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Enthalpy in a box Teaching open v closed system work terms. Abstract

In teaching a general approach to thermodynamics the authors have reduced the equation count in their course to basically one accounting equation, the Reynolds Transport equation. While this has been well received by students it does require that they, in turn, have a greater physical understanding of problems as success is now examined by their skill in application and manipulation of the fundamental equation rather than in successful recall from a large list of problem specific equations. This paper focuses on one area that continually causes problems, specifically the calculation of work done by open and closed systems. Some novel physical explanations are described which have proven successful in eliminating this area of confusion. The paper finishes with a somewhat controversial proposal that ‘flow work’ be taken out of the convected energy terms and instead form part of the net work terms. The benefits of just having internal energy as opposed to internal energy and enthalpy considerations are highlighted.

Introduction

Enthalpy, somewhat like entropy appears to be one of thermodynamic’s mysterious and abstract properties due primarily to difficulty in physical comprehension. Unlike entropy however, one could argue that enthalpy does not bring as much to the table. After all, it is defined, somewhat arbitrarily, from properties that students generally already have a good physical handle on. i.e. Pressure, volume and internal energy. This is particularly true at the author’s institution were a reasonable investment of time is spent on deriving the gas equation for state by using particle dynamics to really ‘knock home’ the link between, temperature, internal energy, pressure, mass and volume. So the question is begged, why bring in a new property that is not required? It is also somewhat surprising to see how vaguely and randomly enthalpy is introduced in many engineering texts, many justifying its introduction simply by saying ‘because the sum U+pV occurs so frequently …. It is convenient to give the combination a name, enthalpy”[1] Some texts allude to it is an energy property that includes ‘flow work’, and this in turn is somewhat diversely defined. Indeed the root of the problem could be traced back to the Greek origins of the word itself, ‘enthalpos’ [2], translated to ‘to put heat into’. Again as most undergraduates will recite, enthalpy is a property and heat is not, so immediately we are running into problems here. Another early discussion by Planck [3] does attempt to be more precise by referring to Gibb’s description of a property, H called ‘the heat function at constant pressure’, while more precise this is a bit of a mouthful and again relates to a specific application of enthalpy due to the ‘heat of reaction’. In contrast, and as can be seen in this paper we can use the property of enthalpy with no particular reference to heat transfer.

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Foley, A., & Plumley, M. (2009, June), Enthalpy In A Box: Teaching Open Vs. Closed System Work Terms Paper presented at 2009 Annual Conference & Exposition, Austin, Texas. 10.18260/1-2--5446