15 it was instead decided to make geometries that would be possible with computer numerical control (CNC) machining. For this a rounded corner design with a flat floor was created as seen in Figure 11 Cutaway of first series of chip design. This general shape would be maintained across all the chip designs with variations on the notch spacing and width. This new 90-degree corner had relevant geometry to the study of vapor bubbles pushing from walls performed by Kandlikar . Form this study we can see an unbalanced force is created forcing the bubble out way from the corner horizontally potentially driving it up the angled ramp in the divot. The purpose of the tightly grouped lines of paired divots in the sintered and un-sintered chips several purposes. Firstly, the small divots, instead of a line, could act as individual nucleation sites. They were also closely paired with the other groups such that any divot could contribute to neighboring vapor bubble with the ramp focusing them toward the center of the pair. With the pairs being in a row, the departing vapor, now in a curtain formation, would not interfere greatly with the incoming cool fluid. As the fluid flows down in between the vapor curtains it meets the chip surface and is forced into the side of the bubbles. This would provide a sweeping fluid flow over the surface increasing the amount of heat that could be carried away improving heattransfer though forced convection. The distance between the machined divots was chose based on  as the depth and width of the channels where decided based on the chip 8 design seen in Table 1 and guidance from Kandlikar. The design would hopefully encourage bubbles to form independently then merge once they passed a certain diameter. This new merged diameter would then depart quickly.
Jaikumar and Kandlikar  employed selectively sintered fin tops with a channel width of 762 µm, channel depth of 400 µm, and fin width of 200 µm to achieve CHF at 313 W/cm 2 and a HTC of 565 kW/m 2 K. Combining microchannels and porous enhance- ments controls both liquid-vapor wetting pathways and the number of nucleation sites via porosity. Surface enhancements shift the boiling curve to the left resulting in higher heat flux dissipation with lower wall superheats. Thiagarajan et al.  created micro- porous surfaces with thicknesses of 100 µm, 360 µm, and 700 µm. HFE-7100 was chosen as the working fluid due to its low global warming potential and good thermodynamic properties. Their findings agree with literature in that thicker porous coatings result in improved HTC. With the 700 µm coating thickness they obtained a 270% HTC im- provement. They postulated this is due to a thicker coating resulting in a larger number of nucleating sites. However, at high heat fluxes the HTC decreases due to the large rate of vapor generation preventing liquid return pathways. Sarangi et al.  explored a “free-particle” technique of adhering loose copper particles to a copper test chip. This method is compared to traditional sintering and they found 95% reduction in wall super- heat for the sintered chip. Furthermore, a powder size between 90 −106 µm was optimal for both the sintered and “free-particle” approaches. It is postulated that thicker coat- ings greatly reduce the wall superheat due to an increased number of nucleation sites. As the coating decreases, the HTC enhancement increases. However, thicker coatings cause coalescence and hinders bubble departure leading to reduced CHF. In general, as the sintered coating thickness decreases, CHF decreases. They suggest this may be due to the reduction in effective heattransfer area having a more dominant effect than the reduced hydraulic resistance associated with a reduction in coating thickness.
In addition to surface enhancements such as pins, fins, or channels, the use of micro or nanoparticle coatings or structures to improve heattransfer have been investigated. A number of different materials can be used for microparticle coatings. Chang and You  applied porous epoxied materials of aluminum, copper, diamond, and silver, which was found to increase the q″ for specified wall superheats. The authors saw CHF values increase approximately 100% as a result of microporous coatings, due to the increased number of active nucleation sites. Kim et al.  also reported CHF and boilingheattransfer improvements using microparticle coatings on wire. Hwang and Kaviany  reported an increase in CHF of 96% using a uniform microparticle coating on a heated surface under poolboiling conditions. In 2014, Dong et al.  studied the effect of micro/nanostructures on bubble nucleation, departure characteristic, and h on poolboiling of ethanol. They found that at low heat fluxes, microstructures enhance bubble nucleation, resulting in a reduction of the ΔT sat and an increase in q″. They also found that nanostructures
The ultimate objective of this work is to identify nanostructures that enhance the poolboiling enhancement at 2D and 3D substrates and investigate the mechanism behind the micro-scale heattransfer phenomenon. To realize this goal, metallic and semiconductor nanowire structures were studied, fabricated and tested on different substrates for boiling enhancement. The effects of nanowire property and substrate geometry were further investigated. Nanowire structure were chosen for constructing nanostructured surface due to their excellent properties of being scalable, cost-effective and able to produce uniform geometrical features at a large scale. Nanowires can be defined as structures that have a thickness or diameter constrained to tens of nanometers or less and an unconstrained length. Nanowire structures possess properties which could be utilized to promoteboiling performance. First, it has been observed that a surface coated with nanowires can be superhydrophilic, which helps in increasing CHF and delaying the dry-out condition. Second, nanowire arrays contain many orders of magnitude more cavities and pores compared to other microfabricated or micromachined surfaces, thereby effectively increasing the nucleation site density. Furthermore, due to the pin fin effect, the effective heattransfer area of nanowires is much higher than that of micro-fabricated surfaces. Finally, nanowire arrays may act as an efficient wicking structure, which provide significant capillary force to hold the liquid.
