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Showing papers on "Subcooling published in 1999"


Journal ArticleDOI
TL;DR: In this article, a detailed and thorough parametric study of the Leidenfrost point (LFP) is presented, which serves as the temperature boundary between the transition and film boiling regimes.
Abstract: Recent demands for superior material properties and more efficient use of materials and production time are forcing manufacturers to develop intelligent processing techniques for enhanced process control in order to better dictate the end product. In the heat treatment and processing of metallic alloys, the desire to obtain parts of enhanced and uniform mechanical properties is requiring increased control over heat removal rates and enhanced temperature control. In particular, spray quenching has been shown to be an effective means to control and enhance the cooling rates of heat treatable aluminum alloys. This study presents a detailed and thorough parametric study of the Leidenfrost point (LFP), which serves as the temperature boundary between the transition and film boiling regimes. Sessile drop evaporation experiments were conducted with acetone, benzene, FC-72, and water on heated aluminum surfaces with either polished, particle blasted, or rough sanded finishes to observe the influential effects of fluid properties, surface roughness, and surface contamination on the LFP. A weak relationship between surface energies and the LFP was observed by performing droplet evaporation experiments with water on polished copper, nickel, and silver surfaces. Additional parameters which were investigated and found to have negligible influence on the LFP included liquidmore » subcooling, liquid degassing, surface roughness on the polished level, and the presence of polishing paste residues. The accumulated LFP data of this study were used to assess several existing models which attempt to identify the mechanisms which govern the LFP. The disagreement between the experimental LFP values and those predicted by the various models suggests that an accurate and robust theoretical model which effectively captures the LFP mechanisms is currently unavailable.« less

361 citations


Journal ArticleDOI
TL;DR: In this paper, the authors obtained ultra-high critical heat flux (CHF) data, with many values exceeding 100 MW m−2, using high mass velocity, subcooled water flow through short, small diameter tubes.

162 citations


Journal ArticleDOI
TL;DR: In this article, the effect of capillary length, capillary diameter, refrigerant subcooling, condensing pressure and type of refrigerant on the mass flow-rates through the capillaries was investigated.

140 citations


Journal ArticleDOI
TL;DR: In this article, it was shown that the water depletion in the mixing zone suppresses steam explosions with corium melts at ambient pressure and in the present pour geometry, however, propagating low energy (about twice the energy of the trigger pulse) events were observed.

132 citations


Journal ArticleDOI
TL;DR: In this paper, a high-CHF (critical heat flux) database for sub-cooled flow boiling of water in tubes was compiled from the world literature and a pressure drop model was developed to determine the pressure at the end of the heated length where burnout (CHF) was observed.

101 citations


Journal ArticleDOI
TL;DR: In this article, the authors measured the average surface temperature of a small square thin metallic film deposited on a silicon substrate and immersed in subcooled water during a voltage pulse of short duration.
Abstract: We describe a method for measuring the average surface temperature of a small square thin metallic film deposited on a silicon substrate and immersed in subcooled water during a voltage pulse of short duration. The thin film studied is a material used in the current generation of commercial ‘desk–jet’printers and comprises a mixture of tantalum and aluminium 65 µm wide and 0.2 µm thick. The experiment uses a bridge circuit with a dynamic amplifier design to measure the evolution of electrical resistance, coupled with a separate calibration of the thin film resistor element with temperature to determine average surface temperature. Voltage pulses of 5 µs typical duration are applied to the thin films. An ‘inflection’ point in the resulting evolution of heater surface temperature identifies bubble nucleation. The calibration of the heater resistance with temperature showed a hysteresis effect that required a burn–in process to stabilize the electrical resistance. With the calibration curve obtained, resistance was converted to temperature and the results analysed. For low power input the average surface temperature exhibited an oscillatory behaviour which indicated a cyclic growth/collapse process often found in nucleate boiling. At higher powers, the oscillatory behaviour disappeared and gave way to an exponential variation of temperature with time similar to a lumped capacitance behaviour of a thermal system. An inflection point in the evolution of surface temperature was found that signified bubble nucleation. The largest heating rate and highest nucleation temperature measured was 0.25 x 109 °C s–1 and 556 K, respectively. This temperature is in good agreement with homogeneous nucleation theory as applied to a surface. The contact angles needed for measured nucleation temperatures to agree with predictions are within the range that is typical for water on metallic surfaces.

