Showing papers on "Subcooling published in 2000"
TL;DR: Onset of nucleate boiling and onset of flow instability in uniformly heated microchannels with subcooled water flow were experimentally investigated using 22 cm long tubular test sections, 1.17 mm and 1.45 mm in diameter, with a 16-cm long heated length.
Abstract: Onset of nucleate boiling and onset of flow instability in uniformly heated microchannels with subcooled water flow were experimentally investigated using 22-cm long tubular test sections, 1.17 mm and 1.45 mm in diameter, with a 16-cm long heated length. Important experimental parameter ranges were: 3.44 to 10.34 bar channel exit pressure; 800 to 4,500 kg/m{sup 2}s mass flux (1 to 5 m/s inlet velocity); 0 to 4.0 MW/m{sup 2} channel wall heat flux; and 7,440--33,000 Peclet number at the onset of flow instability. Demand curves (pressure drop versus mass flow rate curves for fixed wall heat flux and channel exit pressure) were generated for the test sections, and were utilized for the specification of the onset of nucleate boiling and the onset of flow instability points. The obtained onset of nucleate boiling and onset of flow instability data are presented and compared with relevant widely used correlations.
168 citations
TL;DR: In this paper, the results of an experimental study of two-phase flow and boiling heat transfer in channels packed with sintered copper bi-dispersed porous media are presented.
85 citations
Patent•
06 Oct 2000TL;DR: In this paper, a vaporizing refrigerant cycle and a gas expander cycle are utilized in an optimum temperature range which maximizes the efficiency of a particular system, which can be implemented in the design of a new liquefaction plant or can be utilized as a retrofit or expansion of an existing plant.
Abstract: Refrigeration process for gas liquefaction which utilizes one or more vaporizing refrigerant cycles to provide refrigeration below about -40 DEG C and a gas expander cycle to provide refrigeration below about -100 DEG C. Each of these two types of refrigerant systems is utilized in an optimum temperature range which maximizes the efficiency of the particular system. A significant fraction of the total refrigeration power required to liquefy the feed gas (typically more than 5% and often more than 10% of the total) can be consumed by the vaporizing refrigerant cycles. The invention can be implemented in the design of a new liquefaction plant or can be utilized as a retrofit or expansion of an existing plant by adding gas expander refrigeration circuit to the existing plant refrigeration system.
85 citations
TL;DR: A solution for the temperature profile in a cylindrical capillary evaporator subject to a uniform heat flux prior to the initiation of boiling is derived using the Green's Function method.
74 citations
TL;DR: Using a laser interferometer having a resolution of ∼1μm, the authors directly measured the thickness of clathrate-hydrate films at the planar interface between a liquid hydrofluorocarbon, R-134a (CH 2 FCF 3 ), and liquid water.
71 citations
TL;DR: In this paper, a theoretical critical heat flux (CHF) prediction model is developed for the subcooled flow boiling based on the liquid sublayer dryout mechanism. But the model is tested over a large data bank, which is characterized by covering almost the entire physics scope, showing a general good accuracy.
63 citations
TL;DR: In this paper, the phase change behavior in a vertical two-dimensional porous structure made of staggered miniature silver-copper circular cylinders has been carried out using a high-speed video imaging system.
61 citations
TL;DR: In this article, the transition from non-boiling to film boiling at CHF highly subcooled liquids at high pressure measured was not explained by the mechanism of heat transfer crisis based on the hydrodynamic instability (HI) model.
61 citations
TL;DR: In this paper, thermodynamic models of an integrated mechanical sub-cooling system are developed to simulate the actual performance of the sub cooling system, particularly with respect to the subcooler saturation temperature in addition to heat exchanger areas.
60 citations
TL;DR: In this paper, the effects of the imposed wall heat flux, mass flux, liquid subcooling and saturation temperature of R-134a on the resulting nucleate boiling heat transfer and bubble characteristics were examined in detail.
56 citations
Patent•
25 Feb 2000
TL;DR: In this paper, an active regenerative cycle was used for heat transfer in an array of refrigeration elements that are distributed over the temperature gradient of a regenerative bed, where the heat transfer fluid is circulated relative to the working fluid between a thermal load and a heat sink to enact a refrigeration cycle.
Abstract: This application relates to a heat transfer apparatus and method employing an active regenerative cycle. The invention employs a working or “active” fluid and a heat transfer fluid which are physically separated. The working fluid is contained in an array of refrigeration elements that are distributed over the temperature gradient of a regenerative bed. The work for the refrigeration cycle is provided by alternative compression and expansion of the working fluid in each of the refrigeration elements at a temperature corresponding to the element's location in the temperature gradient. The compression and expansion strokes may be coupled together for optimum work recovery. The heat transfer fluid is circulated relative to the working fluid between a thermal load and a heat sink to enact a refrigeration cycle having improved energy efficiency.
