David S.-K. Ting

Desirable airfoil features for smaller-capacity straight-bladed VAWT

In the small scale wind turbine market, the simple straight-bladed Darrieus type vertical axis wind turbine (SB-VAWT) is very attractive for its simple blade design. A detailed aerodynamic performance analysis was conducted on a smaller capacity fixed-pitch SB-VAWT. Brief analyses of the main aerodynamic challenges of this type of wind turbine were first discussed and subsequently the authors conducted further literature survey and computational analysis to shortlist aerodynamic characteristics of a desirable airfoil for a self-starting and better performing SB-VAWT. The required geometric features of the desirable airfoil to achieve the short listed characteristics were also discussed. It has been found out that conventionally used NACA symmetric airfoils are not suitable for smaller capacity SB-VAWT. Rather, it is advantageous to utilize a high-lift and low-drag asymmetric thick airfoil suitable for low speed operation typically encountered by SB-VAWT.

Effects of addition of electrolysis products on methane/air premixed laminar combustion

Abstract In this study, effects of the addition of small amounts of water electrolysis products on laminar premixed methane/air flames have been investigated using chemical kinetic simulation methods. The CHEMKIN kinetic simulation package was used with the GRI kinetic mechanism. Pollutant concentrations, flame speeds, temperature profiles and lean flammability limits of methane/air, methane/hydrogen/air and methane/hydrogen/oxygen/air systems were compared at different addition percentages and equivalence ratios from 1.4 to the lean flammability limit. The addition of 10–20% hydrogen in the fuel was found to have a small effect in improving flame speed and lean flammability limit properties. However, the addition of oxygen and hydrogen in the same ratio as is found in water was shown to be beneficial. Improvements in the flame speeds of methane/air mixtures by the addition of 10% hydrogen and its associated oxygen were equivalent to the improvements obtained by the addition of 20% hydrogen only. In near stoichiometric mixtures, the addition of oxygen substantially increased the NOx concentrations, but for lean mixtures no increase in NOx was predicted. CO emissions were reduced when hydrogen displaced carbon-containing fuels.

Turbulent Flow Downstream of a Perforated Plate: Sharp-Edged Orifice Versus Finite-Thickness Holes

In this study, perforated plates with sharp-edged orificed openings and finite-thickness straight openings were applied to produce nearly isotropic turbulence in a wind tunnel. At the same nominal velocity, the orificed perforated plate was able to produce a higher level of turbulence due to the well-defined flow separation from its sharp edge openings. The integral length, L was found to be related to the square root of the turbulence decay coefficient in the power law decay of turbulence kinetic energy, A. The larger A associated with the orificed perforated plate gave rise to a larger L. The corresponding streamwise autocorrelation functions for the two perforated plates behaved differently, confirming the quantitative disparity in L and further indicates some qualitative difference in the large-scale structures generated.

Distensible air accumulators as a means of adiabatic underwater compressed air energy storage

In the near future, the electricity industry is likely to face historically significant changes. The onset of distributed generation, micro and smart grids will change the entire structured industry. An influx of intermittent renewable generators will make traditional grid balancing notably more difficult. The novel concept of underwater compressed air energy storage is a potentially promising solution that may be used to meet these challenges, especially during the current period of electrical infrastructure renewal and modernisation. Early results from a Lake Ontario Pilot Study point to the potential viability of the concept.

Vibration Analysis of 2.3 MW Wind Turbine Operation Using the Discrete Wavelet Transform

The vibration analysis of operational response data from a 2.3 MW wind turbine is presented. Vibration signals were acquired for two unique environmental conditions with an accelerometer mounted in the turbine tower. A Daubechies 6th order (db6) wavelet was used to perform a 12-level discrete wavelet transform (DWT) revealing trends and similarities within the signals. Full operation signals were segmented into start up and steady state periods. Analysis of turbine start up revealed a common ramping of low frequency energy on the order of rotor rotational frequency. DWT plots were also utilized to reveal high-energy response features related to the mechanical start up of the turbine. Analysis of steady state signals revealed distinct low frequency periodicity evident in the 11th (0.1776Hz) and 12th (0.0888 Hz) decomposition levels. The analysis technique performed shows promise for potential integration into comprehensive structural health monitoring schemes designed to reduce downtime and improve the reliability of commercial wind turbines.

