Convective heat transfer in microchannels with varying flow cross-section

• Main Author: Cheng, Kai Xian
• Other Authors: Ooi Kim Tiow
• Format: Thesis-Doctor of Philosophy
• Language: English
• Published: Nanyang Technological University 2020
• Subjects: Engineering::Mechanical engineering
• Online Access: https://hdl.handle.net/10356/136773
• Institution: Nanyang Technological University

The microchannel heat sinks offer superior heat transfer capabilities. In the past three decades, these microchannels have been studied extensively in terms of design and implementation, fundamental understanding of heat transfer and fluid flow, as well as the enhancement techniques in heat transfer. These thermally-efficient channels have found applications in the industries where space and thermal performance are the key drivers. However, cost-effective manufacturing is a hurdle for the proliferation of microscale heat transfer in the wider commercial applications. Kong and Ooi demonstrated the ability to realise microscale heat transfer by superimposing two macro geometries. In one case, they demonstrated that an annular gap of less than 1 mm in size can be obtained by inserting a macro-sized rod into a macro-sized hollow cylinder. The heat transfer coefficients achieved in these microscale gaps are comparable with that in the existing microchannels. Uniquely, these macro geometries were machined using readily-available conventional machining methods and thereby eliminating costly micro-fabrication processes for the realisation of microscale heat transfer. Goh and Ooi studied nature-inspired configurations for an enhanced heat transfer in these microscale annular gaps. A triangular wavy design, which resembles that of a durian-thorny skin, outperformed the other nature-inspired profiles, in terms of thermo-hydraulic performance. The present Ph.D. study aims to investigate the thermo-hydraulic performance of various microchannel configurations with a varying flow cross-section, implemented by superimposing two macro geometries, realising gap sizes less than 1 mm. In this study, the different configurations of the microchannel are realised by placing solid cylinders of different surface profile into a hollow cylinder of macro-scale with a diameter of 20 mm and a length of 30 mm. Implementation of surface profiles on these cylinders and superimposition of these cylinders at different orientation result in flow channels with a varying cross-section along the flow direction. These parts are machined using simple turning processes. The first configuration is a microchannel with a sinusoidal wavy profile on the non-heated surface of the channel, differentiating this study from the existing works available in open literature. As the heating surface remains flat, this channel features a periodically expanding and contracting flow channel. A parametric study is conducted on the amplitude and wavelength of the waveform. A total of six wavy microchannels are studied and the thermal and hydrodynamic performance are compared against that of a straight microchannel. Experimental measurements are conducted for Re of 1400 to 4600 for a heat flux of 53.0 W/cm2. The channels are also modelled numerically to visualise the flow field and heat transfer. This channel configuration is able to remove up to 64 per cent more heat as compared to the straight microchannel, evaluated at the same pumping power. The amplitude of the waveform shows a more dominant effect on the thermo-hydraulic performance as compared to the wavelength. In addition, working correlations for the average Nusselt number and friction factor are proposed for this channel configuration, with a maximum discrepancy of 15 and 12 per cent respectively, when comparing the predicted data with the measured data. The second configuration is a microchannel with a sinusoidal wavy profile on the heated surface of the channel and the non-heated surface remains flat. This channel is compared with three other channels: the first configuration with a wavy non-heated surface, a serpentine configuration and a straight channel, in terms of thermo-hydraulic performance. The performances are predicted using a numerical model for Re range of 750 to 2200. The predicted Nu and f show an average difference of 4.8 and 18.6 per cent as compared to the measured data gathered for the first configuration. Although the channel with a wavy heated surface has similar pressure drops as that with a wavy non-heated surface, the former has a higher removal capability. This second channel configuration achieves a maximum performance index of 1.88, implying an 88 per cent higher heat removal capability at the same pumping power as the straight microchannel. This figure is 36.8 and 74.1 per cent higher than that of the first configuration and the serpentine channel. The third configuration is a microchannel with a sinusoidal wavy profile of a varied wave amplitude along the flow direction, on both the heated and non-heated surface. For each of the configurations, three types of waveform are implemented: increased-amplitude, decreased-amplitude and uniform-amplitude, yielding a total of six wavy microchannels. Experimental measurements are collected for the channel with a wavy non-heated surface, for the Re range of 550 to 3400 and a heat flux of 42.4 W/cm2. The numerical model, which is validated with the measured results, is used to predict the heat transfer and flow field of the channels with a heated wavy profile, for the Re range of 1580 to 3110. For the channels with a wavy non-heated wall, a decreased amplitude along the flow direction achieves an enhancement of 19.9 and 21.3 per cent of heat removal capability as compared to the waveforms with an increased and uniform amplitude respectively, under a low pumping power condition of 0.5 W. As the pumping power increases, the channel with a uniform-amplitude waveform achieves an improvement in performance index, outperforming those with a varied amplitude along the flow direction. Similar trends are observed with the wavy profile implemented on the heated surface, with the channel with the uniform-amplitude waveform incurs a 44.8 to 49.0 per cent lower pressure drop, despite the similar heat transfer coefficients as the decreased-amplitude waveform. The two other configurations which are being analysed numerically are an eccentric channel and a skewed channel. Three eccentric ratios and three skewed ratios are studied for annular microchannels with radius ratios of 0.95 and 0.97. The predicted Nu and f, for the range of 550 to 2300, are validated with the measured results for the concentric configurations, and the analytical solutions for the eccentric channels. The results showed that the Nu and f of the eccentric and skewed channels deviate more from the concentric channel at a higher radius ratio. The skewed channels have Nu deviated less than 5 per cent from that of the concentric channels, but an increment more than 20 per cent in f for the highest skewed ratio. The eccentric channel with an eccentric ratio of 0.75 has a Nu which is more than 10 per cent lower, and a lower f of similar magnitude range, as compared to the concentric channel. The last configuration that is being studied is a converging flow channel, i.e. a channel with a reducing hydraulic diameter along the flow direction. Four converging gradients are studied, and the thermo-hydraulic performance is compared against a straight channel. Experimental measurements are gathered for Re range of 1400 to 5200. All the converging flow channels possess a performance index less than unity, implying that these channels remove less heat as compared to the straight channel, when operating at the same pumping power. The findings of these channel configurations contribute to the development of a high performance compact microchannel heat exchanger.