Channel-chimney System Research Articles

Effect of surface radiation on natural convection in an asymmetrically heated channel-chimney system

In this paper, a more realistic numerical approach that takes into account the effect of surface radiation on the laminar air flow induced by natural convection in a channel-chimney system asymmetrically heated at uniform heat flux is used. The aim is to enrich the results given in Nasri et al. (Int J Therm Sci 90:122–134, 2015) by varying all the geometric parameters of the system and by taking into account the effect of surface radiation on the flows. The numerical results are first validated against experimental and numerical data available in the literature. The computations have allowed the determination of optimal configurations that maximize the mass flow rate and the convective heat transfer and minimize the heated wall temperatures. The analysis of the temperature fields with the streamlines and the pressure fields has helped to explain the effects of surface radiation and of the different thermo-geometrical parameters on the system performances to improve the mass flow rate and the heat transfer with respect to the simple channel. It is shown that the thermal performance of the channel-chimney system in terms of lower heated wall temperatures is little affected by the surface radiation. At the end, simple correlation equations have been proposed for quickly and easily predict the optimal configurations as well as the corresponding enhancement rates of the induced mass flow rate and the convective heat transfer.

English

The flow field in the hyperbolic natural draft wet-cooling tower, which has great effects on the economy and security of power plant, was studied through numerical simulation. The mathematical model was established and analysed in order to optimize the cooling-tower and to evaluate its efficiency and a Numerical study on a channelchimney system was carried out in order to elucidate the behaviour of heat transfer and fluid flow. A 2D CFD model of natural draught Cooling tower and chimney like heat sink plate is developed the Euler multiphase model was used to simulate the heat and mass transfer in a natural draught cooling tower rain zone and the RNG turbulent model with the dispersed option is used for the turbulence modeling of multiphase flow. The results are presented in terms of local air temperature measurements inside the symmetrically heated channel and between the adiabatic extensions.

Natural convection enhancement in an asymmetrically heated channel-chimney system

In this paper, a numerical study is performed in order to analyze the effect of adding a chimney to a vertical open channel. The channel is heated asymmetrically at uniform heat flux while the chimney is symmetric and wider than the channel. The thermal and dynamic aspects of the channel-chimney system (T chimney) are studied by varying the width and the height of the chimney while the aspect ratio of the channel is kept fixed. The main objective of this work is to determine the optimal geometric parameters of the chimney: the expansion ratio B (chimney width normalized by the channel width) and the extension ratio Er (chimney height normalized by the channel height), that maximize the mass flow rate (G) and the average Nusselt number (Nua). More than four hundred numerical simulations have been carried out at modified Rayleigh numbers ranging from 102 to 5 × 104 (laminar regime). The computations allowed the identification of three types of system responses. The flow structure and the pressure field were also analyzed to elucidate why the increase of the chimney width can improve or deteriorate the mass flow rate and the heat transfer. Finally, appropriate correlations have been proposed for determining the optimal configurations and the corresponding enhancement of the mass flow rate and the heat transfer coefficient.

Numerical investigation of transient natural convection in a vertical channel-chimney system symmetrically heated at uniform heat flux

In the present numerical investigation, a transient numerical analysis for natural convection in air, between two vertical parallel plates (channel), heated at uniform heat flux, with adiabatic parallel plates downstream (chimney), is carried out by means of the finite volume method. The analyzed transient problem is two-dimensional and laminar. The computational domain is made up of the channel-chimney system, and two reservoirs, placed upstream the channel and downstream the chimney. The reservoirs are important because they simulate the thermal and fluid dynamic behaviors far away from the inflow and outflow regions. Results are presented in terms of wall temperature and air velocity profiles. They are given at different Rayleigh number and expansion ratios (chimney gap/channel gap) for a fixed channel aspect ratio (channel height/channel gap) equal to 10 and extension ratio (channel-chimney height/channel height) equal to 2.0. Wall temperature profiles over a period show the presence of overshoots and undershoots. The comparison among the maximum wall temperatures shows that the simple channel is the most critical configuration at steady state condition, but it is the best configuration during the transient heating at the first overshoot. As indicated by the temperature profiles, average Nusselt number profiles over a period of consideration show minimum and maximum values and oscillations before the steady state. Stream function fields allow to observe the development of fluid dynamic structures inside the channel-chimney system, particularly how and when the cold inflow is present and develops.

Thermal management of a symmetrically heated channel–chimney system

A parametric analysis of natural convection in air, in a channel–chimney system, symmetrically heated at uniform heat flux, obtained by means of a numerical simulation, is carried out. The analyzed regime is two-dimensional, laminar and steady-state. Results are presented in terms of wall temperature profiles in order to show the more thermally convenient configurations which correspond to the channel–chimney system with the lowest maximum wall temperature. For the considered Rayleigh number, the difference between the highest and the lowest maximum wall temperatures increases with increasing the channel aspect ratio. The optimal expansion ratio values depend strongly on the Rayleigh number and extension ratio values and slightly on the channel aspect ratio. Correlations for dimensionless mass flow rate, maximum wall temperature and average Nusselt number, in terms of Rayleigh number and dimensionless geometric parameters are presented in the ranges: 5 ⩽ Ra ∗ ⩽ 2.0 × 10 4 , 1.5 ⩽ L / L h ⩽ 4.0 and 1.0 ⩽ B / b ⩽ 4.0 .

