Guarded Hot Box Research Articles

Evaluation of Thermal Performance for Vacuum Glazing by Using Three-Dimensional Finite Element Model

Simulation analysis of thermal performance for vacuum glazing was conducted in this paper. The heat conduction through the support pillars and edge seal and the radiation between two glass sheets were considered. The heat conductance of residual gas in vacuum gap was ignored for a low pressure of less than 0.1Pa. Two pieces of vacuum glazing with sizes of 0.3 × 0.3 m and 1.0 × 1.0 m were simulated. In order to check the accuracy of simulations with specified mesh number, the thermal performance of a small central area (4mm×4mm) with a single pillar in the center was simulated using a graded mesh of 41×41×5 nodes. The heat transfer coefficients of this unit obtained from simulation and analytic prediction were 2.194Wm-2K-1and 2.257Wm-2K-1respectively, with a deviation of 2.79%. The three dimensional (3D) isotherms and two dimensional (2D) isotherms on the cold and hot surfaces of the specimens were also presented. For a validity of simulated results, a guarded hot box calorimeter was used to determine the experimental thermal performance of 1.0m×1.0m vacuum glazing. The overall heat transfer coefficients obtained from experiment and simulation were 2.55Wm-2K-1 and 2.47Wm-2K-1respectively, with a deviation of 3.14%.

Study on transient heat transfer through multilayer thermal insulation: Numerical analysis and experimental investigation

This paper reports on a numerical and experimental study of heat transfer phenomena through two different multilayer fibrous insulations for building applications. The investigated samples were composed of different layers of fibrous materials and aluminium foils, placed between one or two air gaps in the vertical dimension. An experimental apparatus (a guarded hot box) has been used to measure heat transfer through the samples, while a finite volume numerical model combined radiation/conduction heat transfer was developed to predict the temperature distribution and heat transfer in such insulation systems comprised of the materials separated by multiple reflective foils. The model takes into account the coupling between the solid conduction of the fibrous system and the gaseous conduction and radiation. The radiation heat transfer through the insulation system has been modelled via the two flux approximation. The numerical results were compared with the experimental data from the guarded hot box for model validation, as well as to assess the effectiveness of the reflective foils in changing the resistance of the insulations. The comparative verification of the model showed that the numerical results were consistent with the experimental data through the environmental conditions under examination.

Comparison of vacuum glazing thermal performance predicted using two- and three-dimensional models and their experimental validation

Thermal performance of vacuum glazing predicted by using two-dimensional (2-D) finite element and three-dimensional (3-D) finite volume models are presented. In the 2-D model, the vacuum space, including the pillar arrays, was represented by a material whose effective thermal conductivity was determined from the specified vacuum space width, the heat conduction through the pillar array and the calculated radiation heat transfer between the two interior glass surfaces within the vacuum gap. In the 3-D model, the support pillar array was incorporated and modelled within the glazing unit directly. The predicted difference in overall heat transfer coefficients between the two models for the vacuum window simulated was less than 3%. A guarded hot box calorimeter was used to determine the experimental thermal performance of vacuum glazing. The experimentally determined overall heat transfer coefficient and temperature profiles along the central line of the vacuum glazing are in very good agreement with the predictions made using the 2-D and 3-D models.

Validation of data analysis routines for a thermal probe apparatus using numerical data sets

Most thermal properties of construction materials used in the analysis of building performance have been measured under laboratory conditions, using a guarded hot box or hot plate apparatus. As a consequence, these properties seldom reflect the impact of actual conditions (especially moisture content) on the values of thermal conductivity and diffusivity. Hence there is a need to develop techniques that take into account local conditions, and measure building material properties in situ. One option available is the use of a thermal probe. The thermal probe technique is based on creating a line source in a material sample and measuring the temperature rise in the sample in reaction to heat being applied. Obviously the data analysis routines used to calculate thermal conductivity and diffusivity based on the temperature rise observed are crucial to the success of the technique. This work has used transient thermal simulation of a model representing a line source in an infinite material sample to generate a set of numerical data sets to validate analysis routines in conjunction with an experimental thermal probe apparatus. Findings show that by careful application of these routines, a close agreement with simulation input values can be achieved, with errors of less than one percent. This validates the analysis routines and provides a deeper appreciation of the theoretical behaviour of a thermal probe.

