Showing papers on "Subcooling published in 1987"

The effect of void fraction correlation and heat flux assumption on refrigerant charge inventory predictions

Abstract: Ten void fraction correlations and four heat flux assumptions are evaluated for their effect on refrigerant charge inventory predictions Comparisons between mass inventory predictions are made for condensers and evaporators over representative heat pump operating ranges of saturation temperature, mass quality, and mass flux The choice of void fraction model is found to have a major effect on refrigerant inventory prediction The maximum variation of predictions ranges from a factor of 10 for low-ambient, heating-mode evaporators to 42 for cooling-mode evaporators and 17 for high-ambient cooling-mode condensers assuming no subcooling The correlations of Hughmark, Premoli, Tandon, and Baroczy are found to give the highest predictions and closest agreement to measured total system charge The choice of heat flux assumption is shown to be insignificant for forced-flow evaporators and of secondary to possibly equal importance to choice of void fraction model for condensers Implications for charge balancing, off-design and transient performance prediction, and unit reliability are discussed

Nonequilibrium Modeling of Two-Phase Critical Flows in Tubes

Abstract: A nonequilibrium two-phase flow model is described for the analysis of critical flows in variable diameter tubes. Modeling of the two-phase flow mixture in the tube is accomplished by utilizing a one-dimensional form of conservation and balance equations of two-phase flow which account for the relative velocity and temperature differences between the phases. Closure of the governing equations was performed with the constitutive equations which account for different flow regimes, and the solution of the nonlinear set of six differential equations was accomplished by a variable step numerical procedure. Computations were carried out for a steam--water mixture with varying degrees of liquid subcooling and stagnation pressures in the vessel upstream of the tube and for different tube lengths. The numerical resuls are compared with the experimental data involving critical flows with variable liquid subcoolings, stagnation pressures, and tube lengths, and it is shown that the nonequilibrium model predicts well the critical flow rate, pressure distribution along the tube, and the tube exit pressure.

Critical Heat Flux and Flow Characteristics of Subcooled Flow Boiling in Narrow Tubes

Abstract: The critical heat flux (CHF) of subcooled flow boiling in narrow tubes and its flow characteristics were investigated, and the bubble boundary layer concept was discussed. For subcooled flow boiling in narrow tubes of 1 mm inside diameter, the void fraction was estimated to be several ten percent smaller than the predicted by the subcooled void fraction correlations for large-inside-diameter tubes. This caused the CHF in the narrow tubes to increase and the friction pressure drop multiplier to decrease. The Tong CHF correlation based on the bubble boundary layer separation model was examined. Considering the dependence of pressure on the empirical parameter C in the Tong correlation, a modified correlation was proposed.

Multiroom air conditioner

Abstract: A multiroom air conditioner has a plurality of indoor heat exchangers connected to a single outdoor heat exchanger. A reversible expansion valve is installed on a liquid-side branch pipe for each of the indoor heat exchangers, and a solenoid valve is installed on a gas-side branch pipe for each indoor heat exchanger. Temperature sensors are provided on the liquid-side branch pipes, on the gas-side branch pipes, and on the outlet side during cooling of the outdoor heat exchanger. A pressure sensor or a temperature sensor detects the saturation temperature on the high-pressure side of the compressor. Based on the saturation temperature on the high-pressure side of the compressor, the temperature on the outlet side during cooling of the outdoor heat exchanger, and the temperatures in the liquidside branch pipes, a controller controls the expansion valves so as to obtain a target degree of subcooling during cooling and heating. The controller can also control the expansion valves so that the temperatures in each of the branch pipes are approximately equal to one another.

Film Boiling Heat Transfer and Minimum-Heat-Flux (MHF)-Point Condition in Subcooled Pool Boiling : Heat Transfer, Power, Combustion, Thermophysical Properties

Abstract: The minimum-heat-flux (MHF) -point condition and film-boiling heat transfer in subcooled pool boiling of water were experimentally investigated by using a horizontal cylinder and a sphere. First, collapse behavior of vapor films was observed, and it was found that collapse modes can be classified into coherent and propagative collapses. Second, the relation between the MHF-point temperature and the propagation rate of film collapse was experimentally obtained, and the MHF-point temperature at which coherent collapse occurred was defined as its true value. The experimental data obtained show that the true MHF-point temperature is not strongly dependent on liquid subcooling and it does not exceed the liquid's maximum, metastable superheat temperature even at large subcoolings. Finally, using the experimental data of subcooled film-boiling heat transfer for various liquids, a prediction technique based on Hamill's correlation was developed.

