School of Engineering (SoE)

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Open Access
Assessment of the power available in a fixed offshore oscillating water column plant
(Cranfield University, 2012-12) Holzhauer, Eva; Amaral Teixeira, Joao; Trarieux, F.
The early effects of the global warming can be observed and people around the world are beginning to realize the seriousness of the situation. Reducing the CO2 emissions produced by fossil energy seems to be one of the main worldwide technological challenges at the time of writing. Hence, since the oil crisis in the 70s, a growing interest in renewable energies has been noticed. In Europe, the European Commission fixed a target: to produce 20% of the EU energy from renewable sources by 2020. Similar initiatives, in varying degrees, are being considered around the globe. Among all the renewable energy technologies currently on the market, the ocean energy industry is still at an early stage, despite investigations that have been carried out on both tidal and wave energy devices over the past 40 years. The subject of this thesis focuses on one of the wave energy devices: the Oscillating Water Column. The information found in the literature about this type of plants is mainly about onshore and floating offshore OWCs. Very little information about fixed offshore OWC is available. Besides, the availability of large numbers of fixed offshore structures installed in the world oceans suggests that many of these could possibly host an OWC plant. Hence, the present study investigated a fixed offshore OWC. The aim of this thesis is to assess the power available in a fixed offshore OWC plant. To illustrate the procedure of power assessment, the fictional scenario of a platform located in the Santa Maria sea region, off the coast of Californian, is introduced. This work intends to develop a methodology to study the feasibility of such installation and estimate the power extractable through various complementary approaches. From a theoretical approach based on the wave climate of Santa Maria to wave tank experiments with various geometries and shapes of chamber (cylinder and bent duct buoy in frontward and backward position), the viability of a fixed offshore OWC plant is demonstrated for the chosen location. Results highlight the performance of the Backward Bent Duct Buoy (BBDB) for the Santa Maria characteristic sea conditions. With the intention of completing the study with a Computational Fluid Dynamics (CFD) analysis, numerical investigations about the implementation of an alternative method to generate regular waves demonstrates better results of wave propagation than the common wave generation method based on Linear Wave Theory previously used at Cranfield University. In the conclusion, the work achievements and recommendations for future CFD investigations to reproduce the wave tank experiments are discussed.
Open Access
CFD Analyses of Centrifugal Pumps with Emphasis on Factors Affecting Internal Pressure Pulsations
(Cranfield University, 2006-10) Spence, R.; Amaral Teixeira, Joao
The operation of centrifugal pumps can generate instabilities and pressure pulsations that may be detrimental to the integrity and performance of the pump. Until recently these pressure pulsations could only be determined experimentally which resulted in a limited understanding of pressure pulsations around the pump. Industrial pump guarantees are limited to pulsation levels measured at the discharge. However, numerical analysis techniques have advanced to such a stage that they can now be used to explore these effects. The multi-block, structured grid CFD code TASCflow was used to investigate the time variation of pressure within a complete centrifugal pump. A parametric study covered four geometric parameters, namely the cutwater gap, vane arrangement, snubber gap and the sidewall clearance. Taguchi methods allowed the number of transient analyses to be limited to a total of twenty seven. Three flow rates were investigated and the pulsations were extracted at fifteen different locations covering important pump regions. The velocity flow patterns from the transient analyses exhibited important features that were in agreement with two independent sources. The transient flow results compared reasonably with the Weir experimental tests and clearly indicated the pump locations experiencing the largest pulsation levels. It was also noted that monitoring pulsations at the top dead centre of the pump volute casing would provide a better indication of internal pump pulsations than monitoring at the discharge. Taguchi post-processing analysis tools were used to rank the relative importance of the four geometric parameters at each location for each flow rate. The cutwater gap and vane arrangement were found to exert the greatest influence across the various monitored locations and the flow range. However the snubber gap had a dominant influence on the pressure differential across the impeller shroud and pulses in the pressure differential were evident at reduced flows. Through a rationalisation process reductions in pressure pulsations aimed at increased component life and reduced noise/vibration have resulted in a single recommended geometric arrangement. Further analyses confirmed that the new arrangement did indeed produce lesser pulsations levels. Multiple steady state simulations were analysed to determine if they were a viable substitute for the transient analyses. However it was demonstrated that the steady state pulsations did not adequately capture the magnitude and phase of the pulsations shown by the transient results. Likewise the steady state analyses were unable to predict trends for two differing pump geometries. In order to identify the implications of the CFD data for mechanical integrity, the pressure differential predicted by the transient analyses was compared with the pressure loadings currently utilised in Weir design guidelines; this resulted in a new recommendation for use in future designs. Also finite element analyses were conducted using four pressure loadings taken from the numerical results and a centrifugal loading. These supported the recommendation for an increased loading to be used in the design guidelines. The stress levels at the impeller outlet were found to be extremely sensitive to the snubber gap. The completion of this project has allowed a useful set of recommendations to be made regarding the design of high head double entry pumps.
