Showing papers on "Subcooling published in 1998"

Heat Transfer Characteristics in Partial Boiling, Fully Developed Boiling, and Significant Void Flow Regions of Subcooled Flow Boiling

Abstract: Subcooled flow boiling covers the region beginning from the location where the wall temperature exceeds the local liquid saturation temperature to the location where the thermodynamic quality reaches zero, corresponding to the saturated liquid state. Three locations in the subcooled flow have been identified by earlier investigators as the onset of nucleate boiling, the point of net vapor generation, and the location where x = 0 is attained from enthalpy balance equations. The heat transfer regions are identified as the single-phase heat transfer prior to ONB, partial boiling (PB), and fully developed boiling (FDB). A new region is identified here as the significant void flow (SVF) region. Available models for predicting the heat transfer coefficient in different regions are evaluated and new models are developed based on our current understanding

A refrigeration plant

Abstract: A refrigeration plant comprises a plurality of transcritical reverse Rankine-cycle refrigeration units (A1, A2, ....An) and a hydraulic circuit (10) for cooling the exchangers of the refrigeration units which cool the compressed refrigerant gas.

An experimental study on HC290 and a commercial liquefied petroleum gas (LPG) mix as suitable replacements for HCFC22

Abstract: This paper presents an experimental study on the performance of hydrocarbon refrigerants, namely propane and a liquefied petroleum gas (LPG) mix as suitable replacements for the widely used refrigerant HCFC22 in refrigeration and heat pump applications. A cylinder of commercially available LPG from New Zealand market was obtained for this study. The composition of the specific LPG mix (by mass fraction) was propane (HC290)—98.95%, ethane (HC170)—1.007%, iso-butane (HC600a)—0.0397% and other constituents in small proportions. Experiments were carried out in a laboratory heat pump test facility with maximum condenser capacity of approximately 15 kW. Condensing temperatures were held constant at 35, 45 and 55°C, while evaporating temperatures were varied over a wide range from − 15 to + 15°C. All tests were carried out at constant degree of superheat (about 1 K) and subcooling (about 8 K). All appropriate precautions were observed against any leaks or fire. The analysis revealed that the hydrocarbon refrigerants performed better than HCFC22 but with a small loss of condenser capacity. The mass flow rate and compressor discharge temperature were found to be significantly lower than HCFC22. The performance of the specific LPG mix tested was found to be better than HC290 at higher condensing temperatures but poorer at a lower condensing temperature. No adverse effects were found with the LPG mix despite the presence of little moisture (less than 0.01%) in its composition. The study reveals that LPG of the tested composition (i.e. predominantly a mixture of propane, ethane and iso-butane) can be an excellent refrigerant in heat pump/refrigeration applications.

Steady-state model of centrifugal liquid chillers

Abstract: A new steady-state model of vapour-compression type centrifugal liquid chillers is presented. The model has a number of advanced features and is capable of simulating both hermetic and open-drive centrifugal compressors. The model accounts for the real process phenomena such as superheating and subcooling in the heat exchangers as well as a capacity control formulation of the inlet guide vanes. The model algorithm is developed with the aim of requiring only those inputs that are readily known to the design engineer, e.g. the general parameters of the chiller, the chilled water flow temperature out of the evaporator and the return water temperature to the condenser inlet. The outputs include the condenser capacity, the refrigeration capacity (at the evaporator), the coefficient of performance, and also the mass flow rates and thermodynamic states of the refrigerant throughout the cycle. The model is validated with the experimental data on part load to full load performance of three different chillers operating at the University of Auckland and the agreement is found to be within ±10%. The model also demonstrates that the COP of the chillers increases with increasing cooling capacity.

