No abstract

An anthropomorphic phantom study was carried out in 2013-14 in two hospitals, one located in Russia (Mariinsky Hospital, Saint Petersburg) and the other in Sweden (Skåne University Hospital, Malmö). The aim of the study was to investigate the possibilities to reduce the patient dose from digital X-ray chest screening examinations. The existing chest imaging protocols were adjusted by changing the tube voltage, total filtration and grid in order to determine the most dose-effective combination of the examination parameters. It was possible to achieve up to 50 % dose-area product (DAP) and 30 % effective dose reduction by raising the tube voltage from 100 to 125 or 150 kV, and simultaneously decrease the total filtration to the minimum allowed by the X-ray unit (3 mm Al). The absence of a grid allowed to further reduce the DAP and effective dose by up to 80 %. Comparison between Russian and Swedish X-ray units showed the same trend in DAP and effective dose reduction, but the absolute dose values were lower by almost a factor of 10 for the Swedish units due to different image receptors and automatic exposure control settings.

Experiments and CFD-analysis of vaneless diffusers demonstrate that in wide range of relative width there is a risk of flow separation at design flow rate as a flow angle after an impeller is too small. To avoid it the inlet part of a diffuser is made with decreasing width along radius.By the elementwise calculation the diffuser with relative width about 2%, narrowed initial part and conical surfaces with the divergence angle 3 0 between them is turned out to be the best variant of the considered.The diffuser within the stage with narrowed initial part and zero divergence angle has the highest efficiency when relative flow rates are less than design regime flow rate. The stage with diffuser with walls divergence angle 0 0 45' after narrowed part is less effective on 0.5% when relative flow rates are less than design regime. The maximum efficiency of this stage is 1.5% higher and efficiency is sufficiently higher at big flow rates.

Countless combinations of sizes and shapes of the flow path could ensure the specified flow rate and pressure ratio of the axial compressor stage. Programs based on the mathematical models are an effective tool in initial design and preliminary selection of the optimal option. The RROK-GPD-22 software program is briefly being described, it solves the problem on the basis of the pressure loss model and deflection capacity of the A. Komarov lattice, theoretic rigorous calculation of oblique and direct shocks, as well as empirical equations and coefficients. Models were checked by comparison with data on the Rotor 37 NASA test stage. Stages were calculated with different ratio of the cross-sectional area at the outlet of the impeller to that at its inlet and stages with the hub-tip ratios of 0.400 ... 0.728 with different swirl at the impeller inlet. Results are provided in determining the hub-tip ratio influence on the efficiency, pressure ratio and specific stage performance.

Various engineering techniques are used for optimal gas-dynamic design of centrifugal compressors. This includes a universal modelling method that consists of software programs developed at Peter the Great St. Petersburg Polytechnic University. Tangential exit nozzles are elements of the centrifugal compressor flow path. The analysis of the results of the tangential exit nozzle calculations using the current mathematical model showed a need of improvement. The following main provisions formed a basis for a new model: the size of the passage is determined using the flow rate equation at the entrance and exit of the output unit (the calculated cross sections should be increased by 25–35% according to the recognized recommendations by Russian experts); the real nature of the flow in the output unit is taken into account by introducing an empirical coefficient in the equation of the circumferential component of velocity; the output diffuser is designed taking into account the optimal angle of expansion of an equivalent conical diffuser; the scroll tongue is shifted from a section with an angle of expansion of 0° to a section with an angle of expansion of 30°, which aids levelling the circumferential flow parameters and reduces total losses. To simplify the calculation process, a constant density along the scroll length is adopted in the mathematical model. The circumferential component of velocity is also determined approximately using the flow continuity equation without taking viscosity into account. Losses in scrolls and annular chambers are calculated in the radial and meridional planes. In the radial plane, the main losses are friction losses, whereas in the meridional plane, the main losses are due to expansion. For a trapezoid scroll, these pressure losses are determined depending on the scroll’s expansion angle. In the off-design operating modes, incidental losses due to impact flow around the scroll tongue are added. The presented model was implemented in the new version of the universal modeling method. The mathematical model was identified by the results of the commissioning test of the turboexpanders and turbochargers.