Low Pressure Ratio Research Articles

Issues in performance measurement of CO2 compressor near the critical point

Interest in the supercritical carbon dioxide (S-CO2) Brayton cycle technology is gradually increasing due to high efficiency at moderate turbine inlet temperature and potential to achieve high compactness of the whole power conversion system. As a part of research activities on the S-CO2 Brayton cycle technology development, Supercritical CO2 Pressurization Experiment (SCO2PE) facility was constructed for lab scale CO2 compression experiment near the critical point. SCO2PE facility operation is focused on resolving issues in a compressor design and operation near the critical point of CO2. From the SCO2PE operation experience, performance measurement uncertainty issues in a low pressure ratio compressor were identified and these issues become generic if a compressor operates near the critical point. Very high uncertainty on performance measurement was observed due to the peak region in thermodynamic property variation of CO2 near the critical point and low pressure ratio operation of the compressor. High pressure ratio compressor design and operation can mitigate high uncertainty issues in performance measurement. However, low pressure ratio operation cannot be ignored since low pressure ratio operation is always involved during a part load operation or transient operation of a power conversion system. Thus, an alternative approach was suggested to secure reliability of performance estimation from the basic measurement.

Twin Jet Interaction and Reverse Flow

Starting characteristics of the underexpanded twin jets and reverse flow as a consequence of interaction at high altitudes were experimentally investigated in a High Altitude Jet Simulation Facility. The flowfield characteristics were captured through Schlieren flow visualization. Pitot pressure survey upstream of the jet inflection point showed the existence of reverse flow due to jet interaction. The nature of the reverse flow at higher pressure ratios, in the vicinity of the jet inflection point was supersonic and become lower subsonic near the base. On the other hand, at lower pressure ratios, the reverse flow was subsonic throughout the base region. The reverse-flow Mach number was dictated by the dissipation effect and the vented area shape in the base region. Base pressure measurements confirmed that reverse flow was fully diffused at the base.

Waste heat recovery using a novel high performance low pressure turbine for electric turbocompounding in downsized gasoline engines: Experimental and computational analysis

The development of a high performance LPT (Low Pressure Turbine) for turbocompounding applications in downsized gasoline engine is presented in this paper. The LPT was designed to fill the existing technology gap where no commercially available turbines can operate effectively at low-pressure ratios (1.05–1.3) to drive an electric generator with 1.0 kW power output. The newly designed LPT geometry was tested at Imperial College under steady-state conditions; a maximum total-to-static efficiency, ηt−s 75.8% at pressure ratio, PR ≈ 1.08 was found.The LPT performance maps were then used for a validated 1-D engine model in order to assess the effect of turbocompounding on BSFC (Brake Specific Fuel Consumption). Then a prototype of the LPT was tested in the post catalyst position on a 1.0 L gasoline engine for different operating conditions. The test results showed that reduction in BSFC of 2.6% could be achieved.With the post-catalyst position selected, a KP (key-point) engine speed/load analysis was performed in order to project an overall NEDC (New European Drive Cycle) fuel consumption benefit for the LPT in a mechanical turbocompounding configuration, as well as an overall power benefit calculation. Finally, a sensitivity study indicated what the power could be off-cycle.

Exhaust Gas Energy Recovery Via Electric Turbocompounding

Abstract This paper presents the development of a high performance Low Pressure Turbine for turbocompounding applicationsindownsized gasoline engines. The LPT was designed to fill the existing technology gap where no commercially available turbines can operate effectively at low pressure ratios (1.05 - 1.3), in order to drive a small electric generator with continuouspower output of 1.0 kW. The newly designed LPT was tested in a gas test stand and its impact on Brake Specific Fuel Consumption was assessed with on engine testing carried out on a 1.0L EcoBoostgasoline engine.

