Showing papers on "Subcooling published in 2005"

Wall Heat Flux Partitioning During Subcooled Flow Boiling: Part 1—Model Development

Abstract: In this work a mechanistic model has been developed for the wall heat flux partitioning during subcooled flow boiling. The premise of the proposed model is that the entire energy from the wall is first transferred to the superheated liquid layer adjacent to the wall. A fraction of this energy is then utilized for vapor generation, while the rest of the energy is utilized for sensible heating of the bulk liquid. The contribution of each of the mechanisms for transfer of heat to the liquid—forced convection and transient conduction, as well as the energy transport associated with vapor generation has been quantified in terms of nucleation site densities, bubble departure and lift-off diameters, bubble release frequency, flow parameters like velocity, inlet subcooling, wall superheat, and fluid and surface properties including system pressure. To support the model development, subcooled flow boiling experiments were conducted at pressures of 1.03 ‐3.2 bar for a wide range of mass fluxes ~124‐926 kg/m 2 s!, heat fluxes ~2.5‐90 W/cm 2 ! and for contact angles varying from 30° to 90°. The model developed shows that the transient conduction component can become the dominant mode of heat transfer at very high superheats and, hence, velocity does not have much effect at high superheats. This is particularly true when boiling approaches fully developed nucleate boiling. Also, the model developed allows prediction of the wall superheat as a function of the applied heat flux or axial distance along the flow direction. @DOI: 10.1115/1.1842784#

Three-dimensional simulation of impingement of a liquid droplet on a flat surface in the Leidenfrost regime

Abstract: The hydrodynamics and heat transfer phenomena of a liquid droplet impacting upon a hot flat surface are analyzed based on three-dimensional (3D) numerical simulation considering film-boiling behavior. The 3D level-set method is employed to portray the droplet surface variation during its deformation. The governing equations for the droplet and the surrounding gas phase are solved utilizing the finite-volume method with the arbitrary Lagrangian Eulerian technique. To account for the lubrication resistant effects of the vapor cushion formed by the film-boiling evaporation, a separate vapor flow model is developed to simulate the pressure and velocity distribution along the vapor layer between the droplet and the surface. The temperature fields in all phases and the local evaporation rate on the droplet surface are determined by using a full field heat transfer model. Both the water and n-heptane droplets impacting on the solid wall with different Weber numbers are investigated. The comparisons of the simulation results with the experimental results reported in the literatures are made to substantiate the model presented in this study. Specially, the spreading and recoiling motions of the impacting droplets are reproduced accurately for a wide range of the Weber number. The oscillation of the thickness of the vapor layer and the temperature distribution across the interface are also calculated and compared favorably with the experimental results. As the Weber number increases, the extent of the droplet spreading increases, but the residence time of the droplet on the surface is almost unchanged. Compared to the saturated impacts, the subcooled impact yields a thinner vapor layer and a higher heat transfer rate.

Experimental study of boiling phenomena and heat transfer performances of FC-72 over micro-pin-finned silicon chips

Abstract: Experiments were conducted to study the effects of micro-pin-fins on boiling phenomena and heat transfer from square simulated silicon chips immersed in a pool of FC-72. Two kinds of micro-pin-fins having fin thickness of 30 μm and fin heights of 60 and 200 μm, respectively, were fabricated on the silicon chip surface with the dry etching technique. The experiments were conducted at the liquid subcoolings of 3, 25, 35 and 45 K. The effects of dissolved air in FC-72 and chip orientation were also investigated. The boiling curve of the micro-pin-finned chips was characterized by a very small increase in wall superheat with increasing heat flux, and the wall temperatures at the CHF point for all the micro-pin-finned chips were less than the upper limit for the reliable operation of LSI chips (Tw=85°C). Liquid subcooling was very effective in elevating CHF for the micro-pin-finned chips compared to the smooth surface and other treated surfaces. The enhanced boiling heat transfer mechanisms for the micro-pin-finned chips were discussed.

Wall Heat Flux Partitioning During Subcooled Flow Boiling: Part II—Model Validation

Abstract: A mechanistic model for wall heat flux partitioning during subcooled flow boiling proposed in Part I of this two-part paper, is validated in this part. As the first step of the validation process, the developed model was applied to experimental data obtained as part of this study

Nucleation of steam in high-pressure nozzle experiments

Abstract: The nucleation characteristics of high-pressure saturated/subcooled steam were studied in Laval nozzles. By using nozzles designed for different expansion rates and varying the inlet stagnation state in wide limits, the Wilson lines and the fog structure (droplet size and number count) were determined between 0.5 and 5 MPa pressure for expansion rates ranging between 10 000 and 200 000 s -1 . The results show that Wilson point nucleation typically occurs in such fast expansions at supersaturated conditions where 3.5-5 per cent moisture would be present in the case of equilibrium flow. Higher expansion rates entail higher supersaturation, causing the formation of smaller, but more numerous, fog droplets. The reported measurements were made in the early seventies and were originally evaluated on the basis of the IFC-67 Steam Tables. The present evaluation uses the new IF-97 equations and reveals significant differences between the old and the new steam tables, leading to severe discrepancies in the value of subcooling at the Wilson point.

