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A numerical study is conducted of laminar free convection from a vertical window glazing adjacent to an insect screen which is modeled as a layer of porous medium. The numerical results are validated against optical temperature field visualization and local heat flux measurements. The numerical model is also shown to agree well with established correlations for the heat transfer at the upper and lower asymptotic limits of screen permeability. Results are presented for Rayleigh number (based on the glazing height) of Ra H = 10 6 –10 9 , and for window height to screen spacing ratios of H/W = 25, 60 and 100. Under some conditions, the flow resistance of the insect screen is found to reduce the convective heat transfer rate from the glazing by more than 50%. Generally, it was found that the screen has less effect on the convective heat transfer rate as the Rayleigh number increases. Also, in the range of parameters corresponding to most practical window applications, decreasing the screen to window spacing reduces the convective heat transfer rate from the glazing. A general correlation for use in window analysis is presented that accounts for the effect of an insect screen on the average convection coefficient at the glazing. [ABSTRACT FROM AUTHOR]

An experimental study using Mach-Zehnder interferometer has been carried out to investigate the heat transfer from an isothermal horizontal circular cylinder, which is exposed to an air slot jet at different angles of jet impingement. A square edged nozzle is mounted parallel with the cylinder axis and jet flow impinges on the side of the cylinder at angles Θ = 0°, 30°, 60° and 90°. The Reynolds number varied from 240 to 1900 while the Grashof number and slot- to cylinder-spacing is kept constant at Gr = 22,300 and H/w = 7 respectively. The Richardson number varied from 0.006 to 0.4. The flow field is greatly influenced by the slot exit velocity and the buoyancy force due to density change. The local Nusselt number around the cylinder has been calculated using the infinite fringe interferograms at 10° intervals. Average Nusselt number shows that heat transfer is decreased when the angle of jet impingement is increased . [ABSTRACT FROM AUTHOR]

Laser interferometry and flow visualization were used to study free convective heat transfer inside a vertical channel. Most studies in the literature have investigated buoyancy forces in a single direction. The study presented here investigated opposing buoyancy forces, where one wall is warmer than the ambient and the other wall is cooler than the ambient. An experimental model of an isothermally, asymmetrically heated vertical channel was constructed. Intetferometry provided temperature field visualization and flow visualization was used to obtain the streamlines. Experiments were carried out over a range of aspect ratios berween 8.8 and 26.4, using temperature ratios of 0, --0.5, and --0.75. These conditions provide a modified Rayleigh number range of approximately 5 to 1100. In addition, the measured local and average Nusselt number data were compared to numerical solutions obtained using ANSYS FLUENT. Air was the fluid of interest. So the Prandtl number was fixed at 0.71. Numerical solutions were obtained for modified Rayleigh numbers ranging from the laminar fully developed flow regime to the turbulent isolated boundary layer regime. A semi-empirical correlation of the average Nusselt number was developed based on the experimental data. [ABSTRACT FROM AUTHOR]

Abstract: An experimental study has been conducted to examine free convection in a window with an enclosed aluminum venetian-type blind. The unique feature of this experiment was that the blind slats were heated electrically to simulate absorbed solar radiation. Convective heat transfer measurements and temperature field visualization were obtained using a Mach–Zehnder laser interferometer. Optical measurements were made for two glazing spacings, two blind slat angles, two blind heat flux levels, and two glazing temperature differences. Both local and average convective heat flux data were obtained in the center region of the tall air-filled enclosure. At the widest glazing spacing, the temperature field was found to be unsteady. For these cases, the temporal fluctuation of the local convective heat transfer was time-averaged using a high speed camera. The experimental results have been compared to a simplified method in the literature for predicting the center-glass heat flux for this configuration. [Copyright &y& Elsevier]

Abstract: When convective heat transfer measurements are made using a laser interferometer, optical windows are often used to separate the test fluid inside the experimental model from the ambient air. In the present study, the measurement error caused by the thermal boundary layers that form on the exterior surfaces of the optical windows has been analyzed. A convenient analytical expression for the error in the fringe gradient has been derived, using the two-dimensional similarity solution for laminar free convective flow. For typical experimental conditions this source of bias error was found to be generally small, except near the leading edge of the thermal boundary layer. Based on the results, it is recommended that the measurement region be at least five centimeters away from the leading edge of these external thermal boundary layers. [Copyright &y& Elsevier]

