Oscillating flows and multiphase heat transfer processes frequently occur in many engineering and scientific applications and systems, as is the case in enhanced geothermal energy, CO 2 sequestration and storage, and in evaporation in soil, to name a few. Nevertheless, modeling of such flows is a rather challenging task due to the complex interfacial dynamics among different phases and solid porous structures. Over the decades, several types of Lattice Boltzmann (LB) models for multiphase flows have been developed under different physical pictures, for example the color-gradient model, Single-Relaxation-Time (SRT) pseudopotential model, and the HSD model. In this study, a pseudopotential Multiple-Relaxation-Time (MRT) LBM simulation will be utilized to simulate incompressible oscillating flow and condensation in 2D porous media. Initially, the model will be used to optimize the porous structure in order to have the maximum condensation rate of water vapor. Subsequently, the effects of contacting angle, wettability, oscillating frequency and phase angle to the heat flux, the temperature field of porous media, and the condensation rate will be discussed. Moreover, a multiscale approach will be considered in order to couple the heat transfer in macroscale applications. It is expected that such an approach will provide a different perspective regarding the engineering applications involved with oscillating flow and multiphase heat transfer processes.

A variety of engineering applications involve medium Reynolds number ( Re ) oscillating flow around spherical objects (e.g. aerosol dispersion, microorganism motion, sedimentation of small particles, etc.), and they usually require accurate prediction of the movement of particles and the forces. Currently, the popular model of predicting forces acting on an accelerated submerged sphere was developed more than 100 years ago, but only limited to Stokes flow ( Re << 1 ). In the 1960s, Odar and Hamilton conducted experiments and extended the model for Re number up to 64 (OH-64 law). The aim of this study is to further extend the Re number to 300 via numerical simulations using ANSYS Fluent, so that the model is more capable for more varieties of engineering applications. It is expected that the results of this study will be beneficial to advance the fundamental understanding of oscillating flow over spheres and the potential applications.

Compared to Solar Photovoltaics (PV), Concentrated Solar Power (CSP) can store the excess solar thermal energy, extend the power generation at night and cloudy days, and levelize the mismatch between energy demand and supply. To make CSP competitive, Thermal Energy Storage (TES) system filled with phase change material (PCM) is a promising indirect energy storage technique, compared to the TES system using concrete or river rocks. It is of great interests to solar thermal community to apply the latent heat thermal energy storage (LHTES) system for large scale CSP application, because PCMs can store more thermal energy due to the latent heat during the melting/freezing process. Therefore, a comprehensive parametric analysis of LHTES system is necessary in order to improve its systematic performance, since LHTES system has a relatively low energy storage efficiency compared to TES systems using sensible materials. In this study, an 11-dimensionless-parameter space of LHTES system was developed, by considering only the technical constraints (materials properties and operation parameters), instead of economic constraints. Then the parametric analysis was performed based on a 1D enthalpy-based transient model, and the energy storage efficiency was used as the objective function to minimize the number of variables in the parameter space. It was found that Stanton number ( St ), PCM radius ( r ), and void fraction ( ε ) are the three most important ones. The sensitivity study was conducted then based on the three dimensionless-parameter space which will significantly influence the system performance. The results of this study make LHTES system competitive with TES system using sensible materials in terms of energy storage efficiency.

Our recent progress on studying wave interaction with a lift-type rotor is discussed in this paper. The particular focus is on characterization of the rotor’s unidirectional responsiveness in waves. The rotor consists of six hydrofoil blades in two sets. One blade set has three blades laid out as a vertical-axis wind turbine of the Darrieus type. The other blade set has three blades configured like a Wells turbine. In combination, the formed rotor can be driven by flows in any direction to perform unidirectional rotation about its vertically mounted shaft. This unidirectional responsiveness of the rotor also holds in waves, making the rotor a valuable device for wave energy conversion. For parametric study of the rotor, hydrofoil blades using different cross sectional profiles and chord lengths have been employed to configure the rotor. The rotor was then tested in a wave flume under various wave conditions in a freewheeling mode. Experimental results were analyzed and discussed. The yielded research findings will greatly enhance the fundamental understanding on the rotor performance in waves, and effectively guide the prototype rotor development for practical applications.

