Boiling and Two-phase Flow in Microchannels

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... 

J.R. Thome, State-of-the-art overview of boiling and two-phase flows in microchannels. Heat Transfer Eng., 2006, 27, 4-19. 

In this chapter, a review of the literature dealing with the aforementioned topics is presented. Additionally, flow pattern, pressure drop, heat transfer coefficient and critical heat flux predictive methods are presented. The main findings of a recently published study by Ribatski et al. [2] evaluating pressure drop and heat transfer coefficient predictive methods by comparing their results against a broad database from the literature are also discussed. At the end of the chapter, the current leading prediction methods for two-phase flow and boiling in microchannels are presented. The interested reader can also refer to a later referenced website by Thome [55], where numerous videos of two-phase flows in microchannels are available. 

Kandlikar, S.G., (2001), Two-phase flow patterns, pressure drop and heat transfer during boiling in minichannels and microchannels flow passages of compact evaporators. Keynote Lecture presented at the Engineering foundation Conference on Compact Heat Exchangers, Davos, Switzerland, July 1-6. 

Agostini, B., Revellin, R., Thome, J.R., Fabbri, M., Michel, B., Kloter, U. and Calmi, D., High Heat Flux Flow Boiling in Silicon Mnlti-Microchannels Part 111 - Satnrated Critical Heat Flux of R236fa and Two-Phase Pressnre Drop, Int. J. Heat Mass Transfer, 41, 5426-5442, (2008). 

Bubble dynamics in a microchannel refers to the nucleation of a bubble on heated flow channel walls and its subsequent interaction with the single-phase or two-phase flow field. Subcooled flow boiling refers to the boiling process with the mean flow enthalpy below the saturated liquid enthalpy at the local pressure, while saturated flow boiling refers to the boiling process with the mean flow enthalpy at or above the saturated liquid enthalpy but below saturated vapor enthalpy at the local pressure. 

Brutin et al. [9] studied the transition from steady to unsteady flow boiling and developed a transition criterion. This study was performed experimentally in minichannels but the theory also applies to confined geometries such as microchannels. This criterion is based on observations of the unsteady two-phase flow the two-phase flow stops at a location in the microchannel, then all the fluid after this location is expelled toward the exit, whereas all the fluid before this location is sent back to the entrance. This two-phase backflow has been evidenced previously [7] however, the location of this split remained unexplained. 

Thus, ongoing research is focusing on the thermal and fluid properties of nanofluids as well as such mixmre flow in microchannels. How to optimize the cooling capabilities of nanofluids in microchannel heat sinks will be a challenging task. Alternative approaches include fluid jet impingement and two-phase, i.e., liquid-vapor, flow with boiling heat transfer. 

Qu et al. [2] found evidence of two kinds of unsteady flow boiling for 21 parallel microchannels measuring 231 x 713 xm. They observed in their parallel microchannel array either a global fluctuation of the whole two-phase zone for all the microchannels (Fig. 1) or chaotic fluctuations of the two-phase zone (Fig. 2) over-pressure in one microchannel and under-pressure in another. The individual microchannel mass flow rate was not controlled. Hetsroni et al. [3] created an experimental setup to study liquid-gas and liquid-vapor flow in parallel triangular microchannels with diameters of 103 to 161 p.m. They used a fast video camera coupled with a microscope through a Pyrex plate to record the flow patterns. They showed the influence of the injection method (plenum shape) and found evidence for the same inlet conditions. 

Peles YP (2003) Two-phase boiling flow in microchannels -instahUities issues and flow regime mapping. In 1st International Confeence on Microchannels and Minichannels, Rochester... 

Li HY, Lee PC, Tseng FG, Pan C (2003) Two-phase flow instability of boiling in a double microchannel system at high heating powers. 1st International Confsence on Microchannels and Minichannels. ASME, Rochesto, pp 615-621... 

As a fundamental investigation on heat pipes, Tchikanda et al. [54] derived analytical expressions for the mean velocity of a liquid flowing in an open rectangular microchannel. The flow is driven by pressure gradients and shear stress on the liq-uid/vapor interface. Koizumi et al. [55] developed a microheat pipe with asymmetric cross-section and observed boiling two-phase flow. 

The dryout incipience correlation has been presented in the previous section. The generalized correlation for predryout two-phase heat transfer coefficient associated with saturated flow boiling in mini-/microchannels is discussed in the present section. Kim and Mudawar... 

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