Two-Phase Flow for Automotive and Power Generation Sectors

In order to contribute to a more environment-friendly community, a lot of research is still needed in the field of fossil fuels and internal combustion engines. In those applications, fuel injection systems are one of the key subsystems. However, due to their small characteristic sizes and timescale experiments are difficult to carry out. Thus, computational fluid dynamics (CFD) has been a very successful tool to improve engine efficiency during the last years. Several models have been successfully developed to accomplish that goal. One of the latest is the Eulerian or Eulerian–Lagrangian spray atomization model, which has proved to be able to deal with multi-phase flow physics taking place during fuel injection. The key feature of this model is that it is able to seamlessly simulate both the nozzle internal flow and the subsequent spray development into the ambient gas. In this chapter, a review of this model with examples of its applications is performed. Nozzle flow parameters such as fuel mass flow rate and momentum flux are accurately predicted. The flow pattern (pressure, velocity, and temperature) is then analyzed to give ideas about how to improve the nozzle design. At the same time, fuel atomization and mixing with the surrounding gas can also be studied. Spray macroscopic parameters penetration length (both liquid and vapor) and spray angle are again precisely calculated when compared with experimental measurements. Additionally, this model could be also used to analyze microscopic parameters such as droplet size and distribution. This is done by the calculation of the interphase surface density with the addition of a new transport equation. Even though this model has shown great potential in the field of multi-phase flows for engine applications, there is still room for improvement for its sub-models and programming.

Predictive engine simulations are key for rapidly exploring and optimizing the design of cleaner burning and more fuel-efficient engines. Injection strategies in advanced engine concepts are resulting in the injection and atomization of fuel under a wide range of operating conditions in order to meet stringent emission regulations. However, the physics governing the breakup of an injected liquid fuel jet into droplets under these conditions have not been well studied or experimentally characterized to date. It is uncertain whether existing atomization and spray breakup models, historically developed to study conventional diesel operation, can be directly applied within engine CFD simulations to study new advanced engine concepts. This chapter summarizes recent progress made in developing an improved physics-based primary atomization model for use in diesel engine simulations across a broad range of in-cylinder conditions. Physical mechanisms that are likely to contribute to the atomization of diesel sprays are first reviewed, with a particular focus on aerodynamic wave growth on the fuel jet surface and turbulence generated in the injector. Then, recent advances in spray diagnostics that have informed characteristic length scales of primary atomization are highlighted. The chapter concludes with the presentation and validation of a newly developed hybrid spray breakup model, the “KH-Faeth” model, capable of representing both aerodynamic and turbulent breakup mechanisms in the atomization of non-cavitating diesel sprays.

The internal combustion engine has been a major power plant in transportation and industry, and demands continuously advanced technologies to improve its performance and fuel economy, and to reduce its pollutant emissions. Liquid fuel injection is critical to the combustion process in both compression ignition (CI) diesel engines and gasoline direct injection (GDI) engines. Much effort has been focused on modeling of spray atomization, droplet dynamics, and vaporization using a Lagrangian-drop Eulerian-fluid (LDEF) framework, which has been applied in engine computational fluid dynamic (CFD) simulations with success. However, recent experiments have shown the mixing-controlled characteristics of high-pressure fuel injection under vaporization conditions that are relevant to both gasoline and diesel engines. Under such conditions, instead of being dominated by droplet dynamics, the vaporization process of a liquid spray is limited by the entrainment rate of hot ambient gas and a saturated equilibrium phase is reached within the two-phase region. This suggests that an alternative approach of fuel spray modeling might be applicable. An equilibrium phase (EP) spray model was recently proposed for application to engine combustion simulation, based on this mixing-controlled jet theory and assumption of local phase equilibrium. This model has been applied to simulate both diesel fuel injection and GDI sprays, and has shown excellent predictions for transient vapor/liquid penetrations, spatial distribution of mixture fraction, as well as combustion characteristics in terms of flame lift-off length and soot emission. It has also shown better computational efficiency than the classical LDEF spray modeling approach since the dynamic process of droplet breakup, collision, coalescence, and vaporization is not modeled. The model and results relevant to engine simulation are reviewed in this chapter.

