Heat transfer in the turbulent flow through a conduit for removal of combustion products

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Abstract

A numerical model was presented to calculate the velocity and thermal fields of turbulent flow through a conduit for removal of combustion products. This study originated in response to an industrial concern to study dynamic behaviour of a smoke conduit connected to a high-efficiency gas boiler working in permanent running mode (steady state), or cyclical non-static running mode (unsteady state). The momentum and temperature fields were calculated via a finite-volume CFD code using the kε turbulence model. The validation of this calculation was conducted employing a full-scale experimentation. Comparisons of temperature fields were made, and the model was shown to give an acceptable quantitative approach for design within the framework of the coarse approximations adopted.

Introduction

This study originated as an industrial concern of domestic furnace systems using relatively modest power generators (from 10 to 50kW). The developments of new techniques for heating buildings systems (high-performance generators, new burners with modulated functioning, automated management of the systems) allow to design henceforth installations with high thermal efficiency. However, such installations require above all energy recuperation and consequently lower temperatures of removal combustion products. This causes smoke condensation over the wall of the conduit. The condensate is often loaded with acid components contributing to physico-chemical aggressions on the materials [1]. Some problems have been observed in systems in use. Designers put into question the durability of smoke conduits and they are aware of detailed information on the development of this phenomenon. Note that similar situations may also occur in other industrial applications, e.g., exhaust of removal combustion products of motor cars.

The French Scientific and Technical Centre for Building (CSTB)2 has initiated a detailed study in order to establish a classification of the available materials and to prevent the aggressive action of condensate. National and international normalisation must take this phenomenon into account for different type of conduits and different combustibles and burners. Theoretical and experimental approaches have been undertaken in order to fulfil the industrial needs. The problem is complex as it involves coupled physical phenomena: turbulence, unsteady-state, two-phase medium, phase change at different temperatures of saturation of vapour of water and different acids (sulphuric, hydrochloric, nitric, ...etc.). This paper is a first step of the program. Smoke is considered as a single-phase gas, i.e., one neglects the heat and mass transfer by condensation over the wall [2]. The comparison of the predictions of this simplified model with the experimental data allows us to assess the validity of this hypothesis. The aim of this paper is to develop a numerical simulation to study the thermo-convective behaviour of a smoke conduit. It allows the calculation of the thermal field flows and the heat transfer to the wall of the conduit. The simulation takes into account the different parameters: ambient temperature, conduit material properties, dimensions and power of the boiler, etc. In the first part of this paper, a commercially available CFD code [3] is used to predict the temperature field on the basis of an approximate mean velocity field at the entrance of the straight part of the tube. As in practical applications, the boiler works intermittently, the computations are conducted in two cases. The first one is the steady mode where the boiler starts from rest and operates continuously. The second one is the cyclic operating mode of the boiler (referred hereafter as unsteady mode). This case is closer to the actual operating conditions of the system. To validate the computation, an experimental assembly has been developed on the basis of a domestic boiler, working with natural gas connected to a transparent glass conduit. Measurements of temperature distributions are made. In the second part of the study, the experimental data are presented and compared to the model results.

Section snippets

Formulation of the problem

The aim of the numerical model is to predict the gas flow, thermal field and wall temperature in the conduit. This requires the solution of steady- and unsteady-state equations for both flow situations. The physical contributions of conduction in the wall and mixed convection inside the conduit are described by these equations. To simplify the study, the following assumptions are adopted: (i) smoke is assumed to be a part of dry air (Newtonian fluid), (ii) the typical Reynolds number at the

Experimental set-up

This study was carried out in a full-scale experiment (Fig. 1). A glass conduit is connected to a high-efficiency gas boiler operating at low power (23kW). The dimensions of the conduit are 9.20-m height, 0.13-m inner diameter and 5-mm thickness of the wall. The comparison with numerical results is made in a domain of 8-m height.

The boiler works either in continuous established mode (steady state), or in cyclic running mode (unsteady state). The latter case was obtained using a programmed

Measurement uncertainties

Different temperature fields (Fig. 3) were measured (smoke, interior and exterior wall, ambience), in four sections situated at 1, 2, 4 and 8m from the bottom of the conduit.

The measurements of temperatures were difficult to implement owing to the humid and corrosive environment, to which the different sensors were exposed. We used thermocouples type k imbedded in the wall or placed on prongs inside the conduit. An appropriate envelope face to the acid corrosion protected the sensors.

These

Conclusion

The objective of this work is to develop and validate a simplified model for describing and predicting the thermal heat transfer in a smoke conduit. The work includes both established steady regime of operation, which serves as a reference basis, and the cyclic regime. The later is closer to the actual runs of the burner in its daily operation.

The good agreement between computed and measured temperature leads to the conclusion that this numerical model is able to predict qualitatively the

Acknowledgements

The authors thank the Centre Scientifique et Technique du Batiment (CSTB) in Marne La Vallée (France), the Direction of research of Gaz de France and Ugine (division d'Usinor Sacilor) for their technical and financial support. They are also greatly indebted to Mr. Jacques Chandelier who initiated and directed this work.

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    Citation Excerpt :

    Also, the agreement between theoretical and numerical results has been investigated. A numerical simulation has been used to study the thermo-convective behaviour of a smoke conduit [4]. Computational fluid dynamics (CFD) is a branch of fluid mechanics in which problems involving fluid flows are solved using computer simulations.

1

Formerly at Centre Scientifique et Technique du Bâtiment, France (CSTB) where the work has been achieved.

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