Numerical predicting the dynamic behavior of heat exchangers for small-scale Organic Rankine Cycle
Liuchen Liu, Yu Pan, Tong Zhu, Naiping Gao
Session: Poster session & Welcome drinks
Session starts: Wednesday 13 September, 17:30
Liuchen Liu ()
Yu Pan ()
Tong Zhu ()
Naiping Gao ()
Organic Rankine Cycle (ORC) system is the most widely used technique for low-grade waste heat recovery. Its two main advantages are the simplicity and the availability of its components. A typical ORC is formed by assembling heat exchangers (i.e. evaporator, condenser), flow inducing unit (pump), and power extraction unit (expander).
ORC’s steady state behavior has been studied by many researchers since more than 20 years ago. Recently, developing dynamic ORC models played an increasingly important part in the system performance prediction. From the point of view of dynamic simulation, critical components of an ORC system are the heat exchangers since they are the principal media of heat transfer in and out of the unit respectively. And dynamic models for the turbo-machines (pump and expander) are avoided due to their negligible heat transfer irreversibility compared to their mechanical interaction and their relative faster response time to the heat exchangers.
In this paper, moving boundary model and discretized model are used for describing the two-phase flow model of the heat exchangers. The moving boundary (MB) models are low order lumped models particular useful for optimization and control purposes. In this model, the inside part is divided into three parts: sub-cooled liquid region, two phase region and superheated vapour region. On the other side, discretized models based on the balance equations of mass, momentum and energy form an alternative to MB-models when the spatial changes are important. An advantage using discretized models is the possibility of using high accuracy correlations for heat transfer and pressure loss taking the spatial variations into account.
To perform a detailed and realistic design, two concrete configurations of shell-and-tube heat exchanger (condenser) and tube-fin heat exchanger (evaporator) are taken into account. Moreover, steady-state model is established for the expander and working fluid pump. The simulation results of the established model must be compared to each other and are used to find the optimal operating parameters that maximize the waste heat recovery efficiency in response to variations of the boundary conditions such as ambient temperatures and waste heat source temperatures.