Problem Of Forced Convection Research Articles

Analysis of the influence of operating conditions and geometric parameters on heat transfer in water-paraffin shell-and-tube latent thermal energy storage unit

A transient heat transfer phenomenon during charging and discharging of the shell-and-tube latent thermal energy storage system has been analysed in this paper. The mathematical model, regarding the conjugate problem of transient forced convection and solid–liquid phase change heat transfer based on the enthalpy formulation, has been presented. A fully implicit two-dimensional control volume FORTRAN computer code has been developed for solving governing equations with initial and boundary conditions. The numerical model is validated with experimental data obtained by experimental investigations that have been performed on the test unit with technical grade paraffin as the phase change material (PCM) and water as the heat transfer fluid (HTF). Numerical predictions match the experimental results. This pointed out that the presented numerical procedure could be accurately used for transient heat transfer simulation. A series of numerical calculations have been done in order to analyse the influence of several HTF operating conditions and several geometric parameters on the heat transfer process inside the water-paraffin shell-and-tube latent thermal energy storage (LTES) unit. Numerical results, which could be used for operating conditions and geometry optimization, provide guidelines for the design of the latent thermal energy storage system.

Estimation of Time-Varying Gaseous Contaminant Sources in Ventilated Enclosures Through Inversion of a Reduced Model

A method for estimating the time-varying intensity of emitting sources of a gaseous contaminant in ventilated enclosures is proposed in this numerical study. A reduced model linking up a set of control points inside the domain to the contaminant sources is identified using the Modal Identification Method, from simulations carried out using CFD software. This reduced model is then used to solve the inverse forced convection problem consisting of the estimation of sources emission rates as a function of time from simulated contaminant concentration measurements.

A novel thermal sensor concept for flow direction and flow velocity

This paper presents a unified theory for different measurement concepts of a thermal flow sensor. Based on this theory, a new flow sensor concept is derived. The concept allows measuring both direction and velocity of a fluid flow with a heater and an array of temperature sensors. This paper first analyzes the two-dimensional (2-D) forced convection problem with a laminar flow. The two operation modes of a constant heating power and of a constant heater temperature are considered in the analytical model. A novel estimation algorithm was derived for the flow direction. Different methods for velocity measurement were presented: the hot-wire method, the calorimetric method, and the novel average-temperature method. The only geometric parameter of the sensor, the dimensionless position of the sensor array, is optimized based on the analytical results. Furthermore, the paper presents the experimental results of the sensor prototype. In order to verify the analytical model, an array of temperature sensors was used for recording the 2-D temperature profile around the heater. Temperature values are transferred to a computer by a multiplexer. A program running on a personal computer extracts the actual flow velocity and flow direction from the measured temperature data. This paper discusses different evaluation algorithms, which can be used for this sensor. A simple Gaussian estimator was derived for the direction measurement. This estimator provides the same accuracy as the analytical estimator. Velocity results of both the calorimetric concept and the novel average-temperature concept are also presented.

Mathematical Modeling of Flows Inside Rotating Bodies Made of Cellular-Porous Materials

A physicomathematical model is developed, which describes gas flows inside rapidly rotating bodies made of cellular-porous materials. Asymptotic and numerical solutions are obtained for some problems of forced centrifugal convection inside cylindrical cellular-porous bodies. The effect of the governing parameters (drag coefficient and dimensionless length of the cylinder) on characteristics and types of flows is considered.

Forced convection laminar film condensation on an inclined elliptical tube in the absence of gravity

In this paper, the problem of forced convection dominated laminar film condensation, in the absence of gravity, on an inclined elliptical tube is investigated theoretically. In this analysis, the interfacial vapour shear stress is modelled following Shekriladze approach. By employing the method of characteristics, expressions are analytically derived for calculating the local film thickness as well as the local and mean Nusselt numbers. The results show that the mean Nusselt number, $$ {\text{\ifmmode\expandafter\bar\else\expandafter\=\fi{N}u}}/{\sqrt {{\text{ \ifmmode\expandafter\tilde\else\expandafter\~\fi{R}e\;cos}}\varphi } }, $$ enhances with the increase in the tube ellipticity. For the practical ellipticity range: 0.8 ≤ e≤ 0.92, this enhancement is found from 7 to 14% compared to a circular tube with the same length and equivalent condensation surface area.

A Two-Velocity Two-Temperature Model for a Bi-Dispersed Porous Medium: Forced Convection in a Channel

A two-velocity two-temperature model for bi-dispersed porous media is formulated. Using the model, an analytic solution is obtained for the problem of forced convection in a channel between parallel plane walls that are held either at uniform temperature or uniform heat flux. In each case, Nusselt number values are given as functions of a conductivity ratio, a velocity ratio, a volume fraction, and an internal heat exchange parameter.