Energy and exergy based thermodynamic analysis of reheat and regenerative Braysson cycle

The conventional Braysson cycle has not found practical use due to the difficulty in achieving isothermal compression. To make its implementation a reality, the original cycle has been modified by incorporating regenerator and a cooler before the final compression process. Reheating was included for augmenting the power output. Expressions for exergy efficiency and exergy destruction for all the components are derived along with the energy and exergy efficiencies of the complete cycle. The effects of maximum temperature, pressure ratio and number of compression stages on the cycle efficiencies have been evaluated. It has been found that the exergy destruction in the combustion chamber and reheater put together accounts for more than 55% of the total exergy destruction. The cycle efficiency is maximum at an optimum pressure ratio which itself is found to be a function of maximum temperature in the cycle. The energy and exergy efficiency of the cycle equals the efficiency of normal Braysson cycle at a much lower pressure ratio. The efficiency achieved through the modified cycle with 2 stages of compression is only 2.2% less than the efficiency through ideal isothermal compression for a pressure ratio of 3 and turbine inlet temperature of 1200 K.

Investigation of film cooling on nose cone by a forward facing array of micro-jets in Hypersonic flow

A numerical study has been performed to investigate the film-cooling heat reduction performance of a single jet (diameter 2mm and 0.9mm), and an array forward facing of micro-jets (diameter 300μm each) of the same effective area (corresponding to the respective single jet). Helium and/or Nitrogen are injected as coolant gases from the tip of the nose through an array of micro into a Mach 5.9 counterflow free stream. Parametric studies were conducted on injection total pressure with various diameters by using the Reynolds-averaged Navier–Stokes equations with Menter's Shear Stress Transport turbulence model. Complex jet interactions were found in the injection region with a variety of flow features depending upon the specific configuration. These flow features were found to have subtle effects on the overall system performance. Total heat load reductions of up to 40% over a nose were achieved for micro jet with low Pressure Ratio. Significant wall heat transfer reductions were also found in all cases. The heat transfer rates were strongly influenced by injection mass flow rate, and only moderately affected by the number of micro jets. The maximum heat reduction performance was found in the highest injection total jet pressure (consequently more mass flow rate) with the highest micro array.

Analysis of thermally coupled chemical looping combustion-based power plants with carbon capture

A number of CO2 capture-enabled power generation technologies have been proposed to address the negative environmental impact of CO2 emission. One important barrier to adopting these technologies is the associated energy penalty. Chemical-looping Combustion (CLC) is an oxy-combustion technology that can significantly lower this penalty. It utilizes an oxygen carrier to transfer oxygen from air/oxidizing stream in an oxidation reactor to the fuel in a reduction reactor. Conventional CLC reactor designs employ two separate reactors, with metal/metal oxide particles circulating pneumatically in-between. One of the key limitations of these designs is the entropy generation due to reactor temperature difference, which lowers the cycle efficiency. Zhao et al. (Zhao et al., 2014; Zhao and Ghoniem, 2014) proposed a new CLC rotary reactor design, which overcomes this limitation. This reactor consists of a single rotating wheel with micro-channels designed to maintain thermal equilibrium between the fuel and air sides. This study uses three thermodynamic models of increasing fidelity to demonstrate that the internal thermal coupling in the rotary CLC reactor creates the potential for improved cycle efficiency. A theoretical availability model and an ideal thermodynamic cycle model are used to define the efficiency limits of CLC systems, illustrate the impact of reactor thermal coupling and discuss relevant criteria. An Aspen Plus® model of a regenerative CLC cycle is then used to show that this thermal coupling raises the cycle efficiency by up to 2% points. A parametric study shows that efficiency varies inversely with pressure, with a maximum of 51% at 3bar, 1000C and 60% at 4bar, 1400C. The efficiency increases with CO2 fraction at high pressure ratios but exhibits a slight inverse dependence at low pressure ratios. The parametric study shows that for low purge steam demand, steam generation improves exhaust heat recovery and increases efficiency when an appropriate steam production strategy is adopted.

Thermodynamic Analysis of a Power Plant Integrated with Fogging Inlet Cooling and a Biomass Gasification

Biomass energy and especially biofuels produced by biomass gasification are clean and renewable options for power plants. Also, on hot days the performance of gas turbines decreases substantially, a problem that can be mitigated by fog cooling. In the present paper, a biomass-integrated fogging steam injected gas turbine cycle is analyzed with energy and exergy methods. It is observed that (1) increasing the compressor pressure ratio raises the air flow rate in the plant but reduces the biomass flow rate; (2) increasing the gas turbine inlet temperature decreases the air and biomass flow rates; (3) increasing the compressor pressure ratio raises the energy and exergy efficiencies, especially at lower pressure ratios; (4) increasing the gas turbine inlet temperature raises both efficiencies; and (5) overspray increases the energy efficiency and net cycle power slightly. The gas turbine exhibits the highest exergy efficiency of the cycle components and the combustor the lowest. A comparison of the cycle with similar cycles fired by natural gas and differently configured cycles fueled by biomass shows that the cycle with natural gas firing has an energy efficiency 18 percentage points above the biomass fired cycle, and that steam injection increases the energy efficiency about five percentage points relative to the cycle without steam injection. Also, the influence of steam injection on energy efficiency is more significant than fog cooling.

