition
Method of test for burning velocity
2024-05
measurement of A2L flammable gases
Méthode d'essai pour mesurer la vitesse d'inflammabilité des gaz
inflammables A2L
Reference number
© ISO 2024
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ii
Contents Page
Foreword .v
Introduction .vi
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 General test method . 2
4.1 General .2
4.2 Principle of the test method .2
5 Measurement parameters . 4
5.1 General .4
5.2 Flame propagation velocity .4
5.3 Flame front area .5
5.4 Cross-sectional area of the flame base .5
6 Test method . 5
6.1 General .5
6.2 Gas handling and mixtures preparation .6
6.3 The test tube .7
6.3.1 General .7
6.3.2 Dimensions .8
6.3.3 Position .9
6.3.4 Tube ends .9
6.3.5 Interchangeable damping orifices .9
6.3.6 Flame quenching .10
6.3.7 Tube glass type .10
6.3.8 Tube purging with test mixture .10
6.3.9 Tube etching .10
6.4 Ignition .11
6.4.1 General .11
6.4.2 Ignition type .11
6.4.3 Positioning .11
6.4.4 Electrodes .11
6.4.5 Power supply . .11
6.4.6 Ignition time .11
6.5 Flame front visualization . 12
6.5.1 General . 12
6.5.2 Luminous zone and direct photography . 12
6.5.3 Flame emission spectra . 12
6.5.4 Acquisition camera . 12
6.5.5 Exposure Time . 13
6.5.6 Positioning . 13
6.5.7 Scaling and optical distortion. 13
6.5.8 Resolution of the flame images .14
6.6 Purge, exhaust and gas treatment systems .14
6.7 Test temperature setting . 15
6.8 Experimental protocol for mixtures prepared using partial pressure technique.16
7 Evaluation and expression of results . .16
7.1 General .16
7.2 Uncertainty .17
7.2.1 Uncertainty in the burning velocity .17
7.2.2 Uncertainty estimation of concentrations .17
8 Safety precautions. 17
iii
9 Overview on flame shape, propagation regimes and stability .18
9.1 Flame shape.18
9.2 Flame propagation regimes .19
9.3 Flame stability in tubes . 20
9.4 Observations of flames in tubes .21
9.5 Flame quenching in circular tubes .21
9.6 Flame propagation velocity and tube diameter . 22
9.7 Flame area calculation . 22
Bibliography .24
iv
Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards
bodies (ISO member bodies). The work of preparing International Standards is normally carried out through
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The procedures used to develop this document and those intended for its further maintenance are described
in the ISO/IEC Directives, Part 1. In particular, the different approval criteria needed for the different types
of ISO document should be noted. This document was drafted in accordance with the editorial rules of the
ISO/IEC Directives, Part 2 (see www.iso.org/directives).
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This document was prepared by Technical Committee ISO/TC 86, Refrigeration and air-conditioning,
Subcommittee SC 8, Refrigerants and refrigeration lubricants.
Any feedback or questions on this document should be directed to the user’s national standards body. A
complete listing of these bodies can be found at www.iso.org/members.html.
v
Introduction
Safety classification and relative flammability properties of refrigerants are a critical part of ISO 817. The
1)
flammability limits of refrigerant gas in air, as described in Annex B of ISO 817:— give a partial measure of
the relative flammability. Another dimension of flammability is how fast a substance burns, releases energy
and spreads a flame. One can measure the rate at which a flame front moves through a cloud of refrigerant
gas in air, or its burning velocity (BV). This document describes one method that can be useful for BV
measurement, and thereby better quantify and compare relative flame fronts for some refrigerant classes.
In this test, a flame is allowed to propagate upward (vertically) through a well-mixed, quiescent column of
a refrigerant-air mixture enclosed in an open-ended glass tube. Optical systems are used to measure the
upward velocity of the flame front.
The measurement of BV has been widely used in the past to compare highly energetic fluids, such as motor
fuels and rocket propellants. The BV measurement of slow burning fluids, such as ammonia and fluorinated
refrigerants can be more difficult to measure due to the inherent instability of a slow flame. The low rate
of energy evolution from a slow flame makes it susceptible to quenching from a variety of sources. For slow
burning refrigerants, turbulence and convection currents, can break the flame front, and hence quench the
flame. In addition, the test chamber surface can quench free radical flame intermediates as well as extract
some of the heat necessary for flame propagation. Gas-phase thermal radiation is also important for flames
with low burning velocity. These effects are important to note as they tend to diminish and sometimes
quench a weak flame.
