Local scale dispersion model evaluation exercise ... - WIT Press

Transactions on Ecology and the Environment vol 21, © 1998 WIT Press, www.witpress.com, ISSN 1743-3541

Local scale dispersion model evaluation exercise Venetsanos A.G., Andronopoulos S., Statharas J., Bartzis J.G.

Abstract An evaluation study of the Local Scale (CFD) dispersion model ADREA-HF has been performed, by applying the SMEDIS Project evaluation protocol. The model validation for continuous releases, under neutral atmospheric conditions, strong density effects and with or without complex effects (obstacles, aerosols) has been based on the EEC-55 propane experiment. Detailed experimental information on EEC-55 was obtained from the SMEDIS experimental database. The predicted mean concentrations at the experimental sensor positions are compared against the experimental, using various statistical measures. It was found that the model concentration predictions are in good agreement with the experimental data, giving a 70% factor of 2 for the unobstructed case (EEC-550) and an 80% value for the obstructed case (EEC-55 1). The predicted average temperatures were also found in reasonable agreement with the experimental. 1

Introduction

The SMEDIS project is currently underway to develop a model evaluation protocol for the scientific evaluation of dense gas dispersion models [1]. In order to deal with the more realistic release scenarios, particular emphasis is given in SMEDIS to the complex effects, such as obstacles, terrain and aerosols. During the project, a classification of release scenarios has been defined. Well-documented experimental data sets, corresponding to each scenario have been gathered and a validation


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methodology based on flow and dispersion derived quantities has been defined. The ADREA-HF code [2] is a three-dimensional CFD model, developed for predicting the atmospheric dispersion of pollutants, under complex effects conditions. In the framework of SMEDIS the model ADREA-HF has been validated against the EEC-55 continuous two-phase releases with and without fence [6-9]. 2 The ADREA-HF mathematical formulation ADREA-HF is a three-dimensional time dependent computational tool, to be applied for vapour cloud dispersion in complex terrain. The working fluid is considered to be an ideal mixture of two components, the pollutant (fluid 1) and the air (fluid 2). The pollutant can be in two-phase conditions. Thermodynamic equilibrium is assumed for the mixture components, i.e. all phases share, at each point, the same pressure and temperature. The model solves the mass, momentum and internal energy conservation equations for the mixture and the mass conservation equation for the pollutant, in Cartesian form [2,3]. The model also solves the transient one-dimensional temperature equation inside the ground. Temperatures are derived in an iterative way, as a function of the pressure, mixture internal energy and fluid-1 mass fraction. Liquid phase appears, at a given temperature and pressure, when the partial vapour pressure of fluid 1, exceeds the saturation pressure of fluid 1, at the given temperature. The saturation conditions are obtained by application of Raoult's Law. 3

The turbulence model

The turbulence model is a one-equation model for the turbulent kinetic energy described in detail in [3,4]. The length scales are obtained from an algebraic relation, as functions of the distance from the ground, of the stability and of the global pressure gradient. In the present application the turbulent Prandtl number apj is obtained from the following relations:

' f = a—

0)


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(3)

In the above relations G is the mechanical production term of the turbulent kinetic energy equation, Pr is the Prandtl number of air (0.71), cy, cp, are the mixture specific heats for volume and pressure. The functions (%j, OH above are based on the Businger [5] parameterisation of the atmospheric surface layer. The above Richardson (Ri) number formulation takes into account of stability effects both due to temperature and concentration gradients. 4 The EEC-55 experiment The EEC propane field experiments [6-9] consisted of non-isothermal jet releases on flat terrain with an obstacle. They were performed by TV and Riso in Lathen (Germany, 1989) and were sponsored by the European Commission in the framework of the BA Project. In the EEC-55 experiment the liquid propane was released as a jet, for a period of 360 seconds. The nozzle diameter was 15.5mm, and the mass flow rate 3 kg/sec. At a distance of 48 m from the source, a twodimensional (thin) impermeable fence obstructed the flow and dispersion. The 2m high obstacle was removed instantaneously 185 seconds after the start of the release, allowing a comparison with the unobstructed case, under the same meteorological conditions. The wind velocity was 2.6 m/sec, at 10m elevation. The wind direction was 12 degrees from the ideal jet direction (normal to the fence). The stability conditions were neutral. The ground roughness height was 0.006 m. The relative humidity of the air was 99%. The ambient pressure was 102500 Pa Concentrations were measured on 30 sensors in total. Ground concentrations (5 cm from the ground) were measured at 21 sensors. Their locations are shown in Figs. 5, 6, where the sensor number is centred over the sensor position. Concentration profiles were measured at three masts, locations 22, 25 and 28 in Figs. 5, 6. The sensor numbering on each mast corresponds to an increasing distance from the ground (1,2,4 m).


