Journal Articles
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Item A review on recent advances in microchannel heat sink configurations(Bentham Science Publishers, 2018) Narendran, G.; Gnanasekaran, N.; Arumuga Perumal, D.A.A qualitative observation has been undergone to review the various geometries of a microchannel that has been reported for the last two decades in literature majorly for the application of high power devices. Recent research on microchannel is more focused on numerical and experimental work with various configurations of the heat sink. In this paper, a comparative work on different flow geometries used in the microchannel and their influence on heat transfer and pressure drop is investigated with the brief representation of different working fluids used in microchannel heat sink for the purpose of electronic cooling and their associated performance characteristics with various examined parameters. Background: The microchannel cooling is an established cooling technique for high power electronic components which effectively enhances the performance of the high power devices. Objective: This article presents a general overview of microchannels with novel constructional bifurcations structures with related patents. Further, the influential parameter on thermal and flow characteristics with greater depth is also reviewed by authors. Methods: This review directs by presenting standard and benchmark investigation in the microchannel and different working parameters continued with recent studies. Further, it is addressed with the application of electronic cooling with latest patents using bifurcations and fractal microchannels. Result: The current situation of 3D cooling requires a robust cooling system to accommodate increased heat flux without compromising the packaging. Moreover, the recently developed patents also evolved with improved thermal load handling under constrained packaging. Conclusion: The advanced microchannel cooling with an optimized fluid handling system with effective packaging results in a highly effective heat dissipation system. © 2018 Bentham Science Publishers.Item Severity of quenching and kinetics of wetting of nanofluids and vegetable oils(2009) Jagannath, V.; Prabhu, K.N.In the present work, the suitability of vegetable oil blends with mineral oil and alumina based nanofluids as quench media for industrial heat treatment was investigated. Sunflower oil, palm oil, and mineral oil were used for preparing the blends. Alumina based nanofluids of varying concentrations ranging from 0.01-4 % were used. The size of alumina particles was about 50 nm. The severity of quenching and heat transfer coefficients were estimated during quenching of copper probes. Heat transfer coefficients were estimated using a lumped heat capacitance model. The static contact angle was measured on copper substrates having a surface texture similar to the probes used for estimation of heat transfer coefficients. A dynamic contact angle analyzer was used for this purpose. The measured contact angles of nanofluids on copper were high compared to oils, indicating poor wetting by quench media that are polar in nature. Wetting characteristics had a significant effect on heat transfer coefficients estimated during quenching. Copyright © 2009 by ASTM International.Item Critical heat flux enhancement in pool boiling using alumina nanofluids(2010) Hegde, R.N.; Rao, S.S.; Reddy, R.P.The pool boiling characteristics of dilute dispersions of alumina nanoparticles in water were studied. Consistent with other nanofluid studies, it was found that a significant enhancement in critical heat flux (CHF) can be achieved at modest nanoparticle concentrations (<0.1% by volume). During experimentation and subsequent inspection, formation of a porous layer of nanoparticles on the heater surface occurred during nucleate boiling. This layer significantly changes surface texture of the heater wire surface which could be the reason for improvement in the CHF value. © 2010 Wiley Periodicals, Inc.Item Role of surface roughness in pool boiling with Alumina-water nanofluid on a horizontal wire surface(International Information and Engineering Technology Association, 2011) Hegde, R.N.; Rao, S.S.; Reddy, R.P.Boiling heat transfer is one of the major phenomenon which of late, has drawn the attention of many researchers and scientists throughout the world. With nanofluids, further boost is given in heat transfer enhancement. This research paper is the study of heat transfer enhancement using Alumina nanofluid in different volume concentrations ranging from 1 to 9%. The role of surface roughness on critical heat flux enhancement (CHF) in pool boiling with nanofluids was experimentally studied using a 36 gauge NiCr wire at atmospheric pressure. Experimentation included i) investigations on boiling heat transfer subjecting the wire surface to Alumina nanofluid at higher volume concentrations and ii) investigations on surface roughness due to surface coating, subjecting the wire surface to a single heating cycle with different volume concentrations of Alumina nanofluid. Boiling of nanofluid resulted in nanoparticle deposition and subsequent roughning of the wire surface. To substantiate the nanoparticle deposition and its effect on critical heat flux, investigation was done by studying the surface roughness and SEM images of the wire surface. The experimental results show the evidence of nanoparticle deposition on the wire surface and its effect on CHF enhancement and deterioration in pool boiling heat transfer.Item Enhancement of heat transfer characteristics of transformer oil by addition of aluminium nanoparticles(2011) Rajesh, E.; Prabhu, K.N.A two step approach involving the synthesis of Al nanoparticles by mechanical milling followed by dispersion of the nanoparticles in the base fluid is adopted in the present work to prepare transformer oil based nanofluids. Stainless steel (AISI 304) probes of diameter 15 mm and height 70 mm were used to determine the cooling rate intensities of nanofluids. Heat transfer coefficients were determined using Kobasko's method. A dynamic contact angle analyzer was used to determine the contact angle of the droplet on the substrate. The addition of Al nanoparticles to the base fluid decreases the wettability and improves its heat transfer capability. The vapour phase stage existed for a longer period of time for transformer oil than for Al-transformer oil based nanofluids. The dispersion of nanoparticles in the base fluid is believed to disrupt the vapour blanket stage in the early stage of the cooling process. The peak heat transfer coefficient increases with an increase in the Al nanoparticle content in the oil. The addition of 0.5 vol % nanoparticles enhances the peak heat transfer coefficient by about 70 %. Copyright © 2011 