Flow and Separation Performance of Supersonic Gas Separator

This paper presents a CFD-based numerical analysis on the potential benefits of non-radial blading turbine for low speed-low pressure applications. Electric turbocompounding is a waste heat recovery technology consisting of a turbine coupled to a generator that transforms the energy left over in the engine exhaust gases, which is typically found at low pressure, into electricity. Turbines designed to operate at low specific speed are ideal for these applications since the peak efficiency occurs at lower pressure ratios than conventional high speed turbines. The baseline design consisted of a vaneless radial fibre turbine, operating at 1.2 pressure ratio and 28,000rpm. Experimental low temperature tests were carried out with the baseline radial blading turbine at nominal, lower and higher pressure ratio operating conditions to validate numerical simulations. The baseline turbine incidence angle effect was studied and positive inlet blade angle impact was assessed in the current paper. Four different turbine rotor designs of 20, 30, 40 and 50° of positive inlet blade angle are presented, with the aim to reduce the losses associated to positive incidence, specially at midspan. The volute domain was included in all CFD calculations to take into account the volute-rotor interactions. The results obtained from numerical simulations of the modified designs were compared with those from the baseline turbine rotor at design and off-design conditions. Total-to-static efficiency improved in all the non-radial blading designs at all operating points considered, by maximum of 1.5% at design conditions and 5% at off-design conditions, particularly at low pressure ratio. As non-radial fibre blading may be susceptible to high centrifugal and thermal stresses, a structural analysis was performed to assess the feasibility of each design. Most of non-radial blading designs showed acceptable levels of stress and deformation.

The gas and shock wave’s motion in a receiving tube is investigated numerically and experimentally. The results show that, the velocity of the contact face rises rapidly as gas is injected into the receiving tube, and drops sharply after a steady propagation. However, the velocity of the shock wave in the tube is almost linear and the shock wave can be reflected at the close end of the receiving tube. The shock wave and the injecting wave propagate in the different direction. When the reflected wave sweeps, it can heat the gas about 20K under the pressure ratio of 2. If the exhausted gas is used to refrigeration, the reflected wave has negative effect. With increasing of inlet pressure, the velocity of the shock wave and steady velocity of contact face also increase. There is obvious thermal effect as the shock wave sweeps the gas. The reflected shock wave can heat the exhausting gas in the open end. The exhausting gas’ temperature for low pressure ratio is higher than the temperature for high pressure ratio. But as the pressure ratio rises to a value, the temperature drop little. The result shows that the numerical value is a little higher than the experimental value, especially under lower pressure ratio. The reason may lie in the operation condition and the heat exchange. Under lower pressure ratio, the shock wave can heat the gas in the closed end and the heat can easily be transferred to the open end. As a whole, the experimental results are consistent with the numerical simulation. Open image in new window Shock wave’s reflection

The discharge characteristics of orifices at low pressure ratios were investigated. Probe measurements of high-velocity jets confirm and extend the work of Stanton. Expansion factors (Y) at pressure ratios down to r = 0.13 were measured for the flow of air in a 2-in. pipe meter and for the flow of steam in a 3-in. pipe meter. Both meters were installed in accordance with ASME specifications, and operated at a diameter ratio of 0.15. The effects of high approach velocity on the discharge characteristics of square-edged orifices at low pressure ratios were investigated; expansion factors were determined for diameter ratios from 0.2 to 0.8 over a range of pressure ratios from unity to 0.2. A linear Y-r correlation is shown to exist at low pressure ratios; a change of flow regime occurs at a pressure ratio of 0.63. A theoretical solution for supercritical flow, previously established by the author, is further compared with experimental results.

Numerical simulation of supersonic separator with axial or tangential outlet in reflow channel

An internally staged hollow fiber polysulfone membrane is used to generate a permeate stream enriched in CH4H10 from a feed mixture of CH4, C2H6, C4H10, CO2 and N2. Experimental measurements are made as a function of the feed pressure and the reject flow rate. The area ratio of the first and second membrane stages are kept constant as well as the feed composition and the pressure ratio of the first permeate and the feed streams. The plug flow model is used to simulate the separation performance of the multicomponent mixture in the internally staged membrane system. The analysis is focused on determining the local optimum mole fraction for the C4H10 in the second permeate stream over a wide range of operating conditions. The local optimum is determined by varying the feed pressure, first membrane area, pressure and area ratios, and the stage cut. Results show that the local optimum occurs at small membrane area and area ratio, low stage cut and pressure ratio, and high feed pressures. Good agreement is found between measurements and model predictions for the C4H10 mole fraction in the second permeate stream and the stage cut.