The use of the vertical tube method for BV characterization of slow burning refrigerants was the subject
[1]
of doctoral research which was used in Annex C of ISO 817:— and is the basis for this document. In
[2][3]
addition, ASHRAE research notes the use and limitations of the vertical tube technique. While the basic
framework of the method is relatively simple, some sophisticated imaging instrumentation and mathematics
are necessary to extract an average local burning velocity separate from the bulk burning speed as the
flame progresses up the tube. Since 2004 other laboratories have used the basic principle of vertical tube
method and have shown acceptable results for reproducing the measurement of R-32, at 6,7 +/- 0,7 cm/s.
Slower burning velocities (i.e. <4 cm/s) become more difficult to measure reproducibility, so variability may
increase as flame instability is increasing. The lower burning velocity limit of this method, as described, is
between 3 cm/s and 4 cm/s, depending on the actual design and geometry of the apparatus being used. The
uncertainty of the measurement of flames that burn more slowly than R-32 has not yet been determined in
any multi lab comparative testing. The appealing aspects of this test are the relative simplicity and low cost
of its implementation.
1) Under preparation. Stage at the time of publication: ISO/DIS 817:2023.
vi
Publicly Available Specification ISO/PAS 24499:2024(en)
Method of test for burning velocity measurement of A2L
flammable gases
1 Scope
This document specifies a method of measuring the burning velocity (BV) of slowing burning refrigerants
(< 10 cm/s) for use with other standards that utilize the BV for determining safety classification of
refrigerants (e.g. ISO 817) or that use the BV in establishing requirements on the use of slow burning
refrigerants (e.g. ISO 5149).
2 Normative references
There are no normative references in this document.
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
ISO and IEC maintain terminology databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https:// www .iso .org/ obp
— IEC Electropedia: available at https:// www .electropedia .org/
3.1
blends
mixtures composed of two or more refrigerants
3.2
burning velocity
S
u
maximum velocity at which a laminar flame propagates in a normal direction relative to the unburned gas
ahead of it
Note 1 to entry: This value is expressed in centimetres per second.
3.3
combustion
exothermic reaction between an oxidant (e.g. air) and a combustible fuel
3.4
compound
substance composed of two or more atoms chemically bonded in definite proportions
3.5
flame
space where combustion takes place, resulting in a temperature increase and light emission
3.6
flame propagation
combustion, causing a continuous flame which moves upward and outward from the point of ignition
without the influence of the ignition source
3.7
flame propagation velocity
S
s
velocity at which the continuous flame moves upward and outward from the point of ignition without the
point of ignition and without the influence of the ignition source
3.8
flame surface area
A
f
surface area of the flame generated during the combustion of the flammable gas
3.9
flammable
property of a mixture in which a flame is capable of self-propagating
3.10
quenching
effect of extinction of the flame near a surface due to heat conduction losses, absorption of active chemical
species and viscous effects of the surface
3.11
refrigerant
fluid used for heat transfer in a refrigerating system, which absorbs heat at a low temperature and a low
pressure of the fluid and rejects it at a higher temperature and a higher pressure of the fluid usually involving
changes of the phase of the fluid
3.12
stoichiometric concentration
C
st
concentration of a fuel in a fuel-air mixture that contains exactly the necessary quantity of air (approximately
21 % O / 79 % N by volume) needed for the complete oxidation of all the compounds (3.1.4) present
2 2
4 General test method
4.1 General
The test method is based on:
a) the initiation of the combustion of the gas, or blends of gases, in a stagnant homogeneous mixture with
air contained in a vertical cylindrical tube;
b) the observation and the recording of the flame propagation;
c) determining the surface area of the flame.
The burning velocity is a function of the flammable gas concentration in the total mixture with air. The
burning velocity reaches a maximum in the vicinity of the stoichiometric concentration.
This test method involves the use of hazardous substances and therefore requires, for a safe handling and
testing, the knowledge of safety parameters and prevention measures. These measures shall be the user’s
responsibility. However, general safety precautions are given in Clause 8.
4.2 Principle of the test method
The test method consists of initiating the combustion of a homogeneous mixture of a flammable gas (or
a flammable mixture of gases) and air, contained in a vertical tube opened at the lower ignition end, and
propagating a flame upwardly to the upper closed end; see Figure 1. In the early stages of this propagation,
there is a phase of uniform movement during which the shape and the size of the flame are constant.
