Dynamic Thermal Performances of Operating Limits in Heat Pipes
Date Issued
2010
Date
2010
Author(s)
Tsai, Te-En
Abstract
In recent years, a great need to know the thermal performances of the heat pipes emerges due to the flamboyant advancements in semiconductor industry and the associated booming needs of electronic cooling. A representative way to investigate the thermal performance of a heat pipe is to find its maximum heat transport capacity Qmax and the effective thermal resistance Reff. This method is referred to as conventional steady-state test, and is essentially indicative and matured. However, it’s too time/labor consuming and troublesome.
With a view toward shortening the necessary time to examine the thermal performances of heat pipes, a novel dynamic test methodology is originated and developed in the present thesis, and the objective is to substitute the use of conventional steady-state test. A set of dynamic parameters of thermal performances of heat pipes is ideated from the observed transient phenomenon. They are “decreasing slope of temperature difference S”, “dynamic descending rate M”, and “maximum heating temperature Tmax”. The validities of the above parameters are verified by comparing the experimental results with those of the steady-state test through investigating the effects of bending angles, fill ratios, and shapes of heat pipes. It is found that the parameters and the influences of factors between the steady-state test and the dynamic test are remarkably analogous. The same trends could be found between Tmax and Qmax, and between M and Reff no matter which effect is studied. The numerical simulation results also reasonably explain the physical phenomenon of dynamic test.
On the other hand, finding the operating limitations using the dynamic test is also of interests. Therefore, another group of heat pipes and operating conditions are investigated. Experimental results show that the start-up phenomenon and the viscous limitation could be indicated by “minimum transfer limit Qmin” and “minimum heating temperature Tmin”. It is found that the viscous limitation would be easier encountered, observed, and compared at a lower ambient temperature plus a non-circulated water heat source. Heat pipe subjected to a sudden cooling load is also investigated.
Further, the sonic limitation of heat pipe is also investigated by dynamic test. An ultra-thin fibered heat pipe is adopted. Experimental results demonstrate that the operating ranges for sonic limitation and entrainment limitation overlap each other to a respectable extent for this heat pipe. The temperature responses indicate that the heat pipe experiences the transition from viscous limitation, sonic limitation, and sonic-entrainment confounded limitation during the start-up phase. Very non-typical profiles of temperature distributions are found for this heat pipe at all the conditions investigated. The corresponding explanation is addressed in the thesis.
On the whole, the complete methodology and concepts for dynamic investigating the thermal performances and operating limitations of heat pipe is established and developed. The parameters and their physical meaning are testified and expounded. Conclusively speaking, the proposed dynamic test methodology is sufficient to examine the common miniature heat pipes when quality control or a pre-design of the heat pipe are needed. Empirically speaking, only 10~15 minutes are necessary to examine a heat pipe using the proposed dynamic test methodology. This is much more efficient than the steady-state test. Therefore, the proposed dynamic test could be adopted instead of the steady-state test to determine the thermal performance of heat pipes when high efficiency is of prior concern.
Subjects
Heat pipes
Thermal performance
Dynamic test
Operating limitations
Start-up phenomenon
Type
thesis
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