Natural convection at an indoor glazing surface

An experimental study was made to determine correlations that allow the calculation of heat transferred by convection through the window. Three configurations were studied: a hot plate; a cold plate and a window with a single-step frame placed on the wall of a room. We obtained a correlation that can be used to calculate the convection heat transfer through the window. The new correlation in the hot plate configuration differs by 14.5% from the ASHRAE correlation for laminar free convection on a vertical surface, is 27.5% from the cold plate and is 12% from the single step-frame.     

A simplified model of heat transfer at an indoor window glazing surface with a Venetian blind

In this article, a simplified model is developed to predict the radiative and convective heat transfer in a complex fenestration system consisting of a Venetian blind located adjacent to an indoor window glazing. Empirical correlations for natural convection in an asymmetrically heated channel and an isolated flat plate are used in this one-dimensional simplified model. In this simplified model, an energy balance is performed at the blind surface using a mean blind temperature. The radiative heat exchange between the blind, window and room is calculated using a four surface grey-diffuse model, which is coupled to the convective heat transfer. The simplified model has been developed using experimental and numerical data from the literature. Sample results are presented that illustrate the effect of blind slat angle, blind-to-window spacing and absorbed solar heat flux on the heat transfer at the window surface.     

A CFD study of convection in a double glazed window with an enclosed pleated blind

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.     

Natural convection from heated room surfaces

Current convective heat transfer coefficients (CHTC's) for internal room surfaces have, in most cases, been based upon data for small, free-edge heated plates. An extensive survey of CHTC data has shown that a very wide variation exists in CHTC values for vertical and horizontal surfaces. For example, a CHTC value in the range 1–6 W m⁻² K⁻¹ has been obtained for walls. Both building thermal and CFD models require accurate CHTC's for calculations of the thermal conditions and the air movement in a room. However, most such models use convective coefficients obtained for free-edge heated plates. This paper presents convective heat transfer coefficients for the heated surfaces of an environmental chamber and a small box measured under controlled conditions. Using uniformally heated plates attached to an internal surface of the chamber or the box and by accurately measuring the surface and air temperatures, the CHTC's were deduced after allowing for conduction and radiation losses from the plates. Data is presented for a heated wall, a floor and a ceiling for natural convection.     

Experimental study of free convection in a window with a heated between-panes blind

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.     

Key elements of and material performance targets for highly insulating window frames

The thermal performance of windows is important for energy efficient buildings. Windows typically account for about 30-50 percent of the transmission losses though the building envelope, even if their area fraction of the envelope is far less. The reason for this can be found by comparing the thermal transmittance (U-factor) of windows to the U-factor of their opaque counterparts (wall, roof and floor constructions). In well insulated buildings the U-factor of walls, roofs and floors can be between 0.1 and 0.2 W/(m² K). The best windows have U-factors of about 0.7-1.0. It is therefore obvious that the U-factor of windows needs to be reduced, even though looking at the whole energy balance for windows (i.e., solar gains minus transmission losses) makes the picture more complex.In high performance windows the frame design and material use are of utmost importance, as the frame performance is usually the limiting factor for reducing the total window U-factor further. This paper describes simulation studies analyzing the effects on frame and edge-of-glass U-factors of different surface emissivities as well as frame material and spacer conductivities. The goal of this work is to define material research targets for window frame components that will result in better frame thermal performance than is exhibited by the best products available on the market today.     

Radiative and convective heat transfer coefficients of the human body in natural convection

The purpose of this study was to investigate the convective and radiative heat transfer coefficients of the human body, while focusing on the convective heat transfer area of the human body. Thermal sensors directly measuring the total heat flux and radiative heat flux were employed. The mannequin was placed in seven postures as follows: standing (exposed to the atmosphere, floor contact); chair sitting (exposed to the atmosphere, contact with seat, chair back, and floor); cross-legged sitting (floor contact); legs-out sitting (floor contact); and supine (floor contact). The radiative heat transfer coefficient was determined for each posture, and empirical formulas were proposed for the convective heat transfer coefficient of the entire human body under natural convection, driven by the difference between the air temperature and mean skin temperature corrected using the convective heat transfer area.     