Thermo-charger for multiplex air conditioning system

Abstract: A subcooler is provided in heat exchange relationship with the high pressure liquid refrigerant line flowing to the indoor coil, such that when a low pressure condition is sensed on the low pressure side of the system, the subcooler flashes a portion of the liquid refrigerant to the low pressure side to thereby maintain suction pressure to the compressor, while at the same time subcooling the liquid refrigerant flowing to the fan coil to thereby increase its efficiency.

Development of absorption refrigeration units for cold storage of agricultural products

Abstract: To meet the requirements of continuous refrigeration installations, powered by low-grade heat sources, e.g. waste heat, flat-plate solar collectors or solar ponds, and providing cooling or refrigeration at subzero (°C) temperatures, rational selection of suitable refrigerant-absorbent pairs is necessary. It is demonstrated that the use of R22 and suitable organic solvent systems is desirable in absorption refrigeration installations. Since no data were available on the vapour-liquid equilibria of these combinations, they were measured over a broad range of pressures, temperatures and concentrations. A rational selection of the optimal combinations was made using results obtained from a computerized simulation model giving the performance characteristics of a specific refrigeration system operating with selected refrigerant-absorbent combinations. An efficient thermodynamic cycle has been constructed for the chosen refrigerant-absorbent pairs. Within the region of feasible operation of the cycle, optimal operating conditions are determined. In selecting these conditions it is also necessary to take into account the heat transferred in the heat exchangers of the unit and the operating stability. On the basis of the research performed a 15 ton refrigeration prototype unit for the storage of agricultural products has been constructed and is in the final stages of being run in. It operates at a coil temperature of −3°C and maintains the storage chamber at 5°C. Hot water is supplied at 95–98°C, and cooling water is provided at 26°C.

Experimental verification of subcooled flow boiling for tokamak pump limiter designs

Abstract: In fusion energy research devices such as tokamaks, limiters are used to define the plasma boundary, and may serve the additional functions of plasma density and impurity control by removing neutralized particles from the plasma edge region. Because the devices must operate in the plasma edge or ''scrape-off-layer,'' they are subject to high heat fluxes. In this paper, experimental studies for a pump limiter design currently under development are discussed. Subcooled flow boiling of water and twisted tape flow enhancement are combined to enable heat removal of highly peaked local heat fluxes at the tube-water boundary in the 40 to 50 MW/m/sup 2/ range. Experiments were conducted with the use of a rastered 30 kV electron beam apparatus which is capable of producing the desired steady state heat flux levels. Objectives of the experiment were (1) to verify the heat removal model used for finite element thermal and stress analyses, (2) selection of appropriate critical heat flux (CHF) margins and criteria, and (3) development of acoustic techniques to monitor the onset of CHF during actual limiter operation.

Crevice Boiling in Steam Generators

Abstract: Experimental results are presented on the influence of confinement (normal to heated surface) on nucleate boiling in forced flow. The forced flow conditions and confinement geometry studied are similar to those found for boiling between a primary-fluid tube and a tube-support plate in steam generators of pressurized-water-reactor nuclear power plants. Visual observations of the boiling process within the confined region (crevice) between the tube and its support plate, obtained by high-speed photography, are related to simultaneous two-dimensional temperature maps of the hot primary-fluid-tube surface. The results demonstrate the existence of three confinement-dependent boiling regimes in forced flow conditions that are similar to those found in pool boiling conditions. These regimes are shown to be associated with the Weber number.

Methods and systems for magnetohydrodynamic power conversion

Abstract: There is provided a method for wet-vapor, liquid-metal magnetohydrodynamic power conversion. The method includes partially boiling off, using at least a heat source, an electrically conductive, substantially single-phase working liquid, thus generating vapors which, together with the still liquid phase, produce a two-phase mixture, extracting electric power by passing the two-phase mixture through a two-phase MHD generator, causing the vapor phase of the two-phase mixture from the MHD generator to condense, by mixing in a condenser the two-phase stream exiting from the MHD generator with a stream of subcooled liquid metal, splitting the liquid exiting the condenser into a first part and a second part, returning the first part to the heat source to be reheated, subcooling the second part by passing it through a heat-exchanging means, accelerating the subcooled liquid metal to substantially match the velocity of the two-phase mixture in the condenser, and introducing the accelerated and subcooled liquid metal into the condenser to effect the condensing action. There is also provided a system for wet-vapor, liquid-metal magnetohydrodynamic power conversion.