Internal refrigeration for enhanced NGL recovery

Abstract: The present invention is directed to methods for improving the efficiency of processes for the recovery of natural gas liquids from a gas feed, e.g., raw natural gas or a refinery or petrochemical plant gas stream. These methods may be employed with most, if not all, conventional separation methods using distillation towers, e.g., a demethanizer and/or deethanizer column. The methods of the present invention involve installing an internal refrigeration system consisting of an open cycle refrigerant withdrawn from a distillation column and a closed cycle refrigerant derived from the open cycle refrigeration system. A separator is installed downstream of the recycle compressor discharge cooler in the open cycle refrigeration scheme. At least a portion of liquid withdrawn from this separator is used as a closed cycle refrigerant by indirect heat exchange with the inlet gas or other process streams. Thus a closed refrigeration cycle enhances the performance of the open refrigeration cycle.

Thermal‐hydrodynamic modeling of bubbly flows with heat and mass transfer

Abstract: A three-dimensional (3-D) two-fluid model coupled with the population balance equation is used to predict bubbly flows with the presence of heat and mass transfer processes, particularly on subcooled boiling flows at low pressures. To account for the nonuniform bubble distribution in the bulk liquid at subcooled conditions, the CFX MUSIG (MUltiple-Size Group) model is further developed by incorporating the wall nucleation at the heated wall and condensation in the flow regime. Model predictions, covering a wide range of diffrent mass and heat fluxes and inlet subcooling temperatures, are compared against local and axial measurements. For the local case, the bubble size predictions include the comparison of the empirical bubble diameter relationships adopted in the CFX boiling model. The study shows that good agreement is better achieved by the present model with respect to the radial distributions of the bubble Sauter diameter, void fraction, interfacial area concentration, and liquid velocity profiles. However, significant weakness prevails over the vapor velocity distribution. Work is in progress to circumvent the deficiency by considering additional momentum equations or developing an algebraic slip model to account for bubble separation. For the axial case, good agreement is also achieved for the axial distributions of the mean bubble Sauter diameter, void fraction, and interfacial area concentration profiles. Here, the model correctly represents the plateau at the initial boiling stages at upstream, typically found in low-pressure subcooled boiling flows, followed by the significant increase of the void fraction at downstream.

Self-Pressurization of Large Spherical Cryogenic Tanks in Space

Abstract: The pressurization of large cryogenic storage tanks under microgravity conditions is investigated by coupling a lumped thermodynamic model of the vapor region with a complete solution of the flow and temperature fields in the liquid. Numerical results indicate that in microgravity both buoyancy and natural convection are still important and play a significant role in phase distribution and tank pressurization. A spherical vapor region initially placed at the center of the tank deforms and moves to one side of the tank before any significant pressure rise. Long-term results obtained with the vapor region near the tank wall show that, even in microgravity, natural convection leads to thermal stratification in the liquid and significantly alters the initial pressure rise. The final rate of pressure rise agrees with a lumped thermodynamic model of the entire system, but the final pressure levels depart from thermodynamic predictions because of initial transients. The history of the maximum liquid superheat and subcooling is also determined for each configuration.

An interfacial energy mechanism for the complete inhibition of crystal growth by inhibitor adsorption.

Abstract: We present a unified model for complete crystal-growth inhibition based on the thermodynamics of interfaces. The premise for our model is that the adsorption of inhibitor leads to a reduction in interfacial tension or edge energy for the crystal surface or step, respectively. In our formulation, the work to add a layer or grow a step increases due to the difference in interfacial tensions or edge energies for surfaces with and without an adsorbed inhibitor. For a large enough difference in interfacial tensions or edge energies, complete inhibition of growth is realized when the total work does not decrease as more crystals are formed. We demonstrate that our model can provide a theoretical description of critical subcooling data for ice with antifreeze proteins and glycoproteins, critical subcooling data for hydrates and ionic crystals, and critical supersaturation data for various crystal systems.

CFD modelling of subcooled flow boiling for nuclear engineering applications

Abstract: In this work a general-purpose CFD code CFX-5 was used for simulations of subcooled flow boiling. The subcooled boiling model, available in a custom version of CFX-5, uses a special treatment of the wall boiling boundary, which assures the grid invariant solution. The simulation results have been validated against the published experimental data [1] of high-pressure flow boiling in a vertical pipe covering a wide range of conditions (relevant to the pressurized water reactor). In general, a good agreement with the experimental data has been achieved. To adequately predict the lateral distribution of two-phase flow parameters, the modelling of two-phase flow turbulence and non-drag forces under wall boiling conditions have been also investigated in the paper. (author)

Refrigeration system with mechanical subcooling

Abstract: A refrigeration system includes a primary compressor that receives refrigerant from an evaporator and delivers refrigerant to a condenser, a subcooling compressor that delivers refrigerant to the condenser, and a subcooler that receives refrigerant from the condenser. A first refrigerant flow path and a second refrigerant flow path pass through the subcooler. The first refrigerant flow path delivers a portion of the refrigerant to the evaporator, and the second refrigerant flow path delivers a remainder of the refrigerant to the subcooling compressor. The refrigeration system includes a controller operable to control operation of the subcooling compressor such that the refrigeration system operates at a point of highest efficiency.