Numerical and Experimental Study of the Impeller of a Liquid Pump of a Truck Cooling System and the Development of a New Open-Type Impeller

Typically, closed-type impellers are more efficient than open-type impellers, but in the manufacture of closed-type impellers, cost of wheels is higher. This paper describes the development of cost-effective and simple impeller wheel for a fluid pump in the truck cooling system. To perform this task, the numerical computations of a standard impeller wheel were carried out, its characteristics were also obtained from a test bench, the standard impeller wheel model was verified. The open-type impeller wheel was developed according to the current dimensions of standard impeller wheel and then analyzed with the numerical computations by the software ANSYS CFX (Academic license) computational fluid dynamics. The developed open-type impeller wheel works very effectively in spite of performance degradation by 5% in comparison to the closed-type impeller wheel. When working as a part of engine, the pump efficiency is 0.552-0.579. The maximum value of the pump efficiency is 0.579, it can be achieved at the highest speed of the pump (4,548 rpm and 655 l/min).


INTRODUCTION
Reducing CO2 emissions is an urgent task today. Transport is currently responsible for around a quarter of total greenhouse gas emissions, with road transport representing 17.8% of total emissions, arising from the use of vehicles. The European Automobile Manufacturers Association (ACEA) program has been developed to reduce CO2 emissions from trucks by 7% by 2025 and 16% by 2030 [1]. One way to reduce CO2 emissions is to use highly efficient engines. At present, Kama Automobile Plant (KAMAZ PJSC manufacturer located in Naberezhnye Chelny, Russian Federation) is actively developing highly efficient engines, one of which is the 6-cylinder diesel engine to be launched in the nearest future. This engine has reduced CO2 emissions due to its high power density and efficiency. The increasing power leads to the additional requirement to the cooling system. Today, it is planned to use a pump impeller with a cover disk (closed-type). The closed-type impeller wheel has two disks with blades in between. It creates a good pressure and is characterized by small leaks of water from the exit to the entrance. These impellers are produced in several ways: stamping, casting, spot welding or riveting. Despite the high efficiency, the pump impeller with the cover disk is more difficult to manufacture, which affects the cost of the finished product.
Another area to reduce CO2 emissions is to reduce emissions from truck production. Commercial vehicle manufacturer MAN Truck & Bus (Germany) has reduced the CO2 emissions of its production facilities by more than one quarter [2]. The use of a simpler impeller technology for the engine cooling system is also one of the ways to reduce CO2 emissions, which will also reduce the impeller cost.
Therefore, the main goal of this work is developing the cost-effective and simple impeller wheel with the same high efficiency and pressure. The goal of this engineering research is to design the impeller wheel without the cover disk based on the standard impeller wheel geometry with its external dimensions kept unchanged.

Research Model
The new open-type impeller wheel was developed according to the methodology generally accepted in Russia [3], which is also in line with the methods accepted worldwide for calculating impeller machines described in [4]. According to the set dimensions, this methodology allows to calculate angles of the flow entry and flow exit to the impeller, as well as the wheel thickness at the entry and exit points. The methodology of numerical calculation is similar to [5].
Determination of the main parameters of the blade according to the method given in [1]: The initial angle of the flow on blades: The original closed-type impeller wheel is shown in Fig.  1 a), and the modified open-type impeller wheel calculated according to the methodology above is shown in Fig. 1 b).

Creating a Mesh
The mesh is created based on the domain section in the mesh generator software -Ansys Meshing. The hybrid, unstructured mesh consists of tetrahedral and prismatic elements. The prismatic boundary level also consists of seven layers for correct modelling the hydrodynamics of the near-wall flow. Fig. 2 depicts the mesh generated in Meshing software.
The computational model of the liquid pump is created in the Ansys CFX pre-processor based on the generated mesh. The computational model consists of 3 rotor domains and 2 stators coupled through "FLOW -FLOW" interfaces.
The following boundary conditions are determined for this model: -Working medium is water -Environmental pressure is 101325 Pa -Water temperature is 25 °C -Outlet volumetric flow rate corresponds to the pumpworking mode (Tab. 1) -Atmospheric inlet pressure -The angle speed of the domain rotation corresponds to the design mode (Tab. 1).
The work modes of the liquid-ring vacuum pump for researching into its hydrodynamic characteristics are listed in Tab. 1. In general, numerical studies of impeller wheels are carried out using k-e turbulence models with Fluent [6,7] and CFX [8,9], but this k-e model does not ensure the required calculation accuracy. The k-w turbulence model [10,11] is also used, which also gives discrepancies [12]. The greatest accuracy is ensured by the Shear Stress Transport (SST) k-w turbulence model [13][14][15] with the accuracy confirmed by a Particle image velocimetry (PIV) study of the flow structure in [4,16,17]. The convergence of CFD calculation and experimental studies is up to 10% [18], but the SST k-w model allows obtaining high convergence of calculation of 5.1% [6,19] and 3% [20]. In the model creation stage, the semiempirical model of the k-omega SST turbulence was chosen, since the k-omega SST provides an adequate description of the near-wall turbulence and sensitiveness to the border conditions in the external flow. The authors already have experience in performing similar work when designing a centrifugal pump [21] and modeling heat transfer and hydrodynamics [22], where the k-omega SST model also showed good accuracy.