Nucleate boiling is a highly efficient heat-transfer mode for removing large amounts of heat at a low temperature difference. However, a maximum value of the heat flux, known as a critical heat flux (CHF), exists at which nucleate boiling transitions to film boiling. Beyond a critical value of the heat flux (or CHF) a vapour layer develops between the liquid and the solid surface, thus severely deteriorates heattransfer due to poor thermal conductivity of the vapour film, and eventually damaging the heating surface. Therefore, the designs of heattransfer systems that operate in the nucleate boiling regime are limited by CHF. So Critical Heat Flux, or CHF, is an important condition that defines the upper limit of safe operation of heattransfer equipment employing boilingheattransfer in heat flux controlled systems.
As shown in Fig. 3, the boilingheattransfer performance in the porous artery structure is much better than that on a flat surface especially at high heat fluxes. This is possible only when the liquid/vapor two-phase heattransfer occurs on the walls of the arteries; otherwise the thermal conduction through the fins requires a temperature difference of several tens of degrees at high levels of heat flux. The entrainment of liquid is a common phenomenon in conventional heat pipe operation, which will impede the liquid return from the condenser to the evaporator, and degrade the heattransfer performance of the heat pipe. Whereas in this application, the entrainment effect helps to carry multiple liquid droplets to wet the hot walls of the arteries, which significantly benefits the boilingheattransfer performance of the porous artery structure. Acknowledgement
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 heattransfer rate were investigate using experimental work. The vector velocity, heattransfer and temperature distribution were studied using a CFD.
can also alter the mechanism of boilingheattransfer. Yu and Lu  conducted an experiment on copper surfaces with a rectangular fin array, immersed in the saturated fluid FC-72. They studied the effect of geometrical parameters (spacing and length of the fins) on the performance of the boiling process. The surfaces of the test block, made of copper, had a base area of 10 mm x 10 mm. The fins, of four lengths (0.5 mm, 1.0 mm, 2.0 mm and 4.0 mm) were arranged in three spacing patterns, namely 0.5 mm, 1.0 mm and 2.0 mm apart. The investigations were conducted at atmospheric pressure. A flat surface was assumed to be a reference surface, with which finned surfaces were compared. The test results showed that more closely arranged and bigger fins provide greater resistance to flow. Further, when the heat flux was close to the critical one (CHF), numerous vapour bubbles were formed, which departed from the surface, causing the drying of the plate centre. The results also showed that the heattransfer coefficient decreased rapidly when the spacing of the fins was reduced or the fin length increased. The maximum value of CHF on the base surface was 9.8 · 10 5 W m -2 , which was found for the test surface with 0.5 mm spacing and 4.0 mm length of the fins. This value was five times higher than that for the surface without fins.
with the variable φ that indicates the distance from the cell to the interface. A value of φ = 0 corresponds to the interface position, φ > 0 corresponds to phase-1, and φ < 0 corresponds to phase-2. An advection equation changes the values of φ based on the interface velocity. One advantage of the LS lies in the smooth transition of φ across the interface. The smooth transition leads to accurate estimation of the gradients of φ at the interface used to compute surface tension. Therefore, the LS method is capable of capturing multiphase flows where surface tension effects play a primary role (e.g. nucleate boiling). However, the method depends on a reinitialization procedure to ensure that φ remains accurate, which affects mass conservation. The technical literature distinguishes various works that use the LS method to track the interface [17, 19, 20, 34, 35]. Son et al.  simulated nucleate boiling with the LS method. The simulation used φ to define the transition region (three cells thick) around the interface to avoid numerical instabilities. The mass flux at the interface gave the interface velocity. Gradients of φ computed surface tension effects in the momentum equation. Results showed sharp interfaces without appreciable deformations throughout the ebullition cycle (see Figure 6). The interface remained sharp even near the contact line, which is a critical region due to the abrupt changes in curvature and strong evaporation.
