In this paper, the water droplet erosion (WDE) performance of a typical martensitic blade material 17-4 PH (0Cr17Ni4Cu4Nb), surface strengthened laser cladding stellite and brazed stellite alloy is studied based on hi...
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In this paper, the water droplet erosion (WDE) performance of a typical martensitic blade material 17-4 PH (0Cr17Ni4Cu4Nb), surface strengthened laser cladding stellite and brazed stellite alloy is studied based on high-speed water jet test system. The WDE resistances from high to low are the brazed stellite alloy, laser cladding stellite alloy and 17-4 PH. For the same material, the greater the initial roughness of the target surface, the more severe is the WDE damage. Improving the surface finish of the target is conducive to the diversion and expansion of the liquid flow, prolonging the incubation period, and thus enhancing the WDE resistance.
Free surface vortex, especially in a pattern of full air core vortex, causes serious security problems in the residual heat removal system. Vortex in a T-junction pipe system was numerically investigated under the pre...
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Free surface vortex, especially in a pattern of full air core vortex, causes serious security problems in the residual heat removal system. Vortex in a T-junction pipe system was numerically investigated under the premise that the simulation results are in agreement with the experimental results. First, four typical states were obtained in the formation and evolution process of vortex, namely, surface dimple, critical submergence state, air entrainment vortex state, and large air entrainment state. With increasing Froude (Fr) number, the time from an initiate flow field to the critical submergence state decreased, and the time interval between critical submergence state and large air entrainment state increased. Finally, the variation law of vortex intensity was analyzed during the evolution process. The vortex intensity initially increased with time and reached its maximum value and then decreased until it reached a minimum value before the large air entrainment state. With the increase of Fr, the maximum values of vortex intensity gradually increased.
flow and heat transfer of supercritical water in the vertical helically-coiled tube were investigated numerically using RNG k-epsilon model. The inner side of the coil was heated by constant heat flux and the outer si...
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flow and heat transfer of supercritical water in the vertical helically-coiled tube were investigated numerically using RNG k-epsilon model. The inner side of the coil was heated by constant heat flux and the outer side was adiabatic. Heat transfer performance under half-side heating is obtained and compared with that under uniform heating at the same heat transfer rate. Effect of specific heat, secondary flow velocity and buoyancy are discussed. The decrease of specific heat in the near-wall region, and the attenuated turbulence caused by the strong buoyancy force lead to a decrease in heat transfer for half-side heating. The deviations of heat transfer correlations on predicting half-side heating condition are evaluated, and a new heat transfer correlation for supercritical water in the vertical helically-coiled tube under inner half-side heating is proposed. The investigation into different statistical parameters shows that it is superior to the existing correlations.
Cryogen spray cooling (CSC) is commonly applied in laser dermatology to protect the epidermis from thermal damage. Many efforts have attempted to improve the cooling capacity of CSC, among which the use of expansion-c...
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Cryogen spray cooling (CSC) is commonly applied in laser dermatology to protect the epidermis from thermal damage. Many efforts have attempted to improve the cooling capacity of CSC, among which the use of expansion-chambered nozzles is an effectively simple method with considerable potential. This study examined the influences of the expansion-chambered nozzle structure, including the ratios of inlet nozzle diameter to discharge nozzle diameter and of chamber diameter to discharge nozzle diameter on R134a and R404A spray cooling. Fifteen transparent expansion-chambered nozzles with the expansion chamber aspect ratio of 1.0, chamber diameter to discharge nozzle diameter ratios of 5.0-10.0, and inlet nozzle diameter to discharge nozzle diameter ratios of 0.6-1.4 were tested. The internal flow pattern inside the expansion chamber, external spray pattern, and surface heat transfer characteristics of cryogen spray using different nozzles, including the straight-tube nozzle, were investigated. The structure of the expansion chamber was found to have an important effect on the spray patterns and cooling characteristics. The spray radius obviously decreased when the expansion-chambered nozzles were used, and the spray pattern became narrower as the ratio of chamber diameter to discharge nozzle diameter increased. By contrast, the increase in ratio of two nozzle diameters enlarged the spray radius. Surface temperature and heat flux with different nozzles and cryogens showed a similarity, and correlations of surface temperature and heat flux were proposed. The introduction of expansion chambered nozzles could effectively improve the spray cooling capacity. The minimum average surface temperature during the fully developed spray period could be reached for both R134a spray and R404A spray by an expansion-chambered nozzle with a chamber diameter to discharge nozzle diameter ratio of roughly 5.0 and an inlet nozzle diameter to discharge nozzle diameter ratio of roughly 0.6. (C)
This paper focuses on the dynamic modeling of the cold end system of a thermal power plant and its operation optimization during cycling load processes. A revised logarithm mean temperature difference is recommended t...
