Browsing by Author "Hemp, J."
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Item Open Access The character of swirl in turbulent pipe flow with reference to its effect on flowmeters(Cranfield University, 1986-09) Halsey, D. M.; Hemp, J.A theoretical and experimental investigation of the character of in turbulent pipe flow has been carried out and some implications for the use of flowmeters considered. An extensive survey of Industrial users of flowmeters has been made, involving the participation of over 70 companies, which provides information about current industrial practice, attitudes and understanding. Axisymmetric perturbations of fully developed turbulent pipe flow been studied using the mixing-length model of turbulence. A linearised theory finds the character of small perturbations, decaying exponentially in the axial direction, in which the tangential and axial motion are independent. A non-linear similarity theory finds the flow field at a particular cross-section of the pipe assuming it to be determined by the specification of the Reynolds number, the pipe's roughness and the swirl number at that cross-section. Laser Doppler Anemometry was used to measure axial and tangential velocities on 4 equidistant cross-sections of a pipe following a double right-angle bend in two perpendicular planes. It was found the tangential velocity had the form of a solid body rotation decayed exponentially. The axial velocity profile after the bend asymmetrical, having the form of a horseshoe which rotated with swirl whilst returning to its fully developed form. Theoretical studies of the effect of the measured axial velocity profiles on the accuracy of electromagnetic and ultrasonic flowmeters presented. It is suggested that the decay of the level of swirl in turbulent flow after a double bend be modelled by the factor exp(-6fz/D) where f is the friction factor in fully developed flow and z/D the dimensionalised axial distance. The accuracy of flowmeters installed at particular locations downstream can then be predicted the appropriate error characteristics of flowmeters are known. Some indications of these characteristics are presented.Item Open Access A Cryogenic Helium Mass Flowmeter for the Large Haldron Collider(Cranfield University, 2007-09) Serio, Luigi; Hemp, J.This thesis is concerned with the design, development and testing of helium mass flowmeters based on different technologies to be used as diagnostic tools for the cryogenic system of the Large Hadron Collider at CERN, the European laboratory for particle physics. The flowmeters were designed and the performance assessed for liquid, supercritical and superfluid helium down to 1.8 K and with pressures up to 0.3 MPa. A testing regime and equipment to enable the performance of a range of developed flowmeters to be validated at working cryogenic temperatures and pressures is described in the thesis. In particular a novel cryogenic flow test facility with 1.5 % accuracy as well as an irradiation test station designed, constructed and used as part of the evaluation work are described in the thesis. The final validation with metrological certification down to 0.5 % accuracy was performed in collaboration with the Colonnetti metrological laboratory in Italy. Devices based on the thermal exchange and transport properties of the fluid were developed and proven to be reliable and inexpensive but with accuracy poorer than 10 %. As part of this work a new empirical heat transfer correlation for supercritical helium in the piping geometry and operating conditions of the LHC was established. Industrially available Coriolis meters which had not been employed before at such low temperatures were extensively investigated. It was found that with some minor modifications in the electronics (drive system) and in the elasticity modulus (Young’s modulus) value for the flow tubes material at low temperature, accuracy down to 0.5 % could be achieved. Additional features in the mechanical and electrical design were also introduced to improve robustness and reliability at cryogenic temperatures. They mainly consisted in purging the meters secondary containment with helium to avoid gas freezing at low temperatures and increasing the internal gauge wire to improve its fatigue strength. The elasticity modulus of 316 L stainless steel below 20 K was characterised and compared to available literature data. The successful Coriolis flowmeters were validated to meter the cryogenic power distribution in the 27-km long superfluid helium system of the future collider and are now extensively and successfully used in superconducting devices test facilities at CERN and in companion laboratories.Item Open Access Development of baseline stability in an electromagnetic flowmeter for dielectric liquids(Cranfield University, 1998-10) Durcan, L. P.; Hemp, J.; Sanderson, M. L.This thesis is concerned with the development of a prototype electromagnetic flowmeter for dielectric liquids. A initial prototype flowmeter is tested in laboratory conditions with a liquid of low conductivity, the conclusion of these initial tests being that the stability of the flowmeter is below that required for its design applications. This lack of stability is a problem reported on several occasions in similar projects involving electromagnetic flowmeters for dielectric liquids. Therefore the main body of the work is spent investigating the causes of instability and trying to redress the necessary problems in order that a viable working prototype is developed. Problems of thermal instability, electrostatic and electromagnetic shielding are addressed along with the causes and effects of all the