Compressors are essential in the processes required for pressurizing and transporting gases and liquids in industry, aerospace applications, and civil and military fields. It is one of the basic equipment of the heating, ventilation, and air conditioning systems for houses and other structures [
1]. Compressors can be classified into two basic types: positive displacement and dynamic. Positive displacement compressors are divided into four: piston, screw, vane, and lobe compressors. Dynamic compressors are divided into two: axial and radial compressors [
2]. Radial compressors increase the kinetic energy of the fluid with the help of a rotating impeller [
3]. Many design optimizations and numerical analyses have been used to improve compressor aerodynamic designs and increase their performance [
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10]. Hadavandi et al. aimed to improve compressor CFD by providing a complete set of input data for numerical simulations [
11]. Shaban aimed to improve radial compressor performance by optimizing the design of the radial bladeless diffuser. It was proposed, investigated, and numerically optimized to be two bladeless diffuser geometries. The main goal of the optimization was to increase the pressure coefficient and minimize the diffuser loss coefficient. The diffuser reduced the loss coefficient by up to 10 percent and increased the pressure coefficient by 3.8 percent. He performed the simulations by solving the Reynolds-averaged Navier–Stokes equation under 2D axisymmetric conditions [
12]. Based on the ANSYS CFX Version 18.0 workbench finite element analysis software, Sun et al. investigated the numerical simulation of the fluid–solid coupling of pipeline radial compressor impeller by the unstructured grid finite volume method and finite element method. They numerically analyzed the equivalent stress distribution caused by the radial load of the impeller, and the coupling effect of radial load and aerodynamic load [
13]. Xu et al. used a low-flow, single-stage centrifugal compressor with a bladeless diffuser to investigate the position effects of the splitter blade between the two main blades. Splitter blade position optimizations were carried out numerically. They stated that the splitter blade positions are influential on the compressor stage performances. They stated that the traditional splitter blade located in the middle of the two main blades is not the most suitable position for aerodynamic performance [
14]. Li et al. examined the mathematical model of the flow field of the radial compressor, which is one of the basic components of the turbocharger. They stated that when the number of grids in a single channel is over 300,000, the increase in the number of grids has little effect on the compressor performance. They stated that the numerical simulation method they used could be used to predict compressor performance, and the total pressure ratio difference between calculation and test was less than 7% [
15]. Aghaei et al. aimed to show how a good compressor can be designed and modeled with CFD stable models and to explain the reasons for the discrepancies between experiment (1D design) and 3D CFD analysis. They found a good match between CFD and one-dimensional data for a radial impeller with a pressure ratio of approximately 4:1. They stated that, in general, CFD gives a good estimate of performance and adequately resolves local flow details [
16]. Roberts et al. performed a parametric study of the clocking between the TB inducer/exducer to investigate its impact on performance and output stream quality. They showed that the relative environmental position or clock setting between the inducer/exducer has a remarkable effect on TB performance. They noted that the maximum change in operating blade efficiency estimated for various TB configurations was approximately 3.8 [
17]. Cuturi et al. designed and simulated a TB radial compressor for hybrid compound turbocharging. They developed the configurations through a series of improvements guided by the design of experiments (DOE). Full 3D CFD simulations were performed at three different speeds: 2000 rpm, 3500 rpm, and 5500 rpm, respectively. They noted that the final geometry had a higher pressure ratio than the original unit, an improved surge margin, and a lower sensitivity to choke [
18]. Noman et al. present a comprehensive experimental and numerical study on the performance of a medium pressure ratio, shrouded, TB centrifugal compressor compared to a conventional compressor used commercially in China for turbocharging applications. They stated that in all tandem design cases, the surge point shifts towards lower mass flow rates. They stated that a maximum increase of 25 percent was observed in the study range. They emphasized that when a 20 percent reduction in inducer thickness was made, it performed better than conventional designs [
19]. Ju et al. used the CFD method to examine the specific aerodynamic performance of the TB. They said that compared to a single-row blade, a tandem row has the potential to produce a higher pressure ratio with lower losses over a relatively narrow operating range [
20]. Josuhn-Kadner studied the effect of TB geometry on rotor and stage characteristics. He performed measurements at nine points all around the rotor, taking into account three different environmental inducer positions. He stated that the improvement achieved by the TB configuration in the rotor and stage characteristics was slight. Still, there were major differences in the flow field at the rear and outlet of the rotor [
21]. Josuhn-Kadner et al. have mainly investigated experimentally for radial compressor aerodynamic stage optimization. They developed 3D Navier–Stokes calculations in the design to analyze the flow field. They said the inducer setting has a negligible effect on the rotor characteristics. They stated that the maximum rotor efficiency of 93.5% varies within less than 1% depending on different positions of the inducer [
22]. Li et al. improved the compressor performance in highly loaded transonic radial compressors with a TB impeller configuration with rule-surface inducer and exducer blades. Both the impeller and the diffuser achieved a compressor efficiency of 1.4% by reducing flow loss. They stated that using a forward sweep and negative lean design for the impeller, depending on the tandem impeller configuration, would increase efficiency by 2.11 percent compared to a conventional compressor. They said that diffuser performance can be improved with a negative lean design [
23]. Li et al. first examined the flow characteristics of the conventional impeller to determine the causes of compressor instability. Then, they examined the TB impeller and showed the performance of different impellers. They particularly emphasized the variation in impeller tip leakage flow and impeller leading edge separation flow as the mechanism underlying the compressor operating range [
24]. Sadagopan et al. described the mean-line procedure based on isentropic equations for the mixed flow case. They conducted rotor design evaluation studies for 3.5 kg/s mass flow by performing mean-line code and computational analysis based on Bezier curves. They stated that a higher hub load would result in a higher performance of the impeller with a lower tip leakage loss and regular flow at the outlet [
25]. Sadagopan et al. computationally evaluated a supersonic mixed-flow compressor stage with a high-pressure ratio of 6:1 and an efficiency of 75.5%. They carried out the analysis by taking into account the effects of three-dimensionality, viscous flow, and compressibility. They described a new tandem design supersonic diffuser. They stated that this new design outperformed previous supersonic diffusion configurations by 20% in terms of efficiency [
26]. Cravero et al. simulated a two-stage back-to-back radial compressor for refrigerant applications using computational fluid dynamics techniques at operating points close to the surge point. They confirmed the numerical results with experimental results [
27]. Zhu et al. carried out CFD analysis on the optimization of low- and high-pressure radial compressors in a land-use MW-level gas turbine. They increased the pressure ratio and efficiency by reducing the blade diffuser diameter ratio for the low-pressure radial compressor. They stated that they reduced the deviation angle by using a diffuser with splitter blades for the high-pressure radial compressor and thus increased its performance [
28]. Various numerical analysis studies have been carried out to improve the stall margin of the axial compressor [
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In this study, optimization studies for some parameters of the TB radial compressor were carried out numerically. It has been observed that there is a lack of studies on the optimization of these parameters in the literature. The main goal of this paper is to investigate the effect of input parameters determined on pressure–compression ratio and efficiency in tandem blade radial compressors. To ensure the accuracy of the numerical analyses in the optimization studies, the results of the conventional blade centrifugal compressor, whose test results were available in the literature, were examined and confirmed with numerical analyses. After ensuring that our analyses worked, the numerical calculations of the TB centrifugal compressor were performed. The main goal of the optimization studies was to increase the efficiency and total pressure ratio. This goal was achieved with three optimum designs obtained from the analysis results.