Large Pressure Ratio Research Articles

Nucleation, growth, and morphology of dust in plasmas

Abstract

Supercritical heat pump cycles

Supercritical heat-pump cycles suited for high-temperature heat generation and in which heat is delivered in the form of sensible heat of a high-pressure fluid are examined and their energy performance is evaluated. The main variables governing the energy efficiency of the process and the temperatures of the heat produced are recognized to be the fluid critical temperature, the molecular complexity, the top cycle pressure and the amount of internal regeneration of heat. Two cycle configurations are examined: one featuring fluid compression after a regenerative preheating and one that also includes turbine expansion of a fraction of the high-pressure fluid in order to achieve a more effective regeneration. General diagrams giving the operating characteristics of a supercritical heat-pump cycle for any kind of fluid are reported. Some fluids are presented (SF 6, C 3F 8, C 2HF 5, c-C 4F 8), which exhibit a high level of thermal stability and are thermodynamically suitable for supercritical cycles: for each one a detailed performance chart is given. An example application in which a conventional high-temperature cycle is compared with two supercritical solutions is presented. The following conclusions summarize the findings of the thermodynamic analysis. (1) In supercritical cycles high heat-output temperatures are achievable with moderate compressor pressure ratios and with a comparatively simple cycle arrangement, while conventional cycles require a large pressure ratio and a complex cycle organization. Sub-atmospheric pressures, which may be required in conventional cycles, can be avoided. (2) As heat is available in supercritical cycles within a certain temperature range, applications implying the use of heat at variable temperature could benefit from the natural matching between temperature availability and process requirements. (3) The comparatively high pressures at which heat is produced in supercritical cycles could represent a drawback for small-capacity plants but are probably acceptable or even beneficial for large systems. (4) The internal regeneration of a sizeable amount of heat, which is requested in supercritical cycles, represents a definite cost item for this type of heat pump.

Pressure waves in a separated gas-liquid layer in a horizontal duct with a step

Pressure wave propagation into a separated gas-liquid layer in a horizontal duct with a step is investigated analytically. The linear solution is derived assuming a large density ratio of liquid to gas. The solution can be found first for the gas layer and then for the liquid layer. The linear wave in a liquid layer is valid even for fairly large initial pressure ratios, and clearly exhibits the dispersive characteristics of the pressure wave in a liquid layer. As the initial pressure ratio is increased, the pressure wave in the gas layer becomes a shock wave. Thus, its effect on the wave in a liquid layer can be found analytically by modifying the boundary condition in part. The wave in a liquid layer consists of a main wave, which propagates with the shock speed in gas, and a precursor wave, whose front propagates with the speed of sound in liquid. The precursor wave has an oscillatory structure; its amplitude increases with increasing shock strength and also with liquid layer thickness.

Acoustic controlled rotation and orientation

Acoustic energy is applied to a pair of locations spaced about a chamber, to control rotation of an object levitated in the chamber. Two acoustic transducers applying energy of a single acoustic mode, one at each location, can (one or both) serve to levitate the object in three dimensions as well as control its rotation. Slow rotation is achieved by initially establishing a large phase difference and/or pressure ratio of the acoustic waves, which is sufficient to turn the object by more than 45°, which is immediately followed by reducing the phase difference and/or pressure ratio to maintain slow rotation. A small phase difference and/or pressure ratio enables control of the angular orientation of the object without rotating it. The sphericity of an object can be measured by its response to the acoustic energy.

Two-dimensional focusing of a supersonic free jet by a rectangular orifice

Supersonic free jets issuing from rectangular orifices have been observed by using a laser-induced fluorescence technique. Anisotropy of expansion in two directions, the orifice length (z) and width (y), apparently occurs in the jet structure at a large pressure ratio (between reservoir and vacuum chambers); the jet spreads in the y direction whereas it converges in the z direction. This effect is enhanced by interaction of lateral shocks from both ends of the orifice when a small aspect ratio orifice is used. Under a flow condition whereby the shocks reflect normally on the axis, the jet becomes very thin in the z direction.

