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Dynamics of Exhaust Valve Flows and Confined Bluff Body Vortex Shedding
KTH, School of Engineering Sciences (SCI), Mechanics.ORCID iD: 0000-0003-0976-2004
2019 (English)Doctoral thesis, comprehensive summary (Other academic)Alternative title
Dynamik för avgasventilflöden och virvelavlösning från trubbiga kroppar (Swedish)
Abstract [en]

This thesis can be divided into two interconnected topics; engine exhaust-valve flows and confined bluff-body vortex shedding. When optimising engine flow systems it is common to use low order simulation tools that require empirical inputs, for instance with respect to flow losses across the exhaust valves. These are typically obtained from experiments at low pressure ratios and for steady flow, assuming the flow to be insensitive to the pressure ratio and that it can be considered as quasi-steady. Here these two assumptions are challenged by comparing measurements of mass-flow rates under steady and dynamic conditions at realistic pressure ratios. The experiments with a static valve were carried out using a high-pressure flow bench at cylinder pressures up to 500 kPa. For the dynamic-valve experiments the transient flow rate during the blowdown phase of an initially pressurised cylinder was determined. Here a linear motor actuated the valve to obtain equivalent engine speeds in the range 800–1350 rpm. It was shown that neither of the above mentioned assumptions are valid and a new non-dimensional quantification of the steadiness of the process was formulated. Furthermore it was shown through Schlieren visualisation that the shock structures in the exhaust port differ depending on if the system dynamics are included or not. The study shows that reliable results of flow losses past exhaust valves can only be obtained in dynamic experiments at representative pressure ratios. The second topic arose from the need to monitor time-resolved mass-flow rates in conduits. A mass-flow meter based on vortex shedding from bluff bodies was designed where microphones are used to detect the shedding frequency. It consists of a forebody and a downstream mounted tail and the system was shown to be capable of measuring pulsating flow rates. Furthermore, the flow topology associated with different forebody and splitter plates has been characterised, through visualisation of the flow behind the shedder and on the splitter plate. It has been shown that for long splitter plates a “horse shoe” like vortex, which attaches to the tail, is formed. It has also been shown that another energetic mode (denoted mode-II) can interact with and disrupt the primary vortex formation. A hypothesis for the appearance of mode-II has been formulated, linking it to the periodic separation of the boundary layer at the conduit wall.

Place, publisher, year, edition, pages
KTH Royal Institute of Technology, 2019. , p. 77
Series
TRITA-MEK, ISSN 0348-467X ; 2019:16
National Category
Fluid Mechanics and Acoustics
Research subject
Engineering Mechanics
Identifiers
URN: urn:nbn:se:kth:diva-249702ISBN: 978-91-7873-159-6 (print)OAI: oai:DiVA.org:kth-249702DiVA, id: diva2:1305613
Public defence
2019-05-24, F3, Stockholm, 10:15 (English)
Opponent
Supervisors
Note

QC 20190423

Available from: 2019-04-23 Created: 2019-04-17 Last updated: 2019-04-23Bibliographically approved
List of papers
1. On discharge from poppet valves: effects of pressure and system dynamics
Open this publication in new window or tab >>On discharge from poppet valves: effects of pressure and system dynamics
2018 (English)In: Experiments in Fluids, ISSN 0723-4864, E-ISSN 1432-1114, Vol. 59, no 2, article id 24Article in journal (Refereed) Published
Abstract [en]

Simplified flow models are commonly used to design and optimize internal combustion engine systems. The exhaust valves and ports are modelled as straight pipe flows with a corresponding discharge coefficient. The discharge coefficient is usually determined from steady-flow experiments at low pressure ratios and at fixed valve lifts. The inherent assumptions are that the flow through the valve is insensitive to the pressure ratio and may be considered as quasi-steady. The present study challenges these two assumptions through experiments at varying pressure ratios and by comparing measurements of the discharge coefficient obtained under steady and dynamic conditions. Steady flow experiments were performed in a flow bench, whereas the dynamic measurements were performed on a pressurized, 2 l, fixed volume cylinder with one or two moving valves. In the latter experiments an initial pressure (in the range 300–500 kPa) was established whereafter the valve(s) was opened with a lift profile corresponding to different equivalent engine speeds (in the range 800–1350 rpm). The experiments were only concerned with the blowdown phase, i.e. the initial part of the exhaustion process since no piston was simulated. The results show that the process is neither pressure-ratio independent nor quasi-steady. A measure of the “steadiness” has been defined, relating the relative change in the open flow area of the valve to the relative change of flow conditions in the cylinder, a measure that indicates if the process can be regarded as quasi-steady or not.

