Samuel D. Marshall

Heat Exchanger Improvement via Curved Microfluidic Channels: Part 2 — Investigation Into Heat Transfer Enhancement due to the Dynamics of Dean Vortices

The efficiency of conventional heat exchangers is restricted by many factors, such as effectiveness of convective heat transfer and the cost of their operation. The current research deals with these issues by developing a novel method for building a lower-cost yet more efficient heat sink. This method involves using a specially designed curved microchannel to utilise the enhanced fluid mixing characteristics of Dean vortices, and thus transferring heat efficiently.Numerical models have been employed to investigate the heat transfer enhancement of curved channels over straight equivalents, with the aim of optimising the heat exchanger design based on the parameters of maximising heat transfer whilst minimising pressure drop and unit cost. These studies examined the variation of Nusselt Number over the length of the channel, for a range of different curvatures (and hence Dean numbers). The results showed significantly higher heat transfer occurring in curved channels, especially in areas where the generated Dean vortices are strongest, with the variation in Nusselt Number forming the shape of an ‘arc’. In this way, a relationship between the Dean Number and the Nusselt Number is characterised and discussed, leading to suggestions regarding optimal microfluidic heat transfer design.Copyright

Heat Exchanger Improvement via Curved Microfluidic Channels: Part 1 — Impact of Cross-Sectional Geometry and Channel Design on Heat Transfer Enhancement

The efficiency of conventional heat exchangers is restricted by many factors, such as effectiveness of convective heat transfer and the cost of their operation. The current research deals with these issues by developing a novel method for building a lower-cost yet more efficient heat sink. This method involves using a specially designed curved microchannel to utilise the enhanced fluid mixing characteristics of Dean vortices, and thus transferring heat efficiently.Numerical models have been employed to investigate the heat transfer enhancement of curved channels over straight equivalents, with the aim of optimising the heat exchanger design based on the parameters of maximising heat transfer whilst minimising pressure drop and unit cost. A range of cross-sectional geometries for the curved channels were compared, showing significantly higher Nusselt Numbers than equivalent straight channels throughout, and finding superior performance factors for square, circular and symmetrical trapezoidal profiles. Due the difficulty and expense in manufacturing circular microchannels, the relatively simple to fabricate square and symmetrical trapezoidal channels are put forward as the most advantageous designs. These results take into account both constant wall temperature and constant heat flux conditions. For a given set of channel dimensions, an optimal input flow rate condition is also determined.Copyright

Rapid Prototyping of Polymer-Based Rolled-Up Microfluidic Devices

This work presents the simple and rapid fabrication of a polymer-based microfluidic prototype manufactured by rolling up thin films of polymer. The thin films were fabricated via a casting method and rolled up around a center core with the aid of plasma activation to create a three-dimensional (3D) spiral microchannel, hence reducing the time and cost of manufacture. In this work, rolled-up devices with single or dual fluidic networks fabricated from a single or two films were demonstrated for heat sink or heat exchanger applications, respectively. The experimental results show good heat transfer in the rolled-up system at various flow rates for both heat sink and heat exchanger devices, without any leakages. The rolled-up microfluidic system creates multiple curved channels, allowing for the generation of Dean vortices, which in turn lead to an enhancement of heat and mass transfer and prevention of fouling formation. These benefits enable the devices to be employed for many diverse applications, such as heat-transfer devices, micromixers, and sorters. To our knowledge, this work would be the first report on a microfluidic prototype of 3D spiral microchannel made from rolled-up polymeric thin film. This novel fabrication approach may represent the first step towards the development of a pioneering prototype for roll-to-roll processing, permitting the mass production of polymer-based microchannels from single or multiple thin films.

Numerical simulation for optimizing the design of a commonly-used fluid distributor to enhance uniform distribution and reduce dead zones

Two factors are of great importance to the overall performance and efficiency of a fluid distributor: uniform distribution and dead zone volume inside the cavity. From the perspective of improving uniform distribution and reducing dead zones, this research optimized the design of a commonly-used fluid distributor with one side inlet, eight outlets, and a cylinder cavity through numerical simulation. For equal-distribution, results demonstrated that after the inlet diameter was fixed, the distribution became equal continuously with the decrease of the outlet diameter. In addition, compared with one-side-inlet fluid distributor, two-side-inlet fluid distributor and middle-inlet fluid distributor promoted the uniformity of fluid distribution greatly. This study also reported that fluid distributor with an outlet angle of around 30° increased equal-distribution mostly compared with other angles. For dead zones inside the distributor cavity, the conical cavity reduced dead zones significantly compared with the fluid distributor with cylinder cavity. However, for the design of a fluid distributor, it is necessary to make a compromise between improving equal-distribution and reducing dead zones, therefore two optimized fluid distributors combining configurations proposed above were designed. Numerical simulation results illustrated that new fluid distributor designs promoted distribution among outlets more uniformly and reduced dead zones inside the distributor cavity tremendously.

Curvature Induced Improvement In Mixing Performance

Mass and heat transfer are well studied fundamental engineering principles with established relationships between them. Owing to experimental errors and uncertainties, heat transfer studies have been primarily conducted using numerical tools and analytical models. By studying mass transfer behavior, heat transfer characteristics can be obtained through existing correlation. Through this study we aim to understand the mixing performance in various curvature based designs namely a spiral channel, serpentine, saw tooth, square curve, U shaped and simple curved channel, all with a square cross section of hydraulic diameter 600μm. This was conducted through both numerical and experimental investigation over a Reynolds number range of 10-200 for both cases. From this work it can be concluded that a spiral channel is able to generate superior mixing performance in comparison to other curvature designs due to an increase in Dean strength along the channel length from inlet to the outlet. Spiral channels have also been found to be more advantageous due to their low pressure drop and reduced footprint area in comparison to the other curvature designs making it more favorable for microchip device integration. This experiment based investigation would also enable us to tailor micromixer and heat sink designs based on application.