Gas separation in one- or two-step membrane processes

The purpose of these investigations was to find an answer to the question whether gas separation plants with membranes based on a new process engineering concept can be compared with conventional separation methods in terms of economic efficiency. In order to facilitate this task simplified equations were developed or adopted for five different arrangements, assuming an infinitely large pressure ratio between the high and the low pressure side of the membrane. These analytical equations enabled the major data of a plant, such as membrane area required or compressor stream, to be determined very quickly. In the present study the comparison of the separation process was based on oxygen enrichment from the air. For the concentration range investigated, namely, oxygen content of the product gas between 21% and 35%, the system of membrane rectification with dephlegmation was clearly shown to be the most economical solution.

Pressure swing adsorption: Development of an equilibrium theory for gas separations

Two versions of the pressure swing adsorption process are analysed as a means for purification of the light component of a binary feed of arbitrary composition. Local equilibrium with linear, uncoupled isotherms is assumed. The results of the analysis show that a critical pressure ratio must be exceeded before complete purification is possible and that this pressure ratio increases as the light-component content of the feed decreases. When clean up of the light component is complete, pressurization with product leads to higher light-component recoveries than pressurization with feed. This difference becomes large for small separation factors, small fractions of the light component in the feed, and large pressure ratios.

Optimum design of hydrostatic journal bearings Part I: Maximum load capacity

The optimization problem is formulated with a view to maximizing the load-carrying capacity of hydrostatic journal bearings. Equations governing the performance of multi-recess hydrostatic journal bearings are summarized. Practical design limits and operational constraints are also defined. The optimization process is based on the well known Rosenbrock method. Results illustrating the effect of area ratio, axial land width and circumferential land width on load capacity, flow rate and power ration are reported. In conclusion precision bearings with small clearances and low pressure ratios are recommended for applications involving low supply pressures, while bearings with large clearances and pressure ratios close to 0.5 are recommended for applications involving high supply pressures

Gas-Driven Fracture Propagation

A one-dimensional gas-flow drives a wedge-shaped fracture into a linearly elastic, impermeable half space which is in uniform compression, σ∞, at infinity. Under a constant driving pressure, p0, the fracture/flow system accelerates through a sequence of three self-similar asymptotic regimes (laminar, turbulent, inviscid) in which the fracture grows like an elementary function of time (exponential, near-unity power, and linear, respectively). In each regime, the transport equations are reducible under a separation-of-variables transformation. The integro-differential equations which describe the viscous flows are solved by iterative shooting methods, using expansion techniques to accomodate a zero-pressure singularity at the leading edge of the flow. These numerical results are complemented by an asymptotic analysis for large pressure ratio (N = p0/σ∞ → ∞) which exploits the disparity between the fracture length and penetration length of the flow. Since the seepage losses to a surrounding porous medium are shown to be negligable in the late-time long-fracture limit, the results have application to geologic problems such as: containment evaluation of underground nuclear tests, stimulation of oil and gas wells, and permeability enhancement prior to in situ combustion processes.

The effects of trailing vorticity on the flow through highly loaded cascades

This paper presents a procedure whereby three-dimensional inviscid flow through a highly loaded turbomachinery cascade of lifting lines can be treated by methods corresponding to classical aerodynamic theory. In contrast to earlier linearized (thin airfoil) three-dimensional theory, the present study allows analysis of the flow corresponding to the large turning and/or large pressure ratios induced by practical rotors or stators. For the sake of simplicity, the present paper is limited to incompressible flow through a highly loaded rectilinear cascade and to the design problem, i.e. given blade loading. Formulae are derived for both the mean and the three-dimensional components of the flow; in particular, the velocities at the blades induced by the trailing vorticity associated with nonuniform blade circulation are determined.

