Showing papers on "Subcooling published in 2014"
TL;DR: In this paper, a technique to simultaneously measure the liquid-vapor phase boundary, temperature distribution, and heat transfer distribution at a boiling surface is described, and the technique is fully synchronized in time and spatially resolved, and is applied to explore single-bubble nucleate boiling phenomena in a pool of water subcooled by 3°C under atmospheric pressure.
157 citations
TL;DR: In this article, a series of nanoscale molecular dynamics simulations of supersaturated water-methane mixtures was performed and it was shown that the order of the crystalline structure increases by decreasing subcooling.
Abstract: Methane hydrates are crystalline structures composed of cages of hydrogen-bonded water molecules in which methane molecules are trapped. The nucleation mechanisms of crystallization are not fully resolved, as they cannot be accessed experimentally. For methane hydrates most of the reported simulations on the phenomena capture some of the basic elements of the full structure. In few reports, formation of crystalline structures is reached by imposing very high pressure, or dynamic changes of temperature, or a pre-existing hydrate structure. In a series of nanoscale molecular dynamics simulations of supersaturated water–methane mixtures, we find that the order of the crystalline structure increases by decreasing subcooling. Crystalline structures I and II form and coexist at moderate temperatures. Crystallization initiates from the spontaneous formation of an amorphous cluster wherein structures I, II, and other ordered defects emerge. We observe the transient coexistence of sI and sII in agreement with expe...
125 citations
TL;DR: In this paper, the authors investigated the solidification characteristics of water-based NFPCM (nanofluid phase change material), which was prepared by dispersing copper oxide nanoparticles and a nucleating agent in the base PCM (phase change material).
118 citations
TL;DR: In this paper, the forced convective and subcooled flow-boiling heat transfer of CuO/water nanofluid as well as fouling rate were experimentally quantified for different dilute concentrations of CUO nanoparticles in water over a range of mass flux.
110 citations
TL;DR: In this paper, a gas-dominant flowloop was constructed to study hydrate formation in an annular flow regime by exposing warm process fluids to a cold pipe wall.
Abstract: A 130 ft single-pass, gas-dominant flowloop has been constructed to study hydrate formation in an annular flow regime by exposing warm process fluids to a cold pipe wall. Hydrate was formed in six experiments from a natural gas mixture, with 6–18 °F subcooling from hydrate equilibrium. At lower subcooling values a stenosis-type hydrate film growth model without adjustable parameters was used to estimate the resulting pressure drop and yielded an average deviation of 15.8 psi from the experimental value. The accuracy of this model decreases appreciably with increasing subcooling, suggesting the occurrence of a transition after which the pressure drop becomes dominated by additional hydrate phenomena such as particle deposition or wall sloughing. For experiments with 18 °F subcooling, the pressure drop signal contained periodic peak-and-trough behavior and the primary hydrate restriction was observed to migrate downstream at a rate of 3 ft/min over the course of the experiment. Average hydrate growth rates ...
100 citations
TL;DR: In this paper, a comprehensive survey of existing empirical correlations is presented in the first part of this paper to assess the performance of these empirical models and selected combinations of empirical correlations are compared and validated against axial and local radial experiments encompassing a wide range of different mass and wall heat fluxes and inlet subcooling temperatures for subcooled flow boiling at low pressures.
83 citations
TL;DR: In this article, the effects of orientation angle, subcooling, heat flux, mass flux, and pressure on bubble departure diameter in the isolated bubble regime of subcooled flow boiling were studied by high-speed video in a two-phase flow loop that can accommodate a wide range of flow conditions.
73 citations
TL;DR: Graphene flakes are more suitable than spherical oxide nanoparticles for use as nucleating additives in water, and flakes of graphene led to freeze water at a much smaller subcooling degree, and shorter total freezing time as mentioned in this paper.
66 citations
TL;DR: In this paper, the authors investigated the condensation heat transfer enhancement of steam with hybrid surface by integrating the drop-wise and film-wise heat transfer models, and they showed that an optimum value of the maximum droplet radius exists to enhance the condence heat transfer of steam, which is dependent on surface subcooling degree, filmwise region width and contact angle while independent on contact angle hysteresis.
61 citations
TL;DR: In this article, an alternative system architecture, which utilizes a receiver and an additional electronic expansion valve, is used to provide independent control of condenser subcooling, and extremum seeking control is implemented to find the optimal sub cooling in an adaptive, model-free manner.
Abstract: With vapor compression systems consuming a significant portion of the overall U.S. energy consumption each year, extensive efforts have been made toward the development of control strategies which aim to maximize system efficiency while providing the desired cooling. However, previous control strategies under-utilize a degree of freedom corresponding to the amount of refrigerant in the system, which is related to condenser subcooling and can significantly affect system efficiency. In this paper, an alternative system architecture, which utilizes a receiver and an additional electronic expansion valve, is used to provide independent control of condenser subcooling. Simulation and experimental results show there exists an optimal subcooling which maximizes system efficiency; however, this optimal subcooling changes with operating conditions. Therefore, extremum seeking control is implemented to find the optimal subcooling in an adaptive, model-free manner. Experimental results demonstrate a 9% increase in efficiency using the alternative architecture and extremum seeking control.
