Delamination is one of the most common types of defects for carbon fiber reinforced plastic (CFRP) composites. The application of laser techniques to detect delamination faces difficulties with ultrasonic wave excitation because of its low thermal conductivity. Much of the research that can be found in the literature has only focused on the detection of a single delamination. In this study, aluminum foil was pasted onto the surface of the composite so that it was vulnerable to ablation and could acquire a usable signal. Using a fully noncontact system with laser excitation at a fixed point and a scanning laser sensor, the effects of different aluminum foil sizes and shapes on the wavefield were studied for the composites; we decided to use a rectangle with 3 mm length and 5 mm width for laser excitation experiments. Wavefield characteristics of the composite plates were analyzed with single- and multi-layered Teflon inserts. Taking the time window for standard ultrasonic testing as a reference, the algorithms for localized wave energy with appropriate time windows are presented for the detection of single and multiple defects. The appropriate time window is meaningful for identifying each delamination defect. The algorithm performs well in delamination detection of the composites with one or multiple Teflon inserts.
It is widely believed that the behavior of vapor bubble/blanket over heating surface plays a critical role in determining the critical heat flux (CHF) in the subcooled flow boiling. Various CHF models are based on phenomenon observations of vapor bubble/blanket in the flow channel and use vapor bubble/blanket physical parameters to determine CHF values. In this study, subcooled flow boiling tests were conducted on the experiment facility “Test of External Vessel Surface with Enhanced Cooling” (TESEC). A series of natural circulation subcooled flow boiling CHF experiments is performed in a 30 mm by 61 mm rectangular flow channel with a 200 mm long heated surface along the flow direction at various inclination angles of the test section. With the assistance of high speed video technology, the process of flow boiling in the experiments was recorded and analyzed. A novel image processing method based on a MATLAB code is used to analyze high speed images at 999 frames/second and is able to provide detailed statistical information of vapor behavior on the heating surface. By this process, the static and dynamic information of vapor blanket is obtained at the pre-CHF conditions at various inclination conditions of flow channels (30 to 90 degrees). In addition, the Fast Fourier Transform (FFT) algorithm is used to further analyze the dynamic behavior of the vapor blanket.
It is a hot research field to study the Critical Heat Flux (CHF) occurrence mechanism in boiling heat transfer. Although lots of researchers have studied on it, no unified conclusion has been achieved up to now. The proposed CHF occurrence mechanism is also not widely accepted. Because the void fraction close to the heating surface is larger when the heat flux approaches CHF, it is difficult to make visual observations of the boiling heat transfer on the heating surface. Usually the CHF mechanism is based on certain assumptions, and then confirmed by testing. Therefore, it is meaningful to study the occurrence of CHF by experimental methods. Based on the system design of the test system for CHF in a rectangular channel, an experimental facility was set up. The main test section consists of a rectangular flow channel with the copper heating surface downwards mounted into one of the channel walls. The fluid is deionized water. Fluid subcooling is 15 K. The entire test section is mounted on a rotating arm which can be set at different inclination angles from 0° (horizontal) to 90° (vertical). By adjusting the loop, the natural circulation and forced circulation test conditions can be achieved from the experimental facility. Through test research, visual observations are acquired about the bubble growth characteristics in the rectangular channel in the range of 0 ° to 90 °, and the test data about the CHF is also attained. It is found that under the condition of forced circulation and natural circulation, the CHF values is increased effectively with the improved mass flow rate and the increased inclination angles of the test section. Through visual observation, it is found that in forced circulation conditions, CHF occurs first at the entrance location of the test surface, rather than the maximal heat flux location of the test surface (test surface center) or the maximal local void fraction of the test facility (exit location of the test facility). In the natural circulation conditions, CHF occurs first at the exit location of the test facility. This phenomenon may imply that the mechanisms of CHF are different in forced circulation and natural circulation. In forced circulation, the flow plays a main role. In natural circulation, the local void fraction plays a main role. There are some differences in the experimental phenomena compared with the traditional CHF theory, like the bubble crowding theory and the micro fluid layer theory. Through the experiment research, the complexity of the flow boiling heat transfer was found. The flow and boiling heat transfer affect each other. It is can help us to research the CHF theory in flow boiling heat transfer.
In vessel retention (IVR) is one of the key severe accident mitigation strategies to maintain reactor pressure vessel (RPV) integrity. IVR designs utilize the reactor pressure vessel lower head to contain molten fuel and rely on external reactor vessel cooling (ERVC) to remove decay heat. The capacity of ERVC is limited by the critical heat flux (CHF) of flow boiling on the outside of the reactor vessel surface. Therefore, the determination of critical heat flux (CHF) is crucial to predict whether the adoption of IVR would be successful in mitigating severe accidents. In 1999, Celeta et.al proposed a superheated layer vapor replenishment model. In this model they postulated that CHF would occur when the superheated layer was occupied by the vapor blanket coming into contact with the heated wall and they successfully predicted the critical heat flux in subcooled water flow boiling under high mass flux, high liquid subcooling and low/medium pressure conditions. To evaluate the practicability of this model in predicting CHF under IVR conditions, CHF experiments were performed under natural circulation conditions on the experiment facility “Test of External Vessel Surface with Enhanced Cooling” (TESEC). Experiments are conducted in a 30 mm wide, 61mm high rectangular flow channel with a 200 mm long heated surface along the flow direction. Two quartz windows are installed at the sidewalls of the flow channel for visualization. In order to simulate various positions of the reactor lower head, experiments at different inclination angles of the test section were conducted. The high speed visualization data at CHF point at various inclination angles were processed and analyzed by a MATLAB code developed by the author. The vapor blanket thickness at various inclination angles was measured from the visualization data and was also predicted by the Celeta model. By using geometry data from high speed images, CHF values were calculated by Celeta model and compared with the experimental results at various inclination angles. Limitations of the Celeta model in adaptation of predicting CHF under IVR conditions were further discussed.
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