Parallel Air Stream Research Articles

Effects of wall cavity and fuel injection pressure on the performance of a non-reacting supersonic combustor

This study numerically investigates the flow field of a non-reacting cavity-configured scramjet (Supersonic Combustion Ramjet) combustor at various fuel injection pressures by solving the 2D Reynolds-Averaged Navier-Stokes (RANS) equations, species transport equations, and Menter SST k-ω model. The aim of this research is to reveal the effects of wall cavity insertion and fuel injection pressure (FIP) on the crucial performance parameters i.e., fuel-air mixing efficiency (MxE), total pressure recovery (TPR), and mass-averaged Mach number (MAMN). Accordingly, two trapezoidal cavities of aspect ratio 7 are introduced on the opposite walls of a rectangular combustor. The combustor entrance is configured with rearward-facing steps and it intakes finite parallel air streams through finite-width inlets. Gaseous hydrogen jets are injected 30 mm downstream from the combustor entrance and 10 mm upstream from the cavity leading edge. FIP is varied according to the fuel-to-freestream pressure ratios (FFPR) of 4.5, 9.0, 13.5, and 18.0. The results of the cavity-configured combustor are then compared with the performance of the combustor in the absence of the wall cavities. The results delineate the change in flow structures with the inclusion of wall cavities and variation in FIP. Insight physics of mixing, total pressure loss, and MAMN in different regions of the combustor are studied and the results are quantified for comparison. MxE in a cavity-configured combustor does not monotonically increase with decreasing FFPR as found in the combustor without wall cavities. The shock-shear layer interactions (SSLIs) play a dominant role in mixing inside the cavity-configured combustor. The results also demonstrate that the insertion of wall cavities can increase fuel-air MxE through the formation of cavity recirculation zones. In the cavity-configured combustor, a maximum of 45% MxE is achieved for FFPR 4.5, which is 4% higher than the value obtained from the combustor without the cavities with an expense of 3% greater total pressure loss.

Influence of co-axial airflow and volume ratio on thermo-capillary convection in half floating zones

Present work is an extensive numerical investigation of interfacial flow in 5 cSt silicone oil that develops under the action of thermo-capillary force and interaction of this flow with the surrounding air motion using the commercial software ANSYS Fluent 17.2. The physical model is a half floating zone of high Prandtl number fluid (Pr = 56) co-axially surrounded by ambient air under microgravity conditions. The surface of a slender liquid bridge (VR = 0.9) is subjected to parallel air stream which is co-directed and counter-directed with the thermo-capillary flow along the interface. This forced air motion influences the basic flow in the system via shear stress and heat transfer at the interface. The effect of ambient air velocity and the volume ratio of the liquid bridge on the evolution of flow states is examined by performing axisymmetric computations in both liquid and air domains. The considered values of air velocity are comparable or larger than the Marangoni velocity to account for the shear action employed by the parallel airflow. The results show that the heat loss at the interface characterized in terms of local Biot number is significantly higher in case of counter-directed airflow as compared to that of co-directed airflow. By varying the volume ratio of the liquid bridge (0.90 < VR < 1.10), the resulting alteration in surface velocity and temperature of thermo-capillary flow is found to affect the flow dynamics of the system. Thermo-capillary convection inside slender liquid bridge results in more heated core liquid region as compared to that of fat liquid bridge affecting the heat transfer condition at the free surface. The authors report a significant influence of the flow and geometric parameters on the stationary flow solution.

Large eddy simulation of turbulent heat transfer from a rotating disk subjected to a forced parallel air stream

Nowadays, rotating disks are an important part of most industries such as turbo machines, heat exchangers, braking systems, rotating sawing machines, and computer disk drives. Working temperature and consequently heat transfer have a profound effect on performance and lifetime of rotating disks. There is a noticeable difference between numerical and experimental results in the studies of turbulent heat transfer from a rotating disk subjected to parallel air stream. The main objective of this study is numerical simulation of turbulent convective heat transfer from a rotating disk subjected to parallel air stream. In this study, the 2nd order finite volume scheme and large eddy simulation (LES) with the dynamic Smagorinsky procedure in sub-grid-scale is used. Two limit cases called rotating disk in still air (von Karman's swirling flow), stationary disk subjected to parallel air stream are investigated as well. The mean Nusselt number of the disk surface is obtained at statistical steady state for a wide range of rotational and cross flow Reynolds numbers. The influence of flow separation and finite disk thickness on heat transfer is investigated and followed by a detailed discussion. The effect of convection and diffusion on temperature field is illustrated. Comparison with experimental data shows that the LES dynamic Smagorinsky results are by far better than available numerical results. It is discussed in detail that there is a critical value for ratio of crossflow Reynolds number to rotational Reynolds number which above that the influence of disk rotation on mean Nusselt number is negligible.