Open Access
Development of a Port-Hamiltonian Model for use in oscillating water column control scheme investigations.
(Cranfield University, 2014-12) Farman, Judith; Amaral Teixeira, Joao
With global energy demand estimated to rise considerably and global warming accepted by the majority of scientists, the pressure to reduce fossil fuel usage is increasing. To this end, the UK government has set a target of generating 50% of electricity from renewable energy sources by 2050. It can therefore be deduced that decreasing the cost of renewable energy by increasing the energy capture is critical. Oscillating Water Columns (OWCs) employing bidirectional turbines coupled with generators can be used to capture energy from oceanic waves and convert it to electrical energy. This thesis includes a study to quantify the potential power smoothing that can be achieved from a wave farm of ideal OWC devices and from auxiliary hardware such as flywheel energy storage systems. Also detailed are the upgrades to the OWC test facility at Cranfield University, including the world-first capability to simulate polychromatic waves. This test facility has been employed to validate turbine characteristics derived from Computational Fluid Dynamic (CFD) numerical results. This thesis contains a literature review of the existing control strategies for OWCs that concludes that the optimization of power capture from individual components in the energy chain forces system-level compromises. This conclusion drove the development of an unique energy-based model of the complete wave-to-wire system utilizing port-Hamiltonian mechanics which mandated two modifications to the port-Hamiltonian framework. The first modification to the port-Hamiltonian framework resulted in a new generalized means of modeling systems where the potential energy is dependent on the momentum variables. The second modification expands the port-Hamiltonian framework to allow the modeling of ow source systems in addition to effort source systems. The port-Hamiltonian wave-to-wire OWC model enables the future development of a control approach that optimizes power capture at a system level. As a first step to achieving this goal an Injection Damping Assignment (IDA) Passivity Based Control (PBC) strategy was successfully applied to an OWC system and an energy storage flywheel system. These strategies pave the way for future developments utilizing optimization techniques, such as the use of cost functions to identify the peak efficiency operating condition.
Open Access
Development of turbines for use in oscillating water columns.
(2013-01) Tarver, B.; Amaral Teixeira, Joao
Bidirectional impulse turbines for oscillating water column wave power devices have the inherent problem of pressure losses associated with the outlet guide vane, adversely affecting the performance of the turbine. Two methods of guide vane design have been investigated with the aim of improving the performance of bidirectional impulse turbines by reducing these losses. These methods employ the use of optimisation, using computational fluid dynamics as a method of predicting the performance of the turbine. Numerical results are presented of the performance of the datum turbine, these have been validated using previous experimental data. These results are used as a baseline to measure performance improvements. An optimisation system has been adapted for the guide vanes of the datum turbine, allowing optimisation in bi-directional flows. Two novel guide vane design concepts were explored and the results of parameterisation studies investigating the effects of specific guide vane parameters are presented. The results of two global optimisation procedures, demonstrating the predicted performance improvements of the optimum guide vane configurations are presented. Both optimisation methods demonstrate reduced losses associated with the OGV of the turbine. The second concept, investigating guide vane profile optimisation in bi-directional flows, resulted in a respectable gain in turbine efficiency.
