Showing papers on "Subcooling published in 1991"

Development of a Flow Boiling Map for Subcooled and Saturated Flow Boiling of Different Fluids Inside Circular Tubes

Abstract: The thermal behavior of a flow boiling system is represented by a flow boiling map to illustrate visually the relationships among various system parameter. An earlier flow boiling map by Collier (1981) does not include the effect of mass flux and is specific to water at low pressures. For other fluids, significant departures from the parametric trends displayed in Collier's map have been reported in the literature (e.g, Kandlikar). In the present paper, a new flow boiling map is developed to depict the relationships among the heat transfer coefficient, quality, heat flux, and mass flux for different fluids in the subcooled and the saturated flow boiling regions. The trends observed in the experimental data and correlations for water and refrigerants are used in deriving the present map. The particular areas where further investigation is needed to validate the trends are also indicated. In the subcooled investigation is needed to validate the trends are also indicated. In the subcooled boiling region, h{sub TP}/h{sub lo} is plotted against x with Bo as a parameter, while in the saturated boiling region, h{sub TP}/h{sub lo} is plotted against x with {rho}{sub l}/{rho}{sub g} and a modified boiling number Bo* as parameters. It is hopedmore » that the map would prove to be helpful in explaining the role of different heat transfer mechanisms in flow boiling of different fluids.« less

A Model for Correlating Flow Boiling Heat Transfer in Augmented Tubes and Compact Evaporators

Abstract: The additive model for the convective and nucleate boiling components originally suggested by Bergles and Rohsenow (1964) for subcooled and low-quality regions was employed in the Kandlikar correlation (1990a) for flow boiling in smooth tubes. It is now extended to augmented tubes and compact evaporators. Two separate factors are introduced in the convective boiling and the nucleate boiling terms to account for the augmentation effects due to the respective mechanisms. The fin efficiency effects in the compact evaporator geometry are included through a reduction in the nucleate boiling component over the fins due to a lower fin surface temperature. The agreement between the model predictions and the data reported in the literature is within the uncertainty bounds of the experimental measurements.

Refrigeration process with purge and recovery of refrigerant

Abstract: A refrigeration process including a refrigeration cycle, and refrigerant purge and recovery operations is disclosed. The refrigeration cycle may be a vapor compression cycle or an absorption cycle, for example. A purge stream is withdrawn from the refrigeration cycle and subjected to treatment by means of a membrane separation unit. The purge-stream treatment operation produces an essentially pure refrigerant stream, suitable for return to the refrigeration cycle, and an air stream, clean enough for direct discharge to the atmosphere. The process is applicable to most refrigerants, but is particularly useful in minimizing atmospheric emissions of chlorofluorocarbons, such as CFC-11 and CFC-12.

Geometry Effects on Critical Heat Flux for Subcooled Convective Boiling From an Array of Heated Elements

Abstract: The critical heat flux (CHF) condition was experimentally determined for subcooled flow boiling from an array of simulated microelectronic devices on one wall of a vertical rectangular passage. A test apparatus was used in these experiments that allowed visual observation of the boiling process while simultaneously measuring the heat flux and surface temperature for ten heat-dissipating elements. Using R-113 as the coolant, the CHF condition was determined for flush and slightly protruding heated elements. As expected, the element farthest downstream was found to reach the CHF condition first in all cases. For both the flush and slightly protruding elements, the trends in the CHF data are similar to those previously reported for subcooled flow boiling on an isolated element. At moderate flow velocities, the critical heat flux predicted by a proposed correlation for subcooled flow boiling from a single element was found to agree well with the multiple-flush-element data if the local fluid subcooling at the last element was used in the correlation. At lower velocities, however, the data deviated from the predicted values. The data for slightly protruding elements were also found to deviate from those for the flush elements at higher velocities. The apparent physical reasons for thesemore » trends are discussed in detail.« less