101 citations


Journal ArticleDOI
TL;DR: In this paper, the conditions leading to critical heat flux (CHF) were investigated in a straight, rectangular channel with a 5.0×2.5 mm cross-section and 101.6 mm heated length.

89 citations


Journal ArticleDOI
TL;DR: In this article, the effect of density change on the melting process was taken into account in the physical model, and the temperature distributions in the liquid and solid phases were respectively obtained by an exact solution and an integral approximate solution.

87 citations


Journal ArticleDOI
TL;DR: In this article, the authors distinguished two stages of hydrate-crystal growth, which are greatly different from each other both in crystal-growth morphology and in time span, by immersing an R-141b drop in a quiescent pool of water.

87 citations


Journal ArticleDOI
TL;DR: In this article, pressure drop through a capillary tube is modeled in an attempt to predict the size of capillary tubes used in residential air conditioners and also to provide simple correlating equations for practicing engineers.
Abstract: In this paper, pressure drop through a capillary tube is modeled in an attempt to predict the size of capillary tubes used in residential air conditioners and also to provide simple correlating equations for practicing engineers. Stoecker's basic model was modified with the consideration of various effects due to subcooling, area contraction, different equations for viscosity and friction factor, and finally mixture effect. McAdams' equation for the two-phase viscosity and Stoecker's equation for the friction factor yielded the best results among various equations. With these equations, the modified model yielded the performance data that are comparable to those in the ASHRAE handbook. After the model was validated with experimental data for CFC12, HFC134a, HCFC22, and R407C, performance data were generated for HCFC22 and its alternatives, HFC134a, R407C, and R410A under the following conditions: condensing temperature; 40, 45, 50, 55°C, subcooling; 0, 2.5, 5°C, capillary tube diameter; 1.2–2.4 mm, mass flow rate; 5–50 g/s. These data showed that the capillary tube length varies uniformly with the changes in condensing temperature and subcooling. Finally, a regression analysis was performed to determine the dependence of mass flow rate on the length and diameter of a capillary tube, condensing temperature, and subcooling. Thus determined simple practical equations yielded a mean deviation of 2.4% for 1488 data obtained for two pure and two mixed refrigerants examined in this study.

83 citations


Journal ArticleDOI
TL;DR: In this article, the authors investigated the nonlinear dynamics of multiple parallel boiling channels with forced flows by using the Galerkin nodal approximation method and constructed stability maps for multiple-channel systems subject to a constant total mass flow rate on the parameter plane of the subcooling and phase change numbers.

Patent
16 Aug 1999
TL;DR: In this article, a multistage refrigeration system with a thermal storage reservoir disposed in a secondary refrigeration loop of the system is described, where the thermal reservoir material may undergo a liquid/solid phase change, and preferably has a freezing point at about 20° F.
Abstract: A multistage refrigeration system is disclosed, and particularly a system with a thermal storage reservoir disposed in a secondary refrigeration loop of the system. The reservoir stores thermal reservoir material in heat exchange relation with the refrigerant in that secondary refrigeration loop. A primary refrigeration loop transfers heat from the secondary refrigeration loop to ambient. A third refrigeration loop absorbs heat from a product to be cooled and transfers that heat to the secondary refrigeration loop. The thermal reservoir material may undergo a liquid/solid phase change, and preferably has a freezing point at about 20° F. (-7° C.). The multistage refrigeration system of the present invention is particularly adapted for use in stadium or arena settings, where rapid cooling of mass quantities of canned or bottled beverages for immediate consumption is desired.

Journal ArticleDOI
TL;DR: In this paper, the critical heat flux associated with the flow of subcooled water in heated microchannels is investigated and four different channels, all 16 cm in length, were used.
Abstract: Critical heat flux (CHF) associated with the flow of subcooled water in heated microchannels is experimentally investigated. Four different channels, all 16 cm in length, are used: two are circular...

