Himanshu Pokharna

Skin cooling and other challenges in future mobile form factor computing devices

In this paper, we describe several of the cooling challenges in the notebook space as well as outline techniques for evaluating the benefit of new thermal technologies. Some of the challenges described in this paper include thermodynamic limits (total heat within the system), skin temperatures, and component temperatures other than the microprocessor. Specifically, in this paper we will describe a new technology for better cooling skins called laminar wall jets. This technology is useful in reducing bottom skin temperatures of notebooks by up to 20%.

Microchannel Cooling in Computing Platforms: Performance Needs and Challenges in Implementation

Remote cooling is the established cooling scheme in notebook computers, and increasingly, other computing sectors like desktops and servers are evaluating this approach as an option for cooling future platforms. While remote cooling facilitates a larger heat exchanger than the space directly over the processor would allow, it introduces an additional thermal resistance, in particular, θp-f (plate to fluid resistance) — the resistance in getting the heat from the cold plate to the fluid. For any remote cooling system, this resistance needs to be carefully evaluated and minimized. Pumped fluid loops incorporating microchannel heat exchangers are a viable option to achieve low plate-to-fluid resistances. In this paper we will identify a reasonable target for θp-f and subsequently describe two similar but fundamentally different thermal systems to accomplish this target performance: single-phase and two-phase pumped loops. Although two phase flows are traditionally thought of as the way to accomplish the highest heat transfer coefficients and thus the lowest resistances, with microchannel heat sinks the contrast is not so acute. We will present results from our experimental work on single- and two-phase heat transfer from microchannel heat sinks and demonstrate a transition where single-phase performance matches that of two-phase operation. This will be followed by the analysis methods used to predict the heat transfer and the pressure drop data. Moreover, we will discuss system level issues and other hurdles that need to be overcome in commercialization of microchannel technology for cooling computer systems.Copyright

Lid cooling for notebooks

In this paper, we investigate the effective use of the notebook lid area for passive heat dissipation. We address the hinge design challenges associated with this and propose different spreader options for increasing the overall effectiveness of this solution. For transferring the heat to the lid, we have designed and developed a novel thermo-mechanical hybrid hinge that can provide the required torque and simultaneously conduct the heat from the component in the base to the lid. The mechanical and thermal performances and its reliability were evaluated under an accelerated degradation testing environment over a period of 22,000 cycles. The performance degradation observed was within the experimental error of 5%. While a standard Aluminium spreader does increase overall passive dissipation, it was found that this could be further increased by 80% through the use of advanced graphite spreaders. As a second low cost alternative, we also found that an aluminium spreader with 3 flattened heat pipes (1 mm thick) attached to its surface increased cooling by 40% as compared to an Aluminium spreader. Final experiments on a notebook by use of the hybrid hinge and an advanced graphite spreader based lid cooling solution demonstrated a 60% increase in passive dissipation over a regular notebook without lid cooling.

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