D. Gangacharyulu

Heat Transfer and Pressure Drop Characteristics of Dilute Alumina–Water Nanofluids in a Pipe at Different Power Inputs

This work addresses the effect of temperature on the thermophysical properties (i.e., density, viscosity, thermal conductivity, and specific heat capacity) of alumina–water nanofluid over a wide temperature range (25°C–75°C). Low concentrations (0–0.5% v/v) of alumina nanoparticles (40 nm size) in distilled water were used in this study. The pressure drop and the effective heat transfer coefficient of nanofluids were also estimated for different power inputs and at different flow rates corresponding to Reynolds numbers in the range of 1500–6000. The trends in variation of thermophysical properties of nanofluids with temperature were similar to that of water, owing to their low concentrations. However, the density, viscosity, and thermal conductivity of nanofluids increased, while the specific heat capacity decreased with increasing the nanoparticle concentration. The convective heat transfer coefficient of the nanofluid and the pressure drop along the test section increased with increasing the particle concentration and flow rate of nanofluid. Results showed that the heat transfer coefficient increases, while the pressure drop decreases slightly with increasing the power input. This is because of the fact that increasing power input to heater increases the bulk mean temperature of nanofluids, resulting in a decreased viscosity. The prepared nanofluids were found to be more effective under turbulent flow than in transition flow.

An experimental study on stability and some thermophysical properties of multiwalled carbon nanotubes with water–ethylene glycol mixtures

ABSTRACT In this study, the stability and thermophysical properties of multiwalled carbon nanotubes (MWCNTs) with double-distilled water (W) and double-distilled water/ethylene glycol (W/EG) mixtures are investigated. Stability analyses are performed through visual observation, thermal conductivity measurements, spectrophotometry and zeta potential measurement methods. An increase in ethylene glycol ratio in water increases the stability of nanofluid, which helps the nanoparticles disperse uniformly in the base fluid for a longer duration. It is concluded from the results that MWCNT nanofluids with a W/EG system (50:50) has good stability, showing no agglomeration for 36 d as compared with other nanofluids. Thermophysical properties such as thermal conductivity, viscosity and density with temperature were also measured. Maximum thermal conductivity enhancement of 29% was observed for MWCNT-nanofluid with W/EG system (50:50) at 50°C. It is also observed that with the addition of MWCNT in W/EG mixtures, viscosity and density increase but the enhancement was comparatively low with reference to thermal conductivity. From these results, it was interpreted that both stability and thermal conductivity increase with increase in ethylene glycol ratio in water.

Heat Transfer Studies of a Heat Pipe

The present investigation reports a theoretical and experimental study of a wire screen heat pipe, the evaporator section of which is subjected to forced convective heating and the condenser section to natural convective cooling in air. The theoretical study deals with the development of an analytical model based on thermal resistance network approach. The model computes thermal resistances at the external surface of the evaporator and condenser as well as inside the heat pipe. A test rig has been developed to evaluate the thermal performance of the heat pipe. The effects of operating parameters (i.e., tilt angle of the heat pipe and heating fluid inlet temperature at the evaporator) have been experimentally studied. Experimental results have been used to compare the analytical model. The heat transfer coefficients predicted by the model at the external surface of the evaporator and condenser are reasonably in agreement with experimental results.

Augmented Thermal Performance of Straight Heat Pipe Employing Annular Screen Mesh Wick and Surfactant Free Stable Aqueous Nanofluids

abstract Stable surfactant-free Al2O3/deionized (DI) water nanofluids are prepared by a two-step process and are stabilized using an ultrasonic homogenizer. The thermal conductivity enhancement measured by a transient hot wire technique demonstrated a nonlinear relationship with increase in volume fraction of dispersed nanoparticles and attains a maximum enhancement of 15% for 1 vol% of Al2O3 loading in deionized water at 70°C. The stabilized Al2O3/DI water nanofluids were employed as the working fluid in a screen mesh wick heat pipe placed horizontally. The straight heat pipe configuration is altered for more practicality in use, with crimped edges, extended conduction lengths, and minute surface depressions. The heat pipe is tested at various levels of heat inputs and concentrations of Al2O3 nanoparticles. The evaporator section is heated by circulating water through a heating chamber, and the condenser section is cooled under free convection. The experimental results show an optimum reduction of 22% in the thermal resistance value using 1 vol% of Al2O3/DI nanofluids as compared to DI water at low heat input of 12 W. The stabilized operation of the heat pipe is observed at high heat input of 73 W and at low concentration of 0.005 vol% Al2O3/DI water nanofluids. The findings emphasize potential for nanofluids as future heat pipe fluids.

Study of agglomeration in 3.5 MW AFBC using rice husk particles

The combustion of rice husk in fluidised bed combustors seems to be an attractive possibility for power generation in the future, keeping in view the waste disposal problems and the reduction of greenhouse gases. In commercial installations, the occurrence of agglomeration is still one of the main reasons for unscheduled outrages. In this paper, a review of agglomeration mechanism and its prevention has been made by considering the case study of a 3.5 MW rice husk-based power plant situated at Nahar Spinning Mills Ltd. (NSML), Ludhiana, Punjab, India. Proximate, SEM analysis and quantitative elementary analysis of agglomerates samples taken from plant has been done and corrective measures have been suggested. TGA of rice husk taken from the plant has also been done to correlate it with agglomeration. The agglomeration and defluidisation in 3.5 MW NSML Ludhiana using rice husk particles is caused by high potassium content in the fuel. It is not possible to measure localised temperature, which may exceed 900–1000°C, while the plant is running, which may cause agglomeration at above plant. Refreshment of bed with fresh particles and lowering bed temperature are the key measures to prevent agglomeration.

