Rajnish N. Sharma

Characteristics of the turbulence in the flow at a tidal stream power site

This paper analyses a set of velocity time histories which were obtained at a fixed point in the bottom boundary layer of a tidal stream, 5 m from the seabed, and where the mean flow reached 2.5 m s−1. Considering two complete tidal cycles near spring tide, the streamwise turbulence intensity during non-slack flow was found to be approximately 12–13%, varying slightly between flood and ebb tides. The ratio of the streamwise turbulence intensity to that of the transverse and vertical intensities is typically 1 : 0.75 : 0.56, respectively. Velocity autospectra computed near maximum flood tidal flow conditions exhibit an f−2/3 inertial subrange and conform reasonably well to atmospheric turbulence spectral models. Local isotropy is observed between the streamwise and transverse spectra at reduced frequencies of f>0.5. The streamwise integral time scales and length scales of turbulence at maximum flow are approximately 6 s and 11–14 m, respectively, and exhibit a relatively large degree of scatter. They are also typically much greater in magnitude than the transverse and vertical components. The findings are intended to increase the levels of confidence within the tidal energy industry of the characteristics of the higher frequency components of the onset flow, and subsequently lead to more realistic performance and loading predictions.

Computational modelling of the transient response of building internal pressure to a sudden opening

Abstract This paper describes the computational and experimental modelling of the transient response of internal pressures to sudden openings in buildings and models. It is shown that the computer model correctly predicts both the Helmholtz frequency and the decay rate of internal pressure. A study of the flow patterns from the computational results also show that a vena-contracts forms in the oscillating flow following a sudden opening. It is thought that viscous losses may be important and new analytic models have therefore been suggested which have been used to obtain a match with the experimental results.

The effect of roof flexibility on internal pressure fluctuations

This paper discusses the development of an analytic model to describe both the transient and resonant behaviour of internal pressure in flexible low-rise residential and light industrial buildings in the presence of an opening. The model and a linear analogy are used to obtain the response of internal pressure and the roof structure in a typical building and it is shown that building flexibility produces double resonance and a downward shift in the natural (Helmholtz) frequency of internal pressure while increasing the damping. This partially explains the observed vigorous dynamic behaviour of roofs before failure in buildings with a windward opening during the passage of tropical cyclones. In such storms the wind is very turbulent and possesses significant energy at the higher frequency end of the spectrum where typical Helmholtz and structural frequencies lie.

Optimization of valveless micropump for drug delivery

With controlled drug delivery system, appropriate and effective amount of drug can be precisely delivered at appropriate time by a micropump. This is achieved with the use of diffuser and nozzle elements that eliminate the need for valve mechanism simplifying the micropump design and fabrication. A preliminary valveless micropump prototype has been demonstrated and the systematic optimization to improve the performance at low Reynolds number (Re <; 100), typical in drug delivery system, is presented here.

Numerical study of the flow over a circular cylinder in the near wake at Reynolds number 3900

Numerical simulations of flow past a circular cylinder at subcritical Reynolds number Re = 3,900 is carried out using three-dimensional URANS equations. Two-dimensional simulations using SST turbulence model with structured and unstructured meshes are compared as a preliminary study. For three-dimensional simulations, the SST and a recently developed Scale-Adaptive Simulation (SAS-SST) methodology are used for turbulence model. The results obtained by the simulations are compared with recent experimental and numerical results available in the published literature. As expected, two-dimensional results fail to predict global quantities, such as drag force in structured meshes. However, precisely controlled unstructured meshes in the wake region present reasonably good agreement with experimental data in global quantities, although profiles in mean pressure on the cylinder surface and mean streamwise velocity along the wake center line are not simulated very well. The accuracy obtained by three-dimensional simulations with SAS-SST turbulence model is satisfactory considering the computational cost involved in LES for the same test case. In particular, first order turbulence statistics in the very near wake region show fairly good agreement with LES results, although second order quantities are not well produced. It is expected that in real engineering applications, the relatively new turbulence model, SASSST, could be one of the alternative turbulence models when transient turbulence modelling is required with lower cost.

Investigation of the Effects of Flame Holder on the Combustion Characteristics of a Flat Flame Micro Combustor

Different flame holder materials and geometric parameters (pore size, thickness etc.) were experimentally tested in terms of pressure loss and combustion characteristics for the micro combustor of the ultra micro gas turbine (UMGT). Alumina ceramic porous plates, sintered stainless steel and perforated plates were used as flame holders within a 46 mm quartz walled micro combustor. The range of the average pore size of the porous plates and the sintered stainless steel plates are between 100 μm to 580 μm with an average porosity of 43∼54% and thickness of 1 mm to 4 mm. The perforated plate had a hole diameter of 1 mm and the distance between the centres of the holes was 2 mm. It was observed that the pore size and the thickness affect the pressure loss. However, at low incoming flow velocities all the flame holders had pressure losses of less than 5%. Flat flame combustion was achieved with all the different flame holders, but it stabilized at different incoming velocities and air to fuel ratios, according to different flame holder material and geometric parameters.Copyright

