Hazim B. Awbi

Natural convection from heated room surfaces

Abstract Current convective heat transfer coefficients (CHTCs) 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 CHTCs 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 CHTCs 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.

Calculation of convective heat transfer coefficients of room surfaces for natural convection

Abstract Convective heat transfer from internal room surfaces has major effect on the thermal comfort, air movement and heating and cooling loads for the room. Recent studies have shown that the values of convective heat transfer coefficient used in building thermal models greatly influence the prediction of the thermal environment and energy consumption in buildings. In computational fluid dynamics (CFD) codes for room air movement prediction, accurate boundary conditions are also necessary for a reliable prediction of the air flow. However, most CFD codes use ‘wall functions’ derived from data relating to the flow in pipes and flat plates which may not be applicable to room surfaces. This paper presents results for natural convection heat transfer coefficients of a heated wall, a heated floor and a heated ceiling which have been calculated using CFD. Two turbulence models have been used to calculate these coefficients: a standard k — ɛ model using ‘wall functions’ and a low Reynolds number k — ɛ model. The computed results are compared with data obtained from two test chambers.

Application of computational fluid dynamics in room ventilation

Abstract This paper presents the results of a computer program developed for solving 2- and 3-D ventilation problems. The program solves, in finite difference form, the steady-state conservation equations of mass, momentum and thermal energy. Presentation of the fluctuating velocity components is made using the k-ϵ turbulence model. Predicted results of air velocity and temperature distribution in a room are corroborated by experimental measurements. The numerical solution is extended to other room ventilation problems of practical interest.

Mixed convection from heated room surfaces

In most building thermal load calculations natural convective heat transfer is assumed for internal room surfaces. When mechanical ventilation is used, the natural convective heat transfer coefficient (CHTC) will underestimate the convective heat transfer from the room surface. In this paper, measured local and surface-mean convective heat transfer data are presented for convection from a heated surface partially covered by an air jet. Experiments were conducted in a small office-sized, well-insulated environmental chamber with heating plates fixed to the internal surfaces. A fan box with an adjustable nozzle was placed at one end of the surfaces within the chamber to create the convective flow. Cooling was provided at one wall to represent a heat sink. Equations are presented for predicting mixed convection heat transfer as a combination of natural and forced convection for walls, floor and ceiling. When the jet acted on a heated surface the measured CHTC was greater than the equivalent natural convection CHTC, although the increase for the ceiling was much greater than for the floor or wall.

A study of the air quality in the breathing zone in a room with displacement ventilation

Abstract This paper is concerned with the difference in the air quality that is perceived by the occupants (breathing zone) and that existing in the occupied zone as a whole. An environmental chamber with displacement ventilation system has been used to carry out the measurements with the presence of a heated mannequin and other heat sources. Measurements of the age of air distribution, the air exchange index and the ventilation effectiveness were carried out at different points in the chamber for different room thermal loads. CFD simulations were also carried out for the purpose of flow visualisation as well as the calculation of air velocity, temperature and age of air distribution. In addition, CFD simulations were carried out to study the effect of changing the airflow rate to the chamber and the position of air inlet to extend the range of parameters. The results from the CFD simulations were compared with those from measurements and good agreement was obtained in most cases.

Building cluster and shading in urban canyon for hot dry climate: Part 1: Air and surface temperature measurements

Under low latitude conditions, minimization of solar radiation within the urban environment may often be a desirable criterion in urban design. The dominance of the direct component of the global solar irradiance under clear high sun conditions requires that the street solar access must be small. It is well known that the size and proportion of open spaces has a great influence on the urban microclimate

Numerical simulation of the indoor environment

The CFD program VORTEX which has been developed for predicting the indoor environment in occupied spaces is described. The flow equations are the continuity equation, the Navier-Stokes equation, the thermal energy equation, the concentration equation and the equations for the kinetic energy of turbulence (k) and its dissipation rate (e) of the k-e turbulence model. The equations are solved for the 3-D Cartesian system using the SIMPLE algorithm. The program produces a direct simulation of the thermal comfort indices PMV and PPD and the air quality of room air. Some applications involving mechanically ventilated (heating and cooling) and naturally ventilated rooms are presented. Results in the form of velocity vectors and contours for temperature, thermal comfort indices (PMV and PPD) and CO2 concentration are produced for the cases investigated. Simulations using this program can provide design data as required by thermal comfort and indoor air quality standards and guides.

Building cluster and shading in urban canyon for hot dry climate Part 2: Shading simulations

Under low latitude conditions, minimisation of solar irradiance within the urban environment may often be an important criterion in urban design. This can be achieved when the obstruction angle is large (high H/W ratio, H = height, W = width). Solar access to streets can always be decreased by increasing H/W to larger values. It is shown in this paper that the street canyon orientation (and not only the H/W ratio) has a considerable effect on solar shading and urban microclimate. The paper demonstrates through a series of shading simulation and temperature measurements that a number of useful relationships can be developed between the geometry and the microclimate of urban street canyons. These relationships are potentially helpful to assist in the formulation of urban design guidelines governing street dimensions and orientations for use by urban designers.

Design considerations for naturally ventilated buildings

This paper discusses the parameters which should be considered in designing natural ventilation systems and presents a procedure for calculating the air flow rate due to wind and buoyancy. Examples are given of systems using solar-induced ventilation which could have applications in ventilating commercial buildings.

Wind Tunnel and CFD Investigation of the Performance of “Windcatcher” Ventilation Systems

Abstract Experimental investigations and CFD simulations were carried out to evaluate the performance of square and circular section wind-driven systems for natural ventilation applications in buildings. Wind tunnel and smoke visualisation tests were conducted on a full-scale model based on a commercial “windcatcher” design. The experimental set-up consisted of each system being connected to a model test room. The devices are divided internally into four quadrants/segments for the purpose of air supply and extract. Pressure coefficients distribution, internal air speed and volumetric air flow rate were measured for various wind speeds and for different wind directions. In addition, CFD simulations were carried out to obtain the pressure distribution and air flow pattern around the square section system. The results show that the performance of the devices depends mainly on the speed and direction of the prevailing wind. Although the ventilation rate increases with wind speed it was found to decrease with the increase in the wind direction angles from 0° to 45°, measured from the normal to the face of the square system.