Taking into account the mass and species balance through the flame front, the burning velocity, S , is
u
calculated from the knowledge of the flame propagation velocity, S , in the tube and the ratio of the flame
s
front area to its base cross-sectional area. The volume of burned gas per second and per unit area, or the
burning velocity, S , is obtained by dividing the mixture volume which is consumed per second, at the test
u
temperature and pressure, by the flame surface area, A (the subscript “f” denotes the flame). The volume
f
consumption of the mix per second is the volume swept by a cross-sectional area of the flame base, a , with
f
a velocity equal to the flame propagation velocity S . Formula (1) is used to determine volume consumption
s
per unit time.
a
f
SS=× (1)
us
A
f
where
a is the cross-sectional area of the flame base;
f
A is the flame surface area;
f
S is the flame propagation velocity;
s
S is the burning velocity.
u
NOTE The cross-sectional area of the flame base is equal to the tube cross-section reduced by the quenching area
(the area between the edge of the flame and the tube wall).
At a given temperature and pressure, the burning velocity is only a function of the type of flammable
substance and its concentration with the oxidant and is dependent to a limited extent on the experimental
apparatus.
Key
1 direction of flame propagation
2 unburned mixture
3 flame front displacement
4 dx thickness of the combustion region
5 S
u
6 burnt gas
7 ignition
Figure 1 — Schematic of the flame propagation in a vertical tube
5 Measurement parameters
5.1 General
The measurement of the burning velocity requires the knowledge of the following three parameters of
Formula (1):
a) the flame propagation velocity, S ;
s
b) the flame front area A ;
f
c) the cross-sectional area of the flame base a .
f
5.2 Flame propagation velocity
The flame propagation velocity in the tube is required for the measurement of the burning velocity. As a
condition to the derivation Formula (1), only parts of uniform flame propagation shall be considered in the
measurements (constant S ).
s
The linear propagation velocity of the flame is obtained from the direct measurement of the flame front
displacement determined by two successive images with a known time interval (30 Hz to 50 Hz) of the
camera acquisition frequency. More than one succession of images shall be used to check that the flame
propagation is uniform. An image treatment is necessary in order to enhance the flame front shape and to
locate on both images an identical luminous spot (pixels with equal brightness level) that corresponds to
the same location on the front and deduce the flame front displacement. This procedure is proved necessary
with low luminosity flames since any uncertainty in the flame front displacement leads to an uncertainty in
the flame propagation velocity and thus on the burning velocity.
5.3 Flame front area
The flame front shape cannot be generated by the revolution of a parabola nor by the approximation by an
ellipsoid segment, even though in many cases this shape is symmetrical. An accurate method is needed to
calculate the flame front area A . For an upward propagation, the flame usually shows a symmetrical front
f
surface referred to the tube axis. For a uniform propagation, the shape of the flame front remains constant.
Fast moving flames are almost hemispherical, the slower flames are somewhat elongated.
9.7 describes a mathematical and geometrical model to calculate the flame front area. In summary, the flame
front profile is marked with fitting points (20 to 40 fitting points) then divided into two or more horizontal
sections. The fitting points shall be selected on the rim of the most luminous zone on the flame front.
For each section a polynomial fit equation of appropriate order is made in order to give the best fit curve to
the points selected on that section. The best fit gives the minimum deviation of the fit curve to the fitting
experimental points. The area of each section shall then be calculated separately by dividing it into many
small elementary sections. The area of each elementary section is then calculated from the assumption of a
revolution shape., taking into account the bottom edge of the flame not being horizontal.
5.4 Cross-sectional area of the flame base
The cross-sectional area, a , of the flame base shall be calculated from knowledge of the diameter d measured
f
at the base of the flame as illustrated in 9.7. In that case, use Formula (2):
πd
a = (2)
f
where
a is the cross-sectional area of the flame base;
f
d is the diameter of the flame base.
6 Test method
6.1 General
Measuring the burning velocity in a tube consists of
a) propagating a flame in a vertical transparent tube, opened at the lower ignition end, closed at the other
upper end, and filled with the flammable mixture,
b) measuring the velocity of the flame propagating along the tube, and
c) recording the flame front area with a camera.
