Showing papers on "Total pressure published in 2021"

Experimental investigation of ethylene/air combustion instability in a model scramjet combustor using image-based methods

Abstract: The combustion instabilities of supersonic combustion were investigated experimentally in a laboratory-scale scramjet combustor with a cavity flame holder. Ethylene was injected transversely from an orifice to the supersonic flow of Mach 2 with a stagnation temperature of 1900 K and a total pressure of 0.37 MPa. The dynamic pressure, CH* chemiluminescence and shadowgraph images were measured with a pressure sensor and a high-speed video camera. Dynamic pressure was analyzed by fast Fourier transform, and time-resolved CH* chemiluminescence images were modally decomposed by the sparsity-promoting dynamic mode decomposition (SP-DMD). The results indicated that two combustion instabilities were observed for cavity shear-layer stabilized combustion and the oscillation between jet-wake stabilized and cavity shear-layer ram combustions for the power spectral density (PSD) of pressure. In the case of the combustion instability of cavity shear-layer stabilized combustion, a dominant peak of approximately 128 Hz was observed for the PSD of pressure. This instability corresponded to an entire flame oscillation of the cavity shear-layer stabilized combustion, which was validated by the SP-DMD and a low rank reproduction with 10 modes. This was driven by a fuel injection oscillation in the injection orifice. In the case of oscillation between the jet-wake stabilized and the cavity shear-layer ram combustions, peaks around 1600 Hz were observed for the PSD of pressure. This mechanism was also explained by the SP-DMD modes and a low rank reproduction using within 10 modes. The DMD and shadowgraph images indicated that the vortex formed by a separation of the boundary layer induced a strong jet-wake flame, resulting in the temporal thermal choke followed by cavity shear-layer stabilized ram combustion. The data-driven approach with SP-DMD clarified the combustion instability mechanisms of the supersonic combustion in detail.

Single-phase flow distribution in plate heat exchangers: Experiments and models.

Abstract: Maldistribution of fluid among parallel channels is one of the main issues in applications of plate heat exchangers This paper presents an experimental and numerical investigation of the single-phase flow distribution in the brazed plate heat exchangers, but the results can be applied to plate-and-frame and plate-and-shell designs In the experiments, the pressure profile in the heat exchanger is measured by the probes inserted into the headers The flow distribution is determined by the measured pressure drop across the channels and the developed in-channel friction factor correlation The experimental results indicate that in a U-type brazed plate heat exchanger, the channel flow rate first increases for the first several channels near the heat exchanger entrance due to the sudden expansion of flow in the inlet header For the rest channels, the flow rate decreases with the distance away from the entrance/exit of the heat exchanger Such a distribution profile is associated with the axial momentum transfer in the inlet header The influence of the total flow rate on the distribution profile is trivial, but the maldistribution is more severe with an increased number of plates Two distribution models presented are developed based on the principle of equal total pressure drop for all flow paths Two models calculate the pressure profile in the headers by 3-D CFD modeling and 1-D mass and momentum conservation equation The experimental results validate the models

Effect of visco-plastic and shear-thickening/thinning characteristics on non-Newtonian flow through a pipe bend

Abstract: This study investigates the influence of the rheological parameters on the unyielded zone, pressure drop, and secondary flow pattern of non-Newtonian fluid like fresh cement mortar through a pipe bend with different curvatures. The regularized Herschel–Bulkley model is employed in the framework of lattice Boltzmann method to model the effects of the visco-plastic and shear-thickening/thinning characteristics with variations on the yielding stress σ0 and the power-law index n. The sharper curvature, higher power-law index contributes to a smaller and more asymmetric unyielded zone and even vanishes for curvature radius R c = 1 D and n = 1.4, as the width and distribution of plug region are governed by the comparison between yielding stress and shear stress. The increased yielding stress and power-law index lead to an increase in the total pressure drop and additional pressure loss; however, their variations with respect to the curvature radius display an opposite trend. The intensity of the helical secondary flow in the elbow, primarily governed by the competition between the centrifugal and viscous force, is reduced to about one quarter when σ0 and n increase from 0 Pa and 0.6 to 50 Pa and 1.4.