Design of a Special-purpose Airfoil for Smaller-Capacity Straight-Bladed VAWT

Selection of airfoil is one of the most critical factors in achieving better aerodynamic performance and in determining the optimum dimensions of a fixed-pitch straight-bladed Darrieus type vertical axis wind turbine (SB-VAWT), along with solidity, radial arm parasitic drag, aspect ratio etc. Airfoil related design changes also have the potential for increasing the cost effectiveness of SB-VAWTs which is one of the simplest types of wind turbine and they are prospective candidate for diversified urban and rural applications. After identifying the main aerodynamic challenges of this type of wind turbine, the authors short listed the salient aerodynamic and geometric characteristics of a desirable airfoil for a self-starting and better performing SB-VAWT. Based on these findings, detailed performance analyses were done with several prospective airfoils. Finally a special-purpose airfoil has been designed for smaller capacity SB-VAWT.

Implementation Challenges and Solutions for Homogeneous Charge Compression Ignition Combustion in Diesel Engines

Homogeneous charge compression ignition (HCCI) combustion in diesel engines can provide cleaner operation with ultralow NOx and soot emissions. While HCCI combustion has generated significant attention in the last decade, however, till date, it has seen very limited application in production diesel engines. HCCI combustion is typically characterized by earlier than top-dead-center (pre-TDC) phasing, very high-pressure rise rates, short combustion durations, and minimal control over the timing of the combustion event. To offset the high reactivity of the diesel fuel, large amounts of exhaust gas recirculation (EGR) (30–60%) are usually applied to postpone the initiation of combustion, shift the combustion toward TDC, and alleviate to some extent, the high-pressure rise rates and the reduced energy efficiency. In this work, a detailed analysis of HCCI combustion has been carried out on a high-compression ratio (CR), single-cylinder diesel engine. The effects of intake boost, EGR quantity/temperature, engine speed, injection scheduling, and injection pressure on the operability limits have been empirically determined and correlated with the combustion stability, emissions, and performance metrics. The empirical investigation is extended to assess the suitability of common alternate fuels (n-butanol, gasoline, and ethanol) for HCCI combustion. On the basis of the analysis, the significant challenges affecting the real-world application of HCCI are identified, their effects on the engine performance quantified, and possible solutions to overcome these challenges explored through both theoretical and empirical investigations. This paper intends to provide a comprehensive summary of the implementation issues affecting HCCI combustion in diesel engines.

PREMIXED AMMONIA-METHANE-AIR COMBUSTION

ABSTRACT The burning velocity and combustion products of premixed ammonia-methane-air flames at room temperature and pressure were measured using an adiabatic flat flame burner, and also simulated using CHEMKIN, for equivalence ratios of approximately 0.5 to 1.5 and for ammonia concentrations of 0% to 5% by volume in the fuel. Concentrations of NO, NO2 and CO were quantified using an electrochemical gas analyzer, while ammonia concentration was measured by wet chemical analysis. The measured burning velocities agreed well with the simulated results. The addition of 4% ammonia resulted in a 10% to 20% decrease in burning velocities. Both simulations and experiments showed that adding ammonia to methane-air mixtures results in an increase in NO concentrations, especially at stoichiometric composition, and a minimal effect on CO formation. Near the flammability limits, ammonia of less than 0.1% of the incoming amount broke through the flame.

Aerodynamic Factors Affecting Performance of Straight-Bladed Vertical Axis Wind Turbines

Unlike the conventional aerodynamic applications, the straight-bladed vertical axis wind turbines (VAWTs) operate in a circular motion and encounter a wide range of angle of attacks, especially at low tip speed ratios. When the blade angle of attack remains constant or varies slowly with time, it encounters the static stall. However, when the angle of attack changes rapidly with time, it experiences the dynamic stall which is far more difficult to analyze and predict than the static stall. Furthermore, the blade/blade wake interaction in straight-bladed VAWTs also presents modeling problem. In this paper, all of these aforesaid aerodynamic factors are discussed. It was found that these factors need special attention for designing a self-starting straight-bladed VAWT with optimum performance. A numerical method based on Cascade model, proposed by Hirsch and Mandal [1], that gives reasonable correlation with the experimental data available has been used. The effects of dynamic stall and flow curvature on the performance of a straight-bladed VAWT have been analyzed. It is observed from the analysis that aerodynamic forces due to dynamic stall are higher than those due to static stall. As a result, for the performance prediction of straight-bladed VAWTs, especially for the local forces, there can be substantial differences between the experimental data and the calculated values unless the dynamic stall effect is added.Copyright

Modeling operation of HCCI engines fueled with ethanol

A mathematical engine model is developed to study the operation process in homogeneous charge compression ignition (HCCl) engines. Ethanol is used as an alternative fuel for testing in this model. Two-step reaction mechanisms are implemented to model combustion process for which Arrhenius reaction rates are used and heat transfer process is also included. The simulation results are then compared with experimental data from a modified heavy-duty diesel engine which is used to imitate HCCI operation. Results from this model show good correlation with experiment with respect to combustion phasing, pressure rise and peak pressure. Some control strategies could be potentially developed to stabilize HCCI combustion based on this model.