Flow boiling in microchannels: Fundamentals and applications

The rapid advances in performance and miniaturization of electronics and high power devices resulted in huge heat flux values that need to be dissipated effectively. The average heat flux in computer chips is expected to reach 2–4.5 MW/m2 with local hot spots 12–45 MW/m2 while in IGBT modules, the heat flux at the chip level can reach 6.5–50 MW/m2. Flow boiling in microchannels is one of the most promising cooling methods for these and similar devices due to the capability of achieving very high heat transfer rates with small variations in the surface temperature. However, several fundamental issues are still not understood and this hinders the transition from laboratory research to commercial applications. The present paper starts with a discussion of the possible applications of flow boiling in microchannels in order to highlight the challenges in the thermal management for each application. In this part, the different integrated systems using microchannels were also compared. The comparison demonstrated that miniature cooling systems with a liquid pump were found to be more efficient than miniature vapour compression refrigeration systems. The paper then presents experimental research on flow boiling in single tubes and rectangular multichannels to discuss the following fundamental issues: (1) the definition of microchannel, (2) flow patterns and heat transfer mechanisms, (3) flow instability and reversal and their effect on heat transfer rates, (4) effect of channel surface characteristics and (5) prediction of critical heat flux. Areas where more research is needed were clearly mentioned. In addition, correlations for the prediction of the flow pattern transition boundaries and heat transfer coefficients in small to mini/micro diameter tubes were developed recently by the authors and presented in this paper

Experimental study of flow boiling heat transfer and critical heat flux in microchannels

Advancements in microprocessors and other high power electronics have resulted in increased heat dissipation from those devices. In addition, to reduce cost, the functionality of microprocessor per unit area has been increasing. The increase in functionality accompanied by reduction in chip size has caused its thermal management to be challenging. In order to dissipate the increase in heat generation, the size of conventional fin-type heat sinks has to be increased. As a result, the performance of these high heat flux generating electronics is often limited by the available cooling technology and space to accommodate the larger conventional air-cooled heat sinks. One way to enhance heat transfer from electronics without sacrificing their performance is the use of heat sink with many microchannels and liquid passing through it. The present work is aimed toward understanding the flow boiling stability and critical heat flux (CHF) with water and R-123 in microchannel passages. Experimental data and theoretical model to predict the heat transfer and CHF are the focus of this work. The experimental test section has six parallel microchannels with each having a cross sectional area of 1054 x 157 um2 or 1054 x 197 um2. 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 macro-scale empirical equation by Katto et al. [34]. 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 experiemental data with overall MAE of 12.3%. CHF is found to increase with increasing mass flux, and it leads to a change in the flow pattern from a wetted film flow to liquid streams that flow in the core of the flow

