Development of a Simulation and Optimization Tool for Heat Exchanger Design

Abstract

Heat exchangers have been used extensively and play an important role in the capital cost, energy efficiency and physical size of refrigeration and air conditioning systems. In this dissertation, a simulation and optimization tool to improve effectiveness and efficiency in design, rating, and analysis of air to refrigerant heat exchangers including conventional finned tube coils and emerging microchannel heat exchangers, Coil Designer, is developed and investigated using a general-purpose modeling concept and user-friendly interface. It is applicable to design of condensers, evaporators, and heating and cooling coils under any operating conditions. A network viewpoint was adopted to establish the general-purpose model and allow for analysis of arbitrary tube circuitry and mal-distribution of fluid flow inside the tubes. Comprehensive evaluation of solutions to the highly nonlinear system of equations in the local thermal/hydraulic performance within the tube network was conducted and a new solution method to successively approximate the physics of heat and fluid flow was developed to enhance the solution convergence capability. A segment-by-segment approach within each tube was implemented, to account for two-dimensional non-uniformity of air distribution across the exchanger, and heterogeneous refrigerant flow patterns through a tube. A further sub-dividable-segment model was created in order to address the significant change of properties and heat transfer coefficients in the single-phase and two-phase regime when a segment experiences flow regime change. The effectiveness-NTU method for cross-flow configuration was used also for combined heat and mass transfer problem under dehumidification, by defining equivalent thermal resistance and heat capacity. Object-oriented programming techniques were applied in developing Coil Designer to facilitate flexible and customizable design platform and building graphic user-friendly interface. Coupled heat exchangers with multiple fluids inside different subsets of tubes can be modeled and analyzed simultaneously. A wide variety of working fluids and correlations of heat transfer and pressure drop are available at the user’s choice. The tabular and graphic representation of performance simulation results provides convenience in comprehensive and detailed parametric analysis. The model prediction with Coil Designer was verified against experimentally determined data collected from a number of sources. The simulation tool was shown to be able to predict the heat transfer rate for a variety of coils with good accuracy. Parametric studies were conducted to confirm the capability of the program in exploring all aspects of heat exchanger performance under a wide variation of design and operating conditions. A genetic algorithm is introduced and integrated with the simulation tool for single and multi-objective optimization design of heat exchanger to accomplish the following goals quickly and accurately: achieve optimum circuitry selection, minimize volume, minimize the amount of material utilized in the coil and thus minimize overall cost of the coil while achieving the best possible performance.