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Publication Title | Pressurized Carbon Dioxide as Heat Transfer Fluid: Influence of Radiation on Turbulent Flow Characteristics in Pipe

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Manuscript submitted to: Volume 3, Issue 2, 172-182

AIMS Energy DOI:10.3934/energy.2014.2.172 Received 16 January 2014, Accepted 3 April 2014, Published 30 April 2014

Research Article

Pressurized Carbon Dioxide as Heat Transfer Fluid: Influence of Radiation on Turbulent Flow Characteristics in Pipe

Cyril Caliot1 Gilles Flamant1∗

1 Processes, Materials, and Solar Energy Laboratory, PROMES-CNRS,Centre F. Trombe, 7 Rue du Four

Solaire,66120 Font-Romeu-Odeillo, France

∗ Correspondence: Gilles.Flamant@promes.cnrs.fr

Abstract: The influence of radiative heat transfer in a CO2 pipe flow is numerically investigated at different pressures. Coupled heat and mass transfer, including radiation transport, are modeled. The physical models and the high temperature and high pressure radiative properties method of computation are presented. Simulations are conducted for pure CO2 flows in a high temperature pipe at 1100 K (with radius 2 cm) with a fixed velocity (1 m.s−1) and for different operating pressures, 0.1, 1, 5 and 20 MPa (supercritical CO2). The coupling between the temperature and velocity fields is discussed and it is found that the influence of radiation absorption is important at low pressure and as the operating pressure increases above 5 MPa the influence of radiation becomes weaker due to an increase of CO2 optical thickness.

Keywords: Radiation transport, CO2 radiative properties, high pressure carbondioxide spectra, computational fluid dynamics, supercritical carbon dioxide flow, radiation andflow coupling

1. Introduction

Current heat transfer fluids (HTF) for solar concentrating systems are: synthetic oil, steam, molten salt and air. At temperature higher than 565◦C air is the only available HTF, but the poor heat transfer properties of air are well known. Consequently, researches on alternative HTF for the conversion of concentrated solar energy at high temperature (high Carnot efficiency) is an important R&D topic for improving actual technologies. A review of thermodynamic cycles and working fluid has been published in [1] but for low-grade heat. Carbon dioxide appears to be a good candidate because it is non-flammable and non-toxic fluid. The CO2 supercritical state (s-CO2) is observed at 73.8 b and 304.5 K consequently favorable heat transfer and viscous supercritical properties may be built on designing innovative conversion systems. Some works have been done in the field of low and medium temperature solar heat conversion. For example, solar-driven carbon dioxide transcritical power system using evacuated tube type solar collectors was studied in [2] whereas supercritical Rankine cycle was examined in [3] and demonstrated in [4]. In this latter paper evacuated CO2-based solar collectors showed 65-70% solar heat collection efficiency and the measured power conversion efficiency was in the range 8.78-9.45%. At high temperature, it was pointed out in [5] that s-CO2 recompression Brayton cycle can be as efficient as helium Brayton cycle with lower inlet turbine temperature (550◦C for s-CO2 vs. 850◦C for He) but higher inlet pressure (20 MPa for s-CO2 vs 8 MPa for He). In addition, its high working pressure makes the installation more compact and reduces the investment cost. Due to the relatively low inlet turbine temperature, this cycle was mainly studied for power generation in nuclear power plants [5, 6, 7]. However, high temperature s-CO2 cycles may be used to produce power from concentrated solar energy systems. A molten salt solar tower using s-CO2 Brayton cycle to produce electricity was described in [8] and the integration of heat storage to a supercritical

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