TL;DR: In this paper, the authors used thin-film asymptotics to show how a thin vapour layer can support a liquid which is heated from below and cooled from above, a process known as horizontal film boiling.
Abstract: This paper uses thin-film asymptotics to show how a thin vapour layer can support a liquid which is heated from below and cooled from above, a process known as horizontal film boiling. This approach leads to a single, strongly-nonlinear evolution equation which incorporates buoyancy, capillary and evaporative effects. The stability of the vapour layer is analysed using a variety of methods for both saturated and subcooled film boiling. In subcooled film boiling, there is a stationary solution, a constant-thickness vapour film, which is determined by a simple heat-conduction balance. This is Rayleigh–Taylor unstable because the heavier liquid is above the vapour, but the instability is completely suppressed for sufficient subcooling. A bifurcation analysis determines a supercritical branch of stable, spatially-periodic solutions when the basic state is no longer stable. Numerical branch tracing extends this into the strongly-nonlinear regime, revealing a hysteresis loop and a secondary bifurcation to a branch of travelling waves which are stable under certain conditions. There are no stationary solutions in saturated film boiling, but the initial development of vapour bubbles is determined by directly solving the time-dependent evolution equation. This yields important information about the transient heat transfer during bubble development.
TL;DR: In this paper, the experimental study on critical heat flux (CHF) has been performed for water flow in vertical round tubes under low pressure and low flow (LPLF) conditions to provide a systematic data base and investigate parametric trends.
TL;DR: In this paper, the combined effect of sub-cooling and pressure on the performance of an enhanced microstructure based thermosyphon has been investigated, which has shown very high heat transfer rates (up to 100 W/cm/sup 2/ with a wall superheat of 27.8/spl deg/C).
Abstract: The heat dissipation rates at the chip level are projected to reach the 50-100 W/cm/sup 2/ mark for some future high performance electronic systems. Liquid cooling with phase change has been demonstrated to be a very efficient technique for thermal management of such high heat dissipation rates. Past work on liquid immersion cooling using fluorocarbons has shown the advantage of using enhanced structures to reduce boiling incipience excursion and raise the critical heat flux (CHF). Thermosyphons, employing these enhanced structures are an alternative to liquid immersion and are suitable for point cooling applications, where very compact evaporators are needed. This study investigates the combined effect of sub-cooling and pressure on the performance of an enhanced microstructure based thermosyphon, which has shown very high heat transfer rates (up to 100 W/cm/sup 2/ with a wall superheat of 27.8/spl deg/C). The pressure levels tested were partial vacuum (40-101.3 kPa), atmospheric pressure (101.3 kPa) and high pressure (101.3-370 kPa). The experiments were initiated at room temperature, and hence the sub-cooling corresponded to the difference in the liquid saturation temperature at the starting system pressure and room temperature. The results show a reduction in wall superheat values at higher pressures, at a given heat flux. The performance of the system was evaluated by defining a surface-to-ambient resistance. Results show that a partial vacuum at all heat fluxes results in better performance compared to higher pressures. The combined effect of pressure and sub-cooling was also tested for a compact evaporator and the results obtained were similar to the baseline case (larger evaporator).
29 Aug 2000
TL;DR: In this paper, the effects of liquid subcooling, liquid velocity, and sphere superheat on the film boiling heat transfer were investigated in single-and two-phase water flows, respectively.
Abstract: Film boiling on spheres in single- and two-phase flows was studied experimentally and theoretically with an emphasis on establishing the film boiling heat transfer closure law, which is useful in the analysis of nuclear reactor core melt accidents. Systematic experimentation of film boiling on spheres in single-phase water flows was carried out to investigate the effects of liquid subcooling (from 0 to 40 C), liquid velocity (from 0 to 2 m/s), sphere superheat (from 200 to 900 C), sphere diameter (from 6 to 19 mm), and sphere material (stainless steel and brass) on film boiling heat transfer. Based on the experimental data a general film boiling heat transfer correlation is developed. Utilizing a two-phase laminar boundary-layer model for the unseparated front film region and a turbulent eddy model for the separated rear region, a theoretical model was developed to predict the film boiling heat transfer in all single-phase regimes. The film boiling from a sphere in two-phase flows was investigated both in upward two-phase flows (with void fraction from 0.2 to 0.65, water velocity from 0.6 to 3.2 m/s, and steam velocity from 3.0 to 9.0 m/s) and in downward two-phase flows (with void fraction from 0.7 to 0.95, water velocity from 1.9 to 6.5 m/s, and steam velocity from 1.1 to 9.0 m/s). The saturated single-phase heat transfer correlation was found to be applicable to the two-phase film boiling data by making use of the actual water velocity (water phase velocity), and an adjustment factor of (1 - {alpha}){sup 1/4} (with a being the void fraction) for downward flow case only. Slight adjustments of the Reynolds number exponents in the correlation provided an even better interpretation of the two-phase data. Preliminary experiments were also conducted to address the influences of multi-sphere structure on the film boiling heat transfer in single- and two-phase flows.
