Browsing by Author "Moss, J. B."
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Item Open Access Aerodynamic measurements on a small HAWT rotor in axial and yawed flow(Cranfield University, 1990-07) Bellia, J. M.; Hales, R. L.; Moss, J. B.Current wind turbine performance codes are not yet able to predict the rotor aerodynamic behaviour with sufficient certainty. This has led to both the over-design of blades and to operational restrictions in certain wind conditions. Essentially the problem is one of aerodynamic stall. Steady 3-dimensional stall can occur near the blade root in high wind conditions and may produce more power than predicted. Dynamic stall can also be expected due to the effects of yawed operation, turbulence, tower shadow and the earth's boundary layer. The main aim of this work is to provide a coherent set of measured aerodynamic data accounting for both axial/non-axial flow and stall in high winds. These measurements are designed to highlight the effects of both steady and dynamic stall on the rotor aerodynamic performance. In addition, the data will enable current performance prediction codes to be developed and validated. A completely new turbine has been designed and built at Cranfield to make aerodynamic measurements using pressure transducers. The design has been dominated by the requirements of accommodating the transducer signal processing equipment and allowing variation of many of the rotor parameters. Three commercial glass fibre blades were installed and performance curves measured on a conventional field site at a height of 11.5m for three rotor speed settings. These measurements show the turbine to give adequate power performance. A mobile trailer has been used to tow the turbine at a height of 4m along the Cranfield runways. Mobile testing facilitates an accelerated test schedule and allows aerodynamic data to be acquired under controlled wind conditions. A fully instrumented blade, fitted with forty transducers, has been tested under these circumstances and produced a large database of pressure measurements covering operation in winds up to 25 iq/s and yaw angles between -4511 and +55°. Analysis of the data has shown it to be of good quality and allowed some of the effects of yaw and stall to be identified. The use of the data base for performance prediction code validation has also been established.Item Open Access Aerodynamic surface pressure measurement in atmosphere and wind tunnel on a vertical axis wind turbine blade using pressure transducers(1989-09) Oram, C. E.; Garside, A. J.; Hales, R. L.; Moss, J. B.Blade aerodynamic loads during operation of a wind turbine are poorly understood. Better measurements of these forces are needed for the design of more cost effective turbines. Existing estimates have been inferred from strain gauges in the blade structure, but such measurements are confused by the complexity of the blade's structural response. This present work has pressure tapped the blade surface and recorded at high speed, giving a highly detailed picture of dynamic events and blade loads. A modular blade was constructed for the 6m diameter Rutherford Appleton Vertical Axis Wind Turbine and instrumented with 32 semiconductor pressure transducers. A flow probe using a further three transducers projected ahead of the blade during some of the tests. The instrumented module was also operated in the oscillating mounting of the University of Glasgow's 1.6m by 2.4m wind tunnel. 1760 revolutions of data were collected in atmosphere with blade speed ratios down to about 2.0 and with a variety of blade leanout angles. Some 118 runs of a variety of waveform types and Reynolds numbers from 0.3 to 1.1 million, were collected from the blade in tunnel. Analysis of transducer calibrations concluded that the novel temperature correction technique used allowed pressure readings to 40Pa or better with 95% confidence. Confidence in measured pressures has allowed use of leading edge stagnation point pressure and position as a measure of wind in the rotor flowfield and as a surrogate measure of blade angle of attack. Blade surface pressures and integrated forces show the progression of dynamic stall with increase in wind speed, the dominance of the upstream blade pass in usefulness at low and moderate windspeeds and the rough equality at high windspeeds. Comparisons of wind tunnel and on-rotor data show much lower force and moment coefficients in atmosphere. Rotor 'goodness1 ratios of average blade tangential force to maximum blade and rotor force coefficients peak at a blade speed ratio of about 2.8. Goodness ratios show a mixed pattern with blade leanout. Analysis of pressure coefficient traces through stall indicate a mixed stall type starting at mid chord, moving to leading edge and thence to trailing edge.Item Open Access Burning Velocities of Coal-derived Syngas Mixtures(Cranfield University, 2008-01) Bunkute , Birute; Moss, J. B.Summary Integrated gasification combined cycle (IGCC) systems, which utilize coal, petroleum coke, heavy oil, biomass and waste materials as a feedstock, continue to enter the power generation market. The gasification products from gasifiers using these feedstocks are mixtures of hydrogen, carbon monoxide and inerts like nitrogen, carbon dioxide and water. These mixtures are then used as a fuel in low-emission power generation applications. Unlike natural gas or methane, which has been widely used and researched for many years, these mixtures have not been widely investigated. Thus the aim of this study is to provide data on the combustion properties of syngas mixtures, mainly focusing on laminar burning velocities and critical strain rates to extinction. These combustion properties data are essential for gas turbine combustor modelling using turbulent burning velocity closure models. The establishment of such a database in this study mainly relies on numerical computations. Therefore, the experimental campaign was limited to investigation of several CO/H2/N2 fuel mixtures fuel mixtures at different equivalence ratios and operating conditions. The laminar burning velocity values, obtained from the experimental campaign were used mainly for validation of the chemical kinetics model and reaction mechanism. The principal outcome from this study is that at ambient conditions and reactant preheat temperatures up to 400K experimental laminar burning velocity values compare well with numerical predictions. The laminar burning velocity tests at high pressure presented a number of complications due to the formation of cellular flames and the flow in theburner tube entering the transitional laminar to turbulent regime. As a