Enclosure Fire Dynamics

1. 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

2. 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

3. Define mean flame height • Height where flame is observed 50% of the time • Height above which flame appears half the time

4. Froude number in terms of heat release rate • Experiments show mean flame height, L, is a function of the square root of Fr:

5. Normalized flame height versus dimensionless energy release rate • 1< Q* <1000 • See Table 2-1.2 [SFPE] for many different flame height correlations

6. Flame height correlationof Heskestad • Reliable for 0.5 < Q* < 1000

7. Formation of plume and ceiling jet

8. Plume centerline properties

9. The ideal plume (point source plume) • Goal: Derive simple algebraic equations for properties in plume • Assume top hat profile

10. 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]

11. Final form of the equations:

12. Zukoski Plume • Adjusted ideal plume theory to fit with experiments • Generally underestimates plume mass flow rate

13. Zukoski plume experiments

14. 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

15. Heskestad; virtual origin

16. Heskestad plume correlations z>L z>L z<L

17. Measurements of centerline temperatures

18. 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

19. 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)

20. Alpert correlations r/H<0.18 r/H>0.18 r/H<0.15 r/H>0.15

21. Any questions?Next: Unit 5 – Vent flows