WIND INFLUENCE IN NUMERICAL ANALYSIS OF NSHEVS PERFORMANCE

**Abstract.** This paper treats on the subject of including wind as a boundary condition in CFD analysis used in Fire Safety Engineering. Adverse wind effect is observed mostly on the performance of natural smoke and heat ventilators – and often is included in numerical studies performed by engineers. This paper provides with general guidelines on the Computational Wind Engineering, as well as to relevant refer-ences. Paper emphasizes on the necessity of building large enough do-main and performing an angle sensitivity analysis, to determine the worst wind conditions for the vents. Only then the fire related CFD/CWE coupled analysis may be performed, with reasonable and believable results.

Computational Wind Engineering (CWE) is primarily defined as the use of Computational Fluid Dynamics (CFD) for wind engineering applications [3]. In last 50 years, this application went a transition, from emerging field into an increasingly established field in research, practice, and design. CWE is used in prediction of wind comfort, pollution, dispersion or loading on a building [13]. The scale of this analysis may be considered as metrological microscale, but due to the complexity of flows around buildings, especially in urban areas, its requirements may be regarded as one or two orders of magnitude higher, than the requirements for typical CFD application in fire related science. In Fire Safety Engineering (FSE) CFD is used to predict the movement of smoke and heat within building structures. The meeting point between both can be found in complex applications of natural smoke and heat ventilators (NSHEV). NSHEV remove the smoke and heat from the building into the atmosphere due to the small difference in the density of hot gasses inside of the building and atmosphere. For such devices, the wind is an important design factor, that may determine the performance of the system, and as such define the conditions inside in case of the fire.

The interface between CWE and FSE is not well described in the literature; there are insufficient data on validation of such coupled analysis. The most researched areas of CWE are (i) simulations of the Atmospheric Boundary Layer (ABL); (ii) bluff-body aerodynamics; (iii) turbulence modeling and numerical techniques; (iv) verification and validation in CFD for urban physics and wind engineering [4, 21].

The CWE focus lies within the metrological microscale and the lowest part of the ABL. In this field of the atmosphere, the Coriolis force is lowest and does not influence the flow within the model in a way; that would justify modelling it. Scales of the atmospheric phenomena that are investigated range from fractions of centimetre’s (turbulence), metre’s (building wakes and thermal flows) to kilometres (convection, urban heat islands) [4]. The time scale of the phenomena does also scale from fractions of seconds (dissipation of turbulence) to hours and days (metrological phenomena). The medium size phenomena are usually directly simulated, while larger become boundaries of the model and smaller parametrized solutions. For more detailed description of the scales of the analysis, please refer to [27].

The main areas of interest of CWE are (i) structural wind engineering; (ii) pedestrian-level wind and urban flows; (iii) natural ventilation of buildings; (iv) wind-driven rain and snow transport [3].

In FSE, following main areas of the use of CFD may be distinguished: (i) assessment of tenable conditions in the building in fires; (ii) growth and spread of the fire (also forensic); (iii) thermal effects of fires on structures and heat transfer (iv) ventilation system performance.

Modern design methodologies [6,23,33] condition the required amount of smoke ventilators on the size of the design fire and supply air solution. These methods origin in the work of Thomas [30] and others [15,18], who applied Bernoulli’s law to the flow of hot smoke and combustion products from the burning compartment to surrounding. Methods presented below in Eq. 1 [6] and 2-3 [33] require vast knowledge of the designer on the fire itself. Variables that are boundary conditions for the analysis are the depth of smoke layer, the temperature of the smoke or mass flows within the compartment. Even with this detailed information, the result of the calculation is just a general overview of what is the approximate total area of all ventilators required to protect the compartment, but without any information on individual features of these ventilators (e.g. aerodynamic free area, opening angle).

(1)

where: A_{vtot} – total required area of smoke ventiltors [m²], C_{V} – discharge coefficient of smoke ventilators, M_{l} – mass flow of smoke [kg/s], T_{l} – average temperature of smoke [K], ρ – ambient air density [kg/m³], g – gravity [N/kg], Θ – increment of smoke temperature [K], T_{amb} – ambient temperature [K], A_{i} – total area of inlets [m²], C_{i} – discharge coefficient of inlets [-].

(2)

(3)

where: A_{vtot} – total required area of smoke ventilators [m²], C_{v0} – discharge coefficient of smoke ventilators, C_{v0,in} – discharge coefficient of air inlets, T_{amb} – ambient temperature [K] , T_{p} – average temperature of smoke [K], g – gravity [N/kg], w_{i} – flow velocity referred to the geometrical surface area of inlets [m/s], Θ – increment of smoke temperature [K], A_{i} – total area of inlets [m²], m_{p} – mass flow of smoke in fire plume [kg/s], ρ – ambient air density [kg/m³], V – volume flow of air supplied by mechanical means [m³/h].

Despite the complexity of the calculation procedure, it still does not account the wind influence on the system performance, besides the introduction of a discharge coefficient for the ventilator.

Natural smoke and heat exhaust ventilators (NSHEV) are considered safety equipment of the building, and as such are under appropriate supervision, enshrined in the mandate 109 of European Commission [37]. Under Regulation 305/2011 [32] their production, certification, and distribution in member countries of European Union, is governed by the provisions of the harmonized standard EN 12101-2 [7]. The NSHEV performance is dependent on the wind; its negative influence is traditionally stated in the form of discharge coefficient (Cv), varying in value between 0,20 to 0,80. Note must be taken, that this coefficient is different, than ones estimated in pioneering work by Prahl and Emmons [25], as its value is determined always for the same conditions (as described below), and can be considered independent from the Reynolds number of the flow. This is one of the reasons, why practical implementation of this value is difficult in hand calculation methods.