major noise and unwanted signals experienced in such a system. A new prototype is developed incorporating stability driven improvements. This new design is rigorously tested and assessed. The stability of the meter is considered with respect to its flow related output. Several parameters of operational stability are given in the examples considered. A final assessment is then made of the flowmeters baseline performance and a account of any remaining instability is given with respect to the issue developed throughout the thesis. The conclusions indicate that the electromagnetic flowmeter for dielectric fluids does produce a linear output based on Faraday's law of induction. They also indicate that the instability experienced previously can be broken into its constituent parts and each contributor accounted for and reduced, working towards the ideal totally stable flowmeter.Item Open Access Electromagnetic flowmeter for dielectric liquids.(Cranfield University, 1995-07) Amare, T.; Sanderson, M. L.; Hemp, J.Experimental investigation and theoretical analysis of an electromagnetic flowmeter designed for use with dielectric liquids has been carried out. An extensive survey of the industrial users of flowmeters has been made, involving the participation of over 47 companies, which provides information about the current industrial use, attitudes and attributes of electromagnetic and other types of flowmeters. The design of the flowmeter is mainly concerned with overcoming the charge noise that is associated with the flow of dielectric liquids so as to effectively detect the weak flow signal. It is shown that the working of the flowmeter depends heavily on the design of the eddy current free electrostatic shield. The result of the mathematical analysis of the design is a simplified formula for predicting the sensitivity of the meter. It is adjusted to take account of such factors as end-effects and electrical properties of the flow conduit of the meter. Experimental results show that not only an induction flowmeter based on Faraday's principle is possible but also the measured values of the flow signal closely matches those obtained from the formula. It is suggested that the formula can be generalised to include moderately conducting liquids which is supported by results achieved from experiments on water. Finally, recommendations for future work are put forward for further improvement of the design.Item Open Access Electromagnetic flowmeters in multiphase flows(Cranfield University, 1993-09) Krafft, Rainer; Sanderson, M. L.; Hemp, J.The response of an Electromagnetic Flowmeter to a secondary, nonconducting phase in a continuous conducting phase is investigated. Experiments are carried out on a commercial 2" Foxboro flowmeter with sinusoidal mains field excitation and point electrodes. The main aspects which are looked for are the change in flow signal and transformer signal. A mathematical model is developed to simulate the effect of the nonconducting phase in. the form of single spherical particles. The model also allows one to alter the electrode geometry. A second flowmeter is designed to incorporate an improved electrode and coil geometry as well as a newly designed signal processing unit. In a series of experiments, the predictions of the theoretical model have been mostly verified.Item Open Access On non-invasive ultrasonic flowmeasurement(Cranfield University, 1985-11) Spendel, K. D.; Hemp, J.This thesis is concerned with non-invasive ultrasonic flow measurement, using the transit time principle. The errors associated with the transit-time flowmeter are investigated and a design of flowmeter is suggested. A theoretical and experimental study of the transmission of sound through pipe walls is carried out where it is shown that advantage can be taken of the excitation of Lamb modes. A design of transducer arrangement is made from the results of the work. A solution to the difficult problem of measuring very small times is provided in the form of a novel vernier timing system. The benefits and disadvantages of this timing system are discussed along with the design aspects of other electronic circuits required in the construction of the flowmeter. The flawmeter has been built and tested in the laboratory and is shown to be highly repeatable and accurate. The results of testing the flowmeter compare favourably with tests conducted on a commercial instrument. Improvements to the design and construction and suggestions for further work are given.Item Open Access A study of swirling flow in an orifice plate flowmeter(1988-10) Tello-Campos, A.; Hemp, J.This thesis presents a theoretical and experimental study of swirling flow in an orifice plate flowmeter. Firstly, a solid body rotation was considered as the inlet velocity profile of an orifice plate. The beta ratio considered was 0.4. The Reynolds number was 7.82xl04 . As a result, a theoretical value of the discharge coefficient was obtained. Secondly, Laser doppler anemometry was used to determine the axial and tangential velocity profiles at 3 upstream and 2 downstream cross-sections of an orifice plate with a beta ratio of 0.4. A double right angle bend in two perpendicular planes was placed 6.5 diameters upstream of the flowmeter. The axial velocity profile was found to be slightly asymmetrical. The corresponding tangential profile was found to have a form of a solid body rotation. A 20" mercury manometer was used to measure the pressure difference across the orifice. The results showed that the discharge coefficient decreased. Thirdly, a commercial computer program was used to calculate the swirling flow through an orifice plate. A solid body