Steady state characterization of bifacial solar cells at different configurations of air-based photovoltaic thermal solar panels

This paper examines the first and second law efficiency of bifacial photovoltaic thermal (PVT) solar collectors. A mathematical model was developed to evaluate the system performance. Four air-based bifacial photovoltaic thermal panels were also designed based on the bifacial PV cell requirement. Most of the existing PVT designs contain an absorber plate, which in this research is replaced by a reflector to avoid blocking the rear aperture of bifacial cells. The following four air-based panel configurations were considered: single-path; double-path, parallel; double-path, counter flow; and double-path, returning flow. All four panel designs evaluated based on the first law and second law of thermodynamic, at steady state mode. Energy and exergy efficiencies of aforementioned designs strongly depend on packing factor and air mass flow rate. From an energy point of view (first law), the bifacial photovoltaic thermal panel with two parallel air streams exhibited the best efficiency (51%–67%) compared with that of the other three models. However, the exergy analysis indicated that a single air stream panel is preferable with 4.43%–10.15% exergy efficiency. The single-path panel design is the best option if electrical energy is the dominant desired output energy. But double-path parallel design is the best option if thermal energy is the dominant desired output energy.

Numerical Investigation of Laminar Diffusion Flames Established on a Horizontal Flat Plate in a Parallel Air Stream

Numerical investigation of laminar diffusion flames established on a flat plate in a parallel air stream is presented. A numerical model with a multi-step chemical kinetics mechanism, variable thermo-physical properties, multi-component species diffusion and a radiation sub-model is employed for this purpose. Both upward and downward injection of fuel has been considered in a normal gravity environment. The thermal and aerodynamic structure of the flame has been explained with the help of temperature and species contours, net reaction rate of fuel and streamlines. Flame characteristics and stability aspects for several air and fuel velocity combinations have been studied. An important characteristic of a laminar boundary layer diffusion flame with upward injection of fuel is the velocity overshoot that occurs near the flame zone. This is not observed when the fuel is injected in the downward direction. The flame standoff distance is slightly higher for the downward injection of fuel due to increase in displacement thickness of boundary layer. Influence of an obstacle, namely the backward facing step, on the flame characteristics and stability aspects is also investigated. Effects of air and fuel velocities, size and location of the step are studied in detail. Based on the air and fuel velocities, different types of flames are predicted. The use of a backward-facing step as a flame holding mechanism for upward injection of fuel, results in increased stability limits due to the formation of a recirculation zone behind the step. The predicted stability limits match with experimentally observed limits. The step location is seen to play a more important role as compared to the step height in influencing the stability aspects of flames.

Diffusion Flame Stabilized on a Porous Plate in a Parallel Airstream

Effects of an obstacle on the structure and stability of a laminar diffusion e ame established on a porous plate in a parallel airstream have been investigated experimentally. The obstacle, a backward-facing step or a rectangular cylinder, is located upstream of the porous plate through which gaseous methane is injected uniformly. Structures of the e ame are elucidated by the direct and schlieren photography. Flame shapes are described and stability diagrams are plotted for the freestream velocity and the fuel injection velocity, which are discussed with e ow structures.

Diffusion flame stabilized on a porous plate in a parallel airstream

Diffusion flame stabilized on a porous plate in a parallel airstream

Effects of Obstacle on Film Combustion of Methanol in a Parallel Air Stream.

The term film means the diffusive combustion of fuel gas evaporated from a condensed fuel surface with an oxidizer gas flowing along the surface. The present study aims to gain a fundamental understanding of the effects of separated flow created by an obstacle on stabilization and structure of the diffusion flame. As an obstacle, a rod with diameter d is placed upstream of the fuel surface with its axis perpendicular to the flow direction. The distance L from the fuel surface to the axis of the rod as well as the main stream velocity U∞ is varied to change the effects of the separated flow on the fuel surface. It is found that under appropriate conditions, a flame moves forward from the fuel surface and is stabilized behind the rod. A map is obtained which describes the state of flame stabilization in the U∞ plane. Distributions of temperature and main species concentration are measured to elucidate the heat and mass transfer processes in the flame.

