Three sets of transient tests were experimentally carried out to confirm feasibility of the dynamic model. The uncertainties of the temperature and mass flow rate measurements in experiments were within
2 and
6%, respectively, at 95% confidence level. For the first case, the only variation introduced was the hot-side helium inlet temperature. For the second case, both the hot-side and cold-side helium inlet temperatures were varied. Three variations were simultaneously applied in the third case, namely, the hot-side helium inlet temperature, the cold-side helium inlet temperature, and the helium mass flow rate step changes on both sides. To perform temperature transient tests in the HTHF, the helium temperatures at the two electric heaters’ outlets can be alternated by changing the temperature controllers’ setting points. Adjusting the flow rate or inletpressure of the driving air for the gas booster or both would result in changes of the helium flow rate in the HTHF. Both operations were simultaneously applied to the third transient test. Note that all the helium mass flow rates for transient simulations were within the range of the steady-state experimental conditions, which means that the application range of the obtained heat transfer correlations covers all the experimental conditions for both the steady states and transients in the current study.
a. Case 1: Change in the hot-side helium inlet temperature
Tests with the helium temperature step changes at the heat exchanger inlet could not be readily carried out in the HTHF due to the limited heater capability and the facility configuration. The helium mass flow rates were controlled by a gas booster and helium temperatures were governed by the two electric heaters. Although a helium temperature step increase or decrease was not readily achievable, certain helium temperature variations could still be introduced and captured during experiments via the data acquisition system. After the PCHE reached a steady-state condition, helium temperature variations at the inlet of the PCHE hot side were introduced at around 850 seconds by controlling the main-heater power via the temperature controller in the HTHF. During this transient test, the cold-side helium inlet temperature and mass flow rates on both sides were kept constant at about 94.5 °C and 30.5 kg/h, respectively. The fixed cold-side helium inlet temperature can be achieved by maintaining the pre-heater power while bypassing the straight-channel PCHE (upstream of the zigzag-channel test PCHE) in the HTHF as shown in Figure 3-2. Once the straight-channel PCHE was bypassed, the zigzag-channel PCHE cold-side helium inlet temperature was controlled by the pre-heater and the hot-side helium inlet temperature
by the main-heater. A polynomial function was used to fit the measured hot-side helium inlet temperature curve as shown in Figure 5-27 and severed as an input in the dynamic model. The helium outlet temperatures on both the hot and cold sides from the numerical simulation were compared to the experimental data, as shown in Figure 5-28. It was observed that the numerical results predicted the experimental results well. The numerical simulation on the cold side slightly under predicted the experimental value during the initial steady state. After having reached the next steady state, the hot-side numerical and experimental results gave good agreement. Compared to the experimental results, the PCHE helium outlet temperature differences for the initial and final steady state had deviations of 10.1 and 6.5%, respectively.
Figure 5-28. Comparisons of the helium outlet temperature profiles in Case 1: Measurements vs. predictions.
b. Case 2: Change in both the hot-side and cold-side helium inlet temperatures
In the second transient test, the straight-channel PCHE in the HTHF was not bypassed. Since the two PCHEs (i.e., the straight-channel PCHE and the zigzag-channel PCHE) were coupled in the HTHF, as shown in Figure 3-2, the cold-side helium inlet temperature for the zigzag-channel PCHE would increase if its hot-side helium inlet temperature is increased. Unlike the first case, two fitted functions for both helium inlet temperatures of the zigzag-channel PCHE monitored in the experiment were implemented in the dynamic model to simulate the PCHE response. The fitted polynomial curves followed the experimental data well for both the hot-side and cold-side helium inlet temperatures, as can be seen in Figure 5-29. From the figure, the hot-side helium inlet temperature increased from 767.0 to 802.0 °C and was maintained at 802.0 °C for more than an hour. Comparisons of the helium outlet temperatures obtained from experiments and numerical
simulations are shown in Figure 5-30. The cold-side outlet numerical results predicted the experimental data very well with an over-prediction of 0.2 °C for the final steady state. Due to the heat loss from the heat exchanger surfaces, the numerical model over predicted the hot-side experimental results by about 9.0 °C for the initial steady state and by 2.5 °C for the final steady state.
Figure 5-29. Measured hot-side (left) and cold-side (right) helium inlet temperature profiles for Case 2 and their fitted curves.
Figure 5-30. Comparisons of the helium outlet temperature profiles in Case 2: Measurements vs. predictions.
c. Case 3: Helium mass flow rate step change with helium inlet temperature variations
For the third transient test, three variations were simultaneously applied, namely, the hot-side helium inlet temperature, the cold-side helium inlet temperature, and the helium mass flow rate step changes on both sides of the heat exchanger. The hot-side helium inlet temperature was increased from 724.0 to 768.7 °C. As mentioned, the cold-side helium inlet temperature would change accordingly due to the two coupled PCHEs in the HTHF. Two fitted polynomial functions to the experimental data of both the hot-side and cold-side helium inlet temperatures are shown in Figure 5-31. Simultaneously, the helium mass flow rate was subjected to a step decrease from 35.3 to 31.8 kg/h at around 700 seconds and then a step increase from 31.8 to 34.7 kg/h at about 4,700 seconds, as shown in Figure 5-32. There are some oscillations in the helium mass flow rate measurements, but the standard deviation of each operated helium mass flow rate stage was less than 2%
As can be seen from Figure 5-31, the response of the cold-side helium inlet temperature is slower than that of the hot-side helium inlet temperature variations. This is mainly attributed to the thermal inertia of the test loop and the fluid flow lengths in the loop to reach the hot-side inlet and cold- side inlet locations. As can be seen from Figure 3-2, the helium gas was heated in both the pre- and main- heaters. The helium temperature variations in the zigzag-PCHE hot-side inlet were mainly caused by the main heater. The helium flow path is from the zigzag-PCHE cold-side outlet to its hot-side inlet through the main heater. The helium temperature variations in the zigzag-PCHE cold-side inlet are mainly caused by the pre-heater and the straight-channel PCHE at the low- temperature side of the HTHF. The helium gas travel path is from the zigzag-PCHE hot-side outlet,
cooler, and the gas booster, passing the pre-heater and straight-channel PCHE, to the zigzag-PCHE cold-side inlet, which is much longer than that from the zigzag-PCHE cold-side outlet to its hot- side inlet. Therefore, the helium gas travel path from the zigzag-PCHE hot-side outlet to its cold- side inlet has a significantly higher thermal inertia, which in turn results in a much slower transient response for the zigzag-channel PCHE cold-side inlet. The comparisons between the numerical results and experimental data indicated that there were some discrepancies at the initial steady state conditions, however, the numerical results followed the experimental trends well and the two were in good agreement for the final steady state, as shown in Figure 5-33. Compared to the experimental results, the PCHE helium outlet temperature differences for the initial and final steady state had deviations of 6.5 and 2.6%, respectively.
Figure 5-31. Measured hot-side and cold-side helium inlet temperature profiles for Case 3 and their fitted function curves.
Figure 5-32. Measured helium mass flow rate step changes for Case 3.
Figure 5-33. Comparisons of the helium outlet temperature profiles in Case 3: Measurements vs. predictions.
Based on the above three experimental transient tests, the close agreement of the numerical results with the experimental data was achieved. Although there were some discrepancies, it is seen that the numerical solutions are sufficiently accurate, and that feasibility of the dynamic model
developed for predicting the steady-state and transient performance of the reduced-scale zigzag PCHE is confirmed.