Microchannel two-phase cooling is accomplished with the help of a heat sink that consists of a high conductivity material containing parallel, small diameter channels. The simplicity and ease of fabrication of the design are the key reasons behind its unprecedented popularity in the industry. Most microchannel geometries of interest possess diameters in the range of 0.1–0.6 mm. These microchannel devices are therefore very compact and lightweight, and provide high heattransfer coefficients by capitalizing upon the coolant’s latent heat content rather than the sensible heat alone (seen in single phase liquid cooling). This greatly reduces the flow rate required to dissipate the same amount of heat compared to single-phase cooling, which also helps reduce coolant inventory for the entire cooling system. Flowboiling with microchannels also provides better temperature uniformity by maintaining surface temperatures close to the coolant’s saturation temperature. However, two-phase microchannel cooling is not without shortcomings, and their implementation is hindered by the relatively limited understanding of two-phase flow in microchannels.
12 occur in flowboiling at different heat fluxes: bubbly flow, slug flow, annular flow, churn flow, and wispy- annular flow. For increasing values of heat flux the growth of vapor bubbles or slugs in microchannels can be expected to occur at progressively higher rates for any given flow rate. Given a high enough heat flux, the vapor slugs will grow at a high enough rate to grow not only in the downstream, but also in the upstream direction. Kandlikar  suggested that these flow instabilities can be reduced considerably by introducing an upstream pressure drop prior to a microchannel inlet. The flow instabilities and reversals, and high pressure drop were demonstrated visually using high-speed imaging by Balasubramanian and Kandlikar . In their experimental study they found several microchannels in a heat sink exhibited back-flow at moderate heat fluxes and reached an early CHF. In some cases the vapor bubbles traveled upstream and reached the inlet manifold, resulting in severe flow maldistribution, in some cases leading to unexpected channel dry-out and premature onset of CHF. In a numerical study, Mukherjee and Kandlikar  proposed the use of channels with increasing cross-sectional area in the direction of flow to promote the unidirectional growth of vapor slugs and thus prevent flow reversals. Their concept, while simple in nature, significantly complicated the machining of the channels, as non-parallel channel walls with variable aspect ratio were needed. This proposed concept was implemented by Kandlikar et al.  and Kalani and Kandlikar . They used openmicrochannels, allowing a volume of fluid to fill the space above the channels, with a tapered gap manifold, in which the cross-sectional area of the manifold increases in the flow-wise direction. This microchannel geometry was found to have a significantly reduced pressure drop in addition to featuring enhanced CHF, in comparison with microchannels with a uniform manifold. The performance of this geometry was claimed to be a result of the favorable path that is provided for the vapor bubbles to expand into, thereby reducing pressure drop and noticeably reducing flow instability. The advantage of the open microchannel geometry with a taperedmanifold lies in the fact that machining remains simple, as the channel walls can be parallel, with the manifold taper providing the cross-sectional area increase in the flow-wise direction that was proposed by Mukherjee and Kandlikar .
With the rapid development of modern science and technology, flowboiling and two-phase flow in micro- and mini-channels play a significant role in cooling high heat flux in various engineering applications. Over the past decades, VLSI (vast large scale integrated) chips’ heat fluxes have already reached up to 300 W/cm 2 . The traditional cooling technology cannot meet the growing demand of heat dissipation in microelectronics and high-performance computer chips etc. Tuckerman and Pease  investigated heattransfer performance in microchannel heat sinks which have
Effect of horizontal flow (0°) and vertical downflow (-90°) flow orientations on the performance of plain and microchannel heat sinks is presented in Figs. 31(a) and 31(b) in uniform and 4% taper manifold geometries. It is seen that both plain and microchannel heat sinks show superior heat dissipation performance in horizontal orientation as compared to vertical downflow orientation in both manifolds. This difference in performance is attributed to the action of gravitational force which is reflected in the form of buoyancy on the liquid-vapor interface. In downward moving fluid, buoyancy effect forces the vapor to rise in the direction opposite to that of incoming fluid. The effect of buoyancy on flowboiling performance depends on its relative strength to liquid inertia forces. Thus, at low mass fluxes buoyancy effects scale up due to low liquid inertia, leading to intensified flow reversals and pressure fluctuations, based on Eqns. (1) – (3). Similar trends are observed in Figs. 32(a) and 32(b) where, heattransfer coefficient is higher when fluid flows horizontally over the heat sink surface.
