Convection in microchannels

SINGLE-PHASE FORCED CONVECTION IN MICROCHANNELS A State-of-the-Art Review... 

Following the recent developments in microfabrication, a number of major research initiatives have been launched to improve our understanding of the heat transfer and fluid flow phenomena at the micro level. A survey of the literature presented below gives a brief summary of the research carried out in single-phase forced convection in microchannels mostly in the last 15-20 years. 

For fully-developed laminar forced convection in microchannels, Nu is proportional to Re° , while for the fully-developed turbulent heat transfer Nu is predicted by the Dittus-Boelter correlation by modif3ung only the empirical constant coefficient from 0.023 to 0.00805. 

In this section we present said discuss a few numerical results for the two problems considered, transient flow said hmsient convection in microchannels, which were respectively handled by the full and the partial integral hmsformation approaches. The aim is to demonshate file convergence behavior within each strategy and to illushate some physical aspects on file fiansient phenomena at the micro-scale. Although the developed solutions sae readily applicable to dififerent physical situations of eifiier hquid or gas flow, we here concentrate om illustiation of results on typical ex ples of lamina gas slip flow. 

The scientific program starts with an introduction and the state-of-the-art review of single-phase forced convection in microchannels. The effects of Brinkman number and Knudsen numbers on heat transfer coefficient is discussed together with flow regimes in microchannel single-phase gaseous fluid flow and flow regimes based on the Knudsen number. In some applications, transient forced convection in microchannels is important. 

Y. Yener, S. Kakac, M. Avehno, and T. Okutucu, Single phase forced convection in microchannels - State-of art-review, Microscale Heat Transfer-Fundamentals and Applications in Biological Systems and MEMS, edited by... 

R.M. Cotta, M.D. Mikhailov, and S. Kakag, Steady and Periodic Forced Convection in Microchannels, NATO Science Series II Microscale Heat Transfer Fundamentals and Applications, V. 193, S. Kakag et al. (eds.), pp. 49-74 (2005). 

Heat transfer in microchannel flow Single-phase convective flows in microchannels Single-phase forced convection in microchannels... 

Heat transfer to a digitized flow in a microchannel is similar in many ways to single-phase forced convection in microchannels. The thermal boundary conditions that exist are the same however, due to the unique rolling-type flow, DHT behaves in a significantly different fashion. In Fig. 3, the temperature field shows that heat is convected by the vortices and circulates within the droplet. As a droplet rolls down a heated microchannel, cool fluid from the center of the droplet is continually transported to the outer edges of the droplet while hot fluid at the wall is convected inward. Heat gradually diffuses... 

If the thermal energy equation is used in order to study laminar flow forced convection in microchannels, the expression of the thermal energy equation can be simplified. 

T. M. Adams, et al.. An experimental investigation of single-phase forced convection in microchannels. International fournal of Heat and Mass Transfer, 1998, 41, 851-857. 

To study laminar flow forced convection in microchannels under the assumptions of steady state axial pressure distribution and fully developed velocity field, the balance of the thermal energy can be written as follows [1] (see convective heat transfer in microchannels) ... 

TM Adams. Turbulent convection in microchannels. PhD dissertation, Georgia Institute of Technology, Atlanta, GA, 1998, pp. 71-105. 

Figure 6.21 FuUy developed liquid flow forced with convection in microchannel...

Yang, C., Li, D., Masliyah, J. H., Modeling forced liquid convection in rectangular microchannels with... 

The use of convective heat transfer in microchannels to cool microchips has been proposed over the last two decades. Many analytical and experimental studies, involving both liquids and gases, have been carried out to gain a better understanding of huid how and heat transfer phenomena at the micro level. 

