limiting thermal dissipation in a calandria paper for...

Proc. of the International Conference on ‘Advances in Mechanical Engineering’, December 15-17, 2008 S.V. National Institute of Technology, Surat – 395 007, Gujarat, India

Limiting Thermal Dissipation in a Typical Calandria Based Nuclear Reactor

M.H. Dinesh 1, S.D.Ravi 2, G.Premkumar3 and N.K.S.Rajan4

1 M.E Student Bangalore University Bangalore.

2 Project Assistant, CGPL, Dept of Aerospace Engineering, IISc. 3 Faculty Dept of Mechanical Engineering, UVCE, Bangalore.

4 Principal Research Scientist, CGPL, Dept of Aerospace Engineering, IISc. [email protected], [email protected], [email protected]

Abstract

It is established that typically in a nuclear reactor has its channel integrity depending on the coolability of the moderator as the prime heat sink as well as normal working or operating conditions. Hence a thorough understanding of the behavior of the heat transfer involving the moderator is important in the safe design of the reactor. CFD investigations are carried out to study the thermal dissipation estimation in a typical Calandria using a 3–dimensional RANS code. Internal flow computations and experimental studies are carried out for a calandria embedded with a matrix of tubes working together as a reactor. Numerical investigations are carried on the Calandria reactor vessel considering 480 fuel channels, with tangential inlets (14⁰), and outlets located at the bottom with 30⁰ angle to study the flow pattern and the associated temperature distribution. Computations are made for simulations of flow and convective heat transfer for assigned near–to working conditions with different moderator injection rates and reacting heat fluxes in the setup. The results of computation provide an estimate of the tolerance bands for safe working limits for the heat dissipation at different working conditions by virtue of prediction of the hot spots in the calandria. The isothermal CFD results are validated by a set of experiments on a specially designed scaled model conducted over a range of flows and simulation parameters. The results are shown include comparison of the computational and experimental work in this regard. The CFD analysis with heat transfer is carried out using a industry standard CFD tool CFX, using 0.48 million nodes for numerical analysis. The work assumes significance for the design considerations of the reactors and for detailed and critical parametric studies for optimization for the geometry considered.

1. Introduction The present world demands an environmental friendly power generation for which Nuclear power plant is a suitable alternative. The nuclear power is associated with a scare regarding its safety during operation, hence there is need to analyze flow pattern and heat transfer in Calandria or any heat exchanger used in these systems. It is reported that in a Canada Deuterium Uranium (CANDU) reactor[5], fuel channel integrity depends on the coolability of the moderator as an ultimate heat sink under transient conditions such as a loss of coolant accident (LOCA) as well as normal working or operating conditions. The study of thermal and fluid dynamic behaviours in industrial applications using complex geometries and configurations is considered significant to have safety in their designs and to realise optimised performances. In this work have high relevance in the industrial application are considered. Numerical simulation is a very useful tool to compute the distributions of thermal and hydraulic flows in the complex geometry. However, the CFD results need to be supplemented by experimental data in order to validate the different models approaches in obtaining the solution. Heat transfer, fluid flow and boiling are important phenomenon in many industrial applications such as nuclear reactor system, In case of nuclear reactor complete information of the moderator flow and the temperature distribution within the calandria is significant in the safety analysis, particularly with reference to certain postulated accident scenario where the moderator is assumed to be an effective heat sink. It is considered essential to understand the dominating parameters, the flow circulation patterns and the temperature distribution at different operating conditions that would provide vital feedback for the design studies.


To validate CFD results obtained from different models with experimental results, thereby choosing the right model to carryout further heat transfer study. To carryout heat transfer analysis inside a Calandria of actual prototype for different mass flow rate and heat flux using CFD tool. The results of computation provide an estimate of the tolerance bands of safe working limits for the heat dissipation for different working conditions, by virtue of prediction of the hot spots in the calandria. The work assumes significance for preliminary design considerations of the reactors and for detailed and critical parametric analysis that prove to be expensive without CFD tools. 2. Calandria Model

The nature of flow pattern in Calandria has significant effect on safe operation of reactor vessel in PHWR’s. Hence to get an insight into the problem of flow in calandria vessel, laboratory experiments have been conducted.

