Bhalchandra Puranik

Two-Phase Flow Pressure Drop Characteristics in Trapezoidal Silicon Microchannels

This paper focuses on experimentally studying the pressure drop characteristics for two-phase flow in microchannels of hydraulic diameter 109 mum , over a relatively large range of heat flux of (0-30 W/cm2) and mass flow rate values (44-1114 kg/m2-s). Three fluid flow regimes (single-phase, two-phase, and dryout) have been covered in this paper, with deionized water as the working fluid. For a given heat flux, the variation of average pressure drop with flow rate can be classified into three distinct regimes. In the first regime (higher flow rate), the pressure drop decreases linearly with decrease in flow rate. In the second regime (lower flow rate), pressure drop increases with decreasing flow rate and reaches a maximum (with a minimum on either side). Finally, in the very low flow rate regime, pressure drop increases rapidly with decreasing flow rate. The average pressure drop in the two-phase regime is predicted well by the annular flow model. In addition to absolute pressure drop values, we also report pressure fluctuations. The magnitude of pressure fluctuations appears to be correlated to the underlying flow regime, such as bubbly, slug, and annular regimes, which have been identified through the flow visualization. An important outcome of this study is the identification of as many as four operating points with similar pressure drop penalty. This may help to choose the right operating conditions for a microchannel-based heat sink for use in cooling electronics. These detailed experimental results are also expected to be useful for modeling two-phase flow in microchannels.

Heat Transfer Analysis in a Rectangular Duct Without and With Cross-Flow and an Impinging Synthetic Jet

A synthetic jet is a zero-net-mass-flux device, which synthesizes stagnant air to form a jet, and is potentially useful for cooling. Due to the inherent suction and ejection processes in a synthetic jet, its utility in a confined enclosure is not obvious. The synthetic jet impingement heat transfer characteristics inside a rectangular duct are studied in this paper. In addition, the effect of cross-flow created using either fans or another synthetic jet on its heat dissipation capability is examined. Experiments are conducted for different jet Reynolds numbers (Re), in the range of 950-4000, at different offset positions of the synthetic jet with respect to a heated block flush mounted on one surface of the duct. The height of the duct is the same (25 mm) for all measurements while the width is varied between 110 mm and 330 mm in order to examine the effect of confinement on the heat transfer coefficient. The change in the width of the duct is found to have a negligible effect on heat transfer. The heat transfer coefficient is found to be more with synthetic jet direct impingement (150 W/m2 · K) than with combined flow (both impingement and cross-flow) (134 W/m2 · K) or with only cross-flow (45 W/m2 · K) in the duct. The offset of the synthetic jet from the center of the heated block is found to drastically reduce the heat transfer. These results are expected to be useful for designing synthetic jet-based cooling solutions.

Heat Transfer Characteristics of a Heat Sink in Presence of a Synthetic Jet

The heat transfer characteristics of a typical heat sink used in conjunction with an impinging synthetic jet are experimentally investigated in this paper. The experiments are conducted for several excitation frequencies, different shapes of the synthetic jet orifice having the same hydraulic diameter, and several axial distances from the tip of heat sink fins to the orifice plate. In addition, experiments with a fan placed on top of the heat sink are conducted at different input powers to the fan and for different axial distances. The performance of the heat sink in terms of the overall thermal resistance is documented. The heat transfer coefficient with the heat sink is found to be approximately four times greater with the impinging synthetic jet with the impinging synthetic jet than that for the bare surface. Different shapes of the orifice with identical hydraulic diameter have negligible effect on the thermal performance of the heat sink. The synthetic jet is deduced to perform better than a continuous jet but worse than a commercial fan. These results of an impinging synthetic jet on a heat sink have not been reported earlier and are expected to have practical utility.

A hybrid MD-DSMC coupling method to investigate flow characteristics of micro-devices

A new methodology is proposed to couple Molecular Dynamics (MD) and Direct Simulation Monte Carlo (DSMC) methods to simulate high Knudsen number (Kn) flows. For this purpose a two-dimensional hybrid MD-DSMC code is developed. In this method gas-surface interactions are modeled using MD, and gas-gas interactions are modeled using DSMC method. Two-way coupling between MD and DSMC is implemented by employing buffer zones for both MD and DSMC regions. Bootstrap sampling and energy minimization algorithms are employed for dynamic coupling of these two methods since MD utilizes real number of molecules during simulation whereas DSMC utilizes a lesser number of simulated molecules. The hybrid methodology combines the advantages of both methods; it has the capability of modeling the gas-surface interaction accurately considering the effect of the presence of neighboring real number of gas molecules, while in the bulk it utilizes DSMC with only the simulated number of molecules thus increasing the computational efficiency significantly compared to pure MD codes. As a result comparatively large domain sizes can be simulated with realistic behavior at the walls. The utility of the hybrid method is demonstrated by simulating high Kn flows through a micro-channel, micro-nozzle and micro-scale shock tube. The effect of partial accommodation of gas molecules with the wall is seen to be captured dynamically with this approach.