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This paper focuses on the dynamic modeling of the cold end system of a thermal power plant and its operation optimization during cycling load processes. A revised logarithm mean temperature difference is recommended to calculate the heat transfer quantity between exhaust steam and condenser tubes. With this method, a condenser model is developed with a maximum relative error limited within 3%. Coupled with a turbo-generator, a condenser, water pumps, and a cooling tower, a closed cooling system model is established. The total power supply (TPS) during cycling load is calculated and analyzed to optimize the operation during transient processes. A method for obtaining the maximum value of TPS is provided and used to calculate the optimal operating load rate points for switching pumps (OPSP). OPSP increases with the cycling load rate (V-e) during loading up processes decreases with V-e during loading down processes. When V-e is identical, OPSP declines with the ambient temperature for the loading up and down processes. TPS during transient processes is relevant with the switching pump load rate. The maximum value of difference in TPS with different switching pump periods is 691.4 (kW h) for switching pump numbers from 3 to 2 during loading down processes. (C) 2017 Elsevier Ltd. All rights reserved.
Cryogen spray cooling (CSC) has been a widely used auxiliary tool in laser dermatology such as port wine stain to prevent unspecific thermal injury due to laser energy absorption by the melanin in the epidermis. The p...
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Cryogen spray cooling (CSC) has been a widely used auxiliary tool in laser dermatology such as port wine stain to prevent unspecific thermal injury due to laser energy absorption by the melanin in the epidermis. The present paper presents an experimental research on the effect of ambient temperature, relative humidity, and initial substrate temperature on heat transfer performances during R134a spray cooling. Results demonstrated that the cooling capability of R134a spray cooling can be obtained with small ambient temperature (T-a = 10 degrees C) and relative humidity (RH = 25%). Further investigation of cooling mechanism was conducted by studying the temporal and radial heat transfer distributions with different spray distances and nozzles. The heat transfer distribution presented large non-uniformity along radial locations. Two uniform cooling sub-regions of 0 <= r < 2 mm and 6 mm <= r < 10 mm were found under the spray distance of 30 mm and nozzle with an inner diameter of 1.0 mm. The heat transfer barrier was produced due to indirect contact between cold droplets and the substrate surface caused by bubbles and heat transfer is weakened by the low thermal conductivity of these bubbles.
In present study, an experimental system for circulatory flash evaporation was designed and built. Energy and exergy analyses of circulatory flash process were carried out. Basing on the first law of thermodynamics, t...
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In present study, an experimental system for circulatory flash evaporation was designed and built. Energy and exergy analyses of circulatory flash process were carried out. Basing on the first law of thermodynamics, the gained output ratio of circulatory flash evaporation (GOR(CFE)) was defined as an energy ratio between latent heat carried away by flash steam and input excess energy of working fluids. The performance ratio of circulatory flash evaporation (PRCFE) was defined as the mass flow rate divided by exergy destruction rate. The Effects of main factors, namely, superheat degree, circulating flow rates, equilibrium pressure, initial water level and salinity on GOR(CFE) and PRCFE were studied. Results suggested that GOR(CFE) decreased rapidly at the initial stage, then increased with the increase of superheat degree and became flat eventually. The optimal superheat degree was determined wherein the maximum performance ratio was reached. According to the comparison of gained output ratio and performance ratio, the maximum performance ratio was obtained when gained output ratio was great enough and temperature difference was minimal.
In this work, a comprehensive experimental investigation on spray characteristics of four blended fuels, including diesel (D100), diesel-biodiesel (DB), diesel-biodiesel-ethanol (DBE), and diesel-biodiesel-diethyl eth...