Enhancing small-channel convective boiling performance using a microporous surface coating

Abstract: Performance characteristics are experimentally determined for enhanced convective boiling of FC-87 in a horizontal, small-cross-sectional-area, single-channel heater The channel tested has a square cross section with a side length of 2 mm and a heated length of 8 cm. The heated surface of the channel is tested both with, and without, a microporous surface coating. Although few studies have been performed to quantify the effects of internal porous coatings on flow boiling, their application in this area is promising. The coating is shown to provide increases in both heat transfer coefficient and critical heat flux. Convective boiling curves are generated for mass fluxes from 500 to 5000 kg/m 2 s and for inlet subcooling levels ranging from 2 to 31 °C.

Experimental study on dynamic pressure pulse in direct contact condation of steam jets discharging into subcooled water

Abstract: The dynamic pressure resulting from direct contact condensation of steam jets discharging into a subcooled water has been measured as a function of steam mass flux and subcooled water temperature. Steam is discharged in a horizontal direction through four different size nozzles placed in the middle of a quenching tank, which contains water at various temperatures. Four different horizontal nozzles with an internal diameter in the range of 5{approx}20mm were used under the various test conditions of the steam mass flux in the wide range of 24{approx}1190 kg/m{sup 2}-s and the pool water temperature in the range of 20{approx}95 .deg. C. It is observed from the test results that the amplitude of dynamic pressure pulse at condensation oscillation condition becomes greater than that at a stable condensation condition of the same pool temperature . The dynamic pressure tends to increase with pool temperature at the beginning. The amplitude reached a peak at a pool temperature around 60 {approx}80.deg. C depending on nozzle size and steam mass flux and then the amplitude decrased rapidly before the pool water reached saturation temperature. The amplitude of the dynamic pressure pulse at the unstable condition reached it peak at the condition of pool temperature 60.deg. C, but the amplitude peack at the stable condition was found at around 80.deg. C. The chugging phenomena have been observed at low steam mass flux (below 80 kg/m{sup 2}-s). The condensation regime map has been constructed by acoustic/visual observations and dynamic pressure behaviors. The condensation regime map consists of six regimes such as chugging, transient chugging, condensation oscillation, stable condensation, bubble condensation oscillation, and interfacial oscillation condensation. As the pool temperature increases above 90 .deg. C the steam discharging into a subcooled water does not condense completely in the pool and part of it escapes through the free surface of water. The transition from condensation socillations to stable condensation takes place at about 230{approx}270 kg/m{sup 2}-s, but this value increase with increasing pool temperature.

Modelling of plate finned tube evaporators and condensers working with R134A

Abstract: A model to predict the behaviour of finned tube evaporators and condensers working with R134a has been developed. For modelling of the refrigerant phase change, evaporation or condensation, the heat transfer and the pressure drop for the two-phase flow have to be calculated. Therefore, a number of correlations, the most recommended ones in the reviewed literature, have been analysed and compared. The results of this comparison are presented for the evaporation and condensation heat transfer coefficients and for the evaporator frictional pressure drop. Once the correlations have been implemented in the model and compared with the experimental results, the ones that work best for the studied heat exchangers have been ultimately selected. The experimental study to validate the model has been carried out in a small airconditioning unit with cross-flow air-refrigerant type heat exchangers. The results are compared with model predictions for thermal capacity, refrigerant superheat or subcooling, and tube-side pressure drop.

Melting and Resolidification of a Subcooled Mixed Powder Bed With Moving Gaussian Heat Source

Abstract: Melting of a subcooled powder bed that contains a mixture of two powders with significantly different melting points under a moving Gaussian heat source was investigated numerically. Shrinkage induced by the density change on the melting process was taken into account in the physical model. The problem is formulated using a temperature transforming model and solved by the finite difference method. The results show that the effect of surface heat loss due to convection and radiation is not negligible regardless of whether the shrinkage phenomena is considered. The decrease of moving heat source intensity will result in decrease of the sintering depth and the volume of the liquid pool. The increase of the scanning velocity will decrease the sintering depth and the location and shape of the liquid pool is affectea significantly.

