Stojan Petelin

First and Second-Order Accurate Schemes for Two-Fluid Models

The six-equation two-fluid model of two-phase flow taken from the RELAP5/MOD3 computer code has been used to simulate three simple transients: a two-phase shock tube problem, the Edwards Pipe experiment, and water hammer due to rapid valve closure. These transients can be characterized as fast transients, since their characteristic time-scales are determined by the sonic velocity. First and second-order accurate numerical methods have been applied both based on the well-known, Godunov-type numerical schemes. Regarding the uncertainty of the two-fluid models in todays large computer codes for the nuclear thermal-hydraulics, use of second-order schemes is not always justified. While this paper shows the obvious advantage of second-order schemes in the area of fast transients, first-order accurate schemes may still be sufficient for a wide range of two-phase flow transients where the convection terms play a minor role compared to the source terms

Application of CFD Method for Risk Assessment In Road Tunnels

Abstract The definition of the deterministic approach in safety analyses arises from the need to understand the conditions that emerge during a fire accident in a road tunnel. The key factor of the tunnel operations during a fire is ventilation, which during the initial phases of the fire has a strong impact on the evacuation of people and later on the access of the intervention units to the tunnel. The paper presents the use of the CFD (Computational Fluid Dynamics) model in tunnel safety assessment process. The set-up of the initial and boundary conditions and the requirement for grid density found from validation tests of an FDS (Fire Dynamics Simulator) are used to prepare three kinds of fire scenarios—20 MW, 50 MW and 100 MW, with different ventilation conditions: natural, semi-transverse, transverse and longitudinal ventilation. The observed variables, soot density and temperature, are presented in minutes time steps through the entire tunnel length. Comparing the obtained data in a table allows the analyses of the ventilation conditions for different heat releases from fires. The second step is to add additional criteria of human behaviour inside the tunnel (evacuation) and human endurance to the elevated gas concentrations and temperature. What comes out is a fully deterministic risk matrix that is based on the calculated data where the risk is ranged on five levels, from the least to a very dangerous level. The deterministic risk matrix represents the alternative to a probabilistic safety assessment methodology, wherein the fire risk is represented in detail and the CFD model results are physically correct.

Developing Procedure for Nuclear Regulatory Vulnerability Assessment of Nuclear Power Plants

First attempts of developing the procedure and adopting the methodology for nuclear regulatory vulnerability assessment of nuclear power plants are presented. The development of the philosophy and computer tools could be new and important insight for management of nuclear operators and nuclear regulatory bodies who face difficult questions about how to assess the vulnerability of nuclear power plants and other nuclear facilities on external and internal threats. In the situation where different and hidden threat sources throughout the world are dispersed, the assessments of security and safe operation of nuclear power plants are very important. Most risk assessments of nuclear power plants emphasize probability more than consequences. In addition, the probabilities omit terrorists. As of 09/11/2001, the emphasis has changed and this paper tries to show how to do this. Probabilities are enhanced by showing they are dynamic, interactive, and include countermeasures. Consequences are also enhanced by their dynamic nature dividing them on short, intermediate and long term effects. Risk is enhanced by creatively distinguishing between perceived and estimated.Copyright

Large Outdoor Fire Model Analysis

The idea behind the article is how to define fire behavior. The work is based on an analytical study of fire origin, its development and spread. Mathematical fire model called FDS (Fire Dynamic Simulator) in used in a presented work. CFD (Computational Fluid Dynamic) model using LES (Large Eddie Simulation) is used to calculate fire development and spread of combustion products in the environment. The fire source is located in the vicinity of the hazardous plant, power, chemical etc. The article present the brief background of the FDS computer program and the initial and boundary conditions used in the mathematical model. Results discuss output data and check the validity of results. The work also presents some corrections of physical model used, which influence the quality of results.Copyright