Measurements are performed at atmospheric pressure.
The test bench layout is shown in Figure 2. The main elements of the bench are
— mixing vessel,
— ignition system,
— camera,
— test temperature control, and
— gas treatment systems.
NOTE To minimize pressure feedback effects, typically the scrubber system is not attached during the ignition
and burn portion of the testing (see Figure 8).
Key
a
1 mixing vessel From gas supply tanks.
b
2 magnetic stirrer Pressure measurement.
c
3 purging gas line To vacuum pump.
d
4 tube inlet To inlet tube.
e
5 quenching and smoothing screen Temperature measurement.
f
6 test tube Supply power to ignition.
g
7 electrodes Extraction to hood.
8 fitting orifices
9 quenching screen
10 poly(vinyl chloride) pipe
11 igniter
12 gas expansion tank
13 collection tank with neutralizing solution
Figure 2 — Schematic of the test bench
6.2 Gas handling and mixtures preparation
The gas mixture preparation is described in ISO 817:—, 6.1.3. If used, the scrubbing system described in
6.6 should be disconnected so that the expansion volume is not filled with a flammable concentration.
The constant composition blend is then caused to flow through the tube until the gas mixture displaces at
least thirteen times the air volume of the tube. Care should be taken to ensure that the gas mixture exiting
the bottom of the tube is properly vented. Once the desired mixture has been achieved in the tube, the
mixing vessel shall be isolated from the tube before ignition to prevent ignition of the gas in the vessel.
It is good laboratory practice to measure the concentration of the gas mixture in the tube to ensure the
methods employed adequately accomplish this objective. A paramagnetic oxygen analyser is effective for
this determination.
It is recommended that all the components, connections and parts of the test bench be resistant to their use
with corrosive gases, such as ammonia and copper, or other oxidation reactions. Stainless steel can be used,
or any other material identified to be adequate for use with the substances to test.
6.3 The test tube
6.3.1 General
The test tube shall be designed to ease the flame propagation with less possible disturbances, especially at
the ignition level and the first stage of flame propagation; see Figure 3. The design of the test tube should
look into the following points:
a) the ignition system, the quenching screen, and the damping orifice should be designed as close as
possible to the outlet of the tube;
b) the outlet of the tube (at the lower end) should be designed to facilitate its connection to the extraction
and gas treatment systems;
c) the tube should be fixed on a vertical support and at a level below the ignition system to prevent the
fixing support from disturbing the flame propagation (excessive cooling) or any obstruction of the
flame photography;
d) technical limitation with glass design and work should be considered as well.
Dimensions in millimetres
Key
1 fixing housing
2 RIN 10/19 housing for electrodes
3 inlet tube end
Figure 3 — Test tube design and main dimensions
6.3.2 Dimensions
The tube shall be made of glass, 1,2 m long with a 40 mm internal diameter. The diameter has been chosen as
a compromise between narrower tubes that increase the quenching effect but allow more stable propagation
regimes, and larger tubes in which the losses to the walls are smaller but associated with an increase of
[1][4] [5]
instabilities. The choice of the 40 mm diameter has been shown to be the most convenient for
measurement of burning velocities below 40 cm/s. It withstands a pressure of 100 kPa above the atmospheric
pressure even if the overpressure is very limited, the bottom end of the tube being the open end.
NOTE Unstable regimes are frequent with fast propagating flames; see 9.3. The tube length is based on dimensions
from previous research. Any great change in that length affects the flame propagation regimes and its stability only
when working with high burning velocity compounds.
6.3.3 Position
The tube should be placed in a vertical position to reduce possible deformations of the flame front from
buoyant effect and to ensure a more symmetrical shape. In this position the flame propagates upwardly, the
ignition occurring at the lower end of the tube.
6.3.4 Tube ends
The bottom end of the tube shall be open to the atmosphere. At this end are located the ignition system and
the damping orifices. A GL45 cap can be used to maintain the system in place (see Figure 4 and Figure 5).
With harmful components present in the combustion products (toxic or corrosive, e.g. HF, HCl, NH ), the
lower end should be connected to a gas post-treatment system (see 6.6). This design does not allow excessive
pressure build-up and the combustion products can freely exit the tube or expand in a 125 l tank if the gas
treatment system is used.
The upper end of the tube should be connected to the mixing vessel. The mixture flows out from mixing
vessel into the tube and out of its bottom end. A GL45 cap shall be used to fix the inlet system. This end shall
be closed before the ignition and until the end of flame propagation.
