atw 2018-04v6

More documents

Recommendations

Info

atw Vol. 63 (2018) | Issue 4 ı April OPERATION AND NEW BUILD 228 of cases (i) and (ii), respectively. Obviously, a substantial effort was undertaken to include the upstream flow domain, so that the inflow into the fuel assembly and the gap are properly represented in the flow simulations. Since we have less experience with the gap region, and in particular, the applicable turbulence regime we have considered 3 cases corresponding to laminar flow, transitional flow, and fully developed turbulence, respectively. This covers the flow range 0.17 to 0.86 kg/s (Re = 1,250 to 6,250), proposed by the test matrix. For the investigation of the fuel assembly, we consider the nominal flow rate, i.e. the maximum flow rate planned in the test matrix. This corresponds to a flow rate of 3.58 kg/s and Re = 8,910 where Re is based on the bundle hydraulic diameter. All cases considered in the experimental test matrix are within the range of transitional flow according to Cheng and Todreas [4] (see next section on correlations) so that no distinction of various flow regimes is needed for the comparison to correlations. 2.1 Inter-wrapper flow gap region The objective of case study (i) is to investigate the effects of all upstream components on the flow distribution entering the gap, i.e. the inter wrapper flow region. This study employs the computational domain shown in Figure 4 left. For the simulation, a mesh with approximately 0.72 million cells has been generated. The investigated range of flow rates results in turbulent flow in all components upstream of the gap, since the Reynolds- numbers based on pipe diameter varies between 5,200 and 26,000. However, within the gap the Reynolds-number based on gapwidth equates to 1,250 to 6,250 corresponding to the transitional regime of turbulence. The pressure drop along the gap accounts for about 20 % of the total pressure drop. Since we are interested in accurately predicting the upstream flow in the gap region the use of a turbulent model is mandatory. Moreover, in order to judge the uniformity of the flow entering the gap there is no need to use a very accurate result within the gap. Thus, a high- Reynolds-number turbulence model using automatic wall functions is used. Figure 5 shows the velocity vectors in the gap entrance region for the case where the Reynolds-number is 5,200 based on pipe diameter (Reynolds-number is 1,250 based on the gap hydraulic diameter). We observe that the flow within the gap becomes near uniform after a short length, which does not exceed 10 % of the length of the gap region. The heated zone starts further downstream approximately at half the length of the gap region. For higher Reynolds-number a qualitative similar result is obtained. For future simulations aiming at accurately simulating the temperature field, we conclude that the effect of upstream components is negligible. In Table 1 the pressure drop across the simulated region is compared to design values for three selected cases covering the full range of flow rates. Design values are calculated using lumped parameter models. Both results agree reasonably well, indicating that lumped parameter models well describe the flow in the gap. 2.2 Flow within a single wire-wrapped rod bundle As in the previous study, we aim at investigating whether the upstream region that conditions the flow entering the wire-wrapped bundle influences the flow in the heated section of the bundle. Here, i.e. in case study (ii), a single Reynolds-number of 8,900 based on the bundle hydraulic diameter is considered. This corresponds to the nominal flow rate as well as the maximum flow rate intended in the experimental tests. The computational domain of Figure 4 right uses approximately 1 million cells. Figure 6 shows the velocity magnitude within the bundle. At the entrance, we still observe pronounced non-uniformities of the flow distribution. These quickly equilibrate so that a more-uniformly distributed flow is observed well before the heated section of the bundle is reached (for the location of the heated region refer to Figure 3). This result suggests that inflow effects are negligible for the intended thermal analysis of the bundle. Thus in a second simulation we remove the flow-conditioning region to reduce the size of the considered flow domain. To validate our simulation results we use higher mesh resolution within the smaller domain. Figure 7 shows the pressure along two selected axial lines, which are depicted in the small inset. The influence of the wirewrap manifests in the periodical modulation of the pressure profile. Obviously, development effects have decayed at a length of approximately 100 mm. We compute the pressure | | Fig. 5. Velocity vectors within the gap upstream region. Flow rate [kg/s] design Δp tot , [Pa] CFD Δp tot , [Pa] 1. 0.86 15350 13500 2. 0.688 9964 - 3. 0.516 5667 5500 4. 0.344 2586 - 5. 0.172 663 850 | | Tab. 1. Comparison of design values evaluated by lumped parameter model versus computed pressure drop across the test section including the flowconditioning components. | | Fig. 6. Velocity magnitude within bundle showing non-uniformities of flow distribution at leftmost plane and more-uniformly distributed flow in subsequent planes. Operation and New Build Numerical Analysis of MYRRHA Inter- wrapper Flow Experiment at KALLA ı Abdalla Batta and Andreas G. Class
