2D transient natural convection in diode cavities containing an electronic equipment with discrete active bands under constant heat flux

The thermal behavior of airborne electronic equipment submitted to natural convection in closed parallelogrammic air-filled cavities is examined in this study. The cold active wall of the enclosure is maintained isothermal. The hot wall, representing the electronic device, is composed of three parallel discrete bands generating a constant heat flux, separated by two adiabatic bands of equal dimensions. Both walls remain always vertical. The channel is considered adiabatic and the aspect ratio of the cavity is equal to unity. Many configurations are examined while varying the inclination angle of the top and bottom walls of the channel. When the angle is positive the convective heat transfer is favored in comparison with the case of the right cavity, but, on the contrary, it is reduced for negative angles. The resultant enclosures are so called diode cavities in the convective heat transfer sense of the word. The experimental part of the study is achieved with a setup based on electrical data and temperature measurements on the walls. The numerical approach using the finite volume method allows to complete the experimental results with the thermal and dynamical characteristics of the 2D flow. The temperature fields show the thermal behavior of the device during the transient phase after switching it on. The convective results concerning the imposed heat flux treated in this study differ from those corresponding to impose the temperature on the hot bands. The distribution and evolution of the Nusselt number allow to characterize the natural convection occurring in the cavity. The results of this work are consistent with previous studies and allow to predict the thermal behavior of the electronic equipment during the transient phase.     

Alternate rotating walls for thermal chaotic mixing

In this study, we numerically investigate the evolution of two-dimensional mixing and heat transfer enhancement within a two-rod stirring device. The fluid is heated by the walls, which are maintained at a constant temperature. We show by analysis of different stirring protocols that the use of discontinuous wall rotations is necessary to promote heat transfer by chaotic mixing. This condition is also required to avoid hot spots in the vicinity of the walls. The statistics of temperature scalars (mean and standard deviation of dimensionless temperature fields) allow us to determine the influence of geometrical and physical parameters on mixing and heating performance. Thermal strange eigenmodes are revealed during the mixing process by the development of complex recurrent patterns, and the self-similar character of temperature evolutions is confirmed by the probability distribution functions of the rescaled non-dimensional temperature.     

The influence of wall orientation and exterior surface solar absorptivity on time lag and decrement factor in the Greek region

The aim of this study is to determine how time lag and decrement factor are affected by wall orientation and exterior surface solar absorptivity, for specific climatic conditions. Their influence forms a non-sinusoidal periodical forcing function that simulates suitably the outdoor temperature fluctuations. This novel approach, allows the predictability of building's thermal response in an efficient way. The investigation is carried out for various insulated opaque wall formations comprising typical material elements, during the summer period in the mild Greek region. This study that allows proper building planning procedures, at the very early stages of the envelope design, presents great importance. The analysed configurations are assumed to have an orientation that corresponds to each compass point. In addition, the solar absorptivity of surface coatings is assumed to be varying from 0 to 1. The transient thermal analysis is obtained via a thermal circuit that models accurately the fundamental heat transfer mechanisms on both boundaries and through the multi-layered wall configurations. Moreover, the mathematical formulation and solution of this lumped model is achieved in discrete time steps by adopting the non-linear nodal method. The simulation results are focused on the single and combined effects of orientation and solar absorptivity on the dynamic thermal characteristics of various wall configurations.     

A heat transfer model for the analysis of transient heating of the slab in a direct-fired walking beam type reheating furnace

A mathematical heat transfer model for the prediction of heat flux on the slab surface and temperature distribution in the slab has been developed by considering the thermal radiation in the furnace chamber and transient heat conduction governing equations in the slab, respectively. The furnace is modeled as radiating medium with spatially varying temperature and constant absorption coefficient. The steel slabs are moved on the next fixed beam by the walking beam after being heated up through the non-firing, charging, preheating, heating, and soaking zones in the furnace. Radiative heat flux calculated from the radiative heat exchange within the furnace modeled using the FVM by considering the effect of furnace wall, slab, and combustion gases is introduced as the boundary condition of the transient conduction equation of the slab. Heat transfer characteristics and temperature behavior of the slab is investigated by changing such parameters as absorption coefficient and emissivity of the slab. Comparison with the experimental work show that the present heat transfer model works well for the prediction of thermal behavior of the slab in the reheating furnace.     