An efficient spray injection leads to better vaporization and better air–fuel mixing, resulting in the stable combustion and reduced emissions in the internal combustion (IC) engines. The impingement of liquid fuels on chamber wall or piston surface in IC engines is a common phenomenon, and fuel film formed during the impingement plays a critical role in engine performance and emissions, particularly under cold start conditions. Therefore, the study on the characteristics of spray impingement on the chamber wall or piston surface is necessary. However, first, due to the complexity of the practical fuel injection systems, it is difficult to attain the detailed specific information of the spray impingement from the experiments such as droplet size, mass, number, and velocity distributions in the vicinity of wall region. Second, because of the Lagrangian particle/parcel concept (a particle representing a number of droplets in simulations), the spray–wall interaction model under Eulerian–Lagrangian approach is often developed based on the individual droplet. Therefore, the individual droplet’s impingement on wall and the droplet-to-droplet collision have been extensively studied to assist in a profound perception on the spray–wall impingement. In this chapter, the encouraging experimental observations of applying optical diagnostics technology to study droplet–wall impingement are extensively discussed. Single droplet impingement on a solid surface with various conditions was examined to understand the detailed impinging dynamic process. The droplet–wall interaction outcomes, in particular focusing on the splashing criteria, were inspected, and a new correlation of deposition–splashing is developed. Post-impingement characterizations including spreading factor, height ratio, contact line velocity, and dynamic contact angle were then analyzed based on the experimental data at various test conditions. Further, the non-evaporation volume of fluid (VOF) method based on Eulerian approach was used to characterize single droplet impinging on the wall and provide a better understanding of the dynamic impact process. The simulation results of the spreading factor and height ratio matched well with the experimental results during the droplet impingement process. In addition, due to the evaporation drawing more attention during the engine combustion process, an evaporation VOF (e-VOF) sub-model was developed and applied to multi-droplet impingement on a heated surface to qualitatively and quantitatively analyze the vaporizing process as droplets impacting onto the hot surface. The information obtained from VOF simulations can be applied to improve the spray–wall interaction models in the liquid spray Eulerian–Lagrangian method.

In high-pressure fuel injection systems, cavitation is known to affect spray atomization processes. Modeling the cavitation phenomenon has become a necessity to ensure predictive quality and higher fidelity of the fuel spray simulations. Inside the fuel injectors, local pressures drop below the saturation pressure of fuels in regions of flow separations, such as inlet of holes and periphery of needles at low-lift conditions. Several cavitation models and multiphase modeling approaches have been employed in the literature to predict the extent of cavitation in the fuel injection systems. A review of these modeling approaches will be presented. Amongst the cavitation models, bubble-based and semi-empirical timescale-based ones are widely used. Mixture/single-fluid and Eulerian–Eulerian/two-fluid approaches have been adopted for fuel injection cavitation modeling. Two-fluid approach captures the interaction between the two phases, which is usually ignored in single-fluid approach. Comparative studies in the literature will be reviewed here to provide a comprehensive idea of the cavitation modeling approaches to the readers. The advantages and disadvantages of these models will be discussed in depth. Keeping in mind the conflicting requirements of accuracy and constraints of computational cost, recommendations will be provided for suitable cavitation modeling approaches.

Internal combustion engines are widely popular and useful in our life to meet different power requirements. Gaseous and particulate emissions emitted from these engines pose major environmental and health issues. Environment-friendly alternate fuels like biodiesel for diesel engine and alcohols for gasoline engines are gaining popularity steadily in the last decade due to faster depletion of conventional fuels reserves and adaptation of strict emission regulations worldwide. However, it is significantly important to review the spray characteristics of these alternative fuels because engine performance and emissions are largely dependent on air–fuel mixing process to a great extent. This chapter mainly focuses on different optical techniques used for spray characterisation. There are two types of spray characteristics, which are important in the context of internal combustion engines, namely macroscopic spray characteristics and microscopic spray characteristics. Macroscopic spray characteristics such as spray tip penetration and spray cone angle are generally characterised by Mie scattering, shadowgraphy and schlieren techniques, by using high-speed CCD camera. Microscopic characteristics such as spray droplet size distribution and droplet velocity distributions are generally measured using phase Doppler interferometry (PDI) technique.

High fidelity solutions of turbulent flow equations are obtained by large eddy simulation (LES) and direct numerical simulation (DNS). These techniques are devoted for resolving most of the energy-carrying scales in a turbulent flow. Grid resolution in LES or DNS is determined by the lengths of the finest scale of motion which is to be directly simulated. In multiphase flows, further refinement in the grid topology is required to capture the bubble or droplet front and also to resolve the small structures that are created in the wake zone of the bubble/droplet. Owing to the grid size and finer timescales, the computational complexities in LES or DNS are extremely high, and parallel computing resources are often deployed. The present chapter reviews computational efforts involved in turbulent multiphase flow simulation in industrial devices. Several high-performance computing (HPC) strategies like distributed computing using message passing interface (MPI), general purpose graphics processing unit (GPGPU) accelerated computing using CUDA and their hybridizations are also reviewed. Estimations of the computational requirement for simulation of large industrial devices are presented, and potential use of modern computational science and hardware are critically assessed.