In many experimental set-ups we may not know how the "system" under investigation will behave. For a given problem it may be impossible to test for every conceivable condition. Therefore, a limited number of selected tests must be performed and from these tests, for example using a model of a structure, the real full-scale structure has to be designed. To design a particular model, all dimensions of the prototype are required and also the behaviour of the prototype are required. It is also necessary to ensure Geometric similarity, Kinematic similarity and Dynamic similarity between the Model and Prototype before fabricating the model. So the testing can be done after fabricating the model and giving similar boundary conditions of the prototype [5, 6].

The geometric modeling of calandria in CFX was done on same grounds as in experimental study. Since flow in calandria was not necessary to solve flow for entire calandria model. The simulation done on symmetry of the calandria of whole domain. So calandria model was built in Ansys CFX 11 as shown in Fig.1 and simulations carried out using that. A calandria of 305 mm diameter was built with inlet and outlet ports of 6 mm and 12mm respectively. The model can be meshed structured grid.

Fig.1 shows the geometry considered for CFD simulation, Fig.2 shows the 3D- view of the Calandria model and Fig.3 shows the structured mesh for corresponding model. In the core region, 69 fuel channels with 21mm of square pitch are simulated so it is possible to observe the flow behaviors around the channels.

3. Boundary and Initial Conditions

Any numerical simulation can consider only a part of the real physical domain or system. The truncation of domain leads to artificial boundaries, where we have to prescribe values of certain physical quantities. Furthermore, walls that are exposed to flow represent natural boundaries of physical domain. The numerical treatment of boundary conditions requires a particular care. An improper implementation can result in inaccurate simulation of real system. Additionally, stability and convergence speed of solution scheme can be negatively influenced. For boundary conditions, over the axis, symmetry conditions are imposed. Steady state approach with stationary domain, non-buoyant condition, low intensity turbulence and static temperature of 300 K are used as criterion for solution. Water at 250 C is used as domain fluid and reference pressure is taken as 1 atmosphere. ‘No slip’ conditions are used at the wall. The upwind advection scheme is used and a very small physical timescale is specified. At inlet and outlet ports, mass flow rate conditions based on incoming and outgoing incompressible fluid are imposed. The mass flow rate at inlet and outlet are chosen to be equal. The Reynolds number is calculated based on mass flow rate [1]. K-ω Model is used as a turbulence model, and the convergence is obtained with the residuals going down to 10-4. 4. Results and observations

An important observation made during the experimental measurements was that the flow pattern remained almost unaltered over complete range of the measurements and the distribution of velocities over the entire field showed that the magnitude of velocities at any point was closely and linearly proportional to the injection velocity. Thus the distributions of velocities in the entire flow field, normalized with respect the injection velocity remained unchanged within appreciably small margins of errors. Consolidated flow mapping are shown in Figs.4 to 7. Figures shows tracks of stream lines of flow, tracks of vector plots of the flow with the track length representing the scalar magnitude of the velocity and the directions


indicating the direction of the flow. Figures also show the Stream line distribution, velocity distribution mapped over different cross sections of the calandria with CFX results obtained under same boundary conditions using the k-ω. The data obtained from the analysis agreed within 10 % differences in most of the regions, except near the inlet (in main jet stream, from the injection point to about a distance of about one fourth of the calandria diameter on the axis) where the differences were of the order of 20-30% in the range of measurements. These differences near the inlet were due to fact that the velocity gradients being high, it was difficult to know whether a particle being tracked was entrained or resident in the main stream (due to resistance offered by the fuel channels) while mapping the tracks of the particle.