result the numerical model could not be fully validated experimentally for high pressure conditions. A comprehensive combustion properties database has been created using numerical simulations, based on comprehensive descriptions of the chemical kinetics and extensions using neural networks. CFD simulations of reacting flows in a practical combustor geometry demonstrated the importance of obtaining accurate laminar burning velocities and critical strain rates to extinction data.Item Open Access CFD prediction of coupled radiation heat transfer and soot production in turbulent flames(Cranfield University, 1996-06) Bressloff, N. W.; Moss, J. B.The mechanisms governing the formation and destruction of soot in turbulent combustion are intimately coupled to thermal radiation due to the strong dependence of sooting processes and radiative loss on temperature. Detailed computational fluid dynamics (CFD) predictions of the radiative and soot output from turbulent non-premixed flames are normally performed by parabolic algorithms. However, the modelling of combustion systems, such as furnaces and unwanted enclosure fires, often require a fully elliptic description of the flow field and its related physical phenomena. Thus, this thesis investigates the intimate coupling between radiative energy exchange and the mechanisms governing soot formation and destruction within a three-dimensional, general curvilinear CFD code. Thermal radiation is modelled by the discrete transfer radiation model (DTRM). Special emphasis is given to approximate solutions to the radiative transfer equation encompassing various models for the radiative properties of gases and soot. A new algorithm is presented, entitled the differential total absorptivity (DTA) solution, which, unlike alternative solutions, incorporates the source temperature dependence of absorption. Additionally, a weighted sum of gray gases (WSGG) solution is described which includes the treatment of gray boundaries. Whilst the DTA solution is particularly recommended for systems comprising large temperature differences, the WSGG solution is deemed most appropriate for numerical simulation of lower temperature diffusion flames, due to its significant time advantage. The coupling between radiative loss and soot concentration is investigated via a multiple laminar flamelet concept applied within the CFD simulation of confined turbulent diffusion flames burning methane in air at 1 and 3 atm. Flamelet families are employed relating individual sooting mechanisms to the level of radiative loss, which is evaluated by the DTRM formulated for emitting-absorbing mixtures of soot, C02 and H20. Combustion heat release is described by an eddy break-up concept linked to the k-c turbulence model, whilst temperature is evaluated from the solved enthalpy field. Detailed comparisons between prediction and experiment for the critical properties of mixture fraction, temperature and soot volume fraction demonstrate the effectiveness of this novel, coupled strategy within an elliptic flow field calculation.Item Open Access Computational and experimental models of induction flows in spark-ignition engines(Cranfield University, 1988-03) Sanatian, R.; Moss, J. B.The objective of this thesis is to combine computational flow modelling, flow visualization and point measurements of mean flow and turbulence properties to obtain a better, more detailed, understandýing of the effects of alternative throttling devices on mixture preparation and turbulence generation in spark ignition engines. In so doing, it also seeks to assess the wider diagnostic potential of flow field computational techniques in internal combustion engine designs. Full-scale models, comprising simplified representations of the induction tract, throttling device, inlet valve and cylinder, have been manufactured in Perspex for steady-state water analogy tests. The resulting photographs of flow tracers in a variety of viewing planes provide a clear, but qualitative, picture of the princi - pal features of the flow in the models under study. The essentially qualitative data obtained from water analogy tests are complemented by limited hot wire velocity measurements at particular stations in the Perspex models, with air replacing the water as the flow medium. These data, supplemented by information in the literature, provide the framework for comparisons with an extensive computational simulation of induction flows which are performed using the general purpose PHOENICS code developed by CHAM. These studies include both transient and steady state predictions. The statistically stationary turbulent flow field through alternative induction system throttling devices -a conventional butterfly valve and a variable geometry ramp restriction- are modelled computationally and compared with water analogy flow visualization. The principal flow field characteristics are satisfactorily reproduced, including in particular the extent of the recirculation zone in the lee of the throttle and the relative persistence of the turbulence generated downstream for varying throat apertures. That generated by the two-dimensional variable geometry ramp is predicted to be both higher and persist beyond the inlet valve into the cylinder producing discernible swirl at high throttle settings. The limited quantitative comparisons with hot wire velocity measurements lend further support to the more detailed aspects of the computational predictions. Finally, comparisons are made between PHOENICS predictions and Laser-Doppler measurements of velocity for transient flow inside an axisymmetric motored piston-cylinder assembly, for different valve seat angles, reported in the literature. The agreement is again very encouraging, reinforcing the view that general purpose computer codes of the kind investigated can play an important role in detailed design assessment and evaluation.Item Open Access Computational modelling of diesel engine smoke emission.