S.V. Prabhu

Bio: S.V. Prabhu is an academic researcher from Indian Institute of Technology Bombay. The author has contributed to research in topics: Heat transfer & Nusselt number. The author has an hindex of 31, co-authored 168 publications receiving 3766 citations. Previous affiliations of S.V. Prabhu include Nokia & Indian Institutes of Technology.

Performance by Various Water Mist Nozzles in Extinguishing Liquid Pool Fires

Abstract: Water mist based systems have emerged as potentially lucrative alternatives to Halon-based fire suppression systems and have been extensively studied in the literature. In the present work, experimental investigations are carried out on the performance of the water mist nozzles in extinguishing pool fires of diameter 0.1 m, 0.2 m, and 0.3 m with diesel and heptane as fuels. The growth of the fire is studied for these fires by measuring the transient mass burning rates. Water mist is generated by passing plain water at room temperature through single-head solid-cone pressure-swirl nozzles with X-type swirl-inserts. The nozzle is directed vertically downwards and operated at various heights (0.75 m to 2 m) from the fuel surface with the injection pressure ranging from 6 bar to 10 bar. The extinction performances of four nozzles with orifice diameters of 1.2 mm, 1.65 mm, 1.9 mm, and 2.45 mm are investigated through videography. A comparison of the extinction performances by all the nozzles is presented and the optimal nozzle based on the consumption of water is reported for a given fire. An empirical correlation for the extinction time is reported in both the dimensional and non-dimensional form. Observed interactions of the spray with fuel surface are reported depicting probable undesired phenomena in the application of the water-mist system with top injection.

Partitioning of Convective and Radiative Heat Fluxes Absorbed by a Lumped Body Engulfed in a Diffusion Flame

Abstract: A simple model that divides the heat flux to the bodies engulfed in a diffusion flame into different components, namely radiation and convection is studied. Different sizes of brass and stainless steel (SS 304L) rods varying from 25.4 mm to 50.8 mm in length and 25.4 mm in diameter are used as specimens in this study. Experiments are conducted with each body inside a diesel pool fire of different diameters, namely 0.5 m, 0.7 m and 1.0 m. The temperature history of the body engulfed in a pool fire is measured to compute the thermal energy absorbed by the lumped body. Using an energy balance, the total energy is divided into three different components. The gas velocity in the flame is measured to be 1.53 m/s to 1.79 m/s for the diesel pool fires of 0.5 m to 1.0 m in diameter. The dominant mode of heat transfer in this study is radiative in nature. This simple model is reasonably able to predict the heat flux incident on to the lumped bodies engulfed by diesel pool fires using the measured temperature history. A three dimensional formulation for an axi-symmetric pool fire of a measured flame shape, flame temperature and a gray flame absorption coefficient is employed to predict the temperature of the body engulfed in pool fires. This formulation has to be modified to capture the absolute temperature values of the flame.

Experimental Thermal and Fluid Science

Abstract: This paper presents experimental and three-dimensional numerical study of early onset of separation with rarefied gas flow through a tube with single sharp 90 bend. Experiments are conducted for nitrogen gas flowing at low pressures in three conventional size tubes. The flow is dynamically similar to gas flow in a microchannel as the Knudsen number range (0.0003 < Kn < 0.0385) covers part of the continuum and the slip flow regime while maintaining the Reynolds number between 0.27 and 418.5. The static pressures along the inner, outer and top walls are measured for different mass flow rates and analyzed to understand the flow behavior. The static pressure measurement indicates adverse pressure gradient near the bend along the inner and outer walls of the tube at much lower value of Reynolds number as compared to conventional flow. The numerical solution of the Navier–Stokes equations with the Maxwell’s slip boundary condition shows good agreement with experimental data and helps bring out the complex flow behavior near the bend. The adverse pressure gradient, velocity profile, flow streamlines and velocity vectors in the bend plane clearly indicates secondary flows near the bend at as low a Reynolds number as unity. The flow acceleration and the presence of secondary flows near the bend causes a larger pressure drop as compared with a straight tube. Empirical correlations for Poiseuille number and additional pressure drop coefficient are proposed as part of this work. It is noted that limited experimental data exists in the literature for such flows; these results should therefore help enhance the fundamental understanding of gas flow in microchannels with bend.

