THERMAL HYDRAULIC MODELLING OF INTERMEDIATE HEAT EXCHANGER USED IN LIQUID METAL COOLED NUCLEAR REACTOR

Abstract

The thesis is focussed on the numerical simulation of a double-pipe heat exchanger which has potential to be used as an Intermediate Heat Exchanger in a Liquid Metal Cooled Nuclear Reactor. The intermediate heat exchanger has liquid metal and 100% glycerol as the working fluids, where the liquid metal is flowing through the tube section and the glycerol is flowing through the annular section. In this thesis, two liquid metals used are Lead-Bismuth Eutectic (LBE) & Liq. Sodium. The thermophysical properties along with the advantages and disadvantages of liquid metals are explained in the theoretical section of the thesis. The thermal-hydraulic principals of the liquid metal heat transfer are explained by the governing equations and the study of the boundary layer for liquid metals. The geometrical models of the heat exchangers are designed in the Solidworks software and the meshing & the simulations are performed on ANSYS FLUENT. The heat transfer and fluid flow analysis are carried out by varying the inlet conditions and length of the heat exchanger, the inlet boundary conditions like inlet temperature of liquid metal, inlet velocity of the liquid metal, and the inlet temperature of the glycerol were varied, and a total number of 360 cases of heat exchangers were simulated. The thermal performance characteristics like Total rate of heat transfer, Overall heat transfer coefficient, pressure drop, and Logarithmic mean temperature difference were analyzed with the variations in the inlet conditions. All the fluid velocities were in the vi turbulent flow region therefore, realizable 𝑘 − 𝜀 model with enhanced wall treatment was chosen for Turbulent modelling. The thermal boundary layers were analyzed by the pictorial representation of the temperature profile of the axial cross-section of the heat exchanger under steady-state heat transfer process. The heat transfer rate, overall heat transfer coefficient, and LMTD were plotted against various inlet variables in order to assess their variation with the variation in the inlet conditions. It was observed that the overall heat transfer coefficient can be treated as a function of independent variables namely; the thermal conductivity of liquid metal, Reynolds number & Prandtl number of liquid metal, the inlet temperature of glycerol, and the length of the heat exchanger. Non-linear regression analysis was performed by developing a correlation between the overall heat transfer coefficient and the independent variables using the Generalized Reduced Gradient (GRG) algorithm. The predicted values from the correlation for LBE heat exchanger have R2 score of 0.9847 and Liq. sodium heat exchanger has R 2 score of 0.9624. Artificial Neural Network algorithm is used for regression analysis as well, a backpropagation feed-forward network was utilized. The input layer has 5 input variables which are the above-mentioned independent variables and the output layer has 1 variable which is the overall heat transfer coefficient. The prediction done by the ANN algorithm showed some remarkable accuracies, for LBE heat exchanger the R2 score is 0.9964 with the configuration (6, 8, 6) and for Liq. sodium the R2 score is 0.9918 with the configuration (10, 12, 10). The configurations represent the hidden layers with the corresponding number of nodes in themThe analysis of the heat transfer and fluid flow characteristics were analyzed to assess their dependence on the input conditions which were later utilized in developing a correlation and regression analysis was performed. Along with the non-linear regression analysis, the ANN technique was used to predict the overall heat transfer coefficient. It was noted that ANN predictions were relatively more accurate than the predictions by the developed correlations. These techniques are quite robust, precise and relatively easy in use; therefore, these techniques can be used by thermal system design engineers in the future for the double-pipe heat exchanger with liquid metal heat transfer which will inherently reduce the time and cost of CFD simulations.