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This paper analyzes the scale effects that occur in miniature centrifugal flow fans and investigates the possibility of optimizing blade geometry so that performance can be enhanced. Such fans are typically employed in small scale heat sinks such as those used for processor cooling applications or in portable electronics. The specific design parameter varied is the blade chord length, and the resulting fan performance is gauged by examining the flow rate, pressure rise, and power consumption characteristics. The former two are measured using a BS 848 fan characterization rig and the latter, by directly measuring the power consumed. These characteristics are studied for three sets of scaled fans with diameters of 15 mm, 24 mm, and 30 mm, and each set considers six individual blade chord lengths. A novel theory is put forward to explain the anticipated effect of changing this parameter, and the results are analyzed in terms of the relevant dimensionless parameters: Reynolds number, chord length to diameter of fan ratio, flow coefficient, pressure coefficient, and power coefficient. When these characteristic parameters are plotted against the Reynolds number, similar trends are observed as the chord length is varied in all sets of scaled fans. The results show that the flow coefficient for all the miniature fans degrade at low Re values, but the onset of this degradation was observed at higher Re values for longer blade chord designs. Conversely, it was found that the pressure coefficient is elevated at low Re, and the onset Re for this phenomenon correlates well with the drop off in flow coefficient. Finally, the trend in power coefficient data is similar to that for the flow coefficient. The derived theory is used to correlate this data for which all data points fall within 6% of the correlation. Overall, the findings reported herein provide a good understanding of how changing the blade chord length affects the performance of miniature centrifugal fans; hence, providing fan designers with guidelines to aid in developing optimum blade designs, which minimize adverse scaling phenomena. [DOI: 10.1115/1.4001590] Keywords: miniature fans, centrifugal flow, blade design, scaling phenomena, electronics cooling

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. Keywords: air, axial, British standards, cooling, EDM, fan, flow rate, laminar, micro, pressure, Reynolds

In this paper, measurements of surface temperature and air velocity are presented for the Case of a printed circuit board with heated metal elements in a fan-induced flow. Surface Temperature measurements were performed using infra red thermography, while air flow Measurements were performed using Particle Image Velocimetry (PIV). The aim is firstly to provide a physical explanation of the heat transfer process for the board situated in the inlet and exit air flow from the fan, and secondly to provide a set of data to test predictive methods. The results are presented in two parts: first, surface temperature distribution and velocity field data are presented for the case of the fan drawing air from the system. The flow was found to be parallel and uniform. There were symmetrical temperature gradients across the board that were readily explained in terms of board conduction and boundary layer growth. This data set is in strong contrast to that presented in the second part of the paper, which deals with the board mounted in the fan exit flow. In this case, both the flow and the heat transfer changed dramatically. The flow here is unsteady and swirling, giving rise to significantly higher Nusselt numbers over the whole of the board. A simple method of understanding these flows is presented to help the system designer to use them to advantage. The implications for increased system reliability, due to both the measured low component temperatures and the lower operational temperature of the fan are significant. [DOI: 10.1115/1.1649241]

• Thermal energy storage is employed in a novel Rankine cycle cooling solution. • Low Temperature Thermal Storage (LTTS) permits sub-ambient condensation. • A techno-economic model is presented to establish LTTS performance. • LTTS performance is benchmarked by comparison with an air-cooled condenser model. • In suitable climatic conditions, LTTS outperforms air-cooled thermal power plants. This paper presents the essentials of low temperature thermal storage (LTTS), a novel technique whereby thermal energy storage is employed to achieve sub-ambient condensation in air-cooled Rankine cycle power plants. It summarises work which was undertaken to explore the potential and the range of application of LTTS. The technology is most effective at geographical locations with large average daily temperature ranges, and high summertime temperatures. Hourly normal temperature data was sourced for five potential deployment sites, which provided a representative sample of different climate types. A steam turbine, a condenser, an air-cooled heat exchanger, and a chilled water thermal energy storage tank formed the LTTS configuration – a techno-economic model of which was developed to simulate system behaviour. The size of the air-cooled heat exchanger, the fan speed of the air-cooled heat exchanger, and the hours of charge, discharge, and bypass of the thermal energy storage tank were all modelled as variables to determine the effects of component sizes and operating patterns. LTTS performance was benchmarked by comparison with a direct air-cooled condenser model. Results presented in this paper include daily plant output, annual power output, and payback period. This study shows that LTTS can deliver all the advantages of dry-cooling, without suffering the usual performance degradations. The inherent flexibility of LTTS allows for configurations to be customised to exploit the prevailing site climate, and capitalise on the local power demand pattern. There are also clear indications that, in suitable climatic settings, LTTS outperforms traditional air-cooled thermal power plants by offering up to 10% additional generating capacity. Coupled to this are payback periods as short as 2.5 years, ensuring LTTS can be considered a viable alternative to current air-cooling strategies. [ABSTRACT FROM AUTHOR]

The case is made for the continued use of single valued thermal resistances for the prediction of component junction temperature, and, hence, reliability. These values are adjusted using empirically determined influence factors to account for thermal and aerodynamic interactions at board level The paper presents measured values of influence factors for arrays of Plastic Quad Flat Packs (PQFPs) over a range of Reynolds numbers and with a series of board level obstacles modeling upstream passive components. The results are formulated into a novel set of design rules. [DOI: 10.1115/1.1339822] Keywords: Forced Convection Electronic Cooling, Thermal Resistance, Aerodynamic Influence Factors, Plastic Quad Flat Pack (PQFP), Upstream Flow Disturbance, Thermal Design Rules

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 reported 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 [less than or equal to] Re [less than or equal to] 706. Slip factors, in the range 0.92-1.10, have been derived from velocimetry data. [DOI: 10.1115/1.3176985]