Naresh Kumar Maheshwari

Heat Transfer Studies on Lead-Bismuth Eutectic Flows in Circular Tubes

The use of accurate heat transfer model in liquid metal like Lead Bismuth Eutectic (LBE) flow is essential for the designing of the liquid metal cooled nuclear reactor systems. In the present study, the existing physical correlations for heat transfer in LBE flow through circular tube have been reviewed and assessed with the experimental results. In CFD analysis, PHOENICS-3.6 is used to carry out the evaluation of the various turbulence models in the tube geometry and to identify the difference between the numerical results and experimental ones in LBE flows. Based on the assessment of the existing correlations for heat transfer in LBE flow and the CFD results achieved, the best-suited correlation for turbulent Prandtl number is recommended in terms of Peclet number. This Prt can be incorporated in PHOENICS for LBE flow analysis.Copyright

CFD Analysis of Hydrocarbon Fireballs

ABSTRACT Three-dimensional computational fluid dynamics (CFD) simulations of hydrocarbon fireballs have been performed for their detailed characterization, including diameter, lifetime, flame, and internal fireball structure. The diameter and duration of a fireball obtained by CFD simulations and available empirical models have been compared with experiments and video footage data. CFD simulations are essential as empirical models have inherent assumptions and limitation in describing internal fireball structures. The fireball resulting from the loss of a fuel tank exposed to fire (experiment conducted by the German Bundesanstalt für Materialforschung in 1999) and a fireball from an aircraft crash (aircraft ambulance crash in Iceland in 2013) are considered for the CFD analysis. Influence of initial momentum, turbulence, and radiation on fireball structure has been analyzed for both of the cases. To illustrate transient behavior, developed pressure and flame structures are studied during the evolution of a fireball. Fireball diameter and lifting height computed from numerical analysis is found to be in good agreement with available video footage data. The incident radiations on the ground are calculated using a numerical method along the radial position from the center of the fireball, to predict the thermal hazards from the fireball.

Estimating steady state and transient characteristics of molten salt natural circulation loop using CFD

Abstract The steady state and transient characteristics of a molten salt natural circulation loop (NCL) are obtained by 3D CFD simulations. The working fluid is a mixture of NaNO3 and KNO3 in 60:40 ratio. Simulation is performed using PHOENICS CFD software. The computational domain is discretized by a body fitted grid generated using in-built mesh generator. The CFD model includes primary side. Primary side fluid is subjected to heat addition in heater section, heat loss to ambient (in piping connecting heater and cooler) and to secondary side (in cooler section). Reynolds Averaged Navier Stokes equations are solved along with the standard k-∊ turbulence model. Validation of the model is done by comparing the computed steady state Reynolds number with that predicted by various correlations proposed previously. Transient simulations were carried out to study the flow initiations transients for different heater powers and different configurations. Similarly the “power raising” transient is computed and compared with in-house experimental data. It is found that, using detailed information obtained from 3D transient CFD simulations, it is possible to understand the physics of oscillatory flow patterns obtained in the loop under certain conditions.

Mathematical model for prediction of droplet sizes and distribution associated with impact of liquid-containing projectile

Abstract After the events of 9/11, the impact of fast flying commercial aircraft is considered as major hazard threatening the Nuclear Power Plants (NPP) safety. The study of fuel spillage phenomenon and fireball formation is important to understand fire hazards due to burning of dispersed aviation fuel. The detailed analysis of fuel dispersion is very difficult to deliberate because both, large NPP structures and the large size of commercial aircrafts. Sandia National Laboratories, USA conducted impact tests using cylindrical projectiles filled with water to measure the associated parameters. Due to combustion properties and volatile nature of hydrocarbon fuels, the obtained parameters from impact studies using water are incomplete in fire analysis of flammable droplet clouds. A mathematical model is developed for prediction of droplet sizes and distribution associated with the impact of a liquid-containing projectile. The model can predict the transient behavior of droplet cloud. It is validated with experimental data available in literature. In the present study, the analysis has been performed using water and kerosene. The data obtained can be utilized as boundary and initial condition for CFD analysis. This information is useful for fire hazard analysis of aircraft impacts on NPP structures.

Numerical investigation of heat transfer in the vertical annulus between pressure tube and calandria tube of the advanced water cooled reactor

Abstract In advanced water cooled reactors, an annular gap exists between pressure tube and calandria tube. The gap is closed from top but is open from bottom. Due to differential temperature between pressure tube and calandria tube, air flow is induced by natural convection. This leads to heat transfer from pressure tube to calandria tube. The quantification of the heat transfer between pressure tube and calandria tube is numerically carried out with the help of the CFD code PHOENICS. Validation of the CFD code with experimental results and some established computational work from the literature has been done in order to verify the accuracy of the code. The natural convection phenomenon in the annular gap is then simulated. The velocity and temperature fields obtained from the CFD simulation are used to compute local and average heat transfer coefficients. Heat transfer coefficients for various pressure tube temperatures are computed. The effect of water on the heat transfer in the annular gas is also studied.

