National Science Foundation     |     Directorate for Engineering  (ENG)
Division of Chemical, Bioengineering, Environmental, & Transport Systems  (CBET)
CBET Research Highlights 
Notable Accomplishments from CBET Awards
1406 - Part A - High Effectiveness Microscale Condensers and Boilers for Terrestrial and Space Applications
Amitabh Narain  -  Michigan Technological University

Outcome or Accomplishment:  Boilers/Condensers are components of traditional refrigerators, air/conditioners and other cooling systems.  Researchers at Michigan Tech have discovered that gravity has a profound impact on boiler/condenser operations, and these systems cannot be miniaturized (for applications such as electronic-cooling) or used in space.  The underlying cause is shown to be unacceptable liquid-vapor configurations within the devices.  Proposed innovative flow boilers/condensers not only enable highly desirable liquid-vapor configurations but also allow miniaturization at high heat loads.

Amitabh Narain Image A1
  Figure A1.  Liquid-vapor configurations under negligible influence of gravity within traditional and innovative condensers.
Amitabh Narain Image A2
  Figure A2.  Enhancement of average heat transfer rates due to pulsations.
Amitabh Narain Image B1
  Figure B1.  Enhancement of average heat transfer rates due to pulsations.
Amitabh Narain Image Image B2
  Figure B2.  Innovative condenser and boiler implementations with the help of controlled re-circulation of vapor.
Amitabh Narain Image B3
  Figure B3.  Pulsatile condensing flow schematic and resulting heat transfer enhancements.  The two short dynamic signal durations are each representative of 30 minute long quasi-steady data.
Amitabh Narain Image B4
  Figure B4.  This streamline plot is for a steady shear/pressure driven condensing flow in a horizontal channel.  It is obtained from the 2-dimensional scientific computational tool.  The background coloring represents the magnitude of the velocity vectors.
Credit for All Images:  Amitabh Narain, Mechanical Engineering, Michigan Technological University

Impact:  This NSF support helped develop innovative boilers/condensers that make new micro-scale and space-based technologies possible.  Consequently, researchers at Michigan Technological University and NASA-collaborators have initiated development of critically needed space-based technologies with the help of likely NASA-OCT funding.  An adaptation of these innovative devices in traditional applications may also help in addressing the grand challenges of reducing energy consumption (next generation air conditioners) and improving energy generation efficiency (thermal power plants).

Explanation/background:  The traditional boilers/condensers in micro-scale and space-based applications cause bubbles, plugs/slugs, etc. to appear and cover most of the devices' lengths.  These cause large resistances to the flow of heat.  Also, it is identified that the operation of these devices typically appears to be non-repeatable due to extreme sensitivity of these flows to ever-present noises/fluctuations.  As a result, traditional devices cannot be miniaturized or operated in a repeatable manner.  The proposed innovative devices allow miniaturization by keeping the heat-exchange surfaces covered with desired thin liquid film flows (e.g. Figure A1).  Utilizing controlled presence of noises/fluctuations, additional enhancements (>100 %) in heat-transfer rates (Figure A2) are obtained for the innovative devices.

CBET Research Highlight - Part B - Engineering Technical Information

1406 - High Effectiveness Microscale Condensers and Boilers for Terrestrial and Space Applications

Amitabh Narain  -  Michigan Technological University

Background:  Operation of micro-scale or space-based thermal management systems with small heat-exchange areas and high heat loads (~ 1 kW) requires the use of boilers and condensers that employ suitable high latent heat fluids (water, etc.). Since these applications operate under shear/pressure driven conditions (with negligible gravity effects), the traditional device designs pose serious difficulties in keeping the heat-exchange surfaces small and irrigated with thin liquid film flows.  Under these conditions, when vapor bubbles, plugs/slugs, etc. naturally accumulate at or around heat-exchange surface, the fluid flows are relatively slower and conditions are such that they degrade heat flow rates between the heat-exchange surface and the liquid-vapor interface where phase-change occurs.  The flows' sensitivity to ever present fluctuations changes heat transfer rates leading to thermal transients which may influence device/system level stability.
The NSF (and NASA) supported research of Professor Narain and his team at Michigan Technological University, Houghton, Michigan has led to the proposal of innovative devices (Figure B2) that replace undesirable liquid-vapor phase-change configurations with desirable ones (Figures A1 and B1).  New techniques have been and are being developed that further enhance heat-transfer rates (Figure A2).  Fundamental and effective simulation tools have been and are being developed to understand the flow physics and to size the proposed devices for different working fluids.

