المؤلفون: Kingston, Todd A., Olson, Brian D., Weibel, Justin A., Garimella, Suresh V.

المصدر: IEEE Transactions on Components, Packaging & Manufacturing Technology; Oct2021, Vol. 11 Issue 10, p1615-1624, 10p

مصطلحات موضوعية: CHANNEL flow, OSCILLATIONS, SINGLE-phase flow, EBULLITION, PRESSURE drop (Fluid dynamics), MICROCHANNEL flow, FLOW instability, FLOW visualization

مستخلص: Flow boiling provides an effective means of heat removal but can suffer from thermal and hydrodynamic transients that compromise heat transfer performance and trigger device failure. In this study, the transient flow boiling characteristics in two thermally isolated, hydrodynamically coupled parallel microchannels are investigated experimentally. High-speed flow visualization is synchronized to high-frequency heat flux, wall temperature, pressure drop, and mass flux measurements to provide time-resolved characterization. Two constant and two transient heating conditions are presented. For a constant heat flux of 63 kW/m2 into each channel, boiling occurs continuously in both channels and the parallel channel instability is observed to occur at 15 Hz. Time-periodic oscillations in the pressure drop and average mass flux are observed, but corresponding oscillations in the wall temperatures are virtually nonexistent at this condition. At a slightly lower constant heat flux of 60 kW/m2, boiling remains continuous in one of the channels, but the other channel experiences time-periodic flow regime oscillations between single-phase and two-phase flow. At this condition, extreme time-periodic wall temperature oscillations are observed in both channels with a long period (~7 s) due to oscillations in the severity of the flow maldistribution. For the transient heating conditions, square-wave heating profiles oscillating between different heat flux levels are applied to the channels. Because of their relatively high frequency, the heating transients are attenuated by the microchannel walls, resulting in effectively constant heating conditions and flow boiling characteristics like that of the aforementioned constant heating conditions. This study illustrates the susceptibility of parallel two-phase heat sinks to flow maldistribution, particularly when undergoing transient flow regime oscillations. [ABSTRACT FROM AUTHOR]

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المؤلفون: Brendel, Leon PM¹ (AUTHOR) brendel@purdue.edu, Weibel, Justin A¹ (AUTHOR), Braun, James E¹ (AUTHOR), Groll, Eckhard A¹ (AUTHOR)

المصدر: International Journal of Multiphase Flow. Mar2023, Vol. 160, pN.PAG-N.PAG. 1p.

مصطلحات موضوعية: *VAPOR compression cycle, *REDUCED gravity environments, *TWO-phase flow, *CHANNEL flow, *HEAT transfer coefficient, *FLOW instability

مستخلص: • Microgravity two-phase flow fundamentals directly compared with system level research from parabolic flights. • Condensation heat transfer was found to be reduced due to microgravity by fundamental studies and system level testing. • Effects of microgravity on friction and evaporation heat transfer coefficient very small for system level testing. • Decreased gravity dependence for higher flow velocities partially confirmed on system level through inclination testing. • Fundamental and system level research for microgravity two-phase flow should be closer aligned to increase mutual utility. Research on two-phase channel flow in microgravity has extensively investigated flow regimes, pressure drop, and heat transfer during evaporation and condensation processes over several decades. This literature has been primarily motivated by the goal of enabling future space applications for two-phase thermal management systems, among them vapor compression refrigeration cycles. However, relative to the number of two-phase channel flow experiments, research on vapor compression cycles in microgravity has been almost non-existent. This paper reviews two-phase flow research and compares key outcomes with those from system-level measurements obtained from recent testing of a vapor compression cycle at varying inclination angles and on parabolic flights. Previous two-phase channel flow experiments have resulted in several microgravity-specific flow regime maps and have also generally found reduced condensation heat transfer coefficients in microgravity as compared to normal gravity. The flow pattern map of Jayawardena et al. (1997) and generally reduced condensation heat transfer are confirmed by the vapor compression cycle experiments. Regarding evaporation heat transfer coefficients and friction factors, there is not a consensus in the literature on the effects of microgravity in two-phase channel flows. Similarly, the system-level experiments do not show repeatable increases or decreases for those parameters. Lastly, the dependence of system-level behavior on orientation with respect to gravity was tested by performing dynamic inclination experiments, where the inclination angle was changed every 2 min by a 90-degree increment. These tests reveal an increased relative dependence of the vapor compression cycle to orientation with decreasing mass fluxes, in line with two-phase channel flow research that has generally shown that increasing the flow inertia acts to mitigate various flow instabilities and dependence on orientation. However, inclination testing with longer locking times per angle showed a constant relative dependence of the vapor compression cycle to orientation changes. [ABSTRACT FROM AUTHOR]