Performance study of solar power plants with CO2 as working fluid. A promising design window

In this paper a systematic analysis is reported on the use of CO2 as heat carrier fluid in solar thermal receivers and as thermodynamic working fluid. It includes the performance of close-to-critical regenerative Brayton cycles, which opens a broad field of cycle possibilities with low pressure ratios (very simple turbines) complemented with large but standard heat exchangers as regenerators. Radiation intensities needed to reach relevant efficiencies are in the range above 25kW/m2, but receiver efficiencies do not increase significantly beyond that value, featured as a threshold. Receivers are made of multi-tube bundles enclosed in glass-windowed collectors with compensated pressure and dilatation, which eliminates the problem of gas leakage through rotating joints and other non-hermetic fits. This leads to needing concentrators compatible with those collectors, which can be either finely optimized Linear Fresnel Reflectors or central minitowers. CO2 was chosen for this study because its critical temperature (31°C) is very close to environmental temperature, which conveys very positive features for the efficiency of the cycle. The overall result of the theoretical study is the identification of a set of different types of efficient, flexible and robust CSP plants with CO2 as the only fluid which deserves further research at experimental level and in the design and construction of new plant components.

Thermodynamic analysis of manufacturing polysilicon from SiHCl3, SiCl4 and H2

Production of polysilicon by chemical vapor deposition of SiHCl3 with a fluidized bed reactor is a competitive technology. As equilibrium conversion can be approached in a fluidized bed reactor, a reliable thermodynamic analysis is very important. However, inconsistent thermodynamic analysis results have been reported in the literature. The present work studied the effects of thermodynamic data and species selection, and recommended that JANAF was the best Cp data source and the minimum set of species included the following eight species: H2, HCl, SiCl4, SiCl2, SiHCl3, SiH2Cl2, SiH3Cl and Si. Then, the influence of operating conditions on the equilibrium was studied. For the SiHCl3–H2 system, both the yield of silicon and selectivity to silicon reached their maximum at (up to 1100°C), and low pressure and high H2 feed ratio were of benefit for silicon production. For the SiHCl3–SiCl4–H2 system, silicon could be produced only at 900–1400°C, and reducing pressure and increasing H2 feed ratio enhanced the yield of silicon. Meanwhile, the operation map for zero net by-production of SiCl4 by directly recycling the produced SiCl4 was determined.

Analysis of Aerodynamic Design Characteristics of Flade Fan

Numerical simulation was conducted to explore how the load distribution and pre-swirl distribution influence the aerodynamic performance for the Flade fan. The Flade fan faces the design challenge of full span supersonic flow because of the position above the tip of main fan, so it is necessary to introduce full span positive pre-swirl. The full span positive pre-swirl introduced by IGV helped to reduce the rotor inlet relative Mach number and could effectively control the shock loss and the shock-boundary layer loss on the full span. Load distribution could effectively control the conflict between high tangential velocity and low pressure ratio, and change the degree of reaction to make the absolute Mach number at the rotor outlet satisfy the design request of stator. Stall characteristics analysis on Flade show that the tip clearance flow is still far away from the leading edge of blade, and a wide range of low-speed and low-energy region appeared at the root lead to the onset of stall.

