Himadri Chattopadhyay

Large-eddy simulation of flow and heat transfer in an impinging slot jet

The flow field due to an impinging jet at a moderately high Reynolds number, emanating from a rectangular slot nozzle has been computed using a large eddy simulation (LES) technique. A dynamic subgrid-scale stress model has been used for the small scales of turbulence. Quite a few successful applications of the dynamic subgrid-scale stress model use planar averaging to avoid ill conditioning of the model coefficient. However, a novel localization procedure has been attempted herein to find out the spatially varying model coefficient of the flow. The flow field is characterized by entrainment at the boundaries. Periodic boundary conditions could not be used on all the boundaries. The results reveal the nuances of the vortical structures that are characteristic of jet flows. The stress budget also captures a locally negative turbulence production rate. The calibration of the model has been made through prediction of the normalized axial velocity profile and heat transfer on the impingement plate. The computed results compare favorably with the experimental observations, especially in the stagnation zone.

Turbulent flow and heat transfer from a slot jet impinging on a moving plate

Abstract The flow field due to an impinging jet over a moving surface at a moderately high Reynolds number, emanating from a rectangular slot nozzle has been computed using the large eddy simulation technique. A dynamic subgrid-scale stress model has been used for the small scales of turbulence. The velocity of the impinging surface perpendicular to the jet velocity has been varied up to two times the jet velocity at the nozzle exit. Turbulence quantities such as kinetic energy, production rate of turbulent kinetic energy and the Reynolds stresses are calculated for different surface velocities. It has been observed that, while the turbulent kinetic energy increases with increasing velocity of the impinging surface, production rate of turbulence initially increases with increasing surface velocity and then comes down. By analyzing the components of turbulent production it was found that P 33 is the dominant term up to the surface velocity of one unit and when the surface velocity is two times the jet velocity at the nozzle exit, the major contribution to turbulence production comes from P 13 and partly from P 11 . Heat transfer from the plate initially increases with non-dimensional surface velocity up to 1.2 and then comes down.

Augmentation of Heat Transfer by Creation of Streamwise Longitudinal Vortices Using Vortex Generators

This paper summarizes the current state of the art related to improvement of the heat exchanger surfaces using streamwise longitudinal vortices. Primarily, the improvements related to fin-tube cross-flow heat exchangers and the plate-fin heat exchangers have been addressed. Protrusions in certain forms, such as delta wings or winglet pairs, act as vortex generators, which can enhance the rate of heat transfer from the heat-exchanger surfaces that may be flat or louvered. The strategically placed vortex generators create longitudinal vortices, which disrupt the growth of the thermal boundary layer, promote mixing between fluid layers, and hence lead to augmentation in heat transfer. The flow fields are dominated by swirling motion associated with modest pressure penalty. Heat transfer is augmented substantially for all the proposed configurations of the longitudinal vortex generators, such as delta wings, rectangular winglet pairs, and delta winglet pairs, with varying degree of pressure penalty. Both computational and experimental investigations on flow and heat transfer in the heat exchanger passages with built-in vortex generators are revisited and summarized.

Heat Transfer From a Moving Surface Due to Impinging Slot Jets

Turbulent flow field and heat transfer in an array of slot jets impinging on a movingsurface have been numerically investigated by Large-eddy Simulation (LES) technique inthe range of Reynolds numbers between 500 and 3000. The surface velocity, directedperpendicular to the jet, is varied up to two times the jet velocity at the nozzle exit. TheNusselt number distributions for various surface velocities are presented. It has beenobserved that on one hand increasing velocity of the impingement plate reduces heattransfer, while on the other hand distribution of Nusselt number over the impingementsurface becomes more uniform with the increased surface velocity.@DOI: 10.1115/1.1470489#Keywords: Forced Convection, Heat Transfer, Impingement, Jets, Turbulence

Simulation of laminar slot jets impinging on a moving surface

Laminar flow and heat transfer on a moving surface due to a bank of impinging slot jets have been numerically investigated. Two types of jet, namely axial and knife-jet with an exit angle of 60 deg were considered. The surface velocity up to two times the jet velocity at the nozzle exit was imposed on the impinging surface. It has been observed that while with increasing velocity of the impinging surface, the total heat transfer reduces; the distribution pattern becomes more uniform. For the same amount of mass and momentum flux at the nozzle exit, heat transfer from the axial jet is considerably higher than that from the vectored jets at all surface velocities considered