The area of NSHEV multiplied by the discharge coefficient is referred to, as the aerodynamic free area, and is considered as the effective area of an NSHEV through which the flow of hot smoke occurs in wind conditions. As it is the only parameter describing the “performance” of the device, the manufacturers of natural smoke ventilators often improve the value of the discharge coefficient by mounting additional elements, such as fairings, directing jets or increasing the opening angle of the device. Besides the increase in the Cv value, the global efficiency of such solutions in a building remains unknown. By harmonized standard EN 12101-2 [7], the discharge coefficient of a ventilator is evaluated with (Cvw) and without (Cv0) the side wind, Figure 1.

In EN 12101-2 test [3], the ventilator is mounted on the top of the settling chamber, which is located beneath the wind tunnel floor in a way, that its roof is in the line of the wind tunnel floor. The air velocity in the tunnel should be 10 m/s (±0,5 m/s), and the turbulence intensity should not exceed 20% (10% at a certain height). The standard does not limit the uncertainty of the measurement, but it must be sufficient to measure the mentioned limiting values. The wind attack angles are altered by the rotation of the settling chamber, together with the ventilator mounted on it. According to EN 12101-2 the value of discharge coefficient for a single pressure point, at the most difficult wind attack angle, is determined by following the formula (4). Next, with the use of mathematical regression the value of received coefficient can be determined for similar devices of the same producer, depending on their opening angle, the height of the ventilator and the deflector and the aspect ratio of the ventilator throat area.

(4)

where: m_{ing} - mass flow into the settling chamber, A_{v,test} – total area of the tested ventilator, ρ_{air} – ambient air density, Δp_{int} – pressure difference between settling chamber and the wind tunnel.

Determination of a single Cv value for a natural ventilator may be misleading when the performance of a whole system is assessed. This performance is also dependent on the wind velocity, angle and location of inlets to the building. A comprehensive study on this was presented in the past [17], and a result of an extensive numerical study done by the authors is currently in press [34], Figure 4 and Table 1. The performance of multiple combinations of natural ventilators and the wind was also presented as a part of Case Study 2 at SFPE Conference in Warsaw, 2016 [35].

Angle | Wind velocity u_{ref}[m/s] | roof mounted ventilators | roof mounted ventilators with deflectors | wall mounted ventilators (back) | wall mounted ventilators (front) |
---|---|---|---|---|---|

- | 0 | 33,25 | 34,6 | 23,8 | - |

0 | 4 | 30,4 | 31,8 | 22,9 | 8,75 |

45 | 4 | 27,6 | 29,1 | 23,5 | 11,8 |

60 | 4 | 25,4 | 27,1 | 22,2 | 13,7 |

90 | 4 | 29,7 | 29,7 | 19,0 | 18,5 |

60 | 8 | 18,3 | 20,8 | 23,7 | negative |

In [35] authors determined three different approaches to numerical modelling, that are common in FSE/CWE coupling: a) model simplified to include only the interior of the building, outlets modelled as pressure boundary conditions;

b) model simplified to include the interior of the building, and nearest exte-rior, outlets modelled as an opening in the walls along with their most important features, pressure boundaries at the edges of the domain; insufficient size of the domain for CWE;

c) model suitable for wind engineering, with exterior domain large enough to not influence the flow around the building, outlets modelled in details as physical openings with all of their features, pressure boundaries at the edges of the domain with velocity boundary including logarithmic wind profile on velocity inlet.

The first approach (a) is sufficient only for most basic, preliminary analysis, without the wind. The simplification in the modelling of the inlets and outlets will strongly influence the performance of NSHEVs. The second method (b) is valid for NSHEV performance analysis, but without any wind interaction. This method can be used for checking the environmental conditions connected to the evacuation process inside the building, but the designer must add a margin of safety to the results, as in wind conditions they may be significantly worse. The introduction of wind velocity in such approach will lead to increased wind at the walls, due to flow compression, and may not be considered as a valid wind analysis. The third approach (c) is used for precise evaluation of NSHEVs performance in wind conditions, following the requirements of Chapter 4, for different wind angles.

FSE/CWE coupling is a tedious task. It is impossible to guess, which wind direction will be the worst condition for the fire, as local structures or geographical features of the terrain may strongly influence this. For simple buildings and simple combinations of buildings, hand calculation methodology presented in Eurocode 1-4 may be sufficient [9], but for more complex structures and urban environment, CFD analysis is necessary. According to pedestrian level wind guidelines and urban flow guidelines (Chapter 4.1), not less than 12 wind angles should be verified.

Investigation of multiple wind attack angles in a transient, fire simulation is extremely time and resource consuming task. As a simplification, the analysis may be decoupled into two steps.

The first step, wind analysis, is to evaluate the wind influence over external features of the building, without a fire (and often without interior model). This analysis relies on statistical wind data for the area, in which the building is located. Numerical model used in the study should include the building analysed in a domain, according to Chapter 4.2, and allow its free rotation. The analysis itself is steady-state and averaged conditions are estimated. Results of such study are investigated with respect to wind pressure coefficient in areas on which elements of the natural smoke exhaust system are located.