Evaporator with integral liquid sub-cooling and refrigeration system therefor

Abstract: An improved refrigeration evaporator having a first heat exchange element including a fluid inlet and a fluid outlet, for cooling a fluid stream traversing the evaporator by evaporating a volatile refrigerant liquid in heat exchange relation to the fluid stream. The volatile refrigerant liquid is supplied to the evaporator at relatively high saturated condensing temperature and slightly subcooled. The improvement in the refrigeration evaporator comprises a second heat exchange element, positioned in the fluid stream entering the first heat exchange element. The second heat exchange element cools and thereby further subcools the volatile refrigerant liquid prior to the refrigerant liquid entering the first heat exchange element via a pressure reducing device. The fluid stream being cooled may be either gas or liquid and the evaporator may be of the type best adapted for the type of fluid being cooled.

Li-SF(6) Combustion in Stored Chemical Energy Propulsion Systems

Abstract: Appropriate thermodynamic models and thermo-chemical data for multicomponents and immiscible phases have been incorporated into a code for the computation of chemical equilibrium of the reactants used in a stored chemical energy propulsion system. The reactants considered are molten lithium fuel and gaseous sulfur hexafluoride oxidant. Extensive equilibrium state relationships as a function of mixture fraction have been presented over a temperature range of possible operating conditions. They were subsequently employed to predict the complex flow structure of a choked, turbulent, reacting SF6 gas jet submerged in a molten lithium bath. Detailed profiles of velocity, temperature, void fraction and mass fraction of all phases in the jet have been presented. The results show that the reaction is completed within a short distance from the injector but the plume jet penetrates farther beyond due to significant evaporation of lithium fuel in the hot temperature plume, and subsequent condensation of the evaporated fuel and reaction products in the later part of the plume. The effect of lithium subcooling on flow structure has also been examined. It is found that lowering the degree of subcooling by raising the fuel bath temperature increases fuel evaporation, decreases the entrainment rate, delays the completion of reaction and lengthens greatly the jet penetration distance. Finally, for practical use, a simple correlation of the jet penetration length as a function of subcooling has also been presented.

Variation of Superheat With Subcooling in Nucleate Pool Boiling

Abstract: An analysis is presented that explains the variation of superheat with subcooling that has been observed by a number of researchers investigating nucleate boiling heat transfer at constant heat flux. It is shown that superheat initially increases with increasing subcooling near saturated conditions because of the way in which changes in active site density and average bubble frequency with increasing subcooling affect the rate of heat removal from the heater surface by enthalpy transport and microlayer evaporation. As subcooling increases further, natural convection begins to play an increasingly important role in the heat transfer process. Ultimately, natural convection is able to accommodate the entire imposed heat flux, after which superheat decreases as subcooling increases. The success of the analysis in explaining the variation of superheat with subcooling suggests that the rate of the heat removal from the heater surface is completely determined by the mechanisms of enthalpy transport, natural convection, and microlayer evaporation.

Some Measurements in Subcooled Flow Boiling of Refrigerant-113

Abstract: Measurements of local vapor phase residence time fraction, liquid phase temperature, and heated wall temperature were carried out in subcooled flow boiling of Refrigerant-113 through a vertical annular channel. Data are reported for two fluid mass velocities and two pressures over a range of wall heat flux. Estimates of typical vapor bubble size and velocity are given. Some comparisons with a one-dimensional two-fluid model of subcooled boiling flow are also presented.