المؤلفون: Pai, Saeel S.¹ (AUTHOR), Weibel, Justin A.¹ (AUTHOR) jaweibel@purdue.edu

المصدر: International Journal of Heat & Mass Transfer. Oct2022, Vol. 195, pN.PAG-N.PAG. 1p.

مصطلحات موضوعية: *CHANNEL flow, *NUSSELT number, *PRESSURE drop (Fluid dynamics), *LAMINAR flow, *HEAT exchangers, *HEAT sinks, *VORTEX generators

مستخلص: • Artificial neural network-based models are developed for prediction of nusselt number and friction factor. • The predictions apply for fully developed laminar flow in constant cross-section ducts of different cross section shapes. • The machine learning (ML) model predictions are validated and compared against numerical simulations. • Use of ML models in shape optimization of channel cross section for different objective functions is demonstrated. • Guidelines for usage of the developed ML models are discussed. The design optimization of various thermal management components such as cold plates, heat sinks, and heat exchangers relies on accurate prediction of flow heat transfer and pressure drop. During the iterative design process, the heat transfer and pressure drop is typically either computed numerically or obtained using geometry-specific correlations for Nusselt number and friction factor. Numerical approaches are accurate for evaluation of a single design but become computationally expensive if many design iterations are required (such as during formal optimization processes). Correlation-based approaches restrict the design space to a specific set of geometries for which correlations are available. Surrogate models for the Nusselt number and friction factor, which are more universally applicable to all geometries than traditional correlations, would enable flexible and computationally inexpensive design optimization. The current work develops machine-learning-based surrogate models for predicting the Nusselt number and friction factor under fully developed internal flow in channels of arbitrary cross section and demonstrates use of these models for optimization of the cross-sectional channel shape. The predictive performance and generality of the machine learning surrogate models is first verified on various shapes outside the training dataset, and then the models are used in the design optimization of flow cross sections based on performance metrics that weigh both heat transfer and pressure drop. The optimization process leads to novel shapes outside the training data, and so numerical simulations are carried out on these optimized shapes to compare with the surrogate model predictions and show their performance is at least as good as that of shapes with known correlations available. A three-lobed shape was found to reduce friction factor, whereas a pentagon with rounded corners and an ice cream cone-shaped duct, both found using different performance metrics. Although the ML model predictions lose accuracy outside the training set for these novel shapes, the predictions follow the correct trends with parametric variations of the shape and therefore successfully direct the search toward optimized shapes. [ABSTRACT FROM AUTHOR]

المؤلفون: Miglani, Ankur^1,2 (AUTHOR), Weibel, Justin A.¹ (AUTHOR) jaweibel@purdue.edu, Garimella, Suresh V.¹ (AUTHOR)

المصدر: International Journal of Multiphase Flow. Jun2021, Vol. 139, pN.PAG-N.PAG. 1p.

مصطلحات موضوعية: *MICROCHANNEL flow, *PRESSURE drop (Fluid dynamics), *FLOW visualization, *NON-uniform flows (Fluid dynamics), *TWO-phase flow, *CHANNEL flow, *EBULLITION