(Cranfield University, 1995-08) Welch, S.; Moss, J. B.This thesis is addressed to the problem of predicting the emission of exhaust smoke from the diesel engine. A simulation program based on a zonal phenomenological combustion model has been developed, permitting analysis of soot modelling techniques. For the first time, a comparative study of the common soot model expressions has been undertaken. Model sensitivities and behaviour have been critically assessed in order to determine the key model parameters and to establish a more solid predictive capability. Validation of both the combustion and soot predictions was made by means of comparison with the extensive experimental data-set of Kamimoto. The combustion model results showed a very good match between -predicted and experimental heat release curves. The only notable weakness derived from the method chosen to represent the effect of air swirl on the jet. Otherwise, the combustion predictions were deemed to be sufficiently accurate to serve as an effective platform for soot model development and analysis. The predictions of exhaust smoke for different operating conditions revealed the importance of accurately describing the overall air-to-fuel ratio in the spray. The effect of load variation was poorly represented due to neglect of the transfer of combustion products between the model zones. Soot rate predictions were generally quantitatively poor, thus requiring expression calibration. The comparative study of soot expressions identified a ranking of sensitivities of the formation expressions. Though oxidation is conceptually simpler, more distinct qualitative differences were observed in the behaviour of the expressions. The predictions of exhaust soot were found to be highly sensitive to the 'matching' of the formation and oxidation expressions over the period of the combustion process, and with poorly matched expressions, a very high sensitivity to the soot model constants was shown. The best results were obtained by use of simple quasi-chemical rate expressions.Item Open Access Computational Study of Stalled Wind Turbine Rotor Performance(Cranfield University, 1991-01) Rawlinson-Smith, R. I.; Hales, R. L.; Moss, J. B.Simplification of the aerodynamic control of large horizontal axis wind turbines (HAWTs) has been identified as an important step towards improved reliability and reduced cost. At present the majority of large HMrrs use active control to regulate power and loads. A simpler strategy is to use the inherent stalling of the rotor blades in high winds to limit power and loads. Unfortunately the performance of stall regulated HAWTs 1S poorly understood; current performance models often fail to correctly predict peak power levels. The benefits of passive control of power and loads cannot be utilised because of this uncertainty. This study examines the possible reasons for the poor performance of current prediction techniques 1n high winds with the objective of fonmulating a new model. The available experimental evidence suggests that rotor stall is caused by turbulent separation at the rear of the blade aerofoil, growing in extent from the root in increasing wind. This 'picture' of the stalling HAW! rotor forms the basis of the approach. The new model consists of a prescribed vortex wake, first order panel method (extended to represent the viscous region of trailing edge separation) and three dimensional integral boundary layer directly coupled in an iterative scheme. A sensitivity study of rotor indicates that the most important performance to wake geometry factor is the rate at which the wake is convected downstream. However, it is found that stalled power levels are insensitive to wake geometry; the study concludes that the problem of poor prediction of high wind performance lies on the rotor blades. Before using the complete code to calculate the performance of a rotor it 1S first tuned for the aerofoils used on the blade. Aerofoil perfonmance characteristics measured in a wind tunnel are synthesised by the model. Ideally these characteristics should include measured pressure profiles below and above stall. Validation of the complete code against detailed measurements taken under controlled conditions on a three metre diameter machine indicates significant differences in the perfonmance of aerofoil sections on a wind turbine blade when compared to the same section when tested in a wind tunnel. Derived lift coefficients show a reduced lift curve slope and more gentle delayed stall. Similar results are found when the code is applied to two Danish stall regulated machines. These two machines although having very similar geometries and using the same family of aerofoils do however show differences in derived post stall drag. This is thought to be due to the different thickness distributions of the two rotors. The validation and applications of the new model show that it can accurately predict the peak power level of stall regulated machines.Item Open Access Flame Behaviour in an Acoustically Forced Gas Turbine Combustor(Cranfield University, 2009-03) Ruggles, Adam; Moss, J. B.A swirl stabilised dump combustor capable of imposing flow perturbations creating combustion instabilities has been designed and commissioned. The capability of supplying different fuel mixtures (methane hydrogen blends) has been incorporated. Additional capability is the facility to preheat the combustion air prior to chamber entry and to be able to introduce dilution air into the chamber. The chamber itself is of fused silica quartz to allow non-intrusive optical diagnostics. High speed CH* Chemiluminescence has been performed to qualitative characterise the unstable heat release rate of pure methane and methane hydrogen blended flames to allow analysis of the mean deconvoluted flame structure. High speed Stereoscopic Particle Imaging Velocimetry (SPIV) has been used to acquire the flow field throughout the chamber and focusing upon the Annulus entry. These diagnostics have been phase locked to the imposed perturbation. A selection of conditions is presented with three different perturbation frequencies within the low frequency range. These reveal vastly different reacting and flow field structures. The difference of structures is attributed to behaviour of the IRZ (Internal Recirculation Zone) and CRZ (Corner Recirculation Zone) in altering the flame shape. All conditions exhibited the axisymmetric/bubble vortex breakdown mechanism responsible for stabilisation. Both single cell and double cell structures were observed in the mean flow field vector maps. The mechanism of oscillating heat release rate is attributed to oscillations of flame surface area. Profiles of integrated heat release rate and flame exhibit the same profile shape and behaviour correlating very well. The inclusion of hydrogen had no quantifiable impact upon the mean reacting or flow field structures using the current diagnostics. Investigation into the nature of the turbulence of the shear layers close to the annulus is presented for three perturbation frequencies. This highlighted periodic structures within the turbulence corresponding to the imposed perturbation frequency. It was found that excitation of both shear layers for all turbulent components was not always true and depended upon the perturbation frequency and flow structure close to the