Dissociation cross sections for N2 + N → 3N and O2 + O → 3O using the QCT method

Cross sections for the homo-nuclear atom-diatom collision induced dissociations (CIDs): N2 + N and O2 + O are calculated using Quasi-Classical Trajectory (QCT) method on ab initio Potential Energy Surfaces (PESs). A number of studies for these reactions carried out in the past focused on the CID cross section values generated using London-Eyring-Polanyi-Sato PES and seldom listed the CID cross section data. A highly accurate CASSCF-CASPT2 N3 and a new O3 global PES are used for the present QCT analysis and the CID cross section data up to 30 eV relative energy are also published. In addition, an interpolating scheme based on spectroscopic data is introduced that fits the CID cross section for the entire ro-vibrational spectrum using QCT data generated at chosen ro-vibrational levels. The rate coefficients calculated using the generated CID cross section compare satisfactorily with the existing experimental and theoretical results. The CID cross section data generated will find an application in the development of a more precise chemical reaction model for Direct Simulation Monte Carlo code simulating hypersonic re-entry flows.

An Iterative Procedure for the Evaluation of A Conjugate Condition in Heat Transfer Problems

In the present work, an iterative numerical procedure to evaluate the fluid-solid interface condition in a conjugate heat transfer situation is proposed and implemented in a finite volume methodology using both structured and unstructured meshes. Two benchmark problems are solved, and the numerical solutions are found to be in good agreement with the analytical solutions. A parametric study is performed to analyze the physical behavior in the conjugate heat transfer problems, and correlations for the local Nusselt number are developed in one representative case. The conditions for which a conjugate analysis is required are determined.

An ab initio chemical reaction model for the direct simulation Monte Carlo study of non-equilibrium nitrogen flows

A new ab initio based chemical model for a Direct Simulation Monte Carlo (DSMC) study suitable for simulating rarefied flows with a high degree of non-equilibrium is presented. To this end, Collision Induced Dissociation (CID) cross sections for N2+N2→N2+2N are calculated and published using a global complete active space self-consistent field-complete active space second order perturbation theory N4 potential energy surface and quasi-classical trajectory algorithm for high energy collisions (up to 30 eV). CID cross sections are calculated for only a selected set of ro-vibrational combinations of the two nitrogen molecules, and a fitting scheme based on spectroscopic weights is presented to interpolate the CID cross section for all possible ro-vibrational combinations. The new chemical model is validated by calculating equilibrium reaction rate coefficients that can be compared well with existing shock tube and computational results. High-enthalpy hypersonic nitrogen flows around a cylinder in the transition flow regime are simulated using DSMC to compare the predictions of the current ab initio based chemical model with the prevailing phenomenological model (the total collision energy model). The differences in the predictions are discussed.

Convective Heat Transfer Characterization of Aviation Turbine Fuel-Metal Oxide Nanofluids

A thorough experimental investigation of the thermophysical properties (thermal conductivity, viscosity, and specific heat), heat transfer performance, and pressure drop characteristics of aviation turbine fuel (ATF)-based Al2O3, TiO2, and CuO nanofluids was carried out for potential regenerative cooling application in semicryogenic rocket engines. The effects of volume concentration, size, and material of suspended particles, as well as the effect of the temperature of the nanofluid, were investigated. Although an enhanced thermal conductivity led to an increased heat transfer coefficient, the Prandtl number was seen to have a bigger influence on the heat transfer characteristics. In particular, the correlation developed for the Nusselt number showed a much higher influence of the Prandtl number than for single-phase fluids. As a result, theATF-CuOnanofluid performed the best amongst the threemetal oxide nanofluids at a given particle size, concentration, and temperature. The enhancement is seen to improve with temperature, demonstrating the suitability of such a nanofluid at the conditions encountered during the regenerative cooling of the rocket engines ( 150 C).

Global potential energy surface of ground state singlet spin O4

A new global potential energy for the singlet spin state O4 system is reported using CASPT2/aug-cc-pVTZ ab initio calculations. The geometries for the six-dimensional surface are constructed using a novel point generation scheme that employs randomly generated configurations based on the beta distribution. The advantage of this scheme is apparent in the reduction of the number of required geometries for a reasonably accurate potential energy surface (PES) and the consequent decrease in the overall computational effort. The reported surface matches well with the recently published singlet surface by Paukku et al. [J. Chem. Phys. 147, 034301 (2017)]. In addition to the O4 PES, the ground state N4 PES is also constructed using the point generation scheme and compared with the existing PES [Y. Paukku et al., J. Chem. Phys. 139, 044309 (2013)]. The singlet surface is constructed with the aim of studying high energy O2-O2 collisions and predicting collision induced dissociation cross section to be used in simulating non-equilibrium aerothermodynamic flows.

Modifications and Extensions to the Annular Flow Model

Boiling heat transfer to fluid flow in microchannel heat sinks is being looked upon as a promising solution to the problem of cooling microprocessors with large power densities. In the present work, an annular flow model [1] is implemented to investigate the boiling heat transfer and two-phase flow characteristics in microchannel heat sinks. A modification in the model for the deposition mass transfer coefficient is proposed to better compare the existing experimental data [2]. The deposition mass transfer coefficient affects the distribution of liquid in the form of entrained droplets and the liquid film. The liquid film thickness is the most significant parameter in the determination of the heat transfer coefficient. The suggested change ensures consistent results for the behavior of the entrained fraction. We further report pressure drop results obtained using the modified annular flow model and a comparison with existing experimental data. Finally, we present results predicted by the annular flow model for non-uniform heating of a microchannel, in an effort to simulate hot spots on a microprocessor chip. A few preliminary results obtained from the modified model to simulate boiling and two-phase flow in a parallel microchannel device with non-uniform heating are presented.Copyright