Research Interests [College of Engineering Article, 2010]
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.).