Flow Heat Transfer In Microchannels Research Articles

Modulated wall motion approach for augmenting slug flow heat transfer between two micro-parallel plates

Liquid–liquid slug flow heat transfer in microchannels has been an interesting topic of research to many researchers. However, the heat transfer studies available in the existing literature deal with stationary walls of the microchannels. In the present work, a modulated motion is prescribed to the walls of the channel in the transverse direction during oil–water slug flow between micro-parallel plates. The influence of frequency and amplification factor of the modulated wall motion as well as capillary number on the droplet shape, film thickness, pressure drop, and heat transfer rate under uniform wall heat flux conditions is investigated computationally. The heat transfer results for the modulated wall motion case show a significant improvement over liquid-only flow and slug flow without any wall motion. Besides, the effect of slug length on the heat transfer has also been discussed for both modulated and unmodulated wall motions of the channel. A mean absolute deviation of 2%–75% in the pressure drop obtained from the numerical studies and existing semi-empirical models for stationary walls for the studied Capillary numbers is observed. This suggests that a better formulation is required for the pressure drop model. In addition, although Nusselt numbers are found to be in reasonable agreement with the existing model for stationary walls, requirement for the formulation of a generalized model considering the effect of wall oscillations is also suggested. This study proposes a new perspective for heat dissipation in micro-scale channels and promotes flow and heat transfer studies, which could bring benefits to relevant applications.

Numerical study of slug flow heat transfer in microchannels

In the present paper, the heat transfer of slug flow in rectangular microchannels is studied numerically. The Al2O3-water nanofluid with the volume fraction of 1% is used as a homogeneous liquid. Three different configurations are considered to investigate the hydrodynamics and heat transfer in the microchannels. Boundary mesh adaptation method is used to capture the liquid film around gas bubbles. Slug flow characteristics are simulated with a very low computational time compared to previous researches. Slug and bubble lengths, liquid film properties, pressure drop, gas void fraction and heat transfer characteristics are studied. Results show that different inlet configurations impact on slug and bubble lengths. Gas hold-up is influenced by vortices and high frequency fluctuations that are created in the liquid film due to the difference between gas and slug acceleration. It is found that the Nusselt number is an increasing function of slug length and unit cell length. The results reveal that the hottest point on the wall is where the back tail vortex is generated. The nanofluid enhances the heat transfer coefficient by 10% in comparison with the pure water.

Recent Trends, Challenges and Scope in Single-Phase Liquid Flow Heat Transfer in Microchannels-A Review

Miniaturization of electronic gadgets has increased the power density and reduced the sizes of these gadgets drastically. Heat dissipation and stable operation of these electronic devices is challenge for researchers. Microchannel passages providing high surface area to volume ratio gives high heat transfer rates from small areas has emerged as potential heat dissipating and thermal control devices for Micro-Electro Mechanical Systems (MEMS). High performance microchannel heat exchangers are also attractive in applications where space and/or weight constraints dictate the size of a heat exchanger or where performance enhancement is desired. In this review paper a small attempt is made to look into the recent developments and methods to enhance the heat transfer performance of microchannels by using channel geometry, coolant and structural materials. It has been also observed that Nanofluids as coolants in microchannels have excellent potential to enhance the heat transfer performance and are quickly establishing as future coolant to be reckoned with, further challenges faced by researchers and future research scope in the field of single-phase liquid flow heat transfer in microchannels are identified.

Taylor flow heat transfer in microchannels—Unification of liquid–liquid and gas–liquid results

The flow and heat transfer behaviour of liquid–liquid Taylor flow is examined by performing both experiments and CFD simulations for1 and 2 mm vertical tubes with constant wall heat flux boundary conditions. Water and hexadecane are used as the disperse and continuous phases, respectively. The measured heat transfer coefficients are extremely sensitive to experimental uncertainties but, are in overall good agreement with the simulations. The simulations confirm the strong dependence on the flow conditions seen in the experiments.A generalised model of heat transfer in gas–liquid and liquid–liquid Taylor flows is developed from a combination of resistances for wall-to-film, film-to-slug and film-to-bubble or droplet. Good estimates for these individual resistances are described and validated. The overall heat transfer coefficient obtained by a rigorous weighting of the individual resistances correlates the entire set of CFD (liquid–liquid) and experimental (gas–liquid) data with 20% relative standard deviation. The model captures the complex parametric dependencies and sensitivities in the data.