TL;DR: The behavior of vapor bubbles and vapor film during the transition from non-boiling regime such as natural convection or transient conduction regime to film boiling regime on a 1.2mm diameter platinum horizontal cylinder in liquid nitrogen and in water due to exponentially increasing heat inputs, ranging from a quasi-steady state heat input to a very rapidly increasing one, were examined by photographs taken by a high-speed video camera as discussed by the authors.
TL;DR: In this paper, a phenomenological model of critical heat flux (CHF) applicable to both pool boiling and sub-cooled forced convection boiling is developed using the dry-spot model proposed recently and existing correlations for active site density, bubble departure diameter and heat transfer coefficient in nucleate boiling.
TL;DR: In this paper, a theoretical model for the two-phase flow pressure drop type instabilities in an upflow boiling system is presented, where the thermal non-equilibrium effect between the two phases is included assuming the enthalpy profile in the subcooled boiling region.
TL;DR: In this paper, the influence of the buoyancy force on the heat transfer and on the bubble dynamics was investigated on a miniature heater in freon R11 under microgravity conditions during the spacelab mission IML-2 in 1994 and compared with measurements on earth.
TL;DR: In this paper, the forced convection film boiling heat transfer on a vertical 3-mm diameter and 180mm length platinum test cylinder located in the center of the 40-mm inner diameter test channel was measured.
Patent•
01 Aug 2000
TL;DR: In this paper, a system where flashoff losses from cryogenic liquid tankage are reduced wherein fluid from the tankage is condensed and subcooled against refrigeration bearing refrigerant fluid generated by an exogenous refrigeration system is described.
Abstract: A system wherein flashoff losses from cryogenic liquid tankage are reduced wherein fluid from the tankage is condensed and subcooled against refrigeration bearing refrigerant fluid generated by an exogenous refrigeration system.
Patent•
05 Sep 2000TL;DR: In this paper, a portion of the condensed liquid in a condenser is diverted to a generator where it supplies heat to boil off refrigerant from a refrigerant oil mixture and is thereby subcooled.
Abstract: A portion of the condensed liquid in a condenser is diverted to a generator where it supplies heat to boil off refrigerant from a refrigerant oil mixture and is thereby subcooled. The subcooled liquid is supplied to the motor for cooling. The boiling off of refrigerant in the generator results in an “oil rich” liquid which is supplied to the bearings, etc. for lubrication. One, or more, jet or ejector pumps are preferably used to supply the oil rich liquid to the lubrication distribution system.
Patent•
17 May 2000
TL;DR: In this paper, the extraneous-matter trapping means are provided in a refrigerant pipe close to the heat source unit or in a bypass channel connected to the refrigerant manifold.
Abstract: A heat source unit and refrigerant used in an existing refrigeration system are replaced with new refrigerant and a new heat source unit which employs the new refrigerant and is equipped with an oil separator and extraneous-matter trapping means. An indoor unit of the existing refrigeration system may be used, in its present form, or replaced with a new indoor unit. Further, connecting pipes used for the existing refrigeration are reused. After replacement of refrigerant, the refrigeration system performs an ordinary operation after having performed a cleaning operation. The extraneous-matter trapping means is provided in a refrigerant pipe close to the heat source unit or in a bypass channel connected to the refrigerant pipe close to the heat source unit. Alternatively, only the heat source unit of the existing refrigeration system is replaced with a new one, and there is employed refrigeration oil which has no mutual solubility with respect to HFC or has very low mutual solubility.
Patent•
07 Jul 2000
TL;DR: In this paper, a cascade refrigeration system with a high temperature first stage and a low temperature second stage has a compressor, flow control device, evaporator, and heat exchanger.
Abstract: A refrigeration system having a flow control mechanism to selectively increase or decrease refrigerant flow in response to system temperature. In the preferred embodiment, a cascade refrigeration system having a high temperature first stage with a compressor, condenser, flow control device and heat exchanger. The low temperature second stage has a compressor, flow control device, evaporator and heat exchanger. The first stage is in a heat exchange relationship with the second stage through the common heat exchanger, which functions as condenser in the second stage. A controller responsive to temperature sensed at the second stage evaporator outlet operates a valve to increase or decrease refrigerant flow in the first stage. Increased refrigerant flow improves refrigeration system response to large heat loads, while maintaining efficient operation under normal conditions.
