Recent research and development on ramjet and supersonic combustion ramjet (scramjet) engines is concerned with producing greater thrust, higher speed, or lower emission. This is most likely driven by the fact that supersonic/hypersonic propulsion systems have a broad range of applications in military sectors. The performances of such supersonic/hypersonic propulsion systems depend on a series of physical and thermodynamic parameters, such as the fuel types, flight conditions, geometries and sizes of the engines, engine inlet pressure/velocity. As a propulsion system, a stable and efficient combustion is desirable. However, self-excited large-amplitude combustion oscillations (also known as combustion instabilities) have been observed in liquid- and solid-propellant ramjet and scramjet engines, which may be due to acoustic resonance between inlet and nozzle, vortex kinematics (large coherent structures), and acoustic-convective wave coupling mechanisms due to combustion. Such intensified pressure oscillations are undesirable, since they can lead to violent structural vibration, and overheating. How to enhance and predict the engines’ stability behaviors is another challenge for engine manufacturers. The present work surveys the research and development in ramjet combustion and combustion instabilities in ramjet engines. Typical active and passive control of ramjet combustion instabilities are then reviewed. To support this review, a case study of combustion instability in solid-fueled ramjet is provided. The popular mode decomposition algorithms such as DMD (dynamic mode decomposition) and POD (proper orthogonal decomposition) are discussed and applied to shed lights on the ramjet combustion instability in the present case study.

This is a commemoration of the work of Don Nield. His theoretical and applied mathematical models have had a tremendous impact in many fields including convection heat and mass transfer, stability analysis, nanofluid enhanced convection, and flows in porous media. Here the major contributions of Don Nield’s Scopus-indexed outputs are reviewed. His research outputs and related impact are presented chronologically. The citation counts are presented with respect to each work published in that year. The work and impact of Don Nield as single author and in his collaborations are presented.

Hydropower is an early and well-developed form of electricity generation in New Zealand, as well the most important form of electricity generation globally. This paper analyses the world's energy structure, along with New Zealand's energy structure, the distribution of existing hydroelectric power plants, hydroelectric power operators, the distribution of hydroelectric grids, types of hydroelectric power stations and types of hydro turbines. Furthermore, it also compares the hydropower development in New Zealand relative to other nations, such as China and Norway. New Zealand's power plants: hydro, geothermal and wind farms, are compared in terms of environmental impact, investment benefits, levelized generation costs, Māori culture and generation technology. Because countries worldwide are acting towards the 2050 carbon neutral, the use of electric vehicles is gradually increasing. Finally, Therefore, a surge in electricity generation, and its usage, is anticipated. To overcome this issue, upgrading and expanding existing hydropower plants or building small hydropower is considered to be one of the best options for New Zealand to fulfill the electricity demand and commitment towards low carbon emission.

The present study is concerned with numerical investigations on the electrical power output, and NO emissions of an ammonia/hydrogen fuelled micro-thermo-photovoltaic (MTPV) system under extreme operational conditions. For this, three critical parameters are identified and examined. They include: (1) the inlet velocity, (2) the inlet equivalence ratio, and (3) the mole blending ratio of hydrogen. Increasing the inlet velocity markedly raises the electrical power output of the MTPV system. At lower inlet velocities, a higher background temperature helps reducing NO emissions (NO emissions at 2 m/s and 200K are 5.5% higher than at 350K), whereas at higher inlet velocities, lower background temperatures are more effective in reducing NO emissions (NO emissions at 12 m/s and 350K are 6% higher than at 200K). The electrical power output of the MTPV system is maximized, when the inlet equivalence ratio is set to 0.9, yielding a total power output of 15.2W. For optimizing energy efficiencies, an inlet equivalence ratio of 0.8 is preferred, with an energy efficiency of 6.3%. Blending ammonia with hydrogen significantly increases NO emissions (NO emissions at 200K with a hydrogen blend ratio of 0.5 are 26% higher than with the blending ratio of 0.1) and provides limited improvement in the energy output of the MTPV system (energy output at 200K with a hydrogen blend ratio of 0.5 is 10% higher than that with a blending ratio of 0.1, and the energy output at 300K with a hydrogen blend ratio of 0.5 is 7% higher than that at a blending ratio of 0.1).

