Showing papers on "Subcooling published in 2003"

Effect of Pressure, Subcooling, and Dissolved Gas on Pool Boiling Heat Transfer From Microporous Surfaces in FC-72

Abstract: The present research is an experimental study of the effects of pressure, subcooling, and non-condensable gas (air) on the pool nucleate boiling heat transfer performance of a microporous enhanced and a plain (machine-roughened) reference surface. The test surfaces, 1-cm 2 flat copper blocks in the horizontal, upward facing orientation, were immersed in FC-72. The test conditions included an absolute pressure range of 30-150 kPa, a liquid subcooling range of 0 (saturation) to 50 K, and both gas-saturated and pure subcooling conditions. Effects of these parameters on nucleate boiling and critical heat flux (CHF) were investigated

The Direct Contact Condensation of Steam in a Pool at Low Mass Flux

Abstract: The direct contact condensation of steam at low mass flux (in the chugging region) and the pressure oscillation induced by steam condensation were investigated and analyzed. During the experiment, the steam was discharged into a square subcooled water pool in the horizontal direction. The main experimental parameters were the steam mass flux in the range of 10–80 kg/m2·s and the subcooled water temperature in the range of 30–80°C. Also, two different nozzles (5/8, 6/8 inch) were used. In the chugging region, the high pressure pulses were generated with relatively low frequency. The frequency was little affected by the subcooled water temperature, but increased with increasing steam mass flux. A critical value of steam mass flux was found where the pressure pulse generation rate increased suddenly. This critical value for the 5/8 inch nozzle was found to be lower than that for the 6/8 inch nozzle. The pressure pulse was observed to be generated on the occurrence of sudden condensation of the steam bubble w...

Effects of surface roughness, oxidation level, and liquid subcooling on the minimum film boiling temperature

Abstract: The effects of surface roughness, oxidation level, and liquid subcooling on the minimum film boiling temperature, T min, during quenching of a nuclear fuel rod were investigated experimentally. An instrumented nuclear fuel rod simulator (FRS) was designed and fabricated with different combinations of cladding (Inconel and Zircaloy) and filler (BN and Al2O3) materials. Surface roughness was varied by polishing the FRS with different grades of emery cloth, whereas oxidation level was varied by heating the FRS in an oven under carefully controlled conditions for different time periods. An eddy-current device was used to nondestructively measure the oxide layer thickness on the FRS. Quenching experiments were conducted in a subcooled water bath and the value of T min was determined from the measured temperature transients. For the FRS explored in the experiments, the minimum film boiling temperature was found to depend strongly on the liquid subcooling, oxidation level, and surface roughness.

Condensation heat transfer coefficients of R22, R407C, and R410A on a horizontal plain, low fin, and turbo-C tubes

Abstract: In this study, condensation heat transfer coefficients (HTCs) were measured on a horizontal plain tube, low fin tube, and Turbo-C tube at the saturated vapor temperature of 39 °C for R22, R407C, and R410A with the wall subcooling of 3–8 °C. R407C, a non-azeotropic refrigerant mixture, exhibited a quite different condensation phenomenon from those of R22 and R410A and its condensation HTCs were up to 50% lower than those of R22. For R407C, as the wall subcooling increased, condensation HTCs decreased on a plain tube while they increased on both low fin and turbo-C tubes. This was due to the lessening effect of the vapor diffusion film with a rapid increase in condensation rate on enhanced tubes. On the other hand, condensation HTCs of R410A, almost an azeotrope, were similar to those of R22. For all refrigerants tested, condensation HTCs of turbo-C tube were the highest among the tubes tested showing a 3–8 times increase as compared to those of a plain tube.

