Twin-screw compressors are very well known since the last century, but only nowadays, they become largely used in industry. Better performances may achieved, but this is possible only applying sophisticated techniques (CFD) that requires empirical validation. Discovering the flow structure inside a screw compressor and provide empirical data for validating purposes is the aim of the thesis. This was made possible using techniques known as LDV and PIV. It must be empathised that designing an optical compressor and measuring its flow is not an easy task. On one hand, the machine must run without oil therefore, at low speed and low pressure ratio; On the other hand, the compressor has to run in such a way that a representative internal flow is produced. It was found that the best compromise was, running the compressor at 1000 rpm with pressure ratio 1: 1. The LDV, revealed that flow within the rotor chamber can be divided in three zones: • The first, at the leading edge and in the centre, is dominated by the rotor movement, • The second, at the trailing edge, is dominated by the discharge process • The third, near the clearances, is dominated by the leakages. Although LDV is more precise than PIV, the latter proved to have a superior ability to visualise the complexity of the flow at the discharge chamber. It was found that: • The beginning of the discharge process is always unstable and shows jet like flows moving from rotor to discharge chambers • The remaining of the discharge process is stable and, at the top of the discharge chamber, a strong swirl motion takes place. Flow at the inlet port is slow, stable and almost independent of the rotors

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The focus of the thesis is research based around Cranfield University's 3-stage high speed axial compressor test rig. This newly built rig supported by European Commission funding has tested a set of conventionally stacked 2D rotor and stator blades. The results will be used to evaluate the CFD codes; a advanced three dimensional blade set will then be designed by a collaboration of companies. The aim of which is to prove a 2% efficiency gain for the advanced blade set. Due to the limited axial spacing between the measurement planes a unique seven probe assembly and traverse mechanism has been designed. The assembly has the ability to yaw the probes and control the insertion depths. This takes a number of different probes such as cobra probes, fast response (pneumatic) probes and temperature probes. A computer program has been written that completely automates the control of the probes and the data acquisition. This has allowed full area traversing of the compressor between rotor and stator blade rows. Due to the unique design of the seven probes, a high-speed wind turmel section has also been designed and manufactured. This has enabled the probes to be calibrated for mach numbers up to 0.78 as well as for both pitch and yaw. Due to the representative size, blade count and high flow forces of the high-speed compressor, instrumentation to capture the three-dimensional flow field in a high-speed environment has been researched. A small four-hole probe has been designed and used to take three dimensional steady state pneumatic measurements. This combined with a post processing program has provided very detailed results downstream of the stators through the compressor. This includes the complex three-dimensional flow structure and secondary flows associated with tip leakage, end wall boundary layer, wake transportation and blade row interactions. A fast response probe was designed and evaluated to capture the flow field downstream of the rotors but found to be insensitive to yaw angle. The results taken in a high-speed environment at full scale and engine representative speed supports some of the findings taken in a low speed environment. The author considers that the higher turbulence and speeds leads to increased mixing in the blade rows. This leads to almost all the ow being three dimensional in nature, this is not apparent in a low speed representation. The upstream wakes and their interaction with the downstream rows can be clearly seen. This is further demonstrated by the insertion of an upstream probe. The effects of this probe being transported axially downstream through a blade row. The results show the effectiveness and importance of a rigorous mapping procedure. This is particularly useful where the wake thicknesses are small and pressure gradients are high in comparison to the probe size. The size and extent of the areas of loss become more pronounced as results are taken through the compressor. This is true both at peak efficiency as well as near surge. The higher loading at the near surge condition increases these areas of loss still further.

The main objective of the investigation is to provide a physically realistic model describing the movement of liquid phase in an industrial axial compressor during online washing. ln order to achieve this objective, CFD simulations were developed for predicting the water particle trajectories inside an industrial axial compressor, the rate of evaporation of water droplets on axial compressor blades by inertial impaction, turbulent diffusion, pressure and temperature increments. Simulation of water droplet trajectories and evaporation of water droplet content in designed 260MW gas turbine engine were undertaken. Two types of boundary conditions have been considered for a droplet. The first is the inlet boundary condition, which describes the properties of a droplet at the entry/inlet position. It was assumed that the droplets are evenly distributed at the inlet face. For the droplet inlet boundary, there were 4 main factors considered, namely droplet initial temperature, droplet flow rate, droplet axial slip velocity and droplet average initial diameter. Each factor has at least two levels, namely the base line level and some deviation or deviations to the base-line level. The second boundary condition is the wall boundary condition, which models what happens when a droplet interacts with a solid wall. For this second type boundary condition, all simulations in this study, the coefficient of restitution of all solid walls was set at very low number, coefficient of restitution = 0.0005. It is assumed that a droplet that hits a solid wall will undergo a non-perfect rebound. Other than mechanical effects due to the impact of particulates during online washing, the main aero thermal effects are due to the flow changes occurring on account of the two phase nature of the working fluid, heat and mass transfer among the two phases, modification in thermal and transport properties, and changes in chemical action and combustion. Practically, it is not possible, in general, to assure that all of the washing fluid, which entered the engine during online washing, would always be converted to the gas phase by the time the working fluid leaves the high pressure compressor, or even the combustor. Thus, both the state of the fluid and the cross-sectional distribution of washing fluid are of concern throughout the engine flow path, especially the first 3 front stages of the axial compressor. It must be noted that the preceding statements take no account of the presence and effects of bypass doors, vents, and valves. This is a set of devices for which there are no reliable data for air-fluid mixture operation, and thus, no guidance in design or installation. They can introduce major changes in the amount and distribution of the discrete phases of fluid along the flow path in compressors. Finally, however there is a need for at least one set of definitive tests on selected components and engines. At the moment, there is neither clear view nor experimental data available to validate those said objectives. On one hand, there is no validated computational model available to assist with understanding the complex relationship between the injection parameters and the resultant flow pattern and trajectories of the droplet as well as there is no information regarding the droplet build up on the blade surface in a multistage axial compressor during online washing. Nevertheless, even though without experimental validation, this investigation is very important as advancing tools to further understand the axial compressor online washing phenomenon which is still not fully understood even today. Therefore, post-axial-compressor fluid motion is important in understanding the role of air/fluid mixing and fluid evaporations and will be analysed and presented in this study. More precisely, this investigation tries to quantify the effect of injected droplet parameters on generic multistage axial compressor. As this work is focused on the model (droplet parameter) sensitivities rather than the CFD model itself, presented in this investigation herewith are only the main features of the generic droplet trajectories flow pattern in a multistage industrial axial compressor.

Turbo gas compressors are among the most used machines ever invented. In the process industry, they are used intensively to provide compressed air for all types of pneumatic instruments and tools. These compressors suffer from an instability known as surge which usually occurs at low flow rates. The flow becomes seriously unstable and sometimes reverses. Surge has been a major problem for designers and users since the invention of the turbo-compressor. Although many successful echniques have been proposed to tackle the surge problem, most of them seem to offer to control at a single operating point or when nonlinear control is used the aethod tends to be seriously complex. The area remains very challenging for control engineers because of the non-linear nature of the phenomena and the complexity of the machine itself.

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No description available.

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No description available.