Open Access
Diagnosis of low-speed bearing degradation using acoustic emission techniques
(Cranfield University, 2017-01) Alshimmeri, Fiasael; Addali, Abdulmajid; Amaral Teixeira, Joao
It is widely acknowledged that bearing failures are the primary reason for breakdowns in rotating machinery. These failures are extremely costly, particularly in terms of lost production. Roller bearings are widely used in industrial machinery and need to be maintained in good condition to ensure the continuing efficiency, effectiveness, and profitability of the production process. The research presented here is an investigation of the use of acoustic emission (AE) to monitor bearing conditions at low speeds. Many machines, particularly large, expensive machines operate at speeds below 100 rpm, and such machines are important to the industry. However, the overwhelming proportion of studies have investigated the use of AE techniques for condition monitoring of higher-speed machines (typically several hundred rpm, or even higher). Few researchers have investigated the application of these techniques to low-speed machines (<100 rpm), This PhD addressed this omission and has established which, of the available, AE techniques are suitable for the detection of incipient faults and measurement of fault growth in low-speed bearings. The first objective of this research program was to assess the applicability of AE techniques to monitor low-speed bearings. It was found that the measured statistical parameters successfully monitored bearing conditions at low speeds (10-100 rpm). The second objective was to identify which commonly used statistical parameters derived from the AE signal (RMS, kurtosis, amplitude and counts) could identify the onset of a fault in either race. It was found that the change in AE amplitude and AE RMS could identify the presence of a small fault seeded into either the inner or the outer races. However, the severe attenuation of the signal from the inner race meant that, while AE amplitude and RMS could readily identify the incipient fault, kurtosis and the AE counts could not. Thus, more attention needs to be given to analysing the signal from the inner race. The third objective was to identify a measure that would assess the degree of severity of the fault. However, once the defect was established, it was found that of the parameters used only AE RMS was sensitive to defect size. The fourth objective was to assess whether the AE signal is able to detect defects located at either the centre or edge of the outer race of a bearing rotating at low speeds. It is found that all the measured AE parameters had higher values when the defect was seeded in the middle of the outer race, possibly due to the shorter path traversed by the signal between source and sensor which gave a lower attenuation than when the defect was on the edge of the outer race. Moreover, AE can detect the defect at both locations, which confirmed the applicability of the AE to monitor the defects at any location on the outer race.
Open Access
Heat removal in axial flow high pressure gas turbine
(Cranfield University, 2016) Alhajeri, Hamad; Amaral Teixeira, Joao; Addali, Abdulmajid
The demand for high power in aircraft gas turbine engines as well as industrial gas turbine prime mover promotes increasing the turbine entry temperature, the mass flow rate and the overall pressure ratio. High turbine entry temperature is however the most convenient way to increase the thrust without requiring a large change in the engine size. This research is focused on improving the internal cooling of high pressure turbine blade by investigating a range of solutions that can contribute to the more effective removal of heat when compared with existing configuration. The role played by the shape of the internal blade passages is investigated with numerical methods. In addition, the application of mist air as a means of enhanced heat removal is studied. The research covers three main area of investigation. The first one is concerned with the supply of mist on to the coolant flow as a mean to enhancing heat transfer. The second area of investigation is the manipulation of the secondary flow through cross-section variation as a means to augment heat transfer. Lastly a combination of a number of geometrical features in the passage is investigated. A promising technique to significantly improve heat transfer is to inject liquid droplets into the coolant flow. The droplets which will evaporate after travelling a certain distance, act as a cooling sink which consequently promote added heat removal. Due to the promising results of mist cooling in the literature, this research