- 1 -  For any representative application with typical heating/cooling arrangements, the traditional boilers/condensers under shear/pressure driven conditions are larger than those for gravity assisted flows.  This is because the thermal resistances of the non-annular regimes (plug/slug, bubbly, etc.) are significantly larger, and they occupy a larger portion of the length of these devices.  Additionally the fluctuation-sensitivity changes heat transfer rates and, typically, heat-exchange surface temperatures which cause apparent non-repeatability.  If this is not recognized and addressed, it may lead to device/system level instabilities.
- 2 -  The proposed innovative boilers/condensers use re-circulating vapor flows to ensure that the thermally and hydro-dynamically efficient annular flows are realized over most of the devices' heat-exchange surfaces.  When the re-circulating vapor flow rate is within a suitable range, the enhanced interfacial shear stabilizes the flow of the thin liquid film (associated with the annular regime) to keep the heat-exchange surfaces continuously irrigated.
- 3 -  The team's investigations also found that the realized annular flows are sensitive to small pressure-difference pulsations.  These pulsations' amplitudes are comparable to the mean steady pressure-difference but are small relative to the mean absolute pressures in the boilers/condensers.  These, in turn, cause large amplitude mass flow rate fluctuations.  Such pulsations are easily introduced through low energy pulsators in the system.
By controlling the amplitude and frequency of such fluctuations for shear/pressure driven fully condensing flow in a horizontal channel (Figure B3), one can achieve repeatability as well as significant heat-flux enhancements (>100 % in their time averaged values).  The marked end zone (Figure B3) reflects the interfacial waves, and similar effects are being obtained for the proposed innovative condenser by an equivalent exit condition arrangement.  The underlying cause of this enhancement is time-averaged thinning of the liquid film that arises from asymmetric effects of sustained forward and backward moving interfacial waves.
- 4 -  Both an engineering estimation/prediction tool and a sophisticated scientific prediction tool have been and are being developed to assist in the understanding of flow physics and in technology development involving different working fluids and geometry.  The scientific predictive tool (see sample solution in Figure B4) yields nearly exact computational solutions of the full two dimensional steady/unsteady governing equations.  This is to be implemented on the NASA Pleiades supercomputer to predict the stability/sensitivity of these annular flows, to define the flow regime boundaries of the annular regime, and to compare the predictions with experiments.

Scientific Uniqueness:  This NSF-funded research is unique because it has addressed a wide variety of fundamental issues that lead to thermal and hydrodynamic inefficiency of complex non-annular liquid-vapor morphologies that arise in the use of traditional boilers/condensers under shear/pressure driven conditions as opposed to gravity influenced conditions.  Effective ways to control and realize efficient annular flow morphologies under these conditions have been developed through use of innovative flow boiler/condenser arrangements.  Novel ways to further enhance the heat transfer rates for these applications have been experimentally demonstrated.  A unique sophisticated computational tool that accurately predicts the annular regimes' time-varying interface locations in the presence of mass and heat transfer across the interface has been developed and is being implemented.

CBET Strategic Outcome Goals include:
- 1Discovery:  The NSF-funded research is aimed at achieving: (i) a fundamental understanding of problems associated with traditional condenser/boiler arrangements that preclude their uses in micro-scale and space-based applications, (ii) validating new condenser/boiler arrangements that overcome the problems with the traditional arrangements, and (iii) experimentally establishing additional active heat-flux enhancement techniques for the proposed new flow condenser/boiler arrangements.  The primary goal is to translate these research results into development of effective thermal systems for space-based and micro-scale applications.
- 2Learning:  This project is training four PhD candidate graduate students.  It has trained two M.S. graduate students and one undergraduate student (through NSF-REU) who has since joined the team as a graduate student.
- 3Research Infrastructure:  An experimental flow loop has been developed with unique abilities to control and investigate condensing and boiling flows (inclusive of annular and non-annular regimes).  A unique simulation tool (consisting of a blend of the researchers' own novel codes and algorithms with the knowledge and technology available in the latest commercially available tools) has been developed and is to be implemented on national supercomputers.

