Search Results (3)

This paper presents an experimental characterization of liquid nitrogen (LN₂) flow boiling in additively manufactured minichannels. There is a pressing need of concerted efforts from the space exploration and thermal transport communities to design high-performance rocket engine cooling channels. A close observation of the literature gaps warrants a systematic cryogenic flow boiling characterization of asymmetrically heated small (<3 mm) non-circular channels fabricated with advanced manufacturing technologies at mass flux > 3000 kg/m²s and pressure > 1 MPa. As such, this work presents the LN₂ flow boiling results for three asymmetrically heated additively manufactured GR-Cop42 channels of 1.8 mm, 2.3 mm, and 2.5 mm hydraulic diameters. Twenty different tests have been performed at mass flux~3805–14,295 kg/m²s, pressures~1.38 and 1.59 MPa, and subcooling~0 and 5 K. A maximum departure from nucleate boiling (DNB)-type critical heat flux (CHF) of 768 kW/m² has been achieved for the 1.8 mm channel. The experimental results show that CHF increases with increasing LN₂ flow rate (337–459 kW/m² at 25–57 cm³/s for 2.3 mm channel) and decreasing channel size (307–768 kW/m² for 2.5–1.8 mm channel). Finally, an experimental DNB correlation has been developed with 10.68% mean absolute error. Full article

The heat transfer process is an important part of energy utilization and conversion, and boiling heat transfer is one of the most significant and effective heat transfer modes in use. Enhancing boiling heat transfer can directly improve energy use efficiency and promote the sustainable development of the energy industry. Surfaces with mixed wetting topologies have been proven to possess the potential to enhance boiling heat transfer. However, the heat transfer promoting mechanism of these types of surfaces requires further clarification on actual heat exchanger surfaces with macroscale heat transfer enhancement structures, such as millimeter-scale pillars. In this study, the boiling heat transfer enhancement mechanism and the performance of the hybrid wetting surfaces with an array of macropillars were explored using both experimentation and numerical simulation. In the experiment, the single bubble growth dynamics at the onset sites of nucleation of these hybrid wetting surfaces in the initial boiling stage were recorded using a CCD camera with a top view. The boiling heat transfer coefficient was also measured at the stable boiling stage. Within the entire tested range of heat flux (3.75–18 W/cm²), the hybrid wetting surfaces significantly enhanced the boiling heat transfer, and the HPo(bottom)–HPi(top) surface (surf-2) exhibited the best heat transfer performance. At the representative heat flux 12.5 W/cm², the boiling heat transfer coefficient of the HPo (bottom)–HPi (top) surface (surf-2) and the HPi (bottom)–HPo (top) surface (surf-3) were more than 33% and 18% higher than the pure copper flat surface, and more than 16% and 3% higher than the uniform HPi surface (surf-4), respectively. On the one hand, due to the view field of camera being blocked by the fiercely growing bubbles in the stable boiling stage, it was difficult to record bubble numbers and gather statistics at the onset sites of nucleation in order to correlate the bubble dynamics with the mechanism of boiling heat transfer enhancement. On the other hand, the single bubble growth dynamics recorded during the initial boiling stage lacked information about the hybrid wetting surfaces in the vertical cross-sectional plane. Therefore, a two-dimensional VOF-based numerical simulation was adopted to supplement the contribution of hybrid wetting surfaces in the vertical plane. The simulation results indicated that the hybrid wetting surfaces with macropillars can inhibit bubble overgrowth and accelerate bubble departure compared with spatially uniform hydrophobic surface. The bubble radius and departure time on surf-2 were smaller than those on surf-3. These are believed to be the reasons why the surf-2 surface exhibited the best heat transfer performance in the experiment. Both the experiment and numerical analysis proved that the hybrid wetting surfaces with macroscale pillars can promote the boiling heat transfer, thus demonstrating potential applications in actual horizontal or vertical tube boiling heat exchangers. Full article

The pseudopotential Lattice Boltzmann Method has attracted much attention in the recent years for the simulation of boiling heat transfer. Many studies have been published recently for the simulation of the bubble cycle (nucleation, growth and departure from a heated surface). This paper puts forward two-dimensional simulations of bubble nucleation, growth and departure using an improved pseudopotential Lattice Boltzmann Model from the literature at different reduced temperatures, $T_r=0.76$ and $T_r=0.86$. Two different models using the Bhatnagar–Gross–Krook (BGK) and the Multiple-Relaxation-Time (MRT) collision operators with appropriate forcing schemes are used. The results for pool boiling show that the bubbles exhibit axial symmetry during growth and departure. Numerical results of departure diameter and release period for pool boiling are compared against empirical correlations from the literature by varying the gravitational acceleration. Reasonable agreement is observed. Nucleate boiling trends with heat flux are also captured by the simulations. Numerical results of flow boiling simulations are compared by varying the Reynolds number for both reduced temperatures with the MRT model. It was found that the departure diamenter and release period decreases with the increase of the Reynolds number. These results are a direct effect of the drag force. Proper conclusions are commented at the end of the paper. Full article