Heat carrier fluid — TRT Analysis user manual

Heat carrier fluid

The heat carrier fluid circulated during the test sets the flow regime in the pipes and the heat it carries along the loop. Unlike a design tool, the TRT module does not use the fluid properties to size or cost a borehole field; this panel exists for three reasons instead:

  • Confirming the flow regime. The fluid's viscosity and density fix the Reynolds number for the measured flow rate and pipe inner diameter, which is how the loop is confirmed to run turbulent, the regime a thermal response test should be conducted under. A fluid that is too concentrated, or a flow rate too low, pushes the loop toward laminar flow, which inflates the convective resistance Rf and, with it, the apparent borehole resistance Rb*.
  • Computing the theoretical Rb*. That same Reynolds number sets the convective resistance Rf, one of the three resistances that make up the theoretical, geometry-based Rb* reported on the Test borehole panel, the reference the two interpretation methods are compared against.
  • Feeding the short-term g-function. The fluid volumetric capacity Cf = ρf · cp sets how much heat the moving fluid carries along the loop, an input the short-term g-function needs to reconstruct the early-time response. It plays no role in the first-order approximation.

Here you describe the fluid that was actually used during the TRT: the goal is to reproduce the physics of the test as it was performed. From the fluid type and its concentration, the thermal conductivity (kf), dynamic viscosity (μf), density (ρf) and specific heat (cp) are computed from the correlations of Melinder (2007).

TRT Analysis — Heat carrier fluid panel — fluid selector and antifreeze-concentration input, the three phase columns (circulation, heating, recovery) of properties evaluated at each phase mean temperature, and the property charts (kf, μf, ρf, cp) against temperature
Heat carrier fluid panel — fluid selector and antifreeze-concentration input, the three phase columns (circulation, heating, recovery) of properties evaluated at each phase mean temperature, and the property charts (kf, μf, ρf, cp) against temperature

Heat carrier fluid - Five fluids are available: pure water, ethylene glycol, propylene glycol, methyl alcohol and ethyl alcohol. Their thermo-physical properties are computed from the Melinder correlations over the full temperature range. Select the fluid that was circulated during the test.

Water is the default because most test boreholes are filled with it: a TRT is usually run on a purpose-drilled test hole rather than on a completed borehole of the final system, and for that hole water is simpler and cheaper to source, handle and dispose of than a glycol mix. When that is not the case, select the fluid actually in the loop rather than leaving the default in place; the end of this page lists the situations where that applies.

Antifreeze mass concentration - Specify the mass concentration of antifreeze, assuming the solvent is pure water. The units are kilograms of antifreeze per kilogram of solution, reported as a percentage. Enter the concentration of the fluid as it was mixed for the test.

Summary & charts: fluid properties per phase and the resulting flow regime

The three phase columns - In the TRT module the summary table carries one column per phase of the test: Circulation, Heating and Recovery, in place of the cooling and heating design columns of the design module. Each column is headed by the mean fluid temperature measured during that phase, the average of the inlet and outlet readings over its samples, and every value below it is evaluated at that temperature: viscosity, conductivity, density, specific heat, the mean measured flow rate, the resulting Reynolds number and the convective resistance Rf.

Because the columns are keyed to the phases, they follow the phase boundaries. Moving a slider handle on the Thermal response test data panel changes the mean temperatures here, and with them the properties, the Reynolds numbers and the resistance breakdown of the test borehole.

Freezing point, fluid volume and the property charts - The table also reports the freezing temperature of the mixture and the volume of fluid contained in the pipes, and the charts beside it plot each property as a function of temperature. A low Rf is the signature of good, turbulent heat transfer between the fluid and the pipe wall; a high Rf over the range of temperatures reached during the test is a warning that the flow regime may have compromised the measurement.

TRT in practice: enter the fluid that was actually circulated

Match the fluid actually circulated. The fluid entered on this panel must match the one used during the test, at the concentration it was actually mixed to. Substituting water for a glycol mix, or rounding the concentration to a convenient figure, changes the computed viscosity and density and quietly biases the Reynolds number. Antifreeze shows up in two narrower situations: a test run on a production borehole already filled with the fluid planned for the final system, or a winter test where the borehole was charged with antifreeze beforehand to protect it from freezing before the test could start; in both cases enter that fluid rather than the water default. If the fluid or its concentration was not recorded during the test, do not silently assume a value: enter the best available estimate and note the uncertainty in the report, so a later reviewer knows the Reynolds number, and the resistance that depends on it, rests on an assumption rather than on a measurement.