Chapter 3 and 4 describes the enhancement achieved from micromachined surfaces. The area enhanced surfaces also provide convective pathways for bubble removal and liquid recirculation. Also, literature review presented in chapter 1 has shown that porous surfaces provide additional nucleation sites which significantly reduce temperature difference and increase heat dissipation rates. Patil and Kandlikar  have shown that a combination of enhancement techniques results in high heattransfer coefficients. The scope of the current work is developed further by using sintering to create porous structures on open microchannel geometry. Furthermore, porous coatings are deposited on all microchannel regions in the boiling area, channel bottom and fin tops compared to Patil and Kandlikar  where deposition is reported only on microchannel fin tops. Open microchannel is selected as against cross-linked flow passages to establish that porous coatings can be deposited by sintering on these surfaces. Moreover, pool boiling performance on selectively sintered microchannels has never been done before which furthers strengthens the claim of the work presented in this chapter.
transfer became a main issue owing to low thermal conductivity of the most common fluids such as water , oil, and ethylene-glycol mixture. Since the thermal conductivity of solids is often higher than that of liquids, the idea of adding particles to a conventional fluid to enhance its heattransfer characteristics was emerged. Among all the dimensions of particles such as macro, micro, and nano, because of some obstacles in the pressure drop through the system or the problem of keeping the mixture homogeneous, nano-scaled particles have attracted more attention. These tiny particles are fairly close in size to the molecules of the base fluid and, thus, can realize extremely stable suspensions with slight gravitational settling over long periods. The word “nanofluid” was proposed by Choi  to identify engineered colloids composed of nanoparticles dispersed in a base fluid. Following the seminal study of this concept by Masuda et al. , a considerable amount of research in this field has risen exponentially. Meanwhile, theoretical studies emerged to model the nanofluid behaviors. To date, the proposed models are twofold: the homogeneous flow models and the dispersion models. Buongiorno  Nomenclature
This paper presents the method of estimating uncertainty of temperature measurements conducted using K-type thermocouples. Such measurements are conducted in research on flowboilingheattransfer in minichannels. The main aim of this work is to calculate accuracy of fluid temperature measurements by K-type thermocouples using elements of statistical analysis. Estimation of uncertainty of difference temperature was also discussed.
Abstract. The paper focused on flowboilingheattransfer in an annular minigap. This gap of 1 mm width was created between the metal pipe with an enhanced surface contacting fluid and the external glass pipe positioned along the same axis. The heated element for the HFE-649 flowing in the minigap was a cartridge heater. Thermocouples were used to measure the temperature of the metal pipe in the contact surface with a fluid. The local values of the heattransfer coefficient for stationary state conditions were calculated using an one-dimensional method in which the multilayer cylindrical wall was assumed to be planar. The results were presented as a function of the heattransfer coefficient along the minigap length and as boiling curves, prepared for selected values of mass flow rate and five types of the enhanced heated surface and a smooth one. Observations indicated that the highest local values of heattransfer coefficient were obtained with using the enhanced surface produced by electromachining process (spark erosion) at the saturated boiling region. The boiling curves generated for two distances from the minigap inlet have similar plots without a drop in the temperature of the heated surface characteristic for nucleation hysteresis.
On the other hand, various theoretical models have been proposed to predict the local heattransfer related to the condensation annular flow in non-circular channel where the surface tension plays a predominant effect on the condensate flow, more specifically, in the channel corners. Indeed, Zhao and Liao  analyzed annular film condensation heattransfer inside vertical mini tri- angular channel using three zones: the thin liquid film flow on the sidewall, the condensate flow in the corners, and the vapor core flow in the center. Wu and Cheng  carried out a simultaneous visualization and measure- ment experiment to perform condensation flow patterns of steam flowing through an array of trapezoidal silicon microchannels with a hydraulic diameter of 82.8 µm. Wang et al.  and Wang & Rose  proposed a the- oretical model for condensation annular flow in a hori- zontal square and equilateral triangular channel with hy- draulic diameter ranging from 0.5 to 5 mm by taking into account the effects of gravity, surface tension, and inter- facial shear stress. They obtained the local heattransfer coefficient for refrigerants R134a, R22, R152a, CO2, propane, ammonia, and R410a by assuming that the channel wall temperature is uniform. Furthermore, they proposed one correlation for condensation heattransfer in the square and triangular microchannels in which sur- face tension and viscosity are the predominant parame- ters controlling condensate film thickness. Additionally, in references ([15-18]) for which the contents are not detailed here, readers could find more information about the use of various non-circular microchannel shapes.