NUMERICAL INVESTIGATIONS OF HEAT TRANSFER OVER A MOVING SURFACE DUE TO IMPINGING KNIFE-JETS

Turbulent flow field and heat transfer from an array of impinging horizontal knife jets on a moving surface have been investigated using large eddy simulation (LES) with a dynamic subgrid stress model. The surface velocity directed perpendicular to the jet plane is varied up to two times the jet velocity at the nozzle exit. Performance of a horizontal knife jet with an exit angle of 60° is compared with the standard axial jet. It has been observed that increasing surface motion reduces heat transfer for both types of jets. However, the amount of heat transfer from the knife jets is more than that from the axial jets when the surface velocity is within the order of half the jet velocity at the nozzle exit. For further increase in surface velocity, heat transfer from the knife jets is, however, less than that in the case of axial jets if the Reynolds number (Re) is low. For higher Re and higher surface velocity, the heat transfer from either type of jets is of comparable magnitude.Turbulent flow field and heat transfer from an array of impinging horizontal knife jets on a moving surface have been investigated using large eddy simulation (LES) with a dynamic subgrid stress model. The surface velocity directed perpendicular to the jet plane is varied up to two times the jet velocity at the nozzle exit. Performance of a horizontal knife jet with an exit angle of 60° is compared with the standard axial jet. It has been observed that increasing surface motion reduces heat transfer for both types of jets. However, the amount of heat transfer from the knife jets is more than that from the axial jets when the surface velocity is within the order of half the jet velocity at the nozzle exit. For further increase in surface velocity, heat transfer from the knife jets is, however, less than that in the case of axial jets if the Reynolds number (Re) is low. For higher Re and higher surface velocity, the heat transfer from either type of jets is of comparable magnitude.

Eulerian two-phase flow simulation and experimental validation of semisolid slurry generation process using cooling slope

Abstract Experimental and numerical studies of slurry generation using a cooling slope are presented in the paper. The slope having stainless steel body has been designed and constructed to produce semisolid A356 Al alloy slurry. The pouring temperature of molten metal, slope angle of the cooling slope and slope wall temperature were varied during the experiment. A multiphase numerical model, considering liquid metal and air, has been developed to simulate the liquid metal flow along the cooling channel using an Eulerian two-phase flow approach. Solid fraction evolution of the solidifying melt is tracked at different locations of the cooling channel following Schiel’s equation. The continuity, momentum and energy equations are solved considering thin wall boundary condition approach. During solidification of the melt, based on the liquid fraction and latent heat of the alloy, temperature of the alloy is modified continuously by introducing a modified temperature recovery method. Numerical simulations has been carried out for semisolid slurry formation by varying the process parameters such as angle of the cooling slope, cooling slope wall temperature and melt superheat temperature, to understand the effect of process variables on cooling slope semisolid slurry generation process such as temperature distribution, velocity distribution and solid fraction of the solidifying melt. Experimental validation performed for some chosen cases reveals good agreement with the numerical simulations.

Numerical visualization of convective heat transfer from a sphere — with and without radial mass efflux

The conception of a heat function, just like the stream function used in a laminar two dimensional incompressible flow field visualization, has been introduced to visualize the convective heat transfer or the flow of energy around a sphere when the sphere is either being cooled or heated by a stream of fluid flowing around it. The heat function is developed in a spherical polar coordinate and is used to generate the heat lines around the sphere. The heat lines essentially show the magnitude and direction of energy transfer around the sphere with and without the existence of a finite radial velocity at the surface. The steady state hydrodynamic field around the sphere is numerically obtained up to a maximum Reynolds number of 100 and the corresponding thermal field has been obtained by solving the steady state energy equation. The field properties thus obtained are utilized to form the heat function, which becomes an effective tool for visualization of convective heat transfer.

Energy generation from fluidized bed gasification of rice husk

Though gasification of biomass in fluidized bed system is an efficient way of biomass utilization, limited experimental data on the fluidized bed biomass gasification are available in open literature. Therefore, an experimental study of biomass gasification is conducted using a laboratory scale bubbling fluidized bed gasifier. Rice husk is used as the biomass material and air-steam mixture is used as the gasifying agent. As the non-granular nature of rice husk makes it difficult to fluidize, silica sand is used as the inert bed material to help in fluidization. Parametric studies are performed to determine the effects of reactor temperature, equivalence ratio, and steam-to-biomass ratio on the product gas composition and the heating value. The results show that both hydrogen percentage and the heating value of the product gas increase with increase in gasification temperature and steam-to-biomass ratio, but they decrease with increase in equivalence ratio. The maximum heating value (4.26 MJ/Nm3) and hydro...

Effect of pouring temperature on cooling slope casting of semi-solid Al-Si-Mg alloy

Present trend of semi-solid processing is directed towards rheocasting route which allows manufacturing of near-net-shape cast components directly from the prepared semi-solid slurry. Generation of globular equi-axed grains during solidification of rheocast components, compared to the columnar dendritic structure of conventional casting routes, facilitates the manufacturing of components with improved mechanical properties and structural integrity. In the present investigation, a cooling slope has been designed and indigenously fabricated to produce semi solid slurry of Al-Si-Mg (A356) alloy and successively cast in a metallic mould. The scope of the present work discusses about development of a numerical model to simulate the liquid metal flow through cooling slope using Eulerian two-phase flow approach and to investigate the effect of pouring temperature on cooling slope semi-solid slurry generation process. The two phases considered in the present model are liquid metal and air. Solid fraction evolution of the solidifying melt is tracked at different locations of the cooling slope, following Schiels equation. The continuity equation, momentum equation and energy equation are solved considering thin wall boundary condition approach. During solidification of the liquid metal, a modified temperature recovery scheme has been employed taking care of the latent heat release and change of fraction of liquid. The results obtained from simulations are compared with experimental findings and good agreement has been found.