annulus. Two oppositely rotating vorticity structures were revealed attached to the outer and inner circumference of the annulus. These structures protruded into the chamber and spread radially. Frequency analysis of these two structures revealed both were oscillating at the perturbation frequency indicating vorticity shedding. The mean vorticity structures are shown to be influenced also by the behaviour of the recirculation zones.Item Open Access Joint stereo-PIV and NO-LIF in turbulent premixed hydrogen-air flames(Cranfield University, 2009-09) Magand, Christelle C. G.; Moss, J. B.A new technique to simultaneously and instantaneously resolve 3D velocity/2D strain rate fields and scalar/scalar gradient fields was developed and evaluated in this study. This technique combines Planar Laser Induced Fluorescence of the NO radical (NO-PLIF) and Stereoscopic Particle Image Velocimetry (SPIV). It was found that the NO-PLIF technique allowed the determination of various iso-c contours and as such would, in principle, allow the study of the influence of the heat release on various properties, provided a calibration of the NO-PLIF signal as a function of temperature is achieved. It was also shown that the NO-PLIF technique may not be unambiguous at detecting flame extinction. The SPIV technique allowed the determination of the velocities in 3D and of the strain rates in 2D from which the most extensive and the most compressive strain rates but not the intermediate strain rate could be extracted. Information on strain rates and progress variable gradients were of particular interest in this study as they were needed to study the turbulence-scalar interaction which appears explicitly in the transport equation for the scalar dissipation rate which was derived recently. Using the technique above mentioned, this work also aimed at gathering and analysing data such as flame normal orientation, progress variable gradients, velocity change across the flame front and strain rates along the flame contours in turbulent premixed hydrogen/air flames with added nitrogen. The flame normal orientation was found to be consistent with the regime of the flames studied. A new method was designed and presented to infer from the progress variable gradients the component of the flame normal in the third dimension. The velocity change across the flame front, inferred from the SPIV data, was found to be extremely small. It is thought that the (low) heat release of the flames studied contributed more to corrugation of the flame front than acceleration of the gases across the flame front. The strain rates were studied along apparently non-wrinkled and clearly wrinkled flame contours. Their variation could not successfully be linked to curvature solely. Their values were mostly below the value expected for extinction strain rates. Last, this study aimed at investigating the turbulence-scalar interactions in turbulent premixed hydrogen/air flames with added nitrogen via the characteristics of the alignment of the flame normal vectors with the principal strain rates. The results of this study are quite different from earlier experimental results obtained for turbulent premixed ethylene/air flames. The strong preferential alignment of the flame front normal with the most extensive strain rate observed for ethylene/air flames could not be observed for the hydrogen/air flames with added nitrogen studied in the present work. The key outcome of this study was that no preferential alignment could be observed for most of the flames. A slight preferential alignment of the flame front normal with the most compressive strain rate was observed for the flames with very low adiabatic flame temperature. The differences observed were attributed partly to Lewis number effects and partly to the low heat release superimposed on the hydrodynamic fields in the flames studied.Item Open Access Laser diagnosis of gas turbine fuel sprays; scaling effects on NOx emissions and stability(Cranfield University, 2009-12) Hussain, Mazhur; Moss, J. B.This thesis first provided strategic recommendations for the research sponsor, Rolls- Royce plc (RR) and then applied optical diagnostics to measure aero gas turbine fuel spray properties in order to predict Oxides of Nitrogen (NOx) emissions and combustion instability. Analysis of the large civil aero engine sector suggested possible courses of action for RR to protect itself from short-term market volatilities and also prepare for three long term changes in strategic operating context: air traffic growth; tighter United Nations enforced aero engine combustion emissions legislation and entry of civil aviation into the European Union Emissions Trading Scheme. A collaborative game theoretic approach was explored during the pre-competitive, pre-technology, capability acquisition aero engine design phase on unproven future technologies to reduce R&D expenditures, development times and the costs of failure. Lean Prevapourised Premixed combustion demands excellent spray atomisation quality to sustain combustion efficiency, stability and to minimise pollutants. Post development of an improved procedure to calibrate laser signals, methodology to predict NOx and technique to optimise rig operating conditions that minimised fractional discrepancies in two-phase flow behaviour with corresponding engine conditions, this thesis applied quantitative Planar Laser Induced Fluorescence (PLIF) and Laser Sheet Dropsizing (LSD) to measure the fuel placement and dropsize distribution in the near nozzle regions of RR liquid-fuelled hybrid, airblast and pressure-swirl sprays. Measurements were made under non-combusting, low pressure conditions and results were processed to identify fuel injector designs that exhibited low emissions and high stability for the Affordable Near Term Low Emissions (ANTLE) and Instability Control of Low Emission Aero-Engine Combustors (ICLEAC) engine demonstrator programmes. Results also provided validation data and boundary conditions for spray computational codes. Research findings will improve RR core competencies in fuel injection research to accelerate the development and deployment of low emissions aero engine technology.Item Open Access Lean premixed combustion models at elevated pressure for the gas turbine(Cranfield University, 2004-07) Wood, J. P.; Moss, J. B.The latter half of the 20th century has seen great strides made in understanding premixed combustion, and this combined with advances in computing, has lead to significantly improved combustion models. Over the same period changes in the international energy market, allied with major technological advances, have promoted gas turbines