Job Kurian

Pressure Oscillations from Cavities with Ramp

Study on supersonic flow over wall-mounted cavities with different ramp angles is carried out experimentally. Experiments include instantaneous shadowgraph visualization and unsteady pressure measurements. In the case of cavities with ramp angles of 90, 75, and 60 degrees, four different types of waves were observed in the supersonic flow above the cavity. The shear layer appeared predominantly wavy in nature. The wave system observed in the case of cavities of 90, 75 and 60 degrees is distinctly different from that in cavities of 45, 30, and 15 degrees. In the case of cavities with higher ramp angles, high-amplitude tones were observed and the presence of an upstream-traveling acoustic wave could be confirmed. A cavity with a ramp angle of 45 degrees was found to reduce the amplitude of oscillations. An increase in amplitude of oscillations at various locations inside the cavity has been observed for cavities with ramp angles of 30 and 15 degrees. This is in contrast to the cavity with a ramp angle of 45 degrees. The reasons for this ambiguous behavior could not be ascertained. The presence of a forward-moving acoustic wave could not be identified in the cavities with ramp angles of 45,30, and 15 degrees. Temporal mode switching occurring in cavities with higher ramp angles was confirmed by spectrogram studies.

Strut-Based Gaseous Injection into a Supersonic Stream

Introduction M IXING of a secondary jet with the primary supersonic stream is of great importance in many practical applications especially in scramjet combustors. Because the residence time of the high-speed flow in such combustors is only a few milliseconds, it is essential to implement mixing augmentation techniques to provide rapid and uniform mixing of fuel and air. Several injection schemes have been proposed,1−5 and it has been concluded that the vorticity is the main driving mechanism for rapid near-field mixing. The present study is conducted on the strut-based fuel injectors. The main advantage of strut-based injectors is the injection of the fuel into the core of the main flow and uniform spreading of the fuel in the lateral direction. The large-scale structures at the wake region can assist macromixing. Moreover, the shock that emanates from the leading edge of the strut is useful to enhance the mixing via the baroclinic torque mechanism.6 A further advantage of strut-based injection is the formation of a recirculation zone, which can be used for flame holding in combustion.7−9 Four types of fuel injectors are considered in the current experimental investigation, one of them being the plain strut-type injector. Diagnostic methods employed are Mie scattering combined with image processing and the time-averaged schlieren. Numerical simulation of the flowfield using the commercially available software FLUENT has also been carried out.

An experimental investigation of thermal mixing and combustion in supersonic flows

A radially lobed nozzle (petal nozzle) is being increasingly recognised as a potential candidate for promoting mixing in compressible flows. An experimental investigation has been conducted to study its effectiveness in improving thermal mixing and combustion in supersonic flow. A hot gas jet issuing supersonically from a lobed nozzle mixes with a cold supersonic jet in a circular mixing tube. The two jets issue coaxially. A detailed survey of the flow field inside the mixing duct reveals that nearly complete thermal mixing (as exemplified by the nearly uniform temperature distribution) could be achieved in a short distance when a lobed nozzle is employed. The results also indicate the presence of large-scale vortices in the flow field downstream of the lobed nozzle. Having thus created a field in which mixing is good, supersonic combustion was then attempted. Kerosene was introduced into the hot stream issuing from the lobed nozzle and it burned mainly in the mixing tube, which served as a supersonic combustor. Resulting temperature and pressure rises were measured and the supersonic combustion efficiency was found to be of the order of 60%. The performance of a conventional conical nozzle was found to be much inferior to that of the petal nozzle under identical conditions.

Studies of Low-Density Freejets and Their Impingement Effects

A detailed experimental investigation of freejets and their impinging flowfields has been carried out. Studies were conducted for different pressure ratios, and the influence of Reynolds number on flow freezing was examined. A conical convergent-divergent nozzle of large area ratio, typically employed in aerospace applications, was used in the studies. Quantitative data of freejet and impingement flowfields, which could be used as a benchmark for checking the results obtained from numerical simulations such as the direct simulation Monte Carlo method, are presented. Forces due to plume impingement on an adjacent surface were estimated. The impingement effects were found to be considerable at high Reynolds numbers, whereas flowfield properties were found to be unaffected due to the presence of the plate at low Reynolds numbers. The freejet and the impinging flowfield were visualized using a glow discharge technique. The features of transition flow were observed in low Reynolds number flows. The flowfield modifications brought about by the influence of the flat plate could also be visualized.