Two-Phase Heat Dissipation Utilizing Porous-Channels of High-Conductivity Material

Abstract: The results of an experimental study of two-phase heat dissipation in high-conductivity porous channel heat sinks are presented. Porous channels of various sizes were fabricated using sintered copper particles inside rectangular copper channels with base dimensions of 25 mm by 25 mm, either 3 or 10 mm in height. The experiments were conducted using subcooled water as the working fluid and test conditions ranged from an inlet temperature of 85 to 95°C, inlet pressures of 1.062 to 1.219 bars, flow rates of 22.5 to 150 ml/min, and heat fluxes of 10 to 25 W/cm 2 . The experimental results were compared to the results predicted using a previously developed numerical model. For water with inlet subcooling in the range of 6.6 to 10.8°C, heat transfer coefficients for open channel flow were increased from 1.25 to 1.94 W/cm 2 °C to 1.79 to 3.33 W/cm 2 °C, or a 43 to 142 percent improvement through the use of porous channels with mean particle diameters of 0.97, 0.54, 0.39, or 0.33 mm. The results indicate that the high thermal conductivity of the porous material and the large solid-fluid contact area combine to create a highly effective, two-phase heat sink, which may provide an effective mechanism for cooling high heat flux microelectronics

Refrigeration system with variable sub-cooling

Abstract: A refrigeration system has a compressor (10), a condenser (11), a capillary flow control (12) and an evaporator (13). A variable sub-cooling device (15) extracts heat from the refrigerant at a region at or just prior to the capillary flow control (12), to control the position along the capillary at which the refrigerant reaches saturation pressure. The sub-cooling means (15) may comprise a thermo-electric element (19) which may disperse the heat extracted from the refrigerant to the environment at large or to condensed water collected from the outer surface of the evaporator (13). The level of sub-cooling may be varied to provide a flow control which is variable to match the system and the conditions such as the ambient temperature and humidity. Such flow control may be used in conjunction with a compressor (10) that provides variable flow rates.

Method and device for liquefying a natural gas without phase separation of the coolant mixtures

Abstract: A method allowing a gaseous mixture such as a natural gas to be liquefied by using a first compressed coolant mixture M1, at least partially condensed by cooling with the aid of an external coolant fluid, then subcooled, expanded, and vaporized, and a second compressed coolant mixture, cooled with the aid of an external coolant fluid, then cooled by heat exchange with the first coolant mixture M1 during the first cooling stage (I), after which it is in an at least partially condensed state. The second partially condensed coolant mixture is sent without phase separation to a second cooling stage (II) where it is fully condensed, expanded, and vaporized at at least two pressure levels. The subcooled natural gas is expanded to form the LNG produced.

Effect of subcooling on critical heat flux during pool boiling on a horizontal heated wire

Abstract: Critical heat flux(CHF) is measured during pool boiling of water and R113 on a heated horizontal wire submerged in a subcooled liquid. Experiments are conducted over a pressure range from 0.1 to 3.0 MPa and subcooling up to 220 K. CHF data reveal that the CHF increases in a linear fashion with an increase in subcooling, and that the increment of the CHF with increasing subcooling becomes larger with increasing pressure. The characteristics of the CHF obtained differ from those of existing correlations at high pressures, although it is a similar tendency to them in that the CHF is proportional to the subcooling. A new correlation is derived by taking into account the effect of both the density ratio, ρL/ρV, and the Peclet number, Pe, and it succeeds in predicting the CHF data up to higher pressure and higher subcooling ranges, more effectively than previous studies using existing applicable ranges.

New Insights into the Behavior of the Metastable Region of an Operating Capillary Tube

Abstract: This study examines the mass flow rate of R-22 in an adiabatic capillary tube, with an emphasis on the nature of the metastable region. The metastable region of an operating capillary tube was found to be much more predictable than previously reported in the literature. A new data acquisition technique, which takes the history of the system into account, revealed a hysteresis effect in the mass flow rate as the level of inlet subcooling is increased and decreased. This result is attributed to the fact that, as the inlet level of subcooling is decreasing, it is possible to create and lengthen a metastable region in which liquid flow exists where it might otherwise be two phase. It is predicted that this finding will have an impact on the future of capillary tube data acquisition, modeling efforts, and the fundamental understanding of capillary tube flow.