to the forefront of power generation. Lean premixed combustion is key to the low emissions produced in such devices, but with ever more stringent emissions regulations burner design becomes increasingly vital. However there has, perhaps, been a lack of emphasis on research areas that directly influence practical burner design. In particular it is possible to identify high pressure combustion and the development of models that address the pragmatic requirements of industry as areas that have received insufficient attention. This study aims to examine the effectiveness, at elevated pressure, of two models that appear to better meet the demands of industry. The selected models are the turbulent flame speed closure and fractal models. These are implemented in a novel partially premixed form within the CFD research code SOFIE. Comparisons are made between measurements, made by applying particle image velocimetry in a novel manner, and predictions in order to determine the accuracy of these models. The principal conclusion was that although the turbulent flame speed closure model replicated the experimental measurements surprisingly well the fractal model did not. However, a major weakness was identified in extending premixed combustion models, based on the concept of a progress variable, to partially premixed problems. This weakness introduced significant errors into prediction of the fuel distribution, which at the extreme could lead to the apparent 'creation' of fuel. No simple solution to this problem is immediately apparent, and hence it is argued alternative approaches, based on the techniques used in diffusion flames, should be investigated.Item Open Access Modelling of premixed combustion in a gas turbine(Cranfield University, 1998-07) Engelbrecht, Geoffrey E.; Moss, J. B.Three steady state combustion models, two turbulence models and a model for tK'6 prediction of NO., were implemented and investigated on a simple backward facing step experiment as well as an experimental lean prevaporised premixed (LPP) combustor. The three combustion models included the simple Eddy Break-up model as well as a presumed probability density function (pdf) model and a form of the BML crossing frequency flamelet model. These models were adapted to consider a variable mixture fraction to account for a non-homogeneous fuel air mixture. The two turbulence models used were the k-e and second moment models. Despite being unable to capture the flame front spreading in the case of the backward facing step, these predictions provided insight into the performance and implementation of the models. All three of the combustion models, after appropriate tuning, worked well for the LPP test combustor. This illustrates that such time averaged models are useful for flows which do not contain large transient coherent structures, such as that of the LPP test combustor and most practical engine combustors designed today. The second moment closure turbulence model was found to have the greatest impact on the flame front through the flow field predictions rather than through counter gradient diffusion. The Eddy Break-up and BML crossing frequency models both performed very well, qualitatively predicting the correct trends. The additional consideration of flame front straining in the BML crossing frequency model did not appear to significantly influence the flame front. This is because the type of model adopted to predict this effect had a relatively uniform influence everywhere in the flow. The presumed pdf model also performed well and was additionally found to self ignite without the existence of hot products when the inlet temperature was high enough. The NO., model faired well for a simple experimental geometry. However, it considerably over predicted the NO., formed within the LIPP test combustor, which was most probably due to poor boundary conditions. Despite this overprediction, the results give insight into how to improve the NQ, emissions for the experimental combustor.Item Open Access Modelling two-phase flow and transport effects of multi-component fuels(Cranfield University, 2005-07) Maru, Wessenu-Abegaz; Moss, J. B.Three novel multicomponent fuel spray droplet evaporation models are developed by employing the theory of continuous thermodynamics(CT) with the aim of applying them in the design and analysis of various energy conversion devices such as, aircraft jet engines, liquid-fuel rocket engines, diesel engines, and industrial furnaces. The CT methodology seeks to represent complex mixtures - for example,aviation kerosene or JP8 that typically comprise blends of a large number of chemical compounds by using probability distribution functions (PDFs). The components of JP8, which is constituted by the homologous series of paraffin, naphthene, and aromatic hydrocarbons; are each represented by the Pearson-Shultz type three-parameter gamma PDF, where the three (shape, scale, and origin) parameters characterise changes in the mixture composition. The phase transition of the liquid droplet due to evaporation is modelled using both low-pressure (LP) and high-pressure (HP) vapour-liquid equilibrium (VLE) models employing various mixing and combining rules by applying a general cubic equation of state (CEOS). Interestingly enough, the phase transition of the liquid fuel into vapour mixture is characterised by a change in the PDF scale parameter alone. Once the description of the fuel mixture is complete, the traditional species and energy transport equations both for the liquid and vapour phases respectively, are re-written using the composition PDF moments under Lagrangian and Eulerian frameworks. In order to solve the governing equations for the three droplet evaporation models, which characteristically involve phase change and a moving interface, a novel fully Adaptive Method Of Lines using B-Spline Collocation (AMOLBSC) is developed. The models are tested at various pressures, temperatures and convective conditions, including at a lean, premixed, prevaporised (LPP) combustor operating condition. In general, the computational results at an ambient pressure close to atmospheric showed good to excellent agreement against available experimental data in the literature. However, for ambient conditions with elevated-high pressures and temperatures only models that employ the HP formulation gave reliable results. In particular, when the liquid is at or near its critical pressure and temperature it is characterised by faster vaporisation and shorter droplet lifetime, including some evidence of liquid mass diffusion. The liquid model that incorporates the effects of liquid core circulation using semiempirical