Choondal B. Sobhan

Experimental Investigations on the Effects of Cerium Oxide Nanoparticle Fuel Additives on Biodiesel

This paper reports the results of experimental investigations on the influence of the addition of cerium oxide in the nanoparticle form on the major physicochemical properties and the performance of biodiesel. The physicochemical properties of the base fuel and the modified fuel formed by dispersing the catalyst nanoparticles by ultrasonic agitation are measured using ASTM standard test methods. The effects of the additive nanoparticles on the individual fuel properties, the engine performance, and emissions are studied, and the dosing level of the additive is optimized. Comparisons of the performance of the fuel with and without the additive are also presented. The flash point and the viscosity of biodiesel were found to increase with the inclusion of the cerium oxide nanoparticles. The emission levels of hydrocarbon and NOx are appreciably reduced with the addition of cerium oxide nanoparticles.

A review of experimental investigations on thermal phenomena in nanofluids

Nanoparticle suspensions (nanofluids) have been recommended as a promising option for various engineering applications, due to the observed enhancement of thermophysical properties and improvement in the effectiveness of thermal phenomena. A number of investigations have been reported in the recent past, in order to quantify the thermo-fluidic behavior of nanofluids. This review is focused on examining and comparing the measurements of convective heat transfer and phase change in nanofluids, with an emphasis on the experimental techniques employed to measure the effective thermal conductivity, as well as to characterize the thermal performance of systems involving nanofluids.

Heat Transfer Studies in Thermally Conducting and Electrically Insulating Nano-Oils in a Natural Circulation Loop

Mineral oil (MO), a dielectric insulating fluid, is commonly used as coolant and lubricant in various applications, such as in high voltage power transmission systems and machinery. The mode of heat transfer in most of these systems is natural convection. Prolonged operation at higher temperatures leads to the degradation of the dielectric coolant, which leads to diverse problems, such as shortage or breakdown of these devices and apparatuses. Increasing the heat transfer capability of the insulating fluid will minimize the energy consumption of the system, prolonging its useful life. It is proposed to improve the heat transfer performance of insulating fluid by the addition of hexagonal boron nitride (h-BN), which is synthesized and finally obtained in 2D-nanosheets through wet exfoliation technique, without affecting its electrical insulating property. h-BN was reported to have superb effect on thermal conductivity of MO (∼ 80% increase at 0.10wt.%) on addition at very low filler fraction [1], thermal stability of up to 800°C [2], and good electrical insulating properties due to its nature (electron band gap of approx. 4.5eV). The present work reports the application of nano-oil (MO + h-BN 2D-nanosheets) for enhanced heat dissipation. A rectangular thermosyphon loop was modeled as the thermal system in transformer. The aspect ratio of the loop and the positions of the heater and cooler were chosen according to the stability criterion so that the flow remains stable and unidirectional throughout the experiment. The effect on heat removal by varying the concentration of h-BN 2D-nanosheets (h-BNNS) in MO was measured and discussed.Copyright

Development of an Interferometric Method for Measurement of Thermal Conductivity of a Transparent Medium

Interferometric methods are non-intrusive optical measurement techniques, which find extensive use in flow and heat transfer visualization. The present work originates from the idea that by a suitable experimental system and data analysis method, the interferometric technique can be used to estimate its thermal conductivity. A method is developed to obtain the thermal conductivity of a transparent medium using the optical technique of differential interferometry. The basis is of this method is the measurement of the local interference fringe shift values along an isothermal flat plate surrounded by the medium to visualize the heat transfer field. The local Nusselt number distribution along the plate is estimated from fringe shift and compared with theoretical local Nusselt number distribution along an isothermal plate, and this comparison is used to estimate the thermal conductivity of the medium.Copyright

Experimental Investigations on Fluid Flow and Heat Transfer Through Rectangular Mini Channels

Experimental investigation aimed at characterizing fluid flow and heat transfer in mini channels is presented in this paper. Forced convection flow of water through rectangular mini channels with hydraulic diameters in the range 0.28–3.67 mm and aspect ratio (H/W) 0.33–2.5 was investigated, for different flow regimes, and correlations were proposed for the flow friction and heat transfer characteristics. Fully turbulent convective heat transfer was achieved in mini channels at lower Reynolds numbers compared to conventional sized channels. Critical Reynolds number for flow transition was found to decrease with the hydraulic diameter. A comparison of the results in mini channels and microchannels indicate common trends, especially in the dependence of flow transition on the hydraulic diameter. Experimental results were compared with predictions for flow and convective heat transfer in conventional sized channels. Based on the finding that the frictional and heat transfer characteristics of the mini channels deviated from the predictions, a few new correlations are proposed for the laminar and turbulent flow regimes.Copyright