Place, publisher, year, edition, pages
Springer, 2018
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-222028 (URN)10.1007/s00348-017-2478-8 (DOI)000424707100001 ()2-s2.0-85040812357 (Scopus ID)
Funder
Swedish Energy Agency
Note

QC 20180131

Available from: 2018-01-31 Created: 2018-01-31 Last updated: 2019-04-23Bibliographically approved
2. On shock structures in dynamic exhaust valve flows
Open this publication in new window or tab >>On shock structures in dynamic exhaust valve flows
2019 (English)In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 31, no 2, article id 026107Article in journal (Refereed) Published
Abstract [en]

The gas dynamics of the flow past an exhaust valve has been investigated using Schlieren photography. An experimental setup was designed and constructed, which allowed optical access to the valve head and seat region as well as to the exhaust port. The setup was constructed so that the shock structures of a steady flow, with a static valve, could be compared to the structures found in experiments with a more realistic dynamically discharging cylinder, with a moving valve. The steady flow experiments were carried out at a valve lift to a port diameter ratio of 0.155 with cylinder pressures up to 325 kPa. The dynamic valve experiments were performed with an initial cylinder pressure of 300 kPa and at an equivalent engine speed of 1350 rpm. The steady flow experiments belonged to one of the two flow regimes, depending on the cylinder pressure: regime I, a wall-bounded supersonic jet (for low cylinder pressures) or regime II, a fully expanded supersonic nozzle-flow (for high cylinder pressures). By comparing the images from the dynamic valve experiment to those of the steady flow experiments, it was shown that the flow in the dynamic experiments exhibits more similarities with regime I. However, large differences in the shock structures between the steady flow in regime I and the dynamic valve flow remain. This indicates that experiments using a steady flow and a fixed valve lift do not encompass the essential physics found in real engine flows and should be avoided. 

Place, publisher, year, edition, pages
AMER INST PHYSICS, 2019
National Category
Mechanical Engineering
Identifiers
urn:nbn:se:kth:diva-247850 (URN)10.1063/1.5084174 (DOI)000460093800093 ()2-s2.0-85061735704 (Scopus ID)
Note

QC 20190326

Available from: 2019-03-26 Created: 2019-03-26 Last updated: 2019-05-16Bibliographically approved
3. Development of a pressure based vortex-shedding meter: measuring unsteady mass-flow in variable density gases
Open this publication in new window or tab >>Development of a pressure based vortex-shedding meter: measuring unsteady mass-flow in variable density gases
2016 (English)In: Measurement science and technology, ISSN 0957-0233, E-ISSN 1361-6501, Vol. 27, no 8, article id 085901Article in journal (Refereed) Published
Abstract [en]

An entirely pressure-based vortex-shedding meter has been designed for use in practical time-dependent flows. The meter is capable of measuring mass-flow rate in variable density gases in spite of the fact that fluid temperature is not directly measured. Unlike other vortex meters, a pressure based meter is incredibly robust and may be used in industrial type flows; an environment wholly unsuitable for hot-wires for example. The meter has been tested in a number of static and dynamic flow cases, across a range of mass-flow rates and pressures. The accuracy of the meter is typically better than about 3% in a static flow and resolves the fluctuating mass-flow with an accuracy that is better than or equivalent to a hot-wire method.

