Showing papers on "Subcooling published in 1996"

Optimizing and Predicting CHF in Spray Cooling of a Square Surface

Abstract: Spray cooling of a hot surface was investigated to ascertain the effect of nozzle-to-surface distance on critical heat flux (CHF). Full cone sprays of Fluorinert FC-72 and FC-87 were used to cool a 12.7 X 12.7 mm 2 surface. A theoretical model was constructed that accurately predicts the spray's volumetric flux (liquid volume per unit area per unit time) distribution across the heater surface. Several experimental spray sampling techniques were devised to validate this model. The impact of volumetric flux distribution on CHF was investigated experimentally. By measuring CHFfor the same nozzle flow rate at different nozzle-to-surface distances, it was determined CHF can be maximized when the spray is configured such that the spray impact area just inscribes the square surface of the heater. Using this optimum configuration, CHF data were measured over broad ranges of flow rate and subcooling, resulting in a new correlation for spray cooling of small surfaces.

Bubble Behavior and Mean Diameter in Subcooled Flow Boiling

Abstract: Bubble behavior and mean bubble diameter in subcooled upward flow boiling in a vertical annular channel were investigated under low pressure and mass flux conditions. A high-speed video system was used to visualize the subcooled flow boiling phenomenon. The high-speed photographic results indicated that, contrary to the common understanding, bubbles tend to detach from the heating surface upstream of the net vapor generation point. Digital image processing technique was used to measure the mean bubble diameter along the subcooled flow boiling region. Data on the axial area-averaged void fraction distributions were also obtained using a single-beam gamma densitometer. Effects of the liquid subcooling, applied heat flux, and mass flux on the mean bubble size were investigated. A correlation for the mean bubble diameter as a function of the local subcooling, heat flux, and mass flux was obtained.

Second-law-based thermodynamic analysis of two-stage and mechanical-subcooling refrigeration cycles

Abstract: Thermodynamic analysis of HFC-134a vapor-compression refrigeration cycles is investigated by both the first and second laws of thermodynamics. Second-law analysis is carried out for both two-stage and mechanical-subcooling refrigeration cycles. The analysis is performed on each of the system components to determine their individual contribution to the overall system irreversible losses. It is found that most of the losses are due to a low compressor efficiency. Irreversibilities of expansion valves and condenser are also significant. In addition, it is shown that the optimum inter-stage pressure for two-stage and mechanicalsubcooling refrigeration systems is very close to the saturation pressure corresponding to the arithmetic mean of the refrigerant condensation and evaporation temperatures. These results are compared with the existing practice in the industry. Furthermore, theoretical results of a two-stage refrigeration system performance are also compared with experimental values for a CFC-22 system. Copyright © 1996 Elsevier Science Ltd and IIR

An empirical model for sizing capillary tubes

Abstract: This paper presents an empirical model that has been developed to size adiabatic and non-adiabatic capillary tubes for small vapour compression refrigeration systems, in particular, household refrigerators and freezers. The model is based on the assumption that the length of a capillary tube is dependent on five primary variables, namely the capillary tube inner diameter, the mass flow rate of the refrigerant in the capillary tube, the pressure difference between highside and lowside, the refrigerant subcooling at capillary inlet and the relative roughness of the capillary tube material. The model is validated with previous studies over a range of operating conditions and is found to agree reasonably well with the experimental data for HFC134a.

Gas-Saturated Pool Boiling Heat Transfer From Smooth and Microporous Surfaces in FC-72

Abstract: The effects of surface treatments and gassy-subcooling on pool boiling heat transfer are quantified by testing both smooth and treated surfaces at gassy-subcooling levels from 0°C to 40°C (1 atm) and 40°C to 85°C (3 atm). Incipient and nucleate boiling wall superheats decrease over this range of gassy-subcooling. At gassy-subcooling levels greater than 20°C, the boiling curves for the smooth surface indicate two distinct regions governed by different heat transfer mechanisms, one in which the boiling process is influenced by the presence of dissolved gas, the other by boiling of the pure liquid. The critical heat flux (CHF) for each surface continually increases with increased levels of gassy-subcooling and the CHF sensitivity to gassy-subcooling is higher for the treated surface. The CHF increase due to combined surface treatment and gassy-subcooling (85°C) is ∼400 percent (78 W/cm 2 ).