This study addresses the challenges in utilizing ammonia as a CO2-free fuel. However, the drawbacks of ammonia are-low burning speed and nitrogen oxide emissions. To enhance ammonia's flammability in atmospheric micro-combustion, hydrogen is blended in the fuel. First, a simplified chemical reaction mechanism of ammonia consisting of 44-step reactions and 19 species specifically for ammonia is developed and validated using experimental data to reduce computational cost and time. Then, we proposed a heat-recirculating microcombustor fueled by premixed hydrogen/ammonia/air. Five key parameters are identical to numerically studying the thermal performance, entropy generation, and NO emissions. The present findings confirm that higher inlet velocity boosts thermal performance (power output achieving 15.8 W at 7 m/s) and NO emissions peak at 3 m/s (0.0169). Unity equivalence ratio optimizes thermal performance, rich-fuel combustion reduces NO emissions. At the stoichiometric ratio, the power output is 8.34 W, with the highest NO emissions at 0.9 (0.168). Hydrogen blending has a small effect on the performance (8.5 W at xi m = 0.7, 0.51 W above xi m = 0.4), but effectively reduces NO emissions (xi m = 0.7 NO emissions around 30 % lower than xi m = 0.4). Furthermore, changing the material from steel to Corundum enhances power output by approximately 6 %, while longer heat recirculation improves thermal performance.

As conventional fossil fuel is depleting, ammonia has attracted extensive attention as a renewable fuel, which could be made from water, air, and sun. There is current interest in burning ammonia in macro, meso, and micro-combustors. The present work is concerned with the numerical investigations of the entropy production, thermodynamic exergy performance from ammonia/methane-fueled micro-combustors with a single-channel inlet and double-channel outlet (SIDO) in the presence and absence of a porous media (PM). For this, a 3D time-domain model is developed. With the model being validated with experimental data available in the literature, it is then applied to examine the effects of 1) equivalence ratio (0), 2) inlet velocity (Vin), 3) blending/mixing ratio (0b) between the methane and ammonia, and 4) PM porosity (s). In comparison with the combustion system without PM, the application of PM is found to lead to a significant improvement on thermal performances, as Vin is varied. It is found that there is a substantial 37.5% reduction in the standard deviation of the combustor outer wall temperature ST,W at Vin = 2.0 m/s. The optimal thermal performance is achieved, as 0 = 0.9. However, nitrogen oxide emission is shown to be decreased, as 0 is increased. As the entropy production is concerned, the s of PM is shown to exhibit a notable influence. A higher porosity gives rise to a lower entropy production within the PM. The lowest entropy production resulting from heat conduction is shown to be achieved, when s = 0.8. By implementing PM, the exergy efficiency (hexergy) is found to be increased by 23.9% at Vin = 2.0 m/s. In general, the present investigation shed physical insights on the entropy production and thermodynamic exergy performances of ammonia/methane-fueled micro-combustion systems with and without PM.(c) 2023 The Author(s). Published by Elsevier Ltd on behalf of Hydrogen Energy Publications LLC. This is an open access article under the CC BY license (http://creativecommons.org/ licenses/by/4.0/).

Self-actuated bimorph cantilevers are implemented in a variety of micro-electro-mechanical systems. Their tip deflection relies on the unmatched coefficients of thermal expansion between layers. The thermal bimorph phenomenon is dependent on the temperature rise within the cantilever and, while previous studies have investigated variations in the thermal profile along the cantilever length, these have usually neglected variations in the thermal profile along the cantilever thickness. The current study investigates the thermal distribution across the thickness of the cantilever. The exact closed form solution to the one-dimensional problem of heat conduction in the composite (layered) domain subjected to transient volumetric heating is developed using the appropriate Green’s function. This solution is applied to a one-dimensional case study of a 3-layer cantilever with an Aluminium heater, a silicon dioxide resistive layer, and a silicon base layer. The aluminium heater experiences volumetric heating at a rate of 0.2 mW/μm3 of 5 μs duration at 100 μs intervals (10 kHz with a 1/20 duty cycle). Benchmark solutions of the temperature at select times and positions are provided. It is shown that there are negligible temperature gradients across the cantilever thickness during the heating and the first 5 μs afterward. These short-lived temperature differences are positively biased with the unmatched thermal expansion coefficients between the layers, though their relative influence on bending is not clear. A simple parametric analysis indicates that the relative magnitude of the temperature differences across the cantilever (compared to the overall temperature) decreases substantially with increasing duty cycle.

