Graham K. Hargrave

Studies of premixed flame propagation in explosion tubes

An experimental and theoretical study of premixed flame propagation in a number of small-scale, cylindrical vessels is described. The study provides further understanding of flame propagation and the generation of overpressure in explosions, and allows the assessment of a mathematical model of explosions through comparisons with the experimental data obtained. Laser sheet images and data gathered on flame location, shape, and overpressures generated during the course of explosions in an empty vessel and obstacle-containing enclosures elucidate the dynamics of the various combustion processes occurring in the different chambers of the vessels. In particular, flame propagation through the vessels, up until flame front venting, is found to be substantially laminar, with significant overpressure only being generated in the later stages of explosions due to rapid turbulent combustion in the shear layers and recirculation zones induced by the obstacles. Comparisons between measurements and predictions also demonstrate that the mathematical model described provides a reasonable simulation of explosions within obstacle containing enclosures of the type investigated, with rapid turbulent combustion being predicted with sufficient accuracy to yield reasonable results for the overpressures generated.

Forced convective heat transfer from premixed flames —Part 2: Impingement heat transfer

Abstract A study of convective heat transfer from impinging flames is completed with the presentation of heat transfer rates measured in premixed methane-air flames. Unburnt gas equivalence ratios from 0.8 to 1.2 have been examined, with burner exit Reynolds numbers ranging from 2000 to 12 000. Heat fluxes measured at the stagnation point of a body of revolution and a circular cylinder demonstrate that the trends observed in measured heat flux profiles are mainly determined by variations in the mean velocity and temperature within a flame, with peak heat transfer rates occuring within or close to the flame reaction zone. Increases in Reynolds number lead to an increase in the peak heat flux attained within a flame and to a decrease in the axial extent of the flame equilibrium region. Variations in equivalence ratio away from approximately stoichiometric conditions lead to a decrease in the maximum rate of heat transfer from a flame and to a shifting of the position of maximum flux downstream. Theoretical predictions applicable to the equilibrium region of the flames are in reasonable accord with experimental data.

An Experimental Study of a Turbulent Natural Gas jet in a Cross-Flow

Abstract-An understanding of the physical processes involved in the atmospheric venting and flaring of flammable gases is necessary to assessments of the consequences associated with these operations. In order to further such understanding, this paper presents the results of a wind tunnel study of a turbulent natural gas jet in a cross-flow. The results obtained relate the mean concentration field ofa single, non-reacting jet, through the ignition characteristics of that jet to the temperature and radiation fields associated with the cornbusting now. A more detailed study of the ignition characteristics of a number of jets has also been performed. The experimental data obtained provides insight into the connection between those phenomena of interest to consequence assessments, and will be of value to the validation of mathematical models of venting and flaring.

Experimental investigation of flame/solid interactions in turbulent premixed combustion

Abstract An experimental study has been carried out to investigate the interaction between propagating turbulent premixed flames and solid obstacles. The experimental rig was configured specifically to allow detailed measurements with laser-based optical diagnostics. A wall-type solid obstacle was mounted inside a laboratory-scale combustion chamber with rectangular cross-section. The flame was initiated, by igniting a combustible mixture of methane in air at the center of the closed end of the combustion chamber. The flame front development was visualized by a high-speed (9000 frame/s) digital video camera and flame images were synchronized with ignition timing and chamber pressure data. The tests were carried out with lean, stoichiometric and rich mixtures of methane in air. The images were used to calculate highly resolved temporal and spatial data for the changes in flame shape, speed, and the length of the flame front. The results are discussed in terms of the influence of mixture equivalence ratio on the flame structure and resulting overpressure. The reported data revealed significant changes in flame structure as a result of the interaction between the propagating flame front and the transient recirculating flow formed behind the solid obstacle. Combustion images show that the flame accelerates and decelerates as it impinges on the obstacle wall boundaries. It is also found that the mixture concentrations have a significant influence on the nature of the flame/solid interactions and the resulting overpressure. The highest flame speed of 40 m/s was obtained with the unity fuel–air equivalence ratio. Burning of trapped mixture behind the solid obstruction was found to be highly correlated with the flame front length and the rate of pressure rise.

