The marine sector faces a pivotal moment as it undergoes a monumental shift from fossil fuels to emission-neutral solutions as mandated by the International Maritime Organisation (IMO). Simultaneously, digitalisation has emerged as a disruptive force, with simulation-driven ship design (SDSD) replacing outdated design approaches. This article summarises a Siemens white paper that describes the challenges and solutions in the marine industry from conventional alternative fuels to after-treatment and nuclear energy propulsion.
USE SIMCENTER TO MODEL COMPLEXITY
The guiding principle of Simcenter software entails four goals: model the complexity; explore the possibilities; stay integrated and go faster. Simcenter is part of the Siemens Xcelerator business platform of software, hardware and services. Multiphase modelling can be included in design exploration and optimisation projects (explore the possibilities) and can be interfaced with other tools (stay integrated). It is primarily used to model the high complexity of many multiphase flow problems.
AMMONIA DISPERSION
LNG and ammonia are stored in a cryogenic state, which means that any exposure to environmental conditions – such as leakages during bunkering – will lead to instant vaporisation, putting the crew at risk and presenting numerous hazards to the environment. Flash boiling of the liquid spray from a pipe leakage into the environment occurs because the ambient pressure is below the liquid fuel’s saturation pressure.
Depending on the direction of spray and distance to adjacent structures, rainout of the dispersed vapour cloud can occur. Rainout is the impingement of remaining liquid droplets from the mostly vaporised spray that is highly flammable. To better understand the hazards of spray flash boiling and gas dispersion, we studied a replenishment at sea problem, where cryogenic ammonia dispersion from a pipe was simulated with Simcenter STAR-CCM+ software because of a leak during bunkering.
Even though it is unlikely that workers on deck would be exposed to fatal concentration levels exceeding 2,000 parts per million (ppm) over a duration of 30¬ minutes or more, stinging or burning sensations in the eyes and respiratory system can occur from exposure to as little as 70ppm over the same time span according to the National Institute of Health. These levels can be attained inside closed rooms like engine compartments, which underscores the importance of risk assessment. In the present fictitious bunkering scenario, liquid ammonia is discharged from a leak horizontally onto the receiving ship’s bunker infrastructure at a pressure of 8 bar, causing rapid dispersion into the environment. Over the span of 10 seconds, approximately 1ppm could be metered at point locations typical for work operations.
LNG BOIL-OFF
LNG is stored and transported at temperatures as low as -160°C shielded by insulated layers in the tank. Boil-off gas (BOG) is the amount of mass transferred from the liquid to the gaseous phase because of heat energy creeping through the insulation into the tank and kinetic heating due to sloshing. One of three options for aftertreatment of BOG is used to address the problem: release into atmosphere, reliquification or utilisation in engine combustion. To mitigate profit losses associated with BOG, engineers address the insulation problem, sizing of tanks and efficient rerouting methods for aftertreatment. Engineers leverage simulations using hybrid multiphase modelling to resolve thermal dynamics encountered across the entire transportation chain, from filling to emptying and insulation failure at sea, including thermal energy transfer with tank sloshing in response to seakeeping. Due to the complex and multifaceted nature of boiling processes, numerical models are delineated based on the dominating physical characteristics of each problem. Phase transition from a liquid to vapour can occur either at a vapour-liquid interface away from walls; at walls with preexisting clusters of vapour or in the bulk of the liquid due to density fluctuations.
The model selection is therefore driven by several factors, such as the type of boiling (nucleate, molecular or transition) and where it occurs (on or adjacent to walls or in the bulk). The boiling models are embedded in either VOF (volume of fluid) or EMP (electromagnetic pulse). It is not possible to apply VOF for problems requiring phase-specific transport equations, including subcooled boiling or, more generally, non-equilibrium modelling. At the nucleate level, boiling involves generating and growing vapour bubbles on walls, which are incepted at discrete points. The VOF nucleate boiling model is applicable for relatively low solid temperatures. If the wall surface temperature exceeds the maximum valid temperature for the nucleate boiling notion but is still below the temperature range for wall film boiling, a hybrid unstable state of boiling called transition boiling sets in. VOF can be used for this regime. Boiling at high wall surface temperatures falls within the purview of film boiling, which is included in the EMP modelling realm and is present once the critical heat flux has been exceeded, accompanied by a continuous vapour film on the heated surface.
Sought-after metrics are the superheat, the departure from the saturation temperature and the directional transport of vapour. ABS demonstrated how the VOF framework under the equilibrium assumption and homogenous boiling was used for predicting BOG inside a typical LNG tank. Low wall superheating, dominance of heat transfer by convective fluxes and the notion that no evaporation will occur at the molecular level drove the model selection. If such circumstances are given, another factor in favour of VOF is its superiority in capturing the free surface due to sloshing.
SUBCOOLED BOILING
Subcooling is the process of maintaining a liquid below its normal boiling point. In subcooled boiling, the temperature of most of the liquid is below the saturation temperature – and bubbles emerging at the wall have the potential for condensing into the liquid that implies a lucrative increase in heat transfer capability. This scenario has led to widespread application across industries, such as pressurised water reactors of nuclear power plants. The level of pressurisation determines the subcooling margin, defined as the difference between the temperature of the pressurised water and the outlet temperature of the coolant in the core of the reactor. As the bulk water temperature climbs, this margin determines the likelihood of subcooled boiling to set in. The complexity of the underlying physical processes, from evaporation to quenching and convective heating, requires the use of EMP and its multiphase interaction models like the wall boiling model. High heat transfer rates are also encountered in direct contact condensation (DCC) when saturated steam interfaces with water subcooled to a temperature below the steam’s saturation temperature. Such condensers are also prevalent in nuclear reactor systems. If the steam jet enters a water column the phenomenon of geysering is observable, the rapid formation and collapse of pockets of steam through condensation.