Abstract: A numerical study has been conducted of steady free convection in a double glazed window with a between-panes pleated cloth blind. The model geometry is based on a commercial product that is available on the consumer market in North America. The study considers the effects of Rayleigh number, enclosure aspect ratio, and blind geometry on the convective heat transfer. A simplified model of the coupled convective and radiative heat transfer is also presented. Sample results show that the average Nusselt number data from the CFD study can be combined with a one-dimensional model to closely predict the glazing-to-glazing U-value. [Copyright &y& Elsevier]

Abstract: A numerical study has been conducted of free convection in a tall vertical enclosure with an internal louvered metal blind. The study considers the effects of Rayleigh number, enclosure aspect ratio, and blind geometry on the convective heat transfer. The numerical model has been validated against experimental measurements and the results have been presented in terms of an empirical correlation for the average Nusselt number. The correlation is applicable to an enclosure with an internal metal blind. It has been shown that the Nusselt number correlation can be combined with a simple one-dimensional model to closely predict the enclosure U-value. [Copyright &y& Elsevier]

An experimental study has been conducted on natural convective heat transfer in an idealized double-glazed window with a between-panes louvered blind. A Mach-Zehnder interferometer has been used to obtain full-field temperature visualization, as well as local and average convective heat transfer rates. A blind, consisting of 17 horizontal aluminium slats, was mounted inside a tall vertical enclosure formed by two aluminium plates and two acrylic end spacers. Temperature field visualization and measurements were obtained for three plate spacings and three blind slat angles over a Rayleigh number range (based on the enclosure width) of 4.6 × 104 ≤ Ra ≤ 1.3 × 105. Air was the fill gas. The results show that the inter-pane blind has a strong influence on the local and average convective heat transfer rates. In general, a between-panes blind was found to reduce the average convective heat transfer rate across the window, relative to an empty enclosure. At the three plate spacings studied, a closed blind was found to yield the lowest average convective heat transfer rates. [ABSTRACT FROM AUTHOR]

An approximate method is presented for predicting the effect of a louvered blind on the centre-glass thermal performance of a fenestration. The method combines a one-dimensional heat transfer model with data from a numerical simulation of the window and blind. Sample results for a blind mounted on the indoor surface of a window show the effect of blind slat angle on heat transmission. Both summer and winter conditions are considered. The results show that a louvered blind can improve the U-value of a standard double-glazed window by up to 37%. Also, the radiation heat exchange with the room can be dramatically reduced (by up to 60%), which will improve the level of occupant comfort. However, there was found to be a trade-off between U-value and occupant comfort; placing the blind closer to the window improves the U-value, but increases the radiation heat exchange with the room. The predictions from the present simplified method compare well with results from a full two-dimensional computational fluid dynamics solution of the conjugate blind/window interaction. [ABSTRACT FROM AUTHOR]

The presence of a blind adjacent to a window affects the natural convective and radiant heat transfer from the window to the room. As a result, the use of a shading device will change the heat transmission and solar heat gain through the window. A number of numerical and experimental studies of the effects of blinds on the heat transfer from a window have therefore been undertaken, with some of the main features of these studies being described here. In these studies, attention has been given to Venetian, vertical, and plane blinds, although the major attention has been given to Venetian blinds. Initial studies examined the effect of all three types of blinds on the natural convective heat transfer at an indoor glazing surface when there is no solar irradiance. Supporting experimental studies using mainly interferometry were then undertaken, particularly for the Venetian blind case. The numerical and experimental work was then extended to include the effects of solar radiation, in particular the effect of heat generation in the blind resulting from absorbed solar radiation. In addition to providing basic information on the effects of blinds on the heat transfer process, the studies described here will assist in expanding available software for predicting window heat transfer to include the effect of window coverings and assist in the selection of energy-efficient window coverings. [ABSTRACT FROM AUTHOR]

A numerical model has been developed of the conjugate convection, conduction, and radiation heat transfer in a double-glazed window with a between-panes venetian-type blind. The two-dimensional finite-volume model has been validated for a range of slat angles using U value measurements from the literature. Data from this solution have been used to show that the thermal performance of the window/blind enclosure can be closely approximated using a simplified model. Such simplified methods of analysis are of utility in fenestration design software. [ABSTRACT FROM AUTHOR]

Examines the accuracy of beam-averaged interferometric heat transfer rate measurements made in ideal gases. Problem definition; Methods used in the study; Analysis of beam-averaged heat transfer rates.