atw Vol. 63 (2018) | Issue 4 ı April | | Fig. 7. Pressure along two selected axial lines in the wire-wrapped rod bundle. The inset specifies location of lines. drop using data at corresponding wire-wrap positions, i.e. from axial positions 0.065 m to 1.268 m. The mean pressure drop is 946 Pa/m. 2.3 Model validation In this subsection, results of our numerical study are compared to the simplified Cheng and Todreas [1986] correlation. The correlation was recently recommended in (1) to predict pressure drop (Δp) in bundles with an accuracy of ±20 %. It applies for a wide range of Reynolds- numbers. The friction factor (f) is defined in eq. 1, where d h,bdl , L, and u b 2 are hydraulic diameter, length, and average axial bundle velocity, respectively. (1) The correlation for f reads for Re < Re L for Re L ≤ Re ≤ Re T for Re > Re T (2) where Re L = 300 x 10 1.7(P/D−1.0) (3) Re T = 10,000 x 10 0.7(P/D−1.0) (4) ψ = log(Re/Re L ) / log(Re T /Re L ) (5) C fL = (-974.6 + 1612.0(P/D) − 598.5(P/D) 2 )(H/D) .06-0.085(P/D) (6) C fT = (0.8063 − 0.9022(log(H/D)) + 0.3526(log(H/D)) 2 ) × (P/D) 9.7 (H/D) 1.78-2.0(P/D) (7) We compare the nominal flow case of 3.580 kg/s which corresponds to a velocity of 0.2 m/s and Re is 8910, which is in the transient region. According to eqns (3) and (4), Re L and Re T are 902 and 15735, respectively. The calculated friction factor f equates to 0.0557. This corresponds to a pressure drop in the bundle of 1407.2 Pa. The predicted pressure drop resulting from the CFD study is 1,138 Pa. The difference is near 19 %, which lays within the accuracy limits. In future thermos-hydraulic simulations, the current model can be applied. For posttest analysis, additional sensitive studies might be necessary to further reduce the uncertainty. Conclusions The flow in the gap between neighbouring fuel assemblies plays an important role in transients between forced and natural convection. At KALLA an experiment on the interwrapper flow is currently setup and accompanied by pre-test numerical CFD studies. These proof that both the flow in the gap region and the fuel bundle are not influenced by the upstream flow-conditioning region. Moreover, development length are much shorter than the unheated length of the test section, so that the thermal field is uninfluenced by flow non-uniformities. Preliminary comparison of pressure losses computed by CFD and correlation provide reasonable agreement for both the gap and bundle. The result of our study enters pre-test studies of the thermal field within the EU-H2020 SESAME project. There complete simulation of the test section consisting of three bundles connected by the gap region including conjugate heat transfer is performed. Acknowledgement This project has received funding from the Euratom research and training programme 2014-2018 under grant agreement No 654935 and from the AREVA Nuclear Professional School. References: [1] Chen, S.; Todreas, N.; Nguyan, N. (2014). Evaluation of existing correlations for the prediction of pressure drop in wire-wrapped hexagonal array pin bundles. Nuclear Engineering and Design 267, pp. 109 – 131 [2] Rehme, K. (1973). Pressure drop correla tions for fuel element spacers. Nuclear Technology 17, 15–23. [3] Baxi, C.B., Dalle Donne, M., (1981). Helium cooled systems, the gas cooled fast breeder reactor. In: Fenech, H. (Ed.), Heat Transfer and Fluid Flow in Nuclear Systems. Pergamon Press Inc., pp. 410–462. [4] Cheng, S.-K.; Todreas, N. (1986). Hydrodynamic models and correlations for bare and wire-wrapped hexagonal rod bundles - Bundle friction factors, subchannel friction factors and mixing parameters. Nuclear Engineering and Design 92 (2), 227 – 251. [5] Kirillov, P.L., Bobkov, V.P., Zhukov, A.V., Yuriev, Y.S., (2010). Handbook on Thermo hydraulic Calculations in Nuclear Engineering. Thermohydraulic Processes in Nuclear Power Facilities, vol. 1. Energoatomizdat, Moscow. [6] Kamide, H.; Hayashi, K.; Toda, S. (1998). An experimental study of intersubassembly heat transfer during natural circulation decay heat removal in fast breeder reactors. Nuclear Engineering and Design 183, 97 – 106. [7] http://sesame-h2020.eu/ [8] Pacio, J, et. al. (2016), Deliverable 2.10 – KALLA Inter- wrapper flow setup for SESAME (thermal hydraulics Simulations and Experiments for the Safety Assessment of MEtal cooled reactors) project, activity: NFRP-01-2014 Improved safety design and operation of fission reactors, H2020 Grant Agreement Number: 654935. Authors Abdalla Batta Andreas G. Class AREVA Nuclear Professional School Karlsruhe Institute of Technology Karlsruhe, Germany OPERATION AND NEW BUILD 229 Operation and New Build Numerical Analysis of MYRRHA Inter- wrapper Flow Experiment at KALLA ı Abdalla Batta and Andreas G. Class
Page 1 and 2: nucmag.com 2018 4 217 Heat Transfer
Page 3 and 4: atw Vol. 63 (2018) | Issue 4 ı Apr
Page 5 and 6: Kommunikation und Training für Ker
Page 23: atw Vol. 63 (2018) | Issue 4 ı Apr

atw 2018-04v6

Create successful ePaper yourself

Delete template?

Save as template?