NUMERICAL STUDY OF THE 3D FLOWFIELD IN A TRANSONIC COMPRESSOR ROTOR WITH A MODELLING OF THE TIP CLEARANCE FLOW.

This paper describes a computer code aimed at solving the equations of three dimensional viscous compressible flow in turbomachine geometries. The code is applied to the study of the flowfield in a transonic axial compressor rotor at design speed at both maximum flow and towards stall. The predicted flowfield is compared with the laser measurements and the performance of the code discussed. In addition the discussion highlights the change in the predicted endwall and tip clearance flows as the rotor operating point is moved towards stall.

Experimental study of tip clearance flow in a radial inflow turbine

This paper describes an experimental investigation of tip clearance flow in a radial inflow turbine. Flow visualization and static pressure measurements were performed. These were combined with hot-wire traverses into the tip gap. The experimental data indicates that the tip clearance flow in a radial turbine can be divided into three regions. The first region is located at the rotor inlet, where the influence of relative casing motion dominates the flow over the tip. The second region is located towards midchord, where the effect of relative casing motion is weakened. Finally a third region exists in the exducer, where the effect of relative casing motion becomes small and the leakage flow resembles the tip flow behaviour in an axial turbine. Integration of the velocity profiles showed that there is little tip leakage in the first part of the rotor because of the effect of scraping. It was found that the bulk of tip leakage flow in a radial turbine passes through the exducer. The mass flow rate, measured at four chordwise positions, was compared with a standard axial turbine tip leakage model. The result revealed the need for a model suited to radial turbines. The hot-wire measurements also indicated a higher tip gap loss in the exducer of the radial turbine. This explains why the stage efficiency of a radial inflow turbine is more affected by increasing the radial clearance than by increasing the axial clearance.

The effect of clearance on shrouded and unshrouded turbines at two levels of reaction

In this paper, the effect of seal clearance on the efficiency of a turbine with a shrouded rotor is compared with the effect of the tip clearance when the same turbine has an unshrouded rotor. The shrouded versus unshrouded comparison was undertaken for two turbine stage designs one having 50% reaction the other having 24% reaction. Measurements for a range of clearances, including very small clearances, showed three important phenomena. Firstly, as the clearance is reduced, there is a "break-even clearance" at which both the shrouded turbine and the unshrouded turbine have the same efficiency. If the clearance is reduced further, the unshrouded turbine performs better than the shrouded turbine, with the difference at zero clearance termed the "offset loss". This is contrary to the traditional assumption that both shrouded and unshrouded turbines have the same efficiency at zero clearance. The physics of the break-even clearance and the offset loss are discussed. Secondly, the use of a lower reaction had the effect of reducing the tip leakage efficiency penalty for both the shrouded and the unshrouded turbines. In order to understand the effect of reaction on the tip leakage, an analytical model was used and it was found that the tip leakage efficiency penalty should be understood as the dissipated kinetic energy rather than either the tip leakage mass flow rate or the tip leakage loss coefficient. Thirdly, it was also observed that, at a fixed flow coefficient, the fractional change in the output power with clearance was approximately twice the fractional change in efficiency with clearance. This was explained by using an analytical model. © 2010 by ASME.

The effect of clearance on shrouded and unshrouded turbines at two levels of reaction

In this paper, the effect of seal clearance on the efficiency of a turbine with a shrouded rotor is compared with the effect of the tip clearance when the same turbine has an unshrouded rotor. The shrouded versus unshrouded comparison was undertaken for two turbine stage designs one having 50% reaction, the other having 24% reaction. Measurements for a range of clearances, including very small clearances, showed three important phenomena. Firstly, as the clearance is reduced, there is a "break-even clearance" at which both the shrouded turbine and the unshrouded turbine have the same efficiency. If the clearance is reduced further, the unshrouded turbine performs better than the shrouded turbine, with the difference at zero clearance termed the "offset loss." This is contrary to the traditional assumption that both shrouded and unshrouded turbines have the same efficiency at zero clearance. The physics of the break-even clearance and the offset loss are discussed. Secondly, the use of a lower reaction had the effect of reducing the tip leakage efficiency penalty for both the shrouded and the unshrouded turbines. In order to understand the effect of reaction on the tip leakage, an analytical model was used and it was found that the tip leakage efficiency penalty should be understood as the dissipated kinetic energy rather than either the tip leakage mass flow rate or the tip leakage loss coefficient. Thirdly, it was also observed that, at a fixed flow coefficient, the fractional change in the output power with clearance was approximately twice the fractional change in efficiency with clearance. This was explained by using an analytical model. © 2014 by ASME.