Several flow pattern maps are available for predicting two-phase flow regimes in horizontal tubes, but most of them were developed based on air-water data and few were specifically developed for refrigerants. In order to overcome this shortcoming, some empirical factors were introduced to extrapolate these air-water maps to refrigerants. Another important characteristic is that most maps were developed for adiabatic conditions and then extrapolated to diabatic conditions. As it has been pointed out previously, the extrapolation procedure may not always produce reliable results. The original Kattan-Thome-Favrat  flow pattern map and their respective updates were developed specifically for refrigerants under vapour/liquid and gas/liquid conditions, overcoming the two drawbacks previously mentioned. Furthermore, the Wojtan-Ursenbacher-Thome version of the original Kattan-Thome- Favrat flow  pattern map includes the influences of heat flux and dryout on the flow pattern transition boundaries, providing a much more accurate prediction of the flow regimes for horizontal flow. The same has not yet been considered for vertical flows.
The model has been verified by comparison with previous investigations on flowboiling in metal foams filled tube, as shown from Fig. 5 to Fig. 7. In Fig. 5 that the equivalent boilingheattransfer coefficients under different mass flow rates agree mainly with the experimental results by Zhao et al. . The analytical results are a little higher than the experimental data with a maximum deviation of 25%. However, for fixed fibre diameter of metal foams, the results predicted by the model agree well with the experimental data, and this is shown in Fig. 6. The case for the high density heat flux is also investigated for comparison, which is shown in Fig. 7.
This paper presents a comparison of the performance of three smooth heated surfaces with different thicknesses. Analysis was carried out on an experimental stand for flowboilingheattransfer. The test section with a rectangular minichannel, 1.7 mm deep, 16 mm wide and 180 mm long, oriented vertically. The heated element for the working fluid (FC-72) was designated as a Haynes-230 alloy plate: 0.10 mm and 0.45 mm thick or Hastelloy X alloy plate about 0.65 mm thick. Infrared thermography was used to measure the temperature of the outer plate surface.
Saisorn et al. (2010) p erformed flow visualization study for R- 134a refrigerant during flowboiling in a circular channel having a diameter of 1.75 mm. Slug flow, throat-annular flow, churn flow, annular flow and annular-rivulet flow were observed and found t o influence the flowboilingheattransfer process as seen in Fig. 2. Slug flow appeared with the lowest heattransfer coefficient in comparison to the other flow regimes. Annular-rivulet flow showed a relatively high heattransfer coefficient but a local dry-out region was observed at high vapour qualities, which has been undesirable for a thermal design approach dealing with a cooling system implemented with small channels. Moderate values of he at transfer coefficient were given by throat-annular flow, churn flow and annular flow which might be good choices for t he development of t he micro-scale devices. Besides, their flow pattern data were compared with the transition lines by Triplett et al. (1999) for t wo-phase air-water flow through a 1.45 mm diameter channel. In general, the comparisons showed inconsistencies between the flow pattern map established from two-phase gas-liquid flow and that from phase-change process. Such inconsistencies were also reported by Yang and Shieh (2001) and Martin-Callizo et al. (2010). Yang and Shieh (2001) performed flow visualization with air-water mixture and refrigerant R-134a, and the comparison between such two cases were discussed. Martin- Callizo et al. (2010) c onducted the visualization of R -134a during flowboiling in a tube with a diameter of 1.33 m m. Their test section was made from a quartz glass tube coated externally by Indium Tin Oxide (ITO) which was served as the resistive coating over which a potential difference generated by a DC power supply was applied. Their flow pattern data were also compared with the transition lines by Triplett et al. (1999), i ndicating that the agreement was not satisfactory. However, two-phase gas-liquid flow phenomena tend to be compatible with flow mechanisms based on phase-change process in different aspects. In m icro-channels, for instance, Saisorn and Wongwises (2010) re ported the fair agreement between their gas- liquid flow pattern data and the transition lines of G arimella et al. (2002) for condensation flow.
