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. Overview • General on smoke control in buildings • Examples of applications • Derivation of conservation equations • Smoke filling in small room with small opening: Zukoski method • Smoke filling in large room with large opening: Steady-state method • Natural ventilation • Mechanical ventilation • Pressurization of lower layer

3. How do people die in fires?MGM Grand Hotel Many fire deaths far away from fire

4. Reasons for smoke control • Life safety – maintain the escape path as long as possible • Firefighter safety – improve visibility • Firefighter operations – reduce time to put out fire • Property protection – reduce smoke damage • Property protection – reduce temperature and external radiation to fuel packages • Continuity of operations = Open for business sooner

5. Smoke management – preventing smoke spread • Smoke control • Use of mechanical fans for pressurization • Smoke and heat venting • Natural (link operated vents) • Mechanical

6. Locations for smoke control • Atria • Warehouses (rack storage) • High rise buildings (stairwells) • Shopping malls • Performance based designs • Increase travel distance • Substitute for rated construction (smoke barriers) • Elevators for egress

7. Example: Atria • Atrium – singular (one) • Atria – plural (many) • Opening through floors

8. Example: Atrium

9. Example:Preventing smoke spread

10. Example: Smoke management-pressure • Compartmentation with pressurization • We can calculate required pressure difference

11. Example:Single injection point • Fail with door open near injection point • Limit to 8 stories (Klote and Milke)

12. Example:Multiple injection points • More injection points limits local loss of pressure • Determining the design number of open doors can be difficult

13. Example:Smoke control by air flow • Airflow without barriers can control smoke if the air velocity is sufficient • Not usually a recommended method (expensive) • Airflow can enhance burning rate

14. Example: Mechanical exhaust from a large volume space

15. Example:Balcony spill plume • In many cases, the mass flow rate is larger than from an axi-symmetric plume

16. Example:Roof vents

17. Example:Natural smoke venting • Reduced flow due to obstructions, wind and sprinklers • Makeup (replacement) air required

18. Example: Venting of fire gases

19. Activation of smoke control system • Smoke detector • Thermal detector • Smoke density detector • Specified optical density/obscuration • Beam detectors • Specified optical density/obscuration • Avoid manual activation, but provide manual activation for fire department

20. Overview • General on smoke control in buildings • Examples of applications • Derivation of conservation equations • Smoke filling in small room with small opening: Zukoski method • Smoke filling in large room with large opening: Steady-state method • Natural ventilation • Mechanical ventilation • Pressurization of lower layer

21. Conservation equations • Conservation of mass (C of M) • Conservation of energy (C of E) • Conservation of momentum • Conservation of species • (O2, CO2, H2O, soot, fuel)

22. Compartment Fire Environment

23. Finite Control Volume

24. Infinitesimal Control Volume • All four give the same result, but just in a different form

25. Control volume could be one of two layers • Hand calculations • Two-zone computer models

26. Or the control volume could be one small piece of the overall enclosure • Computational Fluid Dynamics • or CFD models

27. Control surface • Surface defining a control volume • Mass, energy etc. pass through the control surface

28. Overview • General on smoke control in buildings • Examples of applications • Derivation of conservation equations • Smoke filling in small room with small opening: Zukoski method • Smoke filling in large room with large opening: Steady-state method • Natural ventilation • Mechanical ventilation • Pressurization of lower layer

29. Smoke filling in a room with a small opening (vent) • Conservation of mass • CV is lower layer • Plume is ignored or could be included in upper layer • Mass leaves CV through plume and vent (positive)

30. Smoke filling in a room with a small opening (vent) • Remember the Zukoski plume equation • Use the energy equation to find • Substitute and into C of M equation

31. Smoke filling in a room with a small opening (vent) • A numerical/graphical solution is possible • Generalize results by using dimensionless numbers

32. Non-dimensional smoke filling equation • Dimensionless height: • Dimensionless HRR: • Dimensionless time: • Dimensionless form of smoke filling equation

33. Result of numerical solution by Zukoski (ceiling and floor leaks)

34. Limitations of Zukoski model • Small room and small opening • Zukoski plume equation • Valid for weak sources • No heat loss • No pressure change with time • Uniform pressure throughout room • Expect approximate results only

35. Overview • General on smoke control in buildings • Examples of applications • Derivation of conservation equations • Smoke filling in small room with small opening: Zukoski method • Smoke filling in large room with large opening: Steady-state method • Natural ventilation • Mechanical ventilation • Pressurization of lower layer

36. Steady state smoke control • Spaces are typically large (tall) • Openings are large, so there is little (no) pressure buildup inside enclosure • Flow rates through vents are a function of hydrostatic pressure differences • Pressure difference varies over height of opening • Two direction vent flow • More directions possible with smoke filling in second compartment • C of M and C of E equations coupled • Must solve at the same time

37. Steady state smoke control • Now do something to stop the upper layer from descending (moving) • Rate of smoke venting (exhaust) equals rate of smoke production (entrainment in plume) • Steady state • Conditions no longer changing with time • Method of Tanaka and Yamana

38. Steady state smoke control • Mass conservation for upper layer • Conservation of energy for upper layer • Heat loss to walls now important

39. Steady state smoke control • We already solved this energy equation in Chapter 6 • But now we know z’, so we know the area of the wall being heated, Aw • Semi-infinite solid with Tg=Ts

40. Look at three cases for removing smoke • Natural ventilation • Mechanical ventilation • Pressurization of lower layer

41. Steady state smoke controlnatural ventilation from upper layer • Mass and energy equations are coupled • Should solve both at same time • Pressure difference at top of layer moves smoke • Thicker layer = greater mass flow • Look at solution by trial and error

42. Steady state smoke controlnatural ventilation from upper layer • Trial and error solution • Guess a layer height • Solve mass and energy balances • Correct layer height guess and density • Resolve mass and energy balance

43. Steady state smoke controlmechanical ventilation from upper layer • A fan is now placed at the top of the compartment to remove smoke • Generally know the fan exhaust rate [m3/s]

44. Steady state smoke controlmechanical ventilation from upper layer

45. Steady state smoke controlpressurization of lower layer • A few cautions about pressurization of the lower layer by mechanical ventilation

46. Full scale test results

47. Some final cautions • There is not universal agreement by fire researchers on these equations • Some, but not much, full scale testing • Fire model results versus correlations • Sometimes easier to just use a fire model • Some models have been validated (tested) under smoke control conditions

48. Any questions?