rotation was considered at the inlet of the orifice. With the pressure field calculated, the corresponding discharge coefficient was determined. An additional simulation was performed with a 1/7 power law inlet profile. The discharge coefficient under swirling flow conditions was smaller than the one under non swirling flow conditions. Based on the experimental and computational results, we can see that the swirling flow produced by a double bend greatly affects the performance of an orifice plate. It is recommended that the flowmeter must be located 70 diameters downstream of a double bend. The BS 1042 does not provide a reasonable straight length for this type of disturbance.Item Open Access Suppression of the vortex induced vibration of slender cylindrical structures(1991-08) Lamb, William Simon; Hemp, J.; King, RogerThis thesis presents the results of a detailed investigation into the performance of devices designed to suppress the vortex induced vibration of slender cylindrical structures. The phenomenon of vortex induced vibration is explained, followed by a literature review of published work on methods of suppression. The review revealed a lack of knowledge in several key areas relating to the performance of suppression devices. Furthermore, there were no analytical or empirical methods of predicting device performance. Slender cylindrical structures, such as marine risers, play a crucial role in the recovery of offshore oil reserves. An economic analysis was carried out which showed that vortex induced vibration can cause operational problems which significantly increase the risks and costs involved in offshore oil field exploitation. There was, therefore, an industrial need for more information on suppression devices and a means of predicting their performance. A large scale experimental investigation was performed to increase the understanding of suppression device performance under a variety of simulated offshore conditions, which included the effects of sheared flow, critical flow and partial suppression device coverage. Of the three traditional suppression devices tested, the fairing performed best, followed by the helical strake and then the perforated shroud. For the strake and shroud, the relationship between coverage and amplitude suppression was linear. For the fairing, however, the relationship became non-linear. With partial device coverage, the position of the device relative to the cylinder’s modal nodes and antinodes was very important. Sheared flow was found to increase the performance of all three suppression devices. In critical flow, vortex induced vibration of a bare cylinder was found to occur, but at reduced levels. Critical flow increased the performance of the suppression device. Computer based mathematical models have been developed which are capable of predicting the amplitude of vibration and drag force on bare cylinders, and cylinders fitted with suppression devices, under a variety of ambient conditions. The models were validated against the data obtained from the experimental programme. Two novel suppression devices have been invented, developed, tested and patented. They offer significant advantages over traditional devices for use in offshore operations.Item Open Access Theoretical and experimental studies of the Coriolis mass flowmeter(Cranfield University, 1990-03) Sultan, G.; Hemp, J.The Coriolis sass flowaeter is modelled using the theory of vibrating beams. Tube deformations for the fundamental mode and the next two nodes of natural (out-of-plane) vibration are worked out for many tube geometries. An improved model taking into account the effects of elastic boundary conditions and added masses of the electromagnetic drive and detectors is also discussed. A method for predicting the optimum detection positions is suggested in relation to signal-to-noise ratio. In addition, an argument for prediction of the effects of a dilute suspension of small bubbles or particles in the fluid is presented. Finally, experimental studies are conducted in order to demonstrate the validity of the theoretical models.Item Open Access Theoretical and experimental study of turbine flowmeters(Cranfield University, 1992-02) Xu, Y. F.; Hemp, J.A vortex shedding model is developed to predict the flow fields around turbine flowmeter blades. This model is used to simulate the flow separation and reattachment in the leading edge areas and the wake flows of the blades. Lewis's inverse blade design method is developed and quite successfully applied to simulate the displacement effect of the separation bubbles in the leading edge areas. A new method is introduced to apply the Kutta condition in unsteady flows around the blades wi th separation points on a blade surface or the flow with blade interaction. This method does not require a large amount of iterative calculation. A model is built up to predict the turbine flowmeter performance when the inlet flow conditions are known. The panel method is applied to predict the inlet flow conditions for the cases without inlet swirl. An experimental study of the flow inside a turbine flowmeter is carried out using Laser Doppler Anemometry(LDA) to measure the throughout velocity fields around a flowmeter at different flowrates and with different inlet swirls. A clear picture of the flow field is thus obtained. The measured resul ts are also used to validate the developed turbine flowmeter performance prediction model. The numerical prediction are tested with experimental results. The theoretical and experimental data agree with each other very well in the cases without inlet swirl and reasonably ",ell in other tested cases with inlet swirl.