Onset of flow instability and critical heat flux in horizontal, thin, uniformly- heated annuli

Abstract

Two-phase flow instability (Ledinegg Excursion) and critical heat flux in heated microchannels are of great concern in the design and operation of numerous practical systems. In order to prevent Ledinegg flow excursion and eventual burnout of the heated channel, limits are typically imposed on the channel power and coolant mass flux to avoid the possibility of operation within the negatively-sloped region of the coolant channel demand curve. The primary purpose of this investigation was to experimentally determine the effect of design and operational parameters on the onset of flow instability (OFI) and critical heat flux (CHE) in thin, horizontal, uniformly-heated annuli. The work was motivated by the need for such data for the design and safety analysis of the Accelerator Production of Tritium (APT) project under development at Los Alamos National Laboratory. To this end, thirteen different experimental setups were employed to determine the OFI and CHF behavior of annular channels with hydraulic diameters on the order of two millimeters (annular gap widths of approximately 1.0 mm). The OFI data were then compiled and used to create two correlations; one comparing OFI heat flux to saturation heat flux and the other comparing OFI mass flux to saturation mass flux. The CHF data acquired in this investigation were compared with several empirical CHF correlations available in the literature and used to demonstrate that OFI, rather than CHF, is the limiting phenomenon in micro channel heat transfer.