The forced mixing layer between parallel streams

The effect of periodic two-dimensional excitation on the development of a turbulent mixing region was studied experimentally. Controlled oscillations of variable ampli- tude and frequency were applied at the initiation of mixing between two parallel air streams. The frequency of forcing was at least an order of magnitude lower than the initial instability frequency of the flow in order to test its effect far downstream. The effect of the velocity difference between the streams was also investigated in this experiment. A typical Reynolds number based on the velocity difference and the momentum thickness of the shear layer was l04.It was determined that the spreading rate of the mixing layer is sensitive to periodic surging even if the latter is so small that it does not contribute to the initial energy of the fluctuations. Oscillations at very small amplitudes tend to increase the spreading rate of the flow by enhancing the amalgamation of neighbouring eddies, but at higher amplitudes the flow resonates with the imposed oscillation. The resonance region can extend over a significant fraction of the test section depending on the Strouhal number and a dimensionless velocity-difference parameter. The flow in the resonance region consists of a single array of large, quasi-two-dimensional vortex lumps, which do not interact with one another. The exponential shape of the mean-velocity distribution is not affected in this region, but the spreading rate of the flow with increasing distance downstream is inhibited. The Reynolds stress in this region changes sign, indicating that energy is extracted from the turbulence to the mean motion; the intensity of the spanwise fluctuations is also reduced, suggesting that the flow tends to become more two-dimensional.Amalgamation of large coherent eddies is resumed beyond the resonance region, but the flow is not universally similar. There are many indications suggesting that the large eddies in the turbulent mixing layer at fairly large Re are governed by an inviscid instability.

Instability and breakup of capillary liquid jets in a parallel airstream

The problem of the development and interaction of nonlinear two-dimensional perturbations in a rotating capillary jet is solved. The main attention is devoted to the study of the nonuniform breakup of the jet with allowance for the influence of the parallel airstream and the rotation. The solution is found by Galerkin's method [1–3]. The nonlinear development and interaction of a large number of perturbations is considered. A significant influence of long-wavelength modulation on the nature of drop formation is established. It is shown that an increase in the velocity of the parallel stream leads to a decrease in the relative size of the satellite (for the characteristic wavelengths). It is also shown that the rotation extends the region of unstable wave numbers in the complete range of flow velocities and air densities.

Film combustion of ethyl alcohol in a parallel air stream

The structure of the diffusion flame formed on a liquid fuel surface with a parallel oxidizer flow was studied both analytically and experimentally. Only the case of ethyl alcohol burning in air has been considered in this paper. The temperature and species concentration profiles across the boundary layer were determined by means of fine, carefully calibrated thermocouples and of gas chromatography. The gas analysis yielded the concentrations for the following species: N 2 , C 2 H 5 OH, O 2 , CO 2 , H 2 O, CO, CH 4 , C 2 H 2 +C 2 H 4 , C 2 H 6 , CH 3 CHO, H 2 . The mathematical model is based on the Shvab-Zeldovic formulation of conservation equations with physical properties considered as functions of temperature and species concentration. The agreement obtained between the experimental and theoretical profiles of temperature and of normalized elemental mass fractions of the atomic species N, O, C, H is satisfactory when the fuel diffusivity in nitrogen replaces the actual species and thermal energy diffusivities in the conservation equations.

Measurement of turbulence quantities in a two-stream mixing layer

Measurements have been made of the time-mean velocity, all three fluctuating velocity components and the turbulence shear stress in the plane mixing layer produced in an open wind tunnel between two nominally parallel air streams with fairly low turbulence levels. The hot-wire measuring system is described, the evaluation equations derived and the principal results presented.

Heat transfer through the unsteady laminar boundary layer on a semi-infinite flat plate Part II: Experimental results from an oscillating plate

An externally heated ceramic flat plate, instrumented on both surfaces with short time response platinum film resistance thermometers, was used to compare the amplitude of the additional unsteady heat transfer rate, δq w during oscillation in its own plane in a steady parallel air stream, of velocity U∞, with the corresponding steady state heat transfer rate, q ws , without oscillation. The plate oscillation frequency, f, was varied between 1 and 60 Hz, the corresponding range of the Strouhal number, S = ωx U∞ ( ω = 2 πf, x is the distance from the leading edge) being 0.006–20, and that of the velocity ratio, ϵ = u pmax U∞ ( u pmax is the maximum plate velocity) being 0.005–6.5. Amplification of periodic electrical signals corresponding to temperature amplitudes in the range 0.003–0.04°F showed that the qualitative trends of several existing theories for the behavior and phase shift (with respect to u pmax ) of H ϵ (where H = δ w q ws ) for both small and large S, were substantiated by the experiment. H ϵ was essentially constant and in phase for S < 1.5, whereas, for S > 1.5, H ϵ ∝ 1 S with the phase lag progressively approaching 90°. In addition, for a limited number of tests, no change in steady state heat transfer rate was detectable up to a value of S = 2.