Natural ventilation in the double skin facade with venetian blind

As a new-style building, the thermal performance of double skin facade with the venetian blind cannot be predicted for the absence of suitable tool. In the paper, a detailed analysis of the thermal process in glass double facade with venetian blind was made. The governing equations were solved by comprising CFD, optical and heat balance model for multi-layer transparent system. It is shown that the more complex natural ventilation exists in the two air gaps divided by venetian blind, which cannot be reflected with the simplified model. Comparing the simulation results with the experiment data in the literature, a good agreement was achieved. Hence, it can be used as a reliable tool to analyze the ventilation in the double skin facade with a venetian blind.     

耦合墙体扩散的室内双扩散混合对流输运过程

研究了建筑墙体传热传质与气流流动综合作用下室内双扩散混合对流过程,介绍了整体求解流体固体区域动量守恒方程、能量守恒方程、污染物组分守恒方程及热函数和质函数方程的数值方法,分析了送风强度、热源强度、污染源强度及墙体热质扩散性能对室内混合对流过程的影响,采用流线、热线与质线反映了热和污染物在墙体和建筑室内的输运过程。     

Evaluation of inside surface condensation in double glazing window system with insulation spacer: A case study of residential complex

Aluminum spacer used to keep the glass panes apart acts as a thermal bridge that increases the risk of inside surface condensation due to its high thermal conductivity. In this study, the inside surface condensation in double glazing window systems with conventional aluminum spacer and insulation spacers made of thermally broken aluminum and thick-walled plastic, respectively, is evaluated. Evaluation method and judgment criteria for preventing inside surface condensation are suggested. Thermal characteristics of window system in residential unit are analyzed and two-dimensional steady state heat transfer simulation is carried out in order to obtain the lowest inside surface temperature. The results show that the application of insulation spacer can substantially increase the lowest inside surface temperature, temperature difference ratio and inside air humidity for preventing inside surface condensation and satisfy the required minimum temperature difference ratio.     

Heat flux profile upon building facade with side walls due to window ejected fire plume: An experimental investigation and global correlation

This paper investigates the heat flux profile upon building facade with side wall constraints due to ejected fire plumes from a window of an under-ventilated compartment fire. A reduced-scale model (1:8), consisting of a cubic fire compartment with a facade wall attached and two side walls located symmetrically at both sides of the window is developed. The window dimensions and the side wall distances are changed in experiments, representing different ventilations and constraints on fire plume entrainment. Five heat flux gauges are employed in measurement of vertical heat flux profile upon the facade wall. Results show that with the decrease in separation distance of side walls, the heat flux increases for small windows where dimensionless excess heat release rate ${\dot{Q}}_{ex}^{⁎}\ge 1.3$ (“(half) axisymmetric fire” regime), meanwhile shows weak dependency on side wall separation distance for large windows where ${\dot{Q}}_{ex}^{⁎}<1.3$ (“wall fire” regime). A new global formula is proposed to characterize the vertical profile of heat flux based on Lee’s model without side walls as further modified by a parameter K in relation to the separation distance of side walls and characteristic length scales of the window. Experimental data for different windows and side wall separation distances are well collapsed by the proposed formula.     

Optimisation and performance analysis of vertical axis wind turbine with blade pitching at best position angle for different tip speed ratios

ABSTRACT

Best pitch position of each blade in the rotation of turbine at different azimuth angle has been calculated at different Tip Speed Ratios (TSRs) by using the aerodynamic analysis. This paper analyses and optimises the various design parameters and its influence on the starting turbine by itself, power coefficient and overall performance of the Vertical Axis Wind Turbine (VAWT). The results indicate that the best pitch position blade method improves the self-starting capacity and power coefficient. The best position blade pitching curve-1 gives maximum power coefficient at 45° pitch angles for TSRs of 0.5 and the best position blade pitching curve-5 with 15° pitch angle gives maximum coefficient of power about 0.49 for TSRs at 2.5. The results indicate that the variation of pitch angle from 45° to 10° with six best position blade pitching curves give the maximum power coefficient for TSRs from 0.5 to 3.