M ultiphase flows are used in a wide range of industrial applications such as heat exchangers, nuclear reactors, food production, chemical processes, and electronic cooling. It is important to understand, model and simulate multiphase phenomena such as boiling, for cooling technology.In boiling systems, a large amount of heat is transferred from the surface within small temperature differences between the surface and saturation temperature of the fluid. Although there are many experimental, computational and analytical studies on boiling phenomena, its highly complex physical mechanism hasn't been clarified yet. In recent years, bubbling characteristics have been observed visually on several surface materials with various surface properties such as roughness, cavity size, shape, and spacing. Zhang and Shoji (2003), made experiments on a silicon surface (15 mm diameter, 0.2 mm thickness) with distilled water [1]. They worked on cylindrical artificial cavities (10 µm diameter and 80 µm depth) arranged as single or twin cavities with 1, 2, 3, 4, 6 and 8 mm spacing. The bubbling behavior is recorded with a high-speed camera and temperature fluctuations beneath and around the cavities are measured with radiation thermometers. According to their observation and analysis, three significant factors involving nucleation site interactions are determined: hydrodynamic interaction between bubbles; thermal interaction between nucleation sites;

Mini and microchannel applications have become an important and attractive research area during the past decades. For micro systems design purposes, numerical and experimental studies have been conducted on flow and heat transfer characteristics of mini and microchannels and various friction factor and Nusselt number correlations have been proposed. Some researchers have tried to apply conventional tube correlations to mini and micro channels, rather than deriving new correlations. In this study, using commercial CFD software, flow and heat transfer characteristics in laminar and turbulent flow through circular channels are analyzed numerically. The applicability of conventional correlations in calculating the friction factor and Nusselt number is investigated. It is concluded that, in laminar regime conventional correlations can be used to calculate the friction factor for the channel sizes considered. In turbulent regime, however, numerical results for friction factor yielded greater values than those calculated by the conventional correlations. Numerical Nusselt numbers are found to be closer to the conventional values in laminar and turbulent regimes. In turbulent regime, on the other hand, Nusselt number values calculated with the microchannel correlations are determined to be greater than the numerical results and the values calculated with the conventional correlations.

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