1. Ph.D Theses
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Item Investigation of Pool Boiling Heat Transfer from Rough Surface and Microchannel Geometry Under Variable Heat Supply(National Institute of Technology Karnataka, Surathkal, 2019) Ashok, Walunj Avdhoot.; A, Sathyabhama.Enormous amount of heat is generated in the Economic Simplified Boiling Water Reactor (ESBWR) due to exponential heat generation from the fuel rod. A core melt accident occurs when the heat generated in the nuclear reactor exceeds the heat removed by the coolant to the point where at least one nuclear fuel element exceeds its melting point temperature. Critical Heat Flux (CHF) is the phase of boiling after which heat transfer coefficient drops resulting in the rapid increase in temperature of core. Hence, understanding the mechanism of CHF is important to control loss of coolant accident (LOCA). CHF enhancement may retard the LOCA in ESBWR. Passive enhancement techniques are the most suitable for the nuclear reactor application. In view of the facts discussed above, the CHF enhancement by two passive techniques namely, rough surfaces and microchannel geometries is investigated. The transient CHF enhancement is compared with the steady-state CHF upto 10 bar pressure. The experimental setup is designed to study the pool boiling of water at 1 bar, 5 bar and 10 bar pressures. The pool boiling experiments are conducted on the thick copper sample of 20 mm diameter at saturated condition of distilled water. The unidirectional scratches are made on the sample to obtain wide range of surface roughness varying from Ra=0.106 μm to Ra=4.03 μm. It is found that steady-state CHF increases with increase in the Ra. Improved wettability and increased nucleation site density resulted in the CHF enhancement by rough surface. The microchannel geometries namely, square (SM-1.0), parabolic (PM-1.6) and stepped (SM-1.6) were fabricated by VMC machining. The improved liquid supply through the channel space and significant bubble growth resulted in the CHF enhancement by the microchannel geometry. The CHF enhancement by SM-1.6 is highest among all the microchannel geometries. The experimental setup is commissioned with programmable power supply to compare the CHF of water during pool boiling on rough surface and microchannel geometry under steady-state and exponential heat supply. The time constant (γ) of exponential heat supply is varied from 1 to 6. It is found that both, rough surfaces and microchannel geometries enhance the transient CHF. However, transient CHF gradually decreasedwith increase in γ due to liquid-vapor instability during exponential heat supply. CHF increased with increase in the pressure at both the condition viz. steady and transient. Steady-state CHF for Ra=4.03 μm and SM-1.6 at P=10 bar is found to be 71.43% and 47.37% higher compared to the CHF at P=1 bar, respectively. The correlation for heat transfer coefficient is developed for prediction of transient boiling performance which includes the non-dimensional time constant γ. Present correlation predicts the experimental values of transient HTC with MAE of 14.91%. CHF model for rough surface, based on force balance approach, is developed incorporating the effect of time constant, bubble angle and roughness parameter viz. Ra, Sm to predict the boiling crisis during pool boiling. It predicts the experimental transient CHF with MAE of 11.89%. Boiling videos are recorded at 1000 fps using high speed camera during the experiments to study the bubble dynamics during pool boiling on rough surface and microchannel geometries upto 10 bar pressure. Bubble dynamics during pool boiling of saturated water is significantly affected by the surface characteristics i.e. surface roughness and microchannel. Prolonged nucleated boiling regime is noticed for rough surface at high pressure due to the capillary wicking in the unidirectional scratches which retards the horizontal coalescence. Forces acting vertically on the growing bubble are considered to predict the bubble departure diameter. The MAE between measured and predicted bubble departure diameter for the rough surface and microchannel geometries at all pressure is 17.09% and 13.30%, respectively.Item Heat Transfer Distribution of Impinging Methane-Air Premixed Flame Jets(National Institute of Technology Karnataka, Surathkal, 2019) Ramkishanrao, Kadam Anil.; Kumar, G. N.Flame jets find importance in industrial and household applications like metal and glass melting/forming and cook stoves respectively. Heat transfer distribution of impinging flame jet was compared with that of the impinging air jet based on the experimental data reported in literature for methane-air flame jet and air jet impingement for Reynolds number, Re = 600 to 1400 and the non-dimensional nozzle tip to impingement plate distance, Z/d = 2 to 6. The comparative data based on mapping experimental data reported in literature suggested that there is a good agreement between the Nusselt numbers for higher Z/d near stagnation region. However, away from the stagnation region, the Nusselt number for flame jet is higher than that of air jet for similar operating conditions of Re and Z/d. A CFD simulation for impinging air jet and impinging flame jet was carried out using FLUENT software to explain the physics and reason for the deviations observed in experimental data. A scale analysis was carried out to identify the dominant forces and their influence on the heat transfer distribution on the impingement plate. Heat transfer from impinging flame jets to a flat plate has been assumed to be onedimensional in most of the investigations and without radiation loss treatment. In the present work, the exact nature of diffusion of heat in the plate is investigated via solution to multidimensional heat conduction problem. Two procedures have been employed – Duhamel theorem and three dimensional transient analytical inverse heat conduction problem (IHCP). The Duhamel theorem which is analytical model for transient one dimensional heat conduction was applied and its application failed the check of linearity requirement of the convection rate equation. From the solution by analytical IHCP for transient three dimensional heat conduction, the distribution of wall heat flux and the wall temperature was perfectly linear. This check confirmed that three dimensional approach has to be used. Experimental data is then analyzed by the three dimensional analytical IHCP for short and larger time intervals. It was found that for short time data, heat transfer coefficient and the reference temperature have oscillatory distribution along the radial direction on the impingement plate and for larger time data the oscillations die out. However, at larger time, radiation loss from the impingement plate becomes significant. The effect of variation in thermal conductivity of the impingement plate with the temperature on heat transfer coefficient and reference temperature is discussed. Anovel method was developed to correct the heat transfer coefficient and reference temperature to incorporate radiation losses. The deviation in heat transfer coefficient and reference temperature estimated without considering variable thermal conductivity and radiation loss for large time interval was upto 50%. The scope of the present technique is examined through its application to impinging jets with various configurations. The present study covers the applications of hot jet, cold jet and multiple jets with distinct Reynolds numbers and the nozzle-to-plate spacing and results confirms the validity of technique to impinging jets as well. Effect of plate thickness on the accuracy of the present technique is also studied. Upto 5 mm thick plates can be used in impinging jet applications without compromising much on accuracy. Use of present technique significantly reduces the experimental cost and time since it works on transient data of just few seconds Experiments were carried out on ribbed plates with three different geometrical shaped rib elements i.e. circular, rectangular and triangular. In addition, numerical simulations were performed to study flow field on and around ribs. During the experiments, Reynolds numbers varied from 600 to 1800 and burner tip to target plate distance from 2 to 4. Heat transfer coefficients were found lower whereas reference temperatures were observed higher on ribbed surfaces than smooth surfaces. Obstruction to the flow, flow separation and decrease in momentum are the reasons attributed for lower heat transfer rate to the ribbed surfaces.