The Experts below are selected from a list of 24285 Experts worldwide ranked by ideXlab platform
Srinivas Garimella - One of the best experts on this subject based on the ideXlab platform.
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investigation of a microchannel heat and mass exchanger for absorption systems
Applied Thermal Engineering, 2019Co-Authors: Dhruv C Hoysall, Srinivas GarimellaAbstract:Abstract An investigation of a microchannel heat and mass exchanger for vapor absorption systems is presented. A sheet with 88 Microchannels of depth 0.5 mm and width 0.76 mm is investigated via visual and thermal measurements. The flow in the Microchannels is predominantly in the slug-flow regime, with distinct vapor and liquid regions. High rates of heat transfer were observed in the absorber. However, poor liquid mass transfer rates limit the performance of the microchannel absorber. The performance of the microchannel absorber is compared with that of an absorber with serpentine microscale passages containing micro-pin fins. A physics-based model for the absorption of ammonia into a dilute solution of ammonia-water in a microchannel absorber with mixing sections is developed using insights from flow visualization. The model is validated by comparing its predictions with the data. The potential of surfactants to enhance absorber performance is also investigated.
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measurement and modeling of condensation heat transfer in non circular Microchannels
International Journal of Refrigeration-revue Internationale Du Froid, 2010Co-Authors: Akhil Agarwal, Todd M Bandhauer, Srinivas GarimellaAbstract:Heat transfer coefficients in six non-circular horizontal Microchannels (0.424 < Dh < 0.839 mm) of different shapes during condensation of refrigerant R134a over the mass flux range 150 < G < 750 kg m−2 s−1 were measured in this study. The channels included barrel-shaped, N-shaped, rectangular, square, and triangular extruded tubes, and a channel with a W-shaped corrugated insert that yielded triangular Microchannels. The thermal amplification technique developed and reported in earlier work by the authors is used to measure the heat transfer coefficients across the vapor-liquid dome in small increments of vapor quality. Results from previous work by the authors on condensation flow mechanisms in microchannel geometries were used to interpret the results based on the applicable flow regimes. The effect of tube shape was also considered in deciding the applicable flow regime. A modified version of the annular-flow-based heat transfer model proposed recently by the authors for circular Microchannels, with the required shear stress being calculated from a non-circular microchannel pressure drop model also reported earlier was found to best correlate the present data for square, rectangular and barrel-shaped Microchannels. For the other microchannel shapes with sharp acute-angle corners, a mist-flow-based model from the literature on larger tubes was found to suffice for the prediction of the heat transfer data. These models predict the data significantly better than the other available correlations in the literature.
Satish G Kandlikar - One of the best experts on this subject based on the ideXlab platform.
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pool boiling heat transfer enhancement over cylindrical tubes with water at atmospheric pressure part ii experimental results and bubble dynamics for circumferential v groove and axial rectangular open Microchannels
International Journal of Heat and Mass Transfer, 2013Co-Authors: Jeet S Mehta, Satish G KandlikarAbstract:Abstract A two-part experimental study is conducted on pool boiling heat transfer over enhanced cylindrical microchannel test surfaces with water at atmospheric pressure. In this Part II of the study, the effects of circumferential V-groove Microchannels and axial rectangular Microchannels are reported. These experiments were performed in the horizontal as well as vertical orientations. The heat transfer performances of the modified surfaces are compared with that of a plain surface. At a heat flux of 1070 kW/m2 a maximum heat transfer coefficient of 96 kW/m2 K was achieved with an axial rectangular microchannel test section in the vertical orientation. Videos captured using a high speed camera were analyzed and the boiling heat transfer mechanisms seen at low and medium heat flux conditions were discussed. The enhancement in the heat transfer performance, and the improvement of the critical heat flux observed with these modified test sections have been attributed to the liquid rewetting phenomenon of the heated surfaces through the Microchannels.