Low emittance coatings and the thermal performance of vacuum glazing

The thermal performances of vacuum glazings employing coatings with emittance between 0.02 and 0.16 were simulated using a three-dimensional finite volume model. Physical samples of vacuum glazings with hard and soft coatings with emittance of 0.04, 0.12 and 0.16 were fabricated and their thermal performance characterised experimentally using a guarded hot box calorimeter. Good agreement was found between experimental and theoretical thermal performances for both a vacuum glazing with a soft coating (emittance 0.04) and those with hard coatings (emittance 0.12 and 0.16). Simulations showed that for a low value of emittance (e.g. 0.02), the use of two low-emittance coatings gives limited improvement in thermal performance of the glazing system. The use of a single high performance low-emittance coating in a vacuum glazing has been shown to provide excellent performance.

Experimental and numerical thermal analysis of a balcony board with integrated glass fibre reinforced polymer GFRP elements

The thermal behaviour of a balcony board with integrated glass fibre reinforced plastic (GFRP) elements replacing the compression reinforcement rods, is investigated by means of measurement as well as numerical analysis. For this reason a specimen consisting of an externally insulated brick wall and a representative part of a balcony is tested under a steady state temperature gradient of 30 K in a guarded hot box. Additionally to the normative requirements, temperature sensors are placed on critical sites within the construction, prior to the pouring of cement, to help the verification of the numerical analysis carried out simultaneously. Measured and calculated results are compared and some numerical parameter studies are carried out to quantify the advantage of glass fibre reinforced plastic elements over conventional balcony boards from a thermal point of view.

Experimental validation of a numerical model for heat transfer in vacuum glazing

Flat vacuum glazings consisting of a narrow evacuated space between two glass panes separated by an array of small support pillars have been fabricated. A guarded hot box calorimeter was designed and constructed to measure their heat transfer coefficients. Experimental measurements of temperatures and rates of heat transfer were found to be in very good agreement with those predicted using a developed finite element model. A method for determining the heat transfer coefficient of the evacuated gap has been established and comparisons are made between the measured and predicted glass surface temperature profiles of the exposed glass area and the heat transfer coefficients of the total glazing system in order to validated the model.

Thermal conductivity of wool and wool-hemp insulation

Measurements have been obtained for the thermal resistance of sheep-wool insulation and wool–hemp mixtures, both in the form of bonded insulation batts, using a calibrated guarded hot-box. The density was 9.6–25.9 kg m−3 for the wool and 9.9–18.1 kg m−3 for the wool–hemp mixtures. The measurements were made at a mean sample temperature of 13.3°C using a calibrated guarded hot-box. The estimated uncertainly in the resistance measurements was of the order of ±7%. The thermal conductivity of the samples, derived from the thermal resistance measurements on the basis of the measured thickness, was well correlated with the density, although the variation with density was larger than that obtained in previous studies. The conductivity of the wool–hemp samples was not significantly different from that of the wool samples at the same density. Moisture uptake produced an increase of less than 5% in the conductivity of the bonded wool insulation for an increase in absorbed moisture content of 20%. The thermal resistance was 1.6% lower on average for samples oriented in the horizontal plane rather than the vertical plane, but this difference is not significant. Copyright © 2005 John Wiley & Sons, Ltd.

Complex multimaterial insulating frames for windows with evacuated glazing

The thermal performance of a complex multimaterial frame consisting of an exoskeleton framework and cavities filled with insulant materials enclosing an evacuated glazing was simulated using a two-dimensional finite element model and the results were validated experimentally using a guarded hot box calorimeter. The analysed 0.5 m by 0.5 m evacuated glazing consisted of two low-emittance film coated glass panes supported by an array of 0.32 mm diameter pillars spaced 25 mm apart, contiguously sealed by a 10 mm wide metal edge seal. Thermal performance of windows employing evacuated glazing set in various complex multimaterial frames were analysed in detail. Very good agreement was found between simulations and experimental measurements of surface temperatures of the evacuated glazing window system. The heat loss from a window with an evacuated glazing and a complex multimaterial frame is about 80% of that for a window comprised of an evacuated glazing set in a single material solid frame.