Study on Surge Characteristics and Internal Flow of a Centrifugal Compressor at Low Pressure Ratio

Study on Surge Characteristics and Internal Flow of a Centrifugal Compressor at Low Pressure Ratio

Miniature Piezoelectric Compressor for Joule-Thomson Cryocoolers

Joule-Thomson (JT) cryocoolers operate with a continuous flow of the working fluid that enters the cooler at a high pressure and leaves it at a lower pressure. Ideally, the temperature of the outgoing fluid equals the temperature of the entering fluid. JT cryocoolers that operate with pure refrigerants require high pressure of a few tens of MPa where the low pressure is usually around 0.1MPa. Circulation of the working fluid in such cases requires high pressure ratio compressors that evidently have large dimensions. JT cryocoolers can operate with much lower pressure ratios by using mixed-refrigerants. Cooling from 300K to about 80K in a single stage cryocooler normally requires a pressure ratio of about 1:25.In the present research a miniature compressor driven by piezoelectric elements is developed in collaboration between Rafael and the Technion. This type of compressor has the advantage of improved long life compared to other mechanical compressors, very low vibrations, and silent operation. In the current case, the design goal of the intake and discharge pressures has been 0.1 and 2.5MPa, respectively, with a flow rate of 0.06g/s. The compressor has two compression stages; 1:5 and 5:25.Several configurations have been considered, fabricated, and tested. The performance of the last configuration approaches the desired specification and is presented in the current paper together with the design concept.

Numerical Study of Unsteady Flow in Centrifugal Cold Compressor

In helium refrigeration system, high-speed centrifugal cold compressor is utilized to pumped gaseous helium from saturated liquid helium tank at low temperature and low pressure for producing superfluid helium or sub-cooled helium. Stall and surge are common unsteady flow phenomena in centrifugal cold compressors which severely limit operation range and impact efficiency reliability. In order to obtain the installed range of cold compressor, unsteady flow in the case of low mass flow or high pressure ratio is investigated by the CFD. From the results of the numerical analysis, it can be deduced that the pressure ratio increases with the decrease in reduced mass flow. With the decrease of the reduced mass flow, backflow and vortex are intensified near the shroud of impeller. The unsteady flow will not only increase the flow loss, but also damage the compressor. It provided a numerical foundation of analyzing the effect of unsteady flow field and reducing the flow loss, and it is helpful for the further study and able to instruct the designing.

Measurement, analysis and modeling of centrifugal compressor flow for low pressure ratios

Increasingly stringent emission legislation combined with consumer performance demands has driven the development of downsized engines with complex turbocharger arrangements. To handle the complexity, model-based methods have become a standard tool, and these methods need models that are capable of describing all operating modes of the systems. The models should also be easily parametrized and enable extrapolation. Both single- and multi-stage turbo systems can operate with a pressure drop over their compressors, both stationary and transient. The focus here is to develop models that can describe centrifugal compressors that operate in both normal region and restriction region from standstill to maximum speed. The modeling results rely on an analysis of 305 automotive compressor maps, whereof five contain measured restriction operation and two contain measured standstill characteristic. A standstill compressor is shown to choke at a pressure ratio of approximately 0.5, and the corresponding choking corrected mass flow being approximately 50% of the compressor maximum flow capacity. Both choking pressure ratio and flow are then shown to increase with corrected speed, and the choking pressure ratio is shown to occur at pressure ratios larger than unity for higher speeds. Simple empirical models are proposed and shown to be able to describe high flow and pressure ratios down to choking conditions well. A novel compressor flow model is proposed and validated to capture the high flow asymptote well, for speeds from standstill up to maximum.

Binary mixed-refrigerants for steady cooling temperatures between 80 K and 150 K with Joule–Thomson cryocoolers

Joule–Thomson (JT) cryocoolers operating with pure gases attain vapor–liquid equilibrium in the evaporator and yield a steady cooling temperature which depends solely on the pressure. Mixed gases often substitute the pure gases as the working fluid in order to allow lower pressure ratios and improved coefficient of performance; however, mixed refrigerants are known for their cooling temperature dependence on the ambient temperature and heat load. It is possible to control the cooling temperature via the compressor operating parameters, heating device at the evaporator, bypass ports for the fluid, and more. The present research aims to achieve a steady cooling temperature with mixed refrigerants in a manner similar to pure refrigerants; meaning, having enough phases to leave a single degree of freedom of the fluid thermodynamic state. Thus, by stabilizing the pressure a stable temperature is attained.The current study focuses on binary mixtures since they can be analytically investigated in a more convenient manner relative to multi-component mixtures. Nitrogen–ethane and nitrogen–propane mixtures are considered based on former research conclusions. The present research includes an analytical method for calculating the cooling temperature, a numerical method for calculating the cooling power of the JT cryocooler in order to evaluate whether the cooling temperature is feasible, and experimental data are presented to verify some of the calculated results.