Investigation of combustion modes and pressure of reflective shuttling detonation combustor

Abstract: Detonation combustors are considered promising alternatives to conventional combustors because they offer high thermal efficiency and fast combustion. However, especially for the rotating detonation combustor, the theoretical propulsive performance has not been confirmed in experimental studies because the highly unsteady flow field hinders the measurements process. To understand the involved phenomena in more detail, a reflective shuttling detonation combustor (RSDC) with a rectangular combustion chamber was developed. The interior of the chamber can easily be visualized owing to its two-dimensional quality. Utilizing the RSDC, several combustion tests with gaseous ethylene and oxygen were conducted for different values of mass flow rates and equivalence ratios. Combustion modes from the tests were classified into four types based on the fast Fourier transform (FFT) analysis of the luminous intensity of the CH* self-luminescence images captured by a high-speed camera and a band pass filter. Simultaneously, the theoretical total pressure of a conventional isobaric combustor was compared to the static pressure measured at the bottom of the RSDC chamber. For the detonation modes, the ratio between experimentally measured static pressure and the theoretical pressure varied depending on the location in the chamber owing to the distribution of the time-averaged static pressure. Furthermore, the pressure ratio of the detonation modes was up to 18% lower than that of the deflagration mode potentially owing to the flow velocity induced by the detonation waves.

The Biot-Stokes coupling using total pressure: formulation, analysis and application to interfacial flow in the eye.

Abstract: We consider a multiphysics model for the flow of Newtonian fluid coupled with Biot consolidation equations through an interface, and incorporating total pressure as an unknown in the poroelastic region. A new mixed-primal finite element scheme is proposed solving for the pairs fluid velocity - pressure and displacement - total poroelastic pressure using Stokes-stable elements, and where the formulation does not require Lagrange multipliers to set up the usual transmission conditions on the interface. The stability and well-posedness of the continuous and semi-discrete problems are analysed in detail. Our numerical study {is framed in} the context of different interfacial flow regimes in Cartesian and axisymmetric coordinates that could eventually help describe early morphologic changes associated with glaucoma development in canine species.

Flow boiling pressure drop characteristics in a multi-microchannel heat sink

Abstract: We make an effort in this study to experimentally investigate the flow boiling pressure drop characteristics in a multi-microchannel heat sink. For the multi-microchannel heat sink, 27 parallel microfluidic channels are considered. Microchannels are fabricated on the copper block and have a hydraulic diameter of 421 µm and length of 40 mm. We perform experiments considering the refrigerant, considered the working fluid in this study. A high mass flux (G ∼ 400 kg/m2 s–1200 kg/m2 s) for a range of wall heat flux (q′′ ∼ 10 kW/m2–170 kW/m2) and varying degrees of saturation temperature (Tsat ∼ 13 °C, 18 °C, and 23 °C) are considered for the experiments. We demonstrate the correlative effect of the total pressure drop on the two-phase flow pattern in the channel. By depicting the interplay between the total pressure drop (∆p) and the frictional pressure gradient, we show that the frictional pressure gradient increases with the increasing mass flux while it decreases with the increasing degree of saturation temperature. We show the significant impact of the heat flux and saturation temperature on the frictional pressure gradient. In addition, we develop a new correlation for the pressure drop characteristics taking the interference effect of the frictional pressure drop in the multi-microchannel heat sink. The inferences of this experimental study will have far-ranging consequences for the design of heat exchangers, leading to the optimization of microscale thermal management equipment.

Well-posedness and discrete analysis for advection-diffusion-reaction in poroelastic media

Abstract: We analyse a PDE system modelling poromechanical processes (formulated in mixed form using the solid deformation, fluid pressure, and total pressure) interacting with diffusing and reacting solutes...

The Implication of Injection Locations in an Axisymmetric Cavity-Based Scramjet Combustor

Abstract: This paper presents the effect of cavity-based injection in an axisymmetric supersonic combustor using numerical investigation. An axisymmetric cavity-based angled and transverse injections in a circular scramjet combustor are studied. A three-dimensional Reynolds-averaged Navier–Stokes (RANS) equation along with the k-ω shear-stress transport (SST) turbulence model and species transport equations are considered for the reacting flow studies. The numerical results of the non-reacting flow studies are validated with the available experimental data and are in good agreement with it. The performance of the injection system is analyzed based on the parameters like wall pressures, combustion efficiency, and total pressure loss of the scramjet combustor. The transverse injection upstream of the cavity and at the bottom wall of the cavity in a supersonic flow field creates a strong shock train in the cavity region that enhances complete combustion of hydrogen-air in the cavity region compared to the cavity fore wall injection schemes. Eventually, the shock train in the flow field enhances the total pressure loss across the combustor. However, a marginal variation in the total pressure loss is observed between the injection schemes.

