This research will aid the development of microscale thermal systems that involve liquid-vapor phase change. To achieve desired system performance, it is often the case that shear-driven quasi-steady condensing flows must be made to behave in a predictable and maintainable fashion. Reliable flow prediction capability is therefore needed for millimeter-to-micrometer scale condensers.
Intellectual Merit: This research seeks to quantify the fluctuation energy transfer mechanisms to and from a condensing flow that might cause significant transient behavior within and outside of a microscale condenser. Experiments involving three different test sections (horizontal gaps or tubes with characteristic dimensions of 200 micrometers to 2 millimeters) in an instrumented flow-loop facility will lead to measured quantitative information on how a small-scale condenser responds - dynamically and thermally - to fluctuation energy transferred through its inlet and outlet. Quantities to be measured include heat transfer coefficients, pressure drops, and other important criteria that are crucial for successful design and operation. The experimental study will be augmented with simulations to generate a predictive capability applicable to a wide array of geometries and various condensing surface temperature conditions.
Broader Impacts: Flow boiling and condensation has promising potential to meet high heat flux needs in many applications. This research will enable a more quantitative prediction of flow and heat transfer in small scale condensers. The research will involve graduate students and undergraduate students recruited from underrepresented groups.
Thermal management issues have hindered advancement in electronic-cooling, power-systems cooling, avionics-cooling, and spaced based operations. Many of these difficulties are associated with scientific and technological challenges in steadily removing large amounts of heat from small areas while keeping device driving power requirements at acceptably low values. These issues are not addressed by straightforward miniaturization of traditional refrigeration or vapor compression cycles. Instead straightforward miniaturization leads to additional device-and-system level instabilities issues. To meet these challenges, it is generally accepted that the need of the hour is to come up with requisite innovations in boiler and condenser operations. While traditional boiler and condenser operations are able to use gravity and large device sizes to achieve their goals for larger-scale refrigeration and power-generation type applications, the above described applications require small heat-exchange surfaces and shear dominated operations with zero to insignificant help from gravity. The outcomes of this NSF-sponsored research has experimentally demonstrated a feasible approach for achieving high heat-flux shear dominated flow condensation with the help of the proposed innovative condenser operations. Furthermore, based on experimental discovery and scientific principles/hypotheses underlying the reported high heat-flux condensation phenomena, the NSF supplement to this grant was used to demonstrate analogous high heat-flux shear dominated annular flow boiling realizations (with nucleation suppression that retains the advantages of nucleation) within the proposed innovative boiler operations. Based on this successful demonstration, a new NSF award is being used to complete the flow boiling investigations and to demonstrate the viability (through a change in working-fluid and experimental set-up) of proposed boiler operations’ ability to handle steady heat-flux values in the vicinity of 1 kW/cm2. Even large industrial-scale gravity-driven boiler operations need to be innovated for the next generation combined cycle (or related) electric power plant technologies – this is towards improving efficiency and reducing size and cost of waste heat recovery boilers. The insights gained from these investigations are being used to propose vertical in-tube annular (wavy)-plus-nucleate boiling (with gravity-drained water films) phenomena in place of existing approaches used in some of the boiler tube-arrays. Combining the innovations for in-tube boiler operations with innovative air-side flow arrangements, it is possible to demonstrate compaction of existing heat-recovery steam generators as well as an output of higher quality (or pressure) steam. The proposed breakthroughs are based on fundamental fluid-physics based experimental discoveries for boiler and condenser operations. For developing scientific knowledge and engineering design tools, these discoveries are also supported by breakthroughs in the associated modeling and simulations research. A key innovative operation procedure introduces passive recirculating vapor flows within the devices. This controls the flows and ensures that very stable boiling and condensing flows occur in a manner where a thin liquid film flow, typically within 0.5 mm thickness, covers the entire heat-exchange surface. A second innovation is in the introduction of large amplitude waves through controlled resonant pulsations in the liquid film - leading to a 200-1000% enhancement of the heat removal rates. Analysis suggests the underlying physics: as the troughs of the waves on the liquid film approach the wetting heat-exchange surface to within 30-50 µm, the specific trough locations start exhibiting solid-liquid-vapor interactions phenomena similar to the high heat-flux contact line locations associated with nucleate boiling or drop-wise condensation. Retaining this physics and changing the working fluid to water, ongoing research plans to demonstrate very high heat removal (> 1 kW/cm2) values over the entire length of the innovative boiler. Supporting published results for 07/10/2013-06/30/2014 period in addition to those (three journal papers, two conference papers, and three separate conference presentations) reported in the earlier annual reports: Naik, R. R., Narain, A., Mitra, Steady and Unsteady Computational Simulations for Annular Internal Condensing Flows in a Channel, Paper No. IMECE2014-38445, Proceedings of 2014 ASME International Mechanical Engineering Congress and Exposition, Montreal, Canada. Kivisalu, M.T., Gorgitrattanagul, P., Narain, A., Results for High Heat Flux Flow Realizations in Innovative Operations of milli-meter Scale Condensers and Boilers, International Journal of Heat and Mass Transfer, 75, 381-398, 2014. Naik R., Mitra S., Narain A., Shankar N., Steady and Unsteady Computational Results of Full Two Dimensional Governing Equations for Annular Internal Condensing Flows, Fluid Dynamics (www.comsol.com/conference2013/usa/presentations/), COMSOL Conference, October 9-11, 2013, Boston. Best Paper Award.