saturation temperatures, mass fluxes, heat fluxes and working fluids etc. Therefore, the extrapolation of the available prediction methods to other fluids and channels do not work properly. It must be stressed here that big discrepancies among the experimental data from different independent laboratories may be caused due to different surface roughness of the test channels, channel dimension uncertainties, improper data reduction methods, flowboiling instabilities, improper designed test facility, test sections and experimental procedures. In some cases, the published results are unreasonable such as too big or too small heattransfer coefficients, some or complete wrong heattransfer behaviours and trends and correlations of various parameters and physical properties even if they have been published in journals. For instance, quite anomaly heattransfer trends are presented but they cannot be explained according to the corresponding flowboiling mechanisms in some papers although it is said that such mechanisms account for the heattransfer behaviours as detailed in a recent analysis in a comprehensive review by Ribatski et al. . Furthermore, some flowboilingheattransfer correlations and models were proposed by simply regressing limited experimental data at limited test parameter ranges without considering the heattransfer mechanisms. Therefore, it is necessary to evaluate these correlations to validate their applicability before using these correlations. In fact, in most cases, such correlations do not work properly for other fluids and conditions. Therefore, it is essential to analyse and evaluate these correlations to identify further research needs in this important field.
is, much lower than those obtained for the saturated boiling region. The lowest coefficients for both liquids were at the inlet to the minichannel, and the highest - at the outlet. The heattransfer coefficient calculated when HFE-7100 was used, started rising with the increasing distance from the minichannel inlet, reaching the highest values at a 2/3 length of the channel from the inlet. Heattransfer coefficients obtained with HFE-7100 at mass flow velocity of Q m = 18∙10 -3 kg∙s -1 at all the heat fluxes
Sobhan and Garimella  presented a comparative analysis of studies on fluid flow and heattransfer in microchannels. They concluded that the differences between the flow and heattransfer in microchannels is quite different from that observed in channels of conventional sizes. Moreover, in spite of the available information in literature, there is no clear evidence to support reasons for these trends. The discrepancies in predictions may very well be due to the entrance region effects, differences in surface roughness elements in different microchannels investigated, non-uniformity of channel dimensions or due to some errors in measurements and high level of uncertainty while performing experiments. There arises a clear need to perform systematic numerical analysis which can consider each parameter influencing fluid flow and heattransfer properties in microchannels.
Azizi et al.  investigated the thermal performance of Cu-water nanofluid through a rectangular microchannel assembled into a cylindrical geometry. The Nusselt number is enhanced up to 23 % for 0.3 wt % nanoparticle addition. Rimbault et al.  reported that the CuO-water nanofluid flow through rectangular microchannels has the higher pressure drop by 70 % with a 4.5 % nanoparticle addition. However, the ratio of the amount of heat transferred to the pumping power required decreases with an increase in nanoparticle volume fraction. Kalteh  has compared different nanofluids which are a combination of nine different nanoparticle and three different base fluids. The highest and the lowest heattransfer coefficients were obtained for diamond-water and SiO 2 -water nanofluid, respectively. However, the
Microchannel structures are utilized widely for more than 20 years in miscellaneous industries. These systems are used by manufacturers who wanted efficient heat exchange in a compact design. Today, the advantages of microchannel systems are beginning to challenge traditional coil technology in different manufactures. As an example, The air conditioning industry faces a continuous challenge to provide higher efficiency levels and greater equipment reliability. This challenge is even more difficult to meet when the aim is simultaneously to maintain equipment size and reduce potential cost influence. Previous engineering solutions designed to compensate these requirements have typically included such changes as improving individual components or increasing the overall heattransfer surface area to increase thermal efficiency. However, each of these improvements tends to increase equipment size, cost, or both. An optional solution for air conditioning applications is microchannel heat exchanger technology. This heat exchanger technology has been widely used in the automotive industry for many years, with considerable success .
To visualize boiling process with high spatial and temporal resolutions digital video camera Vision Research Phantom v.7.0 with frame rate up to 1000 FPS and maximum spatial resolution of 24 µm/pix was used. Visualization was performed from the bottom side of the transparent heater. As it will be shown below, this format of visualization allowed us to study in detail the dynamics of triple contact line in the base of vapor bubbles and nucleation site density at boiling in the range of low heat fluxes. To study the shape of the growing and departing bubbles the video recording from the side of the heater also was performed. In the experiments high-speed thermographic (IR) camera FLIR Titanium was used to measure the non-stationary temperature field of the heating surface. As configured for this study, the thermographic camera had a frame rate of 1000 FPS and maximum resolution of 150 µm/pix.