Compact thermal models for internal convection

Objects undergoing heat transfer by convection internally, i.e., not at their boundaries, need special care in their modeling. An extension of the general formulation made earlier for compact thermal models is discussed here in order to cover the case of internal forced convection problems such as micro-channels. The single resistor model based on the heat transfer coefficient h is replaced with a compact model linking all relevant nodes using h as well as fluid thermal capacitance and inertia, such as to satisfy thermodynamic constraints derived earlier for conduction compact models.

Transpired Turbulent Boundary Layers Subject to Forced Convection and External Pressure Gradients

The problem of forced convection transpired turbulent boundary layers with external pressure gradient has been studied by using different scalings proposed by various researchers. Three major results were obtained: First, for adverse pressure gradient boundary layers with suction, the mean deficit profiles collapse with the free stream velocity, U∞, but into different curves depending on the strength of the blowing parameter and the upstream conditions. Second, the dependencies on the blowing parameter, the Reynolds number, and the strength of pressure gradient are removed from the outer flow when the mean deficit profiles are normalized by the Zagarola/Smits [Zagarola, M. V., and Smits, A. J., 1998, “Mean-Flow Scaling of Turbulent Pipe Flow,” J. Fluid Mech., 373, 33–79] scaling, U∞δ*/δ. Third, the temperature profiles collapse into a single curve using the new inner and outer scalings proposed by Wang and Castillo [Wang, X., and Castillo, L., 2003, “Asymptotic Solutions in Forced Convection Turbulent Boundary Layers,” J. Turbulence, 4(006)], which produce the true asymptotic profiles even at finite Pe´clet number.

C210 Excelの表計算機能を利用した熱流体解析プロセスの可視化

In the numerical analysis using the spreadsheet of Excel, the image of the physical model can be drawn on the worksheet, and both programming and debagging are performed visually via classification by coloring of interior and boundary cells, where corresponding difference equations are embedded. Furthermore, calculated results can be shown and at the same time confirmed immediately by using the accessory graph wizard. In the present study, a newly developed visual-oriented numerical method for solving the two-dimensional steady-state forced convection problem which applies the simple method is explained through a simple physical model of inclined parallel plates with discreet heat sources.

Modeling of Entropy Production in Turbulent Flows

This article presents new modeling of turbulence correlations in the entropy transport equation for viscous, incompressible flows. An explicit entropy equation of state is developed for gases with the ideal gas law, while entropy transport equations are derived for both gases and liquids. The formulation specifically considers incompressible forced convection problems without a buoyancy term in the y-momentum equation, as density variations are neglected. Reynolds averaging techniques are applied to the turbulence closure of fluctuating temperature and entropy fields. The problem of rigorously expressing the mean entropy production in terms of other mean flow quantities is addressed. The validity of the newly developed formulation is assessed using direct numerical simulation data and empirical relations for the friction factor. Also, the dissipation (ε) of turbulent kinetic energy is formulated in terms of the Second Law. In contrast to the conventional ε equation modeling, this article proposes an alternative method by utilizing both transport and positive definite forms of the entropy production equation.

Resolution of linear inverse forced convection problems using model reduction by the Modal Identification Method: application to turbulent flow in parallel-plate duct

Inverse Heat Convection Problems have received attention only recently. They usually involve the use of a high order model corresponding to the spatial discretization of the domain. In this numerical study, the possibility to quickly solve such a problem with a low order model is analysed. The proposed method can be applied to any forced convection problem, whatever the geometry, as far as it is linear. Starting from a Detailed Model (DM) of the system, the Modal Identification Method is applied to build a Reduced Model (RM), which can be used to solve the inverse problem. The inversion procedure is sequential and requires no iterations. The function specification method is used to stabilize the inverse problem. An illustrative application is given. Turbulent forced convection is considered, with a hydrodynamically fully developed, thermally developing, incompressible, turbulent flow of a newtonian and constant property fluid inside a parallel-plate duct. Axial conduction in the flow is neglected. Two wall heat flux densities, varying with time, are estimated from the knowledge of simulated transient temperature measurements inside the fluid. When solving the inverse problem with RM instead of DM, a drastic reduction of computing time is obtained (with a reduction factor up to 11,000 in the present study), without significant loss of accuracy. Effects of functional form of the unknowns, sensors number and position, measurement error, on the accuracy of estimates are examined.

Education and tutorial in modeling of elementary flows, heat and mass transfer during crystal growth in ground-based and microgravity environment

Our experience in education and tutorials regarding the modeling of elementary flows, heat and mass transfer during crystal growth in ground-based and microgravity environment using the computer system convection in microgravity and applications is presented. The system supports free and forced convection problems on the basis of the Navier–Stokes equations for the Boussinesq approach. The computer laboratory as the intellectual shell of this system includes basic double-diffusion gravity-driven and Marangoni problems in enclosures, a Bridgman model, and a Czochralski model for microgravity and ground-based applications.

