Effective thermal management of highheatflux systems has gained widespread importance in the scientific community over the last couple of decades. Two-phase heat transfer, which utilizes latent heat of vaporization of liquid has been studied extensively for its highheat dissipation performance. Kandlikar and Grande  presented a channel classification based on the range of hydraulic diameters passages employed for fluid flow and termed microchannels as flow passages with hydraulic diameter between 10 µm to 200 µm, as shown in Table 1. Flowboilingheat transfer in microchannels has shown great promise in enhancing heat transfer performance due to the inherent ability of microchannels to provide superior heat transfer coefficient . Studies have shown heatflux greater than 100 W/cm 2 being dissipated usingflowboiling in microchannels [3,4]. This makes it a promising candidate for cooling highheat dissipation cooling applications like cooling of servers, turbine blade, solar arrays, boilers and onboard electronics in aircrafts and satellites, to name a few. However, flowboiling performance is affected due to inherent issues like flow instabilities and reversals which lead to surface temperature and pressure fluctuations and intermittent dry-outs, thereby affecting its performance considerably [1,5]. Moreover, utilization of a pump to drive the fluid renders the system cost ineffective, energy inefficient and bulky, wherever weight is a constraint. Hence, it was necessary to address these problems before large scale implementation.
The vapor bubble in the uniform manifoldmicrochannel expands on the microchannel surface and causes dry out state. This dry out results in high pressure drop, low heat transfer coefficient and early CHF state . The tapered manifold is a type of design which provides a much smoother flow of vapor-liquid combination by creating separate liquid vapor flow paths. The increasing cross section area allows bubble growth in the vertical direction thus avoiding a dry out condition to a large extent. The bubble in this type of manifold flows above the liquid region because of a density difference since tapermanifold provides much extra vertical space compared to the uniform manifold. Also, such type of bubble growth and flow provides nucleating sites for new bubbles under fully developed bubbles (Fig.15), thus making the bubble formation continuous and faster
12 occur in flowboiling at different heat fluxes: bubbly flow, slug flow, annular flow, churn flow, and wispy- annular flow. For increasing values of heatflux 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 heatflux, 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 usinghigh-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 open microchannels, 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 openmicrochannel geometry with a tapered manifold lies in the fact that machining remains simple, as the channel walls can be parallel, with the manifoldtaper providing the cross-sectional area increase in the flow-wise direction that was proposed by Mukherjee and Kandlikar .
design. Therefore, the objective of this paper is to investigate the flow phenomenon in a shell and tube heat exchanger to improve the heat exchanger design. The experimental and numerical study of the horizontal shell and tube heat exchanger were successfully conducted. The flow regime and heat transfer rate were investigate using experimental work. The vector velocity, heat transfer and temperature distribution were studied using a CFD.
The study of nanofluids [Wen et al 2009] started over a decade ago aiming to enhance the thermo physical properties of different heat transfer fluids [Buongiorno et al 2009], with late applications in single phase forced convection heat transfer [Wen and Ding 2004; Rea et al 2009] and two-phase convective (pool boiling, flowboiling and in-tube condensation) heat transfer. It has been generally observed the presence of nanoparticles inside a base liquid increased the critical heatflux for pool boiling [Bang and Chang 2005; Kim et al 2007c] and flowboiling in macroscale channels [Kim et al 2008;, Kim et al 2009] and microchannels [Rea et al 2009]. However the quantitative results still differ: the CHF enhancement has been reported to vary from 10-40% [Bang and Chang 2005] to 200-400% [Kim et al 2007c]. It is difficult to explain such wide differences. The possible reasons reside in the stability of nanofluids and the variability of the solid and the liquid phases, as well as the surface wettability change due to complicated interactions between nanoparticles and the heating surface. Most of nanofluids used in boiling experiments tend to be unstable at the high temperatures due to the failure of stabilizers. The stabilizers and particles could agglomerate and deposit on the heating surface, which certainly modifies the heating surface and brings about a number of unforeseen effects, including the change of active nucleate cavities, the modification of surface wettability and the formation of extra thermal resistance on the heating surface that prevents direct contact of liquid with the boiling surface [Wen 2008a,b]
Schematic structure of the studied counter flow mi- crochannel heat exchanger with square channels is shown in Figure 1. To study the entire CFMCHE numerically it is complicated and needs huge of CPU time. Therefore and due to the geometrical and thermal symmetry be- tween hot and cold channels rows, an individual heat exchange unit which consists of two channels containing hot and cold fluids and a separating wall is considered as shown in Figure 2 will be used as a model to represent the complete counter flowmicrochannelheat exchanger since it give an adequate indication about its performance and the heat is transferred from hot fluid to cold fluid through the thick wall separating them.