EXPERIMENT
The standard and new impeller wheels were tested for verification of the numerical model. Figure 4 shows the principal hydraulic scheme of the experimental plant.
The experimental plant provides flow rates from 0 to 800 liters per minute.

Standard Impeller Results
The results of numerical modelling determined integral hydrodynamic parameters of the standard impeller wheel in the different work modes: water streamlines in the pump (Fig. 5), velocity vectors in liquid pump section plane (Fig.  6), velocity profile in liquid pump section plane at mode (Fig.  7) and pressure profile in liquid pump section plane (Fig. 8). The flow structure at the output to the pump is uniform. At the output, you can notice a swirl of the flow, but without obvious flow interruptions, which affects the pump operation.
The cross-section of the pump also shows welldistributed velocity vectors without flow interruptions, but the velocity field shows that flow may disrupt at the end of the blades, which can cause additional flow turbulence and hydraulic losses.  The pressure field shows a uniform pressure increase without obvious flow separations.  In addition, the engine hydraulic characteristic (EHC) was numerically determined. Fig. 9 and Tab. 4 show the comparison results of the numerical calculations and experimental studies. Here we see a good convergence of the numerical and experimental studies results not exceeding 9% in the engine operating modes, which is quite acceptable for this type of work. No further refinement of the numerical model is required.

Modified Impeller Results
Verification of the numerical study and experimental results of the water pump with a standard wheel show the minor error of the computational model. Therefore, this model may be used to test the modified wheel.
The results of numerical modelling determined integral hydrodynamic parameters of the modified impeller wheel at different work modes: water streamlines in the pump (Fig.  10), velocity vectors in liquid pump section plane (Fig. 11), velocity profile in liquid pump section plane (Fig. 12) and pressure profile in liquid pump section plane (Fig. 13).
The flow structure at the pump input is uniform. At the output, you can notice a swirl of the flow without obvious flow interruptions, which affects the pump operation.
Unlike a standard impeller, the speed between the blades of the modified impeller is uniform, which means a more cavitation-resistant design.
The pressure in the modified impeller is increasing even more evenly than that in the standard impeller, which also indicates a good design.
According to the results of the numerical study, the characteristics of the modified impeller are better in the entire range of operation of rotors and fluid flow rates.

Experimental Results
After manufacturing the plastic modified impeller using the FDM (fused deposition modeling) 3D printing method, it was used for the experimental studies, the results of which are presented in Fig. 15. Fig. 15 shows a degradation in the characteristics of the modified impeller with an increase in fluid flow, which is possibly associated with the manufacturing technology and an uneven surface that causes additional turbulence.

DISCUSSION
The verification of the numerical computations with experimental results showed that at the low-pressure values the design values and the experimental values are the same. Based on the results depicted in Tab. 4, the average relative error of the computations is 8.2%. The accuracy of the calculations is a little higher than that in the works [6,19,20], but acceptable, and no further refinements of the numerical model are required.
The verification of the numerical computations with experimental results in the engine assembly showed that EHC and the average relative error is the same and equal to 4.45% (Tab. 4).
The investigated liquid pump is highly efficient in the engine assembly. Its efficiency ranges from 0.552 to 0.579. The maximum efficiency value is 0.579 at 4558 rpm and 655 l/min flow rate. These efficiency characteristics are acceptable for open-type impellers, and in our work they are even better than those in the article [4].
The numerical modelling allows to understand the flow distribution inside the pump and how the velocity and pressure (static and absolute) distribute at steady-state and unsteady-state water flow in the flow part. Obviously, taking into account the manufacturing method of FDM 3D printing, these data could not be obtained in the experimental tests, and it is necessary to choose a more expensive, but high-quality printing technology like SLA.
The computational pump model created in Ansys CFX software has been verified successfully. Consequently, it has proved that the software could be used for numerical computations. Numerical tests in the Ansys CFX software allows to optimize the impeller wheel, flow part of the pump, flow part, scroll for improving the 920.10-700 KAMAZ R6 engine pump. As a result, it is possible to improve the power capacity and steady operation of the cooling system of the 920.10-700 KAMAZ R6 diesel engine. In addition, it allows reducing the time and cost of material resources required for creating and optimizing the elements of the engine cooling system.

CONCLUSION
The study conducted in this paper suggests the possibility of replacing a closed-type impeller with an opentype wheel with a slight loss of efficiency. The main disadvantage of an open wheel is a low efficiency of about 40%. Closed impellers are 10-15% more efficient than opentype impeller wheels, but also have a higher manufacturing cost. In this work, an open-type impeller wheel was developed directly under the required flow rate and head, which reduced the performance decrease by 5% and overall efficiency is about 57% at nominal flow rate. The manufacture of an open-type impeller is much simpler, simple forms can be used, which also reduces energy costs for the manufacture of molds. The closed-type impeller must be made by casting according to the investment casting only and has complex rods. Starting to use the open-type wheels will significantly reduce CO2 emissions in wheel casting in the truck production. As for the cost, the open-type wheel is approximately 2 times cheaper.