Pressure drop, local heat transfer coefficient, and critical heat flux of DNB type for flow boiling in a horizontal straight tube with R-123

Abstract: As far as design safety and operation of heat exchangers with a horizontal flow are concerned, it is necessary to study local heat transfer coefficient, boiling pressure drop, and critical heat flux for flow in a horizontal tube. In the present experimental work, local axial distribution of heat transfer coefficient, two-phase pressure drop, and critical heat flux for the flow boiling in a horizontal straight tube with R-123 as working fluid has been studied. Experiments are performed in horizontal tubes of diameter 11.9 mm and wall thickness 0.4 mm of SS304, having a heated length of 400 mm, 600 mm, and 1000 mm for the mass flux of 180 to 1210 kg/m2s. The local wall temperature is measured using the Infra-Red thermal imaging technique. The local heat transfer coefficients are compared with six different well-known correlations. Also, the two-phase frictional pressure drop is measured and compared with eight different general correlations. In the present study, a sudden rise in wall temperature at any location of a test section is considered as the occurrence of a boiling crisis. The boiling crisis mechanism observed in the present study is of departure from nucleate boiling (DNB). It takes place in the subcooled, or low quality saturated boiling region and in-between the length of the test sections rather than at exit as in the dry-out type of CHF. The critical heat flux is compared with six different predictive correlations. The mechanism of occurrence of CHF in the present work is found to be a departure from nucleate boiling.

Gas flow in ultra-tight shale strata

Abstract: We study the gas flow processes in ultra-tight porous media in which the matrix pore network is composed of nanometre- to micrometre-size pores. We formulate a pressure-dependent permeability function, referred to as the apparent permeability function (APF), assuming that Knudsen diffusion and slip flow (the Klinkenberg effect) are the main contributors to the overall flow in porous media. The APF predicts that in nanometre-size pores, gas permeability values are as much as 10 times greater than results obtained by continuum hydrodynamics predictions, and with increasing pore size (i.e. of the order of the micrometre), gas permeability converges to continuum hydrodynamics values. In addition, the APF predicts that an increase in the fractal dimension of the pore surface leads to a decrease in Knudsen diffusion. Using the homogenization method, a rigorous analysis is performed to examine whether the APF is preserved throughout the process of upscaling from local scale to large scale. We use the well-known pulse-decay experiment to estimate the main parameter of the APF, which is Darcy permeability. Our newly derived late-transient analytical solution and the late-transient numerical solution consistently match the pressure decay data and yield approximately the same estimated value for Darcy permeability at the typical core-sample initial pressure range and pressure difference. Other parameters of the APF may be determined from independent laboratory experiments; however, a pulse-decay experiment can be used to estimate the unknown parameters of the APF if multiple tests are performed and/or the parameters are strictly constrained by upper and lower bounds.

A review on small scale wind turbines

Abstract: Meeting future world energy needs while addressing climatic changes has led to greater strain on conventional power sources. One of the viable sustainable energy sources is wind. But the installation large scale wind farms has a potential impact on the climatic conditions, hence a decentralized small scale wind turbines is a sustainable option. This paper presents review of on different types of small scale wind turbines i.e., horizontal axis and vertical axis wind turbines. The performance, blade design, control and manufacturing of horizontal axis wind turbines were reviewed. Vertical axis wind turbines were categorized based on experimental and numerical studies. Also, the positioning of wind turbines and aero-acoustic aspects were presented. Additionally, lessons learnt from various studies/countries on actual installation of small wind turbines were presented.

A review on the performance of Savonius wind turbines

Abstract: This paper presents a review on the performance of Savonius wind turbines. This type of turbine is unusual and its application for obtaining useful energy from air stream is an alternative to the use of conventional wind turbines. Simple construction, high start up and full operation moment, wind acceptance from any direction, low noise and angular velocity in operation, reducing wear on moving parts, are some advantages of using this type of machine. Over the years, numerous adaptations for this device were proposed. The variety of possible configurations of the rotor is another advantage in using such machine. Each different arrangement of Savonius rotor affects its performance. Savonius rotor performance is affected by operational conditions, geometric and air flow parameters. The range of reported values for maximum averaged power coefficient includes values around 0.05–0.30 for most settings. Performance gains of up to 50% for tip speed ratio of maximum averaged power coefficient are also reported with the use of stators. Present article aims to gather relevant information about Savonius turbines, bringing a discussion about their performance. It is intended to provide useful knowledge for future studies.

Infrared thermography for convective heat transfer measurements

Abstract: This paper deals with the evolution of infrared (IR) thermography into a powerful optical tool that can be used in complex fluid flows to either evaluate wall convective heat fluxes or investigate the surface flow field behavior. Measurement of convective heat fluxes must be performed by means of a thermal sensor, where temperatures have to be measured with proper transducers. By correctly choosing the thermal sensor, IR thermography can be successfully exploited to resolve convective heat flux distributions with both steady and transient techniques. When comparing it to standard transducers, the IR camera appears very valuable because it is non-intrusive, it has a high sensitivity (down to 20 mK), it has a low response time (down to 20 μs), it is fully two dimensional (from 80 k up to 1 M pixels, at 50 Hz) and, therefore, it allows for better evaluation of errors due to tangential conduction within the sensor. This paper analyses the capability of IR thermography to perform convective heat transfer measurements and surface visualizations in complex fluid flows. In particular, it includes the following: the necessary radiation theory background, a review of the main IR camera features, a description of the pertinent heat flux sensors, an analysis of the IR image processing methods and a report on some applications to complex fluid flows, ranging from natural convection to hypersonic regime.