Tham khảo tài liệu 'advances in gas turbine technology part 9', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 230 Advances in Gas Turbine Technology Figures 13 and 14 give the modeling results. Figure 13 shows the evolution of the heat exchange surface versus the inlet radius. The greater the volute the smaller the surface to volume ratio. Small turbomachines therefore have a higher surface to volume ratio. The necessity of taking into account heat transfer in small turbomachines is largely confirmed by Figure 14 the heat losses in the volute are relatively greater. In this study when the inlet radius is halved the surface to volume ratio doubles and the heat losses are multiplied by about Fig. 13. Ratio of heat exchange surface S and the volume V of the volute versus the inlet radius Influence of Heat Transfer on Gas Turbine Performance 231 Fig. 14. Heat transfer in the volute versus the size of the machine 4. Conclusion Internal and external heat transfer induces a drop in the performance of gas turbines. This study shows that the performance of small turbomachines evaluated with the assumption of adiabaticity is not accurate. For a given operating point the mass flow and the compression ratio recorded on the maps and the calculated performance do not correspond to the actual characteristics when the machine operates with heat transfer. The assumption that heat losses represent 15 of the work of adiabatic turbines of which 60 is received by the compressor non-insulated leads to overestimating the power by 35 and the energy efficiency by 23 Insulation of the turbine although it seems to be a solution to maintain the operating characteristics of adiabatic turbines leads in fact to increasing the drop in performance. For the insulated version the net power is overestimated by 51 and efficiency by . In the absence of an adiabatic gas turbine ideal machine which provides the best performance we must avoid insulating the turbine in order not to decrease performance still further. To maintain the level of performance and in particular the net power produced by the gas
