Locations of Unit : mm
RESULTS AND DISCUSSION
This chapter presents the experimental data obtained in the present investigation of an improved CPL system. The effects of the liquid inventory and the relative height between the evaporator and condenser on the heat transfer performances of the CPL with three different improved designs are examined. The liquid inventory is defined as the ratio of the total volume of working fluid in the loop to the total loop volume available for the working fluid. Table 4.1 gives the volume of each component in the CPL system available for the working fluid to pass through. The results obtained here for the grooved channels formed by the square pin fins are compared with those for the grooved channels formed by the plane fins measured in the previous study [34].
Besides, the effects of using the heat spreader and roughing the vertical and bottom surfaces of the grooved channels on the CPL performance will be examined.
In the experiment, the mean evaporator temperature (Tevap) and condenser temperature (Tcond ), which are the average temperatures measured at selected locations in the block containing the grooved channels and the condenser (double-tube heat exchanger), and the temperatures of the working fluid measured at the inlets and exits of the evaporator and condenser shown in Figures 2.2, 2.4 and 2.5 will be inspected. It should be mentioned that the experimental test of the present CPL system will be terminated as the mean evaporator temperature exceeds 80℃.
4.1 Performance Comparison between Vapor Channels Formed by Pin Fins and Plane Fins
In testing the CPL performance with the vapor channels formed by the pin fins
and plane fins, the relative height between the evaporator and condenser is first fixed at 0 cm, and the liquid inventory is at 62%. Figure 4.1 shows the results from this test.
It is first noted that the maximum heat transfer performance of the type-2 pin fin block is about 120W before Tevap exceeds 80℃ and it is lower than that for the type-1 pin fin block and grooved block. This clearly indicates that using the pin fins to form the vapor channels in the copper block is less effective in vapor transport than the grooved channels. This result, in turn, is the consequence of the vapor flows in various parts of channels formed by the pin fins being normal to each other, causing a mutual retarding of the vapor to move slower into the vapor transport line. It was further noted in the test that the amplitude of the temperature oscillation in the evaporator for the pin fin blocks is about 1℃.
Then, comparison is made for different liquid inventories with the relative height between the evaporator and condenser fixed at 0 cm. The liquid inventory is varied from 50% to 75%. The test is conducted for the type-1 pin fin block and the results are shown in Figure 4.2. These results are compared with that for the grooved block in Figures 4.3 - 4.5. Note from the results in Figure 4.2 for the type-1 pin fin block that at the liquid inventory of 62% the CPL has the highest heat transfer capability Qe,max of 141W. For the other two liquid inventories Qe,max is much lower especially for the liquid inventory of 57%. The results in Figures 4.3 - 4.5 indicate that only at the liquid inventory of 75% the CPL heat transfer performance for the type-1 pin fin block is better than the grooved block.
Finally, the effects of the relative height between the evaporator and condenser on the CPL performance with the pin fin and plane fin blocks are tested. Here we only test the type-1 pin fin block with the liquid inventory fixed at 62%. The relative height
results from this comparison are shown in Figures 4.6 - 4.8. It is noted that the CPL system with the pin fin block has a worse heat transfer performance than the system with the plane fin block at all relative heights tested. Figure 4.9 shows the mean temperature at the evaporator and thermal resistance variations with the power input to the evaporator for the relative height varied from 0cm – 10cm for the type-1 pin fin block. Note that the maximum allowable power input to the evaporator Qe,max
increases with the increase in the relative height.
4.2 Installation of Heat Spreader
In examining the performance of the CPL system with the heat spreader containing the grooved channels formed by the plane fins, the liquid inventory is varied from 44% to 71% and the relative height between the evaporator and condenser ranges from 0 cm to 10 cm. It should be noted that in the test of the heat spreader effects, the volume of the whole loop available for the working fluid is larger than the previous test. The volume of each component for the loop with the heat spreader is shown in Table 4.2. The data measured from the CPL system with the heat spreader are inspected in the following.
