Pressure Exchanger for Energy Recovery in a Trans-Critical CO2 Refrigeration System

Trans-critical CO2 vapor compression (VC) refrigeration cycles require a high compression ratio, which is associated with high expansion losses. To recover these expansion losses, a pressure exchange process between the low- and high-pressure sides of the VC cycle is proposed and examined in this study. The proposed pressure exchange system is an open type constant volume process where the high- and low-pressure flows mix inside the system. This prototype is inspired by the pressure exchangers used in reverse-osmosis (RO) desalination systems. In this system, a 2D model was generated and modeled using the computational fluid dynamics (CFD) technique. The numerical model ignored any losses due to leakage or hydraulic friction and the process is considered adiabatic. For the modeling, it was assumed that the inlet conditions for the two pressure exchanger flows are similar to the flow conditions at the evaporator and gas cooler outlets in a VC cycle. Two parameters are examined to test the validity of the system and understand their effect on the performance, including the inlet flow rate represented by the inlet velocity and the process time represented by the speed of rotation. A total of nine cases were simulated and analyzed in this study.

Acoustic Instability with Dynamic Charging in Quantum Plasmas

The time-dependent charging phenomenon of dust particles is timely studied in quantum plasmas with low frequency dust dynamical temporal scales referred to as dust acoustic waves. The quantum effects are incorporated through Fermi pressure, exchange-correlation potential as well as the Bohm potential. The quantum fluid model is employed in getting the dispersion relation pointing to the damping instability. The damping instability is analysed across the whole spectrum k, on varying ion-thermal temperature, electron number density and the dust radii. The applications of this work are pointed out in the laboratory and the astrophysical dense plasma systems.

Improving Refrigeration Performance by Using Pressure Exchange Characteristic of Wave Rotor

Wave rotor with pressure exchange function can be attempted to improve refrigeration performance. The objective of this paper is to verify the feasibility of the method by thermodynamic and experimental analysis. First, a refrigeration process which contains wave rotor pressurization was established. Then, a thermodynamic model which reflects the refrigeration process was designed. The thermal performance was researched under various key parameters. Finally, based on the novel wave rotor refrigeration platform, the experimental work was carried out, and the effects of main parameters of the device were systematically studied. The results showed that it was feasible to enhance the coefficient of performance (COP) by using pressure exchange characteristic of wave rotor. The COP could be improved substantially at relatively small expansion ratio. Under the design point, more than half of the pressure energy could be restored. The performance curve of the novel equipment was also obtained. Enhancing the isentropic efficiency of expansion is the effective means to improve the COP and σ of the system. This paper was designed in a way that contained a novel equipment to enhance the COP of wave rotor refrigeration.

Mechanism Analysis of Weakening Reverse Compression Waves in Gas Wave Refrigerator

Pressure oscillating tubes are core components of the gas wave refrigerator. The reverse compression waves will limit refrigeration efficiency by reheat cooling gas. The promotion from traditional rotation gas wave refrigerator with single opened pressure oscillating tubes to the streamlined pressure exchange gas wave refrigerator with double opened pressure oscillating tubes is mainly in terms of this issue. However, through weakened, reverse compression waves are still inevitable. For further development, the concept of a wave attenuator has been proposed, installed at the high-temperature (HT) port. Numerical simulation has been utilized to analyze mechanism of wave attenuator and the practical effect has been proved by experiment research. The conclusions are as follows: due to structure of wave attenuator, the intensity of reverse compression waves has been weakened; the optimal structure of wave attenuator has been obtained; the refrigeration efficiency of the refrigerator has been significantly increased because of wave attenuator in HT port.