Investigation of mass transport in a spiral microfluidic network with an expansion chamber

In this work, we proposed the new design of two spiral networks interconnected with an expansion chamber to create vortex and disruption of the laminar boundary of intermingled streams which can efficiently boost mass transport in the microchannel. Confocal microscope is used to observe the mixing and fluid motion in the microchannel. We observed the evolution of Dean vortices along the spiral inlet channel and the disruption of stream boundary at the expansion chamber which are further pulled along the outlet spiral channel resulting in higher mixing efficiency compared to that without chamber. Laminar flow of two fluids still maintained at the end of the normal spiral networks but the perfect mixing can be achieved at a given flow rates (Re of 15 to 45) from our design. Furthermore, uniform mixing can be obtained even at the spiral channel with shorter channel length. Unlike other complex designs, the design of the expansion chamber does not increase the pressure drop of the microchannel system and its dimension is larger than that of the main channel allowing to be fabricated by using conventional fabrication approach.

Heat exchanger improvement via curved, angular and wavy microfluidic channels: A comparison of numerical and experimental results

In order to improve upon a conventional straight microchannel heat sink, a range of curved, angular and wavy microchannels were designed in order to increase fluid mixing via the occurrence of secondary flow interactions, in particular Dean vortices, hence augmenting heat transport. Both numerical models conducted in FLUENT and laboratory experiments were employed to investigate the heat transfer enhancement of a range of geometries (single curved, wavy, sawtooth, U-turn and square-wave). In both studies, every channel demonstrated significantly higher Nusselt Numbers and Thermal Performance Factors (TPF) than an equivalent straight channel, despite an increase in pressure drop. The relative order of the channels in terms of TPF was the same for both experiments and numerical simulations, with the exception of the U-turn channel which performed better in the former. However, experimental TPF results were found to be 15–20% of those from the simulation — these differences are associated with the relative simplicity of the numerical model and additional non-linear impacts in the experiments. Overall, wavy channels were found to have superior performance, especially over angular channels with sharp turns, thus it is suggested that wavy microchannels are the most advantageous designs for the development of heat sinks, especially in terms of minimising pressure drop whilst still making use of the enhanced heat transfer properties of Dean vortices. Finally, for a given wavy channel, an optimal input flow rate condition is also determined.

Study of Performance Impact by Thermo-Hydraulic Developing Entrance in Spiral Microchannel With CFD Analysis

Microchannel heat exchangers have become widely employed in modern systems, found within aerospace applications, waste heat recovery, water treatment processes, air conditioning, biomedical treatments and various industrial process applications. The microchannels increase the ratio of heat transfer surface to volume, thus improving the heat transfer performance significantly whilst reducing the overall weight and size. Moreover, by utilizing secondary flow from Dean Vortices induced by curved microfluidic channels, the fluid flow and heat transfer performance can be enhanced even further beyond conventional straight channels. However, since pressure drops found in microchannels are often quite high, channel lengths must be kept relatively short to balance the friction loss and energy consumption. Due to this, the developing region length at the microchannel entrance area has a greater impact than for macroscale channels, in terms of hydrodynamic and thermal performance over the remaining full developed region.The thermo-hydraulic design for heat transfer microchannel surfaces is strongly dependent on several dimensionless performance indicators, namely Nusselt number ‘Nu’ for heat transfer, and Poiseuille number ‘Po’, which is the product of Fanning friction factor ‘f’ and Reynolds number ‘Re’. These parameters are used to characterize and optimize the performance of microchannel surfaces and heat exchangers in general, also can be used to determine both the thermal and hydraulic developing region lengths at the channel entrance area. Whilst many such studies exist for theoretical analysis and experimental verifications, currently there is little literature on the developing region lengths and impacts researched through the method of Computational Fluid Dynamics (CFD). As such, this paper identifies and explores via quantitative analysis the hydraulic and thermal performance changes created by the relevant developing region lengths at the entrance area of spiral microchannels, as well as determinations and comparisons of these effects over straight channels. The numerical results, generated via COMSOL Multiphysics and contrasted with previous literature on the subject, also compared with the effect of the developing region on the effectiveness and efficiency of both spiral and straight microchannels, finding an improved heat transfer performance but an increased impact of hydraulic friction as well for spiral channels against straight counterpart. Furthermore, significant differences between thermal developing region length and hydraulic developing region length can be observed throughout, which illustrates high challenge and the need for compromise in microchannel design. In this way, implications for the configuration and design of industrial microchannels and micro heat exchangers are self-evident. All the key factors given in this paper are dimensionless, and thus the generated results can be utilized for a variety of flow conditions. Hence, this work should permit an increased understanding for and boost the curved microchannel and micro heat exchanger designs subsequently, through reducing the required numbers of tests and experiments and expediting the development for similar applications followed.Copyright

Mixing Enhancement in Spiral Microchannels

Advancements in the field of microfluidics has led to an increasing interest to study laminar flow in microchannel and its potential applications. Understanding mixing at a microscale can be useful in various biological, heating and industrial applications due to the space and time reduction that micro mixing permits. This work aims to study mixing enhancement due to curved microchannel and the influence of varying microchannel cross sectional shape through numerical and experimental investigations. Unlike prior studies which use channel dimensions in the lower microscale range, this work has been conducted on channels with dimensions in the higher end of micrometer range. Using a cross sectional hydraulic diameter of 600 μm enables introduction of flow into the curved channel at a Reynolds Number ranging from 0.15 to 75, the findings of which show considerable improvement in the mixing performance as compared to that of equivalent straight channels, due to the development of secondary flows known as Dean Vortices.Copyright