# CFD and Experimental Comparison for Micro-Pump Performance in Space Applications: A Case Study
## 2 Materials and Methods
### Design Considerations  
Operating specifications of the pump are presented in Figure 2. The working fluid for this study was HFE-7200 (3M, USA) [33]. However, due to the environmental considerations and low availability of the HFE7200 working liquid, the experimental campaign was conducted using deionized water under similitude conditions. This substitution was also applied in the numerical simulations. While the use of deionized water is common in hydraulic testing, it is important to acknowledge the key differences in thermophysical properties between HFE-7200 and deionized water. At room temperature, HFE-7200 has a higher density (\(\approx\)1430 kg/m\({}^{3}\) vs. \(\approx\)997 kg/m\({}^{3}\) for water) and a lower dynamic viscosity (\(\approx\)0.65 mPa-s vs. \(\approx\)0.89 mPa s for water), as well as significantly lower specific heat capacity and thermal conductivity. These differences affect dimensionless parameters such as the Reynolds and Prandtl numbers, which influence flow behavior and heat transfer characteristics.  
In this study, similitude was achieved by preserving Reynolds number similarity in the experimental and numerical setups, thereby ensuring that inertial and viscous forces governing the flow were appropriately scaled. Although thermal similitude could not be achieved due to differences in specific heat and conductivity, this limitation is acceptable because the primary focus was on hydraulic performance rather than thermal effects. As such, deionized water remains a valid substitute for replicating the fluid dynamic behavior of HFE-7200 within the scope of this study.  
The closed impeller and volute geometries were pre-dimensioned using an in-house algorithm, which is based on classical pump theory and analytical formulations [33; 34]. This algorithm calculates key geometric parameters and point coordinates that can be directly exported for CFD analysis. Figure 2 serves as a logical flowchart outlining the decision-making process and fundamental parameters involved in the predesign of a centrifugal pump impeller. Its structure follows a sequential logic rooted in established turbomachinery design principles, as detailed in the cited literature.  
The predesign process is generally governed by the following key concepts:  
* Hydraulic and energy requirements: the process begins with defining the desired head and flow rate, from which hydraulic power and specific speed are determined.
* Selection of specifics (_Ns_): the specific speed guides the choice of impeller type (radial, mixed-flow, or axial) and influences the selection of suitable geometric relationships.  
Figure 1: Design and evaluation methodology for the AM pump components.  
* Velocity triangles and blade angles: blade angles at the inlet and outlet are estimated based on velocity triangles, ensuring efficient energy transfer and proper flow turning.
* Dimensioning of main geometry: core geometric features such as impeller diameter, blade height, and blade number are derived using empirical correlations and design charts, ensuring both hydraulic performance and structural feasibility.
* Loss considerations and corrections: empirical correction factors are incorporated to account for hydraulic losses, slip effects, and blockage, refining the initial estimates.