Patent
07 Dec 1999
TL;DR: In this article, a process and system for liquefying a pressurized gas, especially natural gas, is described, where the process is carried out in two heat exchangers, and cooling for each heat exchanger is provided by a mixed refrigerant which is vaporized at a single essentially constant pressure.
Abstract: Process and system for liquefying a pressurized gas, especially natural gas. The process is carried out in two heat exchangers, and cooling for each heat exchanger is provided by a mixed refrigerant which is vaporized at a single essentially constant pressure. Feed precooling, low level refrigerant precooling, and high level liquid refrigerant subcooling are effected in one of the heat exchanger against low level refrigerant vaporizing at a single essentially constant pressure. The process and system of the invention are especially well-suited for installation on ships, barges, and offshore platforms.

Journal ArticleDOI
TL;DR: In this paper, the authors defined the characteristic performance curves of vapor-compression refrigeration systems as a plot between the inverse coefficient of performance (1/COP) and inverse cooling capacity ( 1/Q evap ) of the system.
Abstract: The characteristic performance curves of vapor-compression refrigeration systems are defined as a plot between the inverse coefficient of performance (1/COP) and inverse cooling capacity (1/ Q evap ) of the system. Using the actual data of a simple vapor-compression system, performance curves of the system are obtained. The curves were found to be linear and this linear relation between 1/COP and 1/ Q evap is explained in the light of various losses of the system, resulting from the irreversibilities losses due to finite rate of heat transfer in the heat exchangers and non-isentropic compression and expansion in the compressor and expansion valve of the system, respectively. A finite-time thermodynamic model which simulates the working of an actual vapor-compression system is also developed. The model is used to study the performance of a variable-speed refrigeration system in which the evaporator capacity is varied by changing the mass-flow rate of the refrigerant, while keeping the inlet chilled-water temperature as constant. The model is also used for predicting an optimum distribution of heat-exchanger areas between the evaporator and condenser for a given total heat exchanger area. In addition, the effect of subcooling and superheating on the system performance is also investigated.

Journal ArticleDOI
TL;DR: In this article, the effects of surface roughness, subcooling temperature, and spray angle on high heat fluxes under steady-state conditions using spray cooling were quantified using a data acquisition system.
Abstract: Spray cooling is a promising technique for the removal of high heat fluxes (SSHHF) under steady state in many practical applications. This article reports on experimental results which quantify the effects that mass flow rate, surface roughness, subcooling temperature, and spray angle have on high heat fluxes under steady-state conditions using the spray cooling technique. An experimental apparatus was built to conduct the experiments consisting of a copper heater with a disk-shaped surface, an atomizer system that uses commercial nozzles, and a data acquisition system. Commercial nozzles were used to generate sprays with mean droplet diameters ranging from 85 to 100 mu m and mass flow rates between 1.48 and 2.91 L/h. The sprays were atomized into a surface that was treated to change its roughness. Two qualitative surface roughnesses were considered: smooth and rough. Curves were generated showing the steady-state boiling process for each parameter considered. A quantitative assessment was conducted based...

Journal ArticleDOI
TL;DR: In this article, the effect of sliding vapor bubbles on forced convection boiling heat transfer was investigated for both vertical upflow and downflow configurations using FC-87 and showed that sliding bubbles enhance the bulk liquid turbulence at the wall, which contributes significantly to the macroscale heat transfer.
Abstract: We describe experimental efforts aimed at examining the effect of vapor bubble sliding on forced convection boiling heat transfer. Flow boiling experiments using FC-87 were conducted for vertical upflow and downflow configurations. Both slightly subcooled single-phase and saturated annular flow boiling were considered. Significantly higher heat transfer rates were measured for vertical upflow than for downflow with the same wall superheat, and slightly subcooled single-phase inlet conditions. This increase in heat transfer is directly attributable to sliding vapor bubbles, which remain attached to the wall during upflow and lift off the wall during downflow. Differences in the measured upflow and downflow heat transfer rates are not as significant for annular flow boiling, which is clue in part to the similar vapor bubble dynamics which have been observed for upflow and downflow. Heat transfer experiments in single-phase subcooled upflow with air bubble injection at the heating surface suggest that sliding bubbles enhance the bulk liquid turbulence at the wall, which contributes significantly to the macroscale heat transfer