مستخلص: • Effect of thermal coupling between parallel microchannels on Ledinegg instability-induced flow maldistribution is studied • Thermal coupling is shown to reduce the wall temperature difference between the channels and dampen flow maldistribution • Thermal coupling reduces the range of input powers with non-uniform flow and the maximum severity of flow maldistribution • Role of thermal coupling in dampening flow maldistribution is confirmed via agreement between model and experiments Two-phase flow boiling is susceptible to the Ledinegg instability, which can result in non-uniform flow distribution between parallel channels and thereby adversely impact the heat transfer performance. This study experimentally assesses the effect of thermal coupling between parallel microchannels on the flow maldistribution caused by the Ledinegg instability and compares the results to our prior theoretical predictions. A system with two parallel microchannels is investigated using water as the working fluid. The channels are hydrodynamically connected via common inlet/outlet plenums and supplied with a constant total flow rate. The channels are uniformly subjected to the same input power (which is increased in steps). Two separate configurations are evaluated to assess drastically different levels of thermal coupling between the channels, namely thermally isolated and thermally coupled channels. Synchronized measurements of the flow rate in each individual channel, wall temperature, and pressure drop are performed along with flow visualization to compare the thermal-hydraulic characteristics of these two configurations. Thermal coupling is shown to reduce the wall temperature difference between the channels and dampen flow maldistribution. Specifically, the range of input power over which flow maldistribution occurs is noticeably smaller and the maximum severity of flow maldistribution is reduced in thermally coupled channels. The data provide a quantitative account of the effect of lateral thermal coupling in moderating flow maldistribution, which is corroborated by comparison to the predictions from our two-phase flow distribution model. This combined experimental and theoretical evidence demonstrates that, under extreme conditions when one channel is significantly starved of flow rate and risks dryout, channel-to-channel thermal coupling can redistribute the heat load from the flow-starved channel to the channel with excess flow. Due to such a possibility of heat redistribution, the coupled channels are significantly less prone to flow maldistribution compared to thermally isolated channels. [ABSTRACT FROM AUTHOR]

المؤلفون: Kingston, Todd A.¹ (AUTHOR) kingston@purdue.edu, Weibel, Justin A.¹ (AUTHOR) jaweibel@purdue.edu, Garimella, Suresh V.¹ (AUTHOR) sureshg@purdue.edu

المصدر: International Journal of Heat & Mass Transfer. Jun2020, Vol. 154, pN.PAG-N.PAG. 1p.

مصطلحات موضوعية: *MICROCHANNEL flow, *HEAT pulses, *HEAT flux, *SINGLE-phase flow, *FLOW visualization, *CHANNEL flow, *HEAT flux measurement

مستخلص: • Microchannel flow boiling under transient heating conditions is studied experimentally. • High-frequency sensor measurements are synchronized to flow visualizations. • Heat flux is pulsed between 15, 75, and 150 kW/m2 using a thin film heater. • The dynamic response to a transient pulse is qualitatively similar to that of a spring-mass-damper system. • Heat flux pulses that induce/arrest boiling cause a temporary wall temperature over/under-shoot. Microchannel flow boiling is an attractive approach for the thermal management of high-heat-flux electronic devices that are often operated in transient modes. In Part 1 of this two-part study, the dynamic response of a heated 500 μm channel undergoing flow boiling of HFE-7100 is experimentally investigated for a single heat flux pulse. Three heat flux levels exhibiting highly contrasting flow behavior under constant heating conditions are used: a low heat flux corresponding to single-phase flow (15 kW/m2), an intermediate heat flux corresponding to continuous flow boiling (75 kW/m2), and a very high heat flux which exceeds critical heat flux and would cause dryout if applied continuously (150 kW/m2). Transient testing is conducted by pulsing between these three heat flux levels and varying the pulse duration. High-frequency measurements of heat flux, wall temperature, pressure drop, and mass flux are synchronized to high-speed flow visualizations to characterize the boiling dynamics during the pulses. At the onset of boiling, the dynamic response resembles that of an underdamped mass-spring-damper system subjected to a unit step input. During transitions between single-phase flow and time-periodic flow boiling, the wall temperature temporarily over/under-shoots the eventual steady operating temperature (e.g. , by up to 20 °C) thus demonstrating that transient performance can extend beyond the bounds of steady performance. It is shown that longer duration high-heat-flux pulses (up to ~50% longer in some cases) can be withstood when the fluid in the microchannel is initial boiling, relative to if it is initially in the single-phase flow regime, despite being at an initially higher heat flux and wall temperature prior to the pulse. [ABSTRACT FROM AUTHOR]

المؤلفون: Drummond, Kevin P.¹ (AUTHOR), Weibel, Justin A.¹ (AUTHOR), Garimella, Suresh V.¹ (AUTHOR) sureshg@purdue.edu

المصدر: International Journal of Heat & Mass Transfer. Jun2020, Vol. 153, pN.PAG-N.PAG. 1p.

