Organic Rankine Cycle
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thermophysical properties transition sharply at a specific tempera- ture termed the pseudocritical temperature (Tpc). The pseudocriti- cal temperature is so named, because it mimics the critical temperature, and exhibits a maximum in specific heat as illus- trated in Figure 1.
Heat transfer augmentation and deterioration occur under cer- tain conditions in this region of drastic property variations for the heating mode . While much work investigating heat transfer in heating modes for supercritical fluids has been done, compara- tively less work exists in the cooling mode; only a few experi- ments to date exist using pure CO2 [3–9]. Many existing heat transfer correlations tend to capture qualitative effects, but quanti- tatively disagree with each other, especially in the near critical region . This work strives to supplement current heat transfer and pressure drop databases, evaluate heat transfer and pressure drop correlations, benchmark current models, and explore proto- typic heat exchanger designs.
Figure 2 shows a schematic representation of the experimental apparatus used during the heat transfer and pressure drop investi- gations. The heat transfer facility consists of two loops: one loop for the recirculation of CO2 and another for the heat exchanger test section. The CO2 loop contains several critical components: the main pump for fluid flow, the inverter for pump control, and the HPLC pump used to control system pressure.
The main pump is a ChemPump (Model GCT-1.5K-20S), capa- ble of flow rates up to 14 m3/h, dynamic head up to 15 m, and sys- tem pressures up to 20 MPa. The pump is used in conjunction with a throttle valve to generate flow to the test section. Once the
University of Wisconsin, Madison, WI 53711
Texas A&M University, College Station, TX 77843
Michael Corradini Aaron Towne University of Wisconsin,
Heat Transfer of Supercritical Carbon Dioxide in Printed Circuit Heat Exchanger Geometries
The increasing importance of improving efficiency and reducing capital costs has led to significant work studying advanced Brayton cycles for high temperature energy conver- sion. Using compact, highly efficient, diffusion-bonded heat exchangers for the recupera- tors has been a noteworthy improvement in the design of advanced carbon dioxide Brayton cycles. These heat exchangers will operate near the pseudocritical point of car- bon dioxide, making use of the drastic variation of the thermophysical properties. This paper focuses on the experimental measurements of heat transfer under cooling condi- tions, as well as pressure drop characteristics within a prototypic printed circuit heat exchanger. Studies utilize type-316 stainless steel, nine channel, semi-circular test sec- tion, and supercritical carbon dioxide serves as the working fluid throughout all experi- ments. The test section channels have a hydraulic diameter of 1.16 mm and a length of 0.5 m. The mini-channels are fabricated using current chemical etching technology, emu- lating techniques used in current diffusion-bonded printed circuit heat exchanger manu- facturing. Local heat transfer values were determined using measured wall temperatures and heat fluxes over a large set of experimental parameters that varied system pressure, inlet temperature, and mass flux. Experimentally determined heat transfer coefficients and pressure drop data are compared to correlations and earlier data available in litera- ture. Modeling predictions using the computational fluid dynamics (CFD) package FLUENT are included to supplement experimental data. All nine channels were modeled using known inlet conditions and measured wall temperatures as boundary conditions. The CFD results show excellent agreement in total heat removal for the near pseudocritical region, as well as regions where carbon dioxide is a high or low density fluid.
Madison, WI 53711
Texas A&M University, College Station, TX 77843
Minimizing capital costs and increasing plant operation allows power to become more affordable. As traditional energy sources in the United States, such as coal, natural gas, and petroleum, con- tinue to rise in price and become scarce, it is clear that we must employ alternative sources of energy to ensure affordable eco- nomic growth in the future. Solar, wind, and nuclear are among the leading alternative energy sources.
The Department of Energy’s program for next generation nu- clear reactors establishes and defines broad goals to help increase nuclear’s role in national and global production of energy . Several of these designs employ a liquid metal or molten salt as the moderator and primary coolant and, therefore, necessitate a secondary power conversion cycle.
The supercritical carbon dioxide (S-CO2) Brayton cycle is one of the recommended power cycles for use with these potential reactors. This Brayton cycle uses CO2 in a supercritical state, at pressures above the fluid’s critical pressure. As single phase is maintained throughout the cycle, it significantly contributes to simplified plant design. In addition, one can take advantage of the high power densities inherent with supercritical power conversion cycles. Highly efficient power conversion (>40%) at moderate temperatures provides additional incentive for closely investigat- ing these cycles.
Many of the cycle designs use a recuperator and a heat exchanger to pre-cool the S-CO2 directly before the compressor. As the S-CO2 decreases in temperature at a given pressure; the
Contributed by the Heat Transfer Division of ASME for publication in the JOUR- NAL OF THERMAL SCIENCE AND ENGINEERING APPLICATIONS. Manuscript received May 11, 2010; final manuscript received May 17, 2011; published online August 10, 2011. Assoc. Editor: Bengt Sunden.
SEPTEMBER2011,Vol.3 / 031002-1
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