Journal ArticleDOI
TL;DR: In this paper, the authors measured space and time resolved heat transfer variations during nucleate pool boiling of FC-72 using a micro-scale heater array in conjunction with a high-speed CCD.
Abstract: The objective of this work is to measure space - and time - resolved heat transfer variations during nucleate pool boiling of FC-72 using a microscale heater array in conjunction with a high-speed CCD. The feedback loops used in this work are vast improvements over those used in previous work, and are described here in detail. The heater array is constructed using VLSI techniques, and consists of 96 serpentine platinum resistance heaters on a quartz substrate. Electronic feedback loops are used to keep the temperature of each heater in the array at a specified value, and the variation in heater power required to do this is measured. Data are obtained with the bulk liquid subcooled by 2 degrees C at a system pressure of 0.8 atm. Isolated bubbles are obtained at a wall superheat of 29 degrees C. One nucleation site occurred in the middle of a 2 x 2 array of heaters and at least three heater lengths away from other bubbles. The heat transfer variation versus time from the four heaters directly around this nu...

Journal ArticleDOI
TL;DR: The superheated layer vapour replenishment model as mentioned in this paper was proposed to predict the CHF in sub-cooled water flow boiling, where a thin elongated bubble, called a vapour blanket, is formed, rising along the near wall region as vertical distorted vapour cylinders.

Journal ArticleDOI
TL;DR: In this article, the critical heat flux (CHF) in the subcooled flow boiling regime was investigated by means of an infrared camera picture, and the experimental results corresponding to various thermal hydraulic conditions were reasonably well predicted by a correlation deduced from the sublayer dryout model proposed by Celata et al.

Journal ArticleDOI
TL;DR: In this paper, experimental analysis was performed for ammonia-water falling film absorption process in a plate heat exchanger with enhanced surfaces such as offset strip fin, and the authors examined the effects of liquid and vapor flow characteristics, inlet subcooling of the liquid flow and inlet concentration difference on heat and mass transfer performance.
Abstract: In this article, experimental analysis was performed for ammonia–water falling film absorption process in a plate heat exchanger with enhanced surfaces such as offset strip fin. This article examined the effects of liquid and vapor flow characteristics, inlet subcooling of the liquid flow and inlet concentration difference on heat and mass transfer performance. The inlet liquid concentration was selected as 5%, 10% and 15% of ammonia by mass while the inlet vapor concentration was varied from 64.7% to 79.7%. It was found that before absorption started, there was a rectification process at the top of the test section by the inlet subcooling effect. Water desorption phenomenon was found near the bottom of the test section. It was found that the lower inlet liquid temperature and the higher inlet vapor temperature, the higher Nusselt and Sherwood numbers are obtained. Nusselt and Sherwood number correlations were developed as functions of falling film Reynolds Re 1 , vapor Reynolds number Re v , inlet subcooling and inlet concentration difference with ±15% and ±20% error bands, respectively.

Journal ArticleDOI
TL;DR: In this paper, the condensation heat transfer coefficients (HTCs) of a plain tube, low fin tube, and Turbo-C tube were measured for the low pressure refrigerants CFC11 and HCFC123 and for the medium pressure (HFC12 and HFC134a) for all the tubes tested.
Abstract: In this study, condensation heat transfer coefficients (HTCs) of a plain tube, low fin tube, and Turbo-C tube were measured for the low pressure refrigerants CFC11 and HCFC123 and for the medium pressure refrigerants CFC12 and HFC134a. All data were taken at the vapor temperature of 39°C with a wall subcooling of 3–8°C. Test results showed that the HTCs of HFC123, an alternative for CFC11, were 8.2–19.2% lower than those of CFC11 for all the tubes tested. On the other hand, the HTCs of HFC134a, an alternative for CFC12, were 0.0–31.8% higher than those of CFC12 for all the tubes tested. For all refrigerants tested, the Turbo-C tube showed the highest HTCs among the tubes tested showing almost an 8 times increase in HTCs as compared to the plain tube. Nusselt's prediction equation yielded a 12% deviation for the plain tube data while Beatty and Katz's prediction equation yielded a 20.0% deviation for the low fin tube data.