investigated its effect on a roughened cooling passage with five levels of mist mass percentages. In order to validate the numerical model, two stages were carried out. First, one single-phase flow case was validated against experimental results available in the open literature. Analysing the effect of the rotational force, on both flow physics and heat transfer, on the ribbed channel was the main concern of this investigation. Furthermore, the computational results using mist injection were also validated against the experimental results available in the literature. Injection of mist in the coolant flow helped achieve up to a 300% increase in the average flow temperature of the stream, therefore in extracting significantly more heat from the wall. The Nusselt number increased by 97% for the rotating leading edge at 5% mist injection. In the case of air only, the heat transfers decrease in the second passage, while in the mist case, the heat transfer tends to increase in the second passage. Heat transfer increases quasi linearly with the increase of the mist percentage when there is no rotation. However, in the presence of rotation, the heat transfers increase with an increase in mist content up to 4%, thereafter the heat transfer whilst still rising does so more gradually. The second part of this research studies the effect of non-uniform cross- section on the secondary flow and heat transfer in order to identify a preferential design for the blade cooling internal passage. Four different cross-sections were investigated. All cases start with square cross-section which then change all the way until it reaches the 180 degree turn before it changes back to square cross-section at the outlet. All cases were simulated at four different speeds. At low speeds the rectangle and trapezoidal cross-section achieved high heat transfer. At high speed the pentagonal and rectangular cross-sections achieved high heat transfer. Pressure loss is accounted for while making use of the thermal performance factor parameter which accounts for both heat transfer and pressure loss. The pentagonal cross-section showed high potential in terms of the thermal performance factor with a value over 0.8 and higher by 33% when compared to the rectangular case. In the final section multiple enhancement techniques are combined in the sudden expansion case, such as, ribs, slots and ribbed slot. The maximum heat enhancement is achieved once all previous techniques are used together. Under these circumstances the Nusselt number increased by 60% in the proposed new design.
Open Access
Optimisation of bidrectional impulse turbines for wve power generation
(Cranfield University, 2009-03) Banks, K; Amaral Teixeira, Joao
The generation of electricity from ocean waves using oscillating water column (OWC) wave energy converters is currently uneconomic due to the high capital cost and low efficiencies of such devices. The bidirectional air turbines utilised in OWCS are one of the principal sources of inefficiency and a significant increase in their performance would improve the prospects of commercial scale wave power generation. The ability of computational fluid dynamics (CFD) to predict the performance of both Wells and impulse type bidirectional turbines for use in OWCS was examined by comparison with experimental results taken from the literature. A design process was then undertaken for a datum impulse turbine and a novel high-efficiency impulse turbine arrangement. Numerical performance predictions are presented with a comparison against experimental data from a large-scale oscillating-flow test rig. An automated design and aerodynamic optimisation system was subsequently developed for application to this novel impulse turbine design. The optimiser employs a hybridised genetic algorithm along with Kriging meta¬models to significantly decrease the number of expensive calls to the 3D-CFD code used to evaluate the objective function. Comparisons to a number of state of the art optimisation algorithms from the literature on some mathematical test functions indicated that the optimiser had equivalent or better performance for most problems. A parameter study was carried out to investigate the effect of various turbine in design variables, before undertaking a 14-variable global design optimisation. A 5-variable optimisation exercise was then performed to investigate the effi- ciency gains that could be achieved by using three-dimensional rotor blades. Substantial gains in performance were attained and the predicted levels of efficiency are significantly higher than those previously reported in the literature for other bidirectional impulse turbine designs.
