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A study of nucleate boiling phenomena on nano/microstructures is a very basic and useful study with a view to the potential application of modified surfaces as heating surfaces in a number of fields. We present a detailed study of boiling experiments on fabricated nano/microstructured surfaces used as heating surfaces under atmospheric conditions, employing identical nanostructures with two different wettabilities (silicon-oxidized and Teflon-coated). Consequently, enhancements of both boiling heat transfer (BHT) and critical heat flux (CHF) are demonstrated in the nano/microstructures, independent of their wettability. However, the increment of BHT and CHF on each of the different wetting surfaces depended on the wetting characteristics of heating surfaces. The effect of water penetration in the surface structures by capillary phenomena is suggested as a plausible mechanism for the enhanced CHF on the nano/microstructures regardless of the wettability of the surfaces in atmospheric condition. This is supported by comparing bubble shapes generated in actual boiling experiments and dynamic contact angles under atmospheric conditions on Teflon-coated nano/microstructured surfaces.

An analytical method was developed to predict the thermal parameters of the VVER-1200 pressurized water reactor (PWR). The peak temperatures of the fuel, outer fuel surface temperature, inner cladding surface temperature, outer cladding surface temperature, and coolant temperature were evaluated along and across the fuel rod, assuming that the hottest fuel rod is located at the reactor core center. This method analyzed the steady-state temperature distributions and was validated against a corresponding steady-state and time-dependent COMSOL Multiphysics model. Additionally, validation was conducted using the VVER-1200 PCtran Simulator for peak fuel temperature and the departure from nucleate boiling ratio (DNBR). The analytical method was employed to increase the reactor’s rated power from 100% to approximately 106% to assess the peak fuel temperature and the minimum departure from nucleate boiling ratio (MDNBR), both of which are crucial safety parameters for the operation of the VVER-1200. Given that the VVER-1200 initiates a scram slightly above 106% rated power, we evaluated the peak fuel temperature and DNBR at 100%, 102%, 104%, and 106% rated powers to determine the safety limits for both peak fuel temperature and MDNBR. The results from both the analytical method and simulations were found to be comparable.

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.

The paper presents the experimental results on transient nucleate boiling on the heater surface with rapidly increasing surface temperature. According to the results of high-speed video recording with a frequency of 180 000 frames per second and a spatial resolution of 5.5 urn per pixel, the input data for existing models of heat transfer during nucleate boiling must be refined to take into account the existence of cluster and pulsating bubbles. It has been established that bubbles, interacting through the exchange of momentum, heat and vapor mass, accelerate activation of neighboring vaporization sites, so the clusters of bubbles can form at the initial stage of covering the heater surface with vapor. The main characteristics of single, cluster and pulsating bubbles have been studied for the wall superheating from 0 to 14 K above the temperature of nucleation beginning and flow subcooling from 23 to 103 K.

In subcooled boiling flows beyond a certain heat flux, heat transfer is hampered due to a phenomenon known as Departure from Nucleate Boiling (DNB). Conducting DNB experiments at one-to-one nuclear reactor operating conditions is highly challenging and expensive. Another alternative approach is to use Look-up table data. However, its applicability is limited due to its dependence on rod bundle correction factors. In the present investigation, a state-of-the-art Eulerian-Eulerian two-fluid model coupled with an extended heat flux partitioning model is used to predict DNB in tubes and rod bundles with square and hexagonal lattices (relevant to Pressurized Water Reactors). In this approach, bubble departure characteristics are modeled using semi-mechanistic models based on force balance analysis. The predicted DNB values are compared with experimental and Look-up table data and found out to be within 1.8% to 20%.

