Now showing 1 - 10 of 28
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    Insights into the evolution of the thermal field in evaporating sessile pure water drops
    (20-02-2021)
    Josyula, Tejaswi
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    We investigate the evolution of the thermal field during evaporation, a fundamental aspect of evaporating sessile drops. With numerous reports in the literature investigating the contact line dynamics, we aspire to identify generalized features in the evolution of the thermal field and ultimately correlate these with the contact line dynamics. Considering a broad range of experimental parameters such as substrate wettability, substrate temperature, initial volume of the drop, and ambient relative humidity results in a wide range of evaporation rates, in turn affecting the strength of internal convective flows. Infrared thermography is utilized to extract the thermal field at the liquid–vapor interface, and optical imaging is used to record the evolution of drop shape during evaporation. We observe that the onset and presence of a convective cell as a cold spot at the interface highlights a non-axisymmetry in the thermal field. In consequence, a hitherto unreported asymmetry in the internal flow field is observed, as evidenced by the particle image velocimetry. Among the multitude of experiments conducted, we report four distinct trends in the evolution of interfacial temperature difference depending on the presence and duration of the presence of the convective cell, which are elucidated by discussing the evolution of maximum and minimum temperatures at the interface. The interplay between heat conducted into the drop and heat released due to evaporation can result in a momentary decrease in temperature of the drop, which is not reported previously. Lastly, a theoretical estimate for the temperature difference within the drop is extracted using vapor diffusion model and energy balance during evaporation. Comparison of this theoretical temperature difference with experimental observations highlights the influence of internal convective flows in homogenizing the thermal field within the drop.
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    Evaluation of candidate strategies for the estimation of local heat transfer coefficient from wall jets
    (02-01-2020)
    Godi, Sangamesh C.
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    Abraham, Satyanand
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    This paper reports results of experimental investigations on planar and three-dimensional wall jets over a flat surface. The local heat transfer coefficient is estimated at transient conditions with a semi-infinite approximation and at steady state conditions with a uniform wall heat flux boundary. Liquid crystal thermography and infrared thermography are used to map the surface temperatures. Experiments are performed for 2000 ≤ Re ≤ 8000 and 0 ≤ x/L ≤ 80. Results show that transient infrared thermography with semi-infinite approximation is a better candidate for the estimation of the heat transfer coefficient from wall jets.
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    Thermal and flow characteristics in a flat plate pulsating heat pipe with ethanol-water mixtures: From slug-plug to droplet oscillations
    (15-09-2022)
    Malla, Laxman Kumar
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    Dhanalakota, Praveen
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    The thermal performance and the internal flow regimes of a closed-loop flat plate pulsating heat pipe (FPPHP) are experimentally investigated. Reports on the FPPHP using the ethanol-water mixtures as working fluids are scarce in the literature. The binary mixtures with different boiling point components are suitable for a wide range of heat fluxes. Therefore, the results are reported for the ethanol-water mixtures of ratios 3:1, 1:1, and 1:3, and the corresponding pure liquids filled in the range of [40–80] % with power inputs given from 40 to 200 W. The effect of different condenser cooling modes, such as forced convective water cooling, forced convective air cooling, and natural convective air cooling on the thermal performance of the FPPHP, is also reported. With the increase in the power input, the observed flow characteristics in the FPPHP channels are: no oscillations, slug-plug oscillations, droplet oscillations, and the evaporator dry out. The binary mixtures with increased ethanol content give better slug-plug flow oscillations with smaller thermal resistances and fewer evaporator drying out instances than the pure working fluids. For power inputs of less than 120 W, the ethanol:water mixture ratio of 3:1 at all filling ratios gives a larger slug departure frequency in the evaporator. The smallest thermal resistance measured is 0.1 K/W, a decrease of 27% over pure ethanol. For power inputs greater than 120 W, the mixture ratio of 1:1 at all filling ratios performs better with continuous droplet oscillations. The smallest thermal resistance measured at the 80% filling ratio is 0.12 K/W, a decrease of 22% over pure ethanol. When the condenser cooling mode is changed to air cooling, the evaporator temperatures reach around 100 °C for power inputs greater than 40 W and 80 W for natural and forced air convection. Thus, the FPPHP filled with ethanol-water mixtures with the water-cooled condenser gives a stable flow regime and better thermal performance for a long-range of power inputs.