5. Computational results Computations for isothermal and non–isothermal cases are made to understand the basic flow physics and convective heat transfer in the calandria [5]. The flow structure and temperature distribution have been captured spatially. The non– isothermal analysis is made with an assumption that the fuel channel surfaces are giving out uniform heat flux. The isothermal flow distribution was used as an initial approximation to the flow field for saving the computation time. This initialisation is also analogous to the typical start–up procedures of the reactor. This observation turns out to be strengthened with the observation that the temperature distribution patterns remaining nearly unchanged at different heat dissipation levels Fig.8 and Fig.9. The hot spots – the region where the temperature is highest in the calandria are found to be near the lower zones of the calandria vessel. These are explainable due to the lack of refreshing mass flux into the zones leading to relatively lower heat removal rate Fig.10.

6. Experimental Results

The developed model constructed by transparent acrylic material for the Experimental study is shown in Fig.13. Fig.14 shows the Different parts of the experimental set-up with Calandria model [3] [4]. A series of photograph of the steady state flow were taken in the range of flow rate 0.75 kg/s of water that correspond to the fluid velocities (at injection) of 26.52m/s and bulk Reynolds number of 8.8 × 106 respectively. Fig.15 shows the Photographs taken with different speed of strobe. 7. Conclusion

CFD analysis is carried out to study the mass flux and temperature distribution in the calandria using CFX-11 as an analysis tool. Internal flow computations are carried out for a calandria embedded with a matrix of tubes carrying nuclear reacting media.

Increase in Reynolds number as mass flow rate is increased, does not have a significant change in the structure of the flow pattern. This is an important input to heat transfer studies to be carried out that indicates the forced convection dominating the heat transfer.

The results of computation provide an estimate of the tolerance bands of safe working limits for the heat dissipation for different working conditions, by virtue of locating the hot spots in the calandria. The work assumes significance for preliminary design considerations of the reactors and for detailed and critical parametric analysis that prove to be expensive without CFD tools. 8. References 1. Y. Nakayama., “Introduction to fluid mechanics”, published by Butterworth-Heinemann,

2000, chapter-16, Pg 274-278. 2. Zhanming Qin., “Vibration and Aeroelasticity of Advanced Aircraft Wings Modelled as

Thin-Walled Beams-Dynamics, Stability and Control”, October 2001, Blacksburg, Virginia, Pg 10-45.

3. Rajan N. K. S., “Experimental and Computational Studies of Fluid Dynamics and Heat Transfer in Spherical Vessels”, PhD Thesis, Department of Aerospace Engineering, Indian Institute of Science, 1989.

4. R J Adrian., Particle-Imaging Techniques for Experimental Fluid Mechanics, Annual Review of Fluid Mechanics, January 1991, Vol. 23, Pages 261-304

5. Ravindra S Tupake, PS Kulkarni and NKS Rajan., “Numerical Analysis of Heat and Mass Transfer in a Calandria Based Reactor”, 5th Asian CFD Conference, Bussan, Korea, (2003)


Fig.5 Experimental results Vs Computational results (streamline plot, m.=0.5kg/s, k-w model)

Fig.4 Experimental results Vs Computational results (vector plots, m.=0.5kg/s, k-w model)

Fig.2 3-D view of the Calandria model Fig.1 Geometry considered for CFD Simulation

Fig.3 Computational Mesh Model


Fig.6 Experimental results Vs Computational results (vector plots.=0.75kg/s, k-w model)

Fig.7 Experimental results Vs Computational results (streamline plots.=0.75kg/s, k-w model)

Fig.8 Thermal analysis for 103MW with 553.6 kg/sec mass flow rate 3.60×108

Fig. 9 Thermal analysis for 1200 MW with 553.6 kg/mass flow rate and Re 3.6×108


Fig.10 Hot-spot mapping at Re 3.6 ×108 with 103 and 1200 MW thermal dissipation

Fig.12 Effect of Inlet Mass flow rate on outlet temperature

Fig.11 Effect of Inlet Mass flow rate on Fuel channel wall Temperature

Fig.13 Model of the calandria fabricated with Acrylic material

Fig.14 Different parts of the experimental set-up-with calandria model.

Fig.15 Photographs taken with different speed of strobe.

limiting thermal dissipation in a calandria paper for...

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