In the case of describing a complex technological processes, considering some generalization, the whole technological system can be handled as the resultant of elementary processes in elementary units. This type of segmentation of the system makes it possible to obtain knowledge about the relatively simple elementary units and makes it easier to analyse them. During the construction of the whole network from these elementary units there are some continuity and conservation rules, namely the conservation of mass, energy and momentum.

The overall goal of this modeling is the development of a dynamical model in order to carry out a successful optimization and a control strategy adapting to the consumer's requirements. The distribution of mass and energy streams is determined by the space and time variation of the state variables of the controlled system, according to thermal declination, the actual consumption of the consumers.

The elementary segment of the network is a pipe-segment between two automatically working condensers. It can be assumed that at the automatic condensers the flowing wet steam gets clear of the condensate flowing on the bottom of the horizontal pipe and becomes saturated steam with quality , where M is the mass and the subscript refers to the corresponding phases (vapor and fluid). For the case when the steam flowing to the elementary segment is superheated, according to the heat loss through the insulated pipe-wall, the gradually decreasing temperature reaches the saturation point at a certain point in the pipe and the condensation begins [1,2,3].

In the case of flowing steam or gas the state of the flow is usually described by two state-parameters, mass-flow and pressure. The dynamics of the mass-stream network can provide information about the impact of the pressure-change at one point of the network on the mass-stream and pressure at other points. In regular steam distribution systems under normal operational conditions the flow is turbulent.

During the modeling of this transport phenomena, the aim of the paper is to focus on the development of the annular flow regime due to the gradual steam condensation on the pipe wall. Thus it is assumed that the two-phase flow can be treated as a hypothetical single-phase flow having volumetric averages of the material properties. In the case of the developed annular flow the separated flow model is recommended instead of the homogenous flow model according to Martinelli and Nelson [4].

For the numerical solution of the established model the finite element method (FEM) has been applied. The governing equations of the phenomenon are the Navier-Stokes equations with the closure and the conservation of heat energy through diffusion and conduction [5,6].

Since the measurement-based identification of the pipe work system under varying operational conditions is very difficult with any of the technologies currently available, the simulation results of the model helps the estimation of the parameters of the network model. These simulation results are applied for the prediction of the condensate loads and the heat transfer coefficients for the larger model of the energy-stream network in order to achieve a consistent energetic and hydraulic modeling and simulation of the whole network.

The major tasks in the future study of the recent problem consist of various parameter studies based on experiments and simulations, building the network from elementary units and performing simulations under various operating conditions, and adapting a geographic information system to the model in order to make the simulations interactive and the results easily interpretable. This will be followed by the design of an energy saving control strategy based on the evaluation of the simulation results.

1

J. El Hajal, J. R. Thome, A. Caballini: Condensation in horizontal tubes,part1: two-phase flow pattern map, International Journal of Heat and Mass Transfer 46. 3349-3363, 2003. doi:10.1016/S0017-9310(03)00139-X

2

J. R. Thome, J. El Hajal, A. Caballini: Condensation in horizontal tubes,part2: new heat transfer model based on flow regimes, International Journal of Heat and Mass Transfer 46. 3365-3387, 2003. doi:10.1016/S0017-9310(03)00140-6

3

A. J. White: Numerical investigation of condensing steam flow in boundary layers, Int.J. of Heat and Fluid Flow 21, 727-734, 2000. doi:10.1016/S0142-727X(00)00030-8

4

R. C. Martinelli, D. B. Nelson, Prediction of pressure drop during forced circulation boiling of water, Transaction of ASME, 70(6), 695-702, 1948.

5

A. Schaffarth et.al.: Modeling of condensation in horizontal tubes, Nuclear Engineering and Design 204. 251-265, 2001. doi:10.1016/S0029-5493(00)00329-0

6

A. G. Gerber: A pressure based Eulerian-Eulerian multi-phase model for non-equilibrium condensation in transonic steam flow, International Journal of Heat and Mass Transfer 47, 2004. doi:10.1016/j.ijheatmasstransfer.2003.11.017

purchase the full-text of this paper (price £20)

go to the previous paper
go to the next paper
return to the table of contents
return to the book description
purchase this book (price £105 +P&P)