Simulation of Convective Heat-Transfer Coefficient in a Buried Exchanger

Abstract

This study presents analytical models allowing to study a forced convection laminar flow in non-established dynamic and thermic regimes. We treated a flow in a bitubular exchanger in permanent thermal contact with a semi-infinite medium, such as the ground. The wall temperature as well as the wall heat flux evolve in the course of time until a quasi-steady mode. The theoretical method is original because it uses Green's functions method to determine the analytical solutions of the heat propagation equation on the wall during the heating phase. These analytical solutions allow to identify the temperature distribution versus time. The complexity of the system geometry as well as the infinity of the medium surrounding the exchanger make the traditional methods of numerical resolution unable to solve the problem. We used, to solve it, the finite volume method coupled with the finite element method at the boundary. We studied the effect of Reynolds number, the fluid entry temperature and the transfer duration on the axial evolution of the heat transfer coefficient. We illustrated also the profile of the temperature field in the fluid medium

FVM-BEM method based on the Green's function theory for the heat transfer problem in buried co-axial exchanger

“…It can be noted that the local heat transfer coefficient is independent of the heating duration for a weak temperature variation where ρ f remains constant. It is constant and preserves the same value practically during the unsteady mode, what is confirmed by the previous correlations presented in the literature [6,7,[13][14][15]. The axial evolution of the local heat transfer coefficient h(z, Re) is given for R i /r ex = 2, L/D H = 50, and various values of Re (Fig.…”

“…The studies that relate to the non-established flow in an annular conduct subjected to a time-dependent heat flux, through the walls, are scarce [1][2][3][4][5][6][7][8]. This type of flow can be found at the inlet of the heat exchangers.…”

“…However, it requires the use of several parameters such as the local heat transfer coefficient, which is difficult to be precisely measured and quantified. Several experimental and numerical studies were carried out [5][6][7][8][9][10][11][12][13][14][15], establishing correlations or empirical expressions of the convective heat transfer coefficient.…”