| Co-Investigator(Kenkyū-buntansha) |
YOSHIZAWA Yoshio Tokyo Institute of technology, Research Laboratory for Nuclear Reactors, Professor, 原子炉工学研究所, 教授 (00016627)
KATO Yukitaka Tokyo Institute of technology, Research Laboratory for Nuclear Reactors, Assistant Professor, 原子炉工学研究所, 助教授 (20233827)
NITAWAKI Takesi Tokyo Institute of technology, Research Laboratory for Nuclear Reactors, Research Associate, 原子炉工学研究所, 助手 (60323838)
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| Research Abstract |
1)A new gas turbine cycle and its application toreactors
A supercritical carbon dioxide (CO_2) gas turbine reactor with a partial pre-cooling cycle attains 4 to 11% higher cycle efficiency compared with a helium(He) gas turbine cycle, providing comparable cycle efficiencies of 45.8% at medium temperature of 650℃ and pressure of 7 MPa with a typical He gas turbine reactor of GT-MHR (47.7%) at high temperature of 850℃. This higher efficiency is ascribed to : reduced compression work around the critical point of CO_2 ; and consideration of variation in CO_2 specific heat at constant pressure, Cp, with pressure and temperature into cycle configuration. Lowering temperature to 650℃ provides flexibility in choosing materials and eases maintenance through the lower diffusion leak rate of fission products from coated particle fuel by about two orders of magnitude. At medium temperature of 650℃, less expensive corrosion resistant materials such as type 316 stainless steel are applicable and thei
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r performance in CO_2 have been proven during extensive operation in AGRs. In the previous study, the CO_2 cycle gas turbomachinery weight was estimated to be about one-fifth compared with He cycles. The proposed medium temperature CO_2 gas turbine reactor is expected to be an alternative solution to current high temperature He gas turbine reactors.
A supercritical CO_2 gas turbine fast reactor(FR) attains slightly higher cycle efficiencies of 41% than conventional sodium(Na) cooled steam turbine cycle FRs(40%) at the same temperature of 530℃. Use of CO_2 cycle instead of steam cycle for the power generation system could remove safety problems due to hazardous water-Na reaction in the event of Na leakage. As the result, extra intermediate cooling loops relative to light water reactors(LWRs) could be eliminated which are necessary to the conventional FRs. This results in reduction of capital cost of the FRs. Therefore, supercritical CO_2 gas turbine FR offers an alternative to sodium-cooled FRs, eliminating problems related to safety and construction cost.
2)Supercritical CO_2 experimental loop and test result of HEATRIC's PCHE
In a gas turbine cycle, a recuperator is one of the most important components since its thermal-hydraulic performance contributes to plant efficiency improvement or one percent change of its temperature effectiveness leads to 0.6 percent change of the cycle efficiency. In addition, its compactification results in plant cost reduction. A promising heat exchanger to serve these purposes is the Printed Circuit Heat Exchanger(PCHE), developed by HEATRIC (UK). A supercritical CO_2 experimental loop was constructed to test thermal hydraulic characteristics of the PCHE and develop a new design of the PCHE. An experimental loop was built to test thermal-hydraulic performance of the PCHE recuperators for the supercritical CO_2 gas turbine cycle. A supercritical CO_2 experimental loop consists of low pressure tank, compressor, oil separator, heaters, heat exchanger, PCHE test section, coolers, pressure-regulating valves, etc. The loop is able to test the PCHE with heat exchange duty of 3 kW under the operating condition xip to 350℃ and 13 MPa.
A3-kW PCHE was purchased from HEATRIC, UK. The PCHE has core dimensions of 71×76×896 mm and a dry mass of 40 kg. The hot and cold sides has 144 and 66 flow channels, respectively. The PCHE has a double banking configuration with 12 hot plates and 6 cold plates. The flow channel may be described as zigzag lines of semicircle cross sections with 1.88 mm diameter. The flow channel pitch is 2.4 mm.
The HEATRIC's PCHE was tested at various inlet temperatures and pressures of 280-300℃ and 2.2-5.2 MPa in the hot side ; and 90-108℃ and 6.5-10.0 MPa in the cold side. The mass flow rate was 40-80 kg/h. The Reynolds numbers in the PCHE were about 2400-6000 and 5000-13000 in the hot side and cold side respectively. The overall heat transfer coefficients were 300 to 700 W/(m^2K), and the local heat transfer coefficients were about 500-1400 W/(m^2K) in the hot side channel. Maximum heat exchanger effectiveness is higher than 99%. Empirical correlations for the friction factor, the overall heat transfer coefficient, and the local heat transfer coefficient are proposed as functions of the average Reynolds number.
The HEATRIC's PCHE has provided excellent heat transfer performance as a recuperator for supercritical CO_2 cycle reactors but rather high pressure drop.
3)New design of PCHE
Three-dimensional computer fluid dynamic (3-D CFD) simulations using the FLUENT code have revealed that the high pressure drop of the HEATRIC's PCHE is ascribed to formation of eddies and swirls around the zigzag corners. Extensive 3-D CFD simulations were done to obtain a new flow channel configuration which reduces pressure drop through eliminating eddies and swirls. The new flow channel configuration has discontinuous fins with an S-shape, similar to a sine curve, in contrast to the continuous zigzag configuration of HEATRIC. Simulation calculations were done changing the fin shape and angle parametrically to obtain an optimal flow channel configuration for the recuperator of the supercritical CO_2 cycle, considering pressure drop and heat transfer performance.
The new configuration has one-fifth of the pressure drop reference to the zigzag configuration with equal heat transfer performance. The pressure drop reduction is ascribed a superior uniform flow velocity profile in the flow area and elimination of eddies and swirls that occur around bend corners of zigzag flow channels in the HEATRIC's PCHE. Less
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