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Explore the intricate dynamics of enclosure fires, from energy release rates to plumes and flames. Learn about pressure and vent flows, gas temperatures, smoke filling, and computer modeling. Discover how to calculate flame heights and plume mass flow, and understand the correlations between Froude number and heat release rate. Dive into the formation of plumes and ceiling jets, and examine ideal plume equations. Delve into the experiments on Zukoski and Heskestad plumes, and gain insights into the interaction of plumes with ceilings.
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Enclosure Fire Dynamics • Chapter 1: Introduction • Chapter 2: Qualitative description of enclosure fires • Chapter 3: Energy release rates, Design fires • Chapter 4: Plumes and flames • Chapter 5: Pressure and vent flows • Chapter 6: Gas temperatures • (Chapter 7: Heat transfer) • Chapter 8: Smoke filling • (Chapter 9: Products of combustion) • Chapter 10: Computer modeling
Flames Calculate flame heights Plumes Calculate plume mass flow (function of height z) Calculate plume centerline temperature (fnct of z) Know Zukoski plume and Heskestad plume Ceiling Jets Use Alperts correlations Goals and expectations
Define mean flame height • Height where flame is observed 50% of the time • Height above which flame appears half the time
Froude number in terms of heat release rate • Experiments show mean flame height, L, is a function of the square root of Fr:
Normalized flame height versus dimensionless energy release rate • 1< Q* <1000 • See Table 2-1.2 [SFPE] for many different flame height correlations
Flame height correlationof Heskestad • Reliable for 0.5 < Q* < 1000
The ideal plume (point source plume) • Goal: Derive simple algebraic equations for properties in plume • Assume top hat profile
Derivation of ideal plume equations • Temperature as a function of height • Difference above T • T(z) [oC or K] • Plume radius as a function of height • b(z) [m] • Upward velocity as a function of height • u(z) [m/s] • Plume mass flow rate as a function of height • [kg/s]
Zukoski Plume • Adjusted ideal plume theory to fit with experiments • Generally underestimates plume mass flow rate
Plume equations that better represent reality • Many researchers have worked on developing plume equations • Derive through dimensional analysis and experiment • Heskestad plume equations • McCaffrey plume equations • etc
Heskestad plume correlations z>L z>L z<L
Plume interaction with a ceiling • Forms a ceiling jet (CJ) • Velocity of CJ driven by buoyancy of plume • Just as with plumes, there are a number of different CJ correlations
Temperature and velocity cross sections are not necessarily the same • Depth of CJ in the range 5%-12% of H • Maximum u and T very near ceiling (1% of H)
Alpert correlations r/H<0.18 r/H>0.18 r/H<0.15 r/H>0.15
