Dynamic Simulation of Heat Exchanger with Multicomponent Phase Change

2017

A dynamic model for a heat exchanger with multicomponent phase change, part of

reliquefaction cycle of natural gas, is developed. The finite control volume method is

used to spatially discretize the heat exchange intro a series of lumps with constant volume.

Vapor-liquid equilibrium is assumed in the vapor-liquid region of the phase envelope.

The model is written in terms of differential and algebraic equations applied to

each lump (or cell).

Different algebraic equations are valid in each of the phase regions (e.g. vapor, liquid,

vapor-liquid), namely the vapor-liquid equilibrium condition is not satisfied in either

of the single phases. Therefore, each phase region has its own set of differential and

algebraic equations. The number required to describe the two-phase region is higher

compared to the single regions. Hence, dummy variables and equations (without a

physical meaning) are used in the single regions to in order to have the same number

of equations in all phases such that the same model can be used for simulating all

phase regions. A logical conditions is implemented to select the corresponding set of

equations.

The model is written and implemented in Matlab® for simulation purposes. The

phase change detection is automatically handled by an event function inside the solver.

A few additional examples are used to investigate how the ode15s solver treats nonsmooth

systems, or how the algebraic equations are solved.

The possibility of formulating the model as a mathematical problem with complementarity

constraints is also investigated.

NTNU