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numerical investigation of heat transfer in rectangular Microchannels under h2 boundary condition during developing and fully developed laminar flow
Journal of Heat Transfer-transactions of The Asme, 2012Co-Authors: V V Dharaiya, Satish G KandlikarAbstract:Study of fluid flow characteristics at microscale is gaining importance with shrinking device sizes. Better understanding of fluid flow and heat transfer in Microchannels will have important implications in electronic chip cooling, heat exchangers, MEMS, and microfluidic devices. Due to short lengths employed in Microchannels, entrance header effects can be significant and need to be investigated. In this work, three dimensional model of Microchannels, with aspect ratios (a ¼a/b) ranging from 0.1 to 10, are numerically simulated using CFD software tool FLUENT. Heat transfer effects in the entrance region of microchannel are presented by plotting average Nusselt number as a function of nondimensional axial length x*. The numerical simulations with both circumferential and axial uniform heat flux (H2) boundary conditions are validated for existing data set for four wall heat flux case. Large numerical data sets are generated in this work for rectangular cross-sectional Microchannels with heating on three walls, two opposing walls, one wall, and two adjacent walls under H2 boundary condition. This information can provide better understanding and insight into the transport processes in the Microchannels. Although the results are seen as relevant in microscale applications, they are applicable to any sized channels. Based on the numerical results obtained for the whole range, generalized correlations for Nusselt numbers as a function of channel aspect ratio are presented for all the cases. The predicted correlations for Nusselt numbers can be very useful resource for the design and optimization of microchannel heat sinks and other microfluidic devices. [DOI: 10.1115/1.4004934]
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critical heat flux measurement and model for refrigerant 123 under stabilized flow conditions in Microchannels
ASME 2006 International Mechanical Engineering Congress and Exposition, 2006Co-Authors: Wai Keat Kuan, Satish G KandlikarAbstract:The present work is aimed toward understanding the effect of flow boiling stability on critical heat flux (CHF) with Refrigerant-123 (R-123) in microchannel passages. Experimental data and theoretical model to predict the CHF are the focus of this work. The experimental test section has six parallel Microchannels with each having a cross sectional area of 1054 × 157 µm 2 . The effect of flow instabilities in Microchannels is investigated using flow restrictors at the inlet of each microchannel to stabilize the flow boiling process and avoid the backflow phenomena. This technique resulted in successfully stabilizing the flow boiling process as seen through a high-speed camera. The present CHF result is found to correlate to mean absolute error (MAE) of 24.1% with a macroscale empirical equation by Katto [13]. A theoretical analysis of flow boiling phenomena revealed that the ratio of evaporation momentum to surface tension forces is an important parameter. For the first time, a theoretical CHF model is proposed using these underlying forces to represent CHF mechanism in Microchannels, and its correlation agrees with the experimental data with MAE of 2.5%.
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evolution of microchannel flow passages thermohydraulic performance and fabrication technology
Heat Transfer Engineering, 2003Co-Authors: Satish G Kandlikar, William J GrandeAbstract:This paper provides a roadmap of development in the thermal and fabrication aspects of Microchannels as applied in microelectronics and other high heat-flux cooling applications. Microchannels are defined as flow passages that have hydraulic diameters in the range of 10 to 200 micrometers. The impetus for microchannel research was provided by the pioneering work of Tuckerman and Pease [1] at Stanford University in the early eighties. Since that time, this technology has received considerable attention in microelectronics and other major application areas, such as fuel cell systems and advanced heat sink designs. After reviewing the advancement in heat transfer technology from a historical perspective, the advantages of using Microchannels in high heat flux cooling applications is discussed, and research done on various aspects of microchannel heat exchanger performance is reviewed. Single-phase performance for liquids is still expected to be describable by conventional equations; however, the gas flow may...
