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Military heat exchanger tracks Coolit predictions

Courtesy of CAS Ltd.

CAS Ltd. was to design a cooling system for a 250 Watt computer built for major aircraft manufacturer. The computer, shown in Figure 1, was housed in an aluminum chassis measuring 265 mm wide x 200 mm high x 365 mm deep. The chassis exterior was cooled by natural convection and radiation. The chassis interior held 12 cards that were fastened to a heat exchanger by Calmark wedge-locks. The plate-fin heat exchanger consisted of 31 horizontal fins mounted on each side of the chassis. The heat exchanger was cooled by forced airflow from the fan, as shown in Figure 1.

Before building a prototype, the system was designed using Coolit. The Coolit model of the computer is shown in Figure 2. Parts of the system in the figure are hidden to show the interior detail such as the cards and the heat exchanger fins. To meet the manufacturer's requirements, four analyses were performed using different mass flow rates corresponding to different flight conditions. The pressure drop across the heat exchanger predicted by Coolit was recorded as a function of the mass flow rate through the system.

Pressure Predictions

Once the system prototype was designed and built, an experiment was conducted to determine the pressure drop across the chassis prototype, D P34 = P4 - P3. The pressure measurement points are shown in the schematic of the experiment in Figure 3. Figure 4 shows a photograph of the set up. Eleven experiments were run with different mass airflow rates as shown in Figure 5 (black dots). The corresponding pressure readings were recorded on a manometer. The test data were fit with the best fit curve (red solid line in Figure 5) and the results were compared with the Coolit predictions (magenta line in Figure 5). Excellent agreement was observed throughout the entire pressure-flow range.

Temperature Predictions

To verify the temperature predictions, a test was performed at one of the designed mass flow rates. The temperature distribution across the "cold plate" was recorded at steady state. The "cold plate" temperature should not exceed 78 degrees C.

The "cold plate" temperature measurement locations are shown in Figure 6. The ambient conditions were measured at the heater are shown in Table 1.

Table 1

Location Test (deg. C) Prediction (deg.C)
Inlet 24.4 24.5
Slot 1 59.1 57.7
Slot 2 62 60
Slot 3 64.2 63.1
Slot 4 66.9 64.4
Slot 5 68.8 65.3
Slot 6 70.2 67.3
Slot 7 72.1 68.8
Slot 8 72.4 70.3
Slot 9 73.6 71.4
Slot 10 74 72.9
Slot 11 75.2 73.2
Slot 12 76.8 77.3
Outlet 67.5 67

Table 2 and the accompanying graph, Figure 7, compare the predicted and actual temperatures for the different locations based on the above boundary conditions. When the test data were compared against the Coolit predictions, the results were within 5%.

Table 2

Air Temperature 71 deg.C
Atmospheric Pressure 1008 mbar
Inlet temperature 24.5 deg.C
Inlet air mass flow 0.86 PPM

Figure 1. General view of chassis.
Figure 2. Coolit model of heat exchanger with top hidden (left) and with top and sides hidden (right).
Figure 3. Labeled diagram of test apparatus system.
Figure 4. Photograph of test apparatus system.
Figure 5. Graph of Flow vs. Pressure.
Figure 6. Thermocouples are visible at each slot location.
Figure 7. Graph of predicted and actual temperatures at each slot position.

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