Zhongning Sun

Fabrication of mineralized electrospun PLGA and PLGA/gelatin nanofibers and their potential in bone tissue engineering

Surface mineralization is an effective method to produce calcium phosphate apatite coating on the surface of bone tissue scaffold which could create an osteophilic environment similar to the natural extracellular matrix for bone cells. In this study, we prepared mineralized poly(D,L-lactide-co-glycolide) (PLGA) and PLGA/gelatin electrospun nanofibers via depositing calcium phosphate apatite coating on the surface of these nanofibers to fabricate bone tissue engineering scaffolds by concentrated simulated body fluid method, supersaturated calcification solution method and alternate soaking method. The apatite products were characterized by the scanning electron microscopy (SEM), Fourier transform-infrared spectroscopy (FT-IR), and X-ray diffractometry (XRD) methods. A large amount of calcium phosphate apatite composed of dicalcium phosphate dihydrate (DCPD), hydroxyapatite (HA) and octacalcium phosphate (OCP) was deposited on the surface of resulting nanofibers in short times via three mineralizing methods. A larger amount of calcium phosphate was deposited on the surface of PLGA/gelatin nanofibers rather than PLGA nanofibers because gelatin acted as nucleation center for the formation of calcium phosphate. The cell culture experiments revealed that the difference of morphology and components of calcium phosphate apatite did not show much influence on the cell adhesion, proliferation and activity.

Flow Characteristics in an Open Two-Phase Natural Circulation Loop

The flow characteristics of an open single-tube (Φ38×2000 mm), two-phase natural circulation (TPNC) loop was investigated, under the condition of constant saturated temperature at the atmosphere (99±0.5°C) at the inlet of heating tube. Five typical circulation modes are observed in the range of experiment heating power (from 2.8 kW to 34.8kW), namely intermittent flashing oscillation, geysering and flashing induced oscillation, flashing boiling eruption and flashing induced oscillation, sustained flashing and steady two-phase flow. It was shown that flashing and geysering or boiling eruption are predominant factors influencing flow characteristics. Moreover, a calculation model is developed to investigate the static flow characteristics of the TPNC system. The calculation results show a good agreement with the experiments. In addition, the calculation results show that the static flow excursion phenomenon may occurs in the open TPNC system, under the higher heating powers.Copyright

Study on the Behaviors of a Conceptual Passive Containment Cooling System

The containment is an ultimate and important barrier to mitigate the consequences after the release of mass and energy during such scenarios as loss of coolant accident (LOCA) or main steam line break (MSLB). In this investigation, a passive containment cooling system (PCCS) concept is proposed for a large dry concrete containment. The system is composed of series of heat exchangers, long connecting pipes with relatively large diameter, valves, and a water tank, which is located at the top of the system and serves as the final heat sink. The performance of the system is numerically studied in detail under different conditions. In addition, the influences of condensation heat transfer conditions and containment environment temperature conditions are also studied on the behaviors of the system. The results reveal that four distinct operating stages could be experienced as follows: startup stage, single phase quasisteady stage, flashing speed-up transient stage, and flashing dominated quasisteady operating stage. Furthermore, the mechanisms of system behaviors are thus analyzed. Moreover, the feasibility of the system is also discussed to meet the design purpose for the containment integrity requirement. Considering the passive feature and the compactness of the system, the proposed PCCS is promising for the advanced integral type reactor.

The Numerical Simulation of Fluid Flow and Heat Transfer in the Pebble Bed

Micro-pebble bed water cooled reactor is a new type of reactor. The characters of fluid flow and heat transfer are very important to the safty of micro-pebble bed water cooled reactor. Build the model of random distribution stacked pebble bed by using disperse element method(DEM). Using the model to simulate the random distribution of fuel pebbles in Micro-pebble bed reactor. By studying the characters of fluid flow and heat transfer in the channel of pebble bed, get its inner parametric regularity of distribution of pressure, velocity, etc. And also get the models heat transfer coefficient when loading the same heat flux as experiment, then contrast the results which fit well with experiment values. Verify the reliability of the model.

Reducing the Flow Distribution in Central-Type Compact Parallel Flow Heat Exchangers Having Optimized Tube Structure

Compact parallel manifolds are widely applied in flow heat exchangers. However, the flow maldistribution exists in the parallel manifolds when the flow is distributed in the header, because of the pressure maldistribution. For a heat exchanger, the uniformity of the flow distribution greatly influence the performance of the heat exchanger. It is significantly important to modify the design of the heat exchanger for a uniform flow distribution. In present study, to reduce the maldistribution in the manifolds, the numerical analysis has been made and a method by varying the insert length of tubes inside the header has been pointed out to solve this problem of flow maldistribution. To illustrate the reliability of this method to reduce the maldistribution, three base cases with different header diameters have been applied to be solved with the method. The results indicates that, by using the method, the maldistribution for three base cases can be reduced by 82% for the case1, 72% for the case2 and 68% for the case3. However, the insert length of tubes inside the header increases more pressure loss. The pressure drop for base cases are respectively increased by 2.83%, 4.83% and 6.46%. This method is proved to be effective under all flow rates.