A phenomenological model of the thermal hydraulics of convective boiling during the quenching of hot rod bundles

Abstract: In this paper, a phenomenological model of the thermal hydraulics of convective boiling in the post-critical-heat-flux (post-CHF) regime is developed and discussed. The model was implemented in the TRAC-PF1/MOD2 computer code (an advanced best-estimate computer program written for the analysis of pressurized water reactor systems). The model was built around the determination of flow regimes downstream of the quench front. The regimes were determined from the flow-regime map suggested by Ishii and his coworkers. Heat transfer in the transition boiling region was formulated as a position-dependent model. The propagation of the CHF point was strongly dependent on the length of the transition boiling region. Wall-to-fluid film boiling heat transfer was considered to consist of two components: first, a wall-to-vapor convective heat-transfer portion and, second, a wall-to-liquid heat transfer representing near-wall effects. Each contribution was considered separately in each of the inverted annular flow (IAF) regimes. The interfacial heat transfer was also formulated as flow-regime dependent. The interfacial drag coefficient model upstream of the CHF point was considered to be similar to flow through a roughened pipe. A free-stream contribution was calculated using Ishii's bubbly flow model for either fully developed subcooled or saturated nucleate boiling. For the drag in themore » smooth IAF region, a simple smooth-tube correlation for the interfacial friction factor was used. The drag coefficient for the rough-wavy IAF was formulated in the same way as for the smooth IAF model except that the roughness parameter was assumed to be proportional to liquid droplet diameter entrained from the wavy interface. The drag coefficient in the highly dispersed flow regime considered the combined effects of the liquid droplets within the channel and a liquid film on wet unheated walls. 431 refs., 6 figs., 4 tabs.« less

A New Critical Heat Flux Correlation for Boiling Liquid Metals

Abstract: This paper reports on correlations for the dryout heat flux for sodium in tube and grid bundles that have been developed based on experimental results and analytical considerations. The main feature of these correlations is that they are derived from thermal, hydrodynamic, and geometrical parameters. These parameters are the subcooling, the inlet and outlet vapor quality, the mass flow rate, the latent heat of vaporization, and the aspect ratio. The correlation proposed for tube geometries is developed from a data base of 170 data points compiled from 11 sources, and the correlation for flow through rod bundles is derived from the results of 13 tests.

Water Vapor Absorption Enhancement in LiBr/H2O Films Falling on Horizontal Tubes

Abstract: An experimental investigation was conducted into the enhancement of water vapor absorption in aqueous lithium bromide solution films falling on horizontal tubes. The variable parameters included the solution flow rate, the inlet temperature of the cooling media, the solution inlet temperature, and the effective extended surface. The experimental results, which were correlated with the film Reynolds number, showed that the cooling effect of the tube wall was the governing factor in the absorption process for small solution flow rates, but the inlet solution subcooling dominated the absorption performance for large flow rates. Also, the R tube, which has a unique ridged fin shape, exhibited the best absorption performance among the tested tubes.

Solvent recovery system with means for reducing input energy

Abstract: The invention is an improvement in a vapor recovery system having a refrigeration system with two refrigerant compressors, a stack with a gaseous mixture flowing through it and a plurality of heat transfer units used to condense vaporized solvent which, along with a gas, is a constituent of the mixture. The improvement includes a heat exchange unit connected to the refrigeration system and disposed downstream of the heat transfer units. The heat exchange unit rejects heat from the refrigeration system to the gas, thereby reducing the input energy required by the refrigeration system.

Boiling incipience in a reboiler tube

Abstract: This heating surface and liquid temperature distributions were experimentally obtained to identify the boiling incipience conditions in a single vertical tube thermosiphon reboiler with water, acetone, ethanol, and ethylene glycol as test liquids. The test section was an electrically heated stainless steel tube of 25.56-mm i.d. and 1900 mm long. The uniform heat flux values were used in the range of 3800--40 000 W/m{sup 2}, while inlet liquid subcooling were varied from 0.2 to 45.5{degrees} C. The liquid submergence was maintained around 100, 75, 50 and 30%. All the data were generated at 1-atm pressure. The maximum superheats attained around boiling incipience were taken from the wall temperature distributions and correlated with heat flux and physical properties of liquids using the expression of Yin and Abdelmessih. The heated sections required for onset of fully developed boiling with net vapor generation were determined assuming a thermal equilibrium model. In this paper a dimensionless correlation relating these values with heat flux, liquid subcooling, and submergence is proposed.