approximation and adaptive mesh refinement (AMR) technique is the most accurate and computationally efficient, although further work is required to establish its ranges of applicability.Item Open Access Numerical simulation of spray combustion using bio-mass derived liquid fuels(Cranfield University, 2007) Rochaya, David; Moss, J. B.The main objective of this work is to create a robust model for two-phase liquid spray combustion flow using vegetable oils, to investigate the flow structure generated by a swirler array with different fuels, and secondly to assess and optimise the capability of the CFD to predict accurately the results obtained experimentally and eventually enhance CFD model development and simulation. Validation is achieved by comparing the numerical results obtained with CFD with the experimental measurements. The purpose of this research is to increase the scientific understanding of the fundamental mechanisms of the spray combustion process using a carbon neutral fuel such as ethanol and biodiesel. In fact, very few numerical simulations of liquid biomass fuels in gas turbine systems are available in the literature. The flames are simulated using the commercial code FLUENT. The combustion/turbulence interaction is modelled using the laminar flamelet approach with detailed chemistry modelled using the OPPDIFF model from CHEMKIN. While the experiments could be carried out only up to 3 atm, the simulations were further extended to a maximum pressure of 10 atm. The FLUENT results were assessed qualitatively and quantitatively between the experimental measurements and the simulation. The cold flow features have been captured by the present simulations with a good degree of accuracy. Effect of air preheating was investigated for the biodiesel, and sensitivity to droplet size and spray angles variation were analysed. Good agreement was obtained for ethanol except in the fuel lean region due to failure of the FLUENT laminar flamelet model to capture local flame extinction while biodiesel simulation resulted in a significant overprediction of the flame temperature especially in the downstream region and satisfactory results further upstream. The results show the importance of setting proper droplet initial conditions, since it will significantly affect the structure of the flame.Item Open Access Prediction of smoke properties and obscuration in compartment fires(Cranfield University, 2002-12) Pierce, J. B. M.; Moss, J. B.This study describes the simulation and experimental investigation of a heptane pool fire, burning within a small compartment, in which interaction between a number of key physical processes is amplified. In particular, the configuration emphasises the coupling of buoyancy induced ventilation, smoke production and radiation heat transfer to the liquid fuel surface, from the luminous flame zone, from the smoke filled ceiling layer and from the confining walls. This study contrasts with those customarily performed for the purpose of model validation in compartment fires, which employ gas burners and so simplify much of the interaction. Initial experiments were carried out using a 0.23m diameter circular pan burning fixed amounts of heptane. Subsequently, a constant supply was used with a smaller circu- lar pan of 0.17m in diameter, in order to introduce experimental longevity under safe, controllable conditions whilst establishing a quasi steady-state system. Issues of non- stationarity in relation to heat-feedback to the fuel surface - an important pool fire mech- anism - are discussed. In addition to probe measurements of velocity and thermocouple temperature, the smoke yield was determined using a light extinction technique employing a 670nm wavelength diode laser and photo-diode detector, housed within a novel fully-traversible water- cooled probe. Data from these experiments illustrate the importance of accounting for room ventilation in terms of overall production of smoke and sound a cautionary note to the labelling of soot by a convenient marker such as temperature. Numerical simulation of the compartment fire is performed using the field model SOFIE, incorporating a simple evaporation model, which relates the mass-loss-rate of fuel to the net heat flux to the fuel surface and heat of gasification. This relationship assumes that heat losses to the pan, re-radiation by the fuel surface and other enthalpy loss terms, are small. Simulations of compartment fire scenarios using this model to calculate the rate of heat release are reported. Further comparisons are made between the industry stan- dard 'Eddy-Breakup' combustion model and the 'Laminar Flamelet' model. In general both the eddy-breakup model and laminar flamelet model tend to underpredict the yields of CO, whilst the eddy-breakup model over-predicts temperature and thus soot. The laminar flamelet approach shows more promise and shows particularly good agreement with the experimental measurements reported here under well ventilated conditions. SORE, the predictive tool employed in this research, has proved invaluable in discern- ing the reason for apparent ambiguities in the experimental measurements of soot con- centration. The results suggest that an alternative simplified zone model approach would overpredict visibility in smoke in terms of concentration, but underpredict in terms of layer depth, due to its inability to capture the important shape of the hot upper layer, which varies significantly from the homogenous, laterally uniform distribution which is assumed. The incorporation of a simple evaporation model which relies on accu- rate prediction of heat transfer in ultimately determining the heat release rate has been shown to be in very good agreement with the experiments. Despite the irregularity in predicted distribution of mass loss rate across the fuel surface - caused mainly due to the 'ray effect' of the radiation model - the main trend of lower heat transfer at the centre of the burner is demonstrated, in agreement with the experiments performed. This phe- nomenon is captured despite the lack of description of fuel vapour radiation blockage above the fuel surface, suggesting that this process may be disregarded. The heat flux distribution which is found here is in contrast to research conducted by other workers for similar sized pans in an open environment, which show a higher measured heat transfer at the centre of the burner. It has been shown that significant improvements could be made in experimental design of compartment fire experiments if CFD prediction is considered for the determination of suitable measurement locations in regions with lower local spatial