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    Heat transfer characteristics of a train of droplets impinging over a hot surface: From film evaporation to leidenfrost point
    (01-06-2021)
    Guggilla, Ganesh
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    Narayanaswamy, Ramesh
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    Stephan, Peter
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    High-performance computing systems are needed in advanced computing services such as machine learning and artificial intelligence. Consequently, the increase in electron chip density results in high heat fluxes and requires good thermal management to maintain the servers. Spray cooling using liquid offers higher heat transfer rates and is efficient when implemented in electronics cooling. Detailed studies of fundamental mechanisms involved in spray cooling, such as single droplet and multiple droplet interactions are required to enhance the process's knowledge. The present work focuses on studying a train of two FC-72 droplets impinging over a heated surface. Experimental investigation using high-speed photography and infrared thermography is conducted. Simultaneously, numerical simulations using opensource CFD package, OpenFOAM are carried out, emphasizing the significance of contact angle hysteresis. The surface temperature is chosen as a parameter, and different boiling regimes along with dynamic Leidenfrost point for the present impact conditions are identified. Spreading hydrodynamics and heat transfer characteristics of these consecutively impinging droplets till the Leidenfrost temperature are studied and compared.
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    Experimental Assessment of the Thermo-Hydraulic Performance of Automobile Radiator with Metallic and Nonmetallic Nanofluids
    (04-02-2020)
    Akash, A. R.
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    Abraham, Satyanand
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    The overall heat transfer of a cross flow heat exchanger can be enhanced by using the nanofluids as coolant, which finds application in reducing the size and weight of automobile radiator. However, improving the heat transfer using nanofluids can be accompanied by simultaneous variations in the required pumping power. This study experimentally evaluates the thermo-hydraulic performance of three nanofluids—metallic (copper, aluminum) and nonmetallic (multiwalled carbon nanotube (MWCNT))—as coolant for an automobile radiator by utilizing an in-house test rig. An enhancement in overall heat transfer coefficient can be observed with nanocoolants (nanofluid as coolant), compared to the de-ionized water at the same Reynolds number. The maximum enhancement in the overall heat transfer coefficient was observed to be 40, 29, and 25% for MWCNT, copper, and aluminum nanofluids, respectively. The thermal performance of coolants was also compared with the same pumping power criterion. The overall heat transfer coefficient of nanofluids were higher than basefluid at low pumping power range and the trend changes with increase in the pumping power. The present study shows that the heat transfer characteristics at the same Reynolds number as well as at the same pumping power needs to be considered for the selection of appropriate nanocoolant for automobile radiator application.
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    Numerical study of thermocapillary migration of a droplet on an oleophilic track
    (01-11-2023)
    Kalichetty, Srinivasa Sagar
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    Sundararajan, T.
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    The thermocapillary migration dynamics of a droplet with a non-circular footprint on an oleophilic track is studied numerically. Three-dimensional simulations showed that reducing the width of the oleophilic track improves the droplet migration velocity only upto a specific track width when compared to the change in velocity of a droplet with circular footprint. Further decrease in track width is seen to depreciate the enhancement in migration velocity. The spread length of the droplet on the oleophilic track is also observed to have a similar variation with track width. The two trends are related because the temperature difference in the droplet, which scales with spread length, determines the driving force governing the migration velocity characteristics. Further, a parametric study is performed to assess the migration behaviour on oleophilic tracks with change in substrate temperature gradient, droplet volume and contact angles within and outside the track. The influence of change in these parameters on the variation in enhancement in spread length and corresponding track width is also investigated. The study identified trends in the minimum track width that may be used to improve migration characteristics, as well as the order of enhancement that can be noticed with changes in each parameter.
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    Droplet dynamics on a wettability patterned surface during spray impact
    (01-03-2021)
    Thomas, Tibin M.
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    Chowdhury, Imdad Uddin
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    Dhivyaraja, K.
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    Tiwari, Manish K.
    Wettability patterning of a surface is a passive method to manipulate the flow and heat transport mechanism in many physical processes and industrial applications. This paper proposes a rational wettability pattern comprised of multiple superhydrophilic wedges on a superhydrophobic background, which can continuously remove the impacted spray droplets from the horizontal surface. We observed that the spray droplets falling on the superhydrophilic wedge region spread and form a thin liquid film, which is passively transported away from the surface. However, most of the droplets falling on the superhydrophobic region move towards the wedge without any flooding. The physics of the passive transport of the liquid film on a wedge is also delved into using numerical modelling. In particular, we elucidate the different modes of droplet transport in the superhydrophobic region and the interaction of multiple droplets. The observed droplet dynamics could have profound implications in spray cooling systems and passive removal of liquid from a horizontal surface. This study’s findings will be beneficial for the optimization of efficient wettability patterned surfaces for spray cooling application.
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    Autonomous transport and splitting of a droplet on an open surface
    (01-09-2021)
    Chowdhury, Imdad Uddin
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    Tiwari, Manish K.