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In this work, a comprehensive experimental investigation on spray characteristics of four blended fuels, including diesel (D100), diesel-biodiesel (DB), diesel-biodiesel-ethanol (DBE), and diesel-biodiesel-diethyl ether (DBDE) has been conducted by using high pressure common rail injection system (up to 200 MPa). The transient spray behavior under various conditions was recorded by high speed photography with scattering light illumination. It is shown that higher injection pressure significantly accelerates the spray tip penetration (STP) evolution due to increased inertia of spray while increase in ambient pressure reduces the STP evolution due to higher gas resistance. With the addition of diethyl ether (DEE) into biodiesel, the STP of blended fuel tends to go down and corresponding projected area increases a lot when compared to DB. By means of particle droplet image analysis (PDIA) optical diagnostic method, spray microscopic parameters such as Sauter Mean Diameter (SMD), droplet diameter distribution probability curve, cumulative volume curve and characteristic diameter have been investigated. Results show that both the injection pressure and ambient pressure have significant influence on the spray microscopic characteristics. In addition, for fixed injection pressure and ambient pressure, when DEE is added into DB blends, the number fraction of smaller droplets increases, though the statistic diameter with peak probability is fixed at a certain value. Furthermore, SMD of the four tested fuels decreases sequentially in the order of DB, D100, DBE, and DBDE, indicating that DEE addition favors the atomization process.
Bubble evolution from photo(electro) catalytic water splitting plays a vital role in the interfacial mass transport on photocatalyst surface. However, little success has been achieved to optimize this process, restric...
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Bubble evolution from photo(electro) catalytic water splitting plays a vital role in the interfacial mass transport on photocatalyst surface. However, little success has been achieved to optimize this process, restricted by the poor understanding. Herein, taking photoelectrochemical (PEC) water splitting over a titanium dioxide (TiO2) nanorod-array electrode as a model system, experiments were performed to study single oxygen bubble dynamics by combining electrochemical measurement and high-speed microscopic imaging. The experimental results indicate that the departure of bubble from photo-electrode is retarded by light irradiation, but the traditional bubble departure criterions fail to predict the bubble departure diameters especially in high light intensity. Additional analysis reveals that the light irradiation causes the Marangoni force acting on the evolving bubble, because it induces temperature rise and generates dissolved gas. A modified force balance model for bubble departure from photoelectrode was developed by adding Marangoni force. This modified model that takes account of the light-induced temperature rise and the dissolved gas, agrees well with the experimental data and can be extended to other photo(electro) catalytic reactions. (C) 2018 Elsevier Ltd. All rights reserved.
Due to limited studies on heat transfer of supercritical CO2 (S-CO2) at low mass flux, an experimental study was conducted on the High-TaP-SCO2 test loop to study heat transfer of S-CO2 in a 16-mm diameter tube with l...
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Due to limited studies on heat transfer of supercritical CO2 (S-CO2) at low mass flux, an experimental study was conducted on the High-TaP-SCO2 test loop to study heat transfer of S-CO2 in a 16-mm diameter tube with low mass flux. Fundamental data were collected at a pressure range of 7.5-10.5 MPa, mass flux of 50-500 kg/m(2)s and heat flux of 5-100 kW/m(2). The effects of heat flux, mass flux, and pressure on heat transfer at low mass flux were analyzed. Results showed that, with increasing heat flux, a completely different heat transfer phenomenon was observed at lower mass flux (G < 300 kg/m(2)s) compared to a normal mass flux (G >= 300 kg/m(2)s). Heat transfer at lower mass flux was not deteriorated but rather enhanced with a rising heat transfer coefficient, which is about 2.6 times higher than single-phase convective heat transfer. With mass flux decreasing from 400 to 100 kg/m(2) s, heat transfer was enhanced about 5 times and turned from deterioration to enhancement. The mechanism of this heat transfer transition was further discussed. Results suggested that the special heat transfer enhancement at lower mass flux was mainly induced by the combined effects of strong buoyancy and high c(p) fluid. Based on the present dataset, existing Nusselt correlations for supercritical fluids were reevaluated, and these correlations failed in capturing the heat transfer enhancement occurring at low mass flux. Finally, a modified correlation for the heat transfer of S-CO2 at low mass flux was proposed within +/- 20% error and is available for the design of relevant devices with S-CO2. (C) 2018 Elsevier Ltd. All rights reserved.
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