Transformative Research:  This research is potentially transformative because it changes the focus from investigating the entire range of complex liquid-vapor morphologies that occur in traditional shear/pressure driven devices to the more limited annular morphologies that are typically present in the innovative devices.  The fundamental knowledge that is being developed is also transformative in the sense that it provides a means for: achieving the desired annular liquid-vapor morphologies, defining flow regime boundaries within which these flows occur, significantly enhancing heat-transfer rates for the desired regimes, and computationally understanding/predicting the flow physics details (aided by experiments, but going beyond what is feasible by experiments alone) of the desired condensing/boiling flow regimes.

Intellectual Merit:  The intellectual merit is notable because it is the first time that a functional method has been proposed for micro-scale and space based thermal management systems that employ boilers/condensers.  Accurate steady/unsteady prediction of these annular regimes has been achieved, through a decade long effort towards blending the researchers' algorithms and codes with sophisticated but available commercial technologies.  It is also the first time that fluctuation-sensitivity (not stability) is identified to be a key issue in developing flow controls that accomplish repeatability (by measuring and controlling fluctuation levels and thereby managing thermal transients).  Additionally, by suitably controlling externally imposed fluctuations, it is shown that heat transfer-rates are significantly enhanced for the stabilized annular flow regimes.

Broader Impacts of this research include:
- 1Benefits to society:
           This NSF (and NASA funded) research will enable high efficiency and stable space-based thermal management systems of interest to NASA for planned in-space systems as well as multi-purpose crew vehicles.  The proposed technology is likely to materialize, over the next 3 years, in a collaborative work of Professor Narain and his team with NASA scientists/engineers.  This will be through a project being actively considered by the NASA Office of the Chief Technologist.
           Development of micro-scale electronic cooling technologies, next generation HVAC systems, aircraft-based vapor compression cycles, etc. will be facilitated by subsequent improvements/miniaturization of the proposed technology.  The potential impact on worldwide energy consumption and generation will be significant if the innovative concepts of the proposed boilers/condensers are adapted and included in the design of the next generation HVAC and Rankine cycle power systems.
           The associated developments of sophisticated engineering and scientific simulation tools make it possible to predict and evaluate the performance of flow boilers/condensers that may be employed by other technology developers dealing with other geometries and working fluids.
           STEM goals are enhanced by training several graduate students and planned post-doctoral researchers.
- 2Advancing discovery and understanding while promoting teaching, training, and learning:   At Michigan Technological University, this project is contributing to the training of four Ph. D. students, two M. S. graduate students, and one undergraduate student (through NSF-REU) who has since joined the team as a graduate student.  The work involves exposure to broad areas of design, fabrication, computations, electronic flow control, etc.  Each contributing member is exposed to the synergistic activities of the project, including discovery-based learning and reporting.
- 3Enhancing the infrastructure for research and education:
           The flow loop infrastructure has been used to expose and inform undergraduate and graduate students of this research activity as well as to relate to courses taught at Michigan Tech (this includes topics such as flow measurements, flow controls, etc.).
           The unique simulation tool (a blend of researchers' own novel codes and algorithms with the knowledge and technology available in the latest commercially available tools) that has been developed is of general value, through minor adaptations, in the investigation of many other two-phase flow problems (air-water flows, melting/freezing problems, etc.).
- 4Disseminating broadly to enhance scientific and technological understanding:
           Four journal papers have been published to report results obtained from this funding. Several more journal papers are in preparation.  These disseminations enhance scientific knowledge (including computational methods) of flow condensation and flow boiling.
           Four international conference presentations (including proceedings publications) and several presentations at government research facilities (NASA-GRC, USAF-Dayton, etc.) have also enhanced dissemination of the scientific results.

Program Director:
Sumanta Acharya
CBET Program Director - Thermal Transport Processes
NSF Award Number:   1033591
Award Title:   Flow Prediction and Fluctuation-sensitivity Investigations for Quasi-steady Shear Driven Condensing Flows in Milli-meter to Micro-meter Scale Two-Phase Systems
Principal Investigator:   Amitabh Narain
Institution Name:   Michigan Technological University
Program Element Code:   1406
CBET Research Highlight:   Fiscal Year 2012
Approved by CBET on:   30 March 2012

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This CBET Research Highlight was Updated on 8 May 2012.