http://hdl.handle.net/1721.1/61395 Wed, 19 Jun 2013 04:07:39 GMT 2013-06-19T04:07:39Z http://hdl.handle.net/1721.1/62142 Film boiling on the inside of vertical tubes with upward flow of the fluid at low qualities Dougall, R. S.; Rohsenow, Warren M. Flow regimes, local heat transfer coefficients, and temperature distributions along the wall have been studied for film boiling inside a vertical tube with upward flow of a saturated liquid. The area of interest has been limited to cases of constant heat flux from the tube wall, small inlet liquid velocities, and film boiling which completely covers the entire heated portion of the tube. The last restriction means that there is no large region of nucleate boiling prior to the film boiling section. A visual test section made of electrically conducting glass tubing has been used for flow visualization studies at low qualities. Visual observations with this test section have indicated that the flow regime is annular with liquid in the center and vapor along the walls of the tube. Based on interpretations of temperature distribution data, it has been concluded that the annular flow regime changes at higher qualities to one of dispersed flow--where the liquid is dispersed in the form of drops through a predominately vapor flow. Two different diameter test sections made of stainless steel and heated electrically have been used to obtain experimental data of temperature distributions along the tube wall and local Nusselt numbers for different heat fluxes and flow rates. The fluid used in all the experimental tests has been freon 113. For the larger tube, 0.408" I.D., heat fluxes have been varied from 14,400 to 25,600 Btu/hr-ft2 , and mass velocities have been varied from 482,000 to 818,000 lbm/hr-ft2 . For these conditions, values of heat transfer coefficients from 24.0 to 41.4 Btu/hr-ft 2 -*F and values of Tw-Ts from 407 to 697*F have been obtained. These conditions have resulted in exit qualities up to 10 per cent. For the smaller tube, 0.180" I.D., heat fluxes have been varied from 22,500 to 41,800 Btu/hr-ft2 , and mass velocities have been varied from 332,000 to 398,000 lbm/hr-ft". For these conditions, values of heat; (cont.) transfer coefficients from 27.0 to 87.5 Btu/hr-ft2 -*F and values of Tw-T from 261 to 883*F have been obtained. These conditions have resulted in exit qualities up to 50 per cent. A theoretical derivation based on an annular flow model with turbulent flow in the vapor film has given good agreement with the experimental data at low qualities. A dispersed flow theory using a modified form of the Dittus and Boelter-McAdams equation seems to be an asymptote which the experimental data approaches with increasing qualities. Tue, 01 Jan 1963 00:00:00 GMT http://hdl.handle.net/1721.1/62142 1963-01-01T00:00:00Z http://hdl.handle.net/1721.1/62141 The mechanism of void formation in initially subcooled systems Griffith, P.; Snyder, George A. When an initially subcooled, water filled system undergoes a transient in heat flux or pressure such that bubbles form, the most important variable which determines the volume of the resulting void is the number of bubbles that is formed. In this report the number of bubbles that are formed is shown to be a function of the surface micro-configuration, the contact angle and the history. A method of specifying the history is developed, experiments are run and the general correctness of the history specification is shown to be correct. Order of magnitude values of the limiting wall superheats as a function of the surface history and configuration are presented, but the reproducibility of the experiments is not found to be high. Tue, 01 Jan 1963 00:00:00 GMT http://hdl.handle.net/1721.1/62141 1963-01-01T00:00:00Z http://hdl.handle.net/1721.1/62023 The mechanism of void formation in initially subcooled systems Griffith, P.; Snyder, George A. When an initially subcooled, water filled system undergoes a transient in heat flux or pressure such that bubbles form, the most important variable which determines the volume of the resulting void is the number of bubbles that is formed. In this report the number of bubbles that are formed is shown to be a function of the surface micro-configuration, the contact angle and the history. A method of specifying the history is developed, experiments are run and the general correctness of the history specification is shown to be correct. Order of magnitude values of the limiting wall superheats as a function of the surface history and configuration are presented, but the reproducibility of the experiments is not found to be high. Tue, 01 Jan 1963 00:00:00 GMT http://hdl.handle.net/1721.1/62023 1963-01-01T00:00:00Z http://hdl.handle.net/1721.1/61503 Model of critical heat flux in subcooled flow boiling Fiori, Mario P.; Bergles A. E. The physical phenomenon occurring before and at the critical heat flux (CHF) for subcooled flow boiling has been investigated. The first phase of this study established the basic nature of the flow structure at CHF. A photographic study of the flow in a glass annular test section was accomplished by using microflash lighting and a Polaroid camera. The results showed that the flow structure at CHF for high heat flux (1 x 106 - 5 x 106 Btu/hr-ft2), high subcooling (50-110 *F), at low pressures (less than 100 psia) was slug or froth flow depending on the mass velocity. Nucleation was shown to exist in the superheated liquid film. Pin-holes in the burned-out test sections suggested that the CHF condition was extremely localized. Flow regime studies in tubular and annular geometries, using an electrical resistance probe, provided further evidence of the slug or froth nature of the flow, and also showed that dryout of the superheated liquid film was not responsible for CHF. Since this evidence was contradictory to previously formulated models of CHF,a new model was proposed: Near the CHF condition, nucleation is present in the superheated liquid film near the surface. As a large vapor clot passes over the surface, these nucleating bubbles break the film and cause a stable dry spot which results in an increased local temperature. As the vapor finally passes the site, the dry spot is quenched by the liquid slug, and the temperature drops. At CHF, the volumetric heat generation, slug frequency, and void fraction are such that the temperature rise resulting from the dry spot is greater than the temperature drop during quenching. An unstable situation results where the temperature of this point continues to rise when each vapor clot passes the site until the Leidenfrost temperature is reached, at which point quenching is prevented and destruction is inevitable.; (cont.) A new method of measuring surface wall temperatures, in conjunction with high speed (Fastax) 16 mm movies, confirmed the microscopic features of the proposed model. At CHF, the wall temperature cyclically increased with the same frequency as the slug-vapor bubble passage. Destruction finally resulted as the temperature increased beyond the Leidenfrost point. An analytical investigation based on an idealized model demonstrated that the cyclical nature of the temperature increase at CHF could be predicted with appropriate flow pattern inputs. A parametric study using the program indicated that heater thickness and heater material should affect the CHF. It was shown that the proposed model appears to be consistent with parametric trends, i.e. mass velocity, pressure, subcooling, diameter, length, and surface tension. The model indicated that the CHF for thicker walled tubes, keeping all other conditions the same, would increase. CHF tests were conducted which confirmed that thicker walled tubes (0.078 vs. 0.012 in. ) had CHF up to 58 percent higher than thin walled tubes. Mon, 01 Jan 1968 00:00:00 GMT http://hdl.handle.net/1721.1/61503 1968-01-01T00:00:00Z