Testing Water Mist Systems Against Large Fires in Tunnels: Integrating Test Data with CFD Simulations

Abstract

The high cost of conducting large, full-scale fire tests for the evaluation of suppression systems in tunnels tends to limit both the extent of the instrumentation provided and the number of tests that are conducted. Because of the variability of the large fires, performance criteria based on single point measurements derived from experience with smaller test fires were not reliable indicators of performance. Yet decisions about the acceptability of suppression systems must be based on the limited amount of performance information available. A means was sought to reduce the reliance on single point instrumentation readings, and to augment the value of the limited amount of test data by integrating the field testing with CFD modeling. In this study a computational fluid dynamic (CFD) model was used to simulate a series of full-scale fire tests of water mist systems conducted in 2006 in a highway test tunnel. The NIST Fire Dynamics Simulator version 4 (FDS4) was used to simulate five of the tunnel fire tests. The task was to confirm that the simulations could achieve a reasonable degree of agreement with the conditions measured in the tests. The model could then be used to evaluate the performance of the water mist system over a broader range of performance indicators than were measured. This paper illustrates what is unique about very large fire tests and presents highlights of the modeling. The level of agreement between simulation and test results is demonstrated for one test. Agreement was deemed to be good enough to establish confidence in applying the model to examine the conditions that would occur with an unsuppressed fire, which had not been tested. CFD modeling can be used to improve the understanding of the performance of the suppression system, and to augment the value of the test results. A second, complementary paper has been submitted to the SFPE Journal of Fire Protection Engineering to provide more detailed information about the FDS4 modeling than can be covered in this paper.

CFD Investigation of Large Scale Pallet Stack Fires in Tunnels Protected by Water Mist Systems

“…Figure 3 compares Rosin-Rammler/log-normal fits with measured data that were obtained in a separate study [57,58] for a Marioff 4S 1MC 8MB 1100 nozzle. The smooth log-normal branch advocated by FDS4 provides a poorer representation of the data than does the nonsmooth branch obtained by the methods presented in [4,52]. This was found to be the case with all of the water mist nozzle data that the authors validated.…”

“…Even though flame height is a distance, because of its dependency on heat release rate and pool fire diameter, the asymptotic relative uncertainty of the flame height scales as ÁL ¼ 1 2 Á _ Q. This is greater than the 2/5 coefficient in Equation (4). In general, the complications associated with large fires with flames impinging on the graded ceiling of a tunnel indicate increased uncertainty.…”

“…The goal of the current article, which only discusses the wood pallet fires, is to quantify and demonstrate the benefits of the suppression system for these fires. The reader's attention is also directed to a companion article, Mawhinney and Trelles [4], which covers certain topics not addressed here in great detail.…”

“…Chan [53] found that this composite analytical distribution provided a good fit to traditional sprinkler data. Reference [4] explains how the distribution parameters were determined for water mist nozzles.…”

“…The time lag was also reconciled against other instrument readings. Refer to [4] for the details. The flame spread methodology was as follows.…”