Using Simcenter STAR-CCM+ offers extensions to the core EMP model that are critical for the modeling of this process, namely the large-scale interface (LSI) and surface tension model.¬
SHIP-ICE INTERACTION
The exploration of large reservoirs of natural gas and oil in arctic environments requires consideration of ship-ice interaction for minimum power requirement and structural strength estimation throughout the design phase and during operation. The DEM, another Eulerian-Lagrangian multiphase model, enjoys the advantage of casting the collective dynamics of a large number of particles within the core finite volume framework of Simcenter STAR-CCM+. It can be used to consider brash ice for the simulation of flows around ships or offshore structures. Using Simcenter STAR-CCM+ offers a bidirectional coupling between high-fidelity fluid dynamics based on the solution of the Navier-Stokes equations and DEM.
Consequently, the effect of flow disturbances induced by the advancing ship hull on the collective behaviour of DEM particles, as well as the modification of the flow field by ice, can be considered. DEM, as implemented in Simcenter STAR-CCM+, extends the Lagrangian formulation of particle tracking to account for the interaction based on the soft-particle formulation, according to which of them are allowed to overlap. Particle dynamics are based on classic momentum conservation equations. The complexity of combined CFD-DEM simulations rests with the coupling of the two continuous, Eulerian phases to the Lagrangian phase. Fluid forces on particles are computed based on simple drag models. The flow velocity required for the evaluation of the drag terms is obtained from the transient CFD solution based on the particle location. For unidirectional coupling, particle dynamics do not affect the CFD solution. For bidirectional coupling, phase displacement, exchange of interphase momentum and mass between the continuous phases of water and air and the dispersed phase of ice are considered. Phase displacement is accounted for by computing the volume fraction of the Lagrangian phase within each control volume (CV). The net exchange of momentum and mass with the continuous phase is found by integrating the dispersed phase equations over all particles crossing a given CV and introducing them to the continuous phase equations by way of source terms. We have applied the coupled CFD-DEM method to study a bulk carrier advancing through a channel of brash ice.
The analysis drew upon two objectives. The first objective was to assess the anticipated increase in resistance arising from contact forces exerted on the ship hull by the ice. Second, it was shown how this computational framework can be used for flow-field analysis to assess the likelihood of impacts to propeller blades.
FLUE GAS CLEANING
Operating flue gas cleaning devices (scrubbers) for aftertreatment has emerged as a viable path for many ship operators to meet mandatory rules in place for sulphur oxide (SOx) and nitrogen oxide (NOx) as per IMO’s Engine International Air Pollution Prevention protocols. Compliance can be achieved in one of two ways: using fuels to emit less SOx or NOx output or operating scrubbers. For the latter, evidence must show that wash water is harmless to the environment. The hybrid multiphase framework introduced in this white paper facilitates predicting flue gas cleaning rates in various kinds of plants encountered in the industry. This is done by leveraging EMP and suitable multiphase interaction models by extending to reactive flow. In the following a demonstration, it is outlined using the example of a spray column for desulphurisation, which makes use of the countercurrent contact effect between liquid and gas. The results of the simulation reveal the desulphurisation rate and slurry concentration, two key performance indicators of the plant. The two Eulerian phases are a multicomponent gas describing the flue gas and a multicomponent liquid describing the slurry. These are connected by a continuously dispersed phase interaction model regime allowing dissolution and interphase mass transfer. The reactant involved is sodium hydroxide (NaOH), which is engaged with a global single-step reaction, for example, SO + NaOH = NaSO¬ + HO, with the eddy break-up unit, which is implemented in Simcenter STAR-CCM+ in a kinetics-only model.
By using simulation, engineers are enabled to quantify how sensitive their design is to droplet size and kinematics, liquid flow rates and tower geometry. This provides important insight into what should drive design to comply with the rule set.
CONCLUSION
The Simcenter STAR-CCM+ multiphase modelling suite is well suited for hybrid operation of otherwise clearly delimited modelling concepts. A single simulation enhances performance for a given problem featuring different flow regimes as opposed to a sequence of segmented simulations, which open up new opportunities for design exploration and integration with other computer-aided engineering (CAE) tools, two cornerstones of simulation-driven ship design. As the shipping industry experiences increasing pressure to maintain or increase
profit margins due to stricter environmental regulations, engineers are struggling to arrange time for thorough analysis and innovation by using simulations to work in disconnected CAE frameworks with segmented specialised tools and lengthy iterative loops. We identified five practical examples from the field within the scope of the green energy transition to underscore the imminent requirement for a CFD framework, which allows for a fast turnaround of the modelling and simulation problem.
Simcenter STAR-CCM+, part of the Simcenter portfolio, brings formerly siloed engineering disciplines together in a cohesive simulation environment. It challenges traditional design methods with a simulation-driven approach combined with next-generation physical testing and data acquisition and gives engineers full confidence that innovative designs will perform according to the requirements.