Increased emission of carbon dioxide into the atmosphere from the fossil fuel-powered automobiles and power plants is one of the major sources of global warming. Using renewable and clean sources of energy as a fuel can control this. Among the other available alternatives, fuel cells have emerged as a promising source of clean energy due to their high efficiency, low/zero emission rate, modular design, and portability. Besides, fuel cells are capable of producing water as a by-product, making them an attractive option for potable water. Recent trends in the global automotive market show a strong trend of gravitating toward hydrogen fuel cell-powered automotive from the existing battery-operated automotive in the coming years. Owing to the high power density characteristics, polymer electrolyte membrane (PEMFC) fuel cell has been considered to be the most attractive one as the primary power source in fuel cell vehicle. Attaining slug-free drainage of water from the gas diffusion layers (GDLs) in PEMFC is one of the key challenges in their commercialization. Water management of hydrogen fuel cell can be optimized by extensive analysis of two-phase heat transfer phenomena like condensation and evaporation happening across GDL of the fuel cell. Excessive accumulation of water droplet on the GDL reduces the overall efficiency of the fuel cell. Thus, water removal from GDL is very important. Studies have shown that the quality of the water from a PEMFC meets the standard health requirements for drinking, indicating the importance of harvesting water from fuel cell exhausts as a sustainable drinking water source. The challenge in such water harvesting lies in achieving high condensation rate with minimum cooling energy penalty. This chapter reviews the fuel cell in general with a focus on multiphase phenomena and its use in water management in the GDL of fuel cells and harvesting drinking water from fuel cell exhaust. The background fundamentals are provided, and the state of art is discussed. Finally, the future perspective of water management and harvesting in fuel cells is provided in a larger backdrop of global energy–water nexus.

Ferrofluids are colloidal suspensions of single-domain magnetic nanoparticles in a nonmagnetic carrier fluid. Despite the presence of both solid (nanoparticle) and fluid (carrier fluid) phases, the suspension behaves as a fluid in the presence of a magnetic field. The superparamagnetic nature of the suspended particles allows for the suspension to be manipulated by a magnetic field ‘from a distance,’ i.e., without any mechanical actuation system. Although ferrofluids were first developed in the 1960s for drawing liquid fuel against gravity in rocket propulsion systems, the application never materialized commercially in a large scale. However, ferrofluids have been extensively used for other applications such as ferrofluidic seals, loudspeakers, magnetic drug targeting, thermomagnetic convection, magnetic shape-shifting optical mirrors, and energy harvesting. Recently, it was shown that ferrofluids can be used in the micropropulsion sector. In the presence of a magnetic field, a ferrofluid surface naturally deforms to form sharp peaks, the phenomenon being known as Rosensweig instability. For ionic ferrofluids, an amplification of the electric field is present at the tip of these peaks, which leads to electrospray emission. The reaction force due to the emitted droplets results in forward propulsion. In the present chapter, a summary of the basics of ferrofluids and their manipulation techniques is presented, followed by a brief review of ferrofluid-based propulsion techniques.

Direct contact condensation (DCC) of steam in subcooled water is a phenomenon which is experienced in many applications such as thermal, chemical and nuclear engineering, particularly in energy generation devices since it enables immense energy transfer via the two-phase interface. However, under certain situations, steam water direct contact can lead to rapid condensation and result in fast (of the order of acoustic time scale) pressure transients which could have serious implications on structural integrity and safety, especially in nuclear power plants. Therefore, understanding of the underlying physics and characteristics of DCC phenomenon has a paramount importance. DCC is a complex thermo-hydraulic event in which the phase change process is governed by the interplay between several thermo-mechanical factors (e.g. local heat transfer coefficient, interfacial area density, turbulence intensity in the liquid phase) across the phasic interface. In this chapter, different situations of the DCC events, their characteristics and underlying mechanisms are discussed in detail. A detailed review of the earlier works which includes both system-level and interface scale modelling of the phenomena is also addressed in this chapter. An emphasis is given on the DCC events which are always associated with the large amplitude pressure spikes such as chugging and condensation-induced water hammer.

A comparative assessment of existing instability models is carried out to find the appropriate length scale in a computationally inexpensive integral model predicting the heat transfer in film boiling over a vertical flat plate. The use of Kelvin–Helmholtz criterion shows good matching to the limited number of experimental data, whereas for high liquid flow velocity the critical film Reynolds number criterion is found as the best. A generalized model covering the range of both the models is then developed by employing a regression analysis. The generalized model is shown to remain accurate within 10% band over a wide range of parameters.

The phenomenon of boiling is visible all around us from cooking to power generation, but despite such all around usages many aspects of boiling are still not very well understood as it is a very complex process and occurs over a wide range of system scales. We often rely on empirical correlations when we want to evaluate different parameters connected with boiling phenomena. Along with the development of empirical correlations for engineering applications, considerable advances are there in understanding the fundamentals of the boiling process. Since the process is very complex and multiple thermal and fluid variables are involved, a complete theoretical model for predicting the boiling heat transfer is yet to be developed. Boiling phenomenon is still being intensively studied and is the focus of research activities in numerous institutions across the world. A better understanding of the physics of boiling can be achieved by either detailed measurements or high-resolution numerical simulation. These two approaches are now complementing each other in understanding the physics of boiling more completely. In recent years, numerical modeling has improved considerably thanks to ever-increasing computational power. With advancing computing capabilities and advent of new numerical techniques for two-phase flow, simulations of boiling heat transfer have become feasible. The main two approaches in numerical simulation of boiling are (i) interpenetrating media approach and (ii) single-fluid approach. In addition to this, some newer techniques like the phase field method and the lattice Boltzmann method have to some extent been used for simulating boiling flows. In this review, we look at the different approaches of numerical simulation of boiling currently being used.