Samalam [43] modeled the convective heat transfer in water flowing through microchannels etched in the back of silicon wafers. The problem was reduced to a quasi-two dimensional non-linear differential equation under certain reasonably simplified and physically justifiable conditions, and was solved exactly. The optimum channel dimensions (width and spacing) were obtained analytically for a low thermal resistance. The calculations show that optimizing the channel dimensions for low aspect ratio channels is much more important than for large aspect ratios. However, a crucial approximation that the fluid thermophysical properties are independent of temperature was made, which could be a source of considerable error, especially in microchannels with heat transfer. 

Peng and Peterson [34] experimentally investigated the single-phase forced convective heat transfer and flow characteristics of water in microchannel plates with extremely... 

Convective heat transfer in microchannels is significantly enhanced, depending on the values of the Knudsen, the Prandtl and the Brinkman numbers and the aspect ratio. Heat transfer characteristics can be significantly different from conventionally sized channels. 

Experimental investigations on convective heat transfer in liquid flows in microchannels have been in the continuum regime. Hence, the conventional Navier-Stokes equations are applicable. 

In the laminar and transition regimes in microchannels, the behavior of convective heat transfer coefficient is very different compared with the conventionally-sized situation. In the laminar regime, Nu decreases with increasing Re, which has not been explained. 

Since the ratio of surface area to volume is large, viscous heating is an important factor in microchannels. It is especially important for laminar flow, where considerable gradients exist. The Brinkman number, Br, indicates this effect. A decrease in Nu for Br > 0 and an increase for Br < 0 have been observed. This is due to the fact that for different cases, Br may increase or decrease the driving mechanism for convective heat transfer, which is the difference between the wall temperature and the average fluid temperature. 

Peng, X.F. and Peterson, G.P., Convective Heat Transfer and Flow Friction for Water Flow in MicroChannel Structures, Int. J. Heat Mass Transfer, 1996, 39(12), 2599-2608. 

Convective heat transfer analysis for a gaseous flow in microchannels was performed in [24]. A Knudsen range of 0.06-1.1 was considered. In this range, flow is called transition flow. Since the eontinuum assumption is not valid, DSMC technique was applied. Reference [24] considered the uniform heat flux boundary condition for two-dimensional flow, where the channel height varied between 0.03125 and 1 micrometer. It was concluded that the slip flow approximation is valid for Knudsen numbers less than 0.1. The results showed a reduction in Nusselt number with increasing rarefaetion in both slip and transition regimes. 

Bayazitoglu, Y., Tunc, G., Wilson, K., and Tjahjono, I., (2005) Convective Heat Transfer for Single-Phase Gases in MicroChannel Slip Flow Analytical Solutions, presented at NATO Advanced Study Institute, Microscale Heat Transfer - Fundamentals and Applications in Biological and Microelectromechanical Systems, July 18-30, Altin Yunus - Qe me, Izmir, Turkey. 

Tunc, G (2002) Convective Heat Transfer in MicroChannel Gaseous Slip Flow, PhD. Thesis, Rice University, Houston, TX. 

CONVECTIVE HEAT TRANSFER FOR SINGLE-PHASE GASES IN MICROCHANNEL SLIP FLOW ANALYTICAL SOLUTIONS... 

Convection and conduction are the two major heat transfer mechanisms that have been investigated at microscale. Convective heat transfer in microchannels has been intensively analyzed by both experimental and analytical means. Conduction studies have focused mostly on thin films in recent years to address such questions as How is the heat transferred How does it differ from large-scale conduction ... 

The present lecture summarizes some of tiie most recent joint research results from tiie cooperation between the Federal University of Rio de Janeiro, Brasil, and tiie University of Miami, USA, on tiie fransient analysis of both fluid flow and heat transfer within microchannels. This collaborative link is a natural extension of a long term cooperation between the two groups, in the context of fimdamental work on transient forced convection, aimed at tiie development of hybrid numerical-analytical techniques and tiie experimental validation of proposed models md methodologies [1- 9]. The motivation of this new phase of tiie cooperation was thus to extend the previously developed hybrid tools to handle both transient flow and transient convection problems in microchannels within the slip flow regime. 

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