مصطلحات موضوعية: *TWO-phase flow, *STAGNATION point, *MICROCHANNEL flow, *FLOW visualization, *SUBCOOLED liquids, *CHANNEL flow, *TEMPERATURE distribution

مستخلص: • Characterization of two-phase flow through a single manifold microchannel. • Test device that mimics the heating conditions and fin effects present in high-aspect-ratio microchannels. • Flow visualizations and spatially resolved temperature measurements along the channel depth. • Evaluation of the effects of channel aspect ratio and flow length on two-phase morphology. • Observation of a vapor blanket at the bottom of deep, short flow channels that governs thermal performance. Manifold microchannel heat sinks can dissipate high heat fluxes at moderate pressure drops, especially during two-phase operation. High-aspect-ratio microchannels afford a large enhancement in heat transfer area; however, the flow morphology in manifold microchannels during two-phase operation, as well as the resulting thermal performance, are not well understood. In this work, a single manifold microchannel representing a repeating unit in a heat sink is fabricated in silicon with a bonded glass viewing window. Samples of different channel lengths (750 μm and 1500 μm) and depths (125 μm, 250 μm, and 1000 μm) are considered; channel and fin widths are both maintained at 60 μm. Subcooled fluid (HFE-7100) is delivered to the channel at a constant flow rate such that the fluid velocity at the inlet is ~1.05 m/s in all cases. A high-speed camera is used to visualize the two-phase flow in the channel through the glass sidewall; an infrared camera measures the temperature distribution on the opposite channel sidewall. The flow visualizations reveal that vapor nucleation occurs at stagnation regions below the manifold near the inlet plenum and at both corners adjacent to the channel base. For deep channels (1000 μm), at sufficiently high heat fluxes, vapor completely covers the base of the channels and liquid does not re-wet the surface in this region. This newly identified vapor blanketing phenomenon causes a significant decrease in performance and an increase in the measured channel wall temperatures. This study reveals the critical role of the two-phase flow morphology in manifold microchannel heat sink design. [ABSTRACT FROM AUTHOR]

المؤلفون: Jain, Aakriti¹ (AUTHOR), Miglani, Ankur¹ (AUTHOR), Weibel, Justin A.¹ (AUTHOR), Garimella, Suresh V.¹ (AUTHOR) sureshg@purdue.edu

المصدر: International Journal of Heat & Mass Transfer. Aug2020, Vol. 157, pN.PAG-N.PAG. 1p.

مصطلحات موضوعية: *CHANNEL flow, *MICROCHANNEL flow, *ANNULAR flow, *HYDRAULICS, *ENTHALPY, *LATENT heat, *MICROFLUIDICS, *TEMPERATURE measurements

مستخلص: • The effect of channel diameter on flow freezing in microchannels is experimentally investigated. • Freezing starts as dendritic ice followed by annular ice growth that causes channel closure. • The closing time is shown to decreases monotonically with a decrease in the channel diameter. • An extremely short closing time of 0.25 s is observed for a 100 μm inner diameter channel. An understanding of the factors that affect the flow freezing process in microchannels is important in the development of microfluidic ice valves featuring well-controlled and fast response times. This study explores the effect of channel diameter on the flow freezing process and the time to achieve channel closure. The freezing process is experimentally investigated for a pressure-driven water flow (0.3 ml/min) through three glass microchannels with inner diameters of 500 μm, 300 μm, and 100 μm, respectively, using channel-wall temperature measurements synchronized with high-magnification, high-speed imaging. Freezing invariably initiates in supercooled water as a thin layer of dendritic ice that grows along the inner channel wall, followed by the formation and growth of a thick annular ice layer which ultimately causes complete channel closure. The growth time of the annular ice layer decreases monotonically with channel diameter, with the 100 μm channel having the shortest closing time. Specifically, the mean closing time for this smallest channel is measured to be 0.25 s, which is markedly shorter compared to other reports in the existing literature using larger channel sizes at similar flow rates. A model-based analysis of the freezing process is used to show that the total latent heat released by the freezing mass (which varies as the square of the channel diameter) is the key factor governing the closing time. Owing to this simple scaling, the study reveals that reducing the channel diameter offers an attractive approach to increasing the responsiveness of ice valves to achieve non-intrusive flow control at high sample flow rates. [ABSTRACT FROM AUTHOR]