Assignment Problem: Explanations should not be more than one paragraph.
Question 1:
(a) Describe the "fire triangle" and discuss the importance of each element in the combustion process.
(b) In your own words explain what "Activation Energy" is. If two polymers had different activation energies, which one would be easier to ignite? Explain why.
(c) If the Heat of Combustion of Benzene (C6H6) is 4.25MJ per mole; how much heat will be released if 250g of Benzene were burnt with combustion efficiency of 80%. Show your workings.
Question 2:
(a) In your own words, explain the relevance of the "Thermal Diffusivity" of a brick to the temperature measured on the hot side of a brick wall heated on one side.
(b) The internal temperature of a cold room required to be maintained at 0oC. The cold room wall is constructed with a special insulation material of which the thermal conductivity is 0.02 W/m.k. On a day the temperature is at 45oC, how much heat energy will be entering in to the cold room by means of conduction through a unit area of the wall. The thickness of the wall is 300 mm.
(c) If a "Black-body Radiator" was at a temperature of 950oC, how much radiant heat energy would it be emitting over every square metre of surface area?
(d) A man is working with his black coloured laptop with its flap open close to a burning barbeque burner. The laptop receives a radiant heat flux of 8 kW/m2 from the burning fire. Assuming the laptop material has an emissivity of 0.8, what temperature would the laptop heat up to?
Question 3:
(a) Hot gases at 480oC from a fire are flowing past a plasterboard wall whose exposed surface has already been heated to 118oC. How much convective heat energy is transferred from the hot gases to the wall per square metre if the heat transfer coefficient is 15W/Km2?
(b) From the question above, if the effective emissivity of the hot gases is 0.8 and the emissivity of the plasterboard wall is 0.9 in the above question, how much net radiant energy is transferred per square metre to the wall?
(c) A room with a flashover fire has a window 1.6 m high and 2.2 m wide which is emitting radiant heat at a mean value of 112 kW/m2. What is the maximum radiant heat flux that would be received at the property boundary 1 m away assuming that the window is a black-body?
Question 4:
(a) A fire is burning near the corner of a 4mx4m room having a ceiling height of 3.0m. The fire is ventilated through an open doorway where hot gases low out of the room and cold air flows into the room. There is an upholstered lounge suite in the centre of the one side of the room that can be assumed to ignite in a relatively short period of time when exposed to a radiant heat flux of 18kW/m2. What temperature would the hot gases beneath the ceiling need to be for the lounge suite to ignite? Assume that the hot layer has an emissivity of 0.9 and that the hot layer is 2.0m above the top of the lounge suite.
(b) Compare your answer with the following statements:
a. "A temperature between 400 and 600oC of the hot layer gases is likely to result in flashover in a compartment."
b. "Flashover is likely to occur when the tip of the flames reach the ceiling".
Question 5:
(a) Plot the heat release rate of a fire that has the following characteristics:
i. Initial growth as a t2 fire at a rate that achieves a convective heat output of 1.5 MW after 150 seconds until sprinkler activation.
ii. Sprinklers activate when the total heat release rate of the fire has grown to 3 MW.
iii. The sprinkler system has been designed to an "Ordinary Hazard" occupancy with a discharge density of 0.08 mm/s. Following sprinkler activation the heat release rate of the fire follows the "NIST" equation fore sprinkler suppression.
(b) Discuss the following with respect to a sprinkler system:
i. Required Delivered Density
ii. Actual Delivered Density
iii. Fire control
iv. Fire suppression
References: International Fire Engineering Guidelines (2005) and Introduction to Fire Dynamics - Drysdale (2011).