NUMERICAL STUDY OF NATURAL CONVECTION IN VERTICAL CHANNELS WITH ADIABATIC EXTENSIONS DOWNSTREAM

ABSTRACT Recent trends in natural-convection research are the finding of new configurations to improve heat transfer parameters or the analysis of standard configurations to determine optimal geometric parameters in order to achieve a better heat transfer rate. In this article a numerical simulation of natural convection in air in a channel–chimney system heated symmetrically at uniform heat flux is carried out. The regime analyzed is two-dimensional, laminar, and steady-state. Wall temperature profiles, air velocity and temperature profiles, and centerline pressure profiles are presented. A comparison with experimental data given in [4] is accomplished and some differences are observed, but thermal and dynamic behavior for different expansion ratios are the same. Results analysis explains how and why the “chimney effect” worsens. The effect is connected to the cold inflow at the outlet section, and this effect is more marked at higher Rayleigh number, Ra. Consequently, optimal thermal configuration for assigned extension ratio presents a larger value of expansion ratio at the lower Ra. values.

Experimental Investigation of Natural Convection in an Asymmetrically Heated Vertical Channel with an Asymmetric Chimney

An experimental investigation on air natural convection, in a vertical channel asymmetrically heated at uniform heat flux, with downstream unheated parallel extensions, is carried out. One extension is coplanar to the unheated channel wall and the distance between the extensions is equal to or greater than the channel gap (geometrically asymmetric chimney). Experiments are performed with different values of the wall heat flux, aspect ratio (Lh∕b), extension ratio (L∕Lh) and expansion ratio (B∕b). For the largest value of the aspect ratio (Lh∕b=10), the adiabatic extensions improve the thermal performance in terms of lower maximum wall temperature of the channel. Optimal configurations of the system with asymmetrical chimney are detected. Flow visualization shows a cold inflow in the channel-chimney system that penetrates down below the channel exit section. Maximum wall temperatures and channel Nusselt numbers are correlated to the channel Rayleigh number, Ra*, and to the geometrical parameters, in the ranges 3.0×102⩽Ra*B∕b⩽1.0105, 1.0⩽B∕b⩽3.0 and 1.0⩽L∕Lh⩽4.0 with Lh∕b=5.0 and 10.0.

Thermal design of symmetrically and asymmetrically heated channel–chimney systems in natural convection

In this paper, design charts for the evaluation of thermal parameters for natural convection with air in a channel–chimney system are proposed. In the thermal analysis of natural convection in channel–chimney systems, the variables that play an important role are: the ohmic heat flux, maximum wall temperatures and geometrical parameters such as the height of the heated channel, the channel spacing and the height and spacing of unheated extensions. A simple numerical procedure to obtain the thermal design charts, a thermal optimization of the system and an uncertainty analysis due to the thermophysical properties are presented. Results are carried out for symmetrically and asymmetrically heated channels with walls at uniform heat flux and a simple estimation procedure is proposed to evaluate the error in the relevant geometrical and thermal parameters due to a different value of the reference temperature. The estimated error, however, is less than the uncertainty of the experimental data employed. Some simple examples are given to show the use of the charts. The proposed results are obtained from experimental data in the following dimensionless parameter ranges: 5.0⩽ L h/ b⩽20; 1.5⩽ L/ L h⩽4; 1⩽ B/ b⩽4; 10 2⩽ Ra⩽10 6.

Heat and fluid flow resulting from the chimney effect in a symmetrically heated vertical channel with adiabatic extensions

An experimental study on a channel-chimney system was carried out in order to elucidate the behavior of heat transfer and fluid flow. The results are presented in terms of local air temperature measurements inside the symmetrically heated channel and between the adiabatic extensions. Different fluid motion regions are observed inside the chimney. Inflows of air are detected in the lower extension ratio, particularly for large values of the ratio of the width of chimney to that of the heated channel. Some typical configurations show the presence of a vortex structure for an expansion ratio greater than one close to the corner regions in the chimney. Some monomial correlation equations between the local Nusselt number, the channel Rayleigh number and the geometric parameters are proposed. The dimensionless parameters are in the following ranges: 10 2⩽Ra ∗(B/b)⩽10 6; 1.5⩽L/L h⩽4.0; 1.0⩽B/b⩽4.0 , in which L is the total height of the system, L h is the height of the heated channel, B is the width of the chimney and b is the width of the heated channel. A good agreement between the correlation and the experimental data is observed.