An experimental study on the subcooled flow boiling pressure drop of R141b in the system of two microchannels

An experimental study of the pressure drop under subcooled flow boiling of the refrigerant R141b in a system with two slotted microchannels was carried out. A copper block with two microchannels 2 mm wide, 0.4 mm deep, and 16 mm long was used as an experimental section for testing. The mass flow rate varied in the range from 1 to 4 g/s, the initial subcooling from 20°C to 50°C. Experimental data show a significant decrease in the pressure drop when the critical heat flux is reached. The experimental data are compared with the model known from the literature. Experimental data show that the occurrence of nucleate boiling incipience at subcooled boiling corresponds to a larger heat flux than that given by the recommended correlation.

The heat transfer and instabilities results during the onset of flow boiling in minichannels

The paper discusses the results of flow boiling heat transfer in minichannels obtained on the basis of time-dependent experiments. The main interest of the work was to investigate the occurrence of the accompanying instabilities during the boiling incipience. The essential part of the experimental standwas a test section with two minichannels, each of 1.7 mm depth. The heated element for FC–72flowing along the minichannels was a thin foil. In the tested minichannel, the temperature of the outer surface of the foil was measured due to thermoelements. The onset of flow boiling in minichannels was induced by increasing the heat flux supplied to the heater. The main aims of the investigation were to determine the heat transfer coefficient by means of the FEM with time—dependent Trefftz–type basis functions based on the Hermite interpolation and to recognize dynamic instabilities during boiling incipience. The results were illustrated as: the heat transfer coefficient, the mass flow rate and the inletpressure versustime and as boiling curves.

Effect of Wettability on Pool Boiling Incipience in Saturated Water

Three different copper surfaces - bare, Al2O3 nano-coated, and Polytetrafluoroethylene (PTFE) coated - are prepared and tested to examine the effect of wettability on the pool boiling incipience in saturated water at 1 atm. A copper surface is coated with Al2O3 particles ranging 25~43 nm in diameter by immersing the surface in Al2O3/ethanol nanofluid (1g/l) and boiled for 3 min. SEM image in Fig. 1 shows the coated Al2O3 nanoparticles on the copper surface, together with the reference bare surface. PTFE coating is also applied to the copper surface using spin coating method with the mixture of Dupont AF 2400 particles and 3M FC-40 solvent. The final coating thickness of the PTFE coating is estimated to be 30 nm. The three surfaces exhibit different static contact angles, 78° (bare), 28° (nano-coated), and 120° (PTFE coated) in Fig. 2, respectively. Wettability affects the boiling incipience heat flux where initial bubble nucleation starts: 15 kW/m2 for the bare surface; 30 kW/m2 for the nano-coated surface; and 2.5 kW/m2 for the PTFE coated surface. Captured images from the high speed camera at 2,000 fps show significantly different bubble shapes and departure frequencies in Fig. 3. During the bubble growth, advancing contact angles are captured and shown qualitatively and found consistent with their static angle measurements for the sessile droplet observed at each surface. The larger bubble is generated on the nano-coated surface compared to that of the bare surface because improved wetting makes promising cavities flood and thus incipience is delayed, resulting in higher superheat. The single bubble life cycle appears to be much longer on the PTFE coated surface due to the increase of the contact angle which becomes hydrophobic (> 90°), resulting in lower bubble departure frequency. Successive tests at the same heat flux of 30 kW/m2 confirmed that life cycle on the PTFE coated surface (88.5 ms) is consistently longer than that on the bare surface (16.5 ms) and nano-coated surface (20 ms).

Experimental Investigation on Boiling Phenomena of Bi-Layer Composite Porous Wicks Textured With Nano-Porous Layer

Miniature loop heat pipes (mLHP) are envisioned as one of the next generation electronic cooling technologies. They are closed loop, phase-change devices where the working fluid evaporates during heat addition and its flow is maintained by capillary forces developed inside the porous wick lining the evaporator. While they have many advantages such as high heat flux rates and heat rejection far from the heat source, potential problems are often associated with bubble nucleation and boiling incipience in the porous wicks. Bi-layer composite porous wicks consisting of a nano-porous layer textured onto the traditional micro-porous material are thought to possess enhanced capillary wicking, which could benefit mLHP applications. In this context, it is important to also understand their boiling characteristics. Therefore, a boiling heat transfer testing apparatus was developed and used to characterize the boiling incipience of bi-layer porous wicks. These results may guide material selection in the design of mLHP evaporators employing bi-layer porous wicks.