Direct refrigerant geothermal heat exchange or multiple source subcool/postheat/precool system therefor

Abstract: First, a system for providing liquid refrigerant subcooling, subsequent to that subcooling accomplished by the primary condenser, by means of direct geothermal heat exchange and if required, secondary (or more) fluid(s) supplementary cooling of the liquid refrigerant contiguous with the direct geothermal subcooling. This system could be utilized with standard condensers for higher efficiency air conditioning, heat pump in the cooling mode, or refrigeration operation of any kind. No modifications to the actual condenser would be required, but a larger evaporator surface could be required. Secondly, a system for providing supplemental heat to the primary evaporator of a heat pump operating in the heating mode, by means of a direct geothermal heat exchange, coupled with heat reclaim of the liquid refrigerant, and if necessary with secondary (or more) fluid(s). This system would require some modification of a standard heat pump in addition to the installation of the direct geothermal liquid refrigerant reclaim, secondary (or more) fluid heat postheater. Next, a system that would provide liquid refrigerant subcooling by means of direct geothermal heat exchange and if required, secondary (or more) fluid(s) supplementary subcooling, coupled with a system that would provide direct geothermal, plus a possible secondary (or more) fluid, supplementary heat, to a postheater of a heat pump in the heating mode. That said postheater becoming a precooler of the hot gas refrigerant when the heat pump is in the cooling mode. Finally, a system that would provide liquid refrigerant subcooling by means of direct geothermal heat exchange and if required, a secondary (or more) fluid(s) supplementary subcooling coupled with a system that would provide direct geothermal heat exchange precooling of the hot gas refrigerant and if required, a secondary (or more) fluid(s) precooling of the hot gas refrigerant. This system could be utilized with standard air source or water source air conditioning or refrigeration condensers for producing more capacity and higher efficiency.

Improvement of Critical Heat Flux Correlation for Research Reactors using Plate-Type Fuel

Abstract: In research reactors, plate-type fuel elements are generally adopted so as to produce high power densities and are cooled by a downward flow. A core flow reversal from a steady-state forced downward flow to an upward flow due to natural convection should occur during operational transients such as “Loss of the primary coolant flow”. Therefore, in the thermal hydraulic design of research reactors, critical heat flux (CHF) under a counter-current flow limitation (CCFL) or a flooding condition are important to determine safety margins of fuel against CHF during a core flow reversal. The authors have proposed a CHF correlation scheme for the thermal hydraulic design of research reactors, based on CHF experiments for both upward and downward flows including CCFL condition. When the CHF correlation scheme was proposed, a subcooling effect for CHF correlation under CCFL condition had not been considered because of a conservative evaluation and a lack of enough CHF data to determine the subcooling effect on CHF. ...

Pressure Drop and Heat Transfer for Flow-Boiling of Water in Small-Diameter Tubes

Abstract: Pressure drop and heat transfer for flow-boiling of water in small-diameter tubes under atmospheric pressure were discussed. The results of experiments were compared with those of several existing correlations and models. The experiments were carried out using atmospheric-pressure water in tubes with inner diameter D ranging from 2.0 to 6.0 mm, heated length L from 4.0 to 680.0 mm, inlet water subcooling ΔT in from 70 to 90 K, and mass velocity G from 100 to 10 170 kg/(m 2 s). The highest heat flux q attained was 33 MW/m 2 . Interesting phenomena were observed in small-diameter tubes with a very short heated length. The most suitable correlations for predicting the pressure drop and heat transfer in small-diameter tubes were also discussed.