Patent•
18 May 2000
TL;DR: In this article, a system for generating refrigeration and providing refrigeration to a heat load at a very cold temperature is described, which includes a forecooling circuit using a multicomponent refrigerant fluid and a magnetic refrigeration circuit.
Abstract: A system for generating refrigeration and providing refrigeration to a heat load at a very cold temperature which includes a forecooling circuit using a multicomponent refrigerant fluid and a magnetic refrigeration circuit which provides refrigeration to the heat load and which rejects heat into the forecooling circuit.
01 Aug 2000
TL;DR: In this article, the performance of a dedicated mechanical-subcooling vapour compression refrigeration system is simulated with respect to the subcooler saturation temperature and the heat exchanger area.
Abstract: Subcooling of the refrigerant at the exit of the condenser in a vapour compression refrigeration system allows the refrigerant to enter the main cycle evaporator with a lower quality and thus allows the refrigerant to absorb more heat in the evaporator, thereby improving the coefficient of performance (COP) of the system. In a dedicated mechanical-subcooling vapour compression refrigeration system, the subcooling is performed by utilizing a small dedicated vapour compression refrigeration cycle. This cycle is coupled to the main cycle at the exit of the condenser. In this paper, thermodynamic models of the dedicated mechanical-subcooling systems are developed to simulate the actual performance of the system, particularly with respect to the subcooler saturation temperature in addition to the heat exchanger areas. It is demonstrated that the performance of the overall cycle is improved over the corresponding simple cycle and this improvement is found to be related to the refrigerant saturation temp...
TL;DR: In this paper, the flow behavior for a wide range of inlet subcoolings, in which the flow experience varies from single-to two-phase, is described in a natural circulation system at different pressures (p = 01, 024, and 15 MPa) Several kinds of flow instability are investigated, including geysering, flashing-related flow instability, and high-frequency flow oscillation at p = 01 and 024 MPa.
Abstract: The experiment was performed on the test loop HRTL-5, which simulates the geometry and system design of the 5-MW nuclear heating reactor developed by the Institute of Nuclear Energy Technology, Tsinghua University The flow behavior for a wide range of inlet subcoolings, in which the flow experience varies from single- to two-phase, is described in a natural circulation system at different pressures (p = 01, 024, and 15 MPa) Several kinds of flow instability are investigated, including geysering, flashing-related flow instability, and high-frequency flow oscillation at p = 01 and 024 MPa, as well as low steam quality density wave oscillation at p = 15 MPa The mechanisms of geysering, which has new features, and flashing-related flow instability, which has never been studied well enough in this field, are particularly interpreted The experimental results show the following: First, for a low-pressure natural circulation system, the two-phase flow is unstable in most inlet subcooling conditions, and the two-phase stable flow can be reached only with very low inlet subcoolings Second, at high inlet subcoolings, the flow instability is dominated by subcooling boiling in the heated section, and at intermediate inlet subcoolings, it is dominated by void flashing in the adiabatic longmore » riser Third, in the two-phase stable flow region, the conditions for boiling out of the core, namely, single-phase flow in the heated section and two-phase flow in the riser due to vapor flashing, can be realized The experimental results are of significance for the design and accident analysis of vessel and swimming pool-type natural circulation nuclear heating reactors« less
TL;DR: In this article, the authors improved their understanding and analysis of direct contact condensation on the gravity injection of CMTs and measured the heat transfer coefficients around steam bubbles using the holographic interferometer and high speed camera.
TL;DR: In this paper, the authors performed a photographic study of the burnout in highly subcooled flow boiling, in order to provide a qualitative description of the flow pattern under different conditions of boiling regime: ONB (onset of nucleate boiling), sub cooled flow boiling and thermal crisis.
Patent•
20 Mar 2000
TL;DR: A simple field installable/removable indicator is usable with a field pressure measurement to indicate the degree of subcooling or superheat of refrigerant in a vapor-compression or refrigeration system.
Abstract: A simple field installable/removable indicator is usable with a field pressure measurement to indicate the degree of subcooling or superheat of refrigerant in a vapor-compression or refrigeration system. The indicator can be attached to a pipe at an appropriate location in the system. The indicator can be used with a pressure measurement obtained at an existing service valve. Temperature-indicating liquid crystals or other chemicals, or alternatively, a conventional thermometer included on the indicator can be used with a scale which shows the superheat or subcooling without requiring saturation curves or tables.