Development Of A Tutorial Software To Serve As A Teaching Aid For Power And Refrigeration Cycles

The paper presents the development and application of a computer based tutorial software to aid instruction and improve problem solving skills in undergraduate “Thermal Systems and Economics” course offered by the Department of Mechanical Engineering. This course is a required design course in the Mechanical Engineering curriculum and an approved technical elective in the Chemical Engineering curriculum. It is offered during both fall and spring semesters with a class size of about 40 students. The expert tutor serves as a concise data base for key concepts learned in the course, and houses property tables and basic equations to interactively solve problems. The benefits of using this computer based instructional aid include: enhanced use of multimedia course materials, more creative thinking exercises for students, reduced time to master new concepts, and coverage of more materials in the course. The application of computer technology to facilitate interactive learning greatly enhances the instruction process. The concept of an expert tutor can be extended virtually to any engineering or science course at the undergraduate level. INTRODUCTION Thermodynamics is a core course in engineering curriculum throughout the nation. All engineering students are required to take the Basic Thermodynamics (usually known as Thermodynamics I) irrespective of their major discipline. Students majoring in Mechanical Engineering are required to take the Applied Thermodynamics (usually known as Thermodynamics II or Thermal Systems) where they apply the principles of thermodynamics to design power plants, reciprocating internal combustion engines, gas turbines and aircraft engines, and refrigeration and air-conditioning machinery. The design exercises usually require repeated calculations using properties of the working fluid (or fluids). The fluid properties are usually provided in a tabulated form and listed as a function of temperature and pressure. Equations correlating the properties are available only for simple substances such as an ideal gas. Calculation of fluid properties for different thermodynamic states usually sum up to a major portion of time needed to solve any problem. Hand calculations are tedious because of interpolation of tabulated data. Therefore, there is a great need for a computer-based property data bank where once a thermodynamic state has been defined by two independent properties, all remaining properties of the state can be readily obtained. In addition, if a student can set up a problem interactively in the computer and can execute the solution steps in an interactive fashion without tedious hand calculations, that definitely increases productivity on the part of the student as well as the instructor. P ge 306.1 The use of computer for the instruction of thermodynamics is not entirely new. In fact most popular textbooks in this area have software diskettes that are sold with the book. Moran and Shapiro 1 uses a software package called Interactive Thermodynamics (IT). It can be used to input the model equations and solve them using a built-in solver. It has thermodynamic property data toolpads and process viewpads that allow the user to develop models of various thermodynamic systems. Black and Hartley 2 offered a software developed by Harper Collins College Software, in which the thermodynamic properties can be obtained for different fluids. Van Wylen et al. 3 offered a software called CATT. The software houses the thermodynamic properties of different substances. It has the graphical capability to visually illustrate the location of the thermodynamic state. Borgnakke and Sonntag 4 offered a second version of this software, called CATT-2, which is more user-friendly and allows pull down menus that permit to move among frames. Cengel and Boles 5 integrated EES (Engineering Equation Solver), which was independently developed to extract thermodynamic properties using equations of state and a non-linear equations solver to solve the process equations. EES has a good plotting package where graphs can be generated from the computed results. There has also been attempts to use commercially available mathematical equation solvers such as MATHCAD or TK Solver for solving thermodynamic problems. Potter and Somerton 6 introduced the use of MATHCAD to model and solve thermodynamics problems. The use of TK Solver has been demonstrated in books by Burghardt and Harbach 7 and Jones and Dugan 8 . Even though the use of computer has been encouraged, a rigorous treatment of computer aided instruction has not been attempted in any work except for Potter and Somerton 6. Particularly, a complete package where a software can augment the instruction by presenting in parallel the key concepts learned in the course and the application of those in an user friendly problem solution module was not available and prompted the developed of the present tutorial software. The software developed here is not only for the calculation of thermodynamic properties, but a complete tutorial package designed to give the students with lecture highlights and additional multimedia course materials that are not available in the textbook, interactive examples where students can become familiar with how to set up and solve a problem, and a solver frame where they can develop their own model. The property manipulation has been made essentially fool proof where warning messages are displayed if certain combinations of data are inconsistent. The software currently contains data for water, R-134a, ammonia, and air. The software has been written using MICROSOFT VISUAL BASIC 3.0 and is compatible with other windows based applications. PROGRAM STRUCTURE The software has a modular structure. The WELCOME SCREEN connects to any of the following four modules: (1) LECTURES, (2) FLUIDS, (3) EXAMPLES, and (4) P ge 306.2 SOLVER. The LECTURE module houses a number of modules containing main course topics and sub-topics as illustrated in Figure 1. Each of these instructional modules were developed using study materials from the text and other reference books. The size of these modules were optimized to provide a clear picture of the subject matter without incorporating excessive written materials. Essentially, the key concepts were highlighted and illustrated with practical applications. By clicking on any of these items, an user can access the text file that contain information about that item. In the present version of the software, students are allowed to open these files using MICROSOFT WORD and copy them to a file to help prepare their customized course note. Figure 1 Items Covered in LECTURES module The FLUIDS module currently contains properties data bank for four different fluids. These are water, air, Refrigerant-134a, and Ammonia. To specify the thermodynamic state of a substance, two independent properties are needed. The relevant properties are pressure, temperature, specific volume, internal energy, enthalpy, entropy, and quality of a liquid-vapor mixture. Once any of these two properties are specified, the program calculates and returns all other properties for that condition. This is illustrated in Figure 2. For air (or any ideal gas), the internal energy and enthalpy are dependent on temperature only. So, only one of these quantities need to be specified to get the relevant information for that condition. The FLUIDS module is the core of all thermodynamic calculations. So, this module is made accessible from a SOLVER screen or from EXAMPLES discussed later. P ge 306.3 Figure 2 Fluid Properties with Two Conditions Specified Figure 3 Items in EXAMPLES Module The EXAMPLES module contains a number of examples as listed in Figure 3. The purpose of these examples is to illustrate the solution procedure for problems related to each course topic. An user is able to use these examples to become familiar with the P ge 306.4 application of course materials to real life problems. In addition, an user can use these examples as the starting point for solving complicated design problems. Figure 4 shows the computer screen which comes up when the example on “Rankine Cycle with Reheating” is selected. It may be noted that a student can start from a state that is completely specified and can walk over the entire cycle by following the step by step instructions. There are several calls to FLUIDS module to extract the water properties at different states during the thermodynamic cycle. This kind of interactive example is believed to stimulate interest in the subject matter and can save a lot of time on the part of a student to learn the course materials. Figure 4 Example on Rankine Cycle with Reheat The SOLVER module is used for the development of mathematical model and numerical solution of any closed or open cycle problem. After entering the SOLVER module and specifying the number of significant thermodynamic states and the working fluid, a work sheet as illustrated in Figure 5 is obtained. The number of rows on the right part of the work sheet corresponds to the number of processes in a particular system. The left part of the work sheet (property table) indicates the state points at the beginning and end of each of these processes. First, known properties corresponding to each thermodynamic state is entered. The connecting device or the process is entered in the column marked “el.” If there is an efficiency value associated with a turbine, compressor, or pump, the corresponding isentropic process is specified by “s” and the actual device and the efficiency is entered in the following line.