SLUG FLOW HEAT TRANSFER IN MICROCHANNELS: A NUMERICAL STUDY

Two-phase microchannel slug flow has attracted significant interest among researchers due to its wide range of applications. This paper will present some as yet overlooked features of slug flow heat transfer in microchannels, using numerical simulation. Numerical simulations are carried out using ANSYS FLUENT with the volume of fluid (VOF) method with a 2D axisymmetric geometry for a 100 µm inner diameter microchannel with constant wall temperature boundary conditions. Effects of slug flow parameters on heat transfer are discussed showing the heat transfer rate follows a similar trend to existing numerical results. Our results show a significant increase in Nusselt number with liquid−liquid two-phase flow compared to single-phase flow, by up to 200%. The study revealed that capillary number, size of the liquid slugs, as well as the liquid film thickness have an important impact on heat transfer.

Impact of tortuous geometry on laminar flow heat transfer in microchannels

An experimental investigation of the hydrodynamic and heat transfer characteristics of water flowing through tortuous microchannels for Reynolds numbers ranging from 50 to 900 has been carried out. The microchannels have semi-circular cross-sections (diameter 2mm) and follow zigzag or sinusoidal pathways consisting of at least five repeating units. Conjugate heat transfer simulations in a straight channel are carried out to understand the complex thermal behaviours present in the current experimental design and to validate the experimental approaches. The integrated measurements of fluid flow and heat transfer offer capabilities for characterising the thermohydraulics of wavy microchannels. Experimental results show that significant heat transfer enhancement is achieved in wavy channels compared with the equivalent straight channel, although an increased pressure-drop penalty is also observed. A detailed flow dynamics study shows that the flow recirculation and secondary flow structures (Dean vortices) induced by the bend help to increase the heat transfer rates. The impact of geometrical parameters on flow and heat transfer performance is assessed. In addition, the stackability of channels on a plate structure is considered. Zigzag channel configurations which provide high heat transfer intensification, are well suited for the use in compact plate heat exchangers due to their significant heat transfer enhancement, as well as good stackability.

Extended Reynolds analogy for slip and transition flow heat transfer in microchannels and nanochannels

It is significant to develop an expression for the rarefaction correction to the continuum solution for heat transfer. An extended Reynolds analogy for slip and transition flow heat transfer in microchannels and nanochannels is proposed. A relationship between the rate of the dimensionless mean wall shear stress reduction and the rate of the dimensionless mean wall heat flux reduction may exist. The extended Reynolds analogy can be utilized to obtain an initial engineering approximation of the Nusselt number. As slip flow heat transfer is inherently complex, the proposed simple model may provide an insight into the mechanism of slip flow heat transfer. This paper cultivates the facility to perform the kind of engineering analysis which, although not exact, still provides useful information concerning real engineering needs.

Numerical simulation of thermofluid characteristics of two-phase slug flow in microchannels

A fundamental study of heat transfer characteristics of two-phase slug flow in microchannels is carried out employing the Volume-of-Fluid (VOF) method. Despite of the fact that numerical simulations of two-phase flows in microchannels have been attempted by many investigators, most efforts seem to have failed in correctly capturing the flow physics, especially those pertaining to the slug flow regime characteristics. The presence of a thin liquid film in the order of 10 μm around the bubble is a contributing factor to the above difficulty. Typically, liquid films have a significant effect on the flow field and heat transfer characteristics. In the simulations reported in this paper, the film is successfully captured and a very high local convective heat transfer coefficient is observed in the film region. A strong coupling between the conductive heat transfer in the solid wall and the convective heat transfer in the flow field is observed and characterized. Results showed that unsteady heat transfer through the solid wall in the axial direction is comparable to that in the radial direction. Results also showed that a fully developed condition could be achieved fairly quickly compared to single-phase flows. The fully developed condition is defined based on the Peclet number ( Pe) and a dimensionless length of the liquid slug. Local and time-averaged Nusselt numbers for slug flows are reported for the first time. It was found that significant improvements in the heat transfer coefficient could be achieved by short slugs where the Nusselt number was found to be 610% higher than in single-phase flows. The study revealed new findings related to slug flow heat transfer in microchannels with constant wall heat flux.