Numerical simulation of hydraulic performance with free overfall flow

Recently, free overfall flows have attracted attention of many researchers in the world. It is because the free overfall can be used as a tool in measuring the discharges and flows in any open channel systems or water collection systems for various flow regime at different shape of channels. In this study, the hydraulic performances of free overfall flow were explored using numerical simulation for three shapes of open channels including uniform rectangular, rectangular circumscribed, and rectangular inscribed steps. The hydraulic performances discussed in this study are including the flow regime, the maximum pressure, drop length and the energy dissipation rate. Numerical simulations were performed under different discharges Q (between 0.0005 m3/s and 0.0015 m3/s) and excise repair densities (y/H) for 1:2, 1:3 and 1:4. Visual observations identified various flow regimes with largely identical flow patterns in different discharges and configurations. Comparisons of properties indicated that there is a better depressurization and energy dissipation performances at the open channel with the shape of rectangular inscribed steps compare to the uniform rectangular in the small charge. Comparative study also revealed that the large excise repair densities of rectangular circumscribed steps might be optimal in terms of energy dissipation performances.

Computational and Experimental Study on Pressure Drop in a Fluidised Bed with Different Air Distributor Designs

Fluidised bed combustion (FBC) has been recognised as a suitable technology for converting a wide variety of fuels into energy. In a fluidised bed, the air is passed through a bed of granular solids resting on a distributor plate. Distributor plate plays an essential role as it determines the gas-solid movement and mixing pattern in a fluidised bed. It is believed that the effect of distributor configurations such as variation of free area ratio and air inclination angle through the distributor will affect the operational pressure drop of the fluidised bed. This paper presents an investigation on pressure drop in fluidised bed without the presence of inert materials using different air distributor designs; conventional perforated plate, multi-nozzles, and two newly proposed slotted distributors (45° and 90° inclined slotted distributors). A 3-dimensional Computational Fluid Dynamics (CFD) model is developed and compared with the experimental results. The flow model is based on the incompressible isothermal RNG k-epsilon turbulent model. In the present study, systematic grid-refinement is conducted to make sure that the simulation results are independent of the computational grid size. The non-dimensional wall distance, is examined as a key factor to verify the grid independence by comparing results obtained at different grid resolutions. The multi-nozzles distributor yields higher distributor pressure drop with the averaged maximum value of 749 Pa followed by perforated, 45° and 90° inclined distributors where the maximum pressure drop recorded to be about one-fourth of the value of the multi-nozzles pressure drop. The maximum pressure drop was associated with the higher kinetic head of the inlet air due to the restricted and minimum number of distributor openings and low free area ratio. The results suggested that low-pressure drop operation in a fluidised bed can be achieved with the increase of open area ratio of the distributor.

Thermal-Fluid Analysis and Optimization of Micro-Scale Heat Sinks With Slip Boundary Condition Using Entropy Generation Minimization Method

Performance of a micro-scale heat sink with slip boundary condition at walls is evaluated using entropy generation minimization (EGM) approach. Optimum geometrical parameters and working conditions are obtained by applying two different constraints of fixed microchannel height and constant maximum pressure drop. It has been observed that with fixed channel height constraint, the optimum aspect ratio and Reynolds number are 2.5 and 700, respectively. Slip boundary condition has negligible effect on optimum working parameters while it enhances the thermal performance of the heat sink by almost 4% at fixed height constraint. At maximum pressure drop constraint, the optimum working parameters are aspect ratio of 2.25 and Reynolds number of 850. Slip on the microchannel walls reduces pressure drop and enhances the thermal performance of the heat sink by 11%.

Catalytic Removal of Ozone by Pd/ACFs and Optimal Design of Ozone Converter for Air Purification in Aircraft Cabin

Ozone in aircraft cabin can bring obvious adverse impact on indoor air quality and occupant health. The objective of this study is to experimentally explore the ozone removal performance of flat-type catalyst film by loading nanometer palladium on the activated carbon fibers (Pd/ACFs), and optimize the configuration of ozone converter to make it meet the design requirements. A one-through ozone removal unit with three different Pd/ACFs space was used to test the ozone removal performance and the flow resistance characteristic under various temperature and flow velocity. The results show that the ozone removal rate of the ozone removal unit with the Pd/ACFs space of 1.5 mm can reach 99% and the maximum pressure drop is only 1.9 kPa at the reaction temperature of 200℃. The relationship between pressure drop and flow velocity in the ozone removal unit has a good fit to the Darcy-Forchheimer model. An ozone converter with flat-type reactor was designed and processed based on the one-through ozone removal experiment, its ozone removal rate and maximum pressure drop were 97% and 7.51 kPa, separately, with the condition of 150℃ and 10.63 m/s. It can meet the design requirements of ozone converter for air purification and develop a healthier aircraft cabin environment.

Analytical and numerical study of entropy generation in small scale heat exchangers

In present work, the entropy generation minimization technique (EGM) is applied to study the performance of a microchannel heat sink combined with a new proposed parameter called irreversibility index and energy harvesting concept. Three different cases have been investigated using geometry of a microchanel heat sink selected from experimental work in the literature. The constraints considered in this study, are fixed channel height and maximum pressure drop. It has been observed that with fixed channel height constraint, while the aspect ratio changes from 1 to 10, the optimum operating condition fall in the range of Reynolds number equal to 2000 and aspect ratio of 2.25. Moreover, the extra constrain on maximum pressure drop imposes a limitation on applicable aspect ratio range. The maximum aspect ratio of the channel for stable flow field in this case cannot be higher than 5 imposed by criteria of laminar flow regime. The obtained optimum values are Reynolds number of 1850 and aspect ratio of 2. Using a combined new defined irreversibility index and Energy Harvesting Concept (EHC), it has been shown that the optimum design values for industrial applications are not necessary ones obtained from EGM method and may shift to a new operating point based on the method considered for energy harvesting.

Analytical solutions for predicting the maximum pressure drop after pump failure in long-distance water supply project

The water hammer caused by pump failure in a long-distance pressurized pipe system generally poses a severe threat to the safety of the whole system. The maximum pressure drop at the pump end of the discharge line is significant for the safety assessment of the pipelines. In this study, the characteristics of the pump-stopping water hammer and its propagation in the pipelines are analyzed. The formula for predicting the maximum pressure drop is deduced based on the Method of Characteristics and the complete characteristics of the pumps. The application conditions of the formula and the solution procedures are presented as well. In addition, two engineering cases are introduced and the results calculated by the formula are compared with those resulting from the numerical simulation, and the agreement is satisfactory. The formula presented in this study is of simple form, practical and of high precision, and can provide a theoretical basis for the water hammer protection scheme of a long-distance water supply project.