Digital Interferometric Measurement of Forced Convection Fields in Compact Channels

The application of digital interferometric techniques in the measurement of forced convection in compact channels is examined. Michelson Interferometry and Mach–Zehnder Interferometry have been compared and contrasted, in terms of the suitability for temperature measurements. A wedge-fringe setting parallel to the heated surface has been utilized as the basis of the fringe analysis, for effective measurement in small dimension channels. Michelson interferometry was found to provide a larger number of data points in the visualization field, thus proving to be a better method of measurement in compact passages. Heat transfer parameters, useful in heat sink design, were also calculated.

Flow Measurements in Metal Oxide-Nanoparticle Suspensions in a Rectangular Natural Circulation Loop

Natural circulation cooling systems, such as the thermosyphon loops are preferred as effective heat dissipation methods where a silent and vibration-free operation is desired in thermal control of devices and processes. Though anomalous enhancement in forced convection heat transfer coefficients have been reported for nanofluids, the effect of addition of nanoparticles to base fluids in natural convection circulation loops is not clearly understood. An experimental study is reported in this work, using aluminum oxide and copper oxide nanofluids with varying concentrations, in a thermosyphon loop. The flow velocity is arrived at from the measured pressure drop. At a nanoparticle concentration of 0.01% by volume Al2O3-water and CuO-water nanofluids shows 88.37% and 42.89% improvement in flow, respectively.

Investigations on Forced Convection in Compact Passages With Surface Irregularities

Two-dimensional, fully developed, convective heat transfer in compact passages is investigated numerically, incorporating the effects of the surface irregularities, to analyze the influence of these irregularities on fluid flow and heat transfer. This analysis helps to bring out the differences in the performance evaluation if regular cross sections are assumed in analyzing compact and mini channels. Forced convection in compact passages with an apparent rectangular shape is analyzed using a finite-difference method. The calculation is validated experimentally using Michelson interferometry. The numerical results for the channel, incorporating surface irregularities, are compared with those assuming a regular cross-sectional geometry. The results indicate that the coefficient of friction and the Nusselt number calculated for channels, considering the irregular cross section, are less than those predicted using an assumption of regular geometry. The results provide some insight into the reasons for the observed deviations in the comparisons. The observations are attributed to the influence of the surface irregularities on the relative dominance of the surface area to the cross-sectional area, which gets pronounced in compact passages. The findings suggest that some of the observed deviations in the performance of compact passages, compared to theoretical results, may be due to the use of regular geometries to define domain shapes while performing theoretical analysis.

Stability and transient performance of vertical heater vertical cooler natural circulation loops with metal oxide nanoparticle suspensions

ABSTRACT Experimental and theoretical investigations on forced convection have generally indicated superior heat transfer capabilities of nanoparticle suspensions (nanofluids) compared to the corresponding base fluids. However, some studies on natural convection heat transfer in nanofluids have shown negative impacts on heat transfer capabilities, while used in vertical fluid columns. The present investigation is aimed at understanding the heat transfer performance of oxide nanofluids in a rectangular natural circulation loop. The circulation in a natural circulation loop is more or less similar to that in a forced circulation system, but with low velocities. Hence, it is important to observe and quantify the heat transfer behavior of nanofluids in natural circulation loops. In the present work, investigations have been performed on a vertical heater-vertical cooler natural circulation loop, designed to operate stably at all operating conditions considered, based on a linear stability analysis, which was confirmed further using the experimental results. Evaluation of the transient heat transfer performance of the experimental system indicates that aqueous nanofluids containing aluminum oxide and copper oxide have superior heat transfer capabilities in the rectangular natural circulation loop, compared to pure water.

An Experimental Investigation of the Refrigerant Adsorption Performance of Carbon Nanotube-Activated Carbon Mixtures

The research reported here presents the results of an experimental investigation of the adsorption performance of different combinations of activated carbon and multi-walled carbon nanotubes mixtures (7.5–25%), as an adsorbent material for refrigerant R-134a, with the application potential in vapor adsorption refrigeration systems. The specific adsorbance was found to increase with the addition of carbon nanotubes to activated carbon. The experimental data were used to calculate the isosteric heat of adsorption using the Clausius–Clapeyron equation, and study its variation with the specific adsorbance for the mixture with the highest percentage of carbon nanotubes, and compare the results with that for activated carbon. The increase in the specific adsorption capacity upon addition of carbon nanotubes was interpreted as due to an increase in the pore volume, which in turn increases the specific surface area of the adsorbent material.