Patent
George A. Swan1
03 Dec 1999
TL;DR: In this article, the subcooled water is injected into the hot liquid, to form a two-phase fluid of the liquid and steam, upstream of the atomization, which is then sparged into the flowing hot oil in a conduit in a riser feed injector.
Abstract: Atomization of a high boiling point, hot liquid, such as a hydrocarbon feed oil for a fluid cat cracker, is enhanced by injecting subcooled water into the hot liquid, to form a two-phase fluid of the liquid and steam, upstream of the atomization. The hot liquid is at conditions of temperature and pressure effective for the injected, subcooled water to vaporize into steam, when the water contacts it. Typically this means that the hot liquid is hotter and at a lower pressure than the water. In an FCC process, the subcooled water is sparged into the flowing hot oil in a conduit (34) in a riser feed injector (10). This produces a spray of hot oil in the riser reaction zone (22) in which the oil drops are smaller and more uniformly distributed in the spray.

Journal ArticleDOI
TL;DR: Onset offlow instability (OFI) in uniformly heated microchannels cooled with subcooled water at very low flow rates was experimentally investigated in this paper, where four different micro-channels, all of which were 22 cm long with a 16 cm-long heated section, were used.
Abstract: Onset offlow instability (OFI) in uniformly heated microchannels cooled with subcooled water at very low flow rates was experimentally investigated. Four different microchannels, all of which were 22 cm long with a 16-cm-long heated section, were used. Two were circular with 1.17- and 1.45-mm diameters. The other two represented flow channels in a microrod bundle with triangular array and had a hydraulic diameter of 1.13 mm; one was uniformly heated over its entire surface, and the other heated only over the surfaces of the surrounding rods. The test parameter ranges were as follows: 220 to 790 kg/m 2 .s mass flux, 240- to 933-kPa channel exit pressure, 30 to 74°C inlet temperature, and 0.1 to 0.5 MW/m 2 heat flux. In addition, the effect of dissolved noncondensables on OFI was examined by performing similar experiments with degassed water and water saturated with air with respect to the test section inlet temperature and exit pressure. Conditions leading to OFI were different from those reported for larger channels and for microchannels subject to higher coolant mass flow rates. In all the experiments, OFI occurred when equilibrium quality at channel exit was close to zero or positive, indicating the possibility of insignificant subcooled voidage in the channel and indicating that the widely used models and correlations that are based on the OFI phenomenology representing larger channels may not apply to microchannels at low-flow rates. The channel total pressure drops were significantly greater in tests with air-saturated water as compared with similar tests with degassed water. The impact of the dissolved noncondensable on the conditions leading to OFI was relatively small, however. With all parameters including heat flux unchanged, the presence of dissolved air changed the mass fluxes that led to OFI typically by only a few percent.

Patent
10 Feb 1999
TL;DR: In this article, an additive which reacts with water may be included in said refrigerating machine oil to reduce the amount of the refrigerant, and the long term reliability of the apparatus can be obtained.
Abstract: In an apparatus having a refrigeration cycle, the refrigeration cycle is formed by annularly connecting a compressor, a condenser, an expansion device and an evaporator with one another, the apparatus uses propane, isobutane or ethane as a refrigerant, and uses refrigerating machine oil having less mutual solubility with the refrigerant, and a dryer is provided in the refrigeration cycle for absorbing water. In stead of the dryer, an additive which reacts with water may be included in said refrigerating machine oil. With this arrangement, the amount of the refrigerant can be reduced, and the long term reliability of the apparatus can be obtained.