On the inverse transient heat-transfer problem in structural elements exposed to solar radiation

The inverse transient heat-transfer problem in walls, whether or not exposed to solar radiation, i.e. the estimation of the thermal properties of a wall if the transient temperature and/or heat flow fields are known, is interesting both from the theoretical and practical point of view. An attempt is made to analyse and solve this problem. Methods are developed for estimating the thermal properties of structural elements which are already parts of existing buildings, i.e. under real transient, non-periodic conditions. The finite-difference and experimental examples presented show that the thermal diffusivity, the thermal conductivity and the overall heat transfer coefficient may be estimated with very good accuracy by taking on-site temperature and heat flow measurements.     

Models for transient conduction in a flat plate subjected to a variable heat flux

This work presents analytical models allowing to identify the transient temperature distribution in a flat plate. The plate is exposed to a convective heat transfer on a face and to a heat flux on the other one. The heating flux is Heaviside (crenel type) and is maintained during a t₁ time. The heating phase is followed by a relaxation one. The theoretical method is original because it uses Green’s functions method to determine the analytical solutions of the heat propagation equation in the plate during the heating and relaxation phases. These analytical solutions allow to identify the temperature distribution as well as wall heat flux versus time. The results of our work can be useful at different levels: during the identification of parameters (such as the thermal conductivity or the thermal diffusivity of a plate), during the identification of the boundary conditions (like the heating flux or the convection coefficient) in industrial processes using this kind of systems, or even with educational intents for teaching transient conduction.     

Optimum wall thickness ratio based on the minimization of entropy generation in a viscous flow between parallel plates

The influence of the geometrical and physical parameters on entropy generation for a viscous flow between infinite parallel walls of finite thickness is studied by solving the momentum and energy conservation equations. The conjugate heat transfer problem in the fluid and solid walls is solved analytically using thermal boundary conditions of the third kind at the outer surfaces of the walls and continuity of temperature and heat flux across the fluid–wall interfaces. Analytic solutions for the velocity and temperature fields in the fluid and walls are used to calculate the local and global entropy generation rate. Conditions under which this quantity is minimized are determined for certain suitable combination of geometrical and physical parameters of the system. Special attention has been given to the effect of the wall thickness on the entropy generation rate. It is found that the global entropy generation reaches a minimum for specific values of the wall thickness ratio, when the other parameters are fixed.     

Conjugate natural convection with radiation in an enclosure

Convective-radiative heat transfer in an enclosure having finite thickness heat-conducting walls at local heating at the bottom of the cavity has been numerically studied. Heat exchange with an environment due to convection and radiation has been considered on one of external sides of the decision region. The effect of parameters such as the Grashof number, the transient factor, the optical thickness and the solid wall thermal conductivity both on the local thermo-hydrodynamic characteristics such as streamlines and temperature fields and on the integral parameter like the average Nusselt number on the heat source surface has been analysed.     

Discharge characteristics of latent heat storage columns packed with cross-linked plastic particles

This investigation deals with experimental and numerical analyses carried out to investigate the discharge characteristics of latent heat storage columns, using cross-linked cylindrical plastic particles as a phase-change type of heat storage material and ethylene glycol as a heat transfer medium. In the experiment, the transient response of the outlet temperature of the heat transfer medium was measured under conditions of varying the initial temperature in the column, the inlet temperature of the heat transfer medium, the mass flow rate of the heat transfer medium, and the mass of the heat storage material packed in the column. In the numerical analysis, the transient temperature distributions in the column was calculated by using an empirical formula for estimating the heat transfer coefficient for a fixed bed, which was recommended in the authors' previous paper. The experimental results were found to be in fair agreement with the numerical results. © 1998 Scripta Technica. Heat Trans Jpn Res, 26(3): 193–206, 1997     

Fluid–structure interaction analysis of mixed convection heat transfer in a lid-driven cavity with a flexible bottom wall

A numerical investigation of steady laminar mixed convection heat transfer in a lid driven cavity with a flexible bottom surface is analyzed. A stable thermal stratification configuration was considered by imposing a vertical temperature gradient while the vertical walls were considered to be insulated. In addition, the transport equations were solved using a finite element formulation based on the Galerkin method of weighted residuals. In essence, a fully coupled fluid–structure interaction (FSI) analysis was utilized in this investigation. Moreover, the fluid domain is described by an Arbitrary-Lagrangian–Eulerian (ALE) formulation that is fully coupled to the structure domain. Comparisons of streamlines, isotherms, bottom wall displacement and average Nusselt number were made between rigid and flexible bottom walls. The results of this investigation revealed that the elasticity of the bottom wall surface plays a significant role on the heat transfer enhancement. Furthermore, the contribution of the forced convection heat transfer to that offered by natural convection heat transfer has a profound effect on the behavior of the flexible wall as well as the momentum and energy transport processes within the cavity. This investigation paves the road for future research studies to consider flexible walls when augmentation of heat transfer is sought.     