r induces a numerical oscillation, whereas excessively small values cause the interfacial temperature to substantially deviate from T sat . In the present study, the source term Src in Eq. (4) is evaluated using Eq. (22), where α l and α v can be obtained within the framework of VOF model. An alternative method of regulating the interface temperature based on artificial parameters was discussed by Zhang et al. (2001). 2.5 Geometric Configuration and Boundary Conditions In the present study, a straight channel with a rectangular cross-section is considered. A vapor-venting porous membrane covers the top of the channel. A uniform heatflux is applied to the bottom and two sidewalls. All other walls are set to be adiabatic. A velocity and a pressure boundary condition are assigned to the channel inlet and outlet, respectively. Pure liquid water is pumped into the channel at a temperature around 3K below the saturation point, creating a sub- cooling boiling environment. The membrane-channel interface has a no-slip boundary condition and an effective contact angle, while the other side of the membrane has a pressure boundary condition. The geometry of the simulation domain is shown in Fig.2 and the parameters are listed in Table 1.
from the local hot spots of the processor . Thin and ultra-thin (less than 10 µm in thickness) liquid films, moving under the action of a forced gas flow in a mini-channel, are promising for the use in the temperature control systems of modern semiconductor devices . The numerical and analytical studies of the heat and mass transfer processes and hydrodynamics of joint motion of intensively evaporating liquid film and gas flow in a mini-channel have been performed in [3-4]. The authors of [5-7] have determined the basic laws of the flow and crisis phenomena in the liquid film, moving under the influence of a gas flow in a horizontal channel, under moderate heat fluxes. The first experiments with locally heated liquid films, moving under the action of gas, showed that they are less subject to the breakdown than the liquid films moving under action of gravity. This is because the film breakdown and formation of dry spots can be controlled by the forced gas flow. Liquid film breakdown and formation of dry spots are important for studying the crisis phenomenon in the liquid films [8-9]. The main goal of this work is to study the breakdown and critical heatflux in the locally heated shear-driven films of water. Experimental data on removal of the heat fluxes higher than 1 kW/cm 2 from a heat source
The existing experimental and mechanistic investigations on the nucleate boilingheat transfer and CHF phenomena in pool boiling are summarized and analyzed here. It is helpful to put the available results in proper perspective. For instance, nucleate pool boiling data are often measured with about 20-30% errors and the experimental data from independent studies on the same pure fluid often disagree by 30-50% or more. The traditional prediction methods and models are sometimes used/modified to predict the nanofluids boilingheat transfer coefficients and CHF. However, they are still limited due to many controlling factors such as inaccurate physical properties, the poor understanding of the physical mechanisms and the lack of systematic and accurate experimental data and so on. Therefore, it is essential to present a comprehensive and deep analysis of the available studies of the important topics. These studies are also the basis to understand more complicated flowboilingheat transfer and CHF with nanofluids which are also discussed in this review. Furthermore, as mentioned in the physical properties of nanofluid, nucleate boiling and CHF phenomena are strongly related to the relevant physical properties. Without proper knowledge of the physical properties, it is difficult to obtain accurate knowledge and reasonable physical mechanisms with nanofluids. In fact, this is the case for most of the available studies in the literature because how to evaluate the relevant physical properties in the reduction of the experimental data is not clearly given in these studies.
