Showing papers on "Film temperature published in 2009"

Natural Convection Heat Transfer in an Inclined Enclosure Filled with a Water-Cuo Nanofluid

Abstract: This article presents the results of a numerical study on natural convection heat transfer in an inclined enclosure filled with a water-CuO nanofluid. Two opposite walls of the enclosure are insulated and the other two walls are kept at different temperatures. The transport equations for a Newtonian fluid are solved numerically with a finite volume approach using the SIMPLE algorithm. The influence of pertinent parameters such as Rayleigh number, inclination angle, and solid volume fraction on the heat transfer characteristics of natural convection is studied. The results indicate that adding nanoparticles into pure water improves its heat transfer performance; however, there is an optimum solid volume fraction which maximises the heat transfer rate. The results also show that the inclination angle has a significant impact on the flow and temperature fields and the heat transfer performance at high Rayleigh numbers. In fact, the heat transfer rate is maximised at a specific inclination angle depending on ...

Boundary layer flow and heat transfer over an unsteady stretching vertical surface

Abstract: The solution to the unsteady mixed convection boundary layer flow and heat transfer problem due to a stretching vertical surface is presented in this paper. The unsteadiness in the flow and temperature fields is caused by the time-dependent of the stretching velocity and the surface temperature. The governing partial differential equations with three independent variables are first transformed into ordinary differential equations, before they are solved numerically by a finite-difference scheme. The effects of the unsteadiness parameter, buoyancy parameter and Prandtl number on the flow and heat transfer characteristics are thoroughly examined. Both assisting and opposing buoyant flows are considered. It is observed that for assisting flow, the solutions exist for all values of buoyancy parameter, whereas for opposing flow, they exist only if the magnitude of the buoyancy parameter is small. Comparison with known results for steady-state flow is excellent.

Heat transfer over an unsteady stretching permeable surface with prescribed wall temperature

Abstract: The unsteady laminar boundary layer flow over a continuously stretching permeable surface is investigated. The unsteadiness in the flow and temperature fields is caused by the time-dependence of the stretching velocity and the surface temperature. Effects of the unsteadiness parameter, suction/injection parameter and Prandtl number on the heat transfer characteristics are thoroughly examined.

Heat Transfer of Aviation Kerosene at Supercritical Conditions

Abstract: The heat transfer characteristics of China no. 3 kerosene were investigated experimentally and analytically under conditions relevant to a regenerative cooling system for scramjet applications. A test facility developed for the present study can handle kerosene in a temperature range of 300-1000 K, a pressure range of 2.6-5 MPa, and a mass How rate range of 10-100 g/s. In addition, the test section was uniquely designed such that both the wall temperature and the bulk fuel temperature were measured at the same location along the flowpath. The measured temperature distributions were then used to analytically deduce the local heat transfer characteristics. A 10-component kerosene surrogate was proposed and employed to calculate the fuel thermodynamic and transport properties that were required in the heat transfer analysis. Results revealed drastic changes in the fuel flow properties and heat transfer characteristics when kerosene approached its critical state. Convective heat transfer enhancement was also found as kerosene became supercritical. The heat transfer correlation in the relatively low-fuel-temperature region yielded a similar result to other commonly used jet fuels, such as JP-7 and JP-8, at compressed liquid states. In the high-fuel-temperature region, near and beyond the critical temperature, heat transfer enhancement was observed; hence, the associated correlation showed a more significant Reynolds number dependency.

Heat and mass transfer in stagnation-point flow towards a stretching surface in the presence of buoyancy force and thermal radiation

Abstract: In this paper an analysis has been made to study heat and mass transfer in two-dimensional stagnation-point flow of an incompressible viscous fluid over a stretching vertical sheet in the presence of buoyancy force and thermal radiation. The similarity solution is used to transform the problem under consideration into a boundary value problem of nonlinear coupled ordinary differential equations containing Prandtl number, Schmidt number and Sherwood number which are solved numerically with appropriate boundary conditions for various values of the dimensionless parameters. Comparison of the present numerical results are found to be in excellent with the earlier published results under limiting cases. The effects of various physical parameters on the boundary layer velocity, temperature and concentration profiles are discussed in detail for both the cases of assisting and opposing flows. The computed values of the skin friction coefficient, local Nusselt number and Sherwood number are discussed for various values of physical parameters. The tabulated results show that the effect of radiation is to increase skin friction coefficient, local Nusselt number and Sherwood number.

