Tentafrågor 2021 svarsfrågor Flashcards
A design fire uses heat release rate (HRR) vs. time curve. The HRR curve can provide a plethora of information regarding the fire behaviour of a material for e.g., its peak heat release rate, time to peak heat release, total heat release, etc. Sometime, a HRR curve can have multiple peaks and also, the property of the material dictates the value/extent of the peak heat release. In fact, peak heat release rate is one of the most important parameters to judge fire behaviour of materials. Based on this knowledge and the information gained in the laboratory tests, match the HRR curves (15 BC; 25 BC, and 30 BC) to the corresponding microscopy images of the samples (A; B; and C). Explain your answer in a few words (2.5
points).
Sometimes, a HRR curve can have multiple peaks, as was seen in the laboratory tests. Please explain in detail, why such multiple peaks occur, and which materials or treatments can
possess the aforementioned
( see exam 2021)
A= 15BC, B=30BC, C=25BC
The formation of char under radiative heat creates a barrier or heat shield. This barrier hinders the transmission of heat, oxygen and fuel from the underlying virgin material to the ambient environment. However, the char barrier can
have fissures and holes, depending on the effectiveness of char forming mechanism, which can be aided by flame retardants or even the inherent property of the material under radiative heat. The char that has a greater number of holes will allow the mass, heat and oxygen transfer more than the char that is more rigid and dense in nature. Therefore, Figure A is for 15BC curve, which has the highest peak heat release rate (PHRR). Similarly, Figure C has holes too but less than Figure A, and thus it is for curve 25BC. Figure B is the densest amongst the three microscopy figures and thus should have the lowest PHRR i.e., 30BC
A material under radiative heat can have dual peaks (or more peaks) due to char formation. The first peak is usually because of the degradation or decomposition or dissociation of the material under radiative heat. However, once char formation is triggered and it builds up, the heat release rate goes down, because of the aforementioned reason (1st paragraph). Nevertheless, under continuous heat radiation, the char can experience breakage of its structure and the underlying material would be exposed once again to the heat. This will cause continued decomposition and combustion of the material, consequently resulting in another (subdued) heat release peak. However, if the char is dense enough to resist breakage, the HRR will reduce after the formation of char and the extinction of fuel and continue a receding pattern until flameout. The dual or more PHRR is observed in materials that have the ability to form char. This can be a result of fire-retardant treatments in materials and/or materials that inherently have char-forming ability like wood (due to the presence of lignin) or wheat gluten or wool.
A simple design fire curve is given below. It is usually characterised by an accelerating growth phase, a steady phase and decelerating decay phase. However, in real-life situations, the design fire is more complex and the curve changes from the one shown below. Draw, by hand, a design fire curve that has 4 fuel packages and explain the progression from the ignition to the final decay (5 points).
A same in one fuel fire the curve acts the same except that there are 4 more peak and growth phases. Then the last material had ignite and burns out the first material will decay. See notes from lesson 4 or 5, drawn by Ulla on the board.
Magnusson and Thelandersson found out a method to determine design fire curve. Write a brief essay on their method, including their assumptions, advantages of their method, and general means by which they reached their goal of systematically presenting the design fire
curves (5 points).
Magnusson and thelandersson (M&T( developed a method to achieve temperature vs. time curve (i.e. design fire curve) of post-flashover fires in a room/enclosure.
Q= q_L + q_w + q_R +q*_B
Assumptions
- even temp. in the room
- ERR is ventilations controlled during the fully developed stage.
- The combustion is fully complete and occurs only within the boundry of the room
- Single surface heat transfer coefficient is used for the entire inner surface area of the room
The advantages of M&T
- ERR as a function of time. Known parameters are fuel load density, opening factor and theral properties of the room boundary materials
- The resluts is presented in a simple way through graphs and tables, removing the need for computer-based calculations.
Developed method
(fyll på)
For a fire plume, its radius, mass flow rate, temperature and velocity can be determined using a number of equations. Many different researchers have put forward their own
versions of the plume equations. These researchers are Heskestad, Zukoski, Mccaffrey, and Thomas, as well as the ideal plume equations. Explain, whose plume equations you would need to use under what special conditions (for all the aforementioned researchers) (5 points).
- The ideal plume eq:
To determine plume radius, velocity, mass flow rate, and temperature when the fire plume is an ideal nature ( top hat profile with no radiative heat losses, density variations throughout the plume height. Velocity/temo. is not dependent on plume height and entrainment velocity specified or known) - Zukoski plume eq:
Generlly not that differant form the Ideal plume eq for mass flow rate. However, the mass flow eq. can be simplifies if the ambient air properties are known. - Heskestad plume eq:
In this case a virtual origin is interduced ( Can be determined from ERR and diameter of the fuel), and Gaussian profile replaces the top hat profile. A large density differences can be taken into account unlike the ideal plume equation. By using heskestad equation we can calculate:
- the plume radius
- velocity
-plume mass flow
- temperature
When the height is above the mean flame height.However, there is only one equation to calculate plume mass flow rate below the mean flame height. - McCaffrey plume eq:
In this case the temperature and plume velocity can be determined using equations and two of the constants can be found from table depending on the flames three regions:
- the continuous
- the intermittent
- far field/plume - Thomas plume eq:
In this case, only plume mass flow rate can be calculated. The equation is especially useful for cases where L/D<1 and cases where the fire source is non-circular.
