Performance prediction model of contra-rotating axial flow pump with separate rotational speed of front and rear rotors and its application for energy saving operation
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| De ZHANG Torishima Pump Mfg. Co., Ltd. |
Yusuke KATAYAMA Waseda University |
Satoshi WATANABE Kyushu University |
Shin-Ichi TSUDA Kyushu University |
Akinori FURUKAWA Kyushu University (Professor Emeritus) |
Abstract
Turbomachines account for a large portion of energy consumption including electricity as well as of energy production, and therefore, even small improvements in performance and efficiency can make a significant contribution to carbon neutrality. The contra-rotating axial flow pump (Fig. 1), consisting of front and rear rotors rotating reversely, has the advantages of smaller size (with the absence of guide vanes) and higher suction performance (with lower rotating speed) than conventional axial flow pumps[1]. Moreover, the performance can be improved by controlling the rotating speeds of front and rear rotors separately[2]. Since the pumps are generally operated in a wide range of flow rates other than the design flow rate depending on the demands, a significant reduction in energy consumption is expected by rotational speed control of the rotors. However, to achieve this, it is necessary to find the optimum combination of front and rear rotor speeds for each operating flow rate, and it is essential to develop a fast performance prediction method that does not rely on costly experiments nor numerical simulations.
In this paper, a low-cost steady-state RANS (Reynolds-averaged Navier-Stokes) analysis for single passage of the both rotors, which is usually performed at the design stage, was firstly performed for the design speed operation. Then, based on the results, a fast and effective performance prediction model for front and rear rotors was developed by considering radial equilibrium conditions at each rotor inlet/outlet, rotalpy and mass conservation laws, empirical laws of deviation angle, blade-blade interaction, and a loss model (Fig. 2). Detailed information can be found in the original paper[3]. The model was constructed for our previously designed contra-rotating rotors, and the comparisons with an experiment under rotational speed controls showed the effectiveness of the model quantitatively over a wide range of flow rates.
Then, the model was constructed for another contra-rotating rotors, and the optimum rotating speed combination of rotors to maximize the system efficiency (the ratio of final output power to the input power) was investigated for two kinds of pipe system resistances to show the effectiveness of separate rotational speed control of rotors in terms of energy saving. Here, one of the results is shown in Fig. 3. Comparing the result of the rotational speed control (RSC) with the conventional valve control (Valve), the system efficiency is found to be significantly improved by the rotational speed control (Fig. 3(c)) with satisfying the required head (Fig. 3(b)). The optimum rotational speed combination can be found in Fig. 3(a).
Although the results of this study were obtained for specific machine, contra-rotating axial flow pump, the idea of constructing performance prediction model can be applied for other turbomachines and is expected to be applied not only to high efficiency operation control but also to control/suppress the unstable flow. The model can also be applied for the design of custom-order machine with taking account of the pipeline system and operation control in advance.
Keywords
Contra-Rotating Axial Flow Pump, Rotating Speed Control, Performance Prediction Model, CFD, Energy Saving
Figures
Fig. 1: Contra-rotating axial flow pump.
Fig. 2: Construction of performance prediction model of contra-rotating axial flow model. The performance under rotational speed control of rotors is predicted by three steps each considering the flow models and physics.
Fig. 3: Example of the application of performance prediction model to predict the optimum rotating speed control against a typical system resistance for energy saving. (a) Optimum rotational speed combination of rotors control, (b) head and pump efficiency and (c) the system efficiencies for rotational speed control (RSC) and valve control (Valve).