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evolution of microchannel flow passages thermohydraulic performance and fabrication technology
Taylor and Francis, 2003Co-Authors: Satish G Kandlikar, William J GrandeAbstract:This paper provides a roadmap of development in the thermal and fabrication aspects of Microchannels as applied in the microelectronics and other high heat-flux cooling applications. Microchannels are defined as flow passages that have hydraulic diameters in the range of 10 to 200 micrometers. The impetus for microchannel research was provided by the pioneering work of Tuckerman and Pease [1] at Stanford University in the early eighties. Since that time, this technology has received considerable attention in microelectronics and other major application areas, such as fuel cell systems and advanced heat sink designs. After reviewing the advancement in heat transfer technology from a historical perspective, advantages of using Microchannels in high heat flux cooling applications is discussed, and research done on various aspects of microchannel heat exchanger performance is reviewed. Single-phase performance for liquids is expected to be still describable by the conventional equations; however the gas flow may be influenced by the rarefaction effects. Two-phase flow is another topic that is still under active research. The evolution of research into microchannel heat sinks has paralleled the advancements made in microfabrication technology. The earliest Microchannels were built using anisotropic wet chemical etching techniques based on alkali solutions. While this method has been exploited successfully, it does impose certain restrictions on silicon wafer type and geometry. Recently, anisotropic dry etching processes have been developed that circumvent these restrictions. In addition, dry etching methods can be significantly faster and, from a manufacturing standpoint, create fewer contamination and waste treatment problems. Advances in fabrication technology will continue to fuel improvements in microchannel heat sink performance and cost for the foreseeable future. Some fabrication areas that may spur advances include new materials, high aspect ratio patterning techniques other than dry etching, active fluid flow elements, and micromolding.Copyright © 2002 by ASME
Y J Cheng - One of the best experts on this subject based on the ideXlab platform.
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numerical simulation of stacked microchannel heat sink with mixing enhanced passive structure
International Communications in Heat and Mass Transfer, 2007Co-Authors: Y J ChengAbstract:Abstract The paper is focused on the investigation of numerical simulation of stacked two-layer microchannel heat sink with enhanced mixing passive microstructure. In contrast to the smooth microchannel studies in the literature, the microchannel with embedded passive microstructure is chosen. The computational fluid dynamics (CFD) will be used to simulate the flow and heat transfer in a stacked two-layer Microchannels with multiple MEMS easy-processing passive microstructures. To simulate the conjugated heat transfer among the heatsink and fluid, the three-dimensional conjugated model is used to solve this problem. The important parameters (e.g. the ratio of embedded structure height to microchannel height and fluid property) are investigated. The ratio of embedded structure height to microchannel height is changed from 0.13 to 0.26. The microchannel Reynolds number is fixed at 14.8. The stacked microchannel with passive structures has better performance than the smooth Microchannels.
Ping Cheng - One of the best experts on this subject based on the ideXlab platform.
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subcooled flow boiling and microbubble emission boiling phenomena in a partially heated microchannel
International Journal of Heat and Mass Transfer, 2009Co-Authors: Guodong Wang, Ping ChengAbstract:Abstract A simultaneous visualization and measurement study has been carried out to investigate subcooled flow boiling and microbubble emission boiling (MEB) phenomena of deionized water in a partially heated Pyrex glass microchannel, having a hydraulic diameter of 155 μm, which was integrated with a Platinum microheater. Effects of mass flux, inlet water subcooling and surface condition of the microheater on subcooled flow boiling in Microchannels are investigated. It is found that MEB occurred at high inlet subcoolings and at high heat fluxes, where vapor bubbles collapsed into microbubbles after contacting with the surrounding highly subcooled liquid. In the fully-developed MEB regime where the entire microheater was covered by MEB, the mass flux, the inlet water subcooling and the heater surface condition have only small effects on the boiling curves. The occurrence of MEB in microchannel can remove a large amount of heat flux, as high as 14.41 MW/m2 at a mass flux of 883.8 kg/m2 s, with only a moderate rise in wall temperature. Therefore, MEB is a very promising method for cooling of microelectronic chips. Heat transfer in the fully-developed MEB in the microchannel is presented, which is compared with existing subcooled flow boiling heat transfer correlations for macrochannels.