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Temperature was measured on the two masts at positions 25 and 28 in Figs 5, 6. Temperature sensors 1-5 on mast 25 correspond to distances 0.1, 1,2,3,4m from the ground. Temperature sensors 6-13 on mast 28 correspond to distances 0.1, 0.5, 1, 1.5, 2, 3, 4, 5 m from the ground. 5 EEC-55 modeling The co-ordinate system X-axis was selected normal to the fence, as shown in Fig. 6. The computational domain extended 150 m in the X, 110m in the Y and 20 m in the Z direction. The grid dimensions were 49x31x32 in the X, Y and Z directions respectively. The grid was nonequidistant. The minimum grid step size was 1 m, in the horizontal plane and 0.05 m in the vertical. The minimum grid sizes were applied close to the ground, fence and source. Far from these locations, the grid was expanded logarithmically with an expansion ratio of 1.12. The source was modelled as a two-phase jet, having area 1.888x10-4 m2. The jet exit velocity was 69.3 m/s, in the X direction. The jet exit temperature was the propane saturation temperature 231.1 K, at the ambient pressure. The exit void fraction was 0.61. These exit conditions correspond to the experimental mass flow rate 3 Kg/s and jet momentum 207.9 N [9]. A time period of one second was assumed for the valve opening time. During this period the jet exit velocity was assumed to change linearly from zero to its nominal value. After this initial period, the release was assumed continuous until the end of the calculation (150 seconds for EEC-550 and 100 seconds for EEC-551). The cloud-ground thermal interaction was taken into account by solving the transient one-dimensional temperature equation inside the ground. The liquid rainout phenomenon was neglected, due to the relatively low latent heat of evaporation of propane and the low value of mass flow rate. 6

Statistical evaluation methodology

A variety of model performance measures have been presented in [10] and are in use in the SMEDIS project. The sets of statistical measures applied in this work are given below. Each set contains two parameters, one for the magnitude of the difference between predicted and experimental average (FB, MRB, MG) and one for the spreading of the values around the mean (NMSE, MRSE, VG).


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Fractional bias (FB). Normalised mean square error (NMSE) 3Z c,,c,

(4) ' '

Mean relative bias (MRB). Mean relative square error (MRSE) MRSE = 4 -£ \Cn+Co)

(5)

Geometric mean bias (MG). Geometric mean variance (VG) MG = exp[li^-hC~] L '•' I' J

FG = exp|fhTc A L\ f/ - I n C/ V' iJ

(6)

In addition to the above, the factor of 2,5,10 measures have also been used. The factor of 2, (FAC2) gives the fraction of data for which l/2 w

i

A

0.0 r 1

A f

A 4

7 10 1 3 1 6 1 9 22 25 28 C o n c e n t r a t i 0 n s e n sc r n u m b er

Fijjure 1 : EEC-55 propane releas e withou ; obstacle. Predictte d ver sus obsei-ved mean c oncentration for a II seiisors . The experimental error"bars repres ent plus/minus sta n dar d de>/iation from meaii.

3.5

E !

3.0

|

2.5 g

t ij(

x E xp M ean I A^D R E A -H r

1 2.0 A

1 '•• 1 i.o !

0.0

i o.o 1

J$ rr £ "1 ^*ft,«««ri%- 1 4 7 1 0 1 3 1 6 19 22 25 28 C o n c e n t r a ti o n s e n s o r n u m her

Figure 2: EEC-55 propane release with obstacle. Predicted versus observed mean concentration for all sensors. The experimental error bars represent plus/minus standard deviation from mean.


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