Flow boiling through v-shape microchannels

Abstract: An experimental facility was set up to study the flow and boiling heat transfer characteristics of water and methanol flowing through V-shape microchannels. The microchannels have hydraulic diameters ranging from 0.2 to 0.6 mm and V-shape groove angles 0 of 30 to 60°. Both the heat transfer and the pressure drop were affected by the thermofluid and geometric parameters such as liquid flow velocity, subcooling, and the hydraulic diameter and groove angle of the microchannels. The experiments indicted that there exists both an optimum hydraulic diameter and an optimum groove angle. The visualization experiments showed that, if there was a good seal between the glass cover and the microchanneled test plate, no bubbles were observed in the microchannels for flow boiling with heat fluxes as high as of the order of 106 W/m2, at which fully nucleate boiling with a large number of bubbles would be expected in conventional situations. Fluctuating liquid flow was induced in the microchannels when many bubbles forme...

Utilization of harvest and/or melt water from an ice machine for a refrigerant subcool/precool system and method therefor

Abstract: A system for providing liquid refrigerant subcooling, subsequent to that subcooling accomplished by the primary condenser of an ice machine, by means of utilizing cold harvest and/or melt water discharge from said ice machine. The subcooler is connected in fluid communication with the output of a pump that pumps stored ice machine discharge water to directly flow through the subcooler from a bottom portion to a top portion in a counter-flow direction and then to discharge such that the subcooler utilizes the pumped and flowing cold discharge water from the ice machine for providing maximum available subcooling to the liquid refrigerant of said ice machine

Exergy analysis of latent heat storage systems with sensible heating and subcooling of PCM

Abstract: The effect of sensible heating of the phase-change material (PCM) before melting and subcooling after solidification, on the performance of the latent heat storage system (LHSS), is studied in terms of first- and second-law efficiencies for the overall charge–discharge cycle. The external heat transfer irreversibilities on account of the interaction between the heat transfer fluid and the storage element are characterized and the optimum phase-change temperature for maximum second-law efficiency is studied. The performance of the LHSS operation is assessed with and without sensible heating before melting and subcooling after solidification. It is observed that the optimum phase-change temperature is higher, and, the overall second-law efficiency is greater for LHSS with sensible heating and subcooling beyond a certain phase-change temperature compared to latent heat storage alone. In addition, the first-law overall efficiency is found to exhibit a minimum and a pre-heated discharge stream is shown to result in a substantial improvement of the second-law efficiency. © 1998 John Wiley & Sons, Ltd.

Immersion cooling of high heat flux microelectronics with dielectric liquids

Abstract: This paper explores the use of direct liquid cooling, by immersion of the components in inert, non-toxic, high dielectric strength perfluorocarbon liquids. Boiling heat transfer with the candidate liquids provides heat transfer coefficients that are as much as two orders of magnitude higher than achievable with forced convection of air. Unfortunately, the highly effective nucleate boiling domain terminates at the so-called Critical Heat Flux, approximately in the range of 20 W/cm/sup 2/ at atmospheric conditions and saturation temperature. Consequently, if immersion cooling is to be used for next generation chips, ways must be found to increase the pool boiling CHF of these dielectric liquids. Use of a pool boiling CHF correlation, developed in this laboratory, points to the possibility of reaching a CHF of nearly 60 W/cm/sup 2/, using elevated pressure and subcooling, along with a dilute mixture of a high boiling point fluorocarbon, and applying a microporous coating to the surface of the chip.

Multistage condensing structure

Abstract: A multistage condensing structure using a liquid dispensing means to transfer droplets of evaporative cooling liquid onto an air-cooled condenser, taking advantage of the conventional air-cooled condensing method and the evaporation method using a liquid coolant. To further increase the efficiency of the air conditioner, two or more condensing units can be arranged in tandem along the blowing direction of a common air blower. With two or more condensing units arranged in tandem, the contacting area between the evaporation surfaces and the air flow can be enlarged to increase the evaporation efficiency without significantly increasing the size of the air conditioner. The multistage condensing structure, with its continuous heat-exchange along the flow of the refrigerant in the coiled pipe, causes an increase in the subcooling condition in the refrigerant. This condensing structure can, therefore, achieve a much higher E.E.R. than the known method of air cooling.