1、Nader Shahcheraghi, PhD, PE is Member ASHRAE and a Senior Program Director at AECOM, Oakland, CA Advanced Analysis Techniques in the Design of Longitudinal Tunnel Ventilation System Using Jet Fans Nader Shahcheraghi, PhD, PE ABSTRACT Application of jet fans in emergency tunnel ventilation, as an eco
2、nomically attractive alternative to traditional fan plants, has become more popular in the US over the past years. Also, recent literature indicates that transportation tunnels are facing increased fire loads. For example the latest (2008) Edition of the NFPA 502, Standard for Road Tunnels, Bridges,
3、 and Other Limited Access Highways, indicates peak Fire Heat-Release Rate (FHRR) of 200-300 MW (683 1025 MBtu/hr) for tankers carrying flammable and combustible liquids. This is two to three times the values reported in the 2004 Edition of the same standard. Larger design FHRR increases the need for
4、 accuracy in accounting for the heat loads on the emergency ventilation equipment such as jet fans in order to avoid significant over design of these equipment and demonstrate their feasibility. Fortunately, this trend continues to grow parallel to reduced cost of computational power. Therefore, it
5、is inevitable that more sophisticated computational analysis tools are used in the analysis of jet fan performance under fire emergency conditions with large design fire sizes of order 100MW. This paper presents a case study, where the three dimensional CFD analysis is used in conjunction with one d
6、imensional SES analysis in order to establish the jet fan configuration and demonstrate performance/compliance of jet fans for the emergency ventilation requirements of a road tunnel. INTRODUCTION Tunnel emergency ventilation is required for emergency evacuation and to support firefighter access in
7、case of a fire emergency in the tunnel. In road tunnels emergency operation generally results from a vehicle accident or a vehicle fire. The most serious is the vehicle fire requiring passenger evacuation. Ventilation may also be required for maintaining air quality during normal, peak or congested
8、traffic conditions. However, this paper does not address the requirements for controlling air quality due to vehicle emissions in the tunnel during various traffic conditions because, typically, fire emergency ventilation is the driving criteria for sizing the tunnel ventilation equipment. Applicati
9、on of jet fans in emergency tunnel ventilation, as an economically attractive alternative to traditional fan plants, has become more popular in the US over the past years. Also, recent literature indicates that transportation tunnels are facing increased fire loads. This trend is reflected in nation
10、al and international literature. For example the latest (2008) Edition of the NFPA 502, Standard for Road Tunnels, Bridges, and Other Limited Access Highways, indicates peak Fire Heat-Release Rate (FHRR) of 200-300 MW (683 1025 MBtu/hr) for tankers carrying flammable and combustible liquids. LV-11-C
11、077 2011 ASHRAE 6372011. American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. (www.ashrae.org). Published in ASHRAE Transactions, Volume 117, Part 1. For personal use only. Additional reproduction, distribution, or transmission in either print or digital form is not permit
12、ted without ASHRAES prior written permission.This is two to three times the values reported in the 2004 Edition of the same standard. Larger design FHRR increases the need for accuracy in accounting for the thermal and mechanical loads on the emergency ventilation equipment such as jet fans in order
13、 to avoid significant over design of these equipment and demonstrate their feasibility. Therefore more accurate analysis methods are necessary to increase the design reliability. Computational Fluid Dynamics (CFD) is one of such analysis methods, which has become an integral part of modern emergency
14、 tunnel ventilation analysis. In this case the three-dimensional CFD analysis is used to compliment the one dimensional Subway Environment Simulation (SES) analysis in order to better predict the performance of jet fans under tunnel fire emergencies. Specifically, CFD analysis is used to capture the
15、 three dimensional temperature stratification patterns in the ventilation flow past a tunnel fire. These three dimensional temperature stratification patterns are used to refine the one dimensional temperature profile along the tunnel as predicted by SES in order to improve the temperature predictio
16、ns down stream of the fire near the tunnel ceiling, where jet fans are installed. The improved temperature predictions near the tunnel ceiling allow for a more accurate analysis of the jet fan performance during fire emergency conditions. Use of CFD Model for fires is used by Rhodes N. for Memorial
17、Tunnel Fire Test (4) and is validated by Miles S. et al (5) using Memorial Tunnel fires. THE TUNNEL The proposed tunnel is approximately 1 km long, has a down grade of 4.8 % in the direction of traffic, and has one-directional traffic entering at the East Portal and exiting at the West Portal. It is