An experimental and numerical investigation of premixed flame deflagration in a semiconfined explosion chamber

A combined experimental and numerical study of turbulent premixed flame propagation over multipleobstacles mounted in a semiconfined explosion combustion chamber is reported. The experimental method used a high-speed laser sheet flow visualization technique to record the progress of the flame front in a stoichiometric methane/air mixture. This allowed calculation of flame speed. Pressure was measured at two locations within the combustion chamber. For the simulations, transient Favre-averaged equations were solved using recent developments of a flamelet model. This model formulates the mean rate of reaction as a function of a transport equation for the flamelet surface density model. Both linear and nonlinear eddy viscosity turbulence models have been used and investigated for the closure of the Reynolds stresses. Excellent agreement of flame structure and pressure impulse was obtained with the use of a nonlinear eddy viscosity turbulence model. It was also found that, irrespective of the eddy viscosity turbulence model, quantitative results of pressure and flame speed were found to be in good agreement with the experimental results. Regimes of combustion covered by the present study have been identified and found to reside in the wrinkled and corrugated (reaction sheets) flamelet regimes.

Forced convective heat transfer from premixed flames: —Part 1: Flame structure

Abstract In this, the first part of a two-part study of convective heat transfer from impinging flames, the aerodynamic structure of four flames was studied. The flames examined were of stoichiometric mixtures of methane and air with a Reynolds number range extending from the laminar to fully turbulent flow regimes. Instantaneous Schlieren photographs revealed that with increasing Reynolds number the flame reaction zone extended further downstream and became thicker and more diffuse. Associated with this, measurements of mean and rms velocities and mean temperatures showed that the properties of the flames became drawn out in the downstream direction as Reynolds number increased. Schlierenstroboscopic techniques also revealed the existence of large scale vortex rings which originated in the shear layer of the flames, and which were found to cause low frequency oscillations in measured instantaneous velocities. These oscillations lead to misleadingly high levels of rms velocities downstream of the flame reaction zone which should not be interpreted as representing turbulence within the flame.

Flame Stability in Underexpanded Natural Gas Jets

Abstract —It is known that, for nozzle diameters greater than some critical value, free jet flames remain stable at all driving pressures. For releases from orifices smaller than this limiting diameter, subsonic jet flames become unstable when the exit velocity is increased above some blow-out stability limit. In these unstable jet flames, it has been postulated that flow conditions suitable for stable flame formation may be obtained by increasing the driving pressure to produce a supercritical (choked and underexpanded) jet. This paper presents an experimental study of the flame blow-out stability limits in high pressure natural gas jets. The critical diameter for unconditionally stable flames is determined and the postulate of high pressure restabilisation confirmed. Also, previous work on the probabilities of subsonic jet light-up resulting from a localised ignition has been extended to supercritical jets. It is shown that any ignition at or beyond that axial location associated with the lower flammabl...

An investigation of string cavitation in a true-scale fuel injector flow geometry at high pressure

String cavitation has been studied in an optical automotive size fuel injector with true-scale flow geometry at injection pressures of up to 2050 bar. The multihole nozzle geometry studied allowed observation of the hole-to-hole vortex interaction and, in particular, that of a bridging vortex in the sac region between the holes. A dependency on Reynolds number was observed in the formation of the visible, vapor filled vortex cores. Above a threshold Reynolds number, their formation and appearance during a 2 ms injection event was repeatable and independent of upstream pressure and cavitation number.

Simultaneous Study of Intake and In-Cylinder IC Engine Flow Fields to Provide an Insight into Intake Induced Cyclic Variations

Simultaneous intake and in-cylinder digital particle image velocimetry (DPIV) experimental data is presented for a motored spark ignition (SI) optical internal combustion (IC) engine. Two individual DPIV systems were employed to study the inter-relationship between the intake and in-cylinder flow fields at an engine speed of 1500 rpm. Results for the intake runner velocity field at the time of maximum intake valve lift are compared to incylinder velocity fields later in the same engine cycle. Relationships between flow structures within the runner and cylinder were seen to be strong during the intake stroke but less significant during compression. Cyclic variations within the intake runner were seen to affect the large scale bulk flow motion. The subsequent decay of the large scale motions into smaller scale turbulent structures during the compression stroke appear to reduce the relationship with the intake flow variations.

Internal Flow and Near-Orifice Spray Visualisations of a Model Pharmaceutical Pressurised Metered Dose Inhaler

The pressurised Metered Dose Inhaler (pMDI) has become the most prescribed drug delivery system for treating the respiratory diseases. However, the spray generation mechanism of these devices has not been extensively researched and there is very little information regarding the two-phase fluid dynamics associated with pre-atomisation inside the valve stem. The aim of the work presented in this paper is to provide high-quality, time-resolved imaging of the internal flow structures of pMDIs in an attempt to link the characteristics of the internal flow to external spray atomization processes. Visualisations of the aerosols in the near-orifice region findings from previous studies of commercial pMDIs and showed the following characteristics: (i) start-up transient, (ii) fully developed spray with slow spray density variations and (iii) rapid spray density pulsations with large droplet production. The results clearly highlighted the potential of optical diagnostics in the development of improved accounts of the state of the flow inside a pMDI valve and its relationship with drop formation.