decreases the numbers of nucleation embryos, contributing to a low heattransfer rate of nucleate boiling. The corresponding boiling curve shifts to the right. It should be noted here that the size of nanoparticles (about 80-100 nm) is much less than that of cavities in which vapor embryos will be activated to initiate boiling. As a result, more nanoparticles deposit on the upside of the heated tube under the impingement and disturbance of acoustic cavitation cluster. Due to the deposition of nanoparticles, the surface roughness of the tube decreases. It would then require elevated applied heat flux to activate vapor embryos at the remaining smaller cavities prior to boiling inception, i.e., the presence of nanoparticles suppresses bubble nucleation within the cavities on the surface of the heated tube. At the same test conditions, heat is dissipated only by convection rather than by latent heat of vaporization and the mode of heattransfer is single-phase convection. Consequently, the incipient boiling superheat increases and the corresponding local boiling curve shifts to the right. This has been verified by the data indicated by thermocouple of T 5 shown in Fig. 6.
The results show that the temperature at the onset of boiling, is strongly linked to the speed and the heating mode. During transient regime, boiling begins with a delay which creates significant overheating. The boiling triggering is abrupt and, before the boiling stabilizes on the surface, a transition to the film boiling regime may be observed. The measurements also show that the nucleation sites can be disabled if a certain time-delay is observed between the preliminary process and the echelon of imposed flux to ribbon. Overheating needed to trigger boiling is higher for long time-delays. This study led to results that can be applied to electrical components cooling in order to increase their efficiency and life-time. In order to avoid a strong overheating trigger boiling, it is possible to always maintain a surface temperature above certain limit. It can thus be recommended to always dissipate low flux to be sure that the surface temperature is sufficient to facilitate the activation of nucleation sites. This study was performed with one type of sample and fluid. It is necessary to diversify the nature of the fluid-sample combinations to test their influence on heat exchange during boiling transient. It is also necessary to vary the size of the sample and to lead a comparative study with wires or massive to assess the influence of the sample inertia.
Also, in contrast to case of uncoated surface, where the departure of vapor phase from the heating surface is accompanied by contact line reduction, at boiling on hydrophobic surface another picture is observed. At the stage of growth, before the achievement of size corresponding to the departure diameter of bubbles on uncoated surface, bubbles coalesce with neighbor nucleation sites with an increase in the area of contact line. It also should be noted, that the coalescence of bubbles into large vapor conglomerates occurs at the some distance from the heating surface, as a result of which liquid separating them may remain after the coalescence under vapor phase in the form of droplets. After the departure of large vapor bubbles from the surface in this place numerous vapor bubbles remain. This bubbles start to grow, merge and form new large bubble.
diameter" and the effect of tube diameter on flow boilingheattransfer coefficient was characterized by the Weber number in gas phase. Results showed that this correlation could be applied to a wide range of tube diameters (0.5–11-mm-ID). In addition, the dryout point and the heattransfer characteristics after the dryout point were also investigated based on the annular flow model. A modified Chen-type correlation for the flow boilingheattransfer was developed that included the effect of tube diameter. In this correlation, the effect of tube diameter on flow boilingheattransfer was characterized by the Weber number. The correlation agreed reasonably well with experimental data for a wide range of tube diameter from 0.51 to 10.92 mm ID. From this study of paper we conclude that the modified chen type correlation better suitable for least MAE compare to kandlikar correlation and gungor and winterton correlation.
However the field which has received relatively less attention is the study of heattransfer involving two phases (air-water) for a stratified flow regime. In the present work, experiments were carried out in a double pipe heat exchanger with hot water as the service fluid (annulus side) and two-phase mixtures of cold water and air in different ratios as the process fluid (tube side). Experimental runs with single-phase fluid (water) and two phase mixtures on the process side were carried out. The heattransfer coefficients on the cold side were correlated with superficial Reynolds numbers.