6.3.5 Interchangeable damping orifices
The flame propagation velocity and the flame shape vary with the type of flammable substance and the
composition of its mixture with the oxidant. Adjusting the exit diameter at the lower open end by insertion
of calibrated orifices helps stabilize the flame front shape by reducing the instabilities and damping the
[6][7][8][9]
acoustic effects and therefore helps to reproduce a better shape of the flame front. The diameters
of the damping orifices for a tube of 40 mm internal diameter vary from 9 mm to 11 mm (see Reference [9]
for detailed calculation). The damping orifices are recommended with relatively high burning velocities (i.e.
higher than 25 cm/s).
Dimensions in millimetres
Key
1 cap for GL 45 tip 5 RIN 10/19 polytetrafluoroethylene (PTFE) stopper
2 polytetrafluoroethylene (PTFE) body 6 test tube
3 quenching screen 7 1 mm diameter electrode
4 polytetrafluoroethylene (PTFE) fitting orifice 8 power supply connection
Figure 4 — Drawing of the lower end of the tube showing the ignition electrodes and the damping
(fitting) orifice
6.3.6 Flame quenching
Quenching screens shall be mounted at both ends of the tube, resistant to the reaction with HF and NH , to
prevent any hazard to the surroundings. The quenching screens shall have a mesh size of 1 (+ 0,5 - 0,1) mm.
6.3.7 Tube glass type
The spectral emissions of most flames are presumed to be in the range of 250 nm to 600 nm. To prevent
excessive losses, it is important to compare the glass transmission profiles before selecting the type of glass
(e.g. silica glass, borosilicate glass).
6.3.8 Tube purging with test mixture
The test tube shall be purged by the mixture under test with a continuous flow from the mixing vessel with
an equivalent volume flow rate which represents at least 13 times the internal tube volume. The gas mixture
shall enter the upper end of the tube and exit from its lower end. The lower end can be closed after purging
to avoid any possible concentration variation by dilution in the neighbourhood of the electrodes. This end is
opened to the atmosphere just before ignition.
6.3.9 Tube etching
The presence of substances such as hydrogen fluoride (HF) or hydrogen chloride (HCl) with water residues
in the combustion products of HFCs or HCFCs results in tube etching so that after several tests (30 to 50
depending on the cleaning process) the tube turns opaque with an almost white colour (see Figure 5).
For this reason, the tube shall be purged immediately after the end of the flame propagation with a stream
of dry nitrogen. Afterwards, a wet wiper may be introduced inside the tube to clear all deposits on the inner
wall. A stream of nitrogen may be again circulated inside the tube to remove water deposits from the wiper.
With this cleaning technique it is possible to use the same tube for a larger number of tests before the etching
effect becomes noticeable and the tube needs to be discarded.
Figure 5 — Tube etching due to hydrogen fluoride
6.4 Ignition
6.4.1 General
The ignition source can affect not only the flammability limit results but also the flame propagation regime.
Analyses of spark ignition have been made by many researchers (Reference [10] gives a survey) and deal
with the electrode arrangement, type (flange electrodes for instance), material and size, the electrode gap,
the spark duration and the breakdown voltage as well as the effect of these on the minimum ignition energy.
The ignition system described in this test method has the same characteristics as the ignition system used
in the ASTM E681 flammability test method in terms of the electrode dimensions, the gap distance, the
ignition time and the power supply. This similarity helps to ensure that the vertical tube burning velocity
method and complements the ASTM flammability method.
NOTE These ignition specifications are also very similar to those specified in DIN 51649-1 (which is meant by the
[9]
flammability limits).
6.4.2 Ignition type
The mixture is ignited with an electrical spark produced by two electrodes.
6.4.3 Positioning
The ignition occurs at the bottom end of the tube. The electrodes are fixed diametrically opposite on the
tube, centred on its axis and positioned 5 mm to 10 mm above the upper surface of the interchangeable
orifices. The electrodes shall be fixed using RIN 10/19 PTFE stoppers lodged in specially conceived RIN
10/19 housing (see Figure 4)
6.4.4 Electrodes
The electrodes are made of tungsten with 1 mm diameter. The gap between the electrodes is 6,4 mm. When
necessary, a special calibrating cylinder can be inserted inside the tube and in-between the electrodes in
order to verify their eccentricity and to ensure a correct gap distance.
