| Owing to two advantages, i.e., mature technique and rich reserves of nuclear fuel, nuclear energy is one of the main ways to solve the present energy problem. However, it also faces the problem of nuclear waste disposal. The "once through" approach which is widely used currently is simple for operation, but causes huge waste of energy. The more serious consequence is that the untreated nuclides will pose a long-term potential threat to human society and natural environment. The Accelerator Driven Subcritical System (ADS) adopting the partition and transmutation techniques can solve this problem effectively. The core cooling system is part of ADS and plays a key role in ADS. Its cooling ability is directly related to the safety and economy of ADS. The core cooling system consists of three loops, i.e., the LBE loop (the first loop) with LBE as core coolant, the helium gas loop (the second loop) which turns thermal energy to power, and the cooling water loop (the third loop) which carries the waste heat out. The first loop exchanges heat with the second loops by the Main Heat Exchanger, and the Cooler is used for the heat exchange between the second and the third loops. In this thesis, the optimization of heat transfer in the core cooling system is conducted to improve the performance of the cooling system.For the traditional thermodynamic analysis, temperature of heat resource is regarded as constant (which means an infinite heat capacity), and the influence of finite heat capacity flow rates (HCFRs) of working fluids on system performance is neglected, which brings the limitations for the application of optimization results. In order to examine the effect of finite HCFRs on system performance, the ideal heat exchanger with infinite heat transfer coefficient and the heat transfer area (which means the infinite heat transfer ability) was explored so as to eliminate the influence of heat transfer ability of heat exchangers on system performance. The three loops were examined on the temperature-entropy diagram as a whole, and the expression for the system performance was obtained by analyzing the relationship among the status parameters of the three loops. The results show that the limited HCFRs of the three loops play important roles on the system performance, and the system achieves its best performance when the HCFRs of the three loops are equal. On this base, the real heat exchanger with the limited heat transfer ability was further explored. For real heat exchangers, there exists a finite temperature difference between cold and hot fluids at the outlet, so the temperature of LBE in the core is higher than that for ideal heat exchanger. The resultant temperature differences among the three loops are larger than that for ideal heat exchangers. Through calculating these temperature differences, the universal expression which describes the influence of such factors as finite HCFRs of the working fluids, finite heat transfer coefficient and finite heat transfer area on the system performance, was obtained for all working conditions. Because of the interaction between hot and cold fluids in heat exchanger, the correlations of Nu for convective heat transfer in single duct under such boundary conditions as constant wall temperature and heat flux are not fully applicable for heat exchangers, and a new method calculating Nu of heat exchanger, i.e., temperature-matching method, was proposed in this thesis. The basis of this method is that the temperature fields of hot and cold fluids have intrinsic relationship because they are continuous and the heat flux is equal on the contacting wall. In order to determine the forms of temperature fields and the corresponding Nu, the convective heat transfer problem in single duct with exponentially-varying boundary condition along the flow direction was explored by considering the fact that the temperature difference of hot and cold fluids varies exponentially along the flow direction, and the temperature fields and Nu for different exponential coefficients were obtained. According to the temperature-matching principle, the matching parameters of hot and cold fluids were obtained, which are the function of HCFR and the exponential coefficient. Thereby, the Nu of hot and cold fluids and the total Nu (the function of the ratio of HCFRs of hot and cold fluids and the ratio of the inherent thermal resistances) of heat exchanger were determined. In order to check the reliability of the temperature-matching method, the Nu calculated by the temperature-matching method under constant wall temperature and constant heat flux was compared with the data from the literature. The result shows that the temperature-matching method is feasible for calculating the Nu of heat exchangers.The core cooling system realizes the conversion of heat to work by means of its second loop, therefore the investigation on the irreversibility of the system was conducted to lay a foundation for the system optimization from the perspective of work production. By studying the entropy generation of the cases with ideal heat exchangers under parallel and counter flows, it is found that both the entropy generation caused by heat transfer irreversibility and the cycle efficiency monotonically varies with the HCFR of each loop and have limit value. This indicates that it is feasible to use work as the objective parameter of the system optimization. During the optimization process, the plate-type heat exchanger was chosen, and the relative flow pattern of hot and cold fluids is counter flow. In addition, the difference between the total work output and the bump work consumption, i.e., Net Work Output (NWO), was defined as the target parameter for the optimization. In order to simplify the expression of the Net Work Output, the HCFRs of the three loops are supposed to be equal. The result shows that, under different constraints, there are the optimal HCFRs for the loops and channel dimensions for heat exchanger, which brings the maximum Net Work Output. On this basis, the approximate analytical solution of the optimum value of the other parameter was obtained when one of such parameters as HCFR and channel thickness is given, as well as the numerical solutions of the optimal parameters when both HCFR and channel thickness are variable.In order to improve the economy of ADS with the constant reactor core heat power, the influence of heat load on the Net Efficiency (the ratio of NWO to heat load) of the system was explored. In order to make the results more comparable, geometric similarity (the ratios of length to width, the ratio of width to height, and the specific area for heat exchanger are constant) and load similarity (the heat load per unit area is constant) are regarded as the constraints. The result shows that the Net Efficiency monotonically decreases with the increasing heat load. This indicates that the cooling scheme with multiple parallel subsystems has better economy than that with only single cooling system under the constant heat load per unit area. Under the above constraints, the optimal HCFR of each loop approximately changes with power function of the heat load, and the optimal thickness of each channel is less affected by the heat load and keeps constant.The optimization of ordinary heat exchanger will be subjected to the problem of "non-synchronous behavior" between minimum heat resistance and minimum pump power consumption, and the optimization from the system level can release such contradiction. In order to further improve the system performance, the improvement and optimization was made for the structure of heat exchanger from the component level, and a scheme of heat-pipe-type heat exchanger was proposed. By adopting the innovatively proposed "thermally short-cut" model, the heat-pipe-type heat exchanger was analyzed. The results show that heat-pipe-type heat exchanger can significantly reduce the "non-synchronous behavior" between heat resistance and pump power consumption, and make the improvement space of the system performance be expanded. |
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