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[Slide 1] 
[Shijin Kozhumal] Hello everyone. Welcome to the second half of the course. In this week, we will primarily focus on heat release rate. So this is one of the most important parameters when dealing with fire. We will quickly look at what it is, how to measure it, what are some reasonable values, how to calculate it, and some related aspects. 
[Slide 2]
So heat release rate is defined as the rate at which heat energy is generated by burning according to NFPA 921. So it's the rate of the heat energy. So we know the unit of energy is either Joule or kilojoule. And if we divide that by second, that is per unit time, we will get watt or kilowatt. So this is the same unit as power. We know the unit of power is also watts which is also energy divided by time. So the unit of heat release rate is power, same as that of power, as watt or kilowatt. And this is used to analyze how big the fire is. We can make some rough estimations on flame height. Municipal sprinkler designs follow the heat release rate to decide when to activate it and some also follow temperature. So in this week we will look at how do we calculate it, how do we measure it, and how to use it. 
[Slide 3]
Here I have a list of some typical values. Actually, these values depend on various other parameters like air availability, even ambient conditions like ambient moisture, temperature, and many parameters. But we can still have some typical approximations for most fires. This will help you give a range of values which one can typically expect. So a burning cigarette is around 5 watt which is very small, you know. And a candle, slightly big candle, is around 80 watt. And mostly, if a wastepaper basket catches on fire that's around 100 kilowatt. And anything above one megawatt is kind of in the dangerous zone because in most compartments anything above 1 megawatt can mostly can lead to flash over and full involvement and it can produce enough heat to ignite most wooden obstacles in the room. So up to this is pretty largely safe to here. Anything above this range we have to be extra cautious say like a burning of 1 meter square pool of gasoline that can lead to a typically around 2.5 megawatt fire which is pretty big. And so on. Again, I repeat, these values are very approximate and depend on many other factors. 
[Slide 4]
So heat release rate. As you know, it's a function of heat divided by time so the value can change with time. So we plot heat release rate as a function of time and such plots are called heat release rate curves. The curve maps out the energy or the release of energy over a duration of the fuel burning. So in typical cases, after ignition fuel starts to burn very slowly. So you can see in the initial stages the heat release rate would be very low as you can see from here it's very close to zero or very small values. But as more and more fuel elements start burning and more heat is released, they further have been heating the nearby fuel elements, and at some stage the heat release rate rapidly increases as you can see here. And then it reaches a peak value. But after that, once the most of the fuel element starts burning out, when it reaches a very high heat release rate, the mass burning rate will also be very high. So most of the fuel will get burned out very soon, and then as the fuel is mostly burned out it will start decaying. The heat release rate will decay and it will slow down, and eventually it’ll come back to zero as all the fuel is burned or fire is extinguished. This is a typical trend for a heat release rate. 
Here I have a video done by the University of Edinburgh using a Christmas tree fire. So they have ignited it. On the right hand side you can see the power and the fuel mass. So as the fire spreads, the fire is growing now, as you can see from the fire power or heat release rate. It's building up because now there is more heat release. It heats everything so the fire spreads very fast and the mass is starting to decrease very fast now because the heat release rate is very high so it can pyrolyze more fuel elements. So after this stage, when now the heat release rate has reached a maximum, almost all the fuel element is involved now. And then, now you can see most of the leaves are already burned out, at this stage heat release rate starts coming down because most of the fuel has burned out. As he can see from the mass here, most of the mass is already burned out. And now, as we saw here, the heat release rate decreases because most of the fuel is already burned out and small parts continue to burn for some more time until it's either extinguished or all the fuel elements are burned out. This is a typical pattern of most of the fires: ignition, a slow growth, and then there is a rapid growth reaching a peak value then there is a decrease. 
[Slide 5]
If we get this same curve of heat release rate we can extract a lot of information from it, a lot of quantitative information from it. Say, for example, if you want to know what is the peak heat release rate, all we need to do is from the peak release point just draw a horizontal line to the heat release rate axis. And wherever it is and if we extrapolate it, you can find that the heat release rate values around 2.2 megawatt or 2200 kilowatt here. And if you want to see how long will it take for the fire to reach that peak heat release rate value all we need to do is draw a vertical line to the time axis. So in this case we can see it's around between 200 and 400. So it's around 375 seconds. So from this graph we can get information like the fire has a peak heat release rate of 2200 kilowatt. It will reach a peak heat release rate value in around 375 seconds after ignition. And at any location if you want to know, or at any time instead if you want to know, the heat release rate from that time instant. Say for example at 600 seconds, draw a vertical line, wherever it meets the curve draw a horizontal line. We can say that at it time, say 600 seconds or 10 minutes from the ignition, this will be the heat release rate value. So I hope you got how to extract data from curves which most of you might already know. 