    Pumpless transport of droplets on open surfaces has gained significant attention because of its applications starting from vapor condensation to Lab-on-a-Chip systems. Mixing two droplets on open surfaces can be carried out quickly by using wettability patterning. However, it is quite challenging to split a droplet in the absence of external stimuli because of the interfacial energy of the droplet. Here, we demonstrate a standalone power-free technique for transport and splitting of droplets on open surfaces using continuous wettability gradients. A droplet moves continuously from a low to a high wettability region on the wettability-gradient surface. A Y-shaped wettability-gradient track - laid on a superhydrophobic background - is used to investigate the dynamics of the splitting process. A three-dimensional phase-field Cahn-Hilliard model for interfaces and the Navier-Stokes equations for transport are employed and solved numerically using the finite element method. Numerical results are used to decipher the motion and splitting of droplet at the Y junction using the principle of energy conservation. It is observed that droplet splitting depends on the configuration of the Y junction; droplets split faster for the superhydrophobic wedge angle of 90∘ and the splitting ratio (ratio of the sizes of daughter droplets) depends on the widths of the Y branches. A critical branch-width ratio (w2w1=0.79) is identified below which the droplet does not split and moves towards the branch of higher width and settles there. The present study provides the required theoretical underpinnings to achieve autonomous transport and splitting of droplets on open surfaces, which has clear potential for applications in Lab-on-a-Chip devices.
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    Influence of fluence, beam overlap and aging on the wettability of pulsed Nd3+:YAG nanosecond laser-textured Cu and Al sheets
    (15-05-2021)
    Vidhya, Y. Esther Blesso
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    Superhydrophobic and hydrophobic surfaces on Al and Cu have a wide range of applications in electronics, industrial devices, air conditioning, and refrigeration plants, and home appliances due to their excellent electrical and thermal characteristics. In many of these applications, the wettability of their surface is essential. This work presents the fabrication of superhydrophobic and hydrophobic surfaces on Al and hydrophobic surfaces on the Cu metal sheets via laser processing and aging without any additional chemical treatment. Texturing with laser spot overlap is used to generate naturally formed nanoscale textures by scanning the whole sample surface. The influence of laser parameters on the dimension and shape of the fabricated surface textures and their impact on wettability is analyzed along with the evolution of wetting behavior over time. The laser textured surfaces, which are initially hydrophilic, are found to transform hydrophobic over time upon exposure to atmospheric conditions. Experimental evidence using X-ray photoelectron spectroscopy (XPS) and ATR-FTIR spectroscopy corroborates this transition is owing to the adsorption of organic molecules present in ambient. The depth profiling using XPS reveals carbon contamination around 60–70 nm from the sample surface. Furthermore, textures exhibiting static contact angles of up to ~154° have been achieved with Al sheets, whereas contact angles up to ~122° have been attained in the Cu sample. These experimental findings enable control of wettability of Al and Cu sheets through precise tuning of laser parameters for desired properties.
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    Fluid flow and heat transfer characteristics of three-dimensional slot film cooling in an annular combustor
    (01-09-2023)
    Revulagadda, Ananda Prasanna
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    Adapa, Buchi Raju
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    Film cooling methodologies in air-breathing gas turbines have been evolving for many decades. In the present work, a combined experimental and numerical parametric study is conducted to investigate the three-dimensional slot film cooling of an annular combustor. The experimental investigation is conducted to understand the fluid flow and heat transfer phenomenon of the film cooling and to validate the numerical study under laboratory conditions (low temperature and pressure). Transient infrared thermography is used to estimate both adiabatic film-cooling effectiveness (ηad) and heat transfer coefficient (h) simultaneously using a semi-infinite approximation method. The blowing ratios considered in the study are in the range of 0.5 to 5. The experimental results showed that film-cooling effectiveness (ηad) enhanced with an increase in the blowing ratio from 0.5 to 2 and deteriorated beyond BR = 2 due to the adverse effects of turbulence. A parametric study is conducted numerically to understand the effect of flow and geometrical parameters under actual engine conditions (high temperature and pressure). The parameters considered are slot Reynolds number (Res), slot jet diameter (d), slot jet pitch (p), lip taper angle (α), lip length (L), and slot jet injection angle (β). For the considered parameters, numerical results showed that the slot jet diameter (d) = 2 mm, dimensionless slot jet pitch (p/d) = 2, slot jet injection angle (β)=20o and dimensionless lip length (L/d) = 5.9 outperformed the other configurations due to the low turbulence and entrainment. Finally, an Artificial Neural Network-based mathematical model is developed that correlates the ηlat as a function of flow and geometrical parameters.