Computational study of moderator flow and temperature fields in the calandria vessel of a heavy water reactor using the PHOENICS code

Abstract Three dimensional CFD simulations of the moderator flow in the calandria vessel of a heavy water reactor are performed using the PHOENICS CFD code. The model includes the entire calandria vessel consisting of three shells, calandria tubes and inlet and outlet nozzle openings. The computational model prepared in PHOENICS consists of (a) standard k-∊ turbulence model, (b) PARSOL technique for handling curved objects in cartesian grids and (c) Boussinesq formulation for handling variable density flows. PHOENICS is validated by applying it to three different flow cases. The flow pattern in the calandria vessel under normal operating conditions is obtained through simulation. The effect of the presence of calandria tubes and heat generation on moderator flow pattern is studied. The simulation is also performed for various heat loads and moderator mass flow rates. The maximum temperature achieved by the moderator flow under various heat loads and moderator mass flow rates is obtained.

Parametric studies on hydrocarbon fireball using large eddy simulations

Occurrences of fireball close to plant buildings due to the release of flammable hydrocarbon fuel caused by the formation of fuel vapour cloud poses severe safety concerns. On the availability of potential ignition source, the induced fireball would cause the damage to the structures of nuclear power plant by direct contact, radiation and/or convection of hot combustion products through the opening of air intakes and ducts. In the present paper, the accidental/ experimental observations and theoretical studies of fireball are summarised. Computational fluid dynamics (CFD) analyses have been carried out to study the behaviour of fireball using OpenFOAM CFD software. The parametric studies are conducted by varying the mass of fuel, inlet velocity and inlet diameter. The new correlations for fireball diameter and duration have been proposed based on the parametric studies using CFD simulations. The fireball with a larger amount of fuel releases the heat slower and for a longer duration. The high heat released rate (HRR) is observed in case of a larger inlet diameter used for the same mass. The incident radiation from the fireball is calculated at different locations to assess thermal hazard. Analysis performed show that various parameters like fireball diameter, duration and the radiative flux falling at different locations can be predicted well using CFD code.

Theoretical studies on fuel dispersion and fireball formation associated with aircraft crash

ABSTRACT The accidental or intentional crash of an aircraft causes the fuel dispersion followed by a fireball formation that may affect the integrity of structures by thermal hazards. The fireball arising from the aircraft crash has been studied and analyzed using Eddy Dissipation Concept (EDC) combustion model in the computational fluid dynamic (CFD) platform openFOAM. The simulations have been performed with two methods. In first method, the fuel is injected in complete vapor form and in the second method, fuel is injected as dynamic fuel spray. The input to analyze fireball by dynamic spray model has been obtained from droplet spray model reported earlier (Shelke et al., 2018). The diameter and lift-off height obtained in CFD analyses are compared with a video footage reported in literature and it is found to be in good agreement. To illustrate the transient behavior of fireball, evolution of mass fractions of fuel and combustion products and developed pressure are studied. It has been found that both methods give similar predictions for fireball diameter and lifting height.

Steady State and Transient Characteristics of Natural Circulation Loop

A natural circulation loop (NCL) consists of a heat source at lower elevation, heat sink at higher elevation and connecting piping. It is representative of primary heat transport system of a nuclear reactor or a solar power plant. The study of steady state and transient characteristics of the NCL is hence important. In this work, a 1D simulation code is developed to model the NCL. The effect of heater power on steady state mass flow rate and temperature rise across heater is studied and compared with experimental data for validation. Further, the code is applied to obtain the flow initiation transients of various configurations of the NCL.

Investigation on Heat Transfer Behaviour of Molten Salt Natural Circulation Loop using Numerical Simulations

Molten salts are used as a coolant/heat transfer fluid in various high temperature engineering systems owing to their high boiling point at low pressure. Natural circulation of molten salt is being preferred in some systems like solar thermal power plant or in some nuclear reactors. Such systems can be studied with the help of a natural circulation loop. In this work, heat transfer characteristics of Molten Salt Natural Circulation Loop (MSNCL) are studied using 3D CFD simulations. Molten Nitrate salt, NaNO3+KNO3 (60:40 ratio by weight), is used as a fluid in MSNCL. In the MSNCL, in heater section, flow is developing and also mixed convection flow regime exists. The local Nusselt number variation in heater is calculated from computed data and is compared with that from Boelter correlation. Steady state heat transfer characteristics are obtained over a wide range of Reynolds number using CFD simulations. Unsteady heat transfer characteristics in the oscillatory flow formed in MSNCL with horizontal heater configuration are also studied and are found to be different as compared to vertical heater placed in vertical heater horizontal cooler configuration.