A theoretical prediction of critical heat flux in subcooled pool boiling during power transients

Abstract: Understanding and predicting critical heat flux (CHF) behavior during steady-state and transient conditions are of fundamenatal interest in the design, operation, safety of boiling and two-phase flow devices. This paper discusses the results of a comprehensive theoretical study made specifically to model transient CHF behavior in subcooled pool boiling. This study is based upon a simplified steady-state CHF model in terms of the vapor mass growth period. The results obtained from this theory indicate favorable agreement with the experimental data from cylindrical heaters with small radii. The statistical nature of the vapor mass behavior in transient boiling also is considered and upper and lower limits for the current theory are established. Various factors that affect the discrepancy between the data and the theory are discussed.

Two types of flow instabilities occurring inside horizontal tubes with complete condensation

Abstract: Data are presented which demonstrate that two different types of condensing instabilities can exist in the horizontal U-tube heat exchangers used in moisture separator reheaters. The first instability was a cyclic type. The thermocouple readings showed that at times the subcooled condensate temperatures leaving certain lubes were cycling in phase and with the same period. The maximum values of these temperature traces never reached the saturation temperature. The second instability was a condensate chugging or water hammer type. The thermocouple readings indicated that vapor was entering some lube ends because of a stratified flow regime. It was then condensed, and all the condensate was discharged as a liquid plug.

A rapid, semiempirical method of calculating the stability margins of superconductors cooled with subcooled He-II

Abstract: A rapid, semiempirical method is presented for calculating the stability margins of superconductors cooled with subcooled He-II. Based on a model of Seyfert et al., the method takes into account both time-dependent Gorter-Mellink heat transport and the effects of interfacial Kapitza resistance. The method has been compared favorably with heat transfer data of Seyfert et al., stability data of Meuris, and stability data of Pfotenhauer and van Sciver.

Power saving refrigeration system

Abstract: A subcooling condenser/receiver is provided upstream of and proximate to an expansion device in a refrigeration loop, the condenser/receiver having an inlet for receiving cooled refrigerant, a receiver for accumulating the refrigerant, an integral suction line to subcool the refrigerant, and an outlet for discharging the accumulated refrigerant to the expansion device. Because the suction line is integral to the receiver, the cooled refrigerant is in thermal communication with the spent refrigerant. In order to condense and accumulate refrigerant in the receiver the subcooling condenser/receiver has a flow restricting structure which produces a pressure drop between the subcooling condenser/receiver inlet and the receiver. A liquid seal is formed at the subcooling condenser/receiver in the high pressure line extending from the compressor to the expansion device. This arrangement reduces pressure losses in the high pressure line thus improving the volumetric flow efficiency. Accordingly, a smaller load is placed on the compressor and the system's power requirements are reduced.

Radiation-induced melting with buoyancy effects in the liquid

Abstract: Experiments have been performed to determine the role of radiant energy absorption in melting in a vertical confined layer of semitransparent phase change material ( n-octadecane). Results show generally that higher incident flux levels increase the melting rale of the layer. More opaque materials melt more rapidly and maintain higher temperatures in the melt region. Initial solid subcooling drastically impedes the melting process. Aspect ratios have been varied and the results obtained underline the importance of natural convection effects. Finally, it is concluded that radiation and natural convection effects are of equal importance in determining the solid-liquid interface velocity and shape and the overall melting rate.

A study of the vapor explosion mechanism with single drops of high-temperature liquids and volatile liquids.

Abstract: The vapor explosion produced by a single drop or plural drops of molten LiNO3 being dropped into tapped ethanol is studied by high-speed photography and pressure trace measurement. The temperature range of fragmentation to occur is firstly examined. The pressure trace with fragmentation is divided into two groups. Under the relatively large subcooling condition of a cold liquid, the typical vapor explosion whose pressure trace has at least three peaks and a period of reduced pressure occurs. The vapor bubble behavior is proved to be well-consistent with the pressure trace. The propagation phenomenon of vapor explosion is photographed under plural-drop experiments. The existence of a lengthy state of coherency is proved, and the pressure behavior is concluded as the control factor of the coherency. A qualitative and hypothetical modeling of the vapor explosion mechanism is made on the basis of the experimental results.

