Ronan Grimes

Thermal Management of Low Profile Electronic Equipment Using Radial Fans and Heat Sinks

There is an increasing need for low profile thermal management solutions for applications in the range of 5-10 W, targeted at portable electronic devices. This need is emerging due to enhanced power dissipation levels in portable electronics, such as mobile phones, portable gaming machines, and ultraportable personal computers. This work focuses on the optimization of such a solution within the constraints of the profile and footprint area. A number of fan geometries have been investigated where both the inlet and exit rotor angles are varied relative to the heat conducting fins on a heat sink. The ratio of the fan diameter to the heat sink fin length was also varied. The objective was to determine the optimal solution from a thermal management perspective within the defined constraints. The results show a good thermal performance and highlight the need to develop the heat sink and fan as an integrated thermal solution rather than in isolation as is the traditional methodology. An interesting finding is that the heat transfer scales are in line with turbulent rather than laminar correlations despite the low Reynolds number. It is also found that while increasing the pumping power generally improves the thermal performance, only small gains are achieved for relatively large pumping power increases. This is important in optimizing portable systems where reduced power consumption is a competitive advantage in the marketplace.

Profile Scaling of Miniature Centrifugal Fans

This paper addresses issues that relate to downscaling the height of centrifugal fans for application in low profile technologies, such as the cooling of portable power electronics. The parameters studied include flow rate, pressure rise, and power consumption characteristics. The former two of these are measured using a fan characterization rig and the latter by directly measuring the power supplied to the fan. These are studied for fan diameters ranging from 15 to 30 mm with numerous profile heights between 0.3 mm and 15 mm. It is found that all of the phenomena encountered are best described in terms of fan aspect ratio. The results show that the conventional scaling laws cannot be accurately applied when blade profile alone is scaled. Indeed, the only parameter reasonably well predicted was the pressure rise attainable, but that was only accurate for fan aspect ratios greater than 0.17. Below this, the pressure rise generated reduces logarithmically toward zero. The study also reveals that no advantage is gained by using fans of aspect ratio greater than 0.3, as the maximum flow rate attainable decreases slightly above this. Overall, the scaling phenomena reported herein provide invaluable information for the future design of efficient low-profile cooling solutions that are to incorporate such fans.

Effect of geometric scaling on aerodynamic performance

Miniaturization of modern electronics and simultaneous elevation in heat dissipation means that future compact electronic systems are likely to be too hot to be held in the users hand. As a result, novel compact cooling technologies are required. In systems such as mobile phones and palmtop computers, macroscale fans cannot be used to overcome this problem because they are too large. As a solution, the implementation of microfan technology is proposed. Aerodynamic scaling issues in microaxial flow fans are addressed. Analysis shows how reduction of fan dimensions to the microscale causes increased local loss. Numerical simulations were performed to investigate the validity of the scaling theory, the results of which give confidence in the scaling analysis. Measurements were carried out on two geometrically similar fans to validate the theory under experimental conditions. Results of these measurements were in good agreement with the analysis. The fundamental finding is that a reduction in scale is accompanied by a reduction in efficiency and, thus, fan performance. It is concluded that geometric scaling alone of macroscale designs is not sufficient to produce microscale cooling fans: Modifications must be made to the geometry that account for changes in flow physics.

The Effect of Reynolds Number on Microaxial Flow Fan Performance

Microscale axial flow fans were investigated in response to the growing cooling requirements of the electronics industry. The two main challenges of this investigation were manufacture of a fully functional fan at the microscale, and performance reduction due to Reynolds number effect. Manufacture of a fully functional axial microfan complete with three-dimensional blade geometry was proven possible using microelectrodischarge machining techniques. Experimental performance measurements proved that Reynolds number effect was not prohibitive at the microscale, and dimensional analysis thereof derived a novel linear scaling method, which quickly and accurately predicted the Reynolds number effect at any fan scale.

The effect of fan operating point and location on temperature distribution in electronic systems

This paper investigates the effects of fan operating point and location on the temperature distribution in a simple test electronic system. First the investigation is performed with the fan mounted at the system outlet and drawing air through the system. Then the investigation is performed with the fan mounted at system inlet, pushing air through the system. Both cases were investigated at three flow rates within the fans recommended operating range. In each case, Particle Image Velocimetry (PIV) was used to measure system air flow and infra red thermography was used to measure PCB surface temperature. At each flow rate examined, PIV showed the air velocity to be uniform in direction and magnitude in the system with the fan mounted at system outlet, with local velocities changing in proportion to the flow rate. PCB temperature increased with reduced flow rate. In the system with the fan mounted at inlet, flow had large tangential components, impingement onto the PCB and reverse flow. Mean PCB temperature was found to be unchanged by flow rate, due to the increased thermal mixing which occurred at lower flow rates. This paper illustrates the gains, which can be made through correct fan placement, and discusses how heat transfer coefficients within fan cooled electronic systems can be optimised throughout the fan operating range.

