Sudhakar Neti

Evaporating drops on patterned surfaces: Transition from pinned to moving triple line

Evaporation of sessile drops on micro-patterned surfaces is investigated over a range of heterogeneity length scales and solid area fractions. The surface topology is generated by a uniform arrangement of square pillars or square holes. The evaporation process is captured using high resolution imaging techniques and later post-processed for such information as contact angle, contact circle diameter and drop volume. It is observed that two distinct phases of evaporation existed for all substrate characteristics: pinned triple line (TL) phase and moving TL phase. In both phases, the process follows a linear decrease of surface area. The dimensionless evaporation rate constant is found to be higher during the moving TL phase in comparison with the pinned TL phase. In addition, it is found that the triple line topology has no effect on the evaporation rate constant.

Measurements of effectiveness in a silica gel rotary exchanger

Desiccant air conditioning systems that incorporate rotary heat and mass exchangers are getting a closer look in view of the pressure on the industry to decrease the use of chlorofluorocarbons and for fuel economies. Heat and mass exchangers that utilize LiCl, silica gel and other desiccants have been considered as possibilities. The performance features of a silica gel rotary mass exchanger wheel, which is the primary component in a desiccant system, has been measured. The process air flow velocities and temperatures ranged from 0.5 to 2.5 m/s, and 20 to 30°C with 30 to 100% relative humidities. Similar velocity and relative humidity conditions were used for regeneration air. The desiccant data are compared to two different theories from the literature; the method of characteristics and a numerical approach. The method of characteristics appears to be good for only a small range of conditions, generally for low values of specific capacities. The numerical approach appears to predict the trends well, though sometimes with large errors.

Binary and Ternary Nitrate Solar Heat Transfer Fluids

Current heat transfer fluids for concentrated solar power applications are limited by their high temperature stability. Other fluids that are capable of operating at high temperatures have very high melting points. The present work is aimed at characterizing potential solar heat transfer fluid candidates that are likely to be thermally stable (up to 500 C) with a lower melting point ( 100 C). Binary and ternary mixtures of nitrates have the potential for being such heat transfer fluids. To characterize such eutectic media, both experimental measurements and analytical methods resulting in phase diagrams and other properties of the fluids are essential. Solidus and liquidus data have been determined using a differential scanning calorimeter over the range the compositions for each salt system and mathematical models have been derived using Gibbs Energy minimization. The Gibbs models presented in this paper sufficiently fit the experimental results as well as providing accurate predictions of the eutectic compositions and temperatures for each system. The methods developed here are expected to have broader implications in the identification of optimizing new heat transfer fluids for a wide range of applications, including solar thermal power systems. [DOI: 10.1115/1.4023026]

Instabilities in viscoelastic flow past a square cavity

Abstract Elastic flow transitions in viscoelastic flow past a square cavity adjacent to a channel are reported. The critical conditions for the onset of flow transitions and the qualitative and quantitative characterization of the secondary flows generated by the instability have been examined using streakline photography and instantaneous pressure measurements. Cellular type of instabilities inside the cavity is observed for flow rates beyond a critical value. Small and large scale eddies are observed at high flow rates. The flow inside the cavity and in the channel upstream and downstream of the cavity becomes weakly time-dependent for high flow rates.

Dynamics of viscoelastic jets of polymeric liquid extrudate

Experimental observations are used to quantitatively describe purely elastic instabilities in viscoelastic jets issuing from a capillary and an orifice. Beyond a critical value of the dimensionless flow rate, or Deborah number, the flow becomes unstable and the purely elastic flow transition occurs. The instabilities result in free surface distortion in the form of vertical stripes laid longitudinally along the axis of the jet. The stripes uniformly spaced along the periphery of the jet are observed by photo-imaging measurements for PIB-based Boger fluid. The critical conditions for the onset of surface distortions and spatio-temporal characteristics of the instabilities are reported. Measurements of the extrudate swell are also conducted for different values of capillary diameter to systematically examine the effect of geometry.

Thermophysical Properties of LiNO3–NaNO3–KNO3 Mixtures for Use in Concentrated Solar Power

One of the major challenges preventing the concentrated solarpower (CSP) industry from occupying a greater portion of theworld’s energy portfolio are unattractive start up and operatingcosts for developers and investors. In order to overcome thesereservations, plant designers must be able to achieve greater effi-ciencies of power production. Molten salt nitrates are ideal candi-dates for CSP heat transfer fluids and have been proposed to offersignificant performance advantages over current silicone basedoil heat transfer fluids. Ternary molten salt nitrates offer highoperating temperatures while maintaining low freezing tempera-tures. However, a shortage of important thermophysical propertydata exists for these salts. Previous work has shown the ternarycompositions of LiNO

Non-isothermal viscoelastic flow through an axisymmetric sudden contraction

Abstract Nonlinear dynamics of non-isothermal viscoelastic flow in an axisymmetric sudden contraction geometry are studied for a highly elastic polyisobutylene (PIB) based polymer solution (referred to as PIB-Boger fluid). Temperature gradient in the flow is generated by the external heating or cooling imposed at the boundaries of the system. The critical conditions for the onset of the elastic instabilities at different wall temperatures are determined by accurate instantaneous measurements of pressure drop across the contraction plane and temperature measurements in the flow. These results along with flow images by streakline photography are presented for various values of wall-to-fluid temperature differences and Deborah numbers (dimensionless flow rates). It is shown that the thermal effects have profound consequences for the onset, development, and structure of the vortical flows in viscoelastic entry flows. Thermal instabilities are not detected in these flows for the range of flow rates and wall temperatures used in the present observations.

Pressure measurements in highly viscous and elastic fluids

Pressure measurements in flows of highly viscous and elastic fluids are of practical importance in polymer processing and rheology systems. Special problems arise during such pressure measurements. High fluid viscosity results in excessive dynamic response time (rise time) of the pressure measuring systems. This is true for systems that consist of manometers as well as pressure transducers attached to the base of a small hole at the wall. We model the dynamic response and examine related disturbing effects. These systematic errors in pressure measurements include hole-pressure effects, instabilities in cavity flow, and the time lag of the disturbance wave

Convective film boiling in a rod bundle: axial variation of nonequilibrium evaporation rates

Abstract An experimental study of convective film boiling (post-CHF) is carried out for two-phase steamwater flow in a nine-rod bundle test section. Measurements of wall heat flux, wall superheat, and vapor superheat are obtained as a function of axial distance from the critical heat flux (quench front) for a range of flow rates and flow qualities. These data permit the evaluation of the effective evaporation ratio (fraction of total heat input causing net vapor generation) as a function of test conditions and axial distance from the quench front. The results indicate the existence of a ‘near region’ with significant evaporation, followed by a far region where the effective evaporation ratio decreases to less than 10% of the total heat input. While the wall and vapor superheats measured in these rod bundle experiments differ in magnitude from those obtained in earlier single tube experiments, qualitative agreement in the axial behavior of the post-CHF process is observed. These data are presented in the hope of aiding attempts to develop improved mechanistic models for convective film boiling heat transfer.

Convective boiling in a rod bundle: transverse variation of vapor superheat temperature under stabilized post-CHF conditions

Abstract Transverse variations of vapor superheat temperatures across a rod bundle are measured for convective boiling of water in the post-CHF regime. Significant differences in superheat (up to 120°C) are observed across the flow subchannels. The steepness of this transverse superheat profile decreases with increasing vapor Reynolds number.