Multiphase flows frequently occur in many important engineering and scientific applications, but modeling of such flows is a rather challenging task due to complex interfacial dynamics between different phases, let alone if the flow is oscillating in the porous media. Using humid air as the working fluid in the thermoacoustic refrigerator is one of the research focus to improve the thermoacoustic performance, but the corresponding effect is the condensation of humid air in the thermal stack. Due to the small sized spacing of thermal stack and the need to explore the detailed condensation process in oscillating flow, a mesoscale numerical approach need to be developed. Over the decades, several types of Lattice Boltzmann (LB) models for multiphase flows have been developed under different physical pictures, for example the color-gradient model, the Shan-Chen model, the nonideal pressure tensor model and the HSD model. In the current study, a pseudopotential Multiple-Relaxation-Time (MRT) LBM simulation was utilized to simulate the incompressible oscillating flow and condensation in parallel plates. In the initial stage of condensation, the oscillating flow benefits to accumulate the saturated vapor at the exit regions, and the velocity vector of saturated vapor clearly showed the flow over the droplets. It was also concluded that if the condensate can be removed out from the parallel plates, the oscillating flow and condensation will continuously feed the cold surface to form more water droplets. The effect of wettability to the condensation was discussed, and it turned out that by increasing the wettability, the saturated water vapor was easier to condense on the cold walls, and the distance between each pair of droplets was also strongly affected by the wettability. It’s expected that this study can be used to optimize and redesign the structure of thermal stack in order to produce more condensed water, also this multiphase approach can be extended to more complicated 3D structures.

A solar thermal-driven multiple-effect thermosyphon distillation (METD) system is proposed for waste-water treatment. The METD system consists of a series of cascaded thermosyphons that are connected by embedding the condenser section of each thermosyphon in the evaporator section of the following device. The input thermal energy is supplied by a solar collector integrated with a latent heat thermal energy storage unit. A thermal network model is developed to predict the heat transfer rates through the METD system and distilled water production rates. METD systems with various number of effects and hot end temperatures are investigated. It is found that the ratio of distilled water production to the input thermal energy increases by increasing the number of effects with no dependence on the hot end temperature. Quantitatively, a METD system with 5 effects can produce 11 kg of fresh water per hour with an input heat of 1.4 kW.

The significant changes of population, environment and climate have led to the increased need for freshwater. However, the shortage of naturally available freshwater is becoming more frequent than ever. Desalination, removal of salt and other minerals from seawater, brackish water, and wastewater, is a promising solution to provide the increasing need of freshwater. Multi stage flashing (MSF) and multi effect distillation (MED) can be classified as conventional thermal-driven desalination technologies. Nevertheless, the disposal of high concentrated brine is a big environmental concern. Despite the tremendous improvements in conventional desalination technology, the desalination process is very energy consuming. Due to high expenses of the conventional energy resources, renewable energy sources can provide alternatives. Considering the fact that Concentrated Solar Power (CSP) has been significantly developed during the past two decades, inorganic salts are the perfect candidates as Heat Transfer Fluid (HTF) for high temperature applications in solar thermal energy storage, fully separating water and salts in the desalination process arises naturally, since the leftover salts can be collected instead of disposing them to the environment. A full separation multi effect distillation (FSMED) system was proposed by other researchers, where the air fed to the full separation tank (FST) is preheated by a heat source, and the highly concentrated brine from the last effect is sprayed in the FST, so that the brine droplets can be completely evaporated by leaving behind inorganic salts. Obviously, the full separation process of salts and water involves multiphase heat and mass transfer. However, a key question is the characterization of the lifetime of brine droplets inside the FST during its entire evaporation process, and the effects from the internal circulation to droplet evaporation. If the droplet has strong internal circulation, the definition of two stages of evaporation becomes inaccurate, it has to be corrected based on the detailed calculation of time scale at each stage. In the current study, a 1D quasi-steady evaporation model was developed to characterize the effects from internal circulation to the life cycle of droplets in the FST, compared with the results using the dynamic evaporation model. To simplify the analysis, water droplet with impurity particles was considered, and a case study was also provided to compare the life cycle of water droplets without internal circulation. Finally, a correction to the definition of two-stage of water droplet evaporation was provided. It is expected that the computation results will be beneficial for thermal driven desalination community.