[Slide 6]
Let's look few more curves for some practical fires. So we have a mattress. So the fire starts building here. As you can see, in the initial stages, here zero is when they really start the fire building up or it’s properly ignited from the source. So it has reached half. I think around this time the heat release rate rapidly showed up to a maximum value and after that stage it will start to decay. You can see the same trend in every heat release curve. After ignition, a slow growth or the fire spreads slowly then there is a rapid growth reaching a peak value and staying there for some time and then a decay. And at this stage most of the fuel has already got consumed. 
And let's look at a case with bunk bed. Very similar trends. Fire spreads and as it grows it spreads faster because there is more heat release rate now. And suddenly the heat release rate value increases and it reaches a maximum value here, a much higher value around 4500 kilowatt or so. And rapidly it decays because most of the fuel gets pyrolyzed. It starts burning. 
So these values are useful because to estimate what is the kind of heat release rate we get from different fuels. And so this was a cotton mattress which typically gives heat release rate within one megawatt or one thousand kilowatt. And here in this bunk bed, here there is a lot of wood. Because of that, the peak heat release rate is much higher around 4500 kilowatt. 
[Slide 7]
Let's look at a few more examples. Here a wooden pallet and a small dresser. Both are similar because both are a similar amount of wood material in it. And so you can see the way the fire grows is pretty similar in both cases. Both are wood. Both are sheets of wood. And in this case, you can see typical values around 1.5 megawatt or slightly more than that, 4600 kilowatt. Here, 1800 kilowatt. So very similar values. And you have to remember 1700, 1800 kilowatt values may be good enough to cause a flash over in some small compartments. And as in both cases, as they reach a peak value, now as most fuel gets burned out they took early decay. 
[Slide 8]
So we’ll look at a few more examples. Here I have a love seat and a sofa. They are similar in construction because these days they use a lot of PU foam inside it. And as we discussed in the previous class PU foam burns very fast, releases a lot of heat, and because of that you can see just one love seat or sofa creating a heat release rate value around three megawatt or three thousand kilowatt which is pretty high. So most of our living rooms, this three megawatt may be big enough to cause a flash over. The decay trends are very similar just like we discussed in all the previous places because as most of the fuel element is consumed it slowly decays. I hope you got a rough idea about the fire growth and the peak values, peak rate release rate values, for common materials that we use in households. 
[Slide 9]
So based on the heat release rate, people have done more in depth experiments to figure out how fast fires can grow for different materials. So this will depend on material properties and also depend on surface to mass ratios. So surface-to-mass ratio, as we discussed in the last class, is significant here also. And this will tell us, based on some t squared curves, what are the materials that are expected to burn slowly or the fire is expected to grow slowly and which are the materials that can burn very fast. So typically the cotton mattresses and all are somewhere in the slow to medium range as you can see here. What that means is, say for example most compartments that have a heat release or flashover around 2.5 megawatt or slightly over 2500 BTU per second. So you can roughly say how much time do we have for different materials before they reach a flashover or a situation where everything can get burned up. So a cotton mattress is slightly on the lower end and the papers or cardboards they are slightly faster. And thin materials, thin wooden materials, plywood, are slightly faster because they have more surface area even though the material is the same. And the highest and the fastest and the most dangerous thing in the modern day housing and offices are upholstered furniture with PU foam inside it. So they burn very fast within like minutes. Then anything can burn down and the heat release rate values, as you can see here, can reach dangerous values within like three or four minutes. 
[Slide 10]
Here I have a typical heat release rate curve just to summarize everything we have explained so far. After ignition, it starts from ignition. And after ignition there is a growth period called a growth phase. And later it reaches almost a steady-state. And then there is a steady state phase were the heat release rate is very close to a constant value and which is around the maximum value of the heat release rate. And the duration of this phase can vary depending on the material, In some fuels it will be a very small period because once it reaches that peak heat release the fuel will start depleting very fast. And eventually, as most of the fuel elements burned out, there will be a decay phase. 
Thank you for listening. In the next video we'll look more into the heat release rate and calculations. 
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