The cryogens generally exhibit different nature of boiling from the conventional liquids. The boiling process in cryogens sometimes don't exhibit the appropriate heat transfer due to transition into film boiling regime. In the present work, film boiling characteristics of liquid nitrogen (a widely used cryogen) were examined at different gravity conditions and under the influence of an electric field. Employing continuum-based numerical simulations and utilizing the coupled level set and volume of fluid (CLSVOF) method for interface tracking, the effects of wall superheat and reduced gravity on bubble dynamics during film boiling near the critical pressure were investigated. The heat flux values obtained from the simulations were compared with the experimental results. It was observed that under lower gravity levels, the number of bubble formation sites decreases, and the apex height of the bubble increases, resulting in slower detachment frequency and reduced average heat transfer rate. An external electric field was found to be effective in controlling the heat transfer rate in such cases. The effect of the electric field was found to be more prominent in lower gravity levels.

The present study employs molecular dynamics (MD) simulations to investigate the boiling phenomenon of liquid nitrogen at molecular level. The simulation involves a nano-scale system with Pt as a heating surface and liquid nitrogen as the boiling medium. The simulation results reveal the intricate details of the boiling process, capturing the evolution of temperature, heat flux and molecular arrangements during the transition from the liquid to vapor phase. By analysing the dynamics of molecules, insights into the mechanisms governing nucleation, bubble formation, and vaporization were explored. The study primarily focuses on the impact of surface modification and temperature on the boiling characteristics such as formation of film boiling regime during nano-scale boiling of liquid nitrogen.

The present study investigates non-premixed combustion using a mixture of coke oven gas (COG) and blast oven gas (BOG) as fuel, with the help of computational fluid dynamics (CFD). The mixing and conversion were compared by employing three turbulent models: standard k-ɛ, renormalization group (RNG), and Reynolds stress model (RSM). Variations in oxidizer inlet angles (0°, 5°, and 10°) were explored for improved fuel-air mixing. Analyzing these models and designs also allowed the understanding of the flow phenomena. Flame patterns and maximum temperature produced were examined with varying fuel-oxidizer velocity ratios. The finding shows the RSM model with a 10° oxidizer inlet angle was outperforming the others, predicting a 4% higher outlet temperature. In the case of analyzing a suitable turbulent model for simulating the combustion of COG-BOG mixture, the study revealed, the RNG model records 35% lower temperatures which means poor conversion due to poor mixing. The produced temperature was closely related to the fuel-oxidizer ratio and a range of this ratio helped to identify the optimized controlling parameter. This research advances knowledge of COG-BOG combustion simulation, aiding in the compatibility of turbulence models, oxidizer-fuel optimization, and improvements to phase flow.

Liquid nitrogen is a widely used cryogenic fluid in various industrial and scientific applications. Film boiling is inevitable when a hot surface is exposed to a cryogenic fluid like liquid nitrogen. This mode of boiling can affect the heat transfer rate and cooling efficiency of the process. The present study investigated the effect of operating pressure on film boiling characteristics of liquid nitrogen using continuum-based numerical simulations. Simulations were performed at atmospheric, intermediate, and near-critical pressures using a coupled level-set and volume of fluid method of interface tracking. Bubble dynamics and heat flux variations were examined at different degrees of wall superheat. The results show that the pressure condition significantly affects the bubble dynamics and boiling of liquid nitrogen. At higher pressure, the specific heat of the vapor phase increases, resulting in a higher heat transfer rate. It was also noted that at high levels of superheat, the bubbles detach from the heating surface faster, resulting in a higher transfer of heat. The effect of pressure on the number of bubble formation sites was also investigated. The simulations also reveal that the local thermodynamic conditions, such as pressure, temperature, and vapor concentration, significantly influence the bubble size.

Present study shows the performance analysis of methanol steam reforming considering different miniature-scale reformer geometries. Three different types of 3D models of steam reformers have been considered: straight annular, spiral, and serpentine-type reactors. Each reformer is packed with a copper-based catalyst to enhance the reaction between steam and methanol. Methanol steam reforming is an endothermic reaction with a two-step mechanism, whose reaction kinetic models were mathematically implemented. The study shows different hydrodynamic aspects of the flow fields through the catalyst bed, the conversion of methanol, and production of hydrogen for each type of reactors. Simulations were carried out at different temperatures and inlet velocities to study the methanol con-version and identify the most optimized reformer design. It was found that in the spiral and serpentine-type reformer, the methanol conversion was more due to the bending in the tubular reactor, which disturbs the flow field with an enhancement of mixing, causing more conversion. Spiral reformer exhibited conversion up to 91.16% at 650 K and producing 0.15 mass fraction of hydrogen, which was found to be 3% more than the conversion achieved in serpentine type and almost 8% more than the straight annular type geometry when the inlet velocity was kept at 0.1 m/s for all the cases.& COPY; 2023 Institution of Chemical Engineers. Published by Elsevier Ltd. All rights reserved.

The study of film boiling of cryogenic fluids has become a topic of interest due to its applications in several areas, including electronic cooling and space vehicles. In this paper, numerical simulations were conducted to investigate the film boiling of liquid nitrogen using a variant of the volume of fluid (VOF)-based algorithm. The effects of wall superheats and electric fields on the bubble dynamics during film boiling at normal atmospheric pressure and near-critical conditions were inves-tigated. It was observed that bubble formation during film boiling of liquid nitrogen at lower wall superheats was governed by Rayleigh-Taylor instability. However, at higher wall superheats, bubble spacing decreased with the appearance of additional bubbles. The average heat transfer rate increased with an increasing wall superheat. The electric field significantly influences the film boiling up to a specific limit of wall superheat. At a higher wall superheat, the influence of the electric field was suppressed by the enhanced thermal effect. It was observed that the impact of the electric field on film boiling is highly dependent on the dielectric permittivity difference between the liquid and vapor phases. Faster bubble detachment and reduced bubble spacing were noticed in the presence of an applied external electric field.

This work deploys the molecular dynamics simulations to study the effect of an external electric field on the structural and dynamic properties of a sessile ionic nano-droplet. The results show that the presence of the ions at different concentrations causes significant deformations in the hydration shells. Such deformations resemble the effect of high pressure on water molecules. Due to the disintegration of the droplet under the influence of high intensity electric fields, the drift velocities and mobilities of the sodium and chlorine ions exhibit a decreasing trend in the presence of the solid substrate which is not observed in free droplets. The change in the ion-water interaction energy increases with the increasing electric field intensity. This implies an underlying reduction in the strength of ionic hydration. The presence of the Pt surface enhances the ion-water and water-water interactions. (c) 2021 Elsevier Ltd. All rights reserved.