Showing papers on "Subcooling published in 1997"

Bubble nucleation and growth characteristics in subcooled flow boiling of water

Abstract: Proceedings of the thirty-second national heat transfer conference. Please see www.asme.org for the complete proceedings.

Pool Boiling Curve in Microgravity

Abstract: Pool boiling experiments using R-113 were conducted in the microgravity of space on a flat heater, consisting of a semitransparent gold film sputtered on quartz substrate, 19.05 x 38.1 mm (0.75 x 1.50 in.). Transient measurements of both the mean heater surface temperature and input heat flux are used to compute the mean heat transfer coefficient at the heater wall. Steady-state pool boiling is achieved in microgravity under conditions in which a large vapor bubble somewhat removed from the heater surface is formed, which acts as a reservoir for the nucleating bubbles. The steady nucleate boiling heat transfer is enhanced materially in microgravity relative to that in Earth gravity, whereas the heat flux at which dryout occurs is considerably less. Using quasisteady data obtained during periods in which some significant portions of the heater surface were dried out, it was possible to construct two distinct composite approximate microgravity pool boiling curves for R-113, one for the higher level of subcooling and one for the lower level of subcooling. These are compared with a reference curve for pool boiling at a/g = +1, constructed from available data and correlations deemed to reasonably represent the circumstances present.

Velocity Field in Turbulent Subcooled Boiling Flow

Abstract: The velocity field was measured in turbulent subcooled boiling flow of Refrigerant-113 through a vertical annular channel whose inner wall was heated. A two-component laser Doppler velocimeter was used. Measurements are reported for two fluid mass velocities and four wall heat fluxes in the boiling layer adjacent to the inner wall as well as in the outer all-liquid layer. The turbulence was found to be inhomogeneous and anisotropic and the turbulent kinetic energy significantly higher than in single-phase liquid flow at the same mass velocity. The axial Reynolds shear stress in the liquid phase was observed to increase sharply near the inner wall. The near-wall velocity field appeared to be quite different from that in single-phase flow. It remains a challenge to separate the contributions of wall turbulence and bubble-induced pseudo-turbulence from the measurements which possibly represent a complicated interaction between the two.

Critical heat flux (CHF) phenomenon on a downward facing curved surface

Abstract: This report describes a theoretical and experimental study of the boundary layer boiling and critical heat flux phenomena on a downward facing curved heating surface, including both hemispherical and toroidal surfaces. A subscale boundary layer boiling (SBLB) test facility was developed to measure the spatial variation of the critical heat flux and observe the underlying mechanisms. Transient quenching and steady-state boiling experiments were performed in the SBLB facility under both saturated and subcooled conditions to obtain a complete database on the critical heat flux. To complement the experimental effort, an advanced hydrodynamic CHF model was developed from the conservation laws along with sound physical arguments. The model provides a clear physical explanation for the spatial variation of the CHF observed in the SBLB experiments and for the weak dependence of the CHF data on the physical size of the vessel. Based upon the CHF model, a scaling law was established for estimating the local critical heat flux on the outer surface of a heated hemispherical vessel that is fully submerged in water. The scaling law, which compares favorably with all the available local CHF data obtained for various vessel sizes, can be used to predict the local CHF limits on large commercial-size vessels. This technical information represents one of the essential elements that is needed in assessing the efficacy of external cooling of core melt by cavity flooding as a severe accident management strategy. 83 figs., 3 tabs.

Condensation of steam on the surface of hard coated copper discs

Abstract: At atmospheric pressure filmwise (FWC) and dropwise (DWC) condensation have been studied on the surface of copper discs which were coated by silicon-modified amorpheous hydrogenated carbon (a-C : H-Si) films of different thickness. On vertically oriented surfaces the DWC heat transfer coefficients were found to be larger by a factor of about 10 than the FWC coefficients which follow as function of surface subcooling temperature quite well Nusselt’s theory. Varying the angle of surface inclination, the DWC coefficient decreased down to about 40% of the vertical-surface values for 180° (face down orientation). The mean value for all inclination angles between 30° and 180° was calculated to be 87.6% of the maximum value for the 90°-orientation. Partly coating of the copper surface indicates a strong heat transfer enhancement of DWC over FWC even for relative small coated parts (e.g., 19%-coating yields an enhancement by a factor 2.3 for a cooling water flow rate of 4 m3/h). The diamond like properties of the a-C : H-coatings promise long stand times and thus application also in real technical condensation systems.

