Heat Flows in a Full Scale Room Exposed to Natural Climate

Abstract

An ambient wall with a window were studied with both theoretical analyses and measurements performed
under conditions with natural climate. The method used wes to estimate the heat flow through wall and window
from measured temperatures and solar radiation. The longwave radiation was calculated from surface temperatures. The convective heat transfer was caculated as the difference between the heat flow through the building element and the longwave radiation. With the one-dimensional dynamic heat transfer model for the window which included shortwave radiation it was possible to measure the continuous heat flow through a window from temperature sensors and solar radiation measurements. With the one-dimensional finite difference model for the heat transfer through the wall it was possible to calculate thre heat flow through a wall from temperature sensors. It was possible to continuously measure the convective heet transfer coeffìcient on the inner surface of a wall or a window, The accurary was not very good: at best +/- 15% for the window and, +/- 20% for the wall. Even with this low accuracy the effect of different heating and ventilation strategies on the inside could clearly be detected. The results showed that the importance of the ventiletion design and the position of the radiaton is crucial. Local convective heat transfer coefficients may be more than 10 times the expected, due to ventilation or position of the radiator.

Abstract (Swedish)

It is, with the thermal models used in today's building simulation progr¿rms,
possible to calculate the major part of the heat transfer in a room with an ambient
outer wall. However, there are some parameters these models calculate
with less or unknown accuracy: heat flows in poorly insulated walls or windows,
heat flows in a room exposed to strong solar radiation, temperatures on
the inside of ambient outer walls and windows.
The reason for these difficulties is mainly that there is a lack of experimental
data for the detailed energy transfer in a window exposed to ambient climate
and the convective energy transport in a room exposed to ambient climate.
The aim of this study was to investigate the detailed energy transfer at an ambient
wall including window. The investigation included both theoretic analysis
and measurements performed under conditions close to the real situation
with, for example, ambient climate.

The most important conclusions regarding the calculation models and measurement
technique used in this study were:
-It is possible to measure the continuous heat flow through a window from
temperature sensors and solar radiation measurements. The accuracy at
least for low angles of incidence of the solar radiation was estimated to +/-
lO%.
-To measure air temperatures in sunlit places thin (0.08 mm in this case)
stripped thermocouples are the only alternative.
-The one-dimensional dynamic heat transfer model for the window which
included shortwave radiation was fairly good except for small temperature
differences and high angles of incidence for solar radiation.

Conclusions for the convective heat transfer coefficient were:
It is possible to continuously measure the convective heat transfer coefficient
on the inner surface of a wall or a window.
The presented results show that the importance of the ventilation design
and the position of the radiator is crucial. Local convective heat transfer
coefficients can be more than l0 times the expected, due to ventilation or
position of the radiator.

It is not obvious how the results of this study should be generalized but some general fomulas were suggested.

• author: Wallentén, Petter
• organization: Division of Building Physics
• alternative title: Heat Flows in a Full Scale Room Exposed to Natural Climate
• publishing date: 1998-05-11
• type: Book/Report
• publication status: published
• subject: Building Technologies; Other Physics Topics; Energy Engineering
• keywords: CONVECTIVE HEAT TRANSFER, heat transfer coefficient, finite difference method, frequency analysis, longwave radiation model, HEAT TRANSFER, CONVECTIVE HEAT TRANSFER, long wave radiation, Field measurements, Window, room temperature
• pages: 150 pages
• publisher: Department of Building Science, Lund Institute of Technology
• report number: TABK 98/3051
• ISBN: 1103-4467
• language: English
• LU publication?: yes
• id: 090ef942-3ccb-4e3a-91e4-89c7c7c51496
• date added to LUP: 2017-06-13 11:25:31
• date last changed: 2018-11-21 21:32:41