Sridhar V. Machiroutu

Remote Heat Pipe Based Heat Exchanger Performance in Notebook Cooling

Remote heat pipe based heat exchanger cooling systems are becoming increasingly popular in cooling of notebook computers. In such cooling systems, one or more heat pipes transfer the heat from the more populated area to a location with sufficient space allowing the use of a heat exchanger for removal of the heat from the system. In analsysis of such systems, the temperature drop in the condenser section of the heat pipe is assumed negligible due to the nature of the condensation process. However, in testing of various systems, non linear longitudinal temperature drops in the heat pipe in the range of 2 to 15 °C, for different processor power and heat exchanger airflow, have been measured. Such temperature drops could cause higher condenser thermal resistance and result in lower overall heat exchanger performance. In fact the application of the conventional method of estimating the thermal performance, which does not consider such a nonlinear temperature variations, results in inaccurate design of the cooling system and requires unnecessarily higher safety factors to compensate for this inaccuracy. To address the problem, this paper offers a new analytical approach for modeling the heat pipe based heat exchanger performance under various operating conditions. The method can be used with any arbitrary condenser temperature variations. The results of the model show significant increase in heat exchanger thermal resistance when considering a non linear condenser temperature drop. The experimental data also verifies the result of the model with sufficient accuracy and therefore validates the application of this model in estimating the performance of these systems. This paper was also originally published as part of the Proceedings of the ASME 2005 Pacific Rim Technical Conference and Exhibition on Integration and Packaging of MEMS, NEMS, and Electronic Systems.Copyright

Challenges and Advances of Heat Pipes in Cooling Notebook Systems

Notebooks represent an increasing percentage of PC client market with growth surpassing that of desktop computers. Heat pipe has been an integral part of notebook computer system cooling and will remain so for the foreseeable future. Heat pipe allows for efficient transport of heat from the CPU and other high power components to a location where there is more room for accommodating motherboard cutout for a fan and a heat exchanger. The thermal resistance along this path must be minimized to enable maximum cooling. This paper first briefly describes the contributing resistance in a heat pipe and ways to measure them for a notebook thermal solution. Since there are several parameters that can affect the performance of the heat pipes, we use an experimental procedure utilizing DOE (Design of Experiments) to first understand the sensitivities of these design, manufacturing and usage parameters on performance and then to arrive at an optimum level of these parameters to minimize various resistances in a heat pipe. We show that for various different wick technologies, it is possible to optimize the heat pipes to achieve an evaporator performance of the level of 0.1 C-cm2/W. Furthermore, we show some simple design rules to minimize the condenser resistance and also results of a design study to optimize the design of heat pipe block at the CPU end to minimize the evaporator resistance. We want to encourage the heat pipe vendor community to use these methods to optimize their products for performance as well as process enhancements to produce higher performing parts, at lower cost.© 2007 ASME