PROCESS INTENSIFICATION BY UTILIZING MULTISTAGE MANIFOLD MICROCHANNEL HEAT AND MASS EXCHANGERS

Much of research and development work has been dedicated to implement the heat and mass transfer using microchannel technology; however, it is not yet cost effective and is limited to higher end applications such as electronics cooling and selected applications in automotive and aircraft heat exchangers. The work on mass transfer application of micro channels also has been very limited, despite the very high potential contribution of micro channels for mass transfer enhancement. Scaling up of microchannel equipment presents several manufacturing and process organization challenges such as flow distribution inside microchannels, cost, fouling, high pressure drops etc. This thesis presents the development of a cost effective and compact tubular manifold microchannel heat and mass exchanger (MMHX) for industrial applications. A novel design for the flow distribution manifolds has been proposed. The proposed manifold helps in the enhancement of heat and mass transfer by creating better flow mixing. The MMHX is designed in such a way that the manifold causes the flow to break into multiple passes of very short flow lengths in the microchannels. These flow lengths are short enough such that the flow in the channels is always into entry zone (developing laminar zone) both hydrodynamically as well as thermally, resulting in higher heat transfer than that in the fully developed laminar flow in conventional microchannel heat exchangers. The pressure drop in the device is low as the fluid flow length into the microchannels is very short. While the manifold design helps in flow distribution, very short flow length inside the microchannels mitigates the problems of flow instability of two-phase heat transfer applications such as that in evaporators and condensers. The mass transfer in gas liquid reaction applications is enhanced due to the multiple passes where continuous breaking of the gas liquid interface as well as mixing of the bulk liquid occurs. A multi-pass microchannel heat and mass exchanger prototype was designed, fabricated and was experimentally tested for the performance as liquid-liquid heat exchanger, evaporator, condenser and gas- liquid absorber. Experiments were carried out by changing the liquid and gas flow rates, geometry of the microchannels and the size of the manifold. Flow visualization studies were also performed to study two phase flow distribution and flow pattern in the manifold. Experimental results have shown that the mass transfer coefficient (using CO2 and DEA-water solution) for the microchannel absorber is 1 to 2 orders of magnitude higher than the conventional absorber. This increase in mass transfer is mainly attributed to high interfacial area to volume ratio of microchannels and good mixing in the manifold. Similarly, heat transfer coefficient for the single phase heat transfer as well as for two phase heat transfer (evaporator and condenser) is about 3 to 8 times higher than the conventional heat exchangers such as shell and tube or plate type heat exchanger. High transfer rates enable us to design compact heat and mass transfer devices for the industrial applications. Industrial processes, such as carbon capture, which are not economically viable due to their high cost, can be feasible with the development of these next generation heat and mass transfer equipment. Due to the simplicity of the component design and the assembly, cost of the industrial scale equipment can be substantially lower as compared other compact heat exchangers. Current work is the continuation of heat and mass transfer work being carried out at the S2TS lab in University of Maryland. While Jha V.(2012) studied the first version of single pass manifold microchannel heat exchanger, Ganapathy H. (2014) studied the absorption of CO2 in DEA solution in single microchannel as well as in parallel microchannels. MMHX studied in this study builds on the previous work by introducing the multipass concept and utilizing commercially available fin tubes as microchannel surfaces

Experimental investigation of heat transfer rate in micro-channels

Metal-based MHEs are of current interest due to the combination of high heat transfer performance and improved mechanical integrity. Efficient methods for fabrication and assembly of functional metal-based MHEs are essential to ensure the economic viability of such devices. The present study focuses on the results of heat transfer testing of assembled Cu- and Al- based microchannel heat exchanger (MHE) prototypes. Efficient fabrication of Cu- and Al- based high-aspect-ratio microscale structures (HARMS) have been achieved through molding replication using surface engineered, metallic mold inserts. Replicated metallic HARMS were assembled through eutectic bonding to form entirely Cu- and Al- based MHE prototypes, on which heat transfer tests were conducted to determine the average rate of heat transfer from electrically heated Cu blocks placed outside the MHEs to water flowing within the molding replicated microchannel arrays. Experimentally observed heat transfer rates are higher as compared to those from previous studies on microchannel devices with similar geometries. Further, infrared thermography was conducted to determine the overall cooling rate and time constants. The time constant for the MHE device was found out to be lower for Cu channels with response times around 1-2 seconds. Al MHE device response time was only slightly lower due to the lower thermal conductivity. Experimental results show a great influence of the type of metal, flow rate and the surrounding conditions on the overall cooling performance of the MHEs. The potential influence of microchannel surface profile on heat transfer rates is discussed. The present results illustrate the potential of metal-based MHEs in wide ranging applications. A two-dimensional thermal lattice Boltzmann model was developed to simulate the heat transfer phenomenon in Cu- and Al- based microchannels. The LBM results were compared with 3D and 2D fluent models. Additionally, attempts were made to visualize the flow field inside an assembled Cu micro-channel at very low flow rates using oil-in-water solution

Experimental study of flow patterns, heat transfer, pressure drop and gravitational orientation during flow boiling in minichannels