61 citations
TL;DR: In this paper, the authors investigated the thermal performance of nanofluid phase change material (NFPCM) for cool thermal energy storage (CTES) system and found that the freezing duration was reduced by 25% in the case of NFPCM along with the elimination of subcooling compared to that of DI water.
Abstract: This study aimed to investigate the thermal performance of nanofluid phase change material (NFPCM) for cool thermal energy storage (CTES) system. The NFPCM was prepared by suspending 0.1 wt. % multiwall carbon nanotubes (MWCNT) in deionized water (DI water) along with pseudomonas as a nucleating agent. The experiments were conducted at various surrounding bath temperatures with the NFPCM. The results evidently showed that the freezing duration was reduced by 25% in the case of NFPCM along with the elimination of subcooling compared to that of DI water. Further, the fraction of mass solidified at various time intervals revealed that 50% of phase change material (PCM) was frozen in 25% of the total freezing time for both the water PCM and NFPCM. The enhanced heat transport properties of the NFPCM along with the elimination of subcooling and accelerating charging will be very useful in designing energy efficient CTES system.
TL;DR: In this article, two ejectors are proposed as expansion devices for a two-stage transcritical CO 2 heat pump cycle to enhance the cycle performance, and the performance is evaluated by using the developed mathematical model, and then compared with those of the basic two stage cycle with a flash tank.
TL;DR: In this article, the authors investigated the effect of the charge amount on the performance of a single-stage and a cascade heat pump system in terms of optimal sub-cooling.
Abstract: The refrigerant charge amount is a key factor for heat pump system optimization, and normally determines the condensing pressure, which affects the subcooling at the exit of the condenser. Heating capacity increases as subcooling increases, however, there is an optimum charge amount for the best coefficient of performance (COP), since the compressor work increases as the charge amount increases. Many studies have focused on optimization of the charge amount for a single-stage heat pump system, while few investigations have been carried out on the charge amount for a cascade heat pump system. The cascade cycle has been suggested as an alternative to a single cycle in extremely cold regions. In this study, air-to-water single and cascade heat pumps were studied to experimentally and numerically verify the effect of the charge amount on the heat pumps. Numerical results showed a good accuracy of experimental results in terms of optimal subcooling.
TL;DR: In this article, a new energy-efficient refrigeration system subcooled by liquid desiccant dehumidification and evaporation was proposed, which could produce very dry air for an indirect evaporative cooler.
TL;DR: In this article, the effect of wall subcooling on the steam condensation heat transfer coefficients has been researched by experiments when the pressure and the air mass fractions are stable. And the empirical correlation is developed for the heat transfer coefficient which covered all data points within 15%.
TL;DR: In this article, an improved heat flux partitioning model based on first principal models of the underlying physics in determining the active nucleation site density, bubble size and bubble frequency is evaluated for subcooled flow boiling in vertical heated channels at low pressures.
TL;DR: In this article, the authors presented the GAX cycle with maximum internal heat recovery, which is derived from a comprehensive method of pinch technology, where the internal integration is clearly shown in a temperature-heat load diagram.
TL;DR: In this article, the authors focused on minimizing the energy consumption of the pure refrigeration cycle in natural gas liquefaction by improving the subcooling system, and the proposed design was deterministically optimized to find the optimal compressing ratio, temperature, pressure, and flow rate.
Abstract: Natural gas liquefaction is an energy-intensive process in which energy reduction is a main concern. This research focused on minimizing the energy of the pure refrigeration cycle in natural gas liquefaction by improving the subcooling system. To minimize energy consumption, a pure refrigeration cycle with a subcooling system was simulated, and the result was thermodynamically analyzed. The thermodynamic analysis identified an opportunity to reduce the energy consumption, and a new design was proposed for the subcooling system. In addition, the proposed design was deterministically optimized to find the optimal compressing ratio, temperature, pressure, and flow rate. As the result, the optimal operating conditions were determined, and the energy consumption was reduced by 17.74%.
TL;DR: In this article, a multi-nodes heat transient model is proposed and evaluated by the theoretical solutions of the adiabatic processes and the high altitude flight test data and the simulation results show that the airship volume, vertical speed and the solar radiation have evident influence on the ascent subcooling descent superheating effects.
TL;DR: In this article, the effects of using magnetic nanofluid and also applying an external magnetic field on the critical heat flux (CHF) of subcooled flow boiling has been studied experimentally.
TL;DR: In this article, an acid-based surfactant was used to improve the thermal properties of myristic acid/palmitic acid (MA/PA) eutectic phase change material (PCM).
TL;DR: In this article, the authors presented an experimental setup which allows the detailed investigation of condensation induced water hammer (CIWH) in horizontal pipes with 1.4° declination, and conclusions regarding the probability of CIWH, the height of pressure peaks and the position of nucleation sites of steam implosion are presented.