Effect of ambient pressure on the extinction limit for opposed flame spread over an electrical wire in microgravity

Abstract: Parabolic flight experiments were carried out to investigate the effect of ambient pressure on the extinction limit for opposed flame spread over an electric wire insulation in microgravity. Low-density polyethylene insulated Nickel-Chrome wires with inner core diameter of 0.50 mm and insulation thickness of 0.30 mm were examined for ambient pressures ranging from 50 kPa to 140 kPa for an external opposed flow of 10 cm/s. The experiments showed that the limiting volumetric oxygen concentration (LOC) increased as the total ambient pressure decreased. This LOC trend can be explained by radiation loss from wire surface. The radiation loss increased as the ambient pressure decreased – a result that can be explained by the increase in preheat length with decreasing ambient pressure. Moreover, when the data was plotted in a partial pressure vs. total pressure space, it became evident that the limiting oxygen partial pressure (LOPP) decreased with decreasing total ambient pressure. This LOPP trend can be explained by the fact that the flame temperature increased as the ambient pressure decreased under constant oxygen partial pressure. In current fire safety design for spacecraft, tentative oxygen concentration criteria in spacecraft suggested by NASA is assumed as 30% of oxygen concentration, and this value is assumed independent of ambient pressure. However, the present result implies that consideration of the effect of ambient pressure on the flammability limit is necessary, especially with respect to the possibility of an extension of the allowable atmosphere condition for spacecraft cabin in the low pressure region.

Orthorhombic Ta3-xN5-yOy thin films grown by unbalanced magnetron sputtering: The role of oxygen on structure, composition, and optical properties

Abstract: Direct growth of orthorhombic Ta3N5-type Ta-O-N compound thin films, specifically Ta3-xN5-yOy, on Si and sapphire substrates with various atomic fractions is realized by unbalanced magnetron sputtering. Low-degree fiber-textural Ta3-xN5-yOy films were grown through reactive sputtering of Ta in a gas mixture of N2, Ar, and O2 with keeping a partial pressure ratio of 3:2:0.1 in a total working pressure range of 5–30 mTorr. With increasing total pressure from 5 to 30 mTorr, the atomic fraction of O in the as-grown Ta3-xN5-yOy films was found to increase from 0.02 to 0.15 while that of N and Ta decrease from 0.66 to 0.54 and 0.33 to 0.31, respectively, leading to a decrease in b lattice constant up to around 1.3%. Metallic TaNx phases were formed without oxygen. For a working pressure of 40 mTorr, an amorphous, O-rich Ta-N-O compound film with a high O fraction of ~0.48, was formed, mixed with non-stoichiometric TaON and Ta2O5. By analyzing the plasma discharge, the increasing O incorporation is associated with oxide formation on top of the Ta target due to a higher reactivity of Ta with O than with N. The increase of O incorporation in the films also leads to a optical bandgap widening from ~2.22 to ~2.96 eV, which is in agreement with the compositional and structural changes from a crystalline Ta3-xN5-yOy to an amorphous O-rich Ta-O-N compound.

Aerodynamic performance of profiled endwalls with upstream slot purge flow in a linear turbine cascade having pressure side separation

Abstract: In aeroengines, purge flow directly fed from the compressor (which bypasses the combustor) is introduced through the disk space between blade rows to prevent the hot ingress. Higher quantity of purge gas fed through the wheel space can provide additional thermal protection to the passage endwall and blade surfaces. However, the interaction of purge flow with the mainstream flow leads to higher secondary losses. Secondary losses inside a turbine blade passage can be reduced effectively by endwall contouring. This paper presents computational investigation on the influence of non-axisymmetric endwall contouring over endwall secondary flow modification in the presence of purge flow with the pressure side bubble (PSB). The experimental analysis was conducted for the base case without purge and base case with purge (BCP) configurations having flat endwalls. The total pressure loss coefficient and exit yaw angle deviation were measured with the help of a five-hole pressure probe. Static pressure distribution over the blade midspan was obtained by 16 channel Scanivalve. Aerodynamic performances of three different profiled endwalls are numerically analyzed and are compared against the BCP configuration. The effects of different contoured endwall geometries on endwall static pressure distribution and secondary kinetic energy were also discussed. Analysis shows that in the first contoured endwall configuration (EC1), the formation of stagnation zones at a contour valley close to the suction surface causes the exit total pressure loss coefficient to increase. The shifting of the contour valley near to the pressure surface (EC2 configuration) has resulted in local acceleration of the diverted pressure side leg of the horseshoe vortex over the hump toward the end of the passage. In the third configuration (EC3 configuration), reduced valley depth and optimum hump height have effectively redistributed the endwall pitchwise pressure gradient. The increased static pressure coefficient at the endwall near to the pressure surface has eliminated the PSB formation. In addition, computational results of unsteady Reynolds averaged Navier–Stokes simulations are obtained for analyzing transient behavior of PSB, with more emphasis on its migration on the pressure surface and transport across the blade passage. The additional work done by the mainstream fluid to transport the low momentum PSB fluid has caused higher aerodynamic penalty at the blade exit region. In this viewpoint, the implementation of contoured endwalls has shown beneficial effects by eliminating the PSB and related secondary vortices. At 27% of axial chord downstream of the blade trailing edge, a 4.1% reduction in the total pressure loss coefficient was achieved with endwall contouring.