Investigation of a radial-inflow bleed as a potential for compressor clearance control

The mismatch in thermal response between a High Pressure Compressor (HPC) drum and casing is a limiting factor in the reduction of compressor clearance. An experimental test rig has been used to demonstrate the concept of radial inflow to reduce the thermal time constant of HPC discs. The testing uses a simulated idle - Maximum Take Off (MTO) - idle transient in order to measure the thermal response directly. The testing is fully scaled in the dimensionless sense to engine conditions. A simple closure model based on lumped capacitance is used to illustrate the scope of potential benefits. The proof-of-concept testing shows that HPC disc time constant reductions of the order 2 are feasible with a radial-inflow bleed of only 4% of bore flow at scaled MTO conditions. Using the experimental results, the simple closure modelling suggests that for a stage with a significant mismatch in thermal response, reductions in 2D axis-symmetric clearance of as much as 50% at MTO conditions may be possible along with significant scope for improvements at cruise conditions. Copyright © 2013 by ASME.

Seasonal variations in growth and clearance rate of the Zhikong scallop Chlamys farreri suspended in the deep water of Haizhou Bay, China

The growth and survival of the Zhikong scallop Chlamys farreri suspended in deep water of Haizhou Bay were studied from July 2007 to June 2008, and the biodeposition method was used to estimate the clearance rate of C. farreri under field conditions. Results showed that the scallop grew fast during all the culture time, with the exception of summer. The condition index of the scallop increased with time and reached the highest value in spring of the second year. The survival of scallops was 60.8 +/- A 3.9% at the end of this study, mortality occurring mainly during the summer and autumn of the first year. The clearance rate fluctuated obviously with season,with the highest value in September 2007, and the lowest value in March 2008. Factors accounting for variations in growth and clearance rate of scallops are also discussed.

HPV clearance and the neglected role of stochasticity.

Clearance of anogenital and oropharyngeal HPV infections is attributed primarily to a successful adaptive immune response. To date, little attention has been paid to the potential role of stochastic cell dynamics in the time it takes to clear an HPV infection. In this study, we combine mechanistic mathematical models at the cellular level with epidemiological data at the population level to disentangle the respective roles of immune capacity and cell dynamics in the clearing mechanism. Our results suggest that chance-in form of the stochastic dynamics of basal stem cells-plays a critical role in the elimination of HPV-infected cell clones. In particular, we find that in immunocompetent adolescents with cervical HPV infections, the immune response may contribute less than 20% to virus clearance-the rest is taken care of by the stochastic proliferation dynamics in the basal layer. In HIV-negative individuals, the contribution of the immune response may be negligible.

The Effect of Diet-Induced Obesity and Subsequent Weight Loss on Body Composition, Glucose Clearance, Metabolite Profile and Liver Amp-Activated Protein Kinase in Mice

Obesity, currently an epidemic, is a difficult disease to combat because it is marked by both a change in body weight and an underlying dysregulation in metabolism, making consistent weight loss challenging. We sought to elucidate this metabolic dysregulation resulting from diet-induced obesity (DIO) that persists through subsequent weight loss. We hypothesized that weight gain imparts a change in “metabolic set point” persisting through subsequent weight loss and that this modification may involve a persistent change in hepatic AMP-activated protein kinase (AMPK), a key energy-sensing enzyme in the body. To test these hypotheses, we tracked metabolic perturbations through this period, measuring changes in hepatic AMPK. To further understand the role of AMPK we used AICAR, an AMPK activator, following DIO. Our findings established a more dynamic metabolic model of DIO and subsequent weight loss. We observed hepatic AMPK elevation following weight loss, but AICAR administration without similar dieting was unsuccessful in improving metabolic dysregulation. Our findings provide an approach to modeling DIO and subsequent dieting that can be built upon in future studies and hopefully contribute to more effective long-term treatments of obesity.