Amitabh Narain

Amitabh Narain

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Professor, Mechanical Engineering–Engineering Mechanics

  • PhD, University of Minnesota

Biography

Narain’s current research interests are both experimental and theoretical/computational in nature and emphasize the area of phase-change flows – especially internal condensing and boiling flows.

Stable, repeatable, and predictable realizations of shear/pressure driven condensing and boiling flows are critical to the development of many high heat transfer rate applications (electronic-cooling, aircraft-based cooling, and space-based thermal management or power systems). These flows are different from the gravity driven (or gravity assisted) flows in their response to typically present pressure fluctuations that are small relative to the mean pressure. The gravity driven (or assisted) flows - commonly occurring in vertical or inclined condensers - have been investigated and found to be largely insensitive to these pressure fluctuations.

It is experimentally demonstrated that extreme sensitivity (large changes) of shear/pressure driven condensate flows result from ever present small pulsations (0-20 Hz) in pressure-difference. Inadvertent or deliberate impositions of such small pressure-difference pulsations (over the annular flow region in the condenser) are often associated with significant mass flow rate pulsations. This leads to changes in mean test-section pressure-difference as well as mean local heat-flux (> 200-300 % enhancements). The underlying cause appears to be flow reversals in the interface region which significantly increase mean shear stress and decrease mean film thickness. There are also some changes in the length of the annular regimes between zero-gravity and horizontal transverse-gravity situations. Significant thermal transients are observed as one sufficiently changes the imposed pressure pulsation levels to go from one quasi-steady flow realization to another.

The above described phenomenon is termed supercritical parabolic sensitivity over time-scales much smaller than the dominant period of an imposed fluctuation (typically in 0-20 Hz range). The phenomenon also implies importance of both inlet and exit conditions – termed elliptic sensitivity – over time-scales much larger than the dominant time-period of the imposed fluctuation.

Significant milestones achieved for the ongoing condensing/boiling flow research are: development of a computational code capable of simulating these annular flows (these include the time-varying locations of the wavy interface), development of a new state of the art experimental facility for investigating various quasi-steady and transient phenomena for internal condensing and boiling flows, and our own invention of fluorescence and fiber-optic based film thickness sensor capable of measuring time-varying thicknesses of a dynamic film. Recent publications discuss the results obtained from experiments and direct computational simulations. The condensing (inside vertical tubes and horizontal mm- to µm-scale channels) and boiling flow experiments, funded by NSF and NASA, employ modern technology (electronic flow control techniques, fiber-optic flow visualization techniques, various sensors, etc.) that allow accurate and effective measurements (heat-flux, pressures, temperatures, etc.) for these phase-change flows.

The new direction on the phase-change flow research focuses on flow prediction as well as system level integration issues associated with condenser, flow boiler and other devices. The effect of the supercritical fluctuations on the system level performance and repeatability is also being investigated.

Narain’s secondary interests are in related areas of transport processes. These include cavitation signatures in an automobile’s torque-converter, melting and solidification processes associated with use of phase-change materials (PCM) for energy storage applications (such as solar power plants), and computational simulations of forced and natural convection turbulent flows (inside heat exchangers, displacement pumps, etc.).

Links of Interest

Research Interests

  • Phase-change flows, especially internal condensing and boiling flows
  • Cavitation signatures in an automobile’s torque-converter
  • Melting and solidification processes associated with use of phase-change materials (PCM) for energy storage applications (such as solar power plants)
  • Computational simulations of forced and natural convection turbulent flows (inside heat exchangers, displacement pumps, etc.)

Areas of Expertise

  • Condensing/Phase-change flows
  • Computational and Experimental Fluid Mechanics/Heat Transfer

Recent Publications

  • J. H. Kurita, M. Kivisalu, S. Mitra, R. Naik, A. Narain: “Experimental Results on Gravity Driven Condensing Flows in Vertical Tubes, their Agreement with Theory, and their Differences with Shear Driven Flows’ Boundary Condition Sensitivities,” International Journal of Heat and Mass Transfer, 54, 2932-2951, 2011. Read More
  • S. D. Kulkarni, A. Narain, M. Kivisalu, S. Mitra, and M. M. Hasan: “Condenser performance, Control, and Heat Transfer Enhancement Issues Resulting from Elliptic-Sensitivity of Shear Internal Condensing Flows,” accepted and in-press for publication in the International Journal of Transport Phenomena, 2011. Read More
  • S. Mitra, A. Narain, R. Naik, and S. D. Kulkarni: “A Quasi One-Dimensional Simulation Method and its Results for Steady Annular/Stratified Shear and Gravity Driven Condensing Flows,” International Journal of Heat and Mass Transfer, 54, 3761-3776, 2011. Read More
  • M. Kivisalu, N. Gorgitrattanagul, S. Mitra, R. Naik, and A. Narain: “Prediction and Control of Internal Condensing Flows in the Experimental Context of their Inlet Sensitivities,” accepted and in-press for publication in Microgravity Science and Technology, Paper number: MGST318R1, 2011. Read More
  • Ng, T. W., A. Narain, M. Kivisalu: “Fluorescence and Fiber-Optics Based Real-Time Thickness Sensor for Dynamic Liquid Films,” ASME Journal of Heat Transfer, Vol. 132, 031603-1 to 12, 2010. Read More
  • S. D. Kulkarni, A. Narain, and S. Mitra:“Forced Flow of Vapor Condensing over a Horizontal Plate (Problem of Cess and Koh*) - Steady and Unsteady Solutions of the Full 2D Governing Equations,” ASME Journal of Heat Transfer, Vol. 132 (10), 101502, 2010. Read More
  • Narain, A., S. D. Kulkarni, S. Mitra, J. H. Kurita, and M. Kivisalu: “Internal Condensing Flows in Terrestrial and Micro-gravity Environments – Computational and Ground-based Experimental Investigations of the Effects of Specified and Unspecified (Free) Conditions at Exit,” Annals of New York Academy of Sciences, Interdisciplinary Transport Phenomena in Space Sciences, Vol. 1161, pp. 321-360, 2009.