Multistage axial compressor has an advantage of lower stage loading as compared to a single stage. Several stages with low pressure ratio are linked together which allows for multiplication of pressure to generate high pressure ratio in an axial compressor. Since each stage has low pressure ratio they operate at a higher efficiency and the efficiency of multi-stage axial compressor as a whole is very high. Although, single stage centrifugal compressor has higher pressure ratio compared with an axial compressor but multistage centrifugal compressors are not as efficient because the flow has to be turned from radial at outlet to axial at inlet for each stage. The present study explores the advantages of extending the axial compressor efficient flow path that consist of rotor stator stages to the centrifugal compressor stage. In this invention, two rotating rows of blades are mounted on the same impeller disk, separated by a stator blade row attached to the casing. A certain amount of turning can be achieved through a single stage centrifugal compressor before flow starts separating, thus dividing it into multiple stages would be advantageous as it would allow for more flow turning. Also the individual stage now operate with low pressure ratio and high efficiency resulting into an overall increase in pressure ratio and efficiency. The baseline is derived from the NASA low speed centrifugal compressor design which is a 55 degree backward swept impeller. Flow characteristics of the novel multistage design are compared with a single stage centrifugal compressor. The flow path of the baseline and multi-stage compressor are created using 3DBGB tool and DAKOTA is used to optimize the performance of baseline as well novel design. The optimization techniques used are Genetic algorithm followed by Numerical Gradient method. The optimization resulted into improvements in incidence and geometry which significantly improved the performance over baseline compressor design. The multistage compressor is more efficient with a higher pressure ratio compared with the base line design for the same work input and initial conditions.

As part of future civil aircraft concepts the integration of jet engines is considered as an additional possibility to increase the efficiency of the whole aircraft. While the ingestion of the fuselage boundary layer reduces the drag of the aircraft it also causes the fan stage of the engine to operate with a distorted inflow. In this study, a fan with a low total pressure ratio, representing future fan designs, and the fan of the International Aero Engines (IAE) V2500 engine, representing a common fan design in today’s jet engines, are imprinted with a parameterized generic but also realistic inlet distortion. The numerical investigation of both fans that employed transient full-annulus simulations showed that regarding different criteria the low pressure ratio fan is more sensitive to the imprinted inlet distortion.

Studies of compressor pressure ratio effect on GAXAC (generator–absorber–exchange absorption compression) cooler

It has been known for some time that when a gas flows through an orifice the coefficient of discharge increases as the ratio of the downstream pressure to the upstream pressure is reduced (Stanton (1)).† This increase continues even after the critical (sonic) pressure ratio is passed, since, unless the discharge coefficient of the orifice is already unity, the mean velocity in the plane of the orifice is still subsonic and can be influenced by the downstream pressure. In a paper to this Institution, Jobson (2) suggested a method by which the variation of discharge coefficient with pressure ratio could be calculated. The method was based on the assumption that the velocity pattern at the walls, upstream of the orifice, was independent of flow rate: it yielded results that were in good agreement with experiments on sharp-edge orifices. The assumption breaks down, however, and leads to demonstrably false results, when the discharge coefficient is greater than about 0·65 and the pressure ratio well away from unity, since the upstream velocity pattern is then affected by compressibility. In the present paper, a simple assumption about the flow pattern at the walls enables allowance to be made for this additional compressibility effect. The resulting curves of discharge coefficient against pressure ratio are then correct for a perfect nozzle, with a discharge coefficient of unity, as well as for a Borda mouth-piece, and so might be expected to be reasonable approximations in between these extremes. Experimental results from a variety of sources have been compared with the theoretical predictions. The agreement is good at pressure ratios up to the critical but very few results at really low pressure ratios are available. It should perhaps be emphasized that neither Jobson's nor the present method of analysis enables the discharge coefficient of a particular orifice to be calculated ab initio. Their object is to predict how the discharge coefficient of a particular nozzle, known under one set of flow conditions, will vary under others. It is therefore to be hoped that the curves presented will prove particularly useful for design purposes, and for performance correlations.