Magneto-hydrodynamic mixed convection of a power-law fluid past a stretching surface in the presence of thermal radiation and internal heat generation/absorption

Abstract: The problem of magneto-hydrodynamic mixed convective flow and heat transfer of an electrically conducting, power-law fluid past a stretching surface in the presence of heat generation/absorption and thermal radiation has been analyzed. After transforming the governing equations with suitable dimensionless variables, numerical solutions are generated by an implicit finite-difference technique for the non-similar, coupled flow. The solution is found to be dependent on the governing parameters including the power-law fluid index, the magnetic field parameter, the modified Richardson number, the radiation parameter, the heat generation parameter, and the generalized Prandtl number. To reveal the tendency of the solutions, typical results for the velocity and temperature profiles, the skin-friction coefficient, and the local Nusselt number are presented for different values of these controlling parameters.

Critical heat flux around strongly heated nanoparticles

Abstract: We study heat transfer from a heated nanoparticle into surrounding fluid using molecular dynamics simulations. We show that the fluid next to the nanoparticle can be heated well above its boiling point without a phase change. Under increasing nanoparticle temperature, the heat flux saturates, which is in sharp contrast with the case of flat interfaces, where a critical heat flux is observed followed by development of a vapor layer and heat flux drop. These differences in heat transfer are explained by the curvature-induced pressure close to the nanoparticle, which inhibits boiling. When the nanoparticle temperature is much larger than the critical fluid temperature, a very large temperature gradient develops, resulting in close to ambient temperature just a radius away from the particle surface. The behavior reported allows us to interpret recent experiments where nanoparticles can be heated up to the melting point, without observing boiling of the surrounding liquid.

Steady mixed convection stagnation-point flow of upper convected Maxwell fluids with magnetic field

Abstract: The steady MHD mixed convection flow of a viscoelastic fluid in the vicinity of two-dimensional stagnation point with magnetic field has been investigated under the assumption that the fluid obeys the upper-convected Maxwell (UCM) model. Boundary layer theory is used to simplify the equations of motion, induced magnetic field and energy which results in three coupled non-linear ordinary differential equations which are well-posed. These equations have been solved by using finite difference method. The results indicate the reduction in the surface velocity gradient, surface heat transfer and displacement thickness with the increase in the elasticity number. These trends are opposite to those reported in the literature for a second-grade fluid. The surface velocity gradient and heat transfer are enhanced by the magnetic and buoyancy parameters. The surface heat transfer increases with the Prandtl number, but the surface velocity gradient decreases.

MHD natural convection flow and heat transfer in a laterally heated partitioned enclosure

Abstract: This study looks at MHD natural convection flow and heat transfer in a laterally heated enclosure with an off-centred partition. Governing equations in the form of vorticity–stream function formulation are solved using the polynomial differential quadrature (PDQ) method. Numerical results are obtained for various values of the partition location, Rayleigh, Prandtl and Hartmann numbers. The results indicate that magnetic field significantly suppresses flow, and thus heat transfer, especially for high Rayleigh number values. The results also show that the x-directional magnetic field is more effective in damping convection than the y-directional magnetic field, and the average heat transfer rate decreases with an increase in the distance of the partition from the hot wall. The average heat transfer rate decreases up to 80% if the partition is placed at the midpoint and an x-directional magnetic field is applied. The results also show that flow and heat transfer have little dependence on the Prandtl number.

