# Safe and Sustainable Design of Composite Smart Poles for Wireless Technologies
## 3 Results
### Thermal Management and Multiple Radio Analysis  
The CFD analysis process was divided into: (1) CFD investigations, (2) feasibility study of 3D printed heat exchangers, and (3) experiments on a 3D printed heat exchanger operating at 200 W. The air-cooling systems by standard plane and pin-fin designs led to heat exchangers with a weight of 3.5-3.8 kg per piece and their volume was overly large compared to the space available inside the demo 5 GPs. With room temperature air, the system's maximum surface temperatures remained below 51 \({}^{\circ}\)C that is substantially below the 65 \({}^{\circ}\)C critical allowable temperature. Concerning the CFD simulations, the constant surface temperature-boundary condition was noted to be misleading as: (1) the surface temperature is not known, and (2) the inserted power cannot be fixed. In the experimental validation, the simulated and experimental velocity fields (using Laser Doppler Anemometry) agreed well. Conjugate heat transfer (CHT) studies are proposed as the next design steps of the design so that temperature transport in both fluid and the solid can be accounted for.  
One of the key drivers for a cooling concept is the compact size and centralized thermal control at an entire 5 GP site. A liquid cooling was a tempting option to be analyzed since the density ratio of water to air is 1000:1 while the specific heat ratio is 4:1. Furthermore, water is much better heat conductor than air with the heat conductivity ratios of approximately 60:1. Motivated by the experiences from the air cooling investigations, we utilized the CHT solver in OpenFOAM called chtMultiRegionFoam. With this procedure, the incompressible Navier-Stokes equations were solved for the fluid phase while the convection-diffusion equation was solved for temperature both in the solid and fluid.  
Figure 12: Measured attenuation in terms of DL; results over the 72-day accelerated UV-ageing period.  
Based on the results, the walls could be made thinner (e.g., 1 mm) to result in a less than 200 g mass of the exchanger while the tested exchanger version (wall thickness 2 mm) had a \(\approx\)393 g mass. It was confirmed by simulations that the system can maintain surface temperatures below 65 \({}^{\circ}\)C for a range of mass flow rates for cooling water's inflow temperature below 50 \({}^{\circ}\)C (see Figure 13). Figure 13 shows the inlet temperature of the cooling water as a function of the mass flow rate that maintains the heat exchangers' heated surface temperatures at 65 \({}^{\circ}\)C. The serial and parallel configurations indicate whether the heat exchangers (inside) of the 5 GP would be connected in series or in parallel to cool all the four radios. The highest allowed inlet water temperature for the serial and parallel configurations of liquid cooling heat exchanger is based on the assumption that the temperature difference between the inlet water and the heated surface is independent of the absolute temperature--based on the mass flow rate sensitivity analysis that has confirmed the independency of transfer performance on the inlet Reynolds number [33]. Furthermore, the functionality of the heat exchanger was tested experimentally, and the measured surface temperatures were in a good agreement with the CFD results.