Transient effects in natural convection cooling of vertical parallel plates

This work presents results from a numerical study of transient natural convection between vertical parallel plates. Two boundary conditions – uniform wall temperature and uniform heat flux – are considered. Results presented include the rate of heat transfer for uniform wall temperature and the maximum wall temperature for uniform heat flux. Also presented are simple correlations to calculate the minimum heat transfer and the maximum wall temperature during the transient period. It is found that for uniform wall temperature the ratio of the minimum heat transfer to the steady state heat transfer decreases with length of the channel, and for uniform heat flux the maximum transient temperature has a maximum of about 9% over the steady state temperature.     

Effect of dynamic load on heat transfer characteristic of a two‐phase pipe boiling flow

An experimental investigation was performed to obtain the flow and heat transfer characteristics of a single‐phase water flow and a two‐phase pipe boiling water flow under dynamic load in the present work. By analyzing the fluid resistance, effective heat, flow pattern, and heat transfer coefficient of the experimental data, the effects of dynamic load on the flow and heat transfer characteristics of single‐phase water and two‐phase boiling water flow were investigated. The results show that the dynamic load significantly influences the flow characteristic and boiling heat transfer of the two‐phase pipe flow. It will enhance the fluid resistance and heat dissipation toward the ambient environment, and reduce the heat transferred to the two‐phase fluid. The impact mixing flow caused by the dynamic load breaks the uniform and varying principle of the wall temperatures. As a result of that, the greater the dynamic load, the lower the wall inner bottom temperature and the higher the wall inner top temperature in a certain extent. © 2011 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley Online Library ( wileyonlinelibrary.com/journal/htj ). DOI 10.1002/htj.20378     

Free Convective Flow in an Open‐Ended Vertical Porous Wavy Channel with a Perfectly Conductive Thin Baffle

The problem of steady two‐dimensional free convective flow of a Walters fluid (model B ′) in a porous medium between a long vertical wavy wall and parallel flat wall in the presence of a heat source is discussed. The channel is divided into two passages by means of a thin, perfectly conductive plane baffle and each stream will have its own pressure gradient and hence the velocity will be individual in each stream. The governing equations of the fluid and the heat transfer have been solved subject to the relevant boundary conditions by assuming that the solution consists of two parts: a mean part and disturbance or perturbed part. Exact solutions are obtained for the mean part and the perturbed part is solved using long wave approximation. Results are presented graphically for the distribution of velocity and temperature fields for varying physical parameters such as Grashof number, wall temperature ratio, porous parameter, heat source/sink parameter, product of non‐dimensional wave number, and space‐coordinate and viscoelastic parameter at different positions of the baffle. The relevant flow and heat transfer characteristics, namely, skin friction and the rate of heat transfer at both walls, has been discussed in detail. © 2013 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley Online Library ( wileyonlinelibrary.com/journal/htj ). DOI 10.1002/htj.21118     

A model of the thermal transient state of a wall of a room during the heating by a heating system

After turning on a room heating system (e.g. central heating) a thermal transient phenomenon takes place on the wall–room system, until it reaches a final thermal equilibrium state. The temperature profiles on the wall cross‐section, starting from an initial profile, corresponding to the initial thermal equilibrium state, come gradually through successive intermediate temperature profiles, to a final temperature profile corresponding to the final thermal equilibrium state. These intermediate, nonlinear and time‐dependent temperature profiles characterize the wall thermal transient state and describe the dynamic thermal behaviour of the wall–room system. The mass of the air in the room is negligible, compared to the mass of the surrounding walls, so the dynamic behaviour of the room–wall system is imposed by the corresponding thermal dynamic behaviour of the walls. The influence of this thermal transient state is important for the room heating behaviour because it acts as a thermal flywheel attenuating and smoothing the room temperature variations. In the present work, using the integral method, analytical expressions yielding the temperature profiles, and the duration of the transient state as a function of thermal and structural characteristics have been developed. Conclusions were drawn on the dynamic thermal behaviour of the room–wall system. Copyright © 2000 John Wiley & Sons, Ltd.     

Unsteady conjugate natural convection in a porous cavity boarded by two vertical finite thickness walls

The objective of this study is to investigate unsteady conjugate natural convection in a porous cavity sandwiched by finite conductive walls considering time-periodic boundary conditions and local thermal non-equilibrium. The top and bottom boundaries are assumed to be isolated and the continuity of temperature and heat transfer are considered in interface boundaries. The effect of varying a plethora of parameters such as Rayleigh number, Thermal conductivity ratio, wall thickness, and non-dimensional frequency on the streamlines, isotherms, and Nusselt number has been studied. It is shown that, apart from non-dimensional frequency and wall thickness, the amplitude of periodic fluid Nusselt number is an increasing function of all aforementioned parameters. Furthermore, aside from Rayleigh number and heat transfer coefficient, the behavior of the solid Nusselt number is the same as fluid Nusselt number. Eventually, the time-averaged Nusselt number and heat transfer through the vertical walls for different values of non-dimensional frequencies are calculated.     