Place, publisher, year, edition, pages
Institute of Physics Publishing (IOPP), 2016
Keywords
vortex-meter, pulsating, mass-flow, measurement, unsteady
National Category
Mechanical Engineering
Identifiers
urn:nbn:se:kth:diva-190551 (URN)10.1088/0957-0233/27/8/085901 (DOI)000380124800037 ()2-s2.0-84979085082 (Scopus ID)
Note

QC 20160815

Available from: 2016-08-15 Created: 2016-08-12 Last updated: 2019-04-17Bibliographically approved
4. Vortex-meter design: The influence of shedding-body geometry on shedding characteristics
Open this publication in new window or tab >>Vortex-meter design: The influence of shedding-body geometry on shedding characteristics
2018 (English)In: Flow Measurement and Instrumentation, ISSN 0955-5986, E-ISSN 1873-6998, Vol. 59, p. 88-102Article in journal (Refereed) Published
Abstract [en]

The periodic vortex shedding from bluff bodies may be used in flow metering applications. However, because the bluff-body is highly confined (typically in a pipe) the shed vortices may interact with the pipe wall; causing an undesirable non-linear behaviour. An experimental investigation has been conducted; examining the vortex-shedding characteristics of highly confined bluff-bodies in pipe flow, at high Reynolds number (ReD=4.4×104 to 4.4×105). The bluff-bodies were comprised of a forebody and tail; both of which affected the primary-shedding characteristics. The shedders typically produced two unsteady modes: Mode-I was associated with the vortex shedding and mode-II resulted from a separation of the pipe-wall boundary layer. The mode-I behaviour allowed two classes of shedder to be defined: long-tails and short-tails. Modes I and II interacted, particularly for long-tailed geometries. When the length-scale of mode-II exceeded 0.8κ (where κ is the physical scale of the primary shedding vortex), mode-II disrupted mode-I, as the mode-frequency ratio (fII/fI) approached an integer value. The coupling of modes I and II caused mode-I to deviate from its preferred Strouhal number. When the deviation exceeded 25–30%, mode-I locked on to the mode-II frequency. This did not happen for short-tailed geometries, as the length-scale of mode-I was always dominant. Mode-coupling for short-tails occurred only when the mode frequencies were equal. 

Place, publisher, year, edition, pages
Elsevier Ltd, 2018
Keywords
Bluff-body, Compressible, Frequency-characteristic, Splitter-plate, Vortex-meter, Vortex-shedding, Boundary layer flow, Boundary layers, Geometry, Reynolds number, Vortex shedding, Bluff body, Coupling of modes, Experimental investigations, Frequency characteristic, High Reynolds number, Nonlinear behaviours, Splitter plates, Vortex flow
National Category
Mechanical Engineering
Identifiers
urn:nbn:se:kth:diva-223133 (URN)10.1016/j.flowmeasinst.2017.12.004 (DOI)000428102800012 ()2-s2.0-85039164249 (Scopus ID)
Note

QC 20190423

Available from: 2018-03-27 Created: 2018-03-27 Last updated: 2019-04-23Bibliographically approved
5. On the scaling and topology of confined bluff-body flows
Open this publication in new window or tab >>On the scaling and topology of confined bluff-body flows
(English)Manuscript (preprint) (Other academic)
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-249700 (URN)
Note

QC 20190423

Available from: 2019-04-17 Created: 2019-04-17 Last updated: 2019-04-23Bibliographically approved
6. Characterization of and correction for pressure-measurement installation
Open this publication in new window or tab >>Characterization of and correction for pressure-measurement installation
2017 (English)Report (Other academic)
Abstract [en]

A method to experimentally determine the dynamic characteristics of pressure measurement installations has been developed and tested. The method involves pressurising a volume enclosed by a rubber membrane until the membrane burst, generating a negative pressure step. The natural frequency and damping ratio of the system can then be found through analysis of the step response of the measurement system.

An example showing how it is possible to compensate for the dynamic characteristics of the installation, for qualitative measurements of highly dynamic processes is also given. This is done by modelling the system as an acoustic oscillator. Since the model requires that the time derivative of the signal is taken it amplifies noise in the signal, meaning that the quantitative values of the corrected measurements should be handled with care.

Publisher
p. 11
Series
TRITA-MEK, ISSN 0348-467X ; 2017.11
Keywords
Pressure measurements, installation characteristics
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-215136 (URN)978-91-7729-569-3 (ISBN)
Note

QC 20171016

Available from: 2017-10-03 Created: 2017-10-03 Last updated: 2019-04-17Bibliographically approved

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