Measurement Of Pressure Profile Of Vortex Flashing Flows In Convergent-Divergent Nozzles

Vortex control is a novel two-phase convergent-divergent nozzle restrictiveness control mechanism which requires no change to the physical dimensions of the nozzle geometry. The control is achieved by adjustable nozzle inlet vortex. A nozzle with inlet vortex was called vortex nozzle (or swirl nozzle). Previous experiments on vortex nozzle with initially subcooled R134a showed that the nozzle becomes more restrictive as the strength of the inlet vortex increases. The maximum vortex control range has been observed to be 42% of maximum choked total mass flow rate through the nozzle. The control range of inlet pressures and mass flow rates that can be achieved by vortex control appears to be large enough to be suitable for numerous technical applications. This novel mechanism can potentially provide flow control with less sacrifice of nozzle efficiency, which is extremely important for ejector cooling cycle performance. It is also less vulnerable to clogging since the flow control is achieved without changing the flow area. However, the underlying mechanism behind the vortex control is still unclear. Measurement of the pressure profile of the vortex flashing flows in convergent-divergent nozzles under different conditions can provide more insights into the vortex nozzle flows and help to explain the above-mentioned phenomena. It also provides validation for modeling of vortex flashing flows. In this study, the experimental investigation of the pressure profile of the vortex flashing flows is presented. The results are discussed and different behaviors of vortex nozzle flow are explained with the insight provided by the pressure measurements

A New Control Mechanism for Two-Phase Ejector in Vapor Compression Cycles Using Adjustable Motive Nozzle Inlet Vortex

Expansion work recovery by two-phase ejector is known to be beneficial to vapor compression cycle performance. However, one of the biggest challenges with ejector vapor compression cycle is that the ejector cycle performance is sensitive to working condition changes which are common in real world applications. Different working conditions require different ejector geometries to achieve maximum performance. Slightly different geometries may result in substantially different COPs under the same conditions. Ejector motive nozzle throat diameter (motive nozzle restrictiveness) is one of the key parameters that can significantly affect COP. This paper presents a new motive nozzle restrictiveness control mechanism for two-phase ejectors used in vapor compression cycles, which has the advantages of being simple, potentially less costly and less vulnerable to clogging. The new control mechanism can possibly avoid the additional frictional losses of previously proposed ejector control mechanisms using adjustable needle. The redesigned ejector utilizes an adjustable vortex at the motive inlet to control the nozzle restrictiveness on the flow expanded in the motive nozzle. An adjustable nozzle based on this new control mechanism was designed and manufactured for experiments with R134a. The experimental results showed that, without changing the nozzle geometry, the nozzle restrictiveness on the two-phase flow can be adjusted over a wide range. Under the same inlet and outlet conditions, the mass flow rate through the nozzle can be reduced by 36% of the full load. This feature could be very useful for the future application of ejector in mobile or stationary systems under changing working conditions

Modeling of Initially Subcooled Flashing Vortex Flow in the Nozzle for Possible Applications in the Control of Ejector Cooling Cycles

Ejectors are known to be beneficial to vapor compression cycle performance as they can recover the kinetic energy released during the expansion instead of dissipating it in a throttling process. It is desirable to introduce an adjustable feature to the ejector so that ejector cycle performance can be optimized under different working conditions, which could make ejector technology more suitable for real world applications. Vortex control is a nozzle control mechanism which can possibly be applied to the control of ejector cooling cycles. It utilizes an adjustable vortex at the nozzle inlet to control the nozzle restrictiveness without having to change physical dimensions of the nozzle geometry. In this paper, two different approaches are employed to model the initially subcooled flashing vortex flow in a convergent nozzle at steady state. The first approach assumes that bubble nucleation during the depressurization in the nozzle all occurs at the nozzle wall. Bubbles are regarded as spherical particles that grow and move in the liquid flow field. The second approach assumes that there is an evaporation wave at the nozzle throat. The bubble generation in the upstream of the evaporation wave is neglected, thus the fluid in the upstream of the evaporation wave is assumed to be single-phase incompressible liquid. The modeling results are presented and compared with the experimental results. It has been concluded that bubble nucleation may not all occur at the nozzle wall at high degree of metastability. Nucleation in the bulk of the liquid might be dominant and should possibly be taken into consideration in the modeling. Pressure reduction is required for the kinetic energy increase of the nozzle flow in the azimuthal direction when there is inlet vortex introduced. For the same mass flow rate through the nozzle, the pressure difference from the nozzle inlet to the center of the nozzle throat increases as the inlet vortex becomes stronger. Therefore, less mass flow rate can be driven through the nozzle with stronger inlet vortex for the same degree of metastability at the throat and the same inlet conditions. The change in total mass flow rate is smaller with larger surface roughness for the same inlet vortex strength

CFD Simulation Of Vortex Flashing Flows In Convergent-Divergent Nozzles

Vortex control is a novel two-phase convergent-divergent nozzle restrictiveness control mechanism by adjustable nozzle inlet vortex. It requires no change to the physical dimensions of the nozzle geometry. A nozzle with inlet vortex was called vortex nozzle (or swirl nozzle). Previous experiments on vortex nozzle with initially subcooled R134a showed that the nozzle becomes more restrictive as the strength of the inlet vortex increases. The maximum vortex control range has been observed to be 42% of maximum choked total mass flow rate through the nozzle. The control range of inlet pressures and mass flow rates that can be achieved by vortex control appears to be large enough to be suitable for numerous technical applications. This novel mechanism can potentially provide flow control with less sacrifice of nozzle efficiency, which is extremely important for ejector cooling cycle performance. It is also less vulnerable to clogging since the flow control is achieved without changing the flow area. However, the underlying mechanism behind the vortex control is still unclear. In this study, 3D CFD simulation of vortex flashing flows in convergent-divergent nozzles has been conducted. The simulation results show increase of nozzle restrictiveness after the application of inlet vortex, which is the same as the experimental results. The vortex control mechanism has also been explained with the insight provided by CFD simulation

Risk-Aware Linear Bandits: Theory and Applications in Smart Order Routing

Motivated by practical considerations in machine learning for financial decision-making, such as risk-aversion and large action space, we initiate the study of risk-aware linear bandits. Specifically, we consider regret minimization under the mean-variance measure when facing a set of actions whose rewards can be expressed as linear functions of (initially) unknown parameters. Driven by the variance-minimizing G-optimal design, we propose the Risk-Aware Explore-then-Commit (RISE) algorithm and the Risk-Aware Successive Elimination (RISE++) algorithm. Then, we rigorously analyze their regret upper bounds to show that, by leveraging the linear structure, the algorithms can dramatically reduce the regret when compared to existing methods. Finally, we demonstrate the performance of the algorithms by conducting extensive numerical experiments in a synthetic smart order routing setup. Our results show that both RISE and RISE++ can outperform the competing methods, especially in complex decision-making scenarios