To ensure good ignition conditions, especially near the lower and upper propagation limits, the electrodes
shall be repeatedly cleaned of any deposit.
6.4.5 Power supply
Power to the ignition electrodes shall be supplied by a transformer with an output of 15 kV, 30 mA. Usually,
such high voltage is not required except with compounds having a high breakdown potential. The power
supply system is connected to the electrodes using insulation rated for at least 15 kV to avoid short circuits
and poor contacts avoiding overheating.
6.4.6 Ignition time
The ignition time shall be set at (0,3 ±0,05) s by adjusting the spark duration with a timer. This time duration
[9]
has been proved to be the most appropriate for flammability limits measurements.
Ignition should not be made immediately after filling the tube with the corresponding mixture, but 5 s to
10 s later, permitting the turbulence to cease in the tube.
NOTE The excessive energy release from this ignition system might be responsible for emitting waves inducing
turbulence in the flame front and the mixture ahead of it. Flame propagation is not steady close to the ignition source.
6.5 Flame front visualization
6.5.1 General
Direct photography is used to record the flame front images. These images are used for the calculation of the
flame propagation velocity as well as its surface area.
6.5.2 Luminous zone and direct photography
The burning velocity measurement Formula (1) is based on the calculation of the flame front area at the
preheat zone layer. With direct photography, the luminous zones of the flame are revealed. Therefore, any
measurement made with this photography technique shall be based on the zone of the flame of most intense
illumination. This zone corresponds to the region of the flame between the point whose temperature is
equal to the ignition temperature and the point at the end of reaction (see Figure 6). The relative uncertainty
in the burning velocity assessed with the flame front area calculation based on flame profiles from direct
photography is 6,5 %.
NOTE The 6,5 % relative uncertainty can be reduced and the correct surface position can be better approached if the
profile of the outer edge of the luminous zone is shifted outwards by a distance equivalent to the luminous zone width.
Key
1 unburned gas
2 burned gas
3 luminous zone
4 pre-heat zone
5 reaction zone
Figure 6 — Temperature profile along a combustion flame and luminous zone
6.5.3 Flame emission spectra
The spectra peaks from combustion depend on the type of substance combusted and the radicals formed
such as OH, HCO, CH, C and C . From a qualitative point of view, it can be stated that the typical peaks for
2 3
maximum emission, and even sometimes a higher-level continuum, are in the range of 250 nm to 600 nm for
HC and HFC flames.
6.5.4 Acquisition camera
A digital camera shall be used to visualize the flame propagation. The flame front images shall be recorded
and saved for further treatment (flame propagation velocity measurement and flame front area calculation).
When identifying the camera to run the tests, the characteristics of exposure time and acquisition rate shall
be selected as a function of the velocity range being measured. With very fast flames, a high acquisition
rate and small exposure time are needed (i.e. <1 ms). The spectral response of the camera shall be also
taken into account and the higher efficiency of the quantum efficiency curve shall cover the range of typical
wavelength of the flames being visualized.
NOTE A set of adjustments and different operating modes, such as the resolution, image enhancements, image
rate, exposure time, number of frames during record, pre-/post-trigger and parameters for image output, performed
via an appropriate interface, can help in adapting the images to the type of flame front being recorded. A set of lenses
can also be used to zoom and focus the optimized photography frame.
6.5.5 Exposure Time
Setting the exposure time is necessary before starting the photography of the flame propagation to best
reproduce the flame front shape and increase the precision of its area measurement.
Since there is no defined relationship between the flame propagation velocity and its more or less luminous
aspect, for fast and low luminous flames the tester has to find a compromise for setting the exposure time.
A higher exposure time compensates the low luminosity but results in an imprecise shape of the flame front
due to its displacement during the exposure time.
For measurements around the stoichiometry, the recommended exposure times are of 1 ms or less. This
value is determined by practical experience and depends on the camera.
6.5.6 Positioning
The camera recording field shall be adjusted to the appropriate position and height of the tube where the
flame movement is known to be uniform. Only images taken at the same level of the lens axis shall be used to
calculate the flame front area and reduce the imprecision on the flame front dimensions.
6.5.7 Scaling and optical distortion
Scaling of the flame images to the real flame front dimensions can be achieved by taking a photo of a
graduated ruler placed along the tube in order that the graduations coincide with a layer cross
...
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