Showing papers on "Subcooling published in 2006"

Heat Transfer and Wall Heat Flux Partitioning During Subcooled Flow Nucleate Boiling—A Review

Abstract: In this paper we provide a review of heat transfer and wall heat flux partitioning models/correlations applicable to subcooled forced flow nucleate boiling. Details of both empirical and mechanistic models that have been proposed in the literature are provided. A comparison of the experimental data with predictions from selected models is also included.

Formation, growth and ageing of clathrate hydrate crystals in a porous medium

Abstract: An experimental study was performed to visually observe the driving force dependence of hydrate growth in a porous medium filled with either liquid water and dissolved CO2 or liquid water and gaseous CO2. The given system subcooling, ΔT sub, i.e. the deficiency of the system temperature from the triple CO2−hydrate−water equilibrium temperature under a given pressure, ranged from 1.7 K to 7.3 K. The fine dendrites initially formed at ΔT sub = 7.3 K changed quickly into particulate crystals. For ΔT sub = 1.7 K, faceted hydrate crystals grew and the subsequent morphological change was hardly identified for an eight-day observation period. These results indicate that the physical bonding between hydrate crystals and skeletal materials becomes stronger with decreasing driving force, suggesting that the fluid dynamic and mechanical properties of hydrate-bearing sediments vary depending on the hydrate crystal growth process.

Effects of Dissolved Air on Subcooled Flow Boiling of a Dielectric Coolant in a Microchannel Heat Sink

Abstract: The effects of dissolved air in the dielectric liquid FC-77 on flow boiling in a microchannel heat sink containing ten parallel channels, each 500 m wide and 2.5 mm deep, were experimentally investigated. Experiments were conducted before and after degassing, at three flow rates in the range of 30– 50 ml/ min. The dissolved air resulted in a significant reduction in wall temperature at which bubbles were first observed in the microchannels. Analysis of the results suggests that the bubbles observed initially in the undegassed liquid were most likely air bubbles. Once the boiling process is initiated, the wall temperature continues to increase for the undegassed liquid, whereas it remains relatively unchanged in the case of the degassed liquid. Prior to the inception of boiling in the degassed liquid, the heat transfer coefficients with the undegassed liquid were 300– 500 % higher than for degassed liquid, depending on the flow rate. The heat transfer coefficients for both cases reach similar values at high heat fluxes 120 kW/ m 2 once the boiling process with the degassed liquid was well established. The boiling process induced a significant increase in pressure drop relative to single-phase flow; the pressure drop for undegassed liquid was measured to be higher than for degassed liquid once the boiling process became well established in both cases. Flow instabilities were induced by the boiling process, and the magnitude of the instability was quantified using the standard deviation of the measured pressure drop at a given heat flux. It was found that the magnitude of flow instability increased with increasing heat flux in both the undegassed and degassed liquids, with greater flow instability noted in the undegassed liquid. DOI: 10.1115/1.2351905

Effect of Surface Orientation on Nucleate Boiling of FC-72 on Porous Graphite

Abstract: Effects of orientations of porous graphite and smooth copper surfaces, measuring 10 mmX 10 mm, on saturation nucleate boiling and critical heat flux (CHF) of FC-72 dielectric liquid and of liquid subcooling (0, 10, 20, and 30 K) on nucleate boiling in the upward facing orientation are investigated. Inclination angles (θ) considered are 0 deg (upward-facing), 60, 90, 120, 150, and 180 deg (downward facing). The values of nucleate boiling heat flux, nucleate boiling heat transfer coefficient (NBHTC), and CHF are compared with those measured on the smooth copper surface of the same dimensions and CHF values on both copper and porous graphite are compared with those reported by other investigators on the smooth surfaces and microporous coatings. Results demonstrated higher NBHTC and CHF on porous graphite, particularly in the downward-facing orientation (θ=180 deg). In the upward-facing orientation, NBHTCs on both surfaces decrease with increased subcooling, but increase with increased surface superheat reaching maxima then decrease with further increase in surface superheat. In saturation boiling on copper and both saturation and subcooled boiling on porous graphite these maxima occur at or near the end of the discrete bubble region, and near CHF in subcooled boiling on copper. Maximum saturation NBHTC on porous graphite increases with decreased surface superheat and inclination angle, while that on copper increases with increased surface superheat and decreased surface inclination. At low surface superheats, saturation nucleate boiling heat flux increases with increased inclination, but decreases with increased inclination at high surface superheats, consistent with previously reported data for dielectric and nondielectric liquids. The fractional decreases in saturation CHF with increased 0 on smooth copper and microporous coatings are almost identical, but markedly larger than on porous graphite, particularly in the downward-facing orientation. In this orientation, saturation CHF on porous graphite of 16 W/cm 2 is much higher than on copper (4.9 W/cm 2 ) and as much as 53% of that in the upward-facing orientation, compared to only ∼18% on copper.

