Mini-channels

The definition of mini-channels and micro-channels has not been clearly and strictly established in the literature although many related studies have been done. For example, for compact heat exchangers, Mehendale et al. (1999) gave a relatively... 

Conventional channels Mini-channels Micro-channels Transitional channels Molecular nanochannels... 

Sobhan CB, Garimella SV (2001) A comparative analysis of studies on heat transfer and fluid flow in micro-channels. Microscale Thermophys Eng 5 293-311 Steinke M, Kandlikar SG (2003) Flow boiling and pressure drop in parallel flow micro-channels. In Kandlikar SG (ed) Proceedings of 1st International Conference on Micro-channels and Mini-channels, Rochester, 24-25 April 2003, pp 567-579 Thome JR (2006) State-of-the-art overview of boiling and two-phase flows in microchannels. Heat Transfer Eng 27(9) 4-19... 

Ide H, Kawahara A, Kawaji M (2006) Comparison of gas-liquid two-phase flow characteristics between mini-channels and micro-channels. In Proceedings of 13th International Heat Transfer Conference, Sydney Convention and Exibition Centre, Sydney, Australia, 13-18 August 2006, MPH-51... 

KandUkar SG (2002) Fundamental issues related to flow boiling in mini-channels and microchannels. Exp Thermal Fluid Sci 26 389-407... 

Kandlikar SG, Steinke ME, Tian S, Campbell LA (2001) High speed photographic observation of flow boiling of water in parallel mini-channels. In 35th Proceeding of National Heat Transfer Conference, ASME, New York... 

Kandlikar SG, Balasubramanian P (2004) An extension of the flow boiling correlation to transition, laminar and deep laminar flows in mini-channels and micro-channels. Heat Transfer Eng 25 86-93... 

Bowers and Mudawar (1994a) performed an experimental smdy of boiling flow within mini-channel (2.54 mm) and micro-channel d = 510 pm) heat sink and demonstrated that high values of heat flux can be achieved. Bowers and Mudawar (1994b) also modeled the pressure drop in the micro-channels and minichannels, using the Collier (1981) and Wallis (1969) homogenous equilibrium model, which assumes the liquid and vapor phases form a homogenous mixture with equal and uniform velocity, and properties were assumed to be uniform within each phase. 

Bowers MB, Mudawar I (1994) High flux boiling in low flow rate, low pressure drop mini-channel and micro-channel heat sink. Int J Heat Mass Transfer 37 321-332 Hsu YY (1962) On the size range of active nucleating cavities on a heating surface. J Heat Transfer 184 207-213... 

Internal heat exchange is realized by heat conduction from the microstructured reaction zone to a mini channel heat exchanger, positioned in the rear of the reaction zone [1,3,4], The falling film micro reactor can be equipped, additionally, with an inspection window. This allows a visually check of the quality of film formation and identification of flow misdistribution. Furthermore, photochemical gas/liquid contacting can be carried out, given transparency of the window material for the band range of interest [6], In some cases an inspection window made of silicon was used to allow observation of temperature changes caused by chemical reactions or physical interactions by an IR camera [4, 5]. 

The micro bubble column comprises internal cooling via heat conduction from the reaction zone to a mini channel heat exchanger [3, 9, 10], Either two such heat exchange plates can encompass the reaction plate, or only one. In the latter case, the free position is occupied by an inspection window which allows direct observation of the quality of the flow patterns. 

Agostini B, Watel B, Bontemps A, Thonon B. Experimental study of single-phase flow friction factor and heat transfer coefficient in mini-channels. CHE Symposium, Grenoble Edizioni ETS, August 2002, 85-89. 

P 60] The observation of the flow patterns was performed using a microscope with a video camera [151]. Flow feed was achieved by hydrostatic pressure. A pH-indicator reaction with bromothymol blue and sodium hydroxide solution was applied, resulting in a dark blue color. Experiments were performed at flow rates of 50, 900 and 4000 pi min-1 for the mini channels and 60 and 6000 pi min4 for the micro channels. 

The bended micro channels had a width of 180 pm and a depth of 25 pm and a reduced length (25 mm) compared with the mini channels [151]. The flow in such channels was characterized at two very low Re (1.0 and 0.1) and compared with the flow in straight channels under some hydrodynamic conditions. In all four cases, undisturbed laminar flow was found. Mixing was only detectable at Re = 0.1 owing to diffusion mixing at a much prolonged residence time. At Re = 1, no mixing could be detected. 

B. Agostini, B. Watel, A. Bontemps, and B. Thonon. Friction factor and heat transfer coefficient of R134a liquid flow in mini-channels . Applied Thermal Engineering, 22, 16, pp. 1821-1834 (2002). 

D. Brutin, D. Topin, and L. Tadrist. Experimental study of unsteady convective boiling in heated mini-channels . International Journal of Heat and Mass Transfer, 46, pp. 2957-2965 (2003). 

In confined space, the heat transfer enhancement is caused by evaporation a thin liquid film in near comer area (see Fig. 12) and dry spot formation on the channel wall. The dry spot formation in this area can explain low dependence of heat transfer coefficient on wall superheat [21]. For this case heat flux in vicinity of liquid-solid-vapor contact line has higher level due to evaporation in ultra thin film area [20]. In that way high level of heat transfer in vicinity of contact line is responsible for the heat transfer enhancement during boiling in mini-channels. The possibility of dry spot formation on the wall for water boiling in narrow annular channel was observed in [35] also. At wall superheat over 4.5 K the drying-out of liquid is responsible for decrease of heat transfer when the size of dry area becomes very large. 

The sample with porous coating has advantage to compare with plain tube. The best results were obtained with heat transfer in circular mini-channel. To understand these phenomena the visual observing and photographing of various stage of process were curried out. The photos are presented in Figure 11. 

Two Phase Flow Heat Transfer in Micro- and Mini-Channels... 

A huge interest has been shown in heat transfer in general and boiling in mini- and micro-channels in particular. Many international meeting and conferences were devoted to the subject. Recently Kandlikar [5] published a literature survey on boiling in micro and mini channels summarizing the main investigations in the area. An adapted version of the summary presented by Kandlikar [5] is shown in Appendix 1. 

Analytical (1993) and experimental (1994) stu(ties comp ing the performance of mini and micro channels. Mini channels are preferable unless liquid inventory or weight constraints are severe. 

In several lectures discuss the measurements of single-phase pressure drop and heat transfer coefficient in micro and mini-channels. Experimental results of pressure drop and heat transfer coefficient of flow boiling are presented in mini-channels. Many correlations for flow boiling heat transfer coefficient in mini-channels have been established. 

Kandlikar, S. G., and Balasubramanian, P., An Extension of the Flow Boihng Correlation to Transition, Laminar, and Deep Laminar Flows in Mini-Channels and Micro-Chaimels, Heat Transfer Engineering, 25(3), 86—93 (2004). 

Zhang, W., Hibiki, T., and Mishima, K., Correlation for Flow Boiling Heat Transfer in Mini-Channels, Int. J. Heat and Mass Transfer, 47, 5749-5763 (2004). 

Liquid Crystai Technique for Measuring Temperature, Fig. 10 Sketch of the mini channel... 

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