Tank Pressure Control Experiment: Thermal Phenomena in Microgravity

Abstract: The report presents the results of the flight experiment Tank Pressure Control Experiment/Thermal Phenomena (TPCE/TP) performed in the microgravity environment of the space shuttle. TPCE/TP, flown on the Space Transportation System STS-52, was a second flight of the Tank Pressure Control Experiment (TPCE). The experiment used Freon 113 at near saturation conditions. The test tank was filled with liquid to about 83% by volume. The experiment consisted of 21 tests. Each test generally started with a heating phase to increase the tank pressure and to develop temperature stratification in the fluid, followed by a fluid mixing phase for the tank pressure reduction and fluid temperature equilibration. The heating phase provided pool boiling data from large (relative to bubble sizes) heating surfaces (0.1046 m by 0.0742 m) at low heat fluxes (0.23 to 1.16 kW/sq m). The system pressure and the bulk liquid subcooling varied from 39 to 78 kPa and 1 to 3 C, respectively. The boiling process during the entire heating period, as well as the jet-induced mixing process for the first 2 min of the mixing period, was also recorded on video. The unique features of the experimental results are the sustainability of high liquid superheats for long periods and the occurrence of explosive boiling at low heat fluxes (0.86 to 1.1 kW/sq m). For a heat flux of 0.97 kW/sq m, a wall superheat of 17.9 C was attained in 10 min of heating. This superheat was followed by an explosive boiling accompanied by a pressure spike of about 38% of the tank pressure at the inception of boiling. However, at this heat flux the vapor blanketing the heating surface could not be sustained. Steady nucleate boiling continued after the explosive boiling. The jet-induced fluid mixing results were obtained for jet Reynolds numbers of 1900 to 8000 and Weber numbers of 0.2 to 6.5. Analyses of data from the two flight experiments (TPCE and TPCE/TP) and their comparison with the results obtained in drop tower experiments suggest that as Bond number approaches zero the flow pattern produced by an axial jet and the mixing time can be predicted by the Weber number.

Blowdown of carbon dioxide from initially supercritical conditions

Abstract: The paper presents experimental investigations of the thermohydraulic phenomena of top vented blowdown processes of initially supercritical carbon dioxide. The initial fluid conditions were chosen such that flashing occurred after saturation conditions were reached. The investigations were focused on pressure and temperature transients. It was observed that the pressure at which flashing occurred first depends mainly on the initial fluid conditions due to the almost isentropic change of state during the supercritical/subcooled blowdown. Furthermore, void fraction profiles along the axis of the vessel were measured by means of a gamma densitometer. The void fraction profile is influenced strongly by phase separation effects. Various stages of characteristic void profile were observed.

Experimental Investigation of Natural Convection Heat Transfer in Volumetrically Heated Spherical Segments

Abstract: External cooling of a light water reactor vessel by flooding of the concrete cavity with subcooled water is one of several management strategies currently being considered for accidents in which significant relocation of core material is predicted to occur. At present, uncertainty exists with respect to natural convection heat transfer coefficients between the pool of molten core material and the reactor vessel wall. In the present work, experiments were conducted to examine natural convection heat transfer in internally heated partially filled spherical pools with external cooling. In the experiments, Freon-113 was contained in a Pyrex bell jar, which was cooled externally with subcooled water. The pool was heated using a 750 W magnetron taken from a conventional microwave. The pool had a nearly adiabatic free surface. The vessel wall temperature was not uniform and varied from the stagnation point to the free surface. A series of chromel-alumel thermocouples was used to measure temperatures in both steady-state and transient conditions. Each thermocouple was placed in a specific vertical and radial location in order to determine the temperature distribution throughout the pool and along the inner and outer walls of the vessel. In the experiments, pool depth and radius were varied parametrically. Both local and average heat transfer coefficients based on maximum pool temperature were obtained. Rayleigh numbers based on pool height were varied from 2 X 10 10 to 1.1 X 10 14 . Correlations for the local heat transfer coefficient dependence on pool angle and for the dependence of average Nusselt number on Rayleigh number and pool depth have been developed.