Resolution of linear inverse forced convection problems using model reduction by the Modal identification Method: application to turbulent flow in parallel-plate duct

Resolution of linear inverse forced convection problems using model reduction by the Modal identification Method: application to turbulent flow in parallel-plate duct

Optimal Heating Algorithm for the Three-Dimensional Forced-Convection Problems

A three-dimensional forced-convection optimal heating algorithm in determining the strength of the unknown optimal surface heating functions utilizing the conjugate gradient method and a general purpose commercial code CFX4.2 is applied successfully in the present study based on the desired surface temperature distributions at the final time of heating. Results obtained by using the conjugate gradient method to solve this three-dimensional optimal heating problem are justified based on the numerical experiments. Two different computational domains and two different desired temperature distributions are given, and the corresponding optimal heating functions are to be determined

Effect of Viscous Dissipation on the Darcy Forced-Convection Flow Past a Plane Surface

The uniform forced-convection flow in a fluid-saturated porous medium adjacent to a plane surface with prescribed temperature distribution Tw = Tw(x) is considered. The effect of viscous dissipation is included in the energy balance equation as a quadratic term of the Darcy velocity (see Bejan, 1984, 1995). It is shown that the usual asymptotic condition T (x, y ® Ґ) = const, єTҐ (as it was applied in some recent publications dealing with mixed-convection problems) contradicts this balance equation. In the present paper, -the appropriate asymptotic condition (a linear function of x) is specified and it is shown that this steady forced-convection problem can formally be reduced to a standard transient heat conduction problem in a homogeneous semi-infinite solid. By exploiting this analogy, several examples solvable in exact analytic form are presented.

Progress in the generalization of wall-function treatments

This paper describes progress in developing an analytical representation of the variation of the dynamic variables and temperature across the near-wall sublayer of a turbulent flow. The aim is to enable the effective “resistance” of the viscous sublayer to the transport of heat and momentum to be packaged in the form of a “wall function”, thus enabling CFD predictions of convective heat transfer to be made without incurring the cost of the very fine near-wall grid that would otherwise have to be adopted. While the general idea is not new, the detailed strategy contains many new features, which have led to a scheme capable of accounting for the effects of buoyancy, pressure gradient and of variations in molecular transport properties. The scheme is applied to the problem of forced and mixed convection in a vertical pipe and to the opposed wall jet with encouraging results.

SIMULTANEOUS ESTIMATION OF INLET TEMPERATURE AND WALL HEAT FLUX IN TURBULENT CIRCULAR PIPE FLOW

An inverse heat convection problem is solved for simultaneous estimation of unknown inlet temperature and wall heat flux in a thermally developing, hydrodynamically developed turbulent flow in a circular pipe based on temperature measurements obtained at several different locations in the stream. The direct problem of turbulent forced convection is solved with a finite difference method with appropriate algebraic turbulence modelling. Although we seek for two unknown functions, we formulate the inverse problem as one of parameter estimation through the representation of the unknown inlet temperature profile and the wall heat flux distribution by one-dimensional finite element interpolation. Nodal values of the inlet temperature and the wall heat flux at chosen positions are determined as unknown parameters through the Levenberg–Marquardt algorithm for minimization procedure. Numerical results for several testing cases with different magnitudes of measurement errors are examined by using simulated experimental data. The effects of the number and the locations of the temperature measurement points are discussed.

Exact analytical solutions of forced convection flow in a porous medium

The forced convection problem in a fluid saturated porous medium is considered. For the self-similar boundary-layer flows past a plane or axisymmetric body with arbitrary shape embedded in this medium and having a power-law surface temperature distribution, analytical solutions are given. In the range λ ≥ 0 of the temperature exponent, the analytical results show an excellent agreement with previous numerical findings. In the range λ < − 1 2 , the existence of a new class of unique solutions, while for − 1 2 < λ < 0 the occurrence of multiple solutions is reported. The heat transfer characteristics and the physical meaning of all these forced convection boundary-layer flows are discussed in detail.

Estimation of unknown wall heat flux in turbulent circular pipe flow

An inverse heat convection problem is solved for the estimation of a nonuniform wall heat flux in a thermally developing, hydrodynamically developed turbulent flow in a circular pipe based on temperature measurements obtained at several different locations in the stream. The direct problem of turbulent forced convection is solved with a finite difference method with appropriate algebraic turbulence modelling. The unknown wall heat flux is represented by a one-dimensional finite element interpolation. Nodal values at several chosen points are determined as unknown parameters by the Levenberg-Marquardt algorithm. The effects of sensor number and position are examined.

Forced convection in thermally developing turbulent flow of drag-reducing fluids within circular tubes

The Integral Transform Technique is used to solve the turbulent forced convection problem for drag-reducing fluids in the thermal developing and fully developed regions within circular tubes. Turbulent effects are taken into account through an algebraic model corresponding to the minimum-drag asymptotic case for viscoelastic fluids. The well-established Sign-Count Method and the Generalized Integral Transform Technique (GITT) are both employed in order to compute the eigenvalues and the respective eigenfunctions of the associated Sturm–Liouville problem. The Nusselt numbers calculated with the present approach are then compared with those obtained from experimental works available in the literature.