Effects of Liquid Inventory

The effects of the liquid inventory, which is defined as the ratio of the total liquid volume of the working fluid input to CPL to the total loop volume available for the working fluid are examined first. Table 4.1 shows the volume of each component in the CPL system available for the working fluid to pass through. Here in the tests of

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the liquid inventory effects, the cooling water temperature in the condenser is set at 25℃ and the relative height between the evaporator and condenser is fixed at 0 cm.

The liquid inventory is varied from 50% to 75%. Besides, in the tests the side and bottom walls of the grooved channels are not covered with the thin cotton gauze layer except in section 4.4 where the effects of the gauze layer covering are investigated.

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 Figs. 4.1- 4.5 for various liquid inventories. It is first noted from Figs.

4.1(a)-4.5(a) that in the sections where the vapor flow dominates at the evaporator outlet and condenser inlet the measured temperatures increase significantly with Q_e, so does the evaporator temperature. While at the condenser outlet and evaporator inlet the liquid flow prevails, the measured temperatures are close to the saturated value.

We also note from Figs. 4.1(b)-4.5(b) that the thermal resistance of the loop decreases substantially with the power input to the evaporator except at the lower liquid inventory of 50%. A close examination of the data given in Fig. 4.6 reveals that for Qe

＞30W the thermal resistance of the loop is only slightly affected by the liquid inventory when the liquid inventory is below 62%. But at the higher liquid inventories of 70% and 75% the thermal resistance is higher. It is important to further note that at the liquid inventory of 62% the power input to the evaporator can be increased to 166W before T_evap exceeds 80℃. This power input is designated as the maximum allowable power input to the CPL for the given liquid inventory and Tcold. The maximum allowable power input to the evaporator Q_e,max, however, reduces to a significant amount when the liquid inventory is lowered to 57% and 50% or is raised to 70% and 75%. More specifically, at the low inventory of 50% the maximum power input Qe,max is only 100W. This large reduction in Qe,max is attributed to the dryout of

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the liquid in the wick due to the insufficient liquid supply from the liquid line at this small liquid inventory. The evaporator thus rises sharply to exceed 80℃at this low Q_e,max. For the high inventories of 70% and 75% the maximum power input to the evaporator are respectively reduced to 107W and 68W. This is the direct consequence of the flooding of the grooved channels at these high liquid inventories. Besides, the liquid appears in the vapor transport line and the CPL operation is somewhat abnormal. Hence at these high liquid inventories the vaporization of the working fluid near the contact surface between the wick and grooved copper block is retarded to a significant degree, causing the deterioration of phase change heat transfer in the evaporator. The above results clearly manifest that an optimal liquid inventory exists for the heat transfer performance of the CPL.

4.2 Effects of the Cooling Water Temperature in the Condenser

Next, results are presented to illustrate the effects of the cooling water temperature in the condenser on the performance of the CPL. The measured data are shown in Figs. 4.7-4.11 for various cooling water temperatures in the condenser for the liquid inventory set at 62% and the relative height between the evaporator and condenser fixed at 0 cm. The results in Fig. 4.7 for Tcold=20℃ indicate that the thermal resistance of the loop decrease significantly with the power input to the evaporator. Similar trends are noted in Figs. 4.8 and 4.9 for the cooling water temperature in the condenser of 25℃ and 30℃. But for the high T_cold of 40℃ the thermal resistance of the loop only changes slightly with Qe. A close inspection of the data given in Fig.4.11 reveals that the maximum allowable power input to the evaporator Qe,max is lowered to a certain degree for an increase in the cooling water temperature in the condenser. This is conjectured to result from the higher pressure in the condenser for a higher Tcold. Consequently, the pressure difference between the

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evaporator and condenser is smaller. This, in turn, results in a lower vapor flow rate in the vapor line and heat transfer in the evaporator is retarded to some extent. Besides, at a higher T_cold the mean evaporator temperature is slightly higher. It is further noted that for Tcold ≦ 30℃ the thermal resistance of the loop is only slightly affected by the cooling water temperature in the condenser except at low Q_e (＜60W).

4.3 Effects of the Relative Height between the Evaporator and Condenser Then, how the relative height between the condenser and evaporator affects the performance of the CPL is examined. The results are presented in Figs. 4.12-4.22 for the cooling water in the condenser set at 25℃ for various relative heights and liquid inventories. Note that for all liquid inventories tested here the thermal resistance of the loop reduces to a noticeable degree for an increase in the height between the condenser and evaporator except at the low liquid inventory of 50%. Most importantly, the maximum allowable power input to the evaporator is substantially augmented when the relative height is increased, as evident from the data given in Figs. 4.13, 4.15, 4.17, 4.19 and 4.21. Moreover, for given Qe and liquid inventory the mean evaporator temperature is considerably lower for a larger relative height.

It is of interest to investigate the effects of the liquid inventory on the performance of the CPL for different relative heights between the condenser and evaporator. This is illustrated in Fig. 4.22. The results indicate that the liquid inventory exhibits slighter effects on the CPL performance for a larger relative height.

But for all condenser-evaporator relative heights tested here highest Qe,max all appears for the liquid inventory of 62%. Note that at this optimal liquid inventory Q_e,max can reach 242W for the condenser-evaporator relative height of 10 cm.

A close inspection of the data given in Figs. 4.12-4.21 reveals that at the liquid inventory of 62% and at Qe,max the thermal resistance of the loop reduces significantly

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from 0.30 ℃/W to 0.20 ℃/W when the relative height is increased from 0 cm to 10 cm. While the corresponding maximum allowable power input to the evaporator is increased from 166W to 242W. Similar situation is noted for other liquid inventories.

The profound influences of the relative height between the condenser and evaporator on the CPL performance presented above can be attributed to the large increase in the gravitational force acting on the downward liquid flow in the liquid transport line for an increase in the relative height. Thus the liquid supply to the wick is greatly enhanced by the relative height increase, which can significantly augment the heat transfer capacity of the evaporator and Q_e,max. Besides, the thermal resistance of the loop is reduced substantially.

We move further to illustrate the effects of the condenser-evaporator relative height on the CPL performance for different cooling water temperatures in the condenser in Figs. 4.23-4.30 for the liquid inventory of 62%. Similar trends are noted from these data. But at the high Tcold of 40℃ the thermal resistance of the loop is less affected by the relative condenser-evaporator height (Fig. 4.30(b)).

Figure 4.31 shows the influence of Tcold on the CPL performance at different relative condenser-evaporator heights. The results indicate that for a larger relative height the thermal resistance of the loop is less affected by Tcold. But the opposite is the case for the mean evaporator temperature.