Microchannels and Minichannels are being used in electronic cooling, fuel cells, automotive heat exchangers, micro refrigeration systems. They are also being considered for high heat flux applications under microgravity environment in space missions. Research laboratories and industries throughout the world are trying to explore and tap the potential uses of these small diameter channels in various products and applications. Before stepping in to any of the product design phase, fundamental issues related to their performance have to be studied and resolved. This thesis work focuses on resolving some of these fundamental issues related to flow boiling in small diameter channels and augmenting the understanding of their basic performance characteristics. The present experimental study is undertaken to determine the flow patterns, heat transfer, pressure drop and effect of gravitational orientation on the flow boiling characteristics of water in a set of six parallel minichannels, each 1054 µm wide by 197 µm deep and 63.5 mm long with a hydraulic diameter of 333 µm. The channels are machined on top of a copper block and are covered with Lexan to permit visual observation. The copper block along with the channel are heated by a cartridge heater. The observed flow patterns provide important information regarding the boiling behavior. The study is also extended to flow patterns under different gravitational orientations. Signal analysis of the associated pressure drop fluctuations is studied and a new correlation for predicting heat transfer coefficients is developed for flow in these geometries and the correlation is tested with other data sets available in the literature

Master of Science

thesisCommercial applications of microfluidic systems have been expanding exponentially over the last decade. Most commercial systems are fabricated using silicon processing; however, development costs remain high. For fundamental process development, a less expensive alternative is desirable. Xurography is an inexpensive rapid prototyping technology for microfluidic systems that is becoming more prevalent in research labs. In this technology, patterns are cut in double-sided adhesive polyimide tape, which is then sandwiched between two substrates. Traditionally, a cutting tool forms the patterns, which are relatively imprecise and subject to defects. To improve the cutting process, a laser has been implemented in this work. Due to the laser energy input, features are found to be more precise, but subject to soot production and melting. Laserbased xurography has been used to create five multilayer heat exchangers to explore the feasibility of thermal processing in devices sealed with the polyimide tape. The crossflow and counterflow heat exchangers were tested under a wide range of conditions; however, turbulent flow was not achieved due to pressure drop limitations. The devices performed leak free at temperatures up to 75 °C and pressures as high as 2520 kPa. Heat exchanger effectiveness matched theoretical predictions within experimental uncertainties. Using an exergy analysis, it was determined that the heat exchangers performed most efficiently at low Reynolds number. This work represents the first time laser-based xurography has been used to develop multilayer microfluidic devices

Flow Boiling in Micro-Passages: Developments in Fundamental aspects and Applications

Flow boiling in mini to micro passages located at the heat source, and as part of a thermal management system, has been identified as a possible way to remove the increasing high heat fluxes generated by high power electronic devices due to their capability of high heat transfer rates with small surface temperature variations. However, some still unresolved fundamental issues hinder the possible full adoption of this technology. These relate to the prevailing flow patterns, heat transfer rates and pressure drop in such geometries, and their dependence on key parameters. The possible major applications of flow boiling in microchannels are first mentioned in this paper, highlighting the requirements and the challenges of the thermal management of each application. The paper then presents new experimental research by the present authors as well as research reported in the literature on flow boiling in single tubes and rectangular multi microchannels to help elucidate the following fundamental issues: the definition of a microchannel, prevailing flow patterns, heat transfer mechanisms, flow instability and reversal and their effect on heat transfer rates, effect of channel material and surface characteristics (including latest research in coatings), effect of different fluid properties, and its relation to channel material, effect of channel length and aspect ratio. An appreciation of the above can help explain the interpretation of the prevailing fluid flow and heat transfer phenomena and the data scatter and discrepancies observed in past studies. In addition, models and correlations predicting flow patterns and heat transfer rates are presented

Two-phase flow dynamics in a micro hydrophilic channel: A theoretical and experimental study

In this paper, two-phase flow dynamics in a micro hydrophilic channel are experimentally and theoretically investigated. Flow patterns of annulus, wavy, and slug are observed in the range of operating condition. A set of empirical models based on the Lockhart-Martinelli parameter and a two-fluid model using several correlations of the relative permeability are adopted; and their predictions are compared with experimental data. It shows that for low liquid flow rates most model predictions show acceptable agreement with experimental data, while in the regime of high liquid flow rate only a few of them exhibit a good match. Correlation optimization is conducted for individual flow pattern. Through theoretical analysis of flows in a circular and 2-D channel, respectively, we obtain correlations close to the experimental observation. Real-time pressure measurement shows that different flow patterns yield different pressure evolutions. © 2013 Elsevier Ltd. All rights reserved