Computational and Experimental Studies of Fluid flow and Heat Transfer in a Calandria Based Reactor

Transcription

1 Computational and Experimental Studies of Fluid flow and Heat Transfer in a Calandria Based Reactor SD Ravi 1, NKS Rajan 2 and PS Kulkarni 3 1 Dept. of Aerospace Engg., IISc, Bangalore, India. ravi@cgpl.iisc.ernet.in 2 Dept. of Aerospace Engg., IISc, Bangalore, India. nksr@cgpl.iisc.ernet.in 3 Dept. of Aerospace Engg., IISc, Bangalore, India. psk@aero.iisc.ernet.in Keywords: Calandria, Reactor, Complex flow, RANS. Abstract: CFD investigations are carried out to study the heat flux and temperature distribution in the 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 with horizontal inlets and outlets located on top and the bottom to study the flow pattern and the associated temperature distribution. The computations have been carried out to simulate fluid flow and convective heat transfer for assigned near-to working conditions with different moderator injection rates and reacting heat fluxes. The results of computations 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 comparison of CFD results with experiments show good agreement. 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. A numerical simulation is a very useful tool to compute the distributions of thermal and hydraulic flows in the complex geometry. However, the CFD

2 2 SD Ravi, NKS Rajan and PS Kulkarni results need to be supplemented by experimental data in order to validate the different models approaches in obtaining the solution. CFD results are validated with results of specially conducted experiments which have further provided optimum operating parameters for numerical simulation for better understanding of the working of nuclear reactor. 2 Calendria Model and Mesh Generation 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. Aprototype of the Calandria for experimental study and a Calandria model for numerical simulation have been designed. It is also necessary to ensure Geometric similarity, Kinematic similarity and Dynamic similarity between the model and prototype to understand flow patterns. Since the flow is axis-symmetric the analysis is carried out only for half portion. A 3-D view of the Calandria model is shown in Fig.1 [Raj89, IAE02]. Fig.1 shows the computational mesh for analysis of the fluid dynamics and heat transfer characteristics inside the calandria 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. The total cell number is about 205, 205 consisting of structured meshes and to save the calculation time. 3 Governing Equations and Boundary Condition A 3D-RANS code having upwinding implicit scheme and k ω approach for turbulence is used for the numerical solution. The Reynolds-Averaged Navier- Stokes Equations are solved for steady, compressible viscous flow. The governing equations used are the conventional standard sets that include: Continuity equation: Momentum equation: Energy equation: U j x j = 0 (1) ( ) ( ) p ρu i + ρu i U j = ( ) τ ij + ρu i t x j x i x u j j (2)

3 CFD and Expt Studies of Fluid Flow and Heat Transfer in Calandria 3 ( ) ( ρh + ρu j h ) = p ( ) Q t x j x j + ρu j h j The boundary conditions and initial conditions used include no slip, impermeable and adiabatic walls; 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. Diminishing residual criteria of the variables is used for the convergence with a limit of RMS residuals falling below (3) 4 Results of Experimental and Computational Analysis It can be seen from Fig.2 and Fig.3 that numerically simulated results match very well with experimental data [RKR03, MSH06]. Steady state observation shows that flow patterns are linearly proportional to the injection of velocities. It can be seen that streamlines and velocity distribution match in the experimental data. It is observe that fuel channels in the vessels enhance the mixing and diffuses the strong circulation zones. 3D-RANS code with various turbulence models has been used for computations. It is found that K ω model perform better than other models when compared with experimental results. 4.1 Computational Results Computations for isothermal and non-isothermal cases are made to understand the basic flow physics and convective heat transfer in the calandria. 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. It is observed that the flow pattern and the velocity profiles Fig4 and Fig.4. This observation turns out to be strengthened with the observation that the temperature distribution patterns remaining nearly unchanged at different heat dissipation levels Figs.5 and 5. It is found that the inlet velocity and heat load are the major parameters affecting on the formation of flow patterns. Fig.6 and Fig.6 shows the effect of inlet velocity on pipe wall temperature and Heat load on pipe wall temperature. 4.2 Experimental Results The developed model constructed by transparent acrylic material for the Experimental study [Raj89] is shown in Fig.7. Fig.7 shows the Different parts of the experimental set-up with Calandria model. A series of photograph of the steady state flow were taken in the range of flow rate kg/s of water that correspond to the fluid velocities (at injection) of 1.3 m/s and bulk Reynolds number of respectively. Fig.8 shows the Photographs taken with different speed of strobe.

4 4 SD Ravi, NKS Rajan and PS Kulkarni 5 Conclusions CFD analysis is carried out to study the mass flux and temperature distribution in the calandria using CFX-10 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. References [Raj89] [IAE02] 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, Bangalore, India(1989) International Atomic energy Agency Vienna: Heavy water reactors status and projected development. Technical reports series no. 407, (2002) [RKR03] Ravindra S. Tupake, Kulkarni P.S and Rajan N.K.S.: Numerical Analysis of Heat and Mass Transfer in a Calandria Based Reactor. 5 th Asian CFD Conference, Bussan, Korea, (2003) [MSH06] Manwoong Kim., Seon-Oh Yu, Hho-Jung Kim. Analyses on fluid flow and heat transfer inside Calandria vessel of CANDU-6 using CFD. Nuclear Engineering and Design (236), (2006) Fig D view of the Calandria model. Structured mesh model.

5 CFD and Expt Studies of Fluid Flow and Heat Transfer in Calandria 5 Fig. 2. Streamline plot comparision. Fig. 3. Vector plot comparision. Fig. 4. Isothermal flow pattern for Re = , with fuel channels. Non-isothermal flow pattern for Re = , with fuel channels. Fig. 5. Temperature distribution at Re = with 300MW thermal dissipation. Temperature distribution at Re = with 1200MW thermal dissipation.

6 6 SD Ravi, NKS Rajan and PS Kulkarni Fig. 6. Effect of inlet velocity on pipe wall temperature. Effect of Heat load on pipewall temperature. Fig. 7. Model of the Calandria fabricated with Acrylic material. Different parts of the experimental set-up-with calandria model. Fig. 8. Photographs taken with different speed of strobe.