Measurement for Surface Tension of Aqueous Inorganic Salt

Bubble columns are effective means of filtration in filtered containment venting systems. Here, the surface tension has a significant influence on bubble size distribution and bubble deformation, which have a strong impact on the behavior of the bubble column. The influence of aqueous inorganic compounds on the surface tension depends on the electrolytic activity, Debye length, entropy of ion hydration and surface deficiencies or excess. In this work the surface tensions of same specific aqueous solutions have been measured by different methods including platinum plate method, platinum ring method and maximum bubble pressure method. The measured surface tensions of both sodium hydroxide and sodium thiosulfate are less than that of water. As solution temperature ranges from 20 ℃ to 75 ℃, the surface tension of 0.5 mol/L sodium hydroxide solution decreases from 71 to 55 mN/m while that of 1 mol/L solution decreases from 60 to 45 mN/m. Similarly during the same temperature range, the surface tension of 0.5 mol/L sodium thiosulfate decreases from 70 to 38 mN/m and that of 1 mol/L sodium thiosulfate is between 68 and 36 mN/m. The analysis for the influence mechanism of aqueous inorganic on surface tension is provided. In addition, experimental results show that the surface tension of solid aerosol suspension liquid has no obvious difference from that of distilled water.

Experimental Study on Decontamination Factor of Methyl Iodide by Bubbling

Once a serious event occurs, plenty of gas should be discharged into environment, so it is in urgent need to filter radioactive material before the ventalition. Bubble columns which belong to a kind of wet filters have a potent ability of filtering methyl iodide. The efficency of filtration depends on interface area between liquid and gas, liquid mass transfer rate and retention time of bubbles. Furthermore the decontamination factor of methyl iodide increases with temperature, pressure and liquid level, but decreases with superficial velocity. The experimental condition in this paper is: maximum pressure 0.5 MPa and maximum temperature 150°C, superficial speed 0.2m/s, gas composed of air and steam, liquid level 0.1–2m. The relationship between DF and experiment condition is deep discussed.Copyright

Simulation Analysis of Condensation Heat Transfer Inside C-type Tubes Based on RELAP5/MOD3.2

The RELAP5/MOD3.2 code was validated experimentally for simulating the condensation heat transfer characteristics inside C-type tubes of the passive residual heat removal heat exchanger. In order to eliminate the effects of the boiling model out of tubes when simulating condensation, the C-type tube condensation simulation, in which the wall temperature was given, was performed using RELAP5/MOD3.2 code. Comparing the simulation results with the experimental data, it is found that the maximum relative deviation between the experimental and calculated condensation heat transfer coefficients exceeds 80% in the range of experimental date, and the change tendency of the average condensation heat transfer coefficient is obviously different with the increase in the outlet condensate Reynolds. RELAP5/MOD3.2 code uses Chato model to calculate the condensation of the horizontal part and uses the Nusselt model to calculate the condensation of the vertical part when simulating the C-type condensation experiment in the range of experimental parameters. The results show that the standard code in RELAP5/MOD3.2 cannot give completely reliable predictions when simulating the condensation heat transfer characteristics inside the C-type tubes.

The Surge Line Stress Analysis Model Setup With the Consideration of Thermal Stratification

In this paper, we deal with a typical pressurizer surge line in a conventional pressurized water reactor (PWR). This study is performed to develop an understanding of thermal stratification phenomenon, which may occur in the surge line during either normal condition or transient process, in the pressurizer surge line. The pressurizer surge line model of Daya Bay nuclear power plant is used as base analysis model, in which the hot leg is taken into account. The transient temperature distribution required to assess the phenomenon along the pressurizer surge line is obtained through CFD analysis technology using ANSYS FLUENT. The temperature loads are transferred to ANSYS Mechanical for stress evaluation for the heat up transient process. Subsequently, the usage factor is calculated on the basis of ASME Section-III design curve. The possible mitigation scheme for the thermal stratification phenomenon of changing the layout angles is also simulated and analyzed in detail. The results show that the thermal stratification phenomenon will occur both in normal operating condition and in heat up transient process. The circumfluent effect makes the thermal stratification phenomenon exhibit unique profile due to the introduction of the hot leg. The continuous spray mass flow rate may influence both the temperature difference and the occurrence range for the thermal stratification phenomenon. The stress analysis incorporating both temperature load and pressure load is performed for pressurizer surge line model with hot leg for the conservative and complete heat up case.Copyright

Subcooled Flow Boiling Heat Transfer in Volumetrically Heated Packed Bed

The volumetrically heated packed bed has been widely utilized in modern industry, however, due to the variability and randomness of packed bed channels, flow boiling heat transfer characteristics is complex. To study subcooled flow boiling heat transfer characteristics of volumetrically heated packed bed, here electromagnetic induction heating method was used to heat the oxidized carbon steel balls adopted to stack packed bed, while water was utilized as refrigerant in the experiment. The mass flux and heat flux effects on the subcooled flow boiling heat transfer were analyzed. A new correlation was developed to predict the subcooled flow boiling heat transfer coefficient in volumetrically heated packed bed.Copyright