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学习进度更新

2008年9月20日:15-26

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15

Air is 99% composed of nitrogen (N) and oxygen (O). It also contains very small quantities of the noble gases, which can have an effect on fire development at an extremely scientific level.

空气主要由氮和氧组成，这里的noble gases：稀有气体

Thus when we consider the carbon, hydrogen and oxygen already existing in the wood and add nitrogen, we can form dangerous mixtures such as hydrogen cyanide 氰化氢.
Although in small percentages, air also contains these noble gases:

If we apply the same process of pyrolysis to all of these elements and mixtures, we can see that there is potential for creating a large number of different gases, with a variety of features such as toxicity and flammability.

And this is only by burning a piece of wood in air. Let’s consider that modem dwellings and workplaces contain furniture, fixtures and contents that are made up of far more complex chemical elements than a wooden bench. There is clearly a potential for firefighters to be operating in an environment composed of a number of chemicals in gaseous form, as smoke, before the fire is even located.

现代火灾，消防员在找到火点前，要处理各种气态物质

For firefighters, how does smoke (gaseous fuels) affect our approach?

If we consider that the majority of these gases are flammable, then we must consider that all smoke is unburned fuel, harmful, flammable and ignitable. A firefighter would not walk through a pool of gasoline and without recognising it as fuel, so why do we continue to walk through unburned fuel in the form of smoke when carrying out our operations at fires?

大部分气体都是可燃的，对消防人有害的，都是未完全燃烧的可燃物。因此在通过这些气体之前，消防员有必要采取一些必要的动作

This environment was termed the '3D environment' by Paul Grimwood, Ed Hartin, John McDonough and Shan Raffel (for more information, see http://www.3dfirefighting.com/).

The presence of fire gases is three dimensional (3D) and requires a 3D or'all around’ approach to successfully address this and reduce risks.

气体都是以三维的形式存在的，因此需要三维或者是全方位的接触，来降低消防员的风险

16

Flammability limits

A relationship between fuel and air
Flammability limits are the ratios of concentration between a fuel in vapour (gaseous) form and the air in the immediate surrounding atmosphere.

燃烧极限是一个比率，可燃性气体占空气中气体的比例

Flammability limits are widely categorised into the following:
■ Lower Explosive Limit
■ Upper Explosive Limit
■ Ideal Mixture (aka Stoichiometric Mixture).

Lower Explosive Limit
This is the lowest concentration of fuel to air that will sustain combustion. It is sometimes referred to as a ‘lean mixture ’. If the concentration dips below this it can be referred to as ‘too lean to burn’.

燃烧下限，最小浓度

Upper Explosive Limit
This is the highest concentration of fuel to air that will sustain combustion. It is sometimes referred to as a ‘**rich mixture **’. If the ratio of fuel to air is too high it can be known as ‘too rich to burn ’.

燃烧上限，最大浓度

This can be a very dangerous situation and if a fire has been or is present it can be referred to as ‘Ventilation controlled’ 一 it’s just waiting for introduction of air to ‘dilute’ it back down to within its flammable limits where it can bum freely.

达到燃烧上限时是非常危险的，这种火灾就是通风控制型火灾，等待空气稀释火场气体的浓度，浓度下降以后，火灾就会发生

Ideal Mixture/Stochiometric Mixture
This is the ratio of fuel to air where a substance can bum most efficiently and with the greatest force.

理想燃烧极限

17

Figure 4: Examples of flammability limits

Courtesy of Merseyside Fire & Rescue Service, UK.

For firefighters, what do the flammability limits mean for us?
As we can see, all fuels need a supply of oxygen to burn. We can take steps to control the amount of air a fuel can mix with through 'flowpath management’ and either ventilating or shutting down compartments.

对消防员来说，难烧极限意味着什么？我们可以通过控制火场的进气量对火势进行控制，比如气流管理，通风，或者是关闭门窗等措施

We also need to be aware that we may be proceeding into danger if we know that a fuel/air mix is above its Upper Explosive Limit and‘too rich', and that if we open doors we may allow airflow to dilute it down to within its flammability limits.

It shows that fuels are in vapour form everywhere in the 3D environment and must be dealt with to reduce our exposure to risk.

可燃物都是以蒸汽的形式存在

We have shown that products of combustion such as carbon monoxide have wide flammable ranges and can burn across a wide ratio of concentrations with air. But modern compartments have more materials than wood releasing C6H10〇5 and fire gases are a combination of a number of fuels in vapour form.