Heat transfer from a local heat source to a subcooled falling liquid film evaporating in a vapor-gas medium

Abstract: An experimental investigation of the heat transfer from a local heat source to a falling on a vertical plate liquid film has been made. The heater has a height of 6.5 mm and a width of 13 mm. The dielectric liquid (Perfluorine-triethyl-amine) is subcooled substantially up to saturation temperature. At the threshold value of heat flux density a horizontal standing wave is formed. Below the wave the film is divided into rivulets flowing with a certain wavelength. Wavelength is a function of the liquid capillary constant and depends weakly on the film Reynolds number. The flow pattern map of the film heating from the local heater in coordinates heat flux − the Reynolds number is first suggested. Four different patterns of liquid film flow have been observed. Three distinctive regimes of heat transfer connected with the film flow pattern have been found. For a light boiling liquid after forming a horizontal standing wave a film breakdown appears between the flowing rivulets. The intensity of the heat transfer abruptly reduces. Upon the evaporation of liquid films in a medium with inert gas in conditions of substantial thermocapillary convection influence and small Reynolds numbers, a specific crisis of heat transfer takes place.

Combined Pressure and Subcooling Effects on Pool Boiling From a PPGA Chip Package

Abstract: This study presents a detailed experimental investigation of the combined effects of pressure and subcooling on nucleate pool boiling and critical heat flux (CHF) for degassed fluorocarbon FC-72 boiling on a plastic pin-grid-array (PPGA) chip package. In these experiments pressure was varied between 101.3 and 303.9 kPa and the subcooling ranged from 0 to 65°C. As expected, lower wall superheats resulted from increases in pressure, while subcooling had a minimal effect on fully developed pool boiling. However, the superheat reductions and CHF enhancements were found to be smaller than those predicted by existing models. The CHF for saturated liquid conditions increased by nearly 17 percent for an increase in pressure from 101.3 to 202.7 kPa. In experiments with both FC-72 and FC-87 further increases in pressure did not produce any significant increase in CHF. At a pressure of 101.3 kPa a subcooling of 30°C increased CHF on horizontal upward-facing chips by approximately 50 percent, as compared to 70 percent on vertically oriented packages. The enhancement in CHF due to subcooling decreased rapidly with increasing pressure, and the data showed that the influence of pressure and subcooling on CHF is not additive. A correlation to predict pool boiling CHF under the combined effects of pressure and subcooling is proposed.

Calcium Carbonate Scale Formation During Subcooled Flow Boiling

Abstract: Scale deposition on the heat transfer surfaces from water containing dissolved salts considerably reduces fuel economy and performance of the heat transfer equipment. In general, this problem is more serious during nucleate boiling due to the mechanisms of bubble formation and detachment. In this study, a large number of experiments were performed to determine the effect of fluid velocity, initial surface temperature, and bulk concentration on the rate of calcium carbonate deposition on heat transfer surfaces during subcooled flow boiling. A physically sound prediction model for the deposition process under these operating conditions has been developed which predicts the experimental data with good accuracy. Two previously published models are also discussed and used to predict the experimental data.

Heat exchange unit for a cryogenic air separation system

Abstract: A heat exchanger having a first counterflow heat exchange unit with first and second segments wherein a flow of at least one product from a cryogenic air separation unit is channeled in a first direction along both the first and second segments and a second counterflow heat exchange unit is juxtaposed to the second segment of the first counterflow heat exchange unit and receives a flow of feed air which is channeled in a second direction that is counter to the first direction and allows heat exchange between the product and the air A crossflow subcooling unit is juxtaposed to the first segment and receives at least one process cryogenic liquid from the air separation system for subcooling

Critical Flow of Initially Highly Subcooled Water Through a Short Capillary

Abstract: Critical discharge of highly subcooled water through a cylindrical channel with a 0.78-mm inside diameter and 0. 78 mm in length was experimentally studied. The range of the initial water subcooling was 76 to 200 K, and the initial water pressure was in the range 0.5 to 5.2 MPa. The measured critical mass fluxes were compared with three models appropriate for application to critical flow in small and short channels. The experimental results confirm the significant effect of pressure losses on critical discharge rates in small channels. They indicate, however, that the frictional pressure losses in cracks may be considerably larger than losses predicted by the widely used correlations for rough channels. It is shown that models and correlations based on isentropic homogeneous equilibrium flow in the channel accurately predict the critical flow data, provided that the liquid initial stagnation pressure is adequately corrected for the channel entrance pressure loss.