Katto and Shoji (1970) utilized radial spreading of a flattened air bubble that was artificially produced by injecting air into a narrow space between a pair of parallel disks (diameter: 30 mm, gap size: 0.5 mm) filled with a transparent liquid (water or methanol), and the behavior of the microlayer thus created on a solid surface was observed. Air bubble growth and the microlayer thickness were simultaneously measured using a light-interference technique and high-speed photography. They found that the increasing rate of the microlayer thickness with the radial distance from the center becomes greater as the spreading velocity of the bubble increases. Their experiments were performed for local spreading velocities within the range 0.05–0.75 m/s, which is considered to be a similar order as the bubbles growth rate in normal nucleate boiling, but slower than that in a microscale boiling system.
As shown in Fig. 3(b), the inlet and the outlet are arranged horizontally for working fluid. The inlet and the outlet are connected with a stainless steel tube to reduce the instability of flow. At the end-piece of the inlet, there is a rectangular sink which is implemented to drop the flow fluctuation of the incoming flow, and the side walls of the sink are designed two holes that are used to measure the pressures and the temperature respectively. The equalization board is set up at the top of the sink, to ensure that the working fluid is distributed in microchannels equably. The O-ring is pressed out between the glass top cover and bottom housing to seal up the working fluid. The bottom housing and the insulating base made of Teflon are used to insulate the copper heating block. When the all parts are mounted. the silicone gaskets and the steel plate can make sure the glass top cover won’t be cracked by the pressure.
Experiments were performed to study the effects of surface wettability on flowboiling of water at atmospheric pressure. The test channel is a single rectangular channel 0.5 mm high, 5 mm wide and 180 mm long. The mass flux was set at 100 and 120 kg/m² s and the base heatflux was varied from 30 to 80 kW/m². Water enters the test channel under subcooled conditions. The sample surfaces are titanium (Ti) and diamond-like carbon (DLC) surfaces having a contact angle of 49° and 63°, respectively. The experimental results show different flow patterns that impact the heat transfer significantly. Compared to the Ti surface, the DLC surface shows a deterioration of 10% in heat transfer.
Li and Haramura  investigated analytically the heat transfer characteristics of the reciprocating laminar ow in a micro-channel type porous-sheets Stirling regenerator, including the entrance eects, to facilitate the ecient design and optimization of Stirling engines. Wand  developed an approach using the Ritz method for slip ow and heat convection that can be applied to any tube cross section. This method for slip ow and constant ux heat transfer in ducts of general shape was established and applied to the isosceles triangular duct, yielding tables for the Poiseuille number and the Nusselt numbers. This approach is a boundary tted method such that curved boundaries and sharp corners do not pose a problem. In addition, the boundary conditions for the slip ow need local normal derivatives, which are dicult to construct for nite dierence or nite element schemes. But, according to the current theorems, these mixed boundary conditions are automatically satised by the minimization of the functional.
shown qualitatively by Kandlikar (2006). This pressure spike can be stabilized by introducing a pressure drop element, in addition to artificial nucleation sites, and by operating the system with an inlet pressure above the pressure spike. Singh et al. (2008) studied the impact of aspect ratio on flowboiling of water in different rectangular microchannels of same hydraulic diameter of 142 µm and length of 20 mm. They observed experimentally that the minimum pressure drop across the rectangular microchannel occurs at an aspect ratio of about 1.56. Furthermore, they suggested that annular flow model is good for approximately square cross-sections but poor for large aspect ratio rectangular cross-section microchannels. Yen et al. (2006) studied convective boiling of HCFC-123 in transparent microchannels of same hydraulic diameter of 214 µm but different cross-sections. They observed that the heat transfer coefficient was higher for square microchannel (because of corners which acted as active nucleation sites) as compared to circular shaped microchannel. Lee et al. (2004) and Li et al. (2004) experimentally investigated bubble dynamics in a single microchannel having a hydraulic diameter of 41.3 μm and two parallel microchannels with a hydraulic diameter of 47.7 μm, respectively using DI water as working fluid. It was observed that the reversed flow occurred more easily in parallel microchannels than in a single microchannel, and also at low mass flux and highheatflux values.