The measured data for the variations of the mean evaporator and condenser temperatures and the temperatures at the evaporator and condenser inlets and exits along with the thermal resistances of the CPL with the power input to the evaporator are shown in Figures 4.10 - 4.16 for various liquid inventories at the relative height of 0 cm. The results indicate that in the sections where the vapor flow dominates at the evaporator outlet and condenser inlet, the measured temperatures increase significantly with the power input, so does the evaporator temperature. Besides, the measured temperatures at the condenser outlet and evaporator inlet are close to the
inventory on the performance of the CPL with the heat spreader, Figure 4.17 compares the data for the liquid inventory varied from 44% to 71%. The results manifest that the maximum allowable power input increases when the liquid inventory decreases from 71% to 59%, and decreases when the liquid inventory decreases further from 59% to 53%. But the maximum allowable power input increases again when the liquid inventory decreases from 53% to 47%. At the optimal liquid inventory of 47% the minimum thermal resistance is about 0.2℃/W and the maximum allowable power input is about 290W. Note that at the lower liquid inventory ranging from 44% to 53%, an unusual phenomenon of a small decrease in the mean evaporator temperature for an increase in the power input is observed at low power input. It is conjectured to be caused by a larger two-phase region in the condenser due to an insufficient liquid in the condenser.
How the relative height between the evaporator and condenser affects the performance of the CPL with the heat spreader is examined next. Figures 4.18 - 4.20 show the test results at the liquid inventory of 59% for various height differences between the evaporator and the condenser. Comparing the results in Figures 4.18-4.20 reveals that the maximum allowable power input increases substantially with the increase in the relative height (Figure 4.21). This is simply due to the fact that the liquid in the condenser flows back to the evaporator more easily at the larger relative height due to the stronger gravity effects. However, at the lower liquid inventory of 47% the data presented in Figures 4.22-4.25 show that there is no significant effect of the relative height on Qe,max. The anomaly at the lower liquid inventory discussed above reduces the effect of the relative height.
Finally, the present data for the CPL system with the heat spreader installed in
inventories for the relative height of 0 cm. Note that the system installed with the heat spreader has a much higher Qe,max at all liquid inventories. We further compare the results for the case of optimal liquid inventories for the systems with and without the heat spreader in Figure 4.27. The results show that at the optimal liquid inventories of 47% and 62% respectively for the systems with and without the heat spreader, Qe,max is increased by nearly 100% simply by using the heat spreader. The effects of heat spreader at the relative heights between evaporator and condenser of 5 cm and 10 cm are shown in Figures 4.28 and 4.29.
4.3 The Effects of Roughing the Surfaces of Grooved Channels
The effects of roughing the vertical and bottom surfaces of the grooved channels on the CPL performance are examined in this section. The heat spreader containing the grooved channels formed by the plane fins is installed in the evaporator in this test.
The measured data from this test are shown in Figures 4.30 - 4.34 for various liquid inventories for the relative height fixed at 0 cm. It is clearly noted that the maximum allowable power input increases when the liquid inventory decreases from 59% to 50%, and decreases when the liquid inventory decreases further from 50% to 47%. The maximum allowable power input at the inventory of 50% is about 325W and the minimum thermal resistance is about 0.17 ℃/W. Note that the anomaly at the lower liquid inventory discussed in section 4.2 is also observed in Figures 4.30 and 4.31.
Comparison of the CPL performance for the grooved channels with and without surface roughing is shown in Figure 4.35. The results indicate that roughing the surfaces of the vapor channels can effectively improve the heat transfer performance.
systems with and without the surface roughing for the vapor channels in Figure 4.36.
The enhancement for Qe,max about 10% can be obtained.
Finally, the test results about the relative height at the optimum liquid inventory of 50% are presented in Figures 4.37 - 4.40. The maximum allowable power input Qe,max also increases with the increase in the relative height. But it can be noted from Figure 4.40 that the enhancement is small at the optimum liquid inventory. Selected test data at other liquid inventories are shown in Figures 4.41 – 4.48. Note that at the other liquid inventories the increase in the relative height between the evaporator and condenser can also noticeably enhance the maximum allowable power input to the evaporator with the surface roughing on the grooved channels. The more effects of vapor channels with surface roughing at the relative heights between evaporator and condenser of 5 cm and 10 cm are shown in Figures 4.49 and 4.50.
In order to simulate cooling a CPU chip, the results presented above are for the cases that the tests are terminated when the mean evaporator temperature reachs 80℃.
The test results for the terminating increased to 85℃ and 90℃ for various liquid inventories are shown in Figures 4.51 – 4.54. Quantitatively, the data show that for the terminating raised from 80℃ to 90℃ the maximum allowable power input to the evaporator can be increased by about 20% on an average (Figure 4.54). It is noted that the maximum allowable power input to the evaporator is increased noticeably for a rise in the terminating . Besides, the thermal resistance almost maintains at a constant for each liquid inventories when the power input is larger than 300W.
Tevap
Tevap
Tevap
Table 4.1 Volume of each component in the CPL system without heat spreader