HEAT TRANSFER CHARACTERISTICS OF THE TWO-PHASE CLOSED THERMOSYPHON (WICKLESS HEAT PIPE) INCLUDING DIRECT FLOW OBSERVATION.

摘要

The two-phase closed thermosyphon is a heat transfer device capable of transferring large quantities of heat from a source to a sink by taking advantage of the high heat transfer rates associated with the evaporation and condensation of a working fluid within the device.; A visual study of such a device was carried out with the objective of delineating the various heat transfer mechanisms and flow phenomena which control its performance. An annular device with a stainless steel inner cylinder and glass outer cylinder was utilized, thus allowing the various flow regimes to be directly observed. Working fluids were Freon-113, ethanol, and water. Pertinent phenomena were photographed with both still and moving pictures. The visual observations are used to interpret the heat transfer characteristics as determined from temperature and pressure measurements for various liquid fill quantities, heat rates, condenser temperatures, and heated-to-cooled length ratios.; For small and intermediate fill quantities, four basic flow regimes were observed in the evaporator section of the thermosyphon during steady-state operation with Freon-113 and ethanol: (1) a smooth continuous film with surface evaporation; (2) the breakdown of the smooth continuous film into a series of rivulets; (3) a wavy film with unstable rivulets; and (4) a wavy film with bubble nucleation occurring in the unstable rivulets. A fifth condition which was observed with all fluids was a "dry-out" (heat transfer limit) where some portion of the evaporator ceases to be cooled effectively by the falling film and rivulets, resulting in a wall temperature excursion or "critical" condition. Three types of dry-out were observed. With the smallest quantities of working fluid, dry-out occurred at the bottom of the device due to the fact that there was not enough fluid available to complete the liquid circuit. With somewhat larger quantities, two types of dry-out, characterized by expulsion of the liquid from the evaporator surface, were observed.; In addition to the visual device, a stainless steel tubular, opaque device was fabricated and its heat transfer characteristics evaluated using Freon-113 as a working fluid. This device was designed to permit a larger range of geometric configurations (heated length, adiabatic length, condenser length, and angle of inclination with respect to the vertical).; Condenser heat transfer characteristics were found to be independent of liquid fill quantity except for the largest fills tested ((psi) > 60%), where liquid carry-over from the two-phase boiling pool into the condenser region occurred. Condenser results correlated in terms of film Nusselt and Reynolds numbers agreed reasonably well with accepted condensation correlations. A thin film evaporation model, similar to the Nusselt condensation theory, was found to give good agreement with local evaporator temperature measurements in those regions of the evaporator where a continuous film persisted. For large fill quantities ((psi) (GREATERTHEQ) 31.0%), evaporator results were indicative of nucleate pool boiling. Evaporator heat transfer characteristics are presented in the form commonly used for nucleate pool boiling results (evaporator heat flux vs. average wall temperature superheat) for various fill quantities and geometric parameters.; The heat transfer limit with Freon-113 for very small fill quantities was found to be in reasonable agreement with a simple model which predicts the heat flux level for which all of the fluid will be in circulation within the device. The limit increased rapidly with liquid fill quantity up to (psi) = 31.0%, for larger fill quantities, the limit was found to be independent of the quantity of fluid in the device.