Single-phase heat transfer in microchannels: The importance of scaling effects

Abstract

Microscale single-phase heat transfer is widely used in industrial and scientific applications and for this reason, many related papers have been published in the last two decades. Nevertheless, inconsistencies between published results still exist and there is no generally accepted model for the prediction of singlephase heat transfer in microchannels. This paper presents a review of the experimental and numerical results available in the open literature. Heat transfer in microchannels can be suitably described by standard theory and correlations, but scaling effects (entrance effects, conjugate heat transfer, viscous heating, electric double layer (EDL) effects, temperature dependent properties, surface roughness, rarefaction and compressibility effects), often negligible in macro-channels, may now have a significant influence and have to be accounted for. Furthermore, measurement uncertainties may be more important, due to the reduced characteristic dimensions, so have to be accurately checked and, where possible, reduced. Experiments with single channels are more accurate and in good agreement with predictions from published correlations, in contrast to multi-(parallel) channel experiments. The latter are subject to maldistribution, 3D conjugate heat transfer effects and larger measurement uncertainties. Sub-continuum mathematical models for fluid dynamics are briefly reviewed and explained. These models are expected to gain a growing interest in the near future due to the rapid descent of microchannel dimensions down to the nano-scale. The paper concludes with a concise set of recommendations for purposes of performance and design. For single channels, available correlations for macro-channels can also give reliable predictions at the micro-scale, but only if all the scaling effects can be considered negligible. Otherwise, when scaling effects cannot be neglected or for the case of heat exchangers with parallel channels, suitable numerical simulations may be the sole alternative to carefully designed experiments to evaluate the heat transfer rates. KeywordsMicrochannels, scaling effects, measurement uncertainties, sub-continuum effects.

Numerical Analysis About the Influence of Inappropriate Shape of the Cross-Section of Microchannels in Laminar Flow

“…Particularly, Figure 5 shows the velocity and temperature contours in the cross-section of the Microchannels 1 and 3 4 for z* ≅ 0.36, in the flow with Re = 200. Figures 5(a-b) show the velocity contours for the Microchannels 1 and 3, respectively, whereas Figures 5(c-d) show the temperature contours for the Microchannels 1 and 3, respectively.…”

“…At the same time, this reduction in physical space is counterweighted by the high performance required at the refrigeration systems in such equipment. Therefore, thermal control is one of the most critical areas for the development of modern microelectronic devices [1][2][3][4][5].…”

“…However, other researchers have indicated that differences in rates and coefficients of heat transfer, as well as in Nusselt number, can be related to the flow velocity and fluid temperature [16,[34][35][36], the Reynolds number [9], the heat transfer conjugated [2,[37][38][39], the viscous dissipation [16,20,26,40,41], the surface roughness [42], the aspect ratio of microchannels [5,43] and the conductivity of the material that compose them [2,[37][38][39], besides experimental uncertainties [4,[30][31][32][33]44]. Some numerical studies [45,46] showed that the Nusselt number is more sensitive to the shape of the cross-section of the microchannels in comparison to other parameters, such as surface roughness, for example.…”

Thermal analysis of a solar collector absorber plate with microchannels

“…Since then, heat transfer in micro-channels has been extensively studied, however, published results are still inconsistent; some studies have found the average Nusselt number to be Reynolds number dependent in the laminar regime [10][11][12], some recorded lower Nusselt numbers [13][14][15] while some recorded higher Nusselt numbers [16][17][18]. Reviews on experimental and numerical studies of heat transfer in microchannels published by Mokrani, Bourouga, Castelain and Peerhossaini [19], Hetsroni, Mosyak, Pogrebnyak and Yarin [20], Rosa, Karayiannis and Collins [21] and Sobhan and Garimella [22] confirm the very large scatter in published results and attribute this to "scaling effects", which arise from neglecting phenomenon which are insignificant in conventional sized channels but become significant with the high channel wall surface to fluid volume ratio in microchannel flow. Some examples of these include surface roughness [14], entrance and exit effects [23,24], axial conduction effects [15,25], thermal boundary conditions [26], viscous dissipation effects [27], electric double layer [28] and increased measurement uncertainties [29].…”

The extended Graetz problem for micro-slit geometries; analytical coupling of rarefaction, axial conduction and viscous dissipation

“…Specifically, Bayazitoglu et al [6], Rosa et al [7], Cetin et al [8], Cetin [9], Haddout and Lahjomri [10] and Yu and Ameel [11] observed that velocity slip increases the Nusselt number (Nu ¼ (h  D H )/k, where h is convective and k is the conductive heat transfer coefficients of fluid), while the temperature jump has a negative effect on heat transfer in single phase microchannel flows.…”