The vaporization of a freely moving drop in a uniform, high-temperature gas stream is investigated through direct numerical simulation. The incompressible Navier-Stokes equations with surface tension and phase change are solved in conjunction with the energy equations of each phase. The sharp liquid-gas interface is tracked using the geometric Volume-of-Fluid (VOF) method and an immersed Dirichlet boundary condition for temperature is imposed at the interface. The simulation approach is validated by simulating water and acetone drops at nearly zero Weber numbers, and the simulation results agree very well with the empirical relation for spherical drops. Parametric simulations were conducted to investigate the aerodynamic breakup of vaporizing drops at low to moderate Weber and Reynolds numbers. The range of Weber numbers considered has covered the vibrational and bag breakup regimes. Through the simulation results, we have characterized the impact of drop deformation and breakup on the drop vaporization rate. When the drop Weber number increases, the windward surface area increases more rapidly over time. As a result, the rate of drop volume reduction also increases. The correlation between the vaporization rate and the windward surface area is examined for different Weber and Reynolds numbers. Using the approximate correlation between the drop vaporization rate and the windward surface area and the TAB model for drop deformation, a new time-dependent drop vaporization model is proposed. The present model agrees well with the simulation results and shows a significant improvement over the conventional model for spherical drops.

With the emergence of global warming and various extreme climates,climate change governance has become one of the focus of attention in the current development process of the international communi-ty.By reducing the emission of carbon dioxide,nitrogen oxides and particles,we can save energy,reduce the damage to the environment,and simultaneously create a more comfortable human living environment.

The vaporization of a freely moving liquid droplet in a uniform high-temperature gas stream is investigated through direct numerical simulation.The sharp liquid-gas interface is tracked using the geometric Volume-of-Fluid (VOF) method.The incompressible Navier-Stokes equations are solved in conjunction with a two-fluid model for the thermal energy advection and conduction, with an immersed Dirichlet boundary condition at the interface to implicitly account for the latent heat absorption.The model is implemented in the open-source solver, Basilisk, which uses adaptive quadtree/octree mesh for spatial discretization and will allow for adaptive mesh refinement of the region near the interface.An acetone droplet at a moderate Weber number is simulated where the drop deforms into a bag shape and experiences breakup.The rate of vaporization of the drop is then increased to study the influence of vaporization on the drop breakup behaviour.By increasing the rate of vaporization, we observe the suppression of droplet breakup, making an otherwise unstable droplet stable.

The present study proposes a reverse flow single-channel inlet and double-channel outlet (SIDO) microcombustor for the analysis of the thermal performance and nitrogen oxide emission characteristics of ammonia/hydrogen-fuelled energy conversion system. Comparison is then made between the proposed system and the conventional single-inlet and single-outlet system. The present findings show that the SIDO combustor is associated with enhanced thermal performances. Increasing the inlet pressure Pin improves thermal performance and the exergy efficiency while reducing nitrogen oxide emissions. Increasing the inlet flow velocity Vin can enhance the temperature uniformity of the combustor wall. It is also found that the ammonia combustion convection heat transfer performance is optimized, when Vin = 1.25 m/s. Increasing the equivalence ratio & phi; leads to a reduction of nitrogen oxide emissions, and the micro-combustor has a better overall performance, when & phi; = 1.0. Finally, Increasing the blending ratio of hydrogen with ammonia & phi;b gives rise to a decayed advection but enhanced diffusion, and the total pressure loss (Ploss) can be reduced. This present study confirms the viability of employing the SIDO reverse flow structure to enhance thermodynamic performances from microcombustion energy conversion systems.

To enhance the hydrogen-fueled micro-combustor's thermal performance for Thermo-photovoltaic applications, we propose and test a double-channel design featuring Y-shaped internal fins. This study investigates three key parameters in the thermal performance of a micro-combustor: the inlet velocity, the inlet equivalence ratio, and the combustor wall material. Further, we develop a new method to calculate the efficiency and exergy of the micro-combustor by considering the entropy generation and exhaust gas. It is observed that increasing the inlet velocity leads to higher mean wall temperature (MWT), standard deviation of wall temperature (SDWT), and radiation heat release rate. Increasing inlet velocity also enhances the thermodynamic second-law efficiency and exergy. The peaks of MWT and SDWT are achieved when the inlet equivalence ratio approaches unity. The inlet equivalence ratio value of unity also corresponds to the maximum radiation heat rate and improved second-law efficiency and exergy. Finally, changing the combustor wall material reveals that the silicon carbide demonstrates better thermal performance by rising MWT and the uniformity of the combustor wall temperature. Moreover, alterations to the combustor wall materials do not appear to have a significant impact on the heat loss resulting from entropy generation.