Conjugate Problems in Convective Heat Transfer

Abstract: Part I: Approximate Solutions Analytical Methods for the Estimation of Heat Transfer from Nonisothermal Walls Basic Equations Self-Similar Solutions of the Boundary Layer Equations Solutions of the Boundary Layer Equations in the Power Series Integral Methods Method of Superposition Solutions of the Boundary Layer Equations in the Series of Shape Parameters Approximate Solutions of Conjugate Problems in Convective Heat Transfer Formulation of a Conjugate Problem of Convective Heat Transfer The Case of Linear Velocity Distribution across the Thermal Boundary Layer The Case of Uniform Velocity Distribution across the Thermal Boundary Layer (Slug Flow) Solutions of the Conjugate Convective Heat Transfer Problems in the Power Series Solutions of the Conjugate Heat Transfer Problems by Integral Methods Part II: Theory and Methods Heat Transfer from Arbitrary Nonisothermal Surfaces in a Laminar Flow The Exact Solution of the Thermal Boundary Layer Equation for an Arbitrary Surface Temperature Distribution Generalization for an Arbitrary Velocity Gradient in a Free Stream Flow General Form of the Influence Function of the Unheated Zone: Convergence of the Series The Exact Solution of the Thermal Boundary Layer Equation for an Arbitrary Surface Heat Flux Distribution Temperature Distribution on an Adiabatic Surface in an Impingent Flow The Exact Solution of an Unsteady Thermal Boundary Layer Equation for Arbitrary Surface Temperature Distribution The Exact Solution of a Thermal Boundary Layer Equation for a Surface with Arbitrary Temperature in a Compressible Flow The Exact Solution of a Thermal Boundary Layer Equation for a Moving Continuous Surface with Arbitrary Temperature Distribution The Other Solution of a Thermal Boundary Layer Equation for an Arbitrary Surface Temperature Distribution Heat Transfer from Arbitrary Nonisothermal Surfaces in Turbulent Flow Basis Relations for the Equilibrium Boundary Layer Solution of the Thermal Turbulent Boundary Layer Equation for an Arbitrary Surface Temperature Distribution Intensity of Heat Transfer from an Isothermal Surface: Comparison with Experimental Data The Effect of the Turbulent Prandtl Number on Heat Transfer on Flat Plates Coefficients gk of Heat Flux Series for Nonisothermal Surfaces Approximate Relations for Heat Flux in a Transition Regime General Properties of Nonisothermal and Conjugate Heat Transfer The Effect of Temperature Head Distribution on Heat Transfer Intensity Gradient Analogy and Reynolds Analogy Heat Flux Inversion Zero Heat Transfer Surfaces Examples of Optimizing Heat Transfer in Flow over Bodies Analytical Methods for Solving Conjugate Convective Heat Transfer Problems A Biot Number as a Criterion of the Conjugate Heat Transfer Rate General Boundary Condition for Convective Heat Transfer Problems: Errors Caused by Boundary Condition of the Third Kind Reduction of a Conjugate Convective Heat Transfer Problem to an Equivalent Heat Conduction Problem Temperature Singularities on the Solid-Fluid Interface Universal Functions for Solving Conjugate Heat Transfer Problems - Solution Examples Reducing the Unsteady Conjugate Convective Heat Transfer Problem to an Equivalent Heat Conduction Problem Integral Transforms and Similar Methods Solutions in Asymptotic Series in Eigenfunctions Superposition and Other Methods Green's Function and the Method of Perturbation Numerical Methods for Solving Conjugate Convective Heat Transfer Problems Analytical and Numerical Methods Approximate Analytical and Numerical Methods for Solving Differential Equations Difficulties in Computing Convection-Diffusion and Flow Numerical Methods of Conjugation Examples of Numerical Studies of the Conjugate Convective Heat Transfer in Pipes and Channels Examples of Numerical Studies of the Conjugate Convective Heat Transfer in Flows around and inside Bodies Part III: Applications Thermal Treatment of Materials Moving Materials Undergoing Thermal Processing Simulation of Industrial Processes Drying of Continuous Moving Materials Technological Processes Multiphase and Phase-Change Processes Drying and Food Processing Manufacturing Equipment Operation Heat Exchangers and Finned Surfaces Cooling Systems Conclusion