Recently, Concentrated Solar Power (CSP) is attracting more research attentions because it can store the excessive heat from the solar field and extend the power generation at night, CSP can also levelized the mismatch between energy demand and supply. To make CSP technology competitive, thermal energy storage (TES) system filled with energy storage media is a critical component in all CSP plant. TES system can be operated by using sensible materials, phase change materials (PCMs) or a combination of both. Because the phase change materials can store more heat due to the latent during the melting/freezing process, it becomes promising to use PCM in latent heat thermal energy storage (LHTES) system for large scale CSP application. Unfortunately, LHSS has relatively low energy storage efficiency compared to SHSS alone because of the fact that LHSS has more parameters to be controlled and optimized. To realize a complete utilization of PCM and a high energy storage/extraction efficiency and a high exergetic efficiency, one approach is to adopt a cascade configuration of multiple PCMs modules in TES tank, which can also be called as a cascade latent heat thermal energy storage (CLHTES) system. The melting temperatures of the PCMs placed in the TES tank should be cascaded from low to high temperature, where the latent heat of PCM can completely be used to absorb the heat from the solar field for energy storage purpose. Due to the complexity of a CLHTES system, it is necessary to provide a comprehensive study from the heat transfer perspective. This paper presents a preliminary parametric study of CLHTES system using a previously developed enthalpy-based 1D transient model for energy storage/extraction in CLHTES system. The effects of material properties (such as latent heat, specific heat at solid and liquid phase) and CSP plant operation conditions (such as charging/discharging time period) are to be explored. The results from the preliminary parametric study is expected to be beneficial to the community of solar thermal engineering.

Because of the capability of large capacity thermal storage, concentrated solar power (CSP) technology is getting more attentions in the recent years. The energy storage allows power generation using solar energy during the late afternoon and evening time. For a large capacity of thermal energy storage, a dual-media system is typically adopted for reducing the use of the heat transfer fluid (HTF), which is usually expensive. In a dual-media system, the solid material must have large heat capacity and be inexpensive. One type of configuration for a dual-media system is that HTF flowing in pipes which are imbedded into the solid material. The present study considers sands, a major component of concrete, as low-cost solid thermal storage materials. The novel approach is that the sand is saturated with high thermal conductive fluid. Compared to using concrete for thermal storage, this method avoids issues of heat transfer degradation associated with the mismatch of thermal expansion of pipes and concrete. Since only sands are porous materials and the heat transfer performance is low, a high conductive fluid (XCELTHERM® 600 hot oil) was used to saturate sands, which then forms a new thermal storage material that can have better heat transfer. Results of thermal storage process with sands only and with the oil-saturated sands are presented and discussed.

Because of the capability of large capacity thermal storage and extended operation during night and cloudy days, concentrated solar thermal power generation is getting more and more attention in the recent years. Dual-media thermal energy storage system is typically adopted in industry for reducing the use of the heat transfer fluid, which is usually expensive. In such a dual-media system, the solid filler material can be a phase change material relying on latent heat or a regular solid material using sensible heat for energy storage. Two strategies of starting-up fluid charge and discharge are considered for the operation of a concentrated solar thermal power plant incorporated with a dual-media thermal storage system. These two strategies include: 1) starting daily cyclic charge and discharge operation with an initially cold tank; 2) to fully charge the thermal storage system before operation of the cyclic discharge/charge for the power plant. The energy storage efficiency and the effects to the power plant operation due to the application of these two strategies are studied in the current work based on an enthalpy-based 1-D model, and significant difference is found in starting-up process of the daily cyclic operations, which will help us decide the best strategy of operating a thermal energy storage system with more electrical energy output.

A group of eutectic ternary halide salts were surveyed and studied for the objective of developing a high temperature heat transfer fluid with a freezing point below 250°C and a low vapor pressure, below 1.0 atm, at temperatures up to 800°C. The studied salts include: 1) NaCl-KCl-ZnCl 2 with a mole fractions of 18.6%-21.9%-59.5% and a melting point of t m =213°C; 2) NaCl-KCl-ZnCl 2 with a mole fraction of 13.4%-33.7%-52.9% and a melting point of t m =204°C; and 3) NaCl-KCl-ZnCl 2 with mole fraction of 13.8%-41.9%-44.3% and a melting point of t m =229 °C. Vapor pressures of these salts at different temperatures were experimentally obtained using an in-house developed test facility. The results show that vapor pressures of all the three eutectic molten salts are below 1.0 atm at a temperature of 800 °C. The salt of ZnCl 2 -KCl-NaCl in mole faction of 44.3%-41.9%-13.8% has lowest vapor pressure which is only about 1.0 atm even at a temperature of 900 °C. Viscosities of these salts were measured in the temperature range from after melting to 850°C. At low temperatures near their melting points of the salts, the viscosities are about 16 × 10 −3 Pa s , while at high temperatures above 700°C the viscosities are around 4 × 10 −3 Pa s , which is satisfactorily low to serve as heat transfer fluid for circulation in a CSP system. Both the vapor pressure and the viscosities of the studied three eutectic salts demonstrated satisfaction to serve as high temperature heat transfer fluids. Other thermal and transport properties of these salts are expected to be reported in the future for screening out a satisfactory high temperature heat transfer fluid.