variations.Item Open Access Reusable launchers(Cranfield University, 1993-07) Berry, W.; Moss, J. B.This research on Reusable Launchers was motivated by the need to reduce substantially the cost of space transportation. The specific objective was to- explore the perception that launcher reusability is the key to achieving these major cost reductions. The exploration was achieved by undertaking a comparative system study on potentially feasible reusable launcher concepts, using a consistent set of design tools, a standard analysis methodology and a standard reference mission. To set the background f or the research, the results of an extensive literature review are 'presented on the vehicle studies and technology developments that are engaged across the world on reusable launchers. Comprehensive vehicle studies appear to be engaged without justification for the choice of selected concepts in the absence of results from comparative system studies of reusable launchers. Technology developments also appear to be engaged without clear links to needs derived from vehicle system studies. The challenge of reusability is then addressed. Firstly, to set the performance and cost targets of reusable launchers, the capabilities of current expendable launchers are derived. Secondly, to establish the operational requirements for reusable launchers, the probable space transportation needs for the early 21st century are derived. Thirdly, the concepts and characteristics of reusable launchers are derived, allowing the selection, on a rationale basis, of a short-list of 13 potentially feasible reusable launcher concepts for analysis in the research. The performance equations of reusable launchers are 'then derived, leading to the preparation of the comparative analysis tools. The major work-of the research, which ''comprises the performance analysis, technical feasibility assessment and cost"analysis of each candidate vehicle are, then presented and compared-. A set of acceptance requirements for performance,, technical feasibility and operational costs - of reusable launchers is then -derived. The results of the comparative analysis for each candidate launcher are then measured , against these requirements. The results of the comparative analysis show that only 2 of the' 13 candidate reusable launcher concepts are able to meet all the acceptance'requirements. These two acceptable vehicles are both rocket-propelled. They are, ýin order of preference: a single-stage-to-orbit, rocket-propelled, vertical launch and vertical landing vehicle; a two-stage-to-orbit, rocket-propelled, vertical launch and horizontal landing vehicle. The operational ''costs per launch for these two'vehicles,, based on a utilisation plan of 3 vehicles operating for 20 years at a launch rate of 12 launches per year, was calculated to be about 20 % of the current costs of the European Ariane 44L expendable launcher. This warrants their further evaluation in a thorough feasibility study. The more complex, air-breathing propelled, horizontal launch and landing vehicles were found to be unable to meet the performance, technical feasibility and cost requirements: Several vehicles were found to be unable to deliver a positive payload mass to orbit; Several vehicles were found to have technology requirements that were deemed to be infeasible to achieve; Several vehicles were found to have operational costs ranging from equal to double that -of the European Ariane 44L expendable launcher,, which -was- adopted as a comparative reference vehicle. The contributions of this research to the advancement of knowledge on reusable launchers are: a clear identification of the performance, capability limits of 13 plausible reusable launcher concepts; an analysis methodology for determining the performance capability limits for any reusable launcher concept; a clear identification of the reasons. for the poor practical performance of air-breathing propulsion systems for Earth-to-orbit launchers, which results from their installed operational characteristics.Item Open Access Servicing polar platforms using electric propulsion(Cranfield University, 1992-02) Welch, C. S.; Moss, J. B.The future of space development has been examined in the context of the infrastructure necessary support it. It is concluded that the selection of propulsion systems for in-orbit transportation requires the development of general computer codes capable of simulating the use of a wide range of propulsion systems on near-Earth missions. It is also concluded that, even if limited infrastructural development occurs, polar orbiting spacecraft will be an important feature of future space activities. Replacing current single-use polar spacecraft with extended-life serviceable platforms is attractive. However, the very limited manned access polar orbits in the mid-term future suggests that such platforms will only be possible if remote telerobotic/autonomous servicing can be carried out. To this extent polar platforms are considered to provide a useful driver and first testbed for the development of technologies designed to extend human capability in those regimes where direct mediation is not possible. Options for such remote servicing are examined, the concept of performing nodal transfers by enhanced differential nodal drift is introduced and the application of electric propulsion to this discussed. Low-thrust orbital manoeuvres are analyzed in this context and the conditions for optimum nodal transfer defined. Particular service vehicle configurations are then defined against a projected infrastructure and baseline polar platform constellation. A model for the service vehicle is defined and its performance investigated using a number of electric propulsion systems. Simulations of transfer manoeuvres; have been carried out and the effects and relative importance of the various orbit perturbations identified. It is concluded that a service vehicle propelled by a Xenon ion system offers the capability required and two final configurations are identified characterising different servicing mission upload schemes.Item Open Access Soot and radiation modelling in buoyant fires(Cranfield University, 1990-10) Syed, K. J.; Moss, J. B.This study seeks to advance present modelling capabilities in respect of soot and thermal radiation emission from fires. Such developments are crucial to the improved estimate of the hazard potential of accidental fires. Radiation calculation requires the prediction of temperature and the concentrations of all radiatively important species. In hydrocarbon combustion, the key species are carbon dioxide, water vapour, carbon monoxide and particulate