Open Access
Transition Modelling for Axial Compressor Flows
(Cranfield University, 2008-11-03) Beevers, A.; Wells, Roger; Amaral Teixeira, Joao; Smart, Palie; Engineering and Physical Sciences (EPSRC)
Abstract The application of Menter's transition model (Menter et al. (2004a), here-after known as the ץ - θ model) available in the CFX CFD code, for use within an axial compressor design group was studied. Simulations of a range of turbomachinery applicable test cases were undertaken, including a range of transitional flat plates and a 2D compressor cascade. Results were com¬pared to experimental data and the results of simulations performed with standard turbulence models. The ץ - θ model significantly improved the prediction of the boundary layer development, compared to the turbulence models. Comparisons with ex¬perimental data were also good. Features such as mid-chord transitional separation bubbles were predicted with the ץ - θ model, but not with the turbulence models. The ץ - θ model offered no consistent improved accuracy over the κ - ω SST turbulence model when predicting leading edge separa¬tion bubbles. The more accurate simulation of the boundary layer enables a closer prediction of viscous losses. 2D and 3D unsteady simulations of a low-speed axial compressor stator blade boundary layer, subject to impinging rotor wakes, were conducted. The pur¬pose was to determine the performance of the ץ - θ model in this environment, as there is no available literature for this. For both simulations, the model gave a good qualitative agreement to experimental data in the prediction of passing rotor wake effects on the suction surface. The effects on the pressure surface transition region due to wake passing were poorly predicted. All models were simulated on low and high-speed axial compressor stages. Results showed no improvement over the turbulence models of the ץ - θ model to predict blade exit parameters. The ץ - θ model does not present a significant enough improvement in the prediction of the flow to warrant its regular use in the design of axial compressor blading. However, it presents a useful tool in the development of high lift compressor blading.
Open Access
Unsteadiness In An Embedded Axial Compressor Stage
(Cranfield University, 2008-02) Naylor, Edward; Ivey, Paul C.; Amaral Teixeira, Joao
Previous research on blade boundary layers in turbomachinery have been recognised to crucially influence the stability and performance of the gas turbine components. The interactions between rotating and stationary blade rows inevitably make the flow environment within a multistage axial compressor unsteady. Research conducted at midspan in Low Pressure turbines has shown that patches of transitional flow can withstand higher levels of deceleration, helping the boundary layer stay attached. An experimental investigation into unsteadiness in a embedded stage was conducted in the third-stage of the Cranfield four-stage Low Speed Research Compressor at two operating points: peak efficiency and near stall. This build of the Cranfield Rig was equipped with three-dimensional blading. A three-hole pressure probe was traversed at the exit of Rotor 3 in the rotating frame of reference and at the exit of Stator 3 in the stationary frame of reference. In addition measurements were made at the exit of both Rotor and Stator 3 using a slanted hotwire rotated about its axis. This measurement technique gave time-resolved three-dimensional velocities. Coupled to the exit traverses a series of boundary layer traverses were performed along Stator 3 suction surface covering the midchord region at midspan and close to the casing endwall. To aid in the understanding and interpretation of the experimental campaign, three-dimensional computations of Stator 3 were made at the two operating points using the commercial Computational Fluid Dynamics code ANSYS-CFX . A two-dimensional unsteady calculation of Rotor 3, Stator 3 and Rotator 4 at midspan and peak efficiency was also performed. The time-resolved measurements downstream of Rotor 3 showed that the rotor wake was characterised by high levels of random unsteadiness and increased incidence onto the stator row. The increase in incidence across the wake was two to three times that experienced with change in flow coefficient. Therefore the increased incidence and turbulence in a rotor wake will have a significant influence on the unsteady development of a downstream boundary layer. Measurements of the boundary layer at design condition at midspan show evidence of laminar and transitional flow up to 50% of the suction surface length. The boundary layer flow periodically undergoes transition due to the convection of the wake-induced strip that was generated close to the leading edge. Towards the casing the picture is altered slightly due to the stator-casing separation region. Boundary layer transition is completed farther forward and the transition length reduced. At off-design the picture is completely altered. Transition is completed upstream of 25% suction surface length and the flow shows only a modulating variation with blade passing. The stator-casing separation region grows in spanwise extent and the boundary layer flow on the stator surface is completely separated aft of 50% of suction surface length.

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