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| Titel |
Analytical simulations on the role of mechanical dispersion in temperature plumes |
| VerfasserIn |
Nelson Molina Giraldo, Peter Bayer, Philipp Blum |
| Konferenz |
EGU General Assembly 2010
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 12 (2010) |
| Datensatznummer |
250040250
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| Zusammenfassung |
| The effect of mechanical thermal dispersion on the simulation of temperature plumes in
aquifers that evolve from vertical ground source heat pump (GSHP) systems is evaluated.
Traditionally, mechanical thermal dispersion is neglected in analytical solutions for the
simulation of heat transport in aquifers, due to the dominance of thermal diffusion. This is
reflected by remarkable thermal diffusion coefficients, which are commonly much higher
than those coefficients describing solute diffusion. One further argument for such
simplification is that acquisition of reliable dispersivity values would imply additional field
measurements and/or calibration procedures. In this study an existing two-dimensional
analytical approach is extended in order to account for mechanical thermal dispersion. The
model is solved for transient and steady state conditions. Moreover, an equation to calculate
the length of the temperature plume for steady state conditions is developed. To study the
interplay between mechanical thermal dispersion and hydraulic conductivity, the
transverse dispersivity is increased from 0.05 m to 0.2 m and the Darcy velocity is
varied from 10-8 m s-1 to 10-4 m s-1. Our criterion for assessing if mechanical
thermal dispersion can be neglected is the calculated error that would be introduced.
Two types of evaluations schemes were used, modeling efficiencies and relative
error. What is more, modeling efficiencies are evaluated as a function of the Peclet
number in order to account for dependencies on the thermal conductivity. All model
results are discussed with respect to their implications for the operation of GSHP
systems. Mechanical thermal dispersion causes dissipation of energy. Temperature
plume lengths become shorter with increasing transverse dispersivity. Apparently, a
dispersion-dominated regime yields lower temperature changes close to the source, i.e. the
borehole heat exchanger (BHE), in comparison to scenarios without mechanical thermal
dispersion. In general, based on a field scale of 10 m, the consideration of the mechanical
thermal dispersion is an important factor for the prediction of shape and extension of
temperature plumes only for coarse sand and gravel aquifers. From the perspective of
environmental regulators, such assumptions might be crucial for further licensing
applications of neighboring GSHP systems. In comparison, ignoring mechanical
thermal dispersion provides appropriate predictions of the temperature plume length
for geological conditions dominated by medium and fine sands, clays, and silts.
Accordingly, the range of hydrogeological conditions, where the thermal dispersivity can
be neglected is quite large. This might be the reason of why mechanical thermal
dispersion has been traditionally neglected in heat transport simulation problems. |
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