The Limerick bubbly flow rig: design, performance, hold-up and mixing pattern

peer-reviewedAs Euler-Euler CFD simulations of bubbly flows suffer from uncertainties due to the many underpinning models, there is an obvious need of accurate experimental data for validation. With this in mind, a new bubbly flow test rig was built to be operated with and without liquid co-flow, with bubble size as uniform as possible in the range 4–7 mm, and with a very even horizontal bubble distribution. We designed the gas sparging system such that we can also produce an essentially bi-modal bubble size distribution. The column consists of two square sections to allow for studying the mixing of two originally separated bubbly flows with either the same or a different bubble size. The bubbles are produced from 2 × 196 needles, bubble sizes are determined with high-speed imaging and with a simple acoustical method, overall volume fractions in the column by means of air chamber pressure measurements. Overall volume fractions are presented as a function of gas and liquid flow rates, with slip velocity mostly increasing with increasing void fraction. First results are obtained on (a) producing bi-model bubble size distributions and the pertinent volume fractions in the column, and (b) flow patterns in the case of unequal aeration

Punishment for 16 Sins

"For Serie XVI, she created Punishment for 16 Sins, where the main protagonist, a metal object in the shape of a gasket, is being tortured and pulled apart, while other objects observe the punishment from the sidelines. Working with rich ochres, she renders a seductive, yet menacing mood by developing light, space, and depth in the print. She cites ancient Chinese painting, Hieronymus Bosch, and surrealist painting as primary influences." - excerpt from Serieproject.org16 x 22 i

Design of a crash energy absorber for a composite aircraft fuselage using a combined analytical-numerical approach

This thesis aims to provide more insight into the crashworthiness behaviour of a composite aircraft fuselage. This is achieved by studying both analytically and numerically the crushing behaviour of composite energy absorbers. The analytical model, which is based on energy dissipation rates, generalises previously derived analytical models to study a wider variety of structural components. It is found that the analytical model can give an estimation of the mean crushing load of square, C-shaped, and corrugated specimens. Next to the analytical study, numerical models are created to gain insight into the behaviour of the studied shapes when subjected to crash loading. Another part of the numerical study investigates the effectiveness of different material models to capture the complex failure mechanism of the composite crash absorber. In LS-Dyna, the MAT054, MAT058, and MAR262 material models are used, which are all able to reproduce the results obtained from reference tests. The ability of the material models to recreate the crushing phenomenon is thought to originate from the opportunity LS-Dyna offers to degrade material properties in the crush frond, which incorporates the formation of cracks and delaminations. In the final stage of this research, a case study is performed, where different absorbers are introduced into a simplified digital twin of the Next Generation Multifunctional Fuselage Demonstration as developed by the STUNNING project. Here, it is found that the fuselage's energy absorption can be increased by introducing energy absorbers. All the simulated fuselage sections show similar behaviour in the first crash stages, characterised by the flattening of the lower section of the frame. Subsequently, the behaviour of the absorber and the surrounding structure highly depends on the absorber's integration. Finally, it is found that by combining the numerical results of a baseline fuselage section and the analytical model for the energy absorbers, one can estimate the energy absorption of the augmented structure with a discrepancy of less than 20$\%$. This proves that the suggested analytical-numerical method can aid engineers during the preliminary design for crashworthiness of a thermoplastic composite aircraft fuselage.Aerospace Engineerin

Technical and economic feasibility assessment of a Car Park as Power Plant offering frequency reserves