A vigilant and well prepared firefighter knows this and treats all vapours in the 3D environment as flammable fuels ready to burn.

消防员只有把各种蒸气当作可燃物来处理，才能确保灭火救援过程中的安全

18

Vapours, and unburned pyrolysis products are flammable!

Note the exiting gases have been ignited (Photograph, Walker IFRA 2015).

Passive agents

The hidden fourth side of the fire triangle

Passive agents are materials within any compartment that absorb energy in the form of heat. All elements within a compartment’s structure or furnishing act as passives for a short time, until temperature increases, pyrolysis begins and the thermal capacity of that material is reached.

温度较低时，所有的可燃物，以被动的形式在吸收热量

At this point the material is no longer passive and actively contributes to the fire as fuel or by transferring heat.

Gases released by passives undergoing pyrolysis can also act as passive agents. Water vapour and carbon dioxide given off in this way can suppress fire development in the early stages and in localised areas.

产生的水蒸气和二氧化碳会抑制火势的发展

19 So what do passives do?

Passive agents take away energy from the fire which can slow its growth for such a time until they can absorb no more energy and start actively contributing to the fire’s growth as fuel.

Heat

Heat is perhaps the most misunderstood of the traditional three elements of the fire triangle. To dispel 消除 a popular misconception we need to know the difference between temperature and heat.

Heat is a measure of the energy contained by a body, both its potential (stored) and kinetic (moving) energy. It is measured in joules (J).

热量用于测量能量

Temperature is a measure of the kinetic (moving) energy of that body. As heat is introduced, the molecules move faster and collide more frequently, producing more heat. Temperature is a measure of how frequently these collisions occur.

温度是测量分子运动的一个指标

However, these are not inextricably linked. For example，the heat created by raising 500 litres of water to 100°C is substantially greater than that created by raising one litre to 100°C although the temperature is the same.

Or，to use another example, a cup of tea at 80°C is a higher temperature than a swimming pool at 30°C, but because the swimming pool is clearly a much larger quantity of water, the total thermal energy it contains (its heat) is a great deal higher.

Measuring heat:

The formula to measure heat is as follows:
Q= CMT
Where Q is heat, C is specific heat capacity, M is mass of the body and T is its temperature.

20 Auto ignition temperatures

When a fuel reaches a certain temperature, there is enough kinetic energy in it to ignite it without the presence of a flame or the introduction of an external ignition source.
Some common examples of auto ignition temperatures are:
插入图表

21 Flashpoints and firepoints

When a liquid substance reaches a certain temperature there can be fire gases (pyrolysates/volatiziles/vapours) released that form an ignitable mixture in air that, when an ignition source is introduced, causes combustion to occur.

物体热解，挥发,蒸发

If the substance does not continue to bum when the ignition source is removed this is known as the‘flash point’. If the substance continues to burn once the ignition source is removed then this is known as the ‘fire point’.

Accepted definitions are:
Flash point: the lowest temperature at which a substance vapourises so that the introduction of an ignition source causes a flame to momentarily ‘flash'across the surface of the fuel.

Fire point: the lowest temperature at which a substance vapourises so that the introduction of an ignition source results in continued combustion once the ignition source is removed.

闪点和燃点的定义

Heat release rate

Heat release rate (or HRR) is a measure of power or the heat energy that is released per measure of time by a fire. It is influenced by many factors including the shape of a fuel and its surface area, for example，a sheet of paper burns more quickly and releases its heat energy more quickly than a log - although the log can generate more heat and greater temperatures over a longer period it releases it more slowly over a greater period of time.

热释放速率是单位时间内，可燃物燃烧释放出的能量；可燃物的形状，以及表面积都会影响HRR。
It should be noted that the heat release rate of a fire dictates its size and its speed of growth. If the HRR is higher, then the pyrolysis process is quicker, more gaseous fuel is released and more oxygen is used up, resulting in incomplete combustion and quantities of unbumed fuel in a ventilation controlled environment.
热释放速率越高，分解的速度越快，会产生更多的可燃性气体，氧气消耗的也会更快，会导致出现不完全燃烧,大量的未燃烧烧的可燃物进入通风控制状态

This is the MOST IMPORTANT factor in the intensity, speed and development of fire.

