Estándares de aprendizaje

As can be seen from the above review of literature, the knowledge and understanding of two-phase flow and heat transfer behavior in microchannels are far from being complete. A review of relevant literature studies demonstrates the necessity to pursue fundamental studies in these relatively new areas where studies devoted to some of these topics remain very limited.

2.4.1 Two-phase Pressure Drop

Most of the microchannels two-phase pressure drop correlations have been reviewed in Section 2.2.1. Some limitations of these correlations are given below:

1. It may be noted from the above that a general correlation for the evaluation of two-phase frictional pressure drop for laminar-liquid laminar-vapor and laminar-liquid turbulent-vapor flow, using multiple refrigerants, does not exist up to now.

2. There is a need to generate a databank for laminar-liquid laminar-vapor flow in microchannels, since this database is less extensive.

All the experimental investigations and correlations of two-phase frictional pressure drop reported in the literature (Lee and Lee, 2001a; Qu and Mudawar, 2003a; Lee and Mudawar, 2005a; Lee and Garimella, 2008), are based on data from microchannels with

hydraulic diameters larger than 349 m. The two-phase pressure drop characteristics and applicability of correlations of frictional pressure drop to a rectangular microchannel with a smaller hydraulic diameter has not yet been investigated, and needs to be clarified. Furthermore, a general correlation to predict the frictional pressure drop in microchannels for different ranges of mass flux and several refrigerants has not yet been developed in the literature.

2.4.2 Flow Boiling Heat Transfer

A significant amount of fundamental research has focused primarily on heat transfer characteristics in straight microchannel heat sinks as shown in the previous review of the literature. Until recently, there are large discrepancies in literature between results reported by different investigators for flow boiling in microchannels. With respect to trends, the presented studies from the literature can be divided into two groups. In the first group of studies, (Kuznetsov and Shamirzaev, 2007; Yun et al., 2006; Lee and Lee, 2001b), the heat transfer coefficient increases with increasing exit quality. The second group of studies (Steinke and Kandlikar, 2004; Lee and Mudawar; 2005b, Ravigururajan, 1998; Yen et al., 2006) shows that as the exit quality increases, a significant decrease in the heat transfer coefficient is observed. Clearly, a comprehensive investigation of microchannel flows is needed to resolve the inconsistencies in the reported results. Furthermore, limited research has been conducted to understand the flow characteristics in microchannel heat sinks using flow visualization. This will provide crucial information to the fundamental understanding of these characteristics and explain discrepancies on experimental data. Nevertheless, although many models and correlations exist for two- phase flow in microchannels, their applicability needs to be clarified.

The majority of past flow boiling studies carried out in microchannel heat sinks were performed using thermocouples to measure the wall temperature on the outer surface of the channels. Thermocouples measure bulk average properties in the vicinity of the transducer, and have a great amount of difficulty measuring highly local temperatures, particularly as dimensions are reduced to the microscale. In some cases, however, flow and temperature measurements through the use of transducers are unavoidable; these measurement devices are typically intrusive and create losses and uncertainties, which are amplified at micro dimensions.

Newer measurement techniques were developed to measure experimental parameters in the micro domain, such as un-encapsulated Thermochromic Liquid Crystals (TLC) for fine wall temperature measurements. TLC’s are a surface coating capable of qualitative temperature measurements through their color indicators and quantitative temperature measurements through a calibration process. This technique is particularly favorable for microscale temperature measurements due to TLC’s non-intrusive nature, capability of full surface mapping and detection of onset of boiling, and very high spatial resolution, particularly in their un-encapsulated form.

2.4.3 Analytical Modeling of Flow Boiling Heat Transfer

A key element in flow boiling in mini- and microchannels – the heat transfer coefficient – is far from properly predicted. Though numerous correlations have been developed to predict the heat transfer coefficient during flow boiling in mini- and microchannel heat sinks (Lee and Lee, 2001b; Lee and Mudawar, 2005b; Bertsch, et al., 2009a; Zhang et al., 2004), very little is known about the validity of these correlations. The available heat transfer correlations for flow boiling in mini- and microchannels

satisfy only limited experimental data and there is little unanimity over a single correlation. Hence, their applicability to other test data and fluids may be questionable. Furthermore, these correlations rely heavily on regression analysis of the tested data, and none were derived based on analytical modeling. Therefore, it is essential to have an analytical model to describe the flow boiling heat transfer in mini- and microchannel heat sinks.

2.4.4 Cross-linked Microchannel Heat Sink

There has been much research on flow boiling in straight microchannel heat sinks up to the present time. Nevertheless, very limited studies have been conducted to investigate flow boiling characteristics in cross-linked microchannel heat sinks (Jiang et al., 2002; Cho et al., 2003; Muwanga and Hassan, 2007; Xu, et al., 2006), although the cross-linked designs have shown better characteristics compared to the straight design in terms of cooling performance, surface temperature, and flow distribution uniformity. Hence, the flow boiling characteristics of cross-linked microchannel heat sinks should be further investigated as such configurations have not been studied extensively.