HETRAN Simulation Code

Introduction

HETRAN (HEat TRansfer ANalysis) is a FORTRAN numerical code utilized for the simulation of the one dimensional transient heat and mass transfer phenomena inside multi-layer porous building materials taking into account phase change phenomena, developed at the Heterogeneous Mixtures and Combustion Systems laboratory of the National Technical University of Athens. The code has been developed within the frame of the EC funded IP titled I-SSB: “The Integrated Safe and Smart Built Concept” with the main aim to serve as a tool for the numerical simulation of heat and mass transfer phenomena in multi-layer building materials. The software solves numerically a form of the energy equation appropriate for the heat transfer, as well as a set of mass balance equations for the mass transfer module inside porous materials, using Finite Volume Techniques (FVT) [Patankar, 1980]. HETRAN takes into account temperature dependent material properties and time dependent boundary conditions. The special features of the code include: broad spectrum of material properties, steady or time dependent boundary conditions for any type of heat transfer problem, mass transfer inside porous materials and phase change, as well as heat transfer in multi-domains. The last one stands for solid to solid domains, as well as solid to fluid domains, e.g. steel stud gypsum board wall assemblies, where is a gap (filled with gas) between the gypsum boards.

Intended Uses

Throughout its development, HETRAN has been aimed to solve heat and mass transfer phenomena in multi-layer building materials. HETRAN can be used to model 1-D transient heat and mass transfer in (multi-layer) building constructions using temperature dependent material properties under time dependent boundary conditions. To date, about half of the applications of the model have been used for investigating the fire resistance of building materials and the other half consist of simulating the thermal behaviour of energy storing building materials.

Required Input

All the input parameters required by HETRAN to simulate a particular test case are given in a user created text file. The file contains information about the numerical grid, ambient environment, building geometry, material properties, boundary conditions and desired output quantities. The numerical grid consists of a number of cells. All the domains used in a scenario must conform to this numerical grid. Domains which theirs bounds do not coincide with mesh nodes are rejected. The total building geometry is input as a series of domains. Boundary conditions are applied to every domain boundary node. Materials are defined by their physical properties, e.g. thermal conductivity, specific heat, density, dynamic viscosity, in case of heat transfer modelling and additionally porosity, permeability and diffusion coefficient when mass transfer is also modelled. There are various ways that this information is conveyed, depending on the desired level of detail. Any simulation of simultaneously heat and mass transfer through building materials involves specifying material properties for the walls. Describing these materials in the input file is the single most challenging task for the user. Thermal properties, such as conductivity, specific heat, density etc. for convectional materials can be found in various handbooks, or in manufacturer literature. For new developed materials, properties are more difficult to describe, as bench-scale measurements are needed.

Output Quantities

HETRAN computes the temperature and the heat flux, within each numerical grid cell at each discrete time step, when only heat transfer phenomena are simulated. In cases, where simultaneously heat and mass transfer phenomena are modelled, HETRAN also computes mass concentrations, mass fluxes and heat fluxes for the gas components inside the porous matrix. Time histories of all the output variables at a single point in space are saved in simple text files that can be plotted using any graphic package.

Features

Following is a brief description of the major features of the HETRAN code.

• Heat Transfer Model: HETRAN solves numerically a form of the energy equation appropriate for heat transfer problems.
• Mass Transfer Model: HETRAN simulates simultaneously heat and mass transfer problems inside porous materials. In this case the code solves numerically a form of the energy equation for the heat transfer and a set of mass balance equations for the mass transfer.
• Reactions: HETRAN simulates problems where reactions and/or phase change phenomena are occurred inside the materials and mass changes are obtained.
• Multiple Domains: This is a term used to describe the capability of using more than one domains in a simulation scenario. It is possible to prescribe more than one domains to handle cases where there is more than one materials (e.g. sandwich materials).
• Boundary Conditions: All boundary nodes are assigned to heat and/or mass boundary conditions. The heat boundary conditions could be adiabatic, prescribed wall temperature, prescribed wall heat flux, convection, radiation, and connection boundary condition (boundary condition when two solid/porous materials are connected). On the other hand, mass boundary conditions can be convection – diffusion with the ambient.
• Table: This feature is utilized for variable input parameters, such as initial conditions, physical properties and boundary conditions. For example, the properties of a material that are function of the temperature are stored in a database and invoked by name. The same procedure is done for variable boundary conditions (function of time) and variable initial conditions (function of space).

Limitations

Although HETRAN can address most of the one dimensional heat and mass transfer scenarios, there are limitations in all of its various algorithms. Some of the more prominent limitations of the model are listed below.

• One Dimensionality: HETRAN solves the one dimensional heat and mass transfer equations. In cases where there is a significant heat or mass transfer to all directions the code cannot be used.
• Constant Mesh Spacing: HETRAN uses constant mesh spacing, so local mesh refinement or coarsening is not possible.
• Convection and Radiation: Inside fluid regions, such as gaps, HETRAN calculates heat transfer only due to conduction. Convection and radiation is taken into account only on boundary nodes and not on the internal nodes. Also, in cases where the material is transparent HETRAN code cannot simulate the radiation inside the material. Furthermore, in these cases mass transfer cannot be solved.
• Shrinkage or Ablation: In cases of fire, building materials may shrink or even destroy. HETRAN cannot take into account the shrinkage or the ablation of the materials.

Requirements

HETRAN requires a relatively fast CPU only in cases with many nodes, materials with variable properties, variable boundary conditions and heat/mass transfer problems. It is not needed big amount of random-access memory (RAM). The amount of output files depends on the interval time used as input parameter. Up to now, HETRAN code has been tested under the Windows XP and Window 7 operating systems.

References

Patankar, S.V., Numerical Heat Transfer and Fluid Flow, Hemisphere: London, 1980.

Release

The release package of HETRAN code contains:

• Documentation: Three different documents, i.e. the Technical Reference Guide, the User Guide and the Verification/Validation Guide are included in the package in order to guide the user how to set up a specific simulation scenario. The Technical Reference Guide contains all the theoretical background of the code that is very usefull to understand how the code performs, the User Guide contains all the approprate steps for setting up a specific simulation scenario and the Verification/Validation Guide describes in detail how to set and run specific examples, as well as includes examples which verify the validity and the accuracy to the HETRAN.
• Help: There is help regarding the installation of the HETRAN code.
• Turorials: Several tutorial files are included inside the release package. The explanation of the setting of each tutorial is described in detailed in the Verification/Validation Guide.
• Executable: The release package includes the executable file which is utilized to run a specific simulation scenario.

The contact information for obtaining the HETRAN code are listed below:

National Technical University of Athens

School of Mechanical Engineering, Thermal Engineering Department

Laboratory of Heterogeneous Mixtures & Combustion Systems

Address: HEROON POLYTECHNIOU 9-15780 ATHENS, POLYTECHNIOUPOLI-ZOGRAFOU

Prof. Dr. Maria Founti (Director)

Tel.: 0030 210 – 772 -3605

Fax: 0030 210 -772-3527

e-mail: mfou@central.ntua.gr

Dr. Dimos Kontogeorgos (HETRAN developer)

Tel.: 0030 210 – 772 – 4002

e-mail: dimkon@central.ntua.gr