In case of dropwise evaporation process, the significant reduction of the Leidenfrost effect has been a challenging task for the current generation research due to the counter current characteristics of drag and inertial forces. The alignment of the above stated forces in one direction reduces the Leidenfrost effect significantly as the vapour flow and droplet impingent occur in the same direction. In case of spray cooling from very high initial temperatures, this is achieved by changing the orientation of spray from the downward direction to upward direction. Furthermore, the upward spray is enhanced by altering the thermo physical properties in the favourable direction of heat transfer. In the current work, benzene is added as an additive for attainment of the above stated. The thermal analysis clearly ensures the enhancement in case of benzene added water upward spray, and the achieved average heat flux is 1.78 MW m −2 . The discussed value is almost 1.3 times higher than the heat flux obtained in case quenching is conducted by downward spray. The theoretical calculation supports the information discussed above. It indicates that in case of upward spray, the contact area and spreading factor are higher than the down case. After the cooling operations, the utilised coolant analysis indicates that the found to (Fe) Total and TDS levels augment. Therefore, the utilised coolant should be treated to achieve lower TDS level before reutilisation or exposure to the open environment.

The ability to predict critical heat flux (CHF) is of considerable interest for high-heat equipment, including nuclear reactors. CHF prediction from a mechanistic model for subcooled flow boiling in rod bundles still remains unsolved. In this paper, we try to predict the CHF in an annulus, which is the most basic flow geometry simplified from a fuel bundle, using a liquid sublayer dryout model. The prediction is validated with both water and R113 data, showing an accuracy within ±30%. After the CHF in an annulus is calculated successfully, a near-wall vapor–liquid structure is proposed on the basis of the liquid sublayer dryout model. Modeling of heat transfer modes over the heating surface at CHF is performed, and predictions of the changes in liquid sublayer thickness and heater surface temperature at the CHF occurrence point are carried out by solving the heat conduction equation in cylindrical coordinates with a convective boundary condition, which changes with the change in flow pattern over the heating surface. Transient changes in the liquid sublayer thickness and surface temperature at the CHF occurrence point are reported.

A three-dimensional hybrid lattice Boltzmann method was used to simulate the progress of a single bubble’s growth and departure from a horizontal superheated wall. The evolutionary process of the bubble shapes and also the temperature fields during pool nucleate boiling were obtained and the influence of the gravitational acceleration on the bubble departure diameter (BDD), the bubble release frequency (BRF) and the heat flux on the superheated wall was analyzed. The simulation results obtained by the present three-dimensional numerical studies demonstrate that the BDD is proportional to g−0.301\\(g^{\\mathrm {-0.301}}\\), the BRF is proportional to g−0.58\\(g^{\\mathrm {-0.58}}\\), and the averaged wall heat flux is proportional to g0.201\\(g^{\\mathrm {0.201}}\\), where g is the gravitational acceleration. These results are in good agreement with the common-used experimental correlations, indicating the rationality of the present numerical model and results.

A model using modified superposition approach is developed to predict the rate of heat transfer in the stagnation region of a planar jet impingement boiling on a hot flat surface. The total heat flux in this model is based on the combination of the single phase forced convection and nucleate pool boiling components. The single-phase component is calculated by using similarity solution approach. The applicability of the model is investigated on the boiling curves under conditions of single phase, partial and fully developed nucleate boiling. The Effect of main parameters of water jet, i.e. jet sub-cooling, jet velocity and nozzle to plate distance on the heat flux, is of concerned. A comparison of the obtained results of the model is made with various published experimental data and good agreement is reported.

We reveal and justify, both theoretically and experimentally, the existence of a unifying criterion of the boiling crisis. This criterion emerges from an instability in the near-wall interactions of bubbles, which can be described as a percolation process driven by three fundamental boiling parameters: nucleation site density, average bubble footprint radius and product of average bubble growth time and detachment frequency. Our analysis suggests that the boiling crisis occurs on a well-defined critical surface in the multidimensional space of these parameters. Our experiments confirm the existence of this unifying criterion for a wide variety of boiling surface geometries and textures, two boiling regimes (pool and flow boiling) and two fluids (water and liquid nitrogen). This criterion constitutes a simple mechanistic rule to predict the boiling crisis, also providing a guiding principle for designing boiling surfaces that would maximize the nucleate boiling performance. Boiling crisis is a physical phenomenon limiting the operation of many technologies cooled by boiling. Zhang et al. reveal theoretically and experimentally the existence of a unifying criterion to explain and predict the boiling crisis.