Combined effects of subcooling and surface orientation on pool boiling of hfe-7100 from a simulated electronic chip

Abstract: The effects of orientation and subcooling on pool boiling of the HFE-7100 dielectric liquid near atmospheric pressure (0.085 MPa) from a 10 × 10 mm smooth copper surface are investigated experimentally. Results are obtained for inclination angles θ = 0° (upward-facing), 30°, 60°, 90°, 120°, 150°, and 180° (downward-facing) and liquid subcoolings ΔTsub = 0, 10, 20, and 30 K. Increasing θ decreases the saturation nucleate boiling heat flux at high surface superheats (ΔTsat > 20 K), but increases it only slightly at lower surface superheats. The critical heat flux (CHF) decreases slowly with increasing θ from 0° to 90°, and then deceases faster with increasing θ to 180°. CHF increases linearly with increased subcooling, but the rate increases from 0.016 K−1 at 0° to 0.048 K−1 at 180°. At θ = 0° and ΔTsub = 30 K, CHF is ∼ 36 W/cm2 and 24.45 W/cm2 for saturation boiling, while at θ = 180° CHF = 10.85 W/cm2 at ΔTsub = 30 K and only 4.30 W/cm2 at saturation. The developed correlation for CHF of HFE-7100, as a fu...

Heat flux correlation for spray cooling in the nucleate boiling regime

Abstract: In this work an experimental study of spray cooling using a spray in which drop sizes and velocities are uniform (monodisperse spray) impinging on a flat and heated surface is reported. The objective of the work was to formulate an empirical model describing the heat flux (HF) for the nucleate boiling regime and for the critical heat flux (CHF). Monodispersed water droplets with a known diameter and velocity, produced by a droplet generator, were directed toward a heated surface and the heat transfer was registered using a data acquisition system. The resulting high heat flux was investigated as function of the droplets’ diameter and velocity, mass flow rate, ambient pressure, subcooling degree of liquid, wall superheat, and surface roughness. The resulting matrix of variables investigated in the experiments included mass flux rate (340 < m˙″ < kg/m2 s), subcooling degree (25 < Tsub < 78°C), ambient pressure (1 < P < 1.8 bar), and surface roughness (79 < Rt < 5 μm). Generalized correlations were developed...

An Experimental Investigation on Flow Boiling of Ethylene-Glycol/ Water Mixtures

Abstract: Mixtures of ethylene glycol and water are used in cooling the engines in automotive applications. To avoid the two-phase flow in the engine, the mixture is subcooled in the radiator before entering the engine block. Heat transfer is therefore essentially under subcooled flow boiling conditions. Very little information is available in the literature on the subcooled flow boiling characteristics of this mixture, and there is no predictive method established in this region. The present work focuses on obtaining experimental heat transfer data for mixtures of ethylene-glycol (0 to 40 percent mass fraction, limited by the maximum allowable temperature in the present setup) and water in subcooled flow boiling region. The experimental setup is designed to obtain local heat transfer coefficient values over a small circular aluminum heater surface, 9.5-mm in diameter, placed at the bottom wall of a rectangular channel 3mm x 40-mm in cross-section. The applicability of the available model for subcooled flow boiling of pure liquids to the mixtures is examined. NOMENCLATURE ” Bo - Boiling number = q I G i,fg cP,r - specific heat of liquid, J/kgK D12 - diffusion coefficient of 1 (ethylene glycol) in 2 (water) Dh - hydraulic diameter of the flow channel, m Fn - Mass diffusion induced suppression factor, eq. (4) Fo - fluid surface parameter in Kandlikar (1990) correlation G - mass flux, kg/m’s i, - latent heat of vaporization, J/kg 4” - heat flux, W/m2 Re - Reynolds number, pVD&t T - temperature, K V - flow velocity, m/s Vt - Volatility parameter, defined by Kandlikar (1998c), eq. (3) xt - liquid mass fraction of ethylene glycol in aqueous solution yt - vapor mass fraction of ethylene glycol in aqueous solution

Solidification in heat packs: I. Nucleation rate

Abstract: The solidification of sodium acetate trihydrate in a commercial heat pack is investigated in a series of three articles. In the first work, the classic nucleation theory is used to predict the range of temperatures and pressures for which a subcooled solution of sodium acetate solidifies spontaneously. The nucleation temperature was determined experimentally and it was shown that it is insensitive to concentration, at atmospheric pressure. Using the theoretical results for high pressure and the experimental measurements at atmospheric pressure, an equation for the nucleation temperature was derived as a function of pressure. This relation takes into account heterogeneous nucleation on ever-present impurities suspended in the solution.