Several scholars have taken numerous studies about the nanofluid over centuries. At present, water and refrigerants are commonly used as working fluids inside microchannel, the mixture of conventional fluids is proposed with the use of nanoparticles (1-100 nm in diameter) to improve the heat transfer performance of conventional fluids by enhancing thermal conductivity inside the microchannel. The use of nanofluids have proved to be a very effective way for cooling systems inside microchannel due to nanoparticle separation to the base fluid. It has been proved that nanofluids have better heat transfer performance than the base liquid and a good substitutional for working fluids inside microchannel (H. Zhang, Shao, Xu, & Tian, 2013).
For all the data conducted with distilled water and benzene, the heating surface showed a distribution of boilingheat transfer coefficient around its circumference. The coefficient decreases from top- to side- to bottom- positions distinctly. The average value of heat transfer coefficient for the boiling of benzene has functional relationship with heatflux. The data analysis shows that the seven-tenth power law relating heat transfer coefficient is valid for the boiling of distilled water and benzene at atmospheric and sub atmospheric pressures. The effect of pressure for the sub atmospheric pressure range, it is found that the heat transfer coefficient increases with pressure raised to the power 0.32. Actually, the heating surface characteristics and the physico- thermal properties of boiling fluids have marked influence
disjoining pressure) both seem to be plausible hypoth- eses for CHF enhancement in nanofluids. However, to understand the principle mechanism of the phenomena, it is necessary to examine the single contribution of each factor to the enhanced CHF performance of nano- fluids. Kim et al. [52,53] carried out an insightful experi- ment to separate the single effect of the nanoparticle deposition layer on the CHF of nanofluids. First, they conducted a pool-boiling test of a nanofluid using a fresh heater wire. A subsequent surface inspection con- firmed the presence of a nanoparticle deposition layer on the heater wire. They then performed an additional CHF test on the nanoparticle-deposited wire submerged in pure water, which resulted in a CHF enhancement of the same magnitude as that of the nanofluids. The experimental results clearly demonstrated that the enhancement of CHF in nanofluids is due to the modifi- cation of surface topology associated with nanoparticle deposition on the heater surface during nanofluid boil- ing. Moreover, Golubovic et al.  and Kwark et al.  recently conducted the same experiments using both thin wire and flat-plate heaters and obtained experimental results consistent with those of Kim et al. [52,53]. Figure 9 shows the experimental results obtained by Kim et al.  and Kwark et al. .
line heat exchangers. The advantages of proposed microchannelheat exchangers were: a) compactness (17.2% and 15.1% volume reduction for evaporator and condenser, respectively), b) weight (2.8% and 14.9% lighter for evaporator and condenser, respectively) and c) cooling capacity and COP increasing about 5% and 8% respectively under high vehicle speed. Khan et al. (2010) investigated experimentally the pressure drop and friction factors as well as their relations corresponding with Reynolds numbers of flowing fluid (50% Ethylene glycol & 50% water mixture) through the multi- port straight microchannel test slab. Experimental results indicated that ∆p follows a linear variation with lower value of Re, and it varies non-linearly with higher Re (>700).The experimental average Poiseuille number (f.Re) is about 5% above the theoretical value (f.Re = 64, in laminar flow regime) within the range 380 ≤ Re ≤ 1650. Jokar et al. (2010) experimentally studied the single phase fluid flow & heat transfer behaviors in meso- channel heat exchangers by flowing fifty percent glycol-water mixture through meso- channels while air is passing on the others side of heat exchanger. They founded that correlations for conventional flow passages do not properly match with their obtained experimental results.
The study of micro-electro-mechanical system (MEMS) and nano-electrical-mechanical systems (NEMS) has attracted much attention to design micro-devices such as micro-motors, micro-sensors, micro-mechanical gyroscopes, micro-pumps, micro valves, micro-rockets, micro-gas- turbines, micro-heat-exchangers, biological and chemical devices etc. Microchannels are used to transport biological material such as protein, DNA, cells and embryos or to transport chemical samples and analytes. Advantage of microchannels is due to their high surface to volume ratio and their small volume. The large surface to volume ratio increases the rate of heat and mass transfer that makes micro devices excellent tools. Flow in heat transfer and chemical reactor devices are usually faster than, those in biological devices and chemical analysis microdevices. These applications have motivated scholars to understand the flow behaviors in these small systems to enhance the performance during the design process.