Numerical Investigation of Flow and Heat Transfer Performance of Nano-Encapsulated Phase Change Material Slurry in Microchannels

Abstract: Microchannels are used in applications where large amount of heat is produced. Phase change material (PCM) slurries can be used as a heat transfer fluid in microchannels as they provide increased heat capacity during the melting of phase change material. For the present numerical investigation, performance of a nano-encapsulated phase change material slurry in a manifold microchannel heat sink was analyzed. The slurry was modeled as a bulk fluid with varying specific heat. The temperature field inside the channel wall is solved three dimensionally and is coupled with the three dimensional velocity and temperature fields of the fluid. The model includes the microchannel fin or wall effect, axial conduction along the length of the channel, developing flow of the fluid and not all these features were included in previous numerical investigations. Influence of parameters such as particle concentration, inlet temperature, melting range of the PCM, and heat flux is investigated, and the results are compared with the pure single phase fluid.

Thermohydrodynamic Analysis of Bump Type Gas Foil Bearings: A Model Anchored to Test Data

Abstract: The paper introduces a thermohydrodynamic (THD) model for prediction of gas foil bearing (GFB) performance. The model includes thermal energy transport in the gas film region, and with cooling gas streams, inner or outer, as in typical rotor-GFBs systems. The analysis also accounts for material property changes and the bearing components’ expansion due to temperature rises and shaft centrifugal growth due to rotational speed. Gas inlet feed characteristics are thoroughly discussed in bearings whose top foil must detach, i.e., not allowing for subambient pressure. Thermal growths determine the actual bearing clearance needed for accurate prediction of GFB forced performance, static and dynamic. Model predictions are benchmarked against published measurements of (metal) temperatures in a GFB operating without a forced cooling gas flow. The tested foil bearing is proprietary; hence its geometry and material properties are largely unknown. Predictions are obtained for an assumed bearing configuration, with bump-foil geometry and materials taken from prior art and best known practices. The predicted film peak temperature occurs just downstream of the maximum gas pressure. The film temperature is higher at the bearing middle plane than at the foil edges, as the test results also show. The journal speed, rather than the applied static load, influences more the rise in film temperature and with a larger thermal gradient towards the bearing sides. In addition, as in the tests conducted at a constant rotor speed and even for the lowest static load, the gas film temperature increases rapidly due to the absence of a forced cooling air that could carry away the recirculation gas flow and thermal energy drawn by the spinning rotor, Predictions are in good agreement with the test data. A comparison of predicted static load parameters to those obtained from an isothermal condition shows the THD model producing a smaller journal eccentricity (larger minimum film thickness) and larger drag torque. A rise in gas temperature is tantamount to an increase in gas viscosity, hence the noted effect in the foil bearing forced performance.Copyright © 2009 by ASME

Heat Transfer in a Micropolar Fluid along a Non-linear Stretching Sheet with a Temperature-Dependent Viscosity and Variable Surface Temperature

Abstract: In this paper, heat transfer characteristics of a two-dimensional steady hydromagnetic natural convection flow of a micropolar fluid passed a non-linear stretching sheet taking into account the effects of a temperature-dependent viscosity and variable wall temperature are studied numerically for local similarity solutions by applying the Nachtsheim-Swigert iteration method. The results corresponding to the dimensionless temperature profiles and the local rate of heat transfer are displayed graphically for important material parameters. The results show that in modeling the thermal boundary layer flow with a temperature-dependent viscosity, consideration of the Prandtl number as a constant within the boundary layer produces unrealistic results and therefore it must be treated as a variable rather than a constant within the boundary layer. The results also show that the local rate of heat transfer strongly depends on the non-linear stretching index and temperature index.