A New Design of Double-Stage Parallel Adding Equipment Used for Dust Suppression in Underground Coal Mines

In order to solve the problems of conventional single-stage adding equipment, such as high adding proportion, easy to generate cavitation, large pressure loss and low pressure ratio, an original design of double-stage parallel adding equipment is proposed. Its performance is investigated experimentally under different working flow and outlet pressure and compared with the single-stage jet equipment. Testing results show that the adding proportion of double-stage parallel adding equipment is less than that of the single-stage one, and the cavitation flow is higher. The new equipment can decrease pressure loss, inlet pressure and improve pressure ratio, which lowers the energy loss of adding equipment and raises its driving ability in application. An adding proportion of 4.6 ‰ and an outlet pressure of 0.16–0.18 MPa were realized when the double-stage parallel adding equipment was used to add foaming agent for dust suppression with foam (DSF). The marked reduction in the adding proportion of foaming agent significantly reduces foam production cost. Therefore, it is believed that the study would lay an important foundation for the widespread application of DSF in underground coal mines.

Heavy-duty engine electric turbocompounding

This paper analyses the performance of a heavy-duty engine when integrating a turbocompound system including a low-pressure-ratio turbine. The main feature of the low-pressure-ratio turbine is that of achieving efficiencies greater than 75% at very low pressure ratios (of approximately 1.1). The engine performance is predicted by a one-dimensionanl simulation model of a 10 l heavy-duty engine. An electric turbocompound system with a low-pressure-ratio turbine is fitted downstream of the main turbocharger to extract the exhaust gas energy. This additional energy supplies the electronic units and batteries. A method to scale an existing low-pressure-ratio turbine, designed for a 1.0 l gasoline engine, is defined and the changes in the turbine losses and the blade stresses are discussed. Low-pressure-ratio turbines of different sizes are analysed and compared in order to obtain the greatest benefit. In all cases the sum of the engine power and the turbocompound power is matched to the baseline engine power output. The full-load results show a power output from the low-pressure-ratio turbine of up to 15.8 kW, an improvement in the brake specific fuel consumption of the engine of up to 4% and a reduction in the brake mean effective pressure of up to 5%. The recovered exhaust gas energy is higher than the engine power required to balance the increased back pressure due to the additional component within the exhaust system. The electrical power produced is sufficiently high to replace the alternator.

Heat reduction using conterflowing jet for a nose cone with aerodisk in hypersonic flow

Numerical simulations of a 2D axisymmetric aerodisked nose cone in hypersonic flow are conducted, and innovative techniques involving forward injection of the gas from the stagnation point of the sphere are investigated; techniques include the injection of various counterflowing jets (helium or carbon dioxide) as a coolant jet from the nose cone behind the aerodisk. In this study, the characteristics of the various jet conditions of a counterflowing jet on a cone surface were investigated numerically to improve performance of the jet on heat reduction at the surface of a nose. The compressible, unsteady, axisymmetric Navier–Stokes equations are solved with SST turbulence model for free stream Mach number of 5.75 at 0° angle of attack with and without gas injection. According to the investigation of various conditions of jets, important phenomena of flow field and some effective jet conditions are found. Heat transfer results show a significant reduction in heat flux, even giving negative heat flux for some low pressure ratio, indicating that the flow wetting the model had a cooling effect which could significantly impact thermal protection system design. The findings suggest that high-speed vehicle design and performance can benefit from the application of counterflowing jets as a cooling system.

Flow and Separation Performance of Supersonic Gas Separator

A study on the flow and separation performance of supersonic gas separator is carried out through numerical simulation and experimentation. The effect of area ratio on the separation performance under low pressure ratio has been researched. The simulation results indicate that with an increase of the area ratio AR, the intensity of the shockwave increases and the location approaches the throat; Shockwaves are absent in the diverging section of the nozzle when area ratio is 1.063 and the pressure ratio is within 1.25-1.75, which reaches the highest separation performance. The calculated and experimental results also show that the separation performance is the highest and can reach 40.82% when pressure ratio is 1.75. The calculated values are in agreement with the experimental results.