In an effort to improve the performance of centrifugal compressors, detailed studies were carried out on three different impeller-diffuser configurations covering a wide range of design flow coefficients. These were two-dimensional impellers chosen from a multi-stage compressor series with identical blade geometry. The impeller exit widths covered were 24.5, 18.5, and 11.5 mm, respectively. Extensive experimental studies have been carried out on the chosen stage configurations with parallel walled vaneless diffusers (VLDs) as well as low solidity vaned diffuser (LSD). For all the tested configurations, the aerofoil shape of the diffuser vanes remained same while the diffuser width, setting angle, and solidity were chosen as variables. The performance for each of the stages with LSDs was compared with that of the corresponding VLD. Significant improvement in performance in terms of polytropic efficiency, head coefficient, and static pressure recovery coefficient was observed with LSD vanes as compared with the VLD. Significant improvement in performance was noticed from low flow till design point for all the tested cases. The study revealed that among the tested configurations, there is an optimum width at impeller exit and diffuser for which the performance of the stage was the best. Performance in terms of static pressure recovery coefficient was also evaluated for all the tested configurations and compared. All the studies were conducted for solidity ratio of 0.81. The effect of setting angle on wall static pressure coefficient was also studied at 24° and 28° for the flow coefficients corresponding to 80, 100, and 120 per cent of design flow. Flow surveys at impeller exit for the three cases covering impellers/parallel walled diffusers of different widths have also been carried out to know the flow behaviour in the passage between the impeller and the diffuser.

Over the past few years there has been increasing utilisation of higher density surface mounting on printed wiring boards. As components and pads decrease in size, the topography of the solder mask relative to the conductors becomes an important solderability issue. There exists convincing evidence that thinner, more conformal solder mask geometries improve soldering yields of both stencilled and wave soldered surface mount components. In order to provide the solder mask coverage required for improved assembly performance, the authors critically compared several commercially available solder mask coating technologies. The coating methods were appraised according to both assembly and printed wiring board manufacturing criteria. Within this programme, seven liquid photoimageable solder masks were also evaluated. The materials were rated according to their final cured properties (electrical, mechanical, chemical performance), their manufacturability in the printed wiring board manufacturing process (maximum throughput, major defects, etc.) and their performance in assembly operations (soldering yields, propensity to ‘solder ball’ formation, white residues, scratches, etc.). The information obtained was used to choose a solder mask strategy which would not only improve assembly efficiency but also increase PWB manufacturing yields and flexibility.