Session: 30-05 Heat Exchangers

Paper Number: 153958

Two-Phase Microchannel Heat Exchangers for Transcritical CO2 Cycles - Investigation of Heat Transfer and Pressure Drop 

Supercritical & Transcritical CO₂ Brayton cycles provide a wide range of design & operational flexibilities relating to heating & refrigeration. Transcritical CO₂ condensing cycles are a preferred choice for temperate climates at lower ambients on account of higher thermodynamic efficiencies and specific power output at lower cycle pressures compared to supercritical CO₂ Brayton cycles. However, equipment design for a transcritical cycle poses additional challenges due to the phase change of the working fluid. Presence of CO₂ liquid-vapor mixture within the precooler and recuperators needs to be appropriately accounted during the heat exchanger sizing and design selection.  This task requires a detailed understanding and modeling of phase change phenomenon within CO₂ to accurately predict the two-phase region along with variables of interest such as temperature, pressure and mixture quality.   

This paper describes the development of a Computational Fluids Dynamics (CFD) based sub-model of a heat exchanger unit cell used in a transcritical CO₂ cycle.  A periodic unit cell of a Printed Circuit Heat Exchange (PCHE) in a counter-flow arrangement comprising of identical semi-circular cross section channels for hot and cold flows is considered.  The CFD model relies on the inbuilt Volume of Fluid (VOF) framework in ANSYS Fluent to predict the phase change behavior of CO₂. Non-linear variations in thermophysical properties of CO₂ within the pseudo critical & transcritical regions is estimated using NIST REFPROP data base incorporated into the CFD solver.  The liquid-vapor interface is tracked using an implicit formulation for computing the volume fractions while operating under steady state. The interface heat & mass transfer through evaporation-condensation is captured using a thermal phase change model. Turbulent flow and the near wall effects within the flow passages are resolved using a SST k-w turbulence model.

The model is validated with experimental measurements from literature performed on a water-cooled sCO₂ tube-in-tube heat exchanger.  The results from the validated CFD model are utilized to characterize the heat transfer and the pressure drop depending on the vapor quality for the heat exchanger unit cell. The model predictions cater to a range of mass flux, heat flux and saturation temperatures (from -40°C to 0°C) which represent both the design & off-design envelope of the transcritical CO₂ cycle. The current work is probably the first to provide a database of heat transfer coefficients and pressure gradients over the two-phase regime thereby aiding in the development of robust correlations for heat exchanger sizing analysis.

Presenting Author: Vyas Duggirala Indian Institute of Science, Bengaluru

Presenting Author Biography: Vyas Duggirala is a PhD scholar at the Indian Institute of Science, Bangalore. His research focuses on development of advanced modeling & simulations methods for supercritical CO2 heat exchangers for power cycle applications. Vyas`s reserach interests are numerical modeling for heat transfer and thermal systems, 2-phase modeling & additive manufacturing of heat exchangers. Vyas is currently working at Boeing Research & Technology, Bangalore as a Mechanical Systems Analysis Engineer, with his research focused on advanced modeling & simulation tools for integrated thermal management applications.

Authors:

Vyas Duggirala Indian Institute of Science, Bengaluru
Venkatanarasimha Hegde Indian Institute of Science
Pramod Kumar Indian Institute of Science, Bengaluru
Venkateswara Reddy Boeing Research & Technology, India