Fluid Dynamic Models Application in Risk Assessment

Risk is a common name for the probability of a hazard turning into a disaster. Vulnerability and hazard are not dangerous in and of themselves, but if they come together, they generate a risk. However, risk can be reduced and managed. If we are careful about how we treat the environment, and if we are aware of our weaknesses and vulnerabilities to existing hazards, then we can take measures to make sure that hazards do not turn into disasters. Hazard from LNG (Liquefied Natural Gas) cargo begins in the first processing stage of natural gas liquefaction and loading the substance into LNG tankers. The transport itself over the sea is the safest part of the distribution process, as is demonstrated by the statistic of nautical accidents in the past 40 years (DNV, 2007, Perkovic et al., 2010 & Gucma, 2007). A review of a Rand Corporation document (Murray et al.) published in 1976 indicates a high level of safety for LNG tankers. The document indicates that in the initial 16-year history (from 1959 up to 1974) there had been no significant accidents. It should be noted, though, that in 1974 the world LNG fleet included only 14 vessels; by November, 2009, there were 327 vessels, a figure expected to increase to 350 vessels sometime in 2010 (LNG Journal, 2008). The DNV (Det Norske Veritas) counts 185 nautical accidents involving LNG tankers, all without severe consequences for the crew. The frequency of LNG tanker accidents is therefore 5.6 x 10-2 per ship year. The findings of the DNV (2007) furthermore demonstrate that the potential loss of life for the LNG crew member is 9.74 x 10-3 or less, considering the occupational fatality rate onboard gas tankers is 4.9 x 10-4. The analysis of the northern Adriatic Sea (Petelin et al. 2009), or, precisely, the gulf of Trieste, demonstrates that nautical accidents should occur with a frequency of 1.25 x 10-2 per year, assuming the current traffic density, and increases to 2.62 x 10-2 if the ship traffic increases by 100%. The hazard associated with LNG is mainly in its potential to cause severe fires resulting in heat radiation. If a large quantity of LNG is spilled into a pool, the cloud that is formed as it evaporates is a mixture of natural gas, water vapour, and air. Initially the cloud is heavier than air (due to its low storage temperature) and remains close to the ground. The buoyancy moves the natural gas upward at a gas temperature of around 170 K (-1030C), as experimentally demonstrated by ioMosaic (2007). The major influences on natural gas diffusion are environmental conditions. The cloud moves in the direction of the wind and the wind causes the cloud to mix with more air. If the concentration of gas in the air is between 5% and 15% it is flammable and burns if it contacts any ignition source. A concentration of gas smaller than 5% will not ignite and if the concentration is over 15% the air becomes saturated. The explosion of natural gas is not possible in open spaces because

Deterministic Risk Methodology for Road Tunnels

The definition of the deterministic approach in safety analyses arises from the need to understand the conditions that emerge during a fire accident in a road tunnel. The key factor of the tunnel operations during the fire is the ventilation, which during the initial phases of the fire have a strong impact on the evacuation of people and later on the access of the intervention units in the tunnel. The paper presents the use of the CFD model in the tunnel safety assessment process. The set-up of the initial and boundary conditions and the requirement for grid density found from validation tests of an FDS (Fire Dynamics Simulator) is used to prepare three kinds of fire scenarios, 20MW, 50MW and 100MW, with different ventilation conditions; natural, semi transverse, transverse and longitudinal ventilation. The observed variables, soot density and temperature, are presented in minutes time steps through the entire tunnel length. Comparing the obtained data in a table allows the analyses of the ventilation conditions for different heat releases from fires. The second step is to add additional criteria of human behaviour inside the tunnel (evacuation) and human resistance to the elevated gas concentrations and temperature. What comes out is a fully deterministic risk matrix that is based on the calculated data where the risk is ranged on five levels, from the lowest to a very dangerous level. The deterministic risk matrix represents the alternative to a probabilistic safety assessment methodology, wherein the fire risk is represented in detail and the CFD (Computational Fluid Dynamics) model results are physically correct.Copyright

Redundancy of Vital Systems for Merchant Ships

When we talk about redundancy we have to compare risks of different options that compete for the same final result and for this result we have to decide what our goal is. Most of the time everything is fine but a ship can experience an event that surpasses its abilities. Either propulsion is lost and the ship cannot control its behavior during a storm, resulting in lost lives or even the loss of the ship, or a polluted environment causes a loss. Most ships are not equipped with a redundant parallel propulsion system. The most common reason is that any redundancy is a cost which burdens the operating company. Even safety costs are not really defined since the compensation for the lost lives is negotiable and also depends on the skills of lawyers. Therefore only the business costs can be defined with relative accuracy.Copyright

Ship’s Engine Room Fire Modelling

When traffic accidents occur, transport systems can result in irreparable negative impacts on people as well as the environment. In maritime transport unexpected fire in the ship’s engine room represents a grave risk. Because such accidents are very often difficult to prevent, modelling of fire propagation bears a vital significance for setting up preventive measures and safety systems, whose task is the suppression of fire danger. The paper describes the CFAST computer model (Building and Fire Research Laboratory - National Institute of Standards and Technology), whose purpose is to solve the problem of fire propagation in a complex multi-compartment environment. In our example it was used in a concrete ship’s engine room, with a fire starting in the ship’s main propulsion engine. The application includes all elements that can be damaged in case of fire at different ventilation conditions of the ship’s engine room. By means of simulation, the analysis and presentation of physical parameters working upon exposed engine components was made.Copyright

What Kind of Propulsion Redundancy Is Needed for Tanker

In the last decade a lot of emphasis was given to the propulsion redundancy of tankers. Since there are several levels of redundancy, it not always clear to what extent these redundancies can help in difficult situations and at last what is the objective of these redundancies. When we look at the costs optimization can be made minimizing the operating costs or it can be done in such a way that probability of spill and the size of its consequences are minimal. With this approach we are minimizing environmental risk. With environmental risk in mind only the higher level of redundancy is acceptable and lower levels are not enough to first prevent and second to mitigate the consequences of possible spills.Copyright