热释放速率对火灾的发展非常非常重要，
My old colleague, a fellow.adventurer, Station Officer Jim Dave of the States of Jersey Fire & Rescue Service, emphasises the importance of heat release rate in the following extract from his masters’ degree dissertation.

23 Heat release rate versus temperature

Underwriters’ Laboratories and the National Institute for Science & Technology (UL/NIST) draw a distinction between HRR and temperature using candles as an example.

One candle and ten candles (of the same type,size and composition) burn at the same temperature but the ten candles release ten times the energy,hence ten times the heat release rate.

十根蜡烛和一根蜡烛，点火热释放速率的对比
The graphic below shows a single candle that will burn at a temperature of 500-1,400°C and give off a HRR of 80 watts.
Also shown are ten candles of the same type and sizs. These also burn at a temperature of 500-1400°C,yet produce a HRR of 800 watts.

Figure 5: Heat release rate of candle

Heat transfer can be measured in terms of the heat transferred over a measured area in a measured amount of time. This is known as‘heat flux’
Let’s briefly examine each of these in turn.

单位时间内通过，可燃物横截面上的热量，热通量

24 Conduction

Conduction is the transfer of heat energy through direct contact of materials.

For example, Greg Gorbett and Jim Pharr in Fire Dynamics cite the example of a metal rod being held in a flame. The part of the metal rod inside the flame is increasing in temperature, so there is more molecular movement (collisions).

These begin to strike and affect the neighbouring molecules and therefore create a chain reaction, with a temperature increase moving along the rod until it reaches the portion where it’s held.

热传导是通过与物质，直接接触，进行传导热量，

Figure 6: Conduction

Firefighters should be aware that certain materials conduct heat extremely well and heat transfer and subsequent ignitions can occur in this way. Other materials do not conduct well and are known as`insulators’ 绝缘.

25 Convection

Convection is the transfer of heat energy through a ‘fluid medium’. Beware here that the term ‘fluid’ does not necessarily mean a liquid. It is far more likely that the fluid medium encountered by firefighters is air.

热对流是热量传导的第二种形式，这里的流体不仅仅指液体，对消防员来说，主要是空气

Figure 7: Convection

Note the heat rises from the heater while cool air is drawn in towards the heat source at the lower levels as it cools and falls.

In a compartment, heated air/fire gas flows past a solid object and there is a temperature difference. The motion of the fluid and the conduction through air of molecules in the fluid, causes increased molecule activity (collisions) in the solid and therefore a rise in temperature (kinetic heat energy) in the solid.

Convection can transfer heat either ‘naturally’ or ‘forced’； consider how a hot pie cools down when you blow onto the surface (forced).

热对流有可能是自然的，也可能是外界的原因

This type of forced heat transfer should also be remembered when managing flowpaths as the actions of the firefighters may change the direction and speed of how the ‘fluid medium’ flows, affecting heat transfer and fire spread.

烟流轨迹管理的时候，消防员要考虑到，灭火过程中，对流体的方向和速度可能会造成的影响

For firefighters, how does convection affect us?

In addition to managing flow paths, our nozzle techniques can rapidly increase a convection speed and fire spread if we misapply them.

如果射水技术有问题的话，会对热对流速度以及火灾的发展造成很大的影响，

26 Radiation

Radiation is the transfer of heat energy through electromagnetic waves.电磁波 This can happen whether a material is in a solid, liquid or gaseous form. Radiation does not require any material such as a conductor or a fluid medium to transfer its energy.

热辐射会出现在固体、液体、气体当中，并不需要任何介质
The transfer of heat energy to earth by the sun is an example of radiation.

Radiant heat transfer is generally responsible for fire spread to other materials in a compartment fire. The radiant heat from the flame can ignite other fuels, and the radiant heat from the smoke layer moving downward can increase the spread of fire and progress it towards becoming a fully developed fire - the flashover stage.

热辐射可以引燃其他可燃物，提高火灾发展速度
Figure 8: Heat radiation

**Firefighters should note **that the biggest heat transfer process (thus the biggest cause of fire spread) in fire is radiation via soot particles in the smoke/ gas. Convection and conduction only form part of heat transfer, in lesser ratios than radiation.

消防员要注意热辐射是火灾发展过程中，提高火灾发展速度最主要的因素，传导和对流反而还是次要