Critical Heat Flux and Heat Transfer Transition for Subcooled Flow Boiling

Abstract: The emphasis in the engineering development of fusion reactor components has been on material development. If high heat fluxes are to be accommodated with the present emphasis, low-pressure thermal data will be needed. The objectives of this experiment were to (1) expand the critical heat flux, W/cm{sup 2} data base near 4.0 kilowatts cm{sup 2} and heated coolant channel length divided by coolant chamber diameter near 100.0 (near-term application), (2) add low-pressure quantitative data to our existing knowledge of the qualitative influence of coolant exit pressure on CHF, and (3) provide thermal data in a region applicable to high heat flux components for assessing existing and evolving CHF and local heat transfer coefficient correlations (long-term).

An Experimental Investigation of Transition Boiling in Subcooled Freon-113 Forced Flow

Abstract: An experimental study of subcooled boiling in a Freon-113 forced flow is presented. The test section is a short tube (length 50 mm, inner diameter 8 mm, and wall thickness 0.3 mm) heated by the Joule effect. Wall temperature profiles along the tube are presented for different operating points and discussed in terms of the upstream propagation of a temperature front separating regions of nucleate and film boiling. This study emphasizes the effect of axial heat conduction on the boiling processes.

Vapor condensation on liquid surface due to laminar jet-induced mixing

Abstract: The effects of system parameters on the interface condensation rate in a laminar jet-induced mixing tank are numerically studied. The physical system consists of a partially filled cylindrical tank with a subcooled jet discharge from the center of the tank bottom toward the liquid-vapor interface which is at a saturation temperature corresponding to the constant tank pressure. Liquid is also withdrawn from the outer part of the tank bottom to maintain the constant liquid level. The steady-state conservation equations in nondimensiohal form are solved by a finite-difference method for various system parameters including liquid height to tank diameter ratio (////>), tank to jet diameter ratio (Did}, liquid outflow to jet area ratio (Aout/A7), and a heat leak parameter (Nh) which characterizes the uniform wall heat-flux. It is found that the average Condensation Stanton number (Stc) is decreasing with increasing H/D, Did, and Nh. For small Nh9 Stc is nearly independent of Aout/A,. For R€J ^ 600, Stc is essentially equal to (d/D)2 if (Did} > 20 and Nh « 1.

Prediction of onset of nucleate boiling, net vapour generation and subcooled CHF in coiled tubes

Abstract: The heat transfer characteristics of a coiled steam generating tube are very different from those of a straight tube. For example, the critical heat flux (CHF) in coiled tubes is less than that in straight tubes for subcooled boiling and significantly higher at higher qualities. In this paper, predictive methods are developed for the determination of the point of onset of nucleate boiling, the point of net vapour generation and the critical heat flux condition in curved ducts. The methods are based on the assumption that these phenomena are governed by the local hydro- and thermodynamic conditions which can be predicted using computational fluid dynamics techniques. Good agreement has been obtained with available experimental data.

Freezing cycle control device

Abstract: PURPOSE: To provide a freezing cycle control device capable of operating with maximum efficiency constantly. CONSTITUTION: A freezing cycle comprises a compressor, a subcooling control condenser 2, an expansion valve, and a vaporizer. Four receiver tubes 11a to 11d are installed to the subcooling control condenser 2 where the number of the receiver tubes 11a to 11d, where refrigerant is collected, is switched over by opening solenoid ON/OFF valves 22 and 23. A pressure switch 24 is used to detect cooling load. A circulation amount of refrigerant in the cycle is controlled by opening the solenoid ON/OFF valves 21, 22, and 23 in conformity with the cooling load, thereby setting an excess cooling degree to obtain the maximum efficiency which is in conformity to the cooling load. COPYRIGHT: (C)1992,JPO&Japio