Simulation of Single Vapor Bubble Condensation with Sharp Interface Mass Transfer Model

Pure numerical simulation of phase-change phenomena such as boiling and condensation is challenging, as there is no universal model to calculate the transferred mass in all configurations. Among the existing models, the sharp interface model (Fourier model) seems to be a promising solution. In this study, we investigate the limitation of this model via a comparison of the numerical results with the analytical solution and experimental data. Our study confirms the great importance of the initial thermal boundary layer prescription for a simulation of single bubble condensation. Additionally, we derive a semi-analytical correlation based on energy conservation to estimate the condensing bubble lifetime. This correlation declares that the initial diameter, subcooled temperature, and vapor thermophysical properties determine how long a bubble lasts. The simulations are carried out within the OpenFOAM framework using the VoF method to capture the interface between phases. Our investigation demonstrates that calculation of the curvature of interface with the Contour-Based Reconstruction (CBR) method can suppress the parasitic current up to one order

Numerical investigation of bubble dynamics and flow boiling heat transfer in cylindrical micro-pin-fin heat exchangers

Micro-pin-fin evaporators are a promising alternative to multi-microchannel heat sinks for two-phase cooling of high power-density devices. Within pin-fin evaporators, the refrigerant flows through arrays of obstacles in cross-flow and is not restricted by the walls of a channel. The dynamics of bubbles generated upon flow boiling and the associated heat transfer mechanisms are expected to be substantially different from those pertinent to microchannels; however, the fundamental aspects of two-phase flows evolving through micro-pin-fin arrays are still little understood. This article presents a systematic analysis of flow boiling within a micro-pin-fin evaporator, encompassing bubble, thin-film dynamics and heat transfer. The flow is studied by means of numerical simulations, performed using a customised boiling solver in OpenFOAM v2106, which adopts the built-in geometric Volume of Fluid method to capture the liquid–vapour interface dynamics. The numerical model of the evaporator includes in-line arrays of pin-fins of diameter of 50 μm and height of 100 μm, streamwise pitch of 91.7 μm and cross-stream pitch of 150 μm. The fluid utilised is refrigerant R236fa at a saturation temperature of 30 ◦C. The range of operating conditions simulated includes values of mass flux = 500–2000 kg∕(m$^2$s), heat flux = 200 kW∕m$^2$, and inlet subcooling $\Delta$$T_{sub}$ = 0–5 K. This study shows that bubbles nucleated in a pin-fin evaporator tend to travel along the channels formed in between the pin-fin lines. Bubbles grow due to liquid evaporation and elongate in the direction of the flow, leaving thin liquid films that partially cover the pin-fins surface. The main contributions to heat transfer arise from the evaporation of this thin liquid film and from a cross-stream convective motion induced by the bubbles in the gap between the cylinders, which displace the hot fluid otherwise stagnant in the cylinders wakes. When the mass flow rate is increased, bubbles depart earlier from the nucleation sites and grow more slowly, which results in a reduction of the two-phase heat transfer. Higher inlet subcooling yields lower two-phase heat transfer coefficients because condensation becomes important when bubbles depart from the hot pin-fin surfaces and reach highly subcooled regions, thus reducing the two-phase heat transfer

Numerical simulation of drop impingement and bouncing on a heated hydrophobic surface

The heat transfer of a single water droplet impacting on a heated hydrophobic surface is investigated numerically using a phase field method. The numerical results of the axisymmetric computations show good agreement with the dynamic spreading and subsequent bouncing of the drop observed in an experiment from literature. The influence of Weber number on heat transfer is studied by varying the drop impact velocity in the simulations. For large Weber numbers, good agreement with experimental values of the cooling effectiveness is obtained whereas for low Weber numbers no consistent trend can be identified in the simulations

Spreading and rebound dynamics of sub-millimetre urea-water-solution droplets impinging on substrates of varying wettability

The interaction of droplets consisting of urea-water solution (UWS) with a wall is of interest for automotive exhaust gas after-treatment of Diesel engines by selective catalytic reduction (SCR). Since the impingement of tiny UWS droplets on the solid substrate is difficult to examine experimentally, little is known about the detailed dynamics of this process. In the present study, the normal impact of single UWS droplets impinging on dry solid substrates of greatly differing wettability is investigated numerically under axisymmetric conditions. Simulations are performed by a diffuse interface phase-field solver developed by the authors where the coupled Cahn–Hilliard Navier–Stokes equations are solved using OpenFOAM. The code is thoroughly validated against a number of experiments from literature considering the rebound of millimetre-sized water droplets from hydrophobic substrates. The numerical simulations on the impact dynamics of UWS droplets cover wide ranges of sub-millimetre droplet sizes and impact velocities that are relevant in technical SCR systems. A strong influence of substrate wettability on droplet dynamics is identified. Reducing wettability from hydrophilic to superhydrophobic conditions reduces spreading and enables drop rebound with reduced drop-surface contact time. The effects of drop diameter, drop impact velocity and equilibrium contact angle on the maximum spreading ratio are quantified, and regime maps on rebound versus non-rebound (deposition) impact outcomes are provided. The results of the present interface-resolving numerical simulations may be useful for development of more advanced drop-wall interaction models as they are required in CFD codes relying on the Euler–Lagrange approach for large-scale computations of UWS spray