Enhancement of thermal mixing in coaxial supersonic jets

An experimental study was conducted to investigate the thermal mixing of a hot, supersonic, primary jet of combustion gases with a coflowing secondary jet of air at ambient temperature. The Mach numbers of the two jets were 1.2 and 1.7, respectively. To enhance the mixing, the core (primary) jet was admitted through a three-dimensio nal, radially lobed nozzle, referred to as the petal nozzle. The uniformity of stagnation temperature in the flowfield was used to characterize the extent of mixing between the jets. The effect of confinement on the mixing was also investigated. The mixing performance of the lobed nozzle and the associated loss in stagnation pressure were compared with those for a conventional conical nozzle. The study confirms the efficacy of the radially lobed nozzle in thermal and momentum mixing of supersonic jets and highlights its potential in supersonic combustion systems. Nomenclature T = stagnation temperature, °C fji = momentum flux, N/m2 cr = standard deviation

Experimental investigation of rarefied gas flow through rectangular slits and nozzles

The flow of a rarefied gas through rectangular configuratons of different geometries has been experimentally studied to determine their discharge coefficient characteristics. The configurations used are a set of sharp-edged slit orifices, a smooth converging nozzle and a tube. The range of the Reynolds number based on the throat conditions varied from 0.01 to 100. The equivalent Knudsen number range based on the upstream conditions and inlet diameter varied from 0.0521 to 2.521. The results for the smooth nozzle are compared with calculations using a numerical method with one-dimensional stream tube approximation based on integrated boundary layer equations. The slit and the tube results are compared with the experimental results of Sreekanth and Davis [1988].

Coaxial Jets from Lobed-Mixer Nozzles

Experiments were conducted to study the jet dynamics of unconfined and confined coaxial high-speed flows from radially lobed nozzles. The shear-layer growth rate of the lobed nozzles was found to be considerably higher than that of a conical nozzle. Pressure profiles indicated increased radial uniformity and transport of properties between flows. Increases in lobe height and number of lobes were associated with enhanced mixing and larger total pressure drops. The thrust loss observed was due primarily to divergent nozzle flow and duct friction losses. Use of a confining duct forced interaction of the flows and enhanced mixing. Compression of the primary flow produced intense compression-expansion regions in the flowfield. Another advance over earlier studies is the visualization of the axial vortices generated by lobed nozzles. These vortices can be linked to the observed cross-plane transport of properties. Nomenclature D - diameter Dp = total pressure drop parameter / = length of mixing tube M = Mach number m = mass flow rate P = total pressure p = static pressure R = radius r = radial coordinate T = thrust u = velocity x = axial coordinate 8 - shear-layer thickness p = density r] = mixing parameter Subscripts c cl e edge ex P S = convective = centerline = equivalent = edge of shear layer = nozzle exit plane = primary stream = secondary stream

Attenuation of Shock Waves in Gas-Particle Mixtures

Our purpose is to add more experimental results on decay process of shock waves in gas-particle mixtures inside a shock tube.

Low density conductance through composite geometries

Abstract Conductance of gases through different short circular passages at low density conditions is experimentally investigated. The main aim of the study is to verify the validity of the commonly used electrical analogy method of calculating flow resistance of a composite passage from the individual elements making the composite geometry. The range of Knudsen numbers covered in the experiments is from 0.01736 to 4.61, whereas the Reynolds number varied from 0.1792 to 14.83.

Shock velocity measurements using microwave Doppler shift

A commercially available 10.587 GHz microwave Doppler module is used for the measurement of shock velocity in a conventional shock tube. With proper electronic circuits the Doppler frequency obtained is found to be quite noisefree and consistent for shock velocities in the range of 1.8 mm/μsec to 2.0 mm/μsec.