The influence of anionic surfactant in water on the quenching process

Understanding the patterns of heat transfer during quenching is important for many technical applications. Of particular interest is the boiling regime, which is characterized by high intensity and occurs at surface temperatures exceeding the temperature of attainable liquid superheat. This work is aimed at studying the effect of surfactants on the onset of intense heat transfer during quenching. For this, experiments were carried out on quenching spheres made of different metals (nickel, stainless steel and zirconium) in water with different concentrations of surfactants. The surfactant was alkylbenzene sulfonate, the concentration of which varied from 0.1 to 2%. The analysis of the obtained cooling thermograms revealed the influence of not only the surfactant concentration on the beginning of the intensive cooling mode, but also the state of the heat transfer surface.

Effect of 2024 Al Alloy Insert on the Grain Refinement of a 2024 Al Alloy Prepared via Insert Mold Casting

In this study, an insert mold casting was fabricated by inserting 2024 Al extruded rods into a 2024 Al melt. The molds were kept at a 2024 Al melt for different times. The 2024 Al extruded rods were used to refine the 2024 Al alloy grains because the advantage of this method is that it is contamination free compared with other grain refiners. Moreover, we investigated the macro and microstructure of the ingots. Further, we analyzed the refinement mechanism of the 2024 Al rod on the 2024 Al alloy. Our result showed that when the immersion time of the 2024 Al insert was 0 s, a metallurgical bonding was partly formed between the 2024 Al insert and the 2024 Al alloy mold cast. When the immersion time of the 2024 Al insert increased to 5 s, the 2024 solid insert was dissolved in the liquid; the coarse dendritic grains were replaced by fine equiaxed grains. The refinement mechanism for the insertion of a 2024 Al rod on the 2024 Al alloy was to melt the 2024 Al insert and have it decrease the degree of the liquid superheat, which thus increased the cooling rate and provided a large number of small particles that acted as the nucleus of heterogeneous nucleation. However, these particles were melted gradually in the high-temperature liquid after an increase of immersion time. Thus, the refinement effect of 2024 Al insert on the solidified structure was weakened.

Study on Flushing Phenomena by Microwave Heating

Recently, the number of nuclear power plants has been increased in many countries. In contrast, uranium fuels used in nuclear power plants are exhaustible resources. Therefore, it is required to exploit uranium resources effectively, and reprocessing of spent fuel is indispensable. To use recovered uranium and plutonium as raw material of nuclear fuel, reprocessing solution (uranium and plutonium mixed nitrate solution) of the spent nuclear fuel is converted to uranium and plutonium mixed oxide (MOX) powder. Microwave heating direct denitration method (MH method) is one of such methods to convert nitrate solution to MOX powder. The cylindrical denitration vessel can be expected to realize high-speed and high-capacity processing against traditional shallow vessel. However, flushing and overflow phenomena of solution have been confirmed in cylindrical vessel. Thus, the safety and the optimization of the vessel shape during microwave heating. In the present study, the purpose of this paper is to clarify generation conditions and generation mechanism of flushing phenomena that is not fully understood. In experiment, flushing phenomena was observed and the liquid temperature was measured using microwave heating device. The main parameters are the vessel diameter, initial water level and the magnetron power. There was tendency of flushing in the case of short vessel diameter and high initial water level when magnetron power was constant. There was also tendency of flushing in the case of large magnetron power when vessel diameter and initial water level was constant. From visualization, it was clarified that generation of singular bubble triggers flushing. If flushing occurs, the liquid is blown up at a burst, and the vessel become almost empty. From temperature measurement results, it was clarified that the liquid had over 10 °C superheat just before flushing. Therefore, it was suggested that the liquid superheat affected flushing significantly. Generation conditions of flushing are different with the vessel diameter, initial water level and the magnetron power because it is considered that these characteristics have influence on the liquid superheat.

Bubble Dynamics and Boiling Heat Transfer in Microsystems

Understanding of micro boiling systems requires multiscale modeling, linking nanoscale fluid-surface interactions and micrometer channels. We present a multiscale model, successfully used for analysis of boundary conditions in microchannels. Slip lengths are found to be mainly dependent on the surface-fluid interactions, weakly on the channel sizes from nanometer to micrometer. The multiscale model is expected to be extended for bubble dynamics in microsystems, considering wall surface roughness, nano bubbles tripped in the cavities of the surface, etc. The second part gives a review on the micro bubble dynamics of pool boiling under pulse heating conditions. The third part reviews boiling heat transfer in silicon microchannels. Bubbles are being nucleated in the channel corners. Flow patterns are repeated in millisecond time-scale. Explosive boiling was found to be triggered by the higher liquid superheat, pushing liquid plugs out of microchannels. Depending on boiling numbers, three distinct heat transfer regions are identified. Heat transfer displays the nucleate boiling behavior at medium boiling numbers, and the convective heat transfer one at higher boiling numbers. The available heat transfer correlations over-predict the heat transfer performance in silicon microchannels, due to lack of nucleation sites in smooth silicon microchannels.