18、 elliptical in cross section and will have two lanes of traffic a standard shoulder and a walkway on the right and an emergency shoulder and walk way on the left side. The tunnel has a cross sectional area of 91.0 m2(979.0 ft2) with a width of 12.8 m (42.0 ft) at widest point and ceiling height of 8
19、 m (26 ft) at the crown of the tunnel as shown in Figures 1a and 1b. This paper describes the design of the tunnel ventilation system for fire emergencies. Figure 1a: Tunnel cross section 638 ASHRAE TransactionsFigure 1b: Tunnel 3-D geometry, jet fans, and worst case fire scenario FIRE EMERGENCY VEN
20、TILATION The tunnel ventilation system was designed to provide sufficient capacity for effective ventilation during a major fire incident in the tunnel. The design was based on site-specific conditions and an advanced design methodology, which uses a combination of CFD and SES analyses to develop an
21、 optimized design. The following sections describe the ambient conditions, the worst case fire scenario, the design methodology, and the analysis (CFD and SES) techniques and results that led to the jet fan thrust and configuration requirements under fire emergency conditions. Methodology The design
22、 of the ventilation system for fire emergency was developed in accordance with the National Fire Protection Association (NFPA) Standard 502-2004, “Road Tunnels, Bridges, and Other Limited Access Highways”. Simulation of emergency ventilation is required for a tunnel fire to ensure sufficient airflow
23、 to prevent backlayering. Backlayering is the movement of smoke and hot gases contrary to the direction of the ventilation airflow in the tunnel. The predicted airflow past the fire is compared to the calculated “critical velocity“. Critical velocity is the ventilation airflow velocity at or above w
24、hich backlayering of smoke and hot gases does not occur and was calculated based on equations provided in reference (2). Ventilation analysis was conducted using U. S. Department of Transportation (USDOT) Subway Environment Simulation (SES) computer program version 4.1 (2). Typically SES analysis is
25、 conducted for a fire in the tunnel near the highest point and ventilating downhill, a worst-case scenario for ventilating against buoyancy and for sizing the jet fans. For single directional traffic flow, it is assumed that vehicles ahead (downstream) of the fire will drive out of the tunnel and pe
26、ople from vehicles behind (upstream) the fire will be evacuated towards the entry portal of the tunnel. Therefore, longitudinal emergency ventilation is in the direction of traffic. A mix of passenger cars, trucks, and busses will use the tunnel. Fuel tankers will be allowed during certain hours. Fo
27、r fire emergency conditions, maximum heat release rate (HRR) of 100MW (341.3 MBtu/hr) was used corresponding to a tanker or a fully loaded heavy goods vehicle (HVG), based on the NFPA Standard 502-2004 recommendation (1). SES 2011 ASHRAE 639simulations were conducted for summer and winter conditions
28、. Emergency Fire Scenario The worst case fire presents the most stringent performance requirements for the emergency ventilation system. This scenario is developed based on the tunnel grade and the configuration of the emergency ventilation system. Specifically, the fire location that results in a c
29、ombination of the largest buoyancy force and reduced fan operating efficiency was selected as the worst-case scenario. The tunnel slope is downward at -4.8% towards the west portal. Therefore, the vehicles were stopped near the anticipated location of the jet fan set at about 120.0 m (393.6 ft) west
30、 of the east portal as shown in Figure 1b. It is assumed that motorists downstream of the fire will drive out of the tunnel towards the west portal. Vehicles upstream of the fire will be trapped and motorists upstream of the fire will be evacuated towards the east portal, in the uphill direction. Ve
31、ntilation will be in the direction of traffic, against buoyancy, providing fresh outside air to evacuating motorists from the portal upstream of the fire (East Portal). The critical velocity and critical flow requirement is in the direction of traffic. The critical velocity requirement is 3.45 m/s (
32、679.0 ft/min) and the critical flow requirement is 314 m3/s (665 kcfm) for the tunnel geometry. The critical velocity and the critical flow values are based on the full area of the tunnel. CFD Analysis and Results The CFD model is based on the tunnel geometry described above and the jet fan configur
33、ation was developed based on preliminary SES analysis. Figure 1b above shows the geometry, jet fan configuration, fire location, and traffic trapped behind the fire in the tunnel. This arrangement was based on the design fire scenario described in the previous section. In addition to the tunnel and
