Mangesh B. Chaudhari

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.

Near-Field Measurements and Cavity Design for Synthetic Jets

Synthetic jet literally means a jet which has been synthesized from the ambient fluid and results because of oscillation of a diaphragm in a cavity. One of the important aspects that have not received much attention in the literature is cavity design which strongly affects the ensuing jet flow. In this paper, we experimentally study the effects of cavity depth, orifice diameter, and excitation frequency, on the exit velocity for an axisymmetric synthetic jet. Velocity measurements in the near field and flow visualization results are presented. The exit velocity indicates a lower and an upper bound on frequency for the formation of a jet and shows resonance at two frequencies. We discuss approaches to manipulate these frequencies from the point of view of cavity design and optimization.

Design, Manufacturing and Validation of Low Cost, Miniature Acoustic Chamber

Limiting the noise exposure of people to limit the risk of hearing damage is the preliminary aim of hearing conservation programs and regulations of most of the countries. Along with the performance and aesthetic considerations; noise reduction carries equal importance in product design and development as manufacturers are obligated for quieter product due to stringent legislations. The most restrictive factor for severity assessment of noise is the cost. This study describes a low cost acoustic test chamber having free working space of 2.28m × 2.28m × 1.12m (7.48ft × 7.48ft × 3.68ft) designed with multilayer absorption treatment by giving emphasis on Green Technology (Green Tech) for sound pressure level measurements of small size sources. The chamber performance has been assured by different methods of validation viz. S/N, 1 Watt, 1 kHz, 1m and Inverse square law (1/r2 or 6 dB/dd). Under realistic conditions of ambient noise the A-weighted background noise level is found to be 21 dB which is low enough to make the use of acoustic test chamber for designed purposes. Researchers can adopt these methodologies with the use of common laboratory equipments to validate the acoustic chamber. This study attempts to bridge the gap between optimal acoustic conditions and cost of acoustic chamber.

Acoustic aspects of synthetic jet generated by acoustic actuator

Thermal overstressing as a result of miniaturization is the foremost challenge for the next generation electronic devices. Shrinking size of electronic devices makes this problem even worse. Conventional cooling module which uses fan and a heat sink seems to be inadequate for cooling of electronic devices due to space constraints and higher circuit densities. Synthetic jet is a novel flow technique which synthesizes stagnant air to form a jet and is potentially useful for cooling applications. It results from periodic oscillations of a diaphragm in a cavity such that there is no net mass addition to the system. It is being recently researched as an effective alternative to fan but the existing rules on noise emissions constitute an impediment to the practical use of synthetic jet. Besides several nonauditory health effects, prolonged exposure to high levels of noise can cause noise-induced hearing loss. To adopt ameliorative solutions and for adherence to legislative regulations, it is essential to assess the noise exposure. Present study is embarked on to investigate the effect of excitation frequency and voltage on sound pressure level of synthetic jet for orifices of different diameter and thickness. This jet acoustic research is performed in an imperative and controlled acoustic environment, i.e. acoustic test chamber. Spectrum analysis clearly indicates impedance mismatch between audio amplifier and electric transducer used to generate the jet flow. Over the past few years extensive experimental and analytical results have led to good understanding of synthetic jet but study on impedance mismatch has always been elusive. The results on synthetic jet heat transfer, flow, and acoustic characteristics cannot be faithfully reproduced due to such hurdle. In order to vanquish or conquest over it, matching the impedance between source and the load is inevitable. The conclusion of this paper unveils the necessity of impedance matching for high fidelity measurements of synthetic jet.

CFD Simulation Studies of High Performance Computing (HPC) Facilities

High performance computing (HPC) facilities consist of a large number of computer servers, which dissipate thermal energy in a data center. Efficient heat dissipation of these high density servers is a major concern to increase the life of electronic devices. Cooling of these devices involve thermal management strategies based on energy efficient cooling in a raised floor configuration, which depends on proper air flow distribution in an under-floor plenum, configuration of perforated tiles and arrangement of servers in racks. The current investigation was carried out with an objective to study air flow pattern inside plenum and to find out a better predictive tile model for air flow distribution through perforated tile. The full scale Computational Fluid Dynamics (CFD) model was used to predict flow over perforated tiles and temperature distribution over server racks inlet at different cold aisles. The governing equations (mass, momentum and energy) were solved to compute flow and temperature distribution in the computational domain. The RANS based k–e turbulence model was used for estimating turbulent kinetic energy (k) and turbulence dissipation rate (e). The under-floor blockages and CRAC locations were found to be significant parameters influencing flow distribution in different cold aisles. The CFD model was validated by carrying out experimental works. The deviation in flow rates obtained by CFD model was within 15 % with respect to experimental values for different rows in cold aisle. Cold aisle containment for over provisioned case of data centre was studied using modified body force model, which predicted better hot air entrainment and momentum rise of cold air as compared to porous jump model. The root mean square error (RMSE) of the temperature predicted by the modified body force model was 1.68 °C, whereas the RMSE predicted by porous jump model was 1.72 °C. Based on the CFD simulation studies, some energy saving opportunities were suggested for improving the thermal performance of the HPC facility.