Micro-combustion systems play a critical role in powering microreactors, micropropulsion and other micro -energy conversion systems. In this work, we have numerically investigated a double-channel counter-flow micro-combustor fuelled by premixed ammonia/hydrogen/oxygen parametric to investigate both NO emissions and thermal performance. The model is used to examine the effects of 1) the inlet velocity, vin, and 2) the inlet equivalence ratio, phi. The enhanced thermal performance is quantified by the increased mean wall temperature and the increased wall temperature uniformity. We find that thermal performance increases with increasing vin. The maximum level of NO is present atvin = 2 m/s. Meanwhile, the mean wall temperature benefits from phi = 1, and further increases in equivalence ratio can lessen the uniformity of the wall temperature. Furthermore, more fuel-rich ammonia combustion can lead to a lower NO emission and improve the thermal performance. To enhance ammonia combustion, further investigations are conducted by blending with different molar fractions of H2 (XH2 ). It is found that mixing ammonia with more H2 can stabilize micro-combustion, and increase the temperature in the combustion field. Additionally, it makes the emission worse. The maximum mean wall temperature occurs atXH2 = 0.25, While the NO emission peaks atXH2 = 0.3. Moreover, the OH mole fraction can affect the formation of NO, positively.

In this work, we develop a 2D numerical model of a Y-shaped bifurcating combustor with a Helmholtz resonator attached. Propane (C3H8) is fueled and burnt with air by applying single-step eddy dissipation combustion model and k−ϵ−RNG turbulence model for simplicity. To validate the numerical findings, experimental measurements are conducted on a bifurcating Y-shaped thermoacoustic combustor with an off-design Helmholtz resonator implemented. It is found that the frequency and amplitude of the dominant mode as experimentally measured agree well with the numerical results. Further agreement is obtained between numerically and theoretically predicted mode-shapes. With the model validated, it is applied to gain insights on the entropy generation and nonlinearity of the pulsating oscillations and the flow fluctuations across the resonator neck. It is found that the nonlinearities are originated in the unsteady heat release rate of the premixed propane flame, and the mass flow rate across the resonator neck. In addition, the rate of the entropy production depends strongly on the temperature fluctuations. Approximately 99% of the entropy production rate involves with the temperature oscillations. Furthermore, it is non-uniformly distributed along the combustor. In addition, the energy conversion rate between total heat release rate and the acoustical energy production rate is less than 0.0001%. Finally, the production of CO2 is increased exponentially, and then reduced gradually in the axial flow direction. In general, the present work provides a low-cost numerical tool of a bifurcating thermoaocustic system. It could be applied to predict the acoustic signature of the combustor and to examine and evaluate the performance of the Helmholtz resonator on attenuating self-sustained thermoacoustic oscillations.

This study presents a numerical analysis of first and second Law performances of coun-terflow double-channel micro-combustors. A parametric analysis is conducted to deter-mine the influence of the inlet velocity, the chamber geometry, and the fuel composition on the mean wall temperature, the uniformity of the wall temperature, and the second law characteristics. We show that the double-channel combustor greatly increases the uni-formity of the wall compared to the single channel configuration. The oval shaped thread configuration results in the highest wall temperature and the wall temperature uniformity. Increasing inlet velocity results in an increased mean wall temperature and a slightly reduced wall temperature uniformity. The greatest contributor to entropy generation is found to be resulting from chemical reaction. The analysis showed that the second law efficiency was just below 0.5 regardless of the thread shapes. The thread shapes had no major influence on the combustor exergy. A blended ammonia-hydrogen fuel is shown to be resulted in a slightly higher mean wall temperature and a less uniform wall temperature.(c) 2022 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

Turbofan engines are one of the most popular propulsion systems used in commercial aircraft due to their high thrust and good fuel efficiency. To reduce noise generated from turbofan engines-powered aircraft, a number of control approaches have been developed. The dominant noise sources include the fan and the high-speed ‘hot’ and ‘cold’ jet. In engineering applications, the noise control approaches include: 1) active control, 2) geometric shape optimization, and 3) passive control (including acoustic boundary control). Because they are considered the most reliable and effective noise reduction methods, the geometric shape optimization and passive control are preferable by the engine manufacturers. In this work, we briefly overview the noise reduction technologies that have great potential to be applied or implemented on turbofan engines. The research and development progress made on the active control, passive control, and geometric shape optimization are reviewed and discussed, aiming to provide an useful guidance on next-generation low-noise turbofan engines. The fundamental noise damping mechanisms of thermos-viscous and vortex shedding are finally overviewed via cases studies.