With a large capacity thermal storage system using phase change material (PCM), Concentrated Solar Power (CSP) is a promising technology for high efficiency of solar energy utilization. In a thermal storage system, a dual-media thermal storage tank is typically adopted in industry for the purpose of reducing the use of the heat transfer fluid (HTF). While the dual-media sensible heat storage system has been well studied, a dual-media latent heat storage system (LHSS) still needs more attention and study; particularly, the sizing of volumes of storage tanks considering actual operation conditions is of significance. In this paper, a strategy for LHSS volume sizing is proposed, which is based on computations using an enthalpy-based 1D model. One example of 60MW solar thermal power plant with 35% thermal efficiency is presented. In the study, potassium hydroxide (KOH) is adopted as PCM and Therminol VP-1 is used as HTF. The operational temperatures of the storage system are 390°C and 310°C, respectively for the high and low temperatures. The system is assumed to operate for 100 days with 6 hours charge and 6 hours discharge every day. From the study, the needed height of the thermal storage tank is calculated from using the strategy of tank sizing. The method for tank volume sizing is of significance to engineering application.

This paper addresses issues of flow field optimization for a water raceway which is used to grow algae for biofuels. An open channel raceway is the typical facility to grow algae in medium to large scales. The algae growth rate in a raceway is affected by conditions of temperature, nutrients, and sunlight intensity etc. These conditions are essentially associated with the fluid mixing in the flow field. Good flow mixing at low consumption of pumping power for the water flow is desirable for an economic algal growth facility. A novel design of an open channel raceway for medium- and large-scale algae growth field has been proposed by the authors previously, which is called High Velocity Algae Raceway Integrated Design (ARID-HV). Optimization analysis using CFD and experimental visualization has been applied to a table-sized ARID-HV test model with various geometries of dams and their spacing in the system. CFD results and flow visualization allow us to understand the flow mixing in the entire raceway. Data is also processed to show the statistics of the locations of ‘fluid particles’ at different height and time period during one flow path. Different flow field designs were thus compared quantitatively based on this statistics according to the understanding that the “tumbling times” of fluid particles at bottom/top of the water is tightly related to the growth rate of algae.

An enthalpy-based model of thermocline operation general to both single phase and encapsulated phase change filler materials was created in MATLAB. The method of characteristics is applied in space and time, mapping fluid temperature and filler enthalpy to a numerical grid, and in the case of a melting filler, allowing accurate tracking of phase state interfaces to fractional positions of the grid. Careful consideration of various possible heat transfer conditions along with placement of phase state interfaces in the numerical grid allows for extreme versatility and accuracy in model application. Input of specific fluid and filler properties, tank size, time of operation, and initial and boundary conditions returns a full representation to any desired amount of charge/discharge processes or cycles. The paper covers mathematical formulation, certain intricacies of numerical implementation, model verification, and the beginnings of application to prove proper operation and generality.

Making fuel from algae is one of the promising approaches of producing biofuels. Open channel raceway is a typical facility of growing algae in medium and large scales. The algae growth rate in water raceway is affected by conditions of water temperature, nutrients, and sunlight. These conditions are essentially associated with the fluid mixing in the flow field. A good mixing of fluid allows better diffusion of nutrients and equal opportunities of exposure to the water surface and therefore sunlight, as well as a uniform temperature everywhere in water raceway so that all algae cells grow in the same rate. While a better fluid mixing is benefit to the growth of algae, it is also desirable that the energy needed to drive the flow and mixing being the minimum. In this work, a novel flow field has been proposed and the flow field was studied through flow visualization and CFD analysis. Optimization of the flow field for better flow mixing and low energy cost for the flow has been considered.