Patent
29 Jan 1999
TL;DR: In this paper, a crossover warm liquid defrost refrigeration system with a medium temperature side for cooling refrigerated goods and a low-temperature side for freezing frozen goods is described.
Abstract: A crossover warm liquid defrost refrigeration system (10) having a medium temperature side (12) for cooling refrigerated goods and a low temperature side (14) for cooling frozen goods The medium temperature side has a primary refrigeration loop (16) and a secondary refrigeration loop (18) The low temperature side also has a primary refrigeration loop (46) and a secondary refrigeration loop (48) Further included in the refrigeration system is a low temperature defrost heat exchanger (28) that is connected to the primary refrigeration loop of the medium temperature side and the secondary refrigeration loop of the low temperature side such that coolant from the low temperature side secondary refrigeration loop can be heated by the refrigerant of the medium temperature side, and then transported to the low temperature side refrigerated space heat exchanger (64) to melt any frost formed thereon Typically, the system further includes a medium temperature defrost heat exchanger (58) that is connected to the primary refrigeration loop of the low temperature side and the secondary refrigeration loop of the medium temperature side such that coolant from the medium temperature side secondary refrigeration loop can be heated by the refrigerant of the low temperature side for defrosting the medium temperature refrigerated space heat exchanger (34)

Journal ArticleDOI
TL;DR: In this paper, an advanced generator absorber heat exchanger cycle (WGAX) was developed to reduce the generator exit temperature as low as possible using waste heat sources, and to compare it with the standard GAX cycle (SGAX).

Journal ArticleDOI
TL;DR: In this paper, a thermal hydraulics computer code was developed to simulate the geysering instability in a natural circulation system starting from subcooled conditions and to assess the impact of the system pressure and channel inlet subcooling on the inception of instability.

Journal ArticleDOI
TL;DR: In this article, the authors investigated the growth and collapse of a long two-dimensional vapour bubble confined between superheated or subcooled parallel plates whose motion is driven by mass transfer effects due to evaporation from the liquid to the vapour and condensation from the vapours to the liquid, and showed that in the asymptotic limit of strong surface tension (small capillary number) the solution consists of two capillary-statics regions (in which the bubble interface is semicircular at leading order) and two thin films attached to the plates
Abstract: In an attempt to model the growth and collapse of a vapour bubble in nucleate boiling this paper investigates the unsteady expansion and contraction of a long two-dimensional vapour bubble confined between superheated or subcooled parallel plates whose motion is driven by mass-transfer effects due to evaporation from the liquid to the vapour and condensation from the vapour to the liquid. It is shown that in the asymptotic limit of strong surface tension (small capillary number) the solution consists of two capillary-statics regions (in which the bubble interface is semicircular at leading order) and two thin films attached to the plates, connected by appropriate transition regions. This generalization of the steady and isothermal problem addressed by Bretherton (1961) has a number of interesting physical and mathematical features. Unlike in Bretherton's problem, the bubble does not translate but can change in size. Furthermore, the thin films are neither spatially nor temporally uniform and may dry out locally, possibly breaking up into disconnected patches of liquid. Furthermore, there is a complicated nonlinear coupling with a delay character between the profiles of the thin films and the overall expansion or contraction of the bubble which means that the velocity with which the bubble expands or contracts is typically not monotonic. This coupling is investigated for three different combinations of thermal boundary conditions and two simple initial thin-film profiles. It is found that when both plates are superheated equally the bubble always expands, and depending on the details of the initial thin-film profiles, this expansion may either continue indefinitely or stop in a finite time. When both plates are subcooled equally the bubble always contracts, and the length of the thin-film region always approaches zero asymptotically. When one plate is superheated and the other subcooled with equal magnitude the bubble may either expand or contract initially, but eventually the bubble always contracts just as in the pure-condensation case.

Journal ArticleDOI
TL;DR: In this paper, the boundary-layer separation of a mixture of a vapor and a lighter non-condensable gas flowing downward along inclined isothermal plates is investigated numerically using the full boundary layer equations for the liquid film and the mixture.