Growth Kinetics of Single Crystal sII Hydrates: Elimination of Mass and Heat Transfer Effects

Abstract: This work presents the results from experiments and modeling of tetrahydrofuran single crystal hydrate growth. The purpose was to study growth kinetics, independent of mass transfer and heat transfer. We used a single crystal apparatus, at stoichiometric concentrations of tetrahydrofuran and water, varying the fluid shear to decrease the boundary layer at the crystal surface. We found that with extreme precautions to totally eliminate mass transfer and to minimize heat transfer via high shear, it is very difficult to obtain reliable kinetic constants for the single hydrate crystal growth system. We eliminated mass transfer, but were only able to reduce the heat transfer resistance to a value of about 10% of the total resistance (i.e., 90% kinetic resistance) at the lowest value of subcooling. We found that growth rate increased with the driving force (i.e., subcooling) and established that the growth process occurred by a step mechanism. We only measured the fluid phases in order to obtain hydrate phase kinetics. The results of this work suggest that assessment of heat transfer, previously ignored in crystal growth kinetic studies is vital for accurate hydrate kinetics.

Experiments Related to the Performance of Gas Hydrate Kinetic Inhibitors

Abstract: (1) The hydrate formation process and growth is described through three different stages: (a) an induction period (nucleation), (b) a slow growth period prior to (c) a final stage described by a catastrophic, fast growth rate. In this paper the effect of kinetic hydrate inhibitors (KI) is characterized by the total delay of the catastrophic growth process at given subcooling ratios at given pressures. (2) The effects of a 20,000 Mw PVCap on a structure I (sI) ethane hydrate and a sII synthetic natural gas (SNG) hydrate have been examined in sapphire cells. A baseline was first established for each of two hydrate forming systems to describe the respective induction periods, slow growth periods, and initial growth rates. Without inhibitor, at similar subcoolings, and at similar pressures a longer induction period and a slower growth rate were observed prior to the catastrophic stage for the sII hydrate system as compared to the sI hydrate system. By the addition of 5,000 ppm PVCap to the aqueous phase the total delay of the catastrophic growth stage increased by a factor 12 for the sII SNG-hydrate system and by a factor 5 for the sI ethane-hydrate system. (3) The effect of different kinetic hydrate inhibitors (KI) at various pressures ranging from about 65 bar and up to about 200 bar has been examined in high pressure sapphire cells using a sII hydrate forming condensate-SNG-synthetic sea water system (SSW, 3.6% salt). For all the experiments reported the degree of subcooling (ΔT) with respect to the hydrate equilibrium properties of the fluid system used was kept at a similar magnitude. This was done to examine KI effects at similar degrees of subcooling within the different pressure regions. The experiments at constant ΔT showed that the KI effect (i.e., total delay of the catastrophic growth stage) decreased with increasing pressure and that a dramatic decrease was observed for increasing pressures above 90 bar. The true driving force of a hydrate forming system is described through a chemical potential, Δμ, which is a function of pressure (i.e., fugacity) and absolute temperature (K). At a given ΔT the chemical potential is greater in the high-pressure region than in the low-pressure region.

Natural Gas Hydrate Formation in an Ejector Loop Reactor: Preliminary Study

Abstract: A natural gas hydrate formation system based on the ejector-type loop reactor (ELR) has been built Three types of flow patterns in the reactor are observed with variation of the gas entrainment rate, ie, single-bubble regime, intermediate regime, and jet regime The flow pattern under the free suction state of the ejector is in the jet regime The microbubble generated from ELR with a static mixer can shorten the induction time for hydrate formation significantly However, it cannot improve the hydrate formation rate as the gas entrainment rate decreased with the static mixer on The hydrate formation rate of the ELR system is dependent on the subcooling, gas entrainment rate, and pressure and is fairly comparable with the formation rate of both water spraying and stirring reactor in the literature