Theory of Exhaust-Plume/Boundary-Layer Interactions at Supersonic Speeds

At supersonic speeds, an under-expande d rocket exhaust jet produces a deflection of the external flow and generates a rise in pressure that communicates upstream through the surface boundary layer. For sufficiently large pressure ratios, the viscous layer separates some distance forward of the base and may seriously affect vehicle stability. The present investigation develops an integral method for predicting some of the main features of the nearfield interaction between the exhaust plume and the surface boundary layer. The analysis is not restricted to laminar flow and extends to turbulent interactions by utilizing a simplified eddy-diffusivity model. The method also applies to the interaction that occurs in the wake of inclined or asymmetric bodies, and results for the flow over a flat plate at angle of attack are given.

Note on an Aerodynamic Seal

Figure 1 is a sketch of an “aerodynamic seal”. This type seal has potential application where it is necessary to control leakage of fluid through a clearance space bounded by one moving wall. For mechanical reasons it is necessary to maintain a clearance, in this case equal to the throat width t, between the inner wall I and the outer wall O. To prevent flow from the high pressure chamber H from passing into the low pressure chamber L, pump P supercharges fluid from H and forces it through nozzle J. The resulting fluid jet, which may be aided in following the curved surface by the Coanda effect, passes through the throat t and into diffuser D which recovers part of the entering velocity head. In the ideal case, with no losses and one-dimensional flow, the per cent recovery of inlet velocity head may be made as near to 100% as desired, since the diffuser recovery coefficient becomes asymptotic to 1-0 with increasing area ratio. Conversely, the dumping loss may be made as near to zero as desired. The attractiveness of this proposal hinges on diffuser performance. The pump is necessary to make up for the velocity head lost at the diffuser exit and for flow losses in the system. The pressure at the diffuser throat should be approximately that of the low pressure chamber for optimum performance. For large pressure ratios it may be necessary to stage two or more seals in series.

Cryogenic turboexpanders for large pressure ratios

Cryogenic turboexpanders for large pressure ratios

Small Perturbation Theory for Shock-Tube Attenuation and Nonuniformity

Closed form expressions are derived, based on the concept of self similarity, for evaluating small perturbations of shock tube flow due to the wall boundary layer. Ideal diaphragm rupture is assumed. Extensive numerical results are obtained for the perturbations induced by the boundary layer in the driven gas, assuming that the boundary layer is wholly laminar or wholly turbulent. The driven gas is taken to be air. Various driver gases are considered. Attenuation results are obtained for shock Mach numbers up to 20. It is shown that the effect of the driver gas boundary layer on perturbations in the driven gas is small for large diaphragm pressure ratios. The results indicate that a shock tube with an efficient driver, e.g., helium, should have less attenuation for a given shock Mach number than would an inefficient driver, e.g., air. The latter result disagrees with existing experimental data. The source of the discrepancy is not known.

Influence of diaphragm opening time on shock-tube flows

Shock waves generated in a shock tube by use of hydrogen or helium as a driver gas and air, nitrogen, oxygen or argon as a driven gas have higher velocities than predicted by simple theory when sufficiently large diaphragm pressure ratios are used. Expected shock-tube performance curves have been constructed using the equilibrium Hugoniot for the driven gas, both for the usual model of shock tube flow, which assumes instantaneous diaphragm removal, and for a suggested model based on a finite rupture time for the diaphragm. Agreement between experiment and the latter model is in general good, and the differences are qualitatively accounted for by the pressure waves expected to result from mixing between driver and driven gases at the contact surface. These waves may be either compression or expansion waves, depending on the relative heat capacities of the two gases. The maximum shock strength observed as a shock goes down the tube was found to occur at a distance from the diaphragm which increases with the shock strength, and the strongest shocks were found to be still accelerating at the end of a 42 ft. long shock tube of 3 1/2 in. square cross-section. Diaphragm breaking time has been measured and found to be consistent with the observations on the shock formation distance.