TL;DR: In this paper, the authors proposed a new subcooling method for vapor compression refrigeration cycle based on expansion power recovery, which makes the hybrid system get much higher COP than the conventional VCS refrigeration system.
Abstract: This study proposes a new subcooling method for vapor compression refrigeration cycle based on expansion power recovery. In a main refrigeration cycle, expander output power is employed to drive a compressor of the auxiliary subcooling cycle, and refrigerant at the outlet of condenser is subcooled by the evaporative cooler, which makes the hybrid system get much higher COP. Various refrigerants, including R12, R134a, R22, R32, R404A, R41, R507A, R717, and R744, are considered. Thermodynamic analysis is made to discuss the effects of operation parameters (expander efficiency and inlet temperature of cooling water) on the system performance. Results show that the proposed hybrid vapor compression refrigeration system achieves much higher COP than the conventional vapor compression refrigeration system, conventional mechanical subcooling system and conventional expansion power recovery system, with maximum COP increments 67.76%, 19.27% and 17.73%, respectively when R744 works as the refrigerant in the main refrigeration cycle. It is most beneficial for R12 and R717 in the auxiliary subcooling cycle and R744, R404A and R507A in the main refrigeration cycle.
Patent•
24 Jan 2014
TL;DR: In this article, the first valve controller monitors an outdoor ambient temperature and a pressure of refrigerant exiting the heat exchanger, determines whether the CO2 refrigeration system is in the subcritical mode or in the transcritical mode.
Abstract: A system and method for a CO2 refrigeration system includes a compressor, a heat exchanger, a liquid receiver, a first valve, and a valve controller. The heat exchanger operates as a gas cooler when the CO2 refrigeration system is in a transcritical mode and as a condenser when the CO2 refrigeration system is in the subcritical mode. The first valve controls a flow of refrigerant from the heat exchanger to the liquid receiver. The valve controller monitors an outdoor ambient temperature and a pressure of refrigerant exiting the heat exchanger, determines whether the CO2 refrigeration system is in the subcritical mode or in the transcritical mode, determines a pressure setpoint based on the monitored outdoor ambient temperature, and controls the first valve based on a comparison of the determined pressure setpoint and the monitored pressure when the CO2 refrigeration system is in the transcritical mode.
TL;DR: In this paper, experimental results on the dynamics of boiling-up at stepwise heat release on a horizontally oriented cylindrical surface in a large volume of freon-21 are presented.
Abstract: Results on experimental investigation of the dynamics of boiling-up at stepwise heat release on a horizontally oriented cylindrical surface in a large volume of freon-21 are presented. Experimental data on the propagation velocity, structure, and other local characteristics of development of self-sustained evaporation fronts at different temperature differences of boiling-up in saturated liquid were obtained. New experimental results on the dynamics of vapor phase incipience and evolution on the surface of a vertical heat releasing tube and on the dynamics of changing the heater temperature and pressure in a flow of liquid (water, ethanol) subcooled to saturation temperature in the channel under nonstationary heat release conditions are represented. It was revealed that the dependence of the expectation time of intense bubble growth on the water motion velocity is nonmonotonic.
TL;DR: In this article, the potential of using hydrogel particles for simultaneously inhibiting the formation of gas hydrates and preventing their agglomeration after they form was investigated.
Abstract: This study investigates the potential to use hydrogel particles for simultaneously inhibiting the formation of gas hydrates and preventing their agglomeration after they form. The particles were synthesized specifically for this application and swell to a controlled degree in water and remain discrete. A stirred autoclave was used to determine the hydrate onset time, subcooling temperature, initial growth rate, and torque changes during hydrate formation with the water absorbed in the hydrogel particles. The system also included a liquid decane phase which simulates a condensate. In comparison to water (with no particles) + decane mixture, the hydrate onset time was delayed and the subcooling temperature increases. In addition, both the initial growth rate and hydrate fraction were lower for polymer hydrogels. Stable torque is observed for polymer hydrogels while there are sharp increases in torque before the stirrer stops for water + decane mixture, suggesting that the hydrogel particles (and hence hydra...
TL;DR: In this article, the authors performed experiments in subcooled flow boiling of water and ZnO-water nanofluids with different nanoparticle concentrations (0.001 −0.01 −vol%) in horizontal annulus at heat fluxes varying from 100 to 550 kW/m2 and flow rates from 0.1 to 0.175 lps at 1 bar inlet pressure and constant subcooling of 20°C to determine the void fraction by image processing technique.
TL;DR: In this article, the authors investigated bubble confinement and elongation in flow boiling in a rectangular microchannel with 0.5mm in width and 1.0mm in height using DI water as the working fluid.
TL;DR: In this article, the transient thermal energy storage characteristics across an air-saturated enclosure packed with microencapsulated phase change material (MEPCM) particles were examined via experimental measurement and numerical simulation.
TL;DR: In this article, the authors investigated the criteria of thermal stratification created by direct contact condensation (DCC) and determined the criteria for the occurrence of the thermal Stratification according to the Richardson number.