Guohui Gan

A parametric study of Trombe walls for passive cooling of buildings

Air movement in a naturally-ventilated room can be induced through the use of a solar chimney or Trombe wall. In this work Trombe walls were studied for summer cooling of buildings. Ventilation rates resulting from natural cooling were predicted using the CFD (computational fluid dynamics) technique. The renoramlization group (RNG) k-e turbulence model was used for the prediction of buoyant air flow and flow rate in enclosures with Trombe wall geometries. The CFD program was validated against experimental data from the literature and very good agreement between the prediction and measurement was achieved. The predicted ventilation rate increased with the wall temperature and heat gain. The effects of the distance between the wall and glazing, wall height, glazing type and wall insulation were also investigated. It was shown that in order to maximize the ventilation rate, the interior surface of a Trombe wall should be insulated for summer cooling. This would also prevent undesirable overheating of room air due to convection and radiation heat transfer from the wall.

A numerical study of solar chimney for natural ventilation of buildings with heat recovery

The performance of a glazed solar chimney for heat recovery in naturally-ventilated buildings was investigated using the CFD technique. The CFD program was validated against experimental data from the literature and good agreement between the prediction and measurement was achieved. The predicted ventilation rate increased with the chimney wall temperature. The effects of solar heat gain and glazing type were investigated. It was shown that in order to maximise the ventilation rate in a cold winter, double or even triple glazing should be used. Installing heat pipes in the chimney for heat recovery not only increased the flow resistance but also decreased the thermal buoyancy effect. To achieve the required air flow rates in naturally-ventilated buildings with heat recovery, use should be made of wind forces.

Effective depth of fresh air distribution in rooms with single-sided natural ventilation

This paper introduces the effective depth of fresh air distribution in rooms with single-sided natural ventilation. A numerical method for the determination of the effective depth is described. The numerical method is based on the CFD technique involving the governing equations for air flow and the transport equation for the local mean age of air. The renormalisation group k−e model of turbulence is used with equations for the conservation of mass, momentum and energy to predict turbulent buoyancy-induced room air movement. The air flow through a large opening is derived from the Bernoulli theory. The predicted air flow pattern, air temperature and local mean age of air are used to determine the effective depth of fresh air distribution in a naturally ventilated room with a window opening for summer cooling. It is shown that the effective depth for thermal comfort may not coincide with that for air quality and in summer, the requirement for thermal comfort is the limiting factor to the effective room depth. The effects of window opening levels and room heat gains on the air flow rate and effective depth are investigated.

Computational fluid dynamics applied to ejector heat pumps

Computational fluid dynamics is applied to the prediction of ejector performance for heat pumps. It is shown that the performance of an ejector varies with types of refrigerant used. It is also shown that the performance depends on the ejector nozzle type and position.

Pressure loss characteristics of orifice and perforated plates

A study was conducted on the pressure loss characteristics of square-edged orifice and perforated plates. Tests were carried out to determine the pressure loss coefficient for thin plates in a square duct for a range of Reynolds numbers. Computational fluid dynamics (CFD) was used to predict the loss coefficient, and the result was compared with experimental measurement. The effect of plate thickness on the loss coefficient for the orifice plate was studied using CFD.

Evaluation of room air distribution systems using computational fluid dynamics

Abstract Computational fluid dynamics (CFD) is used to predict the indoor environment of a mechanically ventilated room and overall ventilation effectiveness of air distribution systems. The prediction of indoor thermal comfort is based on Fangers comfort equations incorporated into the CFD model. A radiation heat exchange model is included for the calculation of mean radiant temperature as well as heat transfer through room boundaries. The overall ventilation effectiveness is related to the energy required for achieving indoor thermal comfort and good air quality. These indices were used to evaluate room air distribution systems for heating and cooling operations. It has been found that the most effective air distribution system for heating operation differs from that for cooling. It is shown that an air distribution system that results in upward displacement ventilation performs better than others in terms of indoor air quality and energy use but may cause local thermal discomfort.

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.

Application of CFD to closed-wet cooling towers

Computational fluid dynamics (CFD) is applied to predicting the performance of closed-wet cooling towers (CWCTs) for chilled ceilings according to the cooling capacity and pressure loss. The prediction involves the two-phase flow of gas and water droplets. The predicted thermal performance is compared with experimental measurement for a large industrial CWCT and a small prototype cooling tower. CFD is then applied to the design of a new cooling tower for field testing. The accuracy of CFD modelling of the pressure loss for fluid flow over the heat exchanger is assessed for a range of flow velocities applied in CWCTs. The predicted pressure loss for single-phase flow of air over the heat exchanger is in good agreement with the empirical equation for tube bundles. CFD can be used to assess the effect of flow interference on the fluid distribution and pressure loss of single- and multi-phase flow over the heat exchanger.

Analytical solution of flow coefficients for a uniformly distributed porous channel

Abstract A general theoretical model is introduced to calculate flow distribution and pressure drop in a channel with porous wall. Analytical solution of nonlinear ordinary differential equations, based on the varying flow coefficients, was obtained, and comparison was made with the solution with flow coefficients. Predicted flow distribution agrees well with experimental data.

Numerical simulation of closed wet cooling towers for chilled ceiling systems

A numerical technique for evaluating the performance of a closed wet cooling tower for chilled ceiling systems is presented. The technique is based on computational flow dynamics (CFD) for the two-phase flow of gas and water droplets. The eulerian approach is used for the gas phase flow and the lagrangian approach for the water droplet phase flow, with two-way coupling between two phases. Numerical simulation indicates that CFD can be used to predict the performance of a closed wet cooling tower, given the appropriate rate of heat generation from the heat exchanger. The technique is suitable for optimization of the design and operation of the cooling tower for chilled ceilings.