soot. In large hydrocarbon fires the latter is usually the dominant radiator. The detailed prediction of the gaseous species in turbulent combustion has previously been shown to be successfully achieved using laminar flamelet modelling in the fast chemistry limit. Soot, however, is governed by relatively slow formation processes which as yet remain poorly understood. The present study proposes a model for soot formation in turbulent non-premixed combustion which aims to address both the slow chemistry and turbulence interaction. In order to circumvent uncertainties in soot formation processes the model relies on empiricism, through the experimental investigation of a sooting laminar diffusion flame. The soot formation model is used to predict soot levels in a jet diffusion flame. Subsequent comparison with experimental data suggests the satisfactory performance of the model, but highlights soot oxidation to be a more significant problem. This stems from uncertainties associated both with instantaneous soot oxidation rate and the highly intermittent nature of this process in turbulent non-premixed flames. The soot formation model is also applied to the prediction of soot levels in a simulated buoyant methane fire, which supplement temperature and gaseous species predictions using a flamelet approach. Detailed predictions of spectrally resolved radiative intensity are then performed and compared with similarly detailed experimental data. The encouraging agreement with experiment allows the assessment of the effect of turbulence-radiation interaction. This is shown to be particularly important in buoyancy-driven fires and is most evident for the luminous radiation. This arises from the soot which is largely confined to narrow sheets that typically lie close to peak temperature zones. A strategy in which more representative soot-temperature correlations may be realised is also described.Item Open Access Soot formation in turbulent vaporised kerosine/air jet flames at elevated pressure(Cranfield University, 1993-02) Young, K. J.; Moss, J. B.The objective of this thesis is to develop and validate a model of soot formation which is capable of being applied to a computational fluid dynamic (CFD) simulation of gas turbine combustion. The work follows previous research by Moss and Co-workers (Moss et al.1987, Syed 1990, Stewart et al.1991) The concept of the study is to generate a detailed set of experimental data in turbulent flames of kerosine in which the complicating factors of gas turbine combustion - that is 3D geometry and droplet combustion - are removed. This allows more confidence in the computational simulation of the flames and therefore more insight into the soot formation process. There are two components to the work: the experimental and theoretical studies. The first involves the compilation of an experimental dataset of key variables in ethylene and vaporised kerosine jet flames at elevated pressure, the second with the simulation of two of the experimentally studied flames using CFD methods. The main achievement of the study is the generation of a formidable and detailed experimental database for flames at a variety of pressures and conditions. The unexpected finding is the extremely large conversion of carbon to soot found in the flames even at low pressure. This results in high radiant heat losses and measurement difficulties. From the data, it is possible to assess the pressure dependence of soot growth in kerosine flames. Although, at the higher pressures, high soot levels created uncertainties in the measurements, in absolute terms growth rate is shown to be independent of pressure up to 6atm pressure. Above this it increases significantly. The soot model of Moss et al.1988 - originally developed in laminar e~hylene flames - was shown to give excellent agreement in turbulent situations. However, owing to the large radiant heat loss and soot levels, its application to the kerosine flames was more problematic since the assumptions that soot is a perturbation to the gaseous field and that temperature may be accurately described by a single perturbed flamelet were no longer valid. Further models to deal with such situations are proposed and tested. Aside from the obvious relevance of this study to the field of gas turbine combustion, the large radiant heat loss and high soot levels observed in the flames studied here imply a further significance for the study of fire hazards. That a laboratory scale flame maybe made to behave in a similar manner to a much larger pool fire flame is a very useful finding.Item Open Access Soot Production and Thermal Radiation from Turbulent Jet Diffusion Flames(Cranfield University, 1996-10-04) Brookes, S. J.; Moss, J. B.The aim of this study is to advance the present capability for modelling soot production and thermal radiation from turbulent jet diffusion flames. Turbulent methane / air jet diffusion flames at atmospheric and elevated pressure are studied experimentally to provide data for subsequent model development and validation. Methane is only lightly sooting at atmospheric pressure whereas at elevated pressure the soot yield increases greatly. This allows the creation of an optically thick, highly radiating flame within a laboratory scale rig. Essential flame properties needed for model validation are measured at 1 and 3 atm. These are mean mixture fraction, mean temperature, mean soot volume fraction, and mean and instantaneous spectrally resolved radiation intensity. These two flames are modelled using the parabolic CFD code GENMIX. The combustion / turbulence interaction is modelled using the conserved scalar / laminar flamelet approach. The chemistry of methane combustion is modelled using a detailed chemistry laminar flame code. The combustion model accommodates the non-adiabatic nature of the flames through the use of multiple flamelets for each scalar. The flamelets are differentiated by the amount of radiative heat loss that is included. Flamelet selection is carried out through the solution of a balance equation for enthalpy, which includes a source term for the radiative heat loss. A new soot model has been developed and calibrated by application to a laminar flame calculation. Within the turbulent flame calculations the soot production is fully coupled to the radiative loss. This is achieved through the use of multiple flamelets for the soot source terms and the inclusion of the radiative loss from the soot (as well as the gases) in the enthalpy source. Spectral radiative emission from the flames has been modelled using the RADCAL code. Mean flame properties from the GENMIX calculations are used as an input to RADCAL.