Pool Boiling Heat Transfer in Microgravity : Experiments with CFC-113 and Water Utilizing a Drop Shaft Facility

Abstract: Microgravity pool boiling experiments were carried out utilizing the drop shaft at the Japan Microgravity Center, which enables control of microgravity to within one-ten-thousandth of terrestrial gravity for a period of 10 seconds. Vapor bubbles were generated on a Joule-heated stainless steel or ITO film backed with a thin alumina or glass substrate, which was immersed in a pool of subcooled or saturated liquid at atmospheric pressure. The CHF (critical heat flux) to CFC-113 was lowered, in microgravity, to one-fifth of that achieved under the normal terrestrial condition. In contrast, the CHF to water remained more than one-half that achieved under the terrestrial condition. This difference in the gravity dependence of CHF is presumably ascribable to the difference in the frequency of bubble detachment from the heater surface between CFC-113 and water in microgravity, which is dependent on the difference in thermophysical properties, particularly the surface tension and the heat of vaporization, between the two fluids.

Capheat: An Homogeneous Model to Simulate Refrigerant Flow Through Non Adiabatic Capillary Tubes

Abstract: **)Department of Mechanical Engineering Federal University of Santa Catarina P. 0. Box 476 88040 900 Florian6polis SC Brazil This work presents a numerical model to simulate refrigerant flow through capillary tube-suction line heat exchangers. The model can predict performance characteristics of geometrically defined capillary tube-suction line heat exchangers under various operating conditions. Lateral and concentric arrangements are considered. The refrigerant mass flow rate and the pressure, temperature and quality profiles along the flow are computed by the model. Calculated results are compared with experimental data reported in the literature and a good agreement is shown. Some illustrative results are presented to indicate the model potentialities. NOMENCLATURE M Temperature difference A' Heat exchange area per unit g Vapor c Specific heat length in Inlet d Capillary tube diameter v Specific volume int Inside D Suction line diameter X Refrigerant quality out Outlet f Darcy friction factor z Distance from capillary inlet p Constant pressure g Gravity acceleration e Capillary tube inclination angle s Suction line G Mass ftux sp Single-phase flow h Convective heat transfer Subscripts sub Subcooling coefficient hx Heat exchanger Specific enthalpy co Ambient tp Two-phase flow L Capillary tube length c Capillary tube v Constant volume ril Mass flow rate cond Condensation p Pressure evap Evaporation t Temperature ext Outside u Overall heat transfer coefficient f Liquid A Heat exchange area fg Latent heat of evaporation INTRODUCTION Even though capillary tubes appear quite simple they are actually very complex expansion devices. At the tube entrance there is a slight pressure drop, due mainly to the abrupt change in cross sectional area. The pressure then decreases linearly along the tube length due to wall friction. This type of regime holds until the ftow reaches saturated conditions. From this point (flash point), where vapor first appears, the pressure drop per unit length increases as the end of the tube is approached. This happens because the pressure drop in the two-phase flow region corresponds to both flow friction and the increase in momentum of the flow media. The cri~ical flow condition or choked flow will occur whenever the fluid velocity reaches the local sonic velocity. After this any further lowering of the evaporating pressure has little effect on the mass flow rate. Many authors have observed experimentally the existence of a delay in vaporization in adiabatic capillary tubes. This metastability can be characterized by the persistence of the liquid state to pressures less than the saturation pressure

A class of internally irreversible refrigeration cycles

Abstract: A Carnot-like irreversible refrigeration cycle is modelled with two isothermal and two non-adiabatic, irreversible processes. The generic source of internal irreversibility, measured by the Clausius inequality, is a general irreversibility term which could include any heat leaks into the Joule - Thompson expansion valve, the evaporator and compressor cold boxes. This cycle is optimized first for maximum refrigeration power and maximum refrigeration load, then for maximum coefficient of performance. Its performances are compared with those of the endoreversible refrigeration cycle, based on a propane stage of a classical cascade liquefaction cycle example. Both cycle models achieve optimum power and maximum refrigeration load at nearly the same refrigeration temperature, but only the coefficient of performance of the irreversible refrigeration cycle reaches a maximum. Moreover, its prediction of heat conductance allocation between evaporator and condenser appears to be not only more conservative, but also more realistic for actual design considerations of refrigeration cycles.