Fluidised bed combustion and gasification of rice husk and rice straw – a state of art review

Rice is cultivated in all the main regions of world. The worldwide annual rice production is 666 million tons for the year 2008. The combustion and gasification of rice husk/rice straw in fluidised bed reactors seems to be an attractive possibility of future for power generation, the solution of waste disposal problems and the reduction of greenhouse gases. In this paper, overview of fluidised bed reactors based on rice husk/rice straw and some data collected/measured from rice husk-rice straw power plant is presented. Combustion efficiency from 60% to 98%, combustion at bed temperature from to 600°C–850°C, carbon conversion efficiency up to 98% and cold gas efficiency up to 67% is reported. Gas heating value up to 6.6 MJ/m 3 and 5.5 MJ/kg is reported. Combustion intensity up to 530 kg/h/m 2 is reported. SOx and NOx emitted from fluidised bed combustors based on rice husk and rice straw are generally very less under normal operating conditions.

Study of Heat Transfer of Aluminium Oxide Nanofluids using Aluminium Split Flow Microchannels

Experimental investigations were performed on Al2O3-H2O nanofluids using split flow type microchannels with particle volume fractions of 0, 0.1%, 0.25%, 0.5% (vol.) with the flow rate ranging from 0.5ml/min to 2.0ml/min. The effect of Prandtl number and Reynolds number is also studied. Experimental results show that thermal conductivity increases and viscosity decreases with increase in temperature. Thermal conductivity and viscosity increases with increase in concentration of nanofluids. Heat transfer coefficient, Prandtl number. It is seen that heat transfer coefficient and Prandtl number increases with increase in concentration of particles whereas Reynolds number decreases with increase in concentration. The heat transfer coefficient increased by maximum of 27% for 0.5% (vol.) concentration and Prandtl number increased by 22% maximum. However Reynolds number had decreased by maximum of 28% for 0.5ml/min flow rate and least decrease by 18% for 2ml/min flow rate. Keywords—Nanofluids; Thermophysical Properties; Split

CFD Analysis of Bubbling Fluidized Bed Using Rice Husk

Rice is Cultivated in all the main regions of world. The worldwide annual rice production could be 666million tons (www.monstersandcritics.com,2008) for year 2008. The annual production of rice husk is 133.2 million tons considering rice husk being 20% of total paddy production. The average annual energy potential is 1.998 *1012 MJ of rice husk considering 15MJ/kg of rice husk. India has vast resource of rice husk; a renewable source of fuel, which if used effectively would reduce the rate of depletion of fossil energy resources. As a result a new thrust on research and development in boilers bases on rice husk is given to commercialize the concept. CFD is the analysis of systems involving fluid flow, heat transfer and associated phenomena such as chemical reactions by means of computer-based simulation. High quality Computational Fluid dynamics (CFD) is an effective engineering tool for Power Engineering Industry. It can determine detailed flow distributions, temperatures, and pollutant concentrations with excellent accuracy, and without excessive effort by the software user. In the other words it is the science of predicting fluid flow, heat and mass transfer, chemical reactions and related phenomena; and an innovate strategy to conform to regulations and yet stay ahead in today’s competitive power market. This paper is divided into two parts; in first part review of CFD applied to the various types of boilers based on biomass fuels/alternative fuels is presented. In second part CFD analysis of fluidized bed boilers based on rice husk considering the rice husk based furnace has been discussed. The eulerian multiphase model has used for fluidized bed. Fluidized bed has been modeled using Fluent 6.2 commercial code. The effect of numerical influence of bed superheater tubes has also been discussed.

Coding, evaluation, comparison, ranking and optimal selection of nanoparticles with heat transfer fluids for thermal systems

ABSTRACT Performance of nanofluids is affected by various parameters and it is important to identify these parameters and their inheritance that determine the overall performance of nanofluids. The 125 attributes are identified and a coding scheme is developed for an in-depth understanding and visual comparison of nanofluids more precisely. A three-stage methodology named multi-attribute decision making (MADM) is used for the evaluation, comparison and ranking of nanofluids and it selects the optimum nanofluid for a given application in less time and effort. In the first stage of MADM, known as elimination search, a long list of alternatives is converged to a manageable list. Later, the procedure is followed for the ranking of nanofluids by employing the Technique for Order Preference by Similarity to Ideal Solution (TOPSIS) approach. This ensures that the selected nanofluid is the closest to the hypothetical best nanofluid and the farthest from the hypothetical worst nanofluids. Finally, the optimum nanofluid is selected by considering the cost factor as well as the government rules for a particular application. This methodology has been explained by an example in which nanofluids are developed in the laboratory to evaluate the heat transfer characteristics for cooling devices such as microchannels. This scheme is user friendly and a simple mathematical procedure is required to use it.

Heat transfer and pressure drop performance of alumina–water nanofluid in a flat vertical tube of a radiator

ABSTRACT This work presents experimental investigation on the effects of nanofluid inlet temperature (40–90°C), Reynolds number (12,000–30,000), particle concentration (0–1 vol.%), and air velocity (0.25–0.55 m/s) on thermal and flow characteristics of water-based alumina nanofluids in a flat vertical tube of a radiator. The specific heat capacity, viscosity, density, and thermal conductivity were measured experimentally. The heat transfer coefficient enhanced (up to 31%) with an increase in fluid inlet temperature, particle volume concentration, Reynolds number as well as air inlet velocity. The pressure drop increased with an increase in the particle volume concentration and Reynolds number, while it decreased slightly with an increase in the fluid inlet temperature. The friction factor and pumping power increased with particle concentration. The friction factor decreased, while the pumping power increased with sn increase in fluid flow rate.