Experiments on the combustion of hydrogen and methane injected transversely into a supersonic air stream

Gaseous hydrogen and methane were injected transversely into a parallel supersonic air stream heated by a plasma burner, and the onset and spread of combustion and its influence on the flow field were investigated by the schlieren method and direct photography. The development of a bulk flame proceeds over a series of isolated reaction zones attached to designated compression zones in the flow field. Upstream of the bulk flame, the flow field itself is not changed by these precursor reaction zones. For a Mach 2 air stream and hydrogen injection, the lowest static air temperature at which stable combustion was observed is about 700°C, whereas the corresponding value for methane injection is about 1300°C.

Study of initial segment of turbulent jets of various gases in a parallel air stream

Results are presented of a study of the gasdynamic parameters and the geometric characteristics of the mixing zone of axisymmetric jets of gases of differing density (Freon-12, air, and helium) propagating in a parallel air stream, within the limits of the initial segment (0≤x/R≤3–30). Experimental data are presented on the effect of different densities (0. 27 ≤n≤8.2) and velocities (0≤m≤≤1.7) of the gas jet and the parallel stream on the mixing process.

The structure of a self-preserving turbulent plane jet

The structure of a self-preserving turbulent plane jet exhausting into a slow-moving parallel airstream is studied. The investigation includes results of turbulence measurements and the structure is compared with that of a self-preserving plane wake. The results show that self-preservation is established at a distance of about thirty jet widths downstream of the jet nozzle and that, in the self-preserving region of the jet, the distributions of the turbulent intensities and shear stress across the jet are very similar to those found in the plane wake. The distribution of the intermittency factor, however, is found to be more like that found in an axi-symmetric jet than in a plane wake. The turbulent energy balance also shows important differences to that of the wake flow. The unsteady irrotational flow outside the turbulent shear layer is investigated and it is found that the experimental results agree with the predictions of the theories of Phillips (1955) and Stewart (1956). Some comments are also made on the eddy structure and the applicability of the simple theories of turbulence.

Sublimation from sharp‐edged cylinders in axisymmetric flow, including influence of surface curvature

AbstractAn experimental investigation was performed on the mass transfer by sublimation from the outer surfaces of hollow naphthalene cylinders, 0.75 and 1.00 in. in diam., in parallel air streams at velocities between 20 and 120 ft./sec. Local mass transfer rates on the cylinders were obtained by a profilometric technique consisting of accurate determinations of changes in radii of the subliming surfaces at points along elements of the cylinders.Local coefficients of mass transfer obtained with laminar boundary layers for Reynolds numbers (based on axial length) between 12,000 and 100,000 were found to be up to 8% greater, because of surface curvature, than corresponding values for flat surfaces. Moreover comparison of the mass‐transfer data with a theoretical prediction for laminar skin friction on circular cylinders indicates an effect of surface curvature on the Chilton‐Colburn anology between momentum and mass transfer amounting to as much as 6% in the range of air velocity employed. For turbulent boundary layers obtained by artificial triggering of turbulence at the leading edges of the cylinders no effect of surface curvature was found. The results obtained for Reynolds numbers of 40,000 to 1,000,000 are lower than previously published correlations of turbulent heat, mass, and momentum transfer, when compared by the Boelter, Martinelli, and Jonassen form of the analogy.

Effect of Vibration on Heat Transfer From a Wire to Air in Parallel Flow

Abstract Studies on the effect of vibration of the heat-transferring surface on convective heat transfer are few and limited to natural-convection conditions. The present investigation deals with the influence of vibration on heat transfer from an electrically heated nichrome wire to parallel air streams. Air velocities ranged from 34 to 63 fps. At each air velocity, frequencies ranging from 75 to 120 cycles per second and different amplitudes were employed as vibrational variables. Both frequency and amplitude increased the heat-transfer coefficient. An increase as high as 130 per cent was obtained. The correlation of the experimental data shows that the proportional increase in heat transfer was controlled by the ratio of the mean vibrational velocity to the air-stream velocity.