# Analysis and Improvement of a Two-Stage Centrifugal Compressor Used in an MW-Level Gas Turbine
## 1 Introduction  
As pressure-generator devices, centrifugal compressors have been applied in the industry since the 19 th century [1]. Centrifugal compressors are commonly used when the flow rate is low, typically 1-4 kg/s. Experimental research on the application of centrifugal compressors to gas turbines began in the early 20 th century. In 1903, [PERSON] [2] successfully built a gas turbine that could transmit power outward. The real motivation for research on centrifugal compressors was the application of these machines in flight propulsion. From 1928-1941, [PERSON], a British engineer, and [PERSON], a German physicist, independently developed the world's first jet engine. They adopted a centrifugal compressor for the selected air compressor device. Gas turbines with centrifugal compressors are frequently used in the engines of tanks or small aircraft due to their limited through-current capacity. The pursuit of a compact engine and a high thrust-to-weight ratio promoted the development of centrifugal compressors decades after the invention of jet engines. In the past60 years, many scholars have investigated centrifugal compressors and obtained a deep understanding of the internal flow phenomenon. In 1976, [PERSON] [3] measured the jet-wake phenomenon in the outlet of a centrifugal compressor with a pressure ratio of 2.1. [PERSON] [4] also measured a centrifugal compressor with a pressure ratio of 4. [PERSON] [5] performed a 3D numerical simulation of a centrifugal compressor. [PERSON] [6; 7] measured the shock wave in the inlet of a centrifugal compressor that had an inlet tip Mach number of 1.3 and a pressure ratio of 6.1. [PERSON] [8] conducted a numerical simulation of this centrifugal compressor. Meanwhile, [PERSON] [9] measured the shock wave and pressure distribution in the inlet of a centrifugal compressor with a pressure ratio of 10. [PERSON] [10; 11] measured a centrifugal compressor with a pressure ratio of 11 and analyzed its flow characteristic. [PERSON] [12] investigated the effects of the radial gap ratio on a high-pressure ratio centrifugal compressor and the flow phenomena inside the compressor components by using numerical simulations. [PERSON] [13] analyzed the matching of the vaned diffuser with the impeller for different working conditions. [PERSON] [14] investigated the flow instability of centrifugal compressors with vaned diffusers experimentally and presented diverse instability patterns and transient behavior in detail. [PERSON] [15] investigated the instability mechanisms of a high-speed turbocharger centrifugal compressor with a vaneless diffuser using the unsteady simulation method. Figure 1 shows the development trend of centrifugal compressors for commercial use during the past few decades. The pressure ratios of single-stage centrifugal compressors have improved continually, and centrifugal compressors with single-stage pressure ratios over 10 have been put into practical use. The required pressure ratio for a tank or helicopter engine is between 8 and 12, and the efficiency is approximately 80%. Centrifugal compressors with multi-stage low-load design are frequently adopted to obtain high efficiency and low cost. Considering that the ratio of a single-stage compressor cannot meet this requirement, gas turbines mostly adopt a two-stage centrifugal or multi-stage axis compressor with a single-stage centrifugal compressor, as shown in Figure 2. For example, the TPE331 turboprop engine developed by Garrett Al in 1959 uses a two-stage centrifugal compressor with a flow mass of 3.49 kg/s and a pressure ratio of 10.8 [16]. The Light Helicopter Turbine Engine Company [17] developed a high-performance two-stage centrifugal compressor for the T800-LHT-800 helicopter  
Figure 1: Development trend of centrifugal compressors: (**a**) Development trend of total pressure ratio; (**b**) Relation between pressure ratio and efficiency. engine. Its design flow mass, compression ratio, adiabatic efficiency, and surge margin are 3.3 kg/s, 14:1, >80%, and >15%, respectively.