Effects of ultrasonic vibration on subcooled pool boiling critical heat flux

Abstract: The effects of ultrasonic vibration on critical heat flux (CHF) have been experimentally investigated under natural convection condition. Flat bakelite plates coated with thin copper layer and distilled water are used as heated specimens and working fluid, respectively. Measurements of CHF on flat heated surface were made with and without ultrasonic vibration applied to working fluid. An inclination angle of the heated surface and water subcooling are varied as well. Examined water subcoolings are 5°C, 20°C, 40°C and the angles are 0°, 10°, 20°, 45°, 90°, 180°. The measurements show that ultrasonic wave applied to water enhances CHF and its extent is dependent upon inclination angle as well as water subcooling. The rate of increase in CHF increases with an increase in water subcooling while it decreases with an increase in inclination angle. Visual observation shows that the cause of CHF augmentation is closely related with the dynamic behaviour of bubble generation and departure in acoustic field.

Visualization Study on Boiling of Nitrogen During Quench for Fault Current Limiter Applications

Abstract: For the development of a 13.2 kV/630 A bifilar winding type high-Tc superconducting fault current limiter (SFCL), a visualization study has been conducted to clarify boiling characteristics during the quench of the high-Tc SFCL. A series of experiments was carried out using a stainless steel strip on a G10 plate as a heating element to simulate the quench state of the high-Tc SFCL. A pulse of DC power input was applied to the strip in saturated and subcooled liquid nitrogen. The magnitude of the heat generation was varied from 10 W/cm2 to 170 W/cm2 and the period of the heat impulse was fixed at approximately 100 ms. Bubble behavior was observed by a high-speed camera through view ports of a cryostat. The boiling phenomena, the temperature rising of the strip and the recovery time were compared for different power densities and liquid nitrogen operating conditions. The bubble suppression was clearly observed with respect to the degree of subcooling

Dissolved gas effects on thermocapillary convection during boiling in reduced gravity environments

Abstract: The mechanisms by which thermocapillary convection arises during boiling of nominally pure fluids in low-g environments are currently not known. It has recently been suggested that small amounts of dissolved gas within the bulk liquid can accumulate within the vapor bubble, forming localized concentration gradients that results in a temperature gradient to form along the liquid–vapor interface that drives thermocapillary convection. This hypothesis was tested by boiling > 99.3% pure n-perfluorohexane with and without noncondensible gas in a low-g environment using a 7.0 × 7.0 mm2 microheater array to measure time and space resolved heat transfer at various wall superheats. The thermocapillary convection around the primary bubble that formed in the gassy fluid was found to be much weaker than in the degassed fluid, and the primary bubble diameter was much larger in the gassy fluid due to the accumulation of noncondensible gas within the bubble. The results suggest that the accumulation of noncondensible gas in the bubble can result in temperature variations along the interface but due to the increased vapor/gas bubble size, the driving thermocapillary temperature gradient along the interface is significantly reduced and result in much weaker thermocapillary flow. The highest CHF values in a reduced gravity environment (19 W/cm2) occurred when the fluid was highly subcooled and degassed.

Model-Based Method of Theoretical Design Analysis of a Loop Heat Pipe

Abstract: Mathematical models accepted so far for loop heat pipes (LHPs) are mentioned, with a brief account of the operating characteristics. The necessity of an analytical model formed in a consistent way is stressed from the viewpoint of design practices. An ordinary differential equation, expressing the heat and mass transfer in a thick-walled porous cylinder, is solved to give a radial temperature distribution of the cylindrical wick, from which the outside-to-inside wick diameter ratio is derived. This ratio depends on the number of transfer units and is finally expressed in terms of the evaporator temperature effectiveness, which serves as a performance index. Positive-powered expressions, including the specified critical Bond number and specified linear pressure loss gradients, are then given to determine the wick inside, vapor line, and liquid line diameters. A pressure-loss model, consisting of theoretically or empirically obtained practical expressions, is presented to specify the wick pore radius. Derived expressions for the wick permeability and conductivity are combined to find an optimal wick porosity. The degree of subcooling, determined from the heat leak and the heat loss or gain, is given in a binominal expression dependent on the heat load, operating temperature, ambient temperature, and modeled coupling conductances. That degree is then converted into the pump efficiency, serving as another performance index. A reasonable method for reservoir sizing is proposed, considering both hot and cold startups. A two-region model representing the surface activity of a condensing radiator is introduced to determine the condenser/subcooler tube diameter, the total tube length, and the radiator half feeder spacing. All the expressions are arranged to develop into an LHP design code of convenience. Results of numerical computations done over a possibly wide range of parameters are graphically shown in the figures with a view to offering LHP design curves of interest.