Microbubble Emission Boiling on a Horizontal Upward-Facing Rectangular Surface in Subcooled Flow : Heat Transfer, Power, Combustion, Thermophysical Properties

Abstract: It is known that there exists a boiling condition of subcooled liquid at which many microbubbles are emitted from a heated surface in the transition boiling regime. We call this phenomenon "microbubble emission boiling" (MEB). MEB occurs easily in systems where the subcooled liquid removes the bubbles naturally, and not against their own motion from the surface soon after their generation. Consequently the subcooling and the velocity of the liquid should be very important factors concerning the occurrence of MEB. In this study, our purpose was to clarify the effects of the degree of liquid subcooling and its approaching velocity to the surface on the occurrence of MEB and the intensity of microbubble emission by measuring temperature fluctuations of the surface and pressure fluctuations in the liquid above the surface.

サプクール沸騰における膜沸騰熱伝達と極小熱流束点条件に関する研究 : 第2報,水平白金円柱-減圧水のプール沸騰系

Abstract: This paper presents experimental results on the real value of the minimum-heat-flux (MHF) -point temperature for subcooled pool boiling of water from a horizontal cylinder and an advanced form of the correlation for subcooled film-boiling heat-transfer proposed in the previous paper. First, experiments to observe the collapse process of film boiling were conducted by using a high speed video. The experimental results show that the collapse mode is classified into the propagative collapse with a precursory local-collapse and the coherent collapse. Second, the propagation velocity of collapse front and the mean surface temperature upon the local collapse followed by the propagative collapse were measured. The experimental results show that, for higher propagation-velocities, the MHF-point temperature decreases and approaches the value upon coherent collapse which has only a slight dependence on liquid subcooling. Finally, an advanced form of the correlation for subcooled film-boiling heat-transfer reported in the previous paper is proposed. This equation is in good agreement with the present data of of water at subatmospheric pressures and the existing data of liquid sodium and freon R-11 at atmospheric pressure and pressurized water.

Heat Transfer of a Liquid-Solid Contact in a Subcooled Pool Boiling System : Transition-Type Boiling Regime and Minute Bubble Emission Boiling Regime

Abstract: This report describes liquid-solid contact behavior in the subcooled pool transition-type boiling regime and the minute bubble emission boiling regime, detected by a void probe that uses a series-resonance circuit. This probe can approach the heated surface within two to three micrometers. The liquid-solid contact state discussed in this report means the nucleate boiling state caused by liquid invasion or Leidenfrost state caused by liquid rush onto the dry part on the heated surface. The dried area on the heated surface was estimated on the basis of the void signal, and the relation between the dried area on the heated surface and the measured heat flux was clarified by introducing a nucleate boiling model.

Evaporation of refrigerant 113 flowing inside smooth tubes

Abstract: Heat transfer coefficients were experimentally measured for R-113 evaporating inside uniformly heated smooth tubes and then compared with six different correlations from the literature. Experiments were performed on 4-m lengths of two different size tubes: an 8.71-mm-I.D. and a 10.92-mm-I.D. The inlet state was subcooled approximately 18/sup 0/C, while the outlet state was at a quality of approximately 0.8. Local heat transfer coefficients along the test tube were obtained from wall temperatures, fluid temperatures (inferred from saturation pressures), and electrical energy input to the fluid. Data were obtained for a range of flow rates and heat fluxes. The experimental results are compared with six of the more commonly used, in-tube evaporation, heat transfer correlations available in the literature. Interestingly, these correlations produced widely differing results. In addition, the agreement with experimental data varied with the quality range in which the comparison was being performed.

Process and arrangement for energy-saving automatic maintenance of the concentration of boiling coolant mixtures

Abstract: The invention deals with a process for the energy-saving automatic control of the concentration of evaporating coolant mixtures, especially of water-containing ammonia solutions in evaporators of refrigerating machines and heat pumps based on the absorption principle, together with different embodiments for carrying out the process. The process makes possible the combined utilization of the vaporization potential of the solution outflow and the cooling potential of the cold vapor withdrawn for subcooling the condensate feed, while selecting the minimum temperature difference between the subcooled not-yet-depressurized condensate feed and the boiling coolant mixture in the evaporator as the control variable for process automatization. This temperature difference is independent of the saturation pressure of the boiling coolant mixture and can be maintained constant for every evaporator load point to obtain an energy-saving solution outflow. Finally, the concentration of the boiling coolant mixture is maintained with this procedure, and the evaporator operates for every evaporation temperature with the highest possible saturation pressure, almost like a single-substance evaporator.