Heat transfer and flow analysis of Al2O3-Water nanofluids in interrupted microchannel heat sink with ellipse and diamond ribs in the transverse microchambers

ABSTRACT A three-dimensional numerical simulation of an interrupted microchannel heat sink is performed to investigate the pressure drop and heat transfer characteristics of a nanofluid flow. In the first part the effect of two different rib shapes on flow characteristics is studied to find the optimum design. In this part water is used as working fluid. For the second part of the present work, an Al2O3/water nanofluid with different volume fractions is modeled as the working fluid, and its performance is compared to water. Analysis shows that using ellipse ribs results in better performance of the microchannel rather than diamond ribs and no ribs. It was also shown that at least 2% of nanoparticles volume fraction is required to achieve an enhancement in heat transfer.

Experimental Study on Flat Flame Combustion for Ultra Micro Gas Turbine Applications

ABSTRACT The requirement for efficient power sources for portable electronics and miniature mechanical devices, such as laptops, micro robots, or micro aerial vehicles, has led to research on the ultra micro gas turbine (UMGT). The ultra micro gas turbine is one of the most promising power sources for small-scale applications due to its higher power and energy densities compared to currently used batteries. In order to realize UMGT as a viable power source, its individual components have to be developed, since downscaling introduces new problems for each component. Since the micro combustor is one of the key components of UMGT, it has to be improved. Until now, it has been very difficult for micro combustors to achieve wide flame stability, high combustion efficiency, and clean combustion with low pressure loss, due to the associated downscaling problems, such as high heat loss and small residence time. In order to achieve wide flame stability, the effect of preheating the reactants on flat flame stabilization was investigated experimentally on a 46-mm inner diameter quartz-walled flat flame combustor. The experimental results showed that preheating improves flat flame stabilization. Since preheating increases the burning velocity, flat flame can stabilize at higher incoming flow velocities as long as they are lower than the burning velocities. From the relationship between the incoming flow velocities and burning velocities, a correlation was obtained for flat flame micro combustors for micro power generation applications. With this correlation, it is possible to determine the minimum combustor diameter required for stable flat flame combustion. Also, in order to have a stable flat flame at higher mass flow rates, the correlation enables the calculation of the required reactants temperature, and is a major contribution to the design of micro combustors.

Thermal Performances of a Small-Scale Regenerative Combustion Chamber for Ultra-Micro Gas Turbine

ABSTRACT New manufacturing techniques and technologies have led to the development of a new research topic focusing on fluid dynamics and combustion at small scale, in particular in the last 20 years. One of the most promising technologies is represented by the ultra-micro gas turbines, which were developed with the aim of providing a portable and clean power source for devices, such as unmanned aerial vehicles and drones, global positioning system, exoskeletons for military applications, and backup emergency power supply. Currently, not many prototypes have been built because of the issues posed by scaling down the system. Among these there are excessive fluid dynamic and thermal losses decreasing the overall efficiency, elevated combined thermal and mechanical stress of components, and the necessity of developing high speed bearings. This study focuses on investigating experimentally a possible solution to effectively recover heat contained in the combustion products to preheat the unburned mixture, increasing the overall efficiency. Thus, an 18-mm internal-diameter regenerative combustion chamber surrounded by two sets of helicoidal channels was developed. The combustion chamber included a sintered steel porous medium to allow the flame to stabilize. The combustion products were recirculated in order to maximize the heat transferred to the cold mixture, with benefits in terms of flammability limits and fuel consumption. Mass flow rate and equivalence ratio were varied in the tests and the gas temperatures at different locations within the combustion chamber were measured, along with the composition of the combustion products. The heat released was included in the range 65 W and 343 W. Tests were run in both a non-insulated and insulated configuration and comparisons were made. Resulted showed the achievement of clean combustion of liquefied petroleum gas with combustion efficiency higher than 99% for lean mixtures. Also, a good level of heat recovery was achieved, reaching 45% and 23% of heat transferred to the reactants from the exhaust gases in the insulated and non-insulated combustion chamber, respectively.

Large Eddy simulation of a NACA0015 circulation control airfoil using synthetic jets

Abstract Large eddy simulation of a NACA0015 circulation control airfoil using synthetic jets is conducted. A chord Reynolds number of 1.10 × 105, the excitation frequency of 175 Hz, the momentum coefficients of 0.0044 to 0.0688, and the angles of attack of 0°, 6°, and 12° are employed in this study. The numerical results presented in this paper show good agreement with the experimental results. Fluctuations in the lift and drag coefficients on the non-actuated airfoil are found to be governed by the vortex shedding frequency. Provided that the momentum coefficient is sufficiently high, the lift and drag coefficients tend to fluctuate at the synthetic jet frequency. The mean lift coefficient linearly increases with the momentum coefficient at a low momentum coefficient range and its incremental rate begins to decline at a high momentum coefficient range. Increasing the angle of attack of the airfoil is observed to slightly reduce the slope of the lift increment.