This study numerically investigates the drug uptake by a population that includes both reversibly and irreversibly electroporated cells. A theoretical continuum model is developed and simulations are conducted in conditions representing low porosity (cells in tissues) and high porosity (cells in suspension). This model considers only passive diffusion following the electroporation pulse and estimates the permeability increases of reversibly electroporated cells using empirically based predictions that relate the long-lived electropore density to the electric field magnitude. A parametric study investigates whether the permeability and resealing rate of irreversibly electroporated cells influence the delivery to the surviving reversibly electroporated cells. The results show that this influence is negligible when the cell number density is low (cells in dilute suspensions). For conditions of cells in tissue when both the fraction of the total cells that are irreversibly electroporated and the permeability of the irreversibly electroporated cells are high enough, the irreversibly electroporated cells rapidly take up the drug and deplete the extracellular space of the available drug. This lowered extracellular concentration can result in less drug delivery to reversibly electroporated cells.

Using a numerical model, this study investigates the spray drying process of whole milk by providing statistics on droplet conditions at exit and first impact with the surfaces of the chamber. A comprehensive four-stage droplet evaporation model was validated against an experiment and then coupled to an Euler–Lagrange model for simulating the milk droplet trajectories inside a dryer. Results show that larger droplets remain for a shorter time in the chamber and contain more moisture on exiting. Higher injection angles result in longer residence times, which leads to lower moisture content for the droplets. By increasing the injection velocity of droplets, their relative velocity compared to the airflow increases, raising the rate of evaporation, and consequently more droplets exit the chamber as fully dried. Furthermore, decreasing the airflow rate and humidity, as well as increasing the airflow inlet temperature, results in a lower moisture content in the final powder. When the droplets impacted the wall of the chamber for the first time, nearly 67% of them were fully dried, and 55% had a velocity magnitude less than 1 m/s. A considerable portion of droplets (close to 52%) impacted the wall at an angle of less than 8°.

The standing-wave thermoacoustic engines(TAE) are applied in practice to convert thermal power into acoustic one to generate electricity or to drive cooling devices. Although there is a number of existing numerical researches that provides a design tool for predicting standing-wave TAE performances, few existing works that compare TAE driven by cryogenic liquids and waste heat, and optimize its performance by varying the stack plate spacing. This present work is primarily concerned with the numerical investigation of the performance of TAEs driven by cryogenic liquids and waste heat. For this, three-dimensional(3-D) standing-wave TAE models are developed. Mesh-and time-independence studies are conducted first. Model validations are then performed by comparing with the numerical results available in the literature. The validated model is then applied to simulate the standing-wave TAEs driven by the cryogenic liquids and the waste heat, as the temperature gradient ΔT is varied. It is found that limit cycle oscillations in both systems are successfully generated and the oscillations amplitude is increased with increased ΔT. Nonlinearity is identified with acoustic streaming and the flow reversal occurring through the stack. Comparison studied are then conducted between the cryogenic liquid-driven TAE and that driven by waste heat in the presence of the same temperature gradient ΔT. It is shown that the limit cycle frequency of the cryogenic liquid system is 4.72% smaller and the critical temperature ΔT cri =131 K is lower than that of the waste heat system(ΔT cri =187 K). Furthermore, the acoustic power is increased by 31% and the energy conversion efficiency is found to increase by 0.42%. Finally, optimization studies on the stack plate spacing are conducted in TAE system driven by cryogenic liquids. It is found that the limit cycle oscillation frequency is increased with the decreased ratio between the stack plate spacing and the heat penetration depth. When the ratio is set to between 2 and 3, the overall performance of the cryogenic liquid-driven TAE has been greatly improved. In summary, the present model can be used as a design tool to evaluate standing-wave TAE performances with detailed thermodynamics and acoustics characteristics. The present findings provide useful guidance for the design and optimization of high-efficiency standing-wave TAE for recovering low-temperature fluids or heat sources.