Effect of silver nanoparticles on the average heattransfer Two experimental conditions are conducted for each sil- ver nanoparticle concentration in water base fluid and pure water. In the first one, the input power is settled at 200 W and the mass flux is varied from 87 to 653 kg/ m 2 s. In the second, the mass flux is settled at 174 kg/ m 2 s and the input power is varied from 120 to 240 W. Figure 13 compares the average heattransfer coefficients of pure water, 25 mg/L and 50 mg/L silver concentration nanofluid under the first experiment conditions. For the same mass flux, the average heattransfer coefficient is larger for nanofluids than that of pure water and it is in- creased with nanoparticle suspension. The maximum enhancement of the average heattransfer coefficient is about 132% for 25 mg/L and 162% for 50 mg/L. Figure 14 illustrates experimental data obtained under the second experiment conditions. It can be seen that the average heattransfer coefficient for pure water and silver-water nanofluids increases by decreasing the input power. For the whole input power range, the heat trans- fer coefficients have almost the same trends for boiling silver-water nanofluids and water. For each fixed power input value, increasing the silver nanoparticle concentra- tion will increase the average heattransfer coefficient. Accordingly, for an input power ranging from 120 to 240 W, the enhancement of the average heattransfer co- efficient for nanofluids relative to pure water is about 30% to 38% for 25 mg/L and 56% to 77% for 50 mg/L silver concentrations, respectively.
Impinging jets have many practical applications in cooling, heating, metal cutting and industrial cleaning. They include different types of flows such as free jet flow, stagnating flow and a wall jet (see Fig. 1.1). In an impinging jet, flow exiting from the nozzle interacts with the ambient flow and due to the Kelvin- Helmholtz instabilities a street of roll-up vortices is generated. There is a frequency for the generation of these vortices which is dependent on different parameters such as boundary conditions, nozzle geometry and Reynolds number. While traveling towards the plate, these vortices interact, break up, pair and coalesce with neighbouring vortices and their symmetrical shape is lost. This results in an unsteady three-dimensional behaviour for pressure and shear stresses in the impingement zone and a vorticity field in the entire domain. The heattransfer from the plate is also influenced by these unsteady three- dimensional structures approaching the plate.
Abstract : We have so many applications related to PoolBoiling. The PoolBoiling is mostly useful in arid areas to produce drinking water from impure water like sea water by distillation process. It is very difficult to distill the only water which having high surface tension. The surface tension is important factor to affect heattransfer enhancement in poolboiling. By reducing the surface tension we can increase the heattransfer rate in poolboiling. Over the past few decades, researchers have investigated different passive enhancement techniques to increase the heattransfer coefficient and the critical heat flux. They have used different working fluids, e.g., water, aqueous surfactant solutions, refrigerants, alcohols, binary mixtures. From so many years we are using surfactants in domestically. It is proven previously by experiments that the addition of little amount of surfactant reduces the surface tension and increase the rate of heattransfer. There are different groups of surfactants. From those I have experimenting with anionic surfactant Sodium Dodecyl Sulfate (SDS), which is most human friendly to test the heattransfer enhancement. The effects, of both SDS concentration, and the excess temperature ΔT on heattransfer performance, are studied. The experimental results show that a small amount of surface active additive enhances the heattransfer coefficient h considerably higher, and that there is an optimum additive concentration for higher heat fluxes. Beyond this optimum point, further increase in additive concentration makes h lower.
Fusion welding technology is widely applied in manufacturing industries. To achieve high quality welding is a challenging task in the fusion welding, as the quality of the weld depends on the fluid dynamics, electrodynamics, heat and mass transfer processes that occur inside a weld pool. These processes are non- linearly dependent on each other and, therefore, demand a robust model for the study of various parameters that affect the weld-pool quality. In this work, we use a commercial software, ANSYS-FLUENT, to model the problem of solidification of tin inside a rectangular cavity. A validation of the developed model shows a reasonable match with existing data. It is shown that the convection is a dominant in deciding the solidification rate of tin inside the weld pool.
One way to enhance heattransfer from electronics without sacrificing their performance is the use of a heat sink with many microchannels and liquid water passing through it. Because of the small size of microchannel heat sink, the performance of a computer system can also be increased by incorporating additional microprocessors at a given space without the issue of over-heated or burned-out chips. The present work involves cooling of electronic devices using two-phase flow in microchannel heat sink. Two-phase heattransfer has significant advantages over single-phase heattransfer because flow rates are smaller through the use of the latent heat of vaporization, pressure drop and pumping power are less, approximately uniform fluid and solid temperatures can be obtained, and it can also be directly coupled with a refrigerant system to provide a lower coolant temperature.