Experimental investigation and numerical analysis of separation performance for supersonic separator with novel drainage structure and reflux channel

Results on noise characteristics and flow-visualization shock structures of 8 × 8 arrays of jets for stagnation pressure ratios in the range 2.4 – 22, and spacing-to-diameter ratios of 1.4 – 4, are presented. Three regimes of noise generation were observed for the arrays tested: regime of weak interactions at low pressure ratios, where the noise spectrum is essentially similar to that for individual jets in the array; regime of combined jet at high pressure ratios, where noise spectrum is similar to that for a single large jet; and, regime of strong interactions, which appears between these two regimes and where individual jets as well as the combined jet significantly contribute to noise generation. The domain of each of these regimes is determined by pressure ratio and spacing-todiameter ratio for the array. The influence of neighboring jets on shock structures of individual jets in the array appears to be insignificant in flow-visualization images at low pressure ratios; however, with increasing pressure ratio, the shock structures increase in size and increasingly influence their neighbors. Large shocks, which span the width of the array and similar to that for a single jet, were observed in the combined jet regime. The combined jet, which is formed by merger of individual jets from the array, dominates noise generation when Mach number in the mixed stream is greater than 0.75. The noise characteristics of the jet arrays correlate well with their corresponding form of shock structures in the three regions described.

Optimisation Potentials for the Heat Recovery in a Semi-closed Oxyfuel-combustion Combined Cycle with a Reheat Gas Turbine

We present experimental results on separation of supersonic flow inside a convergent–divergent (CD) nozzle. The study is motivated by the occurrence of mixing enhancement outside CD nozzles operated at low pressure ratio. A novel apparatus allows investigation of many nozzle geometries with large optical access and measurement of wall and centerline pressures. The nozzle area ratio ranged from 1.0 to 1.6 and the pressure ratio ranged from 1.2 to 1.8. At the low end of these ranges, the shock is nearly straight. As the area ratio and pressure ratio increase, the shock acquires two lambda feet. Towards the high end of the ranges, one lambda foot is consistently larger than the other and flow separation occurs asymmetrically. Downstream of the shock, flow accelerates to supersonic speed and then recompresses. The shock is unsteady, however, there is no evidence of resonant tones. The separation shear layer on the side of the large lambda foot exhibits intense instability that grows into large eddies near the nozzle exit. Time-resolved wall pressure measurements indicate that the shock oscillates in a piston-like manner and most of the energy of the oscillations is at low frequency.

Recently, regenerative refrigerators such as Gifford-McMahon (GM) refrigerators have achieved liquid helium temperature levels using magnetic regenerator materials that have a much larger specific heat capacity below 10K than conventional second regenerator materials of lead. In this investigation a high efficiency 4K GM refrigerator using magnetic regenerator materials was developed and investigated. A refrigeration capacity of 2.04W at 4.2K was obtained for 3.4 kW of compressor input power by optimization of the operating pressure ratio. A maximum coefficient of performance (COP) of 6.04 × 10 −4 was achieved at 4.2K at a pressure ratio of 2.43 and an operating frequency of 30 rpm. In the investigation the refrigerator was operated at various pressure ratios, and at each pressure ratio measurements were made of compressor work, of pressure-volume (P-V) characteristics of the second expansion volume, and of refrigeration capacity at 4.2K. Refrigeration losses were estimated at 4.2K using the experimental results and are discussed. Operating frequency was also optimized to maximize second stage refrigeration performance.

Scroll compressors are seldom used for high pressure ratio applications, despite their rapidly increasing inroads into relatively low pressure ratio air conditioning applications. Semi-hermetic reciprocating machines and, to a lesser extent in larger sizes, twin screw compressors dominate the high pressure ratio market. This is due, in part, to operating characteristics of scroll compressors at these conditions that seem to place scrolls at an inherent efficiency disadvantage. This paper presents an analysis of high pressure ratio scroll compressors and looks at the influences of built-in volume ratios and a dynamic discharge valve on compressor performance. The analysis includes throttling flow losses, over- and under-compression losses, and clearance volume re-expansion losses. Effects of the variable area of the discharge port opening throughout the compression cycle, valve losses (if present), and additional compressibility effects are included. The clearance volume effects, influencing only scroll compressors with dynamic discharge valves, are compared with those in reciprocating compressors. The importance of these various effects is evaluated over a range of saturated evaporator and condenser conditions. Comparison of scroll compressor efficiency with and without the discharge valve is presented over a wide range of pressure ratio conditions. The results of the study show that scroll compressors for high pressure ratio application benefit from a discharge valve. Typically, the scroll compressor with the discharge valve becomes more efficient for system pressure ratios exceeding four. Below this pressure ratio, the compressor without the discharge valve is more efficient because of the absence of discharge valve losses. However, at pressure ratios above four the use of a discharge valve becomes beneficial, because under-compression losses are eliminated. Also, the effect of the discharge port on the throttling losses is less pronounced in high pressure ratio than in low pressure ratio applications, because of reduced volume flow through the discharge port. This makes scroll compressors with a smaller discharge port acceptable for high pressure ratio applications and allows for the design of the scroll compressors with larger built-in pressure ratios and smaller clearance volumes, thus minimizing the losses specific to high pressure ratio applications.