.IDEAS.Fluid.HeatExchangers.RadiantSlab.EmbeddedPipe

Information

Dynamic model of an embedded pipe for a concrete core activation. This model is based on (Koschenz, 2000). In addition the model provides the options to simulate the concrete core activation as if there were multiple parallel branches. This affects the pressure drop calculation and also the thermal calculations.

Assumptions and limitations

The implementation of Koschenz mentions that a minimum discretization (i.e. using nDiscr) is required to avoid violation of the second law of thermodynamics. The model explicitly enforces the second law even for nDiscr=1 by upper bounding the heat flow rate such that this minimum discretization does not apply to our implementation. The parameter nDiscr thus only affects the results at larger flow rates. The example IDEAS.Fluid.HeatExchangers.RadiantSlab.Examples.EmbeddedPipeNDiscr provides an indication of the sensitivity of the results to the value of nDiscr.

The embeddedPipe model is designed to be used together with an IDEAS.Buildings.Components.InternalWall. When nDiscr>1, the wall/floor should also be discretized to be physically correct, although the discretizations can also be connected to the same wall/floor, which gives a reasonable approximation as illustrated by the example IDEAS.Fluid.HeatExchangers.RadiantSlab.Examples.EmbeddedPipeNDiscr.

R_x_val represents thermal resistance between the outer pipe wall temperature and the (fictive) uniform TABS temperature. For small concrete/screed layer thicknesses (d_i ≤ 0.3·T, with T the distance between the pipes), a correction factor needs to be taken into account (see Eq.4-4 and 4-24 in (Koschenz, 2000)).

R_w_val represents the convective thermal resistance between the embedded pipe wall and the water flowing in that pipe. Depending on the Reynolds number rey, laminar or turbulent flow is assumed. For turbulent flow, the convective heat transfer coefficient is determined using a correlation (Eq.4-37) from (Koschenz, 2000). For laminar flow, the convective heat transfer coefficent is calculated using a constant Nusselt number of 4.

Typical use and important parameters

Following parameters need to be set:

• RadSlaCha is a record with all the parameters of the geometry, materials, and even number of discretization layers in the nakedTabs model.
• mFlow_min is used to check the validity of the operating conditions and is by default half of the nominal mass flow rate.
• A_floor is the surface area of (one side of) the radiant slab.
• nDiscr can be used for discretizing the EmbeddedPipe along the flow direction. See above for a more detailed discussion.
• nParCir can be used for calculating the pressure drops as if there were multiple EmbeddedPipes connected in parallel. The total mass flow rate is then split over multiple circuits and the pressure drop is calculated accordingly.
• R_C is the thermal resistivity from the center of the TABS or floor heating system to the zones. Note that the upper and lower resistivities need to be calculated as if they were in parallel. This parameter has a default value based on RadSlaCha but it may be improved if necessary. The impact of the value of this parameter on the model performance is low except in cases of very low mass flow rates.

Options

By default dp_nominal is calculated by making an estimate of the total pipe length. This pressure drop can be an underestimation of the real pressure drop. The used pipe lengths can be changed in the Pressure drop tab. Parameter dp_nominal can be used to override the default calculation.

Validation

A limited verification has been performed in IDEAS.Fluid.HeatExchangers.RadiantSlab.Examples.EmbeddedPipeVerification.

References

EN 15377, Heating systems in buildings – Design of embedded water-based surface heating and cooling systems., 2008.

M. Koschenz and B. Lehmann, Thermoaktive Bauteilsysteme tabs. Dübendorf, Switzerland: EMPA Energyiesysteme/Haustechnik, 2000, ISBN: 9783905594195.

Transsolar, TRNSYS 16 - A TRaNsient SYstem Simulation program, User Manual. Volume 6: Multizone Building modeling with Type56 and TRNBuild. Madison, 2007.

Revisions

• August 12, 2025, by Jelger Jansen:
  Fix correction term and wrong asserts and update documentation.
  See #1381.
• March 20, 2020 by Filip Jorissen:
  Fixed inconsistency in the mass computation of the MixingVolume. See #1116.
• January 31, 2020 by Filip Jorissen:
  Propagated allowFlowReversal in TemperatureTwoPort sensor. See #1105.
• October 19, 2019 by Filip Jorissen:
  Removed discretization assert since we limit the heat flow rate to physically realistic values already using a limit on G_t. Revised documentation. See #863.
• October 18, 2019 by Filip Jorissen:
  Using TemperatureTwoPort sensor. See #1081.
• October 13, 2019 by Filip Jorissen:
  Bugfix for division by zero when dp_nominal=0, See #1031.
• August 14, 2019 by Iago Cupeiro:
  Added output that computes the total TABS heat flow of the EmbeddedPipe.
• April 16, 2019 by Filip Jorissen:
  Added checks for flow reversal. See #1006.
• April 16, 2019 by Filip Jorissen:
  Removed computeFlowResistance=false since this parameter was hidden in the advanced tab and this setting can easily lead to singularities. See #1014.
• June 21, 2018 by Filip Jorissen:
  Set final alpha=0 in prescribedHeatFlow to avoid large algebraic loops in specific cases. See #852.
• April 26, 2017 by Filip Jorissen:
  Removed useSimplifiedRt parameter since this leads to a violation of the second law for small flow rates. See #717.
• November, 2015, by Filip Jorissen:
  Revised implementation for small flow rates.
• 2015, by Filip Jorissen:
  Revised implementation
• March, 2014, by Filip Jorissen:
  IDEAS baseclasses
• May, 2013, by Roel De Coninck:
  Documentation
• April, 2012, by Roel De Coninck:
  Rebasing on common Partial_Emission
• 2011, by Roel De Coninck: First version and validation