In recent years there have been a number of papers that have addressed various topics within the general subject area of thermal modeling in a pipeline. These have all been worthy papers and certainly present the PSIG membership with a reasonably comprehensive view of the subject. However, the approach to solving the temperature equation together with the hydraulic equations has only been briefly discussed and relatively few comparisons made between the alternative strategies. This paper investigates the differences between a fully coupled system, in which all three equations used to describe the flow of fluid in a pipe are solved simultaneously, and a decoupled system, in which the thermal equation is solved separately from the hydraulic equations. The advantages of such a decoupling are reduced complexity and improved computational speed. But what is the cost? Is the accuracy of a decoupled system compromised? However, can a properly constructed decoupled system produce solutions that are indistinguishable from those produced by a fully coupled system? To compare the different approaches a comprehensive set of test cases has been developed. As well as highlighting specific thermal modeling phenomena, the results of these tests demonstrate where differences in the solutions lie and the magnitude of such differences: ultimately the tests are used to determine the credence of decoupling the thermal solution from the hydraulic solution.


Over the past 30 years there have been many papers presented at PSIG on the subject of thermal modeling ranging from tutorials on the physics and thermodynamics[1],[2], to comparison of different solution methods[3],[4], verification[5] and accuracy[6] and why thermal modeling is important in the real world[7],[8]. A number of these papers[3],[4],[8] also present investigations into various simplifications that can be made to the physics of the thermal model and to what extent these simplifications affect the accuracy.

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