Flow Regime Driven Thermal Enhancement in Internally - Grooved Tubes

Title of Document: FLOW REGIME DRIVEN THERMAL ENHANCEMENT IN INTERNALLY-GROOVED TUBES Darin Sharar, Doctor of Philosophy, 2016. Directed by: Professor Avram Bar-Cohen Department of Mechanical Engineering Internally-grooved refrigeration tubes maximize tube-side evaporative heat transfer rates and have been identified as a most promising technology for integration into compact cold plates. Unfortunately, the absence of phenomenological insights and physical models hinders the extrapolation of grooved-tube performance to new applications. The success of regime-based heat transfer correlations for smooth tubes has motivated the current effort to explore the relationship between flow regimes and enhanced heat transfer in internallygrooved tubes. In this thesis, a detailed analysis of smooth and internally-grooved tube data reveals that performance improvement in internally-grooved tubes at low-tointermediate mass flux is a result of early flow regime transition. Based on this analysis, a new flow regime map and corresponding heat transfer coefficient correlation, which account for the increased wetted angle, turbulence, and Gregorig effects unique to internally-grooved tubes, were developed. A two-phase test facility was designed and fabricated to validate the newlydeveloped flow regime map and regime-based heat transfer coefficient correlation. As part of this setup, a non-intrusive optical technique was developed to study the dynamic nature of two-phase flows. It was found that different flow regimes result in unique temporally varying film thickness profiles. Using these profiles, quantitative flow regime identification measures were developed, including the ability to explain and quantify the more subtle transitions that exist between dominant flow regimes. Flow regime data, based on the newly-developed method, and heat transfer coefficient data, using infrared thermography, were collected for two-phase HFE-7100 flow in horizontal 2.62mm 8.84mm diameter smooth and internally-grooved tubes with mass fluxes from 25-300 kg/m2s, heat fluxes from 4-56 kW/m2, and vapor qualities approaching 1. In total, over 6500 combined data points for the adiabatic and diabatic smooth and internally-grooved tubes were acquired. Based on results from the experiments and a reinterpretation of data from independent researchers, it was established that heat transfer enhancement in internallygrooved tubes at low-to-intermediate mass flux is primarily due to early flow regime transition to Annular flow. The regime-based heat transfer coefficient outperformed empirical correlations from the literature, with mean and absolute deviations of 4.0% and 32% for the full range of data collected. FLOW REGIME DRIVEN THERMAL ENHANCEMENT IN INTERNALLYGROOVED TUBES By: Darin James Sharar Dissertation Submitted to the Faculty of the Graduate School of the University of Maryland, College Park in partial fulfillment of the requirements for the degree of Doctor of Philosophy 2016 Advisory Committee: Dr. Avram Bar-Cohen, Chair Dr. Bongtae Han Dr. Michael Ohadi Dr. Patrick McCluskey Dr. Mario Dagenais, Dean’s Representative This work was supported by the U.S. Army Research Laboratory under Contract W911QX-15-D-0023, Task 0001. The views and conclusions contained in this document are those of the author, Darin J. Sharar, and should not be interpreted as presenting the official policies or position, either expressed or implied, of the U.S. Government.