Thermal properties of a variable cavity wall

An experimental investigation of heat transfer through a variable aspect ratio cavity wall has been conducted with the aid of a guarded hot box. A block and brick cavity wall measuring 1.2×1.2 m 2 was tested at cavity depths of 78, 60 and 40 mm. The influence of cavity flow upon the rate of heat transfer was established by correlation of convective cavity flows, measured by means of laser Doppler anemometry and by measuring surface temperatures on the four cavity faces. The experimental results show that with increasing aspect ratio, flow magnitude falls, circulation intensity reduces and the thermal resistance of the air cavity is thereby increased. Computational predictions were also used for the purpose of gaining an insight into cavity heat transfer and extrapolating models for further configurations.

Environmental Chamber for Investigation of Building Envelope Performance

This paper describes the design and development of an environmental chamber for investigation of building envelope performance as a function of heat, air, and moisture transfer, and the interaction between the building envelope and the indoor environment. The environmental chamber is the first of its kind in Canada and is unique in concept. It is located in a specially constructed laboratory of the Centre for Building Studies (CBS), in downtown Montreal. Experiments can be performed on envelope assemblies up to 7.2 m high and 4.1 m wide. The environmental chamber consists of two chambers representing dynamic indoor and outdoor conditions, denoted as hot and cold boxes, respectively. Between the hot and cold boxes, a rectangular frame is sandwiched, in which building envelope components can be built up. A metering box is also located on the hot box side of the test specimen to measure heat flow and temperature drop across it for thermal resistance measurement based on the guarded hot box method. Without the metering chamber, the facility can be used for tests similar to the calibrated hot box method. The second major function of the facility is its use as one large environmental chamber for testing full rooms.

Determining the heat flow through building walls under simulated actual weather patterns

We present a methodology for determining the transient heat flow through building walls. The methodology involves the use of a guarded hot box operating under a set of simulated conditions representative of actual weather patterns. Results validate the use of the guarded hot box for the measurement of heat flow through building constructions operating under different characteristic weather patterns. Outdoor heat flow measurements are made using an isothermal-heat-flux measuring plate and the same outdoor air temperature and solar radiation effects are simulated in a guarded hot box using the sol-air temperature. Finally, these heat flow values are compared with predictions made using the DOE-2 computer code. This comparison provides a means for validating computer predicted heat flows through building walls.

Transition test method: New approach to determining thermal properties

A transition test method is developed for determining the thermal properties of building materials using a guarded hot box apparatus. The model is presented and the results of the tests are analysed. The results are compared with the results from the standard guarded hot box test method (GHB). They are accurate and time saving. Compared with the conventional test method, this new approach can reduce the test time from 24 hours to within only 2 hours. For determining the temperature-dependent thermal conductance (equivalent value) of a sample of transparent insulation materials, several days can be saved. The method can be also used to measure the insulation properties of materials with low thermal conductance, to which the conventional method cannot be applied with a GHB.

Field Measurements of Thermal Resistance in Poultry Buildings

Thermal resistances (R values) were measured using a portable heat flux meter (HFM) in a guarded hot box and in heated broiler houses. Hot box tests showed HFM measurements to be within 10% of hot box values. Field tests indicated that R values of aged fiberglass insulation did not change from the initial rating, while R values of aged expanded polyurethane insulation tended toward lower values as indicated by ASHRAE (1985) and MRCA (1985). Field tests of extruded polystyrene insulation indicated a reduction of R value below the initial rating, but consideration of sources of HFM error precluded determination of the extent of such reduction.

An Operational Procedure for Guarded Hot Box Testing

Abstract An operational procedure for guarded hot box (GHB) testing has been developed in the Thermal Measurements Laboratory (TML) of the Construction Products Division of W. R. Grace and Co. The GHB operational procedures consisting of specimen preparation, “calibration”, testing, data analysis, and reporting are presented in this report. These procedures follow the general guidelines given in ASTM Test for Steady-State Thermal Performance of Building Assemblies by Means of a Guarded Hot Box (C 236). In the specimen preparation procedure, material handling, specimen construction, specimen mounting into the specimen frame, and instrumentation placement and method of attachment are discussed in detail. The “calibration” procedure developed accounts for any variations in the thermal conductance of the metering box walls caused by design or construction. The experimentally determined setting for the thermopile electromotive force is controlled during testing so that the net heat flow through the metering box walls is negligible. The “calibration” procedure does not require knowledge of the thermal conductance of the specimen. A testing procedure is discussed that provides steady-state operation as defined by six steady 4-h periods of temperature and heat flow data within 1% of each other. The test data are analyzed and a GHB test report is generated by a computer software package. Examples of a typical GHB test report and a testing procedure data sheet are presented.