by ASTM International.Item Comparison of Grossmann and lumped heat capacitance methods for assessment of heat transfer characteristics of quench media(2011) Prabhu, K.N.; Ali, I.The suitability of Grossmann and lumped heat capacitance (LHC) methods for estimation of quench severity of nanofluids, brine solutions and a laboratory detergent based medium was investigated. The study involved the assessment of the effect of section thickness of the probe on heat transfer coefficients for different quench media. Computer aided cooling curve analysis during quenching of type 304 stainless steel probes was carried out. The measured thermal history data were used to estimate heat transfer coefficients by Grossmann and LHC methods. The LHC method based on a 10 mm diameter stainless steel probe was found to be applicable for characterising media having quench severity less than 20 m-1. Although Grossmann method is based on the concept of average heat transfer coefficient, it can be used for quench media having varying severity of quenching. Grossmann method is found to be more sensitive to the effect of section thickness on heat transfer. © 2011 IHTSE Partnership.Item Behavioral study of alumina nanoparticles in pool boiling heat transfer on a vertical surface(2011) Hegde, R.N.; Reddy, R.P.; Rao, S.S.Experiments were carried out to investigate the pool boiling of alumina-water nanofluid at 0.1 g/l to 0.5 g/l of distilled water, and the nucleate pool boiling heat transfer of pure water and nanofluid at different mass concentrations were compared at and above the atmospheric pressure. At atmospheric pressure, different concentrations of nanofluids display different degrees of deterioration in boiling heat transfer. The effect of pressure and concentration of nanoparticles revealed significant enhancement in heat flux and deterioration in pool boiling. The heat transfer coefficient of 0.5 g/l alumina-water nanofluid was compared with pure water and clearly indicates deterioration. At all pressures the heat transfer coefficients of the nanofluid were lower than those of pure water. Experimental observation revealed particles coating over the heater surface and subsequent SEM inspection of the heater surface showed nanoparticles coating on the surface forming a porous layer. To substantiate the nanoparticle deposition and its effect on heat flux, investigation was done by measuring the surface roughness of the heater surface before and after the experiment. While SEM images of the heater surface revealed nanoparticle deposition, surface roughness of the heater surface confirmed it. Based on the experimental investigations it can be concluded that an optimum thickness of nanoparticles coating favors an increase in heat flux. Higher surface temperature due to the presence of nanoparticles coating results in the deterioration of boiling heat transfer. © 2011 Wiley Periodicals, Inc.Item Characterisation of water base copper nanoquenchants by standard cooling curve analysis(2011) Ramesh, G.; Prabhu, K.N.Water base copper nanofluids having concentrations varying from 0?001 to 0?1 vol.-% were prepared and used as quench media for immersion quenching. Cooling curve analyses were carried out by using a standard ISO/DIS 9950 quench probe. An inverse heat conduction model is employed to estimate the metal/nanoquenchant interfacial heat flux transients from the measured temperature field and thermophysical properties of the quench probe material. The addition of copper nanoparticles had a significant effect on the occurrence of the vapour blanket stage and nucleate boiling stage. Furthermore, all six cooling curve parameters were found to be altered by adding nanoparticles to water. The contact angle of water decreased from 67 to 39° by adding 0?1 vol.-% of copper nanoparticles indicating the improved wettability of nanofluids. The heat flux curve shows a maximum initially then drops rapidly during quenching. The peak cooling rate and heat flux of water increased by adding copper nanoparticles up to 0?01 vol.-%. Both parameters decreased with further increase in concentration of nanoparticles. The results suggest that the quench severity of water could be altered by adding copper nanoparticles. © 2011 IHTSE Partnership.Item Flow visualization and study of critical heat flux enhancement in pool boiling with Al2O3-water nanofluids(Serbian Society of Heat Transfer Engineers, 2012) Hegde, R.N.; Rao, S.S.; Reddy, R.P.Pool boiling heat transfer characteristics of Al2O3 -water nanofluids is studied experimentally using a NiCr test wire of 36 standard wire guage diameter. The experimental work mainly concentrated on (1) change of critical heat flux with different volume concentrations of nanofluid and (2) flow visualization of pool boiling using a fixed concentration of nanofluid at different heat flux values. The experimental work revealed an increase in critical heat flux value of around 48% andflow visualization helped in studying the pool boiling behaviour of nanofluid.Out of the various reasons which could affect the critical heatflux enhancement, surface roughness plays a major role in pool boiling heat transfer.Item Flow visualization, critical heat flux enhancement, and transient characteristics in pool boiling using nanofluids(ASTM International, 2012) Hegde, R.N.; Rao, S.S.; Reddy, R.P.This paper presents the experimental outcome of a study of the pool boiling heat transfer characteristics of alumina and CuO nanofluid in distilled water using a 0.19 mm diameter NiCr wire. A series of experiments were conducted in order to visualize the flow, critical heat flux (CHF) enhancement, and transient characteristics of nanofluid. The boiling phenomenon was visualized using a 0.1 g/l concentration of alumina nanofluid. The average bubble diameter was measured and was found to increase with increased heat flux. The average bubble contact angle decreased from 69° during the initial stages of boiling to 33° at CHF. Massive vapour bubbles were observed on the test heater surface near the CHF, inducing vapour blankets and forming hot/dry spots. The increase in the CHF could be well explained by the hot/dry spot theory. Pool boiling experiments conducted using low volume concentrations of CuO-water nanofluid at atmospheric pressure in distilled water showed an increase in the CHF by 30 % at a 0.3 g/l concentration. The transient behaviour of nanofluid, examined by exposing the heater surface at a constant heat flux of 700 kW/m2, indicated CHF enhancement of 5.21 % to 6.77 % for the two time durations. Based on the experimental investigations, it was concluded that the CHF enhancement is due to nanoparticle coating, which changes the thickness of the surface as a function of time and surface wettability and corroborates the hot/dry spot theory. Copyright © 2012 by ASTM International.