Pool boiling critical heat flux on a horizontal cylinder in subcooled water for wide ranges of subcooling and pressure and its mechanism

Abstract: The correct understanding of the cooling limits of high heat fluxes encountered in many important engineering and scientific systems especially in plasma-facing components in fusion reactors becomes necessary; many experimental investigations for highly subcooled flow boiling of water were carried out at fusion reactor coolant conditions recently. On the other hand, though the critical heat fluxes in a pool of water up to high subcoolings and pressures are important as the fundamental database to understand those at zero velocity of water, no systematic studies on the critical heat fluxes in pool boiling of water for wide ranges of subcooling and pressure exist till quite recently. The authors carried out the experimental studies as a part of transient boiling for high subcoolings at high pressures in water due to quasi-steadily increasing heat inputs recently. They suggested that there exist two different mechanisms for heat transfer crisis at critical heat flux for low and high subcooled regions for water. The objective of present research is to clarify as the extension of the previous research the effects of the test heater shapes in water, and of the kinds of liquids such as He I, and ethanol on the critical heat fluxes for widemore » ranges of subcoolings and pressures which are due to the different two mechanisms. The effect of water velocities under forced convection condition on the critical heat fluxes under the conditions of high subcoolings at high pressures will appear elsewhere in the near future.« less

An Analytical Solution for the Wilson Point in Homogeneously Nucleating Flows

Abstract: The calculation of conditions at the Wilson point is the key to both theoretical and numerical studies of the condensation of pure vapours by homogeneous nucleation. Nucleation and droplet growth occur in a very short period of time, during which the changes of many vapour properties due to the normal thermofluid dynamic processes are negligible compared with the change of the heat release rate. This feature is exploited in an analysis leading to an approximate solution for the maximum subcooling and other properties at the Wilson point. The analysis is general but attention is focused on the main application of interest, which is the condensation of steam in high-speed flows by homogeneous nucleation. Crucial approximations are justified over a wide range of steam pressures and the analytical results reveal the dependency of steam properties at the Wilson point on controlling parameters such as the rate of pressure decrease. A direct link is established between the steam properties at the saturation point and those at the Wilson point, which, when used in multidimensional condensation flow calculations, should remove the need for very fine meshes and excessive computing resources which are otherwise required.

A Mechanistic Model for the Prediction of Water-Subcooled-Flow-Boiling Critical Heat Flux at High Liquid Velocity and Subcooling

Abstract: A new model is presented for the prediction of the critical heat flux (CHF) of subcooled flow boiling based on a liquid-sublayer dryout mechanism, i.e., the dryout of a thin, liquid layer beneath a...

Pre-fractionation of cracked gas or olefins fractionation by one or two mixed refrigerant loops and cooling water

Abstract: The present invention is a plurality of stages for partial condensation and phase separation a process gas stream, preferably containing substantial amounts of light olefins, methane, and hydrogen, but containing initially at least substantial amounts of methane and ethylene. Each condensation and separation stage is refrigerated with a subcooled and flashed high pressure liquid of a mixed refrigerant refrigeration loop, wherein throughout the loop the relative component ratios of the mixed refrigerant components are constant, contrasted with the several mixed refrigerant processes of the prior art where for all the streams of each closed refrigeration loop. In addition, rectification and/or stripping sections of an ethylene separation train have a common support construction taking advantage of relatively similar column diameters to reduce the total number of columns in the process.

The 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 semi-transparent gold film sputtered on quartz substrate, 19.05 mm x 38.1 mm (0.75" x 1.50"). 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, while the heat flux at which dryout occurs is considerably less. Using quasi-steady 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-l 13, 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 all available data correlations deemed to reasonably represent the circumstances present. L sys q'T t t* nucleation Nomenclature