34、its portals, a portion of the outdoor space was included in the CFD model in order to account for the external effects, such as inlet and exit pressure losses and wind conditions. The wind data for the closest available airport was used to estimate the worst case opposing wind at the exit portal. In
35、 the CFD model, a combination of opposing wind, as high as 38.6 km/hr (21 knots), and the number of operating jet fans was used to control the ventilation flow rate through the tunnel. The jet fan configuration was based on SES analysis, which provides the bulk temperature profile down stream of the
36、 fire location. Due to initial uncertainty about the temperature stratification patterns, which effects the total number of required operational jet fans, the CFD model includes a total of 25 jet fans (5 sets of 3 each plus 5 sets of 2 each), which are positioned in the tunnel ceiling arch, see Figu
37、re 1b. However, not all of the jet fans were turned on for the CFD analysis. With the exception of the set near the east portal (three jet fans), only two jet fans per set were turned on in each operating row. Table 1 shows the location, number of jet fans, and operational mode of the jet fans (on/o
38、ff) used for the CFD analysis. The jet fan dimensions are based on 1.6 m (5.25 ft) diameter (D) jet fans with a discharge velocity of 38.1 m/s and 2D silencers. The fire region is positioned at a location of approximately 120 m (393.6 ft) from the east portal based on the design fire scenario. The f
39、ire region dimensions are approximately 4 m x 3.6 m x 18.3 m (13.1 ft x 11.8 ft x 60 ft) based on a pool fire from combustible liquid fuel spill. The fire is positioned in the right lane of traffic and is made of 3 zones in order to model the volumetric growth of the fire. Table 1: Jet Fan Configura
40、tion in CFD Model Jet Fan Set No. of Jet Fans Jet Fan On/Off Location* m, (ft) 1 3 On 12 (39) 2 2 Off 112 (367) 3 2 Off 212 (6954 2 Off 312 (1023) 5 2 Off 412 (13516 2 Off 512 (1679) 640 ASHRAE Transactions7 2 On 612 (2007) 8 2 O 712 (23359 2 On 812 (2663) 10 2 On 912 (2991) * Location is distance f
41、rom entrance portal The computational domain was discretized using hexahedral control volumes. The mesh is distributed in non-uniform body-fitted format in order to efficiently resolve the large gradient regions. The mesh points are clustered in areas near the tunnel wall and the road surface close
42、to the fire and in the vicinity of the jet fans in order to properly account for the mixing action due to buoyancy and the high velocity discharge at the jet fans. Some representative grid sections are shown in Figures 2a-d. The computational mesh is made of approximately 253,000 grid points. Figure
43、 2: CFD mesh: a) fire region, b) exit portal, c) entrance portal, d) tunnel The flow in the tunnels is subsonic, turbulent with multi-component fluid mixture (air and smoke), and involves heat transfer. Therefore, appropriate physical models were activated in order to solve the relevant equations, i
44、.e. mass continuity, momentum, component transport, and thermal energy. The two-equation k-epsilon turbulence model and the scalable wall function were used in order to model the turbulence in the simulation. The CFD model included the tunnel concrete liner in order to accurately account for tunnel
45、wall convective heat exchange, which was modeled using the conjugate heat transfer. CFD Boundary Conditions. Based on the physical models used in the CFD model, the boundary conditions can be 5 2011 ASHRAE 641divided into three groups; fluid mechanics, heat transfer, and mass transport. These bounda
46、ry conditions are described as follows: Fluid Mechanics 1. Ambient pressure opening (inflow/outflow) at the horizontal and vertical faces outside the tunnel. 2. No slip conditions at all solid surfaces, e.g. tunnel/road surfaces, jet fan housing, etc. 3. For operating jet fans; velocity inflow condi
47、tions 38.1 m/s (7498.1 ft/min) at the jet fan discharge. 4. For operating jet fans; velocity outflow conditions 38.1 m/s (7498.1 ft/min) at the jet fan intake. 5. For non-operating jet fans; local relative pressure opening (inflow/outflow) at jet fan discharge locations. 6. For non-operating jet fan
48、s; outlet conditions (with average component velocity at jet fan discharge) for jet fan inlet locations. Heat Transfer 1. Ultrafast t2growing heat source with peak HRR of 100 MW (341.3 MBtu/hr) at the fire region. 2. Ambient summer design temperature of 33.9 oC (93.0 oF) at inflow surfaces outside t
49、he tunnel. 3. Fluid-solid interface at tunnel wall and road surfaces. 4. Deep sink temperature of 12 oC (53.6 oF) for the tunnel concrete liner outer surface. 5. For operating jet fans; outflow thermal conditions at jet fan intake sides. 6. For operating jet fans; average intake temperature at all jet fan discharge sides. 7. For non-operating jet fans; opening (inflow/outflow) conditions at jet fan intake and discharge locations. Inflow temperature equals to average outflow temperature. Mass Transport 1. Ultrafast t2growing mass/smoke source based on lower heating value (LHV) o
