individual
What campus are you from?
Daytona Beach
Authors' Class Standing
Christopher Bochenek, Senior
Lead Presenter's Name
Christopher Bochenek
Faculty Mentor Name
Dr. Sandra Boetcher
Abstract
Heat transfer of supercritical fluids is found in a wide range of applications, including novel power cycles, electronics cooling schemes, waste heat recovery, and green HVAC designs. Among these, supercritical carbon dioxide (sCO2) is especially promising due to its favorable but complex thermodynamic properties near the critical point. These properties strongly influence heat transfer but are highly sensitive and uncertain, requiring further experimental and numerical research to develop accurate predictive models for engineering use. Conventional turbulence models face challenges in predicting sCO2 heat transfer accurately. Direct Numerical Simulation (DNS) provides detailed insight but is limited to low-to-moderate Reynolds numbers, requires fine meshing, high computational power, and long runtimes. Reynolds-Averaged Navier-Stokes (RANS) models offer greater mesh flexibility and are suited for all turbulent regimes but predict local sCO2 heat transfer and flow physics with lower accuracy. Prandtl's secondary flows of the second kind are important turbulent phenomena occurring in non-circular ducts, caused by Reynolds stress anisotropy near corners. These flows generate transverse vortices near walls, redistributing momentum and energy across the duct cross-section outside the viscous sublayer, impacting heat transfer. Standard RANS turbulence models frequently implemented in the sCO2 literature assume isotropic Reynolds stresses, but this isotropic assumption is unable to capture the physics of anisotropy and the effect in the fluid's heat transfer. Reynolds Stress Models (RSMs), available in commercial CFD solvers like ANSYS Fluent, individually solve for each Reynolds stress component and can capture these anisotropic effects, including Prandtl’s secondary flows. ANSYS Fluent's RSMs are first validated against DNS data for a circular duct cooling scenario with a bulk Reynolds number Re_b = 5400, wall heat flux q_w = -30.78 kW m^-2, diameter D = 2 mm, length L = 30$D$, operating pressure P = 8 MPa, and inlet temperature T_0 = 342.05 K, respectively. The models are then applied to a square duct with the same hydraulic diameter and conditions to evaluate the effects of Prandtl’s secondary flows on sCO_2 heat transfer.\\ Key outputs include streamwise wall temperature, local Nusselt number, radial property gradients, velocity profiles, and Reynolds stresses. This study uniquely investigates the impact of these secondary flows on local sCO2 heat transfer, which remains unexplored in current literature.
Did this research project receive funding support from the Office of Undergraduate Research.
No
Investigating the Effects of Prandtl's Secondary Flows on sCO2 Heat Transfer Using Reynolds Stress Models
Heat transfer of supercritical fluids is found in a wide range of applications, including novel power cycles, electronics cooling schemes, waste heat recovery, and green HVAC designs. Among these, supercritical carbon dioxide (sCO2) is especially promising due to its favorable but complex thermodynamic properties near the critical point. These properties strongly influence heat transfer but are highly sensitive and uncertain, requiring further experimental and numerical research to develop accurate predictive models for engineering use. Conventional turbulence models face challenges in predicting sCO2 heat transfer accurately. Direct Numerical Simulation (DNS) provides detailed insight but is limited to low-to-moderate Reynolds numbers, requires fine meshing, high computational power, and long runtimes. Reynolds-Averaged Navier-Stokes (RANS) models offer greater mesh flexibility and are suited for all turbulent regimes but predict local sCO2 heat transfer and flow physics with lower accuracy. Prandtl's secondary flows of the second kind are important turbulent phenomena occurring in non-circular ducts, caused by Reynolds stress anisotropy near corners. These flows generate transverse vortices near walls, redistributing momentum and energy across the duct cross-section outside the viscous sublayer, impacting heat transfer. Standard RANS turbulence models frequently implemented in the sCO2 literature assume isotropic Reynolds stresses, but this isotropic assumption is unable to capture the physics of anisotropy and the effect in the fluid's heat transfer. Reynolds Stress Models (RSMs), available in commercial CFD solvers like ANSYS Fluent, individually solve for each Reynolds stress component and can capture these anisotropic effects, including Prandtl’s secondary flows. ANSYS Fluent's RSMs are first validated against DNS data for a circular duct cooling scenario with a bulk Reynolds number Re_b = 5400, wall heat flux q_w = -30.78 kW m^-2, diameter D = 2 mm, length L = 30$D$, operating pressure P = 8 MPa, and inlet temperature T_0 = 342.05 K, respectively. The models are then applied to a square duct with the same hydraulic diameter and conditions to evaluate the effects of Prandtl’s secondary flows on sCO_2 heat transfer.\\ Key outputs include streamwise wall temperature, local Nusselt number, radial property gradients, velocity profiles, and Reynolds stresses. This study uniquely investigates the impact of these secondary flows on local sCO2 heat transfer, which remains unexplored in current literature.