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effects of inlet outlet configurations on flow boiling instability in parallel Microchannels
International Journal of Heat and Mass Transfer, 2008Co-Authors: Guodong Wang, Ping Cheng, A E BerglesAbstract:A simultaneous visualization and measurement study has been carried out to investigate effects of inlet/outlet configurations on flow boiling instabilities in parallel Microchannels, having a length of 30 mm and a hydraulic diameter of 186 μm. Three types of inlet/outlet configurations were investigated. Fluid flow entering to and exiting from the Microchannels with the Type-A connection was restricted because the inlet and outlet conduits were perpendicular to the Microchannels. The fluid flow had no restriction in entering to and existing from the Microchannels with the Type-B connection. In the Type-C connection, fluid flow was restricted in entering each microchannel but was not restricted in exiting from the Microchannels. It is found that amplitudes of temperature and pressure oscillations in the Type-B connection are much smaller than those in the Type-A connection under the same heat flux and mass flux conditions. On the other hand, nearly steady flow boiling exists in the parallel Microchannels with the Type-C connection under the experimental conditions. Therefore, this configuration is recommended for high-heat-flux microchannel applications. As predicted, the stability threshold is determined by the minimum in the pressure-drop-versus-flow-rate curve. The pressure drop and heat transfer coefficient versus vapor quality for flow boiling in Microchannels with the Type-C connection are presented. It is found that experimental data of pressure drop are higher and heat transfer coefficients are lower for boiling flow at high vapor quality in Microchannels than those predicted from correlation equations for boiling flow in macrochannels, due to local dryout.
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unstable and stable flow boiling in parallel Microchannels and in a single microchannel
International Journal of Heat and Mass Transfer, 2007Co-Authors: Guodong Wang, Ping ChengAbstract:Abstract A simultaneous visualization and measurement study have been carried out to investigate flow boiling instabilities of water in Microchannels at various heat fluxes and mass fluxes. Two separate flow boiling experiments were conducted in eight parallel silicon Microchannels (with flow interaction from neighboring channels at headers) and in a single microchannel (without flow interaction), respectively. These Microchannels, at a length of 30 mm, had an identical trapezoidal cross-section with a hydraulic diameter of 186 μm. At a given heat flux and inlet water temperature, it was found that stable and unstable flow boiling regimes existed, depending on the mass flux. A flow boiling map, in terms of heat flux vs mass flux, showing stable flow boiling regime and unstable flow boiling regime is presented for parallel Microchannels as well as for a single microchannel, respectively, at an inlet water temperature of 35 °C. In the stable flow boiling regime, isolated bubbles were generated and were pushed away by the incoming subcooled liquid. Two unstable flow boiling regimes, with long-period oscillation (more than 1 s) and short-period oscillation (less than 0.1 s) in temperature and pressure, were identified. The former was due to the expansion of vapor bubble from downstream while the latter was owing to the flow pattern transition from annular to mist flow. A comparison of results of flow boiling in parallel Microchannels and in a single microchannel shows that flow interaction effects from neighboring channels at the headers are significant.
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bubble cavitation in a microchannel
International Journal of Heat and Mass Transfer, 2004Co-Authors: Ping ChengAbstract:Abstract The effects of microchannel size, mass flow rate and heat flux on boiling incipience or bubble cavitation in a microchannel are studied in this paper. The well-known concept of survival of a bubble cavity is extended, and the classical kinetics of nucleation is introduced to study bubble nucleation in water flowing in a silicon microchannel. The effects of contact angle, dissolved gas, and the existence of microcavities and corners in the microchannel on bubble’s nucleation/cavitation temperature are estimated. With this information, a numerical solution is carried out to study the effects of mass flow rate and heat flux on bubble nucleation/cavitation in Microchannels. The results of this analysis provide further physical insight on boiling heat transfer in Microchannels.
Lioubov Kiwiminsker - One of the best experts on this subject based on the ideXlab platform.
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influence of flow regime on mass transfer in different types of Microchannels
Industrial & Engineering Chemistry Research, 2011Co-Authors: M N Kashid, A Renken, Lioubov KiwiminskerAbstract:The performance of microstructured reactors (or Microchannels) for mass-transfer-controlled liquid-liquid reactions depends on flow regimes that define the specific interfacial area for the mass transfer. In the present work, experiments were carried out to investigate the two phase-flow regimes and the mass transfer at relatively high throughput for a single microchannel (of 1-18 mL/min) in five generic microchannel designs (with and without structured internal surfaces), using a nonreacting water-acetone-toluene system. When the flow results were analyzed collectively in all Microchannels, six different flow regimes such as slug, slug-drop, deformed interface, parallel/annular, slug-dispersed, and dispersed flow were observed. The mass-transfer comparison shows that the microchannel with structured internal surfaces shows better performance, because it creates a very fine dispersion, providing high interfacial area, compared to other Microchannels. Finally, the mass-transfer data were correlated, which can be used for a priori predictions of mass-transfer rates in Microchannels.