# Design and Additive Manufacturing of TPMS Heat Exchangers
## 3 Additive Manufacturing of Gyroid HXs
### Performance Test of Gyroid HXs  
After checking and assuring that there were no water leakages for the fabricated HXs, connectors were made for thermal testing. As shown in Figure 22, four ports of the HX were threaded manually with a 3% NPT male thread. Four adapters were used to covert the male thread to a female thread. Rubber rings were installed on each adaptor in order to prevent water leakage. Figure 21: Relative density measurement results at four randomly selected locations. Figure 20: Gyroid wall thickness and relative density measurement results. A result of 98% relative density and 500 \(\upmu\)m thickness was achieved, with an excellent uniformity control. Figure 23 shows the schematic of the experimental setup. The heat exchangers being tested were mounted vertically on a metal plate. Water pressure was measured by pressure transducers that were tapped into the pipe located four-pipe-diameters upstream to the heat exchanger inlet and ten-pipe-diameters downstream to the heat exchanger outlet. Water temperatures were measured using T-type thermocouples mounted through a pipe elbow in the center of the upward pipe. The water volumetric flow rate was measured by using paddlewheel flowmeters located downstream of the heat exchanger, with a straight pipe length of twenty-pipe-diameters upstream to the flowmeter and ten-pipe-diameters downstream to the flowmeter. While hot water was generated using a gas-fired boiler with a nominal input capacity of 58.3 kW, cold water was generated using an air-cooled chiller with a nominal capacity of 10 ton. Figure 23 shows the target testing conditions. The experimental test was conducted in accordance with ANSI/ AHRI Standard 400 (I-P). The experimental data were considered to be valid only if the heat balance was within \(\pm\)5%. Errors in the tested heat capacity caused by the inaccuracy of the sensor can be within \(\pm\)1.2%, which is relatively low. It should be  
Figure 22: HX assembled with adaptors. Figure 23: Schematic diagram of the HX testing setup (T means temperature sensor and P means pressure sensor). noted that some combined conditions of flow and hot side inlet temperature were found to be unachievable in the testing process due to constraints in heating equipment capacity and pumping power in the testing laboratory. Table 8 were the testing conditions including the temperature and flow rate at the inlet of cold side and hot side. The hot side heat transfer rate \(\dot{q}_{hot}\) was calculated as  
\[\dot{q}_{hot}=\dot{M}_{hot}\times Cp_{hot}\times(T_{hot,in}-T_{hot,out}) \tag{3}\]  
where  
\(\dot{q}_{hot}\) = hot side heat transfer rate, kW;  
\(\dot{M}_{hot}\) = hot side water volumetric flow rate, kg/s;  
\(Cp_{hot}\) = hot side water specific heat, J/(kg*K);  
\(T_{hot,in}\) = hot side water inlet temperature, K;  
\(T_{hot,out}\) = hot side water outlet temperature, K.  
The hot side heat transfer rate \(\dot{q}_{cold}\) was calculated as  
\[\dot{q}_{cold}=\dot{M}_{cold}\times Cp_{cold}\times(T_{cold,out}-T_{cold,in}) \tag{4}\]  
where  
\(\dot{q}_{cold}\) = cold side heat transfer rate, kW;  
\(\dot{M}_{cold}\) = cold side water volumetric flow rate, kg/s;  
\(Cp_{cold}\) = cold side water specific heat, J/(kg*K);  
\(T_{cold,in}\) = cold side water inlet temperature, K;  
\(T_{cold,out}\) = cold side water outlet temperature, K.  
The water density and specific heat were determined using the average of inlet and outlet water temperatures. The average heat transfer \(\dot{q}_{average}\) was calculated as  
\[\dot{q}_{average}=\frac{\dot{q}_{hot}+\dot{q}_{cold}}{2} \tag{5}\]  
The heat balance (\(HB\)) was calculated as  
\[HB=\frac{\dot{q}_{hot}-\dot{q}_{cold}}{\dot{q}_{average}}\times 100\% \tag{6}\]  
The heat transfer performances were tested and the results are shown in Figure 24. The summary of the testing results is listed in Table 9. They match the modeling results. This shows that the HX70-555 (5 mm lattice) has a significantly high HTC, thanks to its operating in high turbulent mode.