Numerical Modeling of a Wave Turbine and Estimation of Shaft Work

Wave rotors are periodic-flow devices that provide dynamic pressure exchange and efficient energy transfer through internal pressure waves generated due to fast opening and closing of ports. Wave turbines are wave rotors with curved channels that can produce shaft work through change of angular momentum from inlet to exit. In the present work, conservation equations with averaging in the transverse directions are derived for wave turbines, and quasi-one-dimensional model for axial-channel non-steady flow is extended to account for blade curvature effects. The importance of inlet incidence is explained and the duct angle is optimized to minimize incidence loss for a particular boundary condition. Two different techniques are presented for estimating the work transfer between the gas and rotor due to flow turning, based on conservation of angular momentum and of energy. The use of two different methods to estimate the shaft work provides confidence in reporting of work output and confirms internal consistency of the model while it awaits experimental data for validation. The extended wave turbine model is used to simulate the flow in a three-port wave rotor. The work output is calculated for blades with varying curvature, including the straight axial channel as a reference case. The dimensional shaft work is reported for the idealized situation where all loss-generating mechanisms except flow incidence are absent, thus excluding leakage, heat transfer, friction, port opening time, and windage losses. The model developed in the current work can be used to determine the optimal wave turbine designs for experimental investment.

Impact of Relativistic Electron Beam on Hole Acoustic Instability in Quantum Semiconductor Plasmas

We studied the influence of the classical relativistic beam of electrons on the hole acoustic wave (HAW) instability exciting in the semiconductor quantum plasmas. We conducted this study by using the quantum-hydrodynamic model of dense plasmas, incorporating the quantum effects of semiconductor plasma species which include degeneracy pressure, exchange-correlation potential and Bohm potential. Analysis of the quantum characteristics of semiconductor plasma species along with relativistic effect of beam electrons on the dispersion relation of the HAW is given in detail qualitatively and quantitatively by plotting them numerically. It is worth mentioning that the relativistic electron beam (REB) stabilises the HAWs exciting in semiconductor (GaAs) degenerate plasma.

Mixing Control in an Isobaric Energy Recovery Device of Seawater Reverse Osmosis Desalination System

Mixing phenomena in an isobaric energy recovery device (ERD) of a seawater reverse osmosis (SWRO) desalination system are investigated experimentally and numerically using Particle Image Velocimetry (PIV) and Computational Fluid Dynamics (CFD). The ERD, which recovers energy from high-pressure brine discharged from RO membranes, is one of the most important mechanical devices in a SWRO desalination system. In this ERD, seawater is introduced into a vertical chamber from the top, and then high-pressure brine is introduced into the chamber from the bottom. The high-pressure brine pressurizes the seawater through direct liquid-to-liquid contact, transferring high-pressure energy of the brine to the seawater. This enables a sharp reduction in the electric energy consumption, typically 50%, of high-pressure pumps used to elevate seawater pressure for RO membranes. The energy recovery efficiency of the present ERD is over 98%, which is extremely high compared to a conventional turbine-type energy recovery device, such as a Pelton turbine, which has a system energy recovery efficiency of 60 to 80%. The possible weakness of the present ERD is the amount of mixing between brine and seawater around the direct contact surface, because mixing phenomena increase the salinity of seawater supplied to RO membranes. A higher pressure is required to keep the same amount of permeate from the membrane, which results in an energy loss in the system. To minimize mixing, a set of unique flow distributors was invented and placed at both ends of the pressure exchange chamber, which stabilizes the contact surfaces and suppresses excessive mixing. Mixing phenomena in the pressure-exchange chamber are investigated experimentally in detail with PIV and numerically with CFD, and the effectiveness of the flow distributors is clarified.

Pressure Exchange Heat Pump: Natural Gas Residential Air Conditioning

There is a strong societal need for a thermally-based air conditioning system which can provide cooling in the summer to small homes and businesses. There is currently no technology available today that can fill this vast market in a cost-effective manner. This paper discusses a potentially important technology which has the potential of contributing to this need, as well as helping to reduce carbon emissions. The paper discusses this technology, it potential, and describes some of the preliminary computational work that will lead to its ultimate development.