Experimental and numerical study of unsteady non-periodic wall heat transfer under step,ramp and cosine temperature perturbations

An experimental and numerical study of the transient non-periodic wall heat transfer problem is presented. A computer-controlled indoor/outdoor environment simulation system produces any desired variation of the air temperature, thus allowing measurement of the dynamic thermal behaviour of any test wall under the desired boundary conditions. Measurements of the temperature field within the wall, of the heat flow and of the convection coefficients at the wall surfaces are performed during step, ramp and cosine perturbations of the outdoor air temperature. The measurements are in very good agreement with the numerical predictions obtained by a developed finite difference solution procedure. The results showed that in building heat transfer applications, for example in air conditioning, the usual assumption of periodic outdoor conditions may lead to considerable errors in case of a significant temporary deviation of the temperature from periodicity.     

Comparison of Mixed Convection in a Square Cavity with an Oscillating versus a Constant Velocity Wall

This article presents a numerical investigation of unsteady laminar mixed convection heat transfer in a two-dimensional square cavity. The cavity is configured such that one of the vertical walls is cooled and slides either with a constant speed or with a sinusoidal oscillation. A portion of the opposite stationery wall is heated by a constant temperature heat source while, the remaining walls of the cavity are thermally insulated. Different configurations of sliding wall movement and a series of Richardson numbers and Strouhal numbers are tested. The results indicate that the direction and magnitude of the sliding wall velocity affect the heat transfer rate. At low Richardson numbers, the average heat transfer rate for the cavity with an oscillating wall is found to be lower compared to that for the cavity with a constant velocity wall. In addition, at a fixed Richardson number, as the Strouhal number decreases the oscillation frequency of average Nusselt number on the vertical walls decreases; however, the oscillation amplitude of average Nusselt number increases.     

Studying the effect of the force convection boundary condition and radius magnetic field on fluid flow and heat transfer between coaxial pipes

An investigation of the heat transfer of Newtonian fluid flow through coaxial two pipes with variable radius ratio has been conducted with the boundary conditions of forced convection on the inner pipe walls and a radius magnetic field. This paper presents an exact analytical solution to the momentum equation and a novel semi-analytic collocation method for solving the full-term energy equation that takes Joule heating into account as well as viscous dissipation. Based on the results of the numerical fourth-order Runge–Kutta method, it was found that increasing the magnetic parameter decreased the amount of friction on the surface of the pipe walls and the rate of heat transfer. As the radius ratio of the two pipes increases, so does the skin friction and heat transfer rate on the internal pipe walls. As Eckert (Ec) and Prandtl (Pr) numbers increase, the mean temperature as well as the dimensionless temperature between the two pipes increases. The increase in Biot number (Bi) has the opposite impact on the mean temperature. As Ec, Pr, and Bi increase, so does the rate of heat transfer on the inner wall of the pipe.     

Transient natural convection in enclosures filled with humid air,including wall evaporation and condensation

Transient natural convection combined heat and mass transfer in enclosures filled with humid air, including evaporation from or condensation to the walls, which are subjected to time varying prescribed temperatures, is studied numerically. Emphasis is given to the two-dimensional enclosures of circular cross-section, emulating horizontal containers or ducts filled with humid air. Starting from uniform temperature and concentration distributions, wall temperature decrease with time leads to some water condensation at the walls, and wall temperature increase with time leads to evaporation of some liquid water from the walls if it exists there. During the final period of both situations temperature ceases changing, and a final steady-state regime is reached. Situation of prescribed time-periodic wall temperature is also considered, and a periodic solution is obtained after some few periods of operation. Results reveal the flow structure and also the temperature and concentration time evolutions in the enclosure. Special attention is given to the time evolution of the overall Nusselt and Sherwood numbers over the walls, and also to the time and space evolutions of the condensate layer over the walls.     

小型直喷柴油机传热过程的研究

本文在实测１９０Ａ直喷柴油机缸盖表面瞬态温度的基础上，对该柴油机缸内传热过程进行了分析。利用有限元数值计算方法，针对改变结构设计和冷却方式的不同情况，进行了活塞温度场模拟计算，通过对不同隔热方案和隔热机理进行了研究，为减少该柴油机散热损失提供了理论基础和可行方案。