An experimental study on the performance of miniature heat sinks for forced convection air cooling

In recent years the design of portable electronic devices must incorporate thermal analyses to ensure the device can be adequately cooled to acceptable temperatures. Consumer demand for smaller, more powerful devices has lead to an increase in the heat required to be dissipated and a reduction in the surface area both of which result in an increased heat flux. In this paper, an experimental study is performed on one of the smallest commercially available miniature fans, suitable for cooling portable electronic devices, used in conjunction with both finned and finless heat sinks. Previous analysis has shown that due to fan exit angle, flow does not enter the heat sinks parallel to the fins or bounding walls. This results in a non uniform flow rate within the channels of the finned and finless heat sink along with impingement of the flow at the entrance giving rise to large entrance pressure losses. In this paper straightening diffusers were attached at the exit of the fan which resulted in aligning the flow entering the heat sinks with the fins and channel walls. In designing the finned heat sink current optimization criterion for finned heat exchangers has been applied to ensure maximum heat transfer rates; the finless heat sink was designed to the same specifications. The maximum overall footprint area of the cooling solution is 534 mm2 with a profile height of 5 mm. The thermal performance of each cooling solution was investigated by quantifying its thermal resistance over a range of fan speeds and comparing the results to cases without diffusers. In order to investigate the flow field, detailed velocity measurements were obtained using Particle Image Velocimetry, which provided a further insight into the physics of the flow in such miniature geometries and in designing the straightening diffusers. The thermal analysis results indicate that the cooling power of the solution is increased by up to 20% through the introduction of a diffuser. Hence, demonstrating the need for integrated fan and heat sink design of low profile applications.

The performance of active cooling in a mobile phone

Power dissipation levels in mobile electronics devices are heading towards five watts and above. With this power dissipation level, products such as mobile phones will require active cooling to ensure that the devices operate within an acceptable temperature envelop from both user comfort and reliability perspectives. To the authors knowledge no studies to date have been carried out to determine the potential performance of fans within mobile phone architectures. In this paper a centrifugal fan is implemented into a Nokia mobile phone. Its performance is compared in terms of aerodynamic characteristics, maximum phone surface temperature, and allowable phone heat dissipation, for various levels of blockage in the phone, which are simulated using perforated plates with varying porosity. The results show that for the best case scenario, with minimal blockage increased power dissipation levels of order 75% can be achieved but with realistic blockages this value is more likely to be in the region of 50%.

Film Thickness for Two Phase Flow in a Microchannel

The issue of contamination of micro channel surfaces by bio fluids is a significant impediment to the development of many biomedical devices. A solution to this problem is the use of a carrier fluid, which segments the bio fluid and forms a thin film between the bio fluid and the channel wall. A number of issues need to be addressed for the successful implementation of such a solution. Amongst these is the prediction of the thickness of the film of carrier fluid which forms between the bio sample and the channel wall. The Bretherton and Taylor laws relate the capillary number to the thickness of this film. This paper investigates the validity of these laws through an extensive experimental program in which a number of potential carrier fluids were used to segment aqueous droplets over a range of flow rates. The aqueous plugs were imaged using a high speed camera and their velocities were measured. Film thicknesses were calculated from the ratio of the velocity of the carrier fluid to the velocity of the aqueous plug. The paper concludes that significant discrepancies exist between measured film thicknesses and those predicted by the Bretherton and Taylor laws.Copyright

An Experimental Investigation of the Flow Fields Within Geometrically Similar Miniature-Scale Centrifugal Pumps

Flow fields within two miniature-scale centrifugal pumps are measured and analyzed to facilitate an understanding of how scaling influences performance. A full-scale pump, of impeller diameter 34.3 mm and blade height 5 mm, and a half-scale version were fabricated from a transparent material to allow optical access. Synchronized particle-image velocimetry (PIV) was performed within the blade passage of each pump. Pressure-flow characteristics, hydrodynamic efficiencies, and high-resolution flow field measurements are repbrted for six rotational speeds over a Reynolds number range 706―2355. Fluidic phenomena occurring in the impeller passage at both pressure and suction surfaces are identified. Efficiencies are evaluated from direct measurement to be between 10% and 44% and compared with inner efficiencies calculated from the PIV data. Hydrodynamic losses as a percentage of overall efficiency increase from 12% to 55% for 2355 ≤ Re ≤ 706. Slip factors, in the range 0.92―1.10, have been derived from velocimetry data.

Performance Analysis of a Modular Air Cooled Condenser for a Concentrated Solar Power Plant

The use of air cooled condensers in power generation facilities is increasing in arid regions around the world. There is a specific requirement for more efficient air cooling technologies to be developed for Concentrated Solar Power (CSP) plants. This paper aims at determining the effects of various condenser design features on CSP plant output. In particular this paper considers a modular condenser and focuses on designing a suitable compact heat sink to be coupled with a variable speed fan array. Tube banks with radial fins have been used for decades to heat and cool gases and numerous correlations exist to predict the performance of such a heat exchanger. The initial design of this air-cooled condenser is essentially a tube bundle consisting of 6 rows of helically finned round tubes in an equilateral staggered arrangement. A laboratory-scale steady state test facility was designed to investigate the accuracy of the relevant correlations for the given design. Due to an undesired phenomenon which exists in multi-row condensers known as backflow, an investigation was performed to analyze the performance of the tube bank with fewer tube rows. The thermal and hydraulic performance for a tube bundle with a different number of tube rows was measured and found to be within 10–18% of the existing correlations. New correlations for heat transfer and pressure drop for the given design are presented for greater accuracy in the calculation of the condenser performance. These correlations, based on the measured data were combined with performance characteristics from a steam turbine to model the thermodynamic plant performance incorporating the various condenser designs. The investigation shows that for each condenser size, design and ambient temperature, an optimum fan speed exists which maximizes plant output. Further analysis shows that for a 1000 module condenser, a 4 row condenser results in the highest plant output, with a loss in efficiency due to condenser operation of 1.85%. A 2 row condenser also performs relatively well with 600 or more modules. This analysis shows that a condenser consisting of a series of such modules, can tightly control and optimize the net plant output power by simply varying fan speed.Copyright