Page 18 - Design and Operation of Heat Exchangers and their Networks

Heat exchangers and their networks: A state-of-the-art survey 5

exchanger with constant overall heat transfer coefficient and fluid properties,
if we take the whole exchanger as one cell, the results are correct. However,
if we divide the exchanger into only a few cells, the calculated outlet tem-
peratures are approximated. The deviation can be reduced by increasing the
number of cells, similar to the finite-volume method.
The finite-volume method has been more commonly used for the
numerical simulation of steady-state heat transfer performance and dynamic
temperature response. In the finite-volume method, one-dimensional flow
is assumed. The mass and momentum balance equations of the fluids and
energy balance equations of the fluids and solid materials are solved numer-
ically by means of the finite-difference method or finite-volume method. As
an example, Pavkovic and Vilicic (2001) used a dynamic numerical model to
simulate a compression vapor cycle heat pump. In the finite-volume
approach, the discretized mass and energy conservation equations for an
evaporator or condenser are determined from mass and energy balances
for control volumes of the evaporator or condenser. Differing from the cell
method, the finite-volume method does not need to take each control vol-
ume as a small heat exchanger unit. The relationship between the inlet and
outlet parameters of the control volume is set by applying a conservation
scheme of the finite-difference method.
For dynamic two-phase flows in heat exchangers, that is, evaporators and
condensers, the moving boundary method (Grald and MacArthur, 1992;
Jensen and Tummescheit, 2002) would be a suitable model, which is numer-
ically fast compared with discretized models and very robust to sudden
changes in the boundary conditions, and therefore, it is suitable for the
design, testing, and validation of advanced control schemes for evaporators
and condensers. Bonilla et al. (2015) presented a mathematical formulation
based on physical principles for two-phase flow moving boundary evapora-
tor and condenser models that support dynamic switching between the flow
configurations, for example, disappearance of an existing superheated vapor
region or appearance of a new subcooled liquid region. Kim et al. (2017)
developed a more reliable moving boundary approach to treat temperature
glide for a binary mixture. Recently, Chu and Zhang (2019) presented a
coupling algorithm that combines the advantages of moving boundary
method and finite-volume method, in which the moving boundary cells
(cells for single-phase and two-phase regions) are further divided into several
finite-volume cells.
A comparison of the finite-volume and the moving boundary
approaches to simulate the dynamics behavior of an evaporator was given