As a promising 3D bioprinting process, laser induced forward transfer (LIFT) has attracted attention in the last decade due to its advantages of non-contact, nozzle-free, high dropping rate and high resolution. However, the mechanism of bubble/jet formation under laser inducement has not been well comprehended yet. To better understand the multiphase process, the bubble formation and jet process under single laser pulse was explored in this study, using both the Computational Fluid Dynamics (CFD) model and experimental study. The results showed that under a laser pulse with the Gaussian distribution, a vapor bubble was formed around 0.1μs, then the bubble was expanded over time. During the bubble expansion process, the maximum magnitude of velocity could reach as high as 22m/s. The pressure near the laser interaction area was around 4.72 × 10 7 Pa, which is 470 times of the ambient pressure. After increasing the pulse energy and focal spot area, the liquid bubble layer moved downward to complete the bioink transfer process after the collapse of glycerol vapor bubble, which showed similar flow characteristics as the experimental results under the same laser fluence (1.4J/cm 2 ). When the laser fluence was decreased to 0.8 J/cm 2 , a regular jet flow could be observed. The proposed multiphase numerical model can be used to understand the mechanism of bubble/jet formation under laser inducement and provide some insights into the bioink transfer during LIFT process, in order to eventually optimize the LIFT 3D-printing process with greater cell viability.

In this study, a three-dimensional solid oxide fuel cell stack numerical model was developed and validated by experimental data. Five-cell SOFC stacks with different inlet and outlet manifold arrangement were investigated employing the numerical model. The stack polarization curve, current density, temperature, and flow distributions were analyzed. The results show that the effect of different manifold arrangement is almost neglected on the polarization curves and flow distribution. However, the current density and temperature distribution in individual unit of the stack can be influenced by the locations of inlets and outlets in the manifolds. Considering current density and temperature distribution uniformity as well as ease manufacturing and installation, the manifold arrangement with inlets at the top and outlets at the bottom for both fuel and air is recommended for SOFC stacks.

In this study, both electro-dynamic balance (EDB) method and pendant droplet method were employed to study the evaporation and crystallization process of brine droplet. The EDB setup was used to levitate the charged micrometer sized droplets to study the evaporation process. The pendant droplet method could hang millimeter sized droplet to study the crystallization process. The evaporation of brine droplets with different mass concentrations was visualized by a high speed camera under different temperatures and relative humidity in the test chamber. The results showed that EDB method could get the accurate droplet evaporation results which obeyed the classic D2-law. It was found that the evaporation was increased with the decrease of relative humidity. Due to the attractive force provided by sodium and chloride ion on water molecules, evaporation rate of brine droplet was smaller than pure water, therefore the evaporation rate decreased with the increase of mass concentration by showing a linear relationship. In addition, a salt shell could be formed at the outside of droplet which still contained some amount of salt solution inside. Consequently, the water molecules need to overcome the pressure difference inside and outside the salt shell, and diffuse through the shell for further evaporation. For a higher relative humidity, a more round shell would be formed during the evaporation, and the growth of crystallization along the filament is weaker than that of smaller relative humidity. We hope this study can provide a different perspective to the heat transfer community about the evaporation of brine droplet.

Ocean wave is a rich source of renewable energy with much higher power density than winds. Various WEC technologies have been proposed or are under development. In this study, we developed a 2-dimensional (2D) model and analyzed the rotational motion of the lift-type rotor’s blade under steady flow and unsteady flow. The numerical model was validated by experiments under steady flow. Fast Fourier Transform (FFT) analysis was performed to identify the major contribution of frequency in terms of vortexes generated in the flow field. A comparative study was also performed by comparing all the cases in terms of energy conversion efficiency under different wave conditions. It turns out that the efficiency of energy conversion has a maximum value in the steady flow, while the efficiency for unsteady flow keeps decreasing, therefore this is highly due to the increased dissipation because of the oscillating. When the flow is oscillating, the rotational speed of the rotor under periodic condition is lower than the rotational velocity with steady flow, and a curve fitting was performed in this study to predict the periodic average rotational speed. We conclude that for oscillating flow a minimum of 1.6% energy conversion efficiency can be expected, but it may vary for the actual ocean waves. It is expected the current 2D simulation results can contribute to the wave energy community, especially when the rotor design and optimization is required.

Thermoacoustics (TA) deals with the conversion of heat into sound and vice versa. The device that transfers energy from a low temperature reservoir to a high temperature one by utilizing acoustic work is called TA cooler (TAC). The main components of a typical TA device are a resonator, a regenerator (stack of parallel plates) and two heat exchangers. The thermoacoustic phenomenon takes place in the stack when a nonzero temperature gradient imposed along the regenerator (i.e. parallel to the direction of the sound wave propagation) interacts with the sound wave oscillations. The low temperature at the cold of TAC can be used to condense humid water from the air and also reduce the moisture in the air at some humid areas. In the current study, the high intensity sound waves was produced by the speaker to drive a TA cooler to produce cooling power at a cold temperature of around 18°C. The drainage of condensate in the regenerator is the key for the system performance, because if the porous structure will be blocked by the condensate, TA phenomenon cannot take place in the regenerator. This work is dedicated to investigate the effect from temperature gradient created in TAC for condensation enhancement. 3D printer was used to design and fabricate different structures of regenerator, and then the systematic cooling capacity was measured and compared with different designs of regenerators. Energy balance was also discussed for each type of regenerator. The potential application of this investigation can be an autonomous thermoacoustic cooler system for water harvesting in arid areas. This work can be used to evaluate how the TA effect can be affected by the condensation if humid air is used as the working fluid.

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.

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.

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.