Vinod Srinivasan

Nanowires for Enhanced Boiling Heat Transfer

Boiling is a common mechanism for liquid-vapor phase transition and is widely exploited in power generation and refrigeration devices and systems. The efficacy of boiling heat transfer is characterized by two parameters: (a) heat transfer coefficient (HTC) or the thermal conductance; (b) the critical heat flux (CHF) limit that demarcates the transition from high HTC to very low HTC. While increasing the CHF and the HTC has significant impact on system-level energy efficiency, safety, and cost, their values for water and other heat transfer fluids have essentially remained unchanged for many decades. Here we report that the high surface tension forces offered by liquids in nanowire arrays made of Si and Cu can be exploited to increase both the CHF and the HTC by more than 100%.

Enhanced Heat Transfer in Biporous Wicks in the Thin Liquid Film Evaporation and Boiling Regimes

Biporous media consisting of microscale pin fins separated by microchannels are examined as candidate structures for the evaporator wick of a vapor chamber heat pipe. The structures are fabricated out of silicon using standard lithography and etching techniques. Pores which separate microscale pin fins are used to generate high capillary suction, while larger microchannels are used to reduce overall flow resistance. The heat transfer coefficient is found to depend on the area coverage of a liquid film with thickness on the order of a few microns near the meniscus of the triple phase contact line. We manipulate the area coverage and film thickness by varying the surface area-to-volume ratio through the use of microstructuring. Experiments are conducted for a heater area of 1 cm 2 with the wick in a vertical orientation. Results are presented for structures with approximately same porosities, fixed microchannel widths w � 30 lm and w � 60 lm, and pin fin diameters ranging from d ¼3‐29 lm. The competing effects of increase in surface area due to microstructuring and the suppression of evaporation due to reduction in pore scale are explored. In some samples, a transition from evaporative heat transfer to nucleate boiling is observed. While it is difficult to identify when the transition occurs, one can identify regimes where evaporation dominates over nucleate boiling and vice versa. Heat transfer coefficients of 20.7 (62.4) W/cm 2 -K are attained at heat fluxes of 119.6 (64.2) W/cm 2 until the wick dries out in the evaporation dominated regime. In the nucleate boiling dominated regime, heat fluxes of 277.0 (69.7) W/cm 2 can be dissipated by wicks with heaters of area 1 cm 2 , while heat fluxes up to 733.1 (6103.4) W/cm 2 can be dissipated by wicks with smaller heaters intended to simulate local hot-spots. [DOI: 10.1115/1.4006106]

Suppression of global modes in low-density axisymmetric jets using coflow

Experiments conducted in helium axisymmetric jets with an annular coflowing air stream yield critical values of the velocity ratio U2∕U1 needed to suppress global instability inherent in these low-density flows. Global mode suppression was achieved for coflowing velocities less than approximately 20% of the jet centerline velocity, though the critical velocity ratio displayed a nonmonotonic relationship with the initial shear layer momentum thickness. The experiments are supported by spatio-temporal inviscid stability theory, where the convective-absolute transition was tracked in an operating domain including U2∕U1 and D∕θ. For initially thick shear layers, the experimental observations are in good agreement with linear theory, but deviate considerably as the separating shear layer thickness is reduced.

Viscous linear stability of axisymmetric low-density jets: Parameters influencing absolute instability

Viscous linear stability calculations are presented for model low-density axisymmetric jet flows. Absolute growth transitions for the jet column mode are mapped out in a parametric space including velocity ratio, density ratio, Reynolds number, momentum thickness, and subtle differences between velocity and density profiles. Strictly speaking, the profiles used in most jet stability studies to date are only applicable to unity Prandtl numbers and zero pressure gradient flows—the present work relaxes this requirement. Results reveal how subtle differences between the velocity and density profiles generally used in jet stability theory can dramatically alter the absolute growth rate of the jet column mode in these low-density flows. The results suggest heating/cooling or mass diffusion at the outer nozzle surface can suppress absolute instability and potentially global instability in low-density jets.

Numerical and Experimental Evaluation of Ceramic Honeycombs for Thermal Energy Storage

ABSTRACT Thermal energy storage at high temperature is a challenging research area with typical applications like regenerative heating in steel production plants and auxiliary energy source in solar thermal plants. Honeycomb structures made of ceramics are used as high temperature thermal energy storage units because of their large heat transfer surface area per unit volume, large thermal capacity and good thermal shock resistance. The material properties and geometric parameters of these units determine the storage capacity and heat addition/retrieval rate. A thorough understanding of the thermal response of storage unit at different process conditions is crucial for designing the system. In this work, new compositions of mullite and chromite based ceramic honeycombs were developed for high temperature thermal storage application. An experiment was designed to evaluate the performance of the ceramic honeycomb in the temperature range of 773-1273 K by studying the storing and discharging characteristics in cyclic mode. Numerical studies using ANSYS Fluent have been presented to predict the effect of honeycomb design, material properties and flow rates on thermal energy storage and heat transfer characteristics. This data are used to validate the experimental results and for designing an optimum thermal energy storage system. GRAPHICAL ABSTRACT

Development of a Ceramic Pressurized Volumetric Solar Receiver for Supercritical CO2 Brayton Cycle

Recently, the supercritical CO2 (s-CO2) Brayton cycle has been identified as a promising candidate for solar-thermal energy conversion due to its potentially high thermal efficiency (50%, for turbine inlet temperatures of ∼ 1000K). Realization of such a system requires development of solar receivers which can raise the temperature of s-CO2 by over 200K, to a receiver outlet temperature of 1000K. Volumetric receivers are an attractive alternative to tubular receivers due to their geometry, functionality and reduced thermal losses. A concept of a ceramic pressurized volumetric receiver for s-CO2 has been developed in this work. Computational Fluid Dynamics (CFD) analysis along with a Discrete Ordinate Method (DOM) radiation heat transfer model has been carried out, and the results for temperature distribution in the receiver and the resulting thermal efficiency are presented. We address issues regarding material selection for the absorber structure, window, coating, receiver body and insulation. A modular small scale prototype with 0.5 kWth solar heat input has been designed. The design of a s-CO2 loop for testing this receiver module is also presented in this work.Copyright

Film Cooling Effect of Rotor-Stator Purge Flow on Endwall Heat/Mass Transfer

Mass transfer measurements on the endwall and blade suction surfaces are performed in a five-blade linear cascade with a high-performance rotor blade profile. The effects of purge flow from the wheelspace cavity entering the hot gas path are simulated by injecting air through a slot upstream of the blade row at 45° to the endwall, for Reynolds number of 6×105 based on blade true chord and cascade exit velocity, and blowing ratios of 0.5, 1 and 1.5. Detailed maps of cooling effectiveness on the passage endwall and blade suction surface are generated for the cases of injection of naphthalene-free and naphthalene-saturated air. Oil-dot visualization indicates that with injection, a recirculation region is set up upstream of the leading edge, and the growth of the passage vortex is altered. The coolant exiting from the slot is drawn to the suction side of the blade and is pushed up along the suction surface of the blade by the secondary flow. For blowing ratios of 0.5 and 1.0, only a little coolant reaches the pressure side in the aft part of the passage. However, at a blowing ratio of 1.5, there is a dramatic change in the flow structure. Both the oil dot visualization and the cooling effectiveness maps indicate that at this blowing ratio, the coolant exiting the slot has sufficient momentum to closely follow the blade profile, and is not significantly entrained into the passage vortex. As a result, high cooling effectiveness values are obtained at the pressure side of the endwall, well into the mid-chord and aft portions of the blade passage.Copyright