In the Netherlands the main sources of electricity are currently coal and gas fired power plants. Due to the increasing share of electricity that is produced from renewable energy sources, the operational hours of these conventional power plants are decreasing. However, these power plants are also the main source of frequency reserves, which are required to guarantee the stability of the electricity grid. Stability is maintained if there is a real time balance between the electricity production and consumption. Due to the decreasing availability of conventional power plants, the possibility to offer frequency reserves with other power sources must be explored. A possible provider is the Car Park as Power Plant (CPPP). This is an aggregation of Fuel Cell Electric Vehicles (FCEV) parked in a car park and operating in Vehicle to Grid (V2G) mode. This thesis contains a technical and economic feasibility assessment of a Car Park as Power Plant offering frequency reserves in a future power system with a low share of conventional power plants.The dynamics of the frequency was analysed considering the reduction of the operational hours of the conventional power plants. This causes the inertia in the system to decrease. As a consequence, this increases the rate of change of frequency (RoCoF). A high RoCoF makes the frequency react faster which will make it more feasible that the maximum instantaneous frequency deviation will be reached. When this maximum deviation is reached, the system enters the alert state, which could endanger the global security of the system. To prevent the frequency from reaching the maximum instantaneous frequency deviation under conditions of a high RoCoF, the full activation time (FAT) of the frequency reserves could be reduced. The FAT is the maximum time between the moment that the signal for a change in power output is given and the actual moment that the required power output is reached. Reserves with a reduced FAT are referred to as fast frequency reserves. By measuring the FAT of the FCEV in V2G mode with an experimental setup, it was tested if the FCEV could offer the fast frequency reserves. The FCEV appeared to be a suitable power source to offer fast frequency reserves. When the power output of the battery and the fuel cell are combined, an even higher power gradient and thus a shorter full activation time, can be obtained. However, improvements must be implemented in the V2G discharge unit and the energy management system of the FCEV to optimise the operation in V2G mode. The economic feasibility of the CPPP as provider of frequency reserves was then analysed. The factor that has the highest impact, is the position of the CPPP on the bid ladder. Only when the price for frequency reserves is relatively high, the reserves of the CPPP will be activated. This is caused by the high price for hydrogen, which is the dominant factor in the marginal costs of the CPPP. Most of the time the prices for frequency reserves and for electricity sold on the spot market are lower than the marginal costs of the CPPP.From a technical point of view the CPPP is a suitable power source to offer fast frequency reserve. The profitability of the CPPP is, however, strongly related to the occupation pattern of the car park and the price for frequency reserves. The occupation pattern can be influenced by the aggregator by giving incentives to the car owners, combining different car parks with deviating occupation patterns or by operating company car parks with, for example, autonomous driving cars. The price for frequency reserves is dependent on the quickly evolving market and can not be influenced by the aggregator of the CPPP. The aggregator can influence the marginal costs of the CPPP. By adding relevant components to the system, like hydrogen production units or storage facilities, the aggregator can offer reserves with different power sources, which will have different marginal costs. From the point of view of the transmission system operator, it could be possible to add an extra product for fast frequency reserves instead of changing the requirements of the existing reserves. This could have a positive effect on the profitability of the CPPP. The specific requirements such as the minimum bid size, validity period and payment mechanism for this product should be evaluated in further studies

Experiments on the motion of gas bubbles in turbulence generated by an active grid

The random motion of nearly spherical bubbles in the turbulent flow behind a grid is studied experimentally. In quiescent water these bubbles rise at high Reynolds number. The turbulence is generated by an active grid of the design of Makita (1991), and can have turbulence Reynolds number R[lambda] of up to 200. Minor changes in the geometry of the grid and in its mode of operation improves the isotropy of the turbulence, compared with that reported by Makita (1991) and Mydlarski & Warhaft (1996). The trajectory of each bubble is measured with high spatial and temporal resolution with a specially developed technique that makes use of a position-sensitive detector. Bubble statistics such as the mean rise velocity and the root-mean-square velocity fluctuations are obtained by ensemble averaging over many identical bubbles. The resulting bubble mean rise velocity is significantly reduced (up to 35%) compared with the quiescent conditions. The vertical bubble velocity fluctuations are found to be non-Gaussian, whereas the horizontal displacements are Gaussian for all times. The diffusivity of bubbles is considerably less than that of fluid particles. These findings are qualitatively consistent with results obtained through theoretical analysis and numerical simulations by Spelt & Biesheuvel (1997)