Refrigerant compression system with selective subcooling

Abstract: The compressor (11) of a transport refrigeration system is divided into two sections, with each section having a suction inlet (24, 39). The refrigerant vapor from the evaporator (14) is passed to one of the suction inlets (24), whereas the refrigerant vapor from a subcooler (26) circuit is passed to the other suction inlet (39). A valve (33) is operated to selectively insert or remove the subcooler (26) as capacity requirements vary. In one embodiment, a six cylinder (41, 42, 43, 44, 46, 56) reciprocating compressor (11) is used with five cylinders (41, 42, 43, 44, 46) compressing within a first section, and a single cylinder (56) compressing in the other section containing the subcooler (26) circuit.

Heat transfer at the subcooled‐scraped surface with/without phase change

Abstract: Continuous heat extraction is important for the process of freeze concentration of aqueous solutions, in which water is removed as solid ice. Three typical unsteady heat transportation patterns were distinguished at the subcooled surface of a scraped-surface heat exchanger (SSHE) in this study. They were found in different stages of freeze concentration. Experimental measurement of the heat-transfer coefficient in an SSHE showed that the overall heat-transfer coefficient of stage III, which was characterized by ice formation on the cooler surface, was about 1.5 times higher than stage I, where no ice formed. Although the ice layer (also known as ice fouling) on a heat exchanger surface may be considered disadvantageous for heat transfer, the initial ice formation actually boosted up the heat transportation in an SSHE. The mechanism analysis and mathematical modeling of this phenomenon, however, have not been found in the literature. A mathematical model is developed and a unified expression of the heat-transfer coefficient in an SSHE with/without phase change is presented. The model predicts a step increase of heat transfer occurs at the onset of ice formation and the maximum heat-transfer coefficient exists in a narrow range right after reaching the freezing point. These are consistent with the experimental results of this study.

Ultra High Critical Heat Flux During Forced Flow Boiling Heat Transfer With an Impinging Jet

Abstract: An ultra high critical heat flux (CHF) was attempted using a highly subcooled liquid jet impinging on a small rectangular heated surface of length 5∼10 mm and width 4 mm. Experiments were carried out at jet velocities of 5∼60 m/s, a jet temperature of 20°C and system pressures of 0.1 1∼1.3 MPa. The degree of subcooling was varied from 80 to 170 K with increasing system pressure. The general correlation for CHF is shown to be applicable for such a small heated surface under a certain range of conditions. The maximum CHF achieved in these experiments was 211.9 MW/m 2 , recorded at system pressure of 0.7 MPa, jet velocity of 35 m/s and jet subconling of 151 K, and corresponds to 48% of the theoretical maximum heat flux proposed by Gambill and Lienhard.

Refrigeration system with bypass subcooling and component size de-optimization

Abstract: A refrigeration system having a primary refrigerant path including a compressor (12), a condenser (24), a primary expansion device (16), and an evaporator (28) connected together to form a closed loop system with a refrigerant circulating therein; and a bypass path (27) coupled to an outlet of the condenser The bypass path includes a secondary expansion device (23); and a heat exchanger (22) thermally coupled to the primary refrigerant path between the condenser outlet and the primary expansion device inlet to remove heat from the refrigerant discharged from the condenser The condenser is downsized such that lacks the heat transfer capacity to provide some or all of the required subcooling as provided according to conventional practice, and the heat exchanger provides some or all the required subcooling according to the capacity of the condenser A pressure differential accommodating device (38) operative to mix two vapors at different pressures may also be provided to connect the outlets of the evaporator and the heat exchanger to an inlet of the compressor

