Bülent Ünsal

Laminar-to-turbulent transition of pipe flows through puffs and slugs

(Received 16 August 2007 and in revised form 7 July 2008) Laminar-to-turbulent transition of pipe flows occurs, for sufficiently high Reynolds numbers, in the form of slugs. These are initiated by disturbances in the entrance region of a pipe flow, and grow in length in the axial direction as they move downstream. Sequences of slugs merge at some distance from the pipe inlet to finally form the state of fully developed turbulent pipe flow. This formation process is generally known, but the randomness in time of naturally occurring slug formation does not permit detailed study of slug flows. For this reason, a special test facility was developed and built for detailed investigation of deterministically generated slugs in pipe flows. It is also employed to generate the puff flows at lower Reynolds numbers. The results reveal a high degree of reproducibility with which the triggering device is able to produce puffs. With increasing Reynolds number, ‘puff splitting’ is observed and the split puffs develop into slugs. Thereafter, the laminar-to-turbulent transition occurs in the same way as found for slug flows. The ring-type obstacle height, h, required to trigger fully developed laminar flows to form first slugs or puffs is determined to show its dependence on the Reynolds number, Re = DU/ν (where D is the pipe diameter, U is the mean velocity in the axial direction and ν is the kinematic viscosity of the fluid). When correctly normalized, h + turns out to be independent of Reτ (where h + = hUτ /ν, Reτ = DUτ /ν and Uτ = √ τw/ρ; τw is the wall shear stress and ρ is the density of the fluid).

Mass flow rate control system for time-dependent laminar and turbulent flow investigations

Most experimental flow investigations are carried out under imposed steady-state flow conditions. The major reason for this is that there is a lack of experimental facilities to impose well-controlled time-dependent inlet and outlet conditions on flows. There is apparently no equipment available to supply, in a well-controlled manner, the mass flow rate for time-dependent internal flow investigations. The work described in this paper remedies this situation. It introduces the basic ideas for a mass flow rate control system for time-dependent laminar and turbulent flow investigations. A first unit was built for controlled mass flow rate variations in the range of 0–217.8 g min−1 under atmospheric conditions corresponding to 0–180 l min−1. With this first unit the authors demonstrate that the basic ideas put forward in the paper can be used to build mass flow rate control units for experimental fluid mechanics studies. The flow rate can be simply controlled by a voltage input. The system was designed to work up to frequencies of 125 Hz.

Method for defined mass flow variations in time and its application to test a mass flow rate meter for pulsating flows

In a previous publication, the authors presented a transient mass flow rate metering technique for pulsating pipe flows and its outstanding performance was demonstrated experimentally for fuel injection nozzles. In the present paper, the application of the mass flow rate metering technique is described for pulsating air flows. The basics of the measuring technique are summarized and the corresponding experimental setup is explained. For verification experiments, a mass flow rate control unit was developed that permits time-varying mass flow rates to be provided proportional to the voltage of an electronic input signal to the unit. The basic ideas of this unit and its performance are summarized. Its application to verify the performance of the developed mass flow rate measuring instrument represents the major part of the paper. Performance tests were carried out for various time variations of mass flow rate pulsations. It is shown that the mass flow rate of the mean flow and that of the pulsating flow can be separated and both can be accurately measured. Comparative measurements show that the mass flow rate measuring technique works very well and reproduces the mass flow rate variations in time imposed by the mass flow rate control unit.

Instantaneous mass flowrate measurements through fuel injection nozzles

Abstract A method of measuring the instantaneous mass flowrates of fuels passing through fast operating injection nozzles is described. The flowrate information is deduced from instantaneous pressure gradient measurements. Through a Fourier transformation, the Fourier coefficients of the pressure gradient are determined, which are used for the calculation of the transient mass flowrate of fuel through injection nozzles, utilizing a special reconstruction algorithm. The theoretical background of the measurement method is provided and the development work to yield a practically applicable measuring system is summarized. The latter is used for verification experiments, providing time-resolved flowrates for a magnetically driven injection valve operating at a pressure of approximately 6 bar. Online measurement of the injection mass flowrates can be recorded with this measuring system and information can be obtained for individual injection events. Cycle-to-cycle variations can be detected in this way.

Fast-operating injection valves with virtually pressure wave-free supply lines

FMP Technology GmbH has developed a new injection valve for internal combustion engines with a modification of the fuel guidance. The valve does not exhibit the high pressure fluctuations that generally occur in the supply lines, and the entire fuel injection system works virtually without pressure waves. Due to this new method of fuel guidance, the fuel is no longer accelerated or decelerated when the valves open and close. Instead, the fuel is diverted. Because of this, the major part of the fuel in the supply lines does not change its state of motion when the valve is opened and closed. With the proposed valve, the injected mass of the fuel can be determined simply by the duration of the opening time. This is shown by numerous numerical studies carried out by the authors. Experimental verifications still need to be performed.

Schnell schaltende Einspritzventile mit praktisch druckwellenfreien Kraftstoff-Zuführleitungen

Die FMP Technology GmbH entwickelt ein neuartiges Einspritzventil fur Verbrennungsmotoren mit einer geanderten Kraftstofffuhrung. Dieses Ventil zeigt nicht die ublicherweise in den Kraftstoffzuleitungen auftretenden hohen Druckpulsationen; das gesamte Einspritzsystem arbeitet praktisch druckwellenfrei. Dies bringt beachtliche Vorteile bezuglich der Kontrolle des Einspritzvorganges mit sich, da druckwellenbedingte Schwankungen der eingespritzten Kraftstoffmenge nicht mehr auftreten. Durch die neue Weise der Kraftstofffuhrung wird die bei Ventiloffnung und -Schliesung zugefuhrte Kraftstoffmenge nicht mehr beschleunigt oder abgebremst. Stattdessen wird der Kraftstoff umgelenkt. Dadurch andert der groste Teil des Kraftstoffes, im Ventil und in den Zuleitungen, seinen Bewegungszustand beim Offnen und Schliesen des Ventils nicht. Mit Ventilen dieser Art lasst sich die eingespritzte Kraftstoffmenge allein uber die Offnungszeiten des Ventils bestimmen. Dies zeigen umfassend numerische Berechnungen. Experimentelle Verifikationen stehen noch aus.