ORCID Number

0009-0004-9777-8165

Date of Award

Summer 6-18-2026

Embargo Period

6-18-2031

Access Type

Thesis - Open Access

Degree Name

Master of Science in Mechanical Engineering

Department

Mechanical Engineering

Committee Chair

Sandra K.S. Boetcher

Committee Chair Email

boetches@erau.edu

Second Committee Member

Mark Ricklick

Second Committee Member Email

ridlickm@erau.edu

Third Committee Member

Marc Compere

Third Committee Member Email

comperem@erau.edu

College Dean

James W. Gregory

Abstract

Supercritical carbon dioxide (sCO2) has emerged as a promising working fluid for power generation and refrigeration systems due to its favorable thermophysical behavior near the pseudocritical region (Lemmon et al., 2018). However, in this region abrupt variations in density, viscosity, and specific heat introduce strong buoyancy and acceleration effects that complicate the heat transfer and pressure drop prediction (Van Nieuwenhuyse et al., 2023). Conventional turbulent flow correlations (Petukhov and Gnielinski) were developed for constant property fluids and often result in large errors when used for sCO2 flow (±30%). Improved correlations have been created for use with sCO2 but they are limited mainly to small diameter smooth tubing. Limited research has been conducted examining the thermohydraulic performance of sCO2 in non standard or enhanced surface geometry despite their increasing use in high performance heat exchangers. Recent research has explored enhanced surface tubing (twisted oval, spirally fluted, and dimpled tubing) for use in sCO2 systems yet the accuracy of existing prediction methods and usefulness of these configurations is largely unknown (Ehsan et al., 2018). To address these gaps, an experimental flow loop was designed and constructed to characterize the heat transfer and pressure drop behavior of sCO2 in enhanced surface tubing and conventional smooth tubing. The test section consisted of a 1 m long stainless steel tube with an outer diameter of 15.875 mm (5/8 in) and a wall thickness of 1.245 mm (0.049 in). The data collected provides insight into how surface disruptions from enhanced geometry interact with secondary flows and strong gradients present in sCO2 flows. This research helps validate computational models and aids in the development of improved prediction methods for sCO2 flow in nonstandard geometry. Conclusions from this research determine the usefulness of nonstandard tube geometry in sCO2 heat exchangers and help guide the development of next generation heat exchangers. Results showed that the dimpled tube increased the average heat transfer coefficient by up to approximately 38% near and above the pseudocritical region while producing little to no additional pressure drop penalty. The local results provided similar improvements averaged across all conditions, αlocal rose from 1205 W/m2K for the smooth tube to 1585 W/m2K for the dimpled tube, a 32% increase. Wall temperature measurements showed that enhanced surface tubing reduced thermal stratification of sCO2 flow.

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Available for download on Wednesday, June 18, 2031

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