Boiling characteristics of cylindrical heaters in saturated, gas saturated, and pure-subcooled FC-72

Abstract: A series of pool boiling experiments are performed in thermodynamically saturated, gas saturated, and pure-subcooled FC-72 using 25-, 50-, 75-, and 390-μm-diameter platinum wires. Comparisons of nucleate boiling characteristics are made between the various fluid-state/wire-size combinations. The effects of dissolved gas concentration on heat transfer are significant and are found to depend on length scale and heat flux. This discovery ofa heater size and heat flux impact on dissolved-gas boiling enhancement provides insight into experimental results from previous investigations.

Pool boiling curve in microgravity

Abstract: Pool boiling experiments using R-113 were conducted in the microgravity of space on a flat heater, consisting of a semitransparent gold film sputtered on quartz substrate, 19.05 x 38.1 mm (0.75 x 1.50 in.). Transient measurements of both the mean heater surface temperature and input heat flux are used to compute the mean heat transfer coefficient at the heater wall. Steady-state pool boiling is achieved in microgravity under conditions in which a large vapor bubble somewhat removed from the heater surface is formed, which acts as a reservoir for the nucleating bubbles. The steady nucleate boiling heat transfer is enhanced materially in microgravity relative to that in Earth gravity, whereas the heat flux at which dryout occurs is considerably less. Using quasisteady data obtained during periods in which some significant portions of the heater surface were dried out, it was possible to construct two distinct composite approximate microgravity pool boiling curves for R-113, one for the higher level of subcooling and one for the lower level of subcooling. These are compared with a reference curve for pool boiling at a/g = +1, constructed from available data and correlations deemed to reasonably represent the circumstances present.

Comparison between various thermal hydraulic tube concepts for the ITER divertor

Abstract: High heat flux tests on CuCrZr actively water cooled elements were performed with geometric and thermal hydraulic parameters relevant to ITER (International Thermonuclear Experimental Reactor) divertor conditions. Different types of mock-ups with the same width were tested and compared: double smooth tubes (SM2), swirl tubes (ST2; ST4), annular flow tubes (AF1; AF3) and hypervapotron tubes (HV1; HV3). Analyses of tests were done using the CEA method [1;2] first developed by Sandia Laboratory [ 3 ]. Finite Element calculations were used with a set of correlations in order to express the wall heat flux as a fonction of wall temperature in the convective regime as well as in the subcooled boiling regime (this set is now available in the EUPITER code). Maximum wall heat flux was compared with modified TONG-75 correlation. In terms of ICHF (Incident Critical Heat Flux) and for the same thermal hydraulic conditions, results gave this decreasing order: HV1, HV3, ST2, AF1, ST4, AF3, SM2. Versus lineic pumping power, the previous order was slightly changed: HV1, HV3, ST2, ST4, AF1, SM2, AF3. A typical HV1 result is a 38 MW/m 2 ICHF for 135°C local subcooling, 10 m/s water velocity, 3.5 MPa local pressure, 0.3 MPa/m lineic pressure drop and 380 W/m lineic pumping power.

Heat transfer and heat transfer fouling during subcooled flow boiling for electrolyte solutions

Abstract: Boiling heat transfer to electrolyte solutions is an essential operation in most concentration, crystallization and separation processes. The reliable prediction of heat transfer coefficients is, therefore, of major importance for optimum and economical overall plant design. While extensive research efforts have been devoted to the mechanisms of boiling of pure liquids and of liquid mixtures, hardly any information is available on the effects of dissolved salts on the boiling heat transfer coefficient. In subcooled flow boiling this lack of data is even more pronounced. Accurate prediction of clean heat transfer coefficients is also a pre-requisite for any in depth study on fouling in these processes, because of the strong influence of surface temperature on the deposition rates. Fouling of heat transfer surfaces during subcooled flow boiling is a frequent engineering problem in process industries. Nevertheless, only few experimental and theoretical investigations on this subject can be found in the literature and there is a lack of experimental evidence and physical understanding with respect to heat transfer to electrolyte solutions under clean and fouled conditions. The purpose of this study was to investigate 'clean' heat transfer and fouling phenomena under subcooled flow boiling heat transfer conditions for some electrolyte solutions. In this investigation the effect of operating parameters on the deposition of calcium sulphate and calcium carbonate on heat transfer surfaces during subcooled flow boiling was investigated. A large number of experiments were performed to determine the mechanisms which control deposition. Fluid velocity, surface and bulk temperature, concentration and ionic strength of the solutions were varied systematically. Finally, a mechanistic model is suggested for prediction of fouling resistances, which is in good agreement with the experimental data. Furthermore clean heat transfer to several electrolyte solutions under subcooled flow boiling conditions was studied and a model was developed to predict heat transfer coefficients for these conditions.