Automated high-speed video analysis of the bubble dynamics in subcooled flow boiling

Abstract: Subcooled flow boiling is a commonly applied technique for achieving efficient heat transfer In the study, an experimental investigation in the nucleate boiling regime was performed for water circulating in a closed loop at atmospheric pressure The test-section consists of a rectangular channel with a one side heated copper strip and a very good optical access For the optical observation of the bubble behaviour the high-speed cinematography is used Automated image processing and analysis algorithms developed by the authors were applied for a wide range of mass flow rates and heat fluxes in order to extract characteristic length and time scales of the bubbly layer during the boiling process Using this methodology, a huge number of bubble cycles could be analysed The structure of the developed algorithms for the detection of the bubble diameter, the bubble lifetime, the lifetime after the detachment process and the waiting time between two bubble cycles is described Subsequently, the results from using these automated procedures are presented A remarkable novelty is the presentation of all results as distribution functions This is of physical importance because the commonly applied spatial and temporal averaging leads to a loss of information and, moreover, to an unjustified deterministic view of the boiling process, which exhibits in reality a very wide spread of bubble sizes and characteristic times The results show that the mass flux dominates the temporal bubble behaviour An increase of the liquid mass flux reveals a strong decrease of the bubble life- and waiting time In contrast, the variation of the heat flux has a much smaller impact It is shown in addition that the investigation of the bubble history using automated algorithms delivers novel information with respect to the bubble lift-off probability

Refrigeration system with de-superheating bypass

Abstract: A refrigeration system includes a compressor, a condenser, an expansion device and an evaporator connected in a closed circuit through which a refrigerant is circulated. A portion of liquid refrigerant exiting the condenser is diverted through a secondary expansion valve and a heat exchanger, which is thermally coupled between compressor and condenser, thereby allowing the superheated refrigerant vapor to be cooled at a temperature at or close to its saturation temperature when it enters the condenser. Hence, a de-superheating process inside the condenser is eliminated, and the condenser operates more efficiently, resulting in increased subcooling and thus increased cooling capacity. Also, the more efficient condenser decreases condenser pressure, a phenomenon which results in the reduction of the compressor work and accordingly increases the efficiency. The refrigerant vapor from the bypass line is maintained at an intermediate pressure between evaporator and condenser pressures and is combined with the low-pressure vapor from the evaporator through a pressure differential accommodating device, which may generate a vacuum by vortex flow of the superheated vapor from the bypass line, by flow of the superheated vapor through the throat of a venturi device, or in any other comparable manner. By increasing the amount of diverted refrigerant beyond that required for de-superheating, reduced cooling capacity can be achieved without the need for frequent on-off cycling of the compressor. The refrigeration system may employ a single refrigerant or a mixture of refrigerants such as R-134 a , R-32 and R-125.

Effects of Noncondensable Gas and Subcooling on the Spray Cooling of an Isothermal Surface

Abstract: Spatially resolved heat transfer measurements have been made on a nominally isothermal surface that is spray cooled. The effects of dissolved gas and subcooling were studied. The local measurements were obtained using a microheater array with ninety-six heaters each about 700 microns in size. The heater array consisted of thin-film platinum resistors deposited on a quartz substrate. Electronic feedback circuits were used to keep each of the heaters at a constant temperature and the voltage required to do this was measured. The array was cooled with FC-72 using a spray nozzle from ISR. The heat transfer data was correlated with high-speed images of the flow structure obtained from below. In this paper, heat transfer mechanisms in spray cooling will be discussed, with a particular emphasis on the effects of a non-condensable gas on critical heat flux in comparison to the comparable effects of thermally subcooling the liquid prior to atomization. NOMENCLATURE