A classical or holographic interferometer can be used to measure the average fluid temperature integrated in the direction of the test/object beam. However, such beam-averaged measurements are not exact for a three-dimensional temperature field, when a standard two-dimensional analysis technique is applied. In the current study, an analysis is performed to determine the error in the average fluid temperature measurement caused by temperature variations in the light beam direction. The error has been calculated for both an ideal gas and water as the test fluid. The results show that for typical experimental conditions, the measurement error introduced by this temperature variation is less than 2%, even for the most adverse temperature distribution in the light beam direction. For reasons discussed in this article, this result may promote the wider application of interferometry to three-dimensional heat transfer problems. [ABSTRACT FROM AUTHOR]

Laser interferometry is combined with high-speed digital cinematography to measure time-averaged transient and turbulent convective heat transfer rates. The method is applied to study free convection in a tall vertical air-filled enclosure. Measurements are made at three wall spacings in the turbulent flow regime (5.2×104 ⩽ Raw ⩽ 2.8×105). An automated image processing algorithm is used to calculate the instantaneous local heat flux from a sequence of interferograms that is captured by a high-speed camera. The local Nusselt number distributions on the hot and cold walls are obtained by time-averaging the fluctuations in local heat flux. The effects of key experimental parameters, such as the camera frame rate and the total image capture time, are investigated. For the current problem, it is shown that a total capture interval of about 10 s is required to accurately measure the time-average local Nusselt number. Within the measurement uncertainty, the average Nusselt number results are in agreement with a widely used empirical correlation from the literature. [ABSTRACT FROM AUTHOR]

Examines the influence of heated, horizontal and rotatable louvers on the convective heat transfer from a heated or cooled vertical isothermal surface. Determination of the temperature field and local surface flux data; Comparison of results with that from a conduction/convection/radiation finite element model; Effect of the heated louvers on the heat transfer rate from the plate surface.

Examines the influence of heated, horizontal and rotatable louvers on the convective and radiative heat transfer from a heated or cooled vertical isothermal surface. Description of the physical model used in the study; Governing equations; Discussion of results.

Heat transfer to horizontal cylinders immersed in fluidized beds has been extensively studied, but mainly in the context of heat transfer to boiler tubes in coal-fired beds. As a result, most correlations in the literature have been derived for cylinders of 25-50 mm diameter in vigorously fluidizing beds. In recent years, fluidized bed heat treating furnaces fired by natural gas have become increasingly popular, particularly in the steel wire manufacturing industry. These fluidized beds typically operate at relatively low fluidizing rates (G/Gmf<5) and with small diameter wires (1-6 mm). Nusselt number correlations developed based on boiler tube studies do not extrapolate down to these small size ranges and low fluidizing rates. In order to obtain reliable Nusselt number data for these size ranges, an experimental investigation has been undertaken using two heat treating fluidized beds; one a pilot-scale industrial unit and the other a lab-scale (300 mm diameter) unit. Heat transfer measurements were obtained using resistively heated cylindrical samples ranging from 1.3 to 9.5 mm in diameter at fluidizing rates ranging from approximately 0.5xgmf (packed bed condition) to over 10xgmf using aluminum oxide sand particles ranging from dp=145-330 µm (50-90 grit). It has been found that for all cylinder sizes tested, the Nusselt number reaches a maximum near 2xgmf, then remains relatively steady (±5-10%) to the maximum fluidizing rate tested, typically 8-12xgmf. A correlation for maximum Nusselt number is developed. [ABSTRACT FROM AUTHOR]

Abstract: Natural convection of air in a tall vertical cavity was studied using a smoke patterns and interferometry. Experiments covered Rayleigh numbers of 4850< Ra <54,800 and aspect ratio A ≈40. Secondary cells were noted at Ra as low as 6228. The flow was stable at Ra <104. As Ra exceeded 104 the flow became irregular, the core flow became increasingly unsteady and 3-D motion became evident. Interferometry showed that most of the temperature drop exists in boundary layers near the walls. The core is well mixed and of relatively uniform temperature with little or no vertical stratification. [Copyright &y& Elsevier]