June 18, 2006
June 18, 2006
June 21, 2006
Computers in Education
11.810.1 - 11.810.11
An interactive method for using spreadsheets to calculate and graphically display the properties of various thermodynamic working fluids is presented; the detailed calculation of individual property values is accomplished via Thermal Fluids Toolbox, a freeware program provided by Spreadsheet World, Inc. Interface with the cells containing calls to Thermal Fluids Toolbox is provided with Visual Basic for Applications sliding toolbars which allow a smooth variation of property values. Properly used by students, these worksheets have the potential to reinforce and enhance understanding of the fundamental interrelationships among various properties; students are free to change various parameters such as pressure, temperature, and quality, and immediately view the effect of these changes on the associated Mollier and T-s diagrams. Students may quickly visualize the effect of these changes, rather than being mired in the minutiae of table lookups, interpolation, transcription, and manual plotting. An additional benefit of the rapid and very accurate plotting of thermodynamic properties is a better understanding of approximations frequently employed in the compressed liquid region, wherein the effect of pressure may be neglected (i.e., properties may be evaluated principally as functions of temperature); the relationship between saturation pressure and saturation temperature is also immediately discernable, as well as property variations in the compressed liquid and superheated vapor regions. In addition to the dynamic visualization of the Mollier and T-s diagrams, students may be tasked to use the worksheet to construct their own Mollier and T-s diagrams for prescribed property values using a “copy and paste” technique. A detailed description of the spreadsheet construction for Mollier and T-s diagrams for water is provided, along with suggested tutorial questions for students and recommendations for additional applications. Feedback comments are provided from seniors who used the Mollier and T-s diagrams, and from juniors who used a similar application which provided a tutorial on p-v and T-v diagrams. Several screen shots of the worksheets are provided. The spreadsheet files are available from the author via email.
Introduction To the budding thermodynamics student, nothing can be quite as daunting as the initial exposure to the property tables for gases and (even more so) two-phase fluids. Many undergraduates, even at the end of their careers as mechanical engineering degree candidates, are uncertain of the relationships between the properties of various thermodynamic working fluids: pressure (p), temperature (T), specific volume (v), specific entropy (s), quality (x), and specific enthalpy (h) are the most commonly referenced properties. While gases are generally not the problem, two- phase fluids are usually more difficult to understand--the interrelationships between graphic and tabular presentations of the data are also often not fully appreciated. One possible solution is to have students plot tabular data manually; this becomes exceptionally tedious, especially if several different plots are required (p-v, T-v, T-s, h-s, etc.). By actually constructing the graph of interest, and investigating how lines of constant property plot and change (isobars, isotherms, isentropes, etc.), it is hoped that students will better understand and retain the nature of these diagrams. What is needed, however, is some method of relieving the students of the tedium associated with the construction of these diagrams, yet not totally divorcing them from the fundamental nature of these plots. Many students are “visual learners” and would benefit greatly from a dynamic, graphical depiction of how properties vary.
Maixner, M. (2006, June), Interactive Graphic Depiction Of Working Fluid Thermal Properties Using Spreadsheets Paper presented at 2006 Annual Conference & Exposition, Chicago, Illinois. 10.18260/1-2--140
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