Angelo Delsante

Natural ventilation induced by combined wind and thermal forces

Abstract Analytical solutions are derived for calculating natural ventilation flow rates and air temperatures in a single-zone building with two openings when no thermal mass is present. In these solutions, the independent variables are the heat source strength and wind speed, rather than given indoor air temperatures. Three air change rate parameters α, β and γ are introduced to characterise, respectively, the effects of the thermal buoyancy force, the envelope heat loss and the wind force. Non-dimensional graphs are presented for calculating ventilation flow rates and air temperatures, and for sizing ventilation openings. The wind can either assist the buoyancy force or oppose the airflow. For assisting winds, the flow is always upwards and the solutions are straightforward. For opposing winds, the flow can be either upwards or downwards depending on the relative strengths of the two forces. In this case, the solution for the flow rate as a function of the heat source strength presents some complex features. A simple dynamical analysis is carried out to identify the stable solutions.

Prediction of natural ventilation in buildings with large openings

Abstract This paper first presents a consistent pressure-based formulation for natural ventilation of single-zone and multi-zone buildings with multiple openings. Pressure-based multi-zone formulation is made easier to implement by introducing an auxiliary concept of external pressure, which allows all the formulas to be presented in an integrated form. Multi-zone situations considered include vertically interconnected zones, and horizontally interconnected zones with same heights and different heights. The formulation includes the combined effect of wind, thermal buoyancy and mechanical ventilation, and it can be used for both external and internal large openings. A simple and easy implementation method was then presented. Single-zone and multi-zone analytical solutions are revisited or developed by the pressure-based formulations and used for the validation of the implementation method. A CFD method is also used to cross-check the implementation method in a single-zone building with very large external openings. A reasonable agreement has been found between the results predicted by the pressure-based formulation and those predicted by the analytical solutions and CFD methods.

Some examples of solution multiplicity in natural ventilation

Abstract This paper shows that under certain conditions, multiple solutions for the flow rate exist in a natural ventilation system, induced by the non-linear interaction between buoyancy and wind forces. Under certain physical simplifications, the system is governed in steady state by a non-linear algebraic equation or a system of equations. Three examples are given here: a single-zone building with two openings, a channel with two end openings, and a two-zone building with two openings in each zone. Analytical and numerical solutions are presented. It is shown that in all three cases the flow rate exhibits hysteresis. These results have significant implications for multi-zone modelling of natural ventilation and smoke spread in buildings. An experimental investigation using a small-scale model in a water tunnel confirms that two steady-state solutions exist for a single-zone building.

Derivation of capture efficiency of kitchen range hoods in a confined space

Abstract An existing derivation of capture efficiency of a kitchen range hood in a confined flow system, based on a two-zone mixing model, is shown to be inconsistent and inadequate. A new derivation is proposed, which shows that the capture efficiency equals the ratio of captured flow rate to the total plume flow rate at the front canopy height. The new capture efficiency equals the direct capture efficiency, if we assume that the hood captures contaminant directly from the source as efficiently as it captures contaminant that is entrained from the room.

Steady-state heat losses from a building floor slab with vertical edge insulation—II

Abstract The steady-state heat loss from an infinitely long slab-on-ground floor, insulated at its edges by vertical insulation into the ground, is calculated in two dimensions from a Fourier series solution of the temperature field in the ground. The temperature at the surface of the ground is assumed to change linearly from the inside of the building to the outside over a distance representing the wall thickness. The heat loss is calculated as a function of γ=ki/ks, d/L and δ/L, where ki and ks are the conductivities of the insulation material and soil respectively, d is the insulation depth, δ is the insulation thickness, and L is the building half-width. The heat loss for small but nonzero values of γ and δ/L can be considerably greater than for the idealized situation of infinitesimally thin, perfectly insulating vertical edge insulation.

Theoretical calculations of the steady-state heat losses through a slab-on- ground floor

Abstract A new analytical expression for the two-dimensional steady-state heat loss from a slab-on-ground floor is derived as a function of the floor width, wall thickness, ground conductivity, and surface film conductance. Two approximate forms, which are easier to calculate, arealso given. Analogous approximate expressions are obtained for the three-dimensional case. Comparisons with previous theoretical work generally show good agreement, except for very small floors, for which the expressions derived here predict somewhat lower heat losses.

Steady-state heat losses from the core and perimeter regions of a slab-on-ground floor

Abstract In order to assess the likely effectiveness of insulating a concrete slab floor laid on the ground, it is useful to be able to estimate the relative magnitudes of the heat losses from the core and perimeter regions, where the latter is usually a strip of constant width. New analytical expressions are derived for such losses in the steady-state, as a function of the slab dimensions, wall thickness, soil conductivity, and average surface film conductance. A simplified expression, useful for hand calculations, is given for rectangular floors, and the ratio of core to total heat loss is presented graphically for some typical cases. The results support the generally held view that most of the losses occur from a relatively narrow perimeter region.

The effect of water table depth on steady-state heat transfer through a slab-on-ground floor

Abstract The effect of water table depth and temperature on the total heat flux through a slab-on-ground floor has not been previously analysed explicitly. In this paper the two-dimensional problem is solved using a conformal transformation. An expression for the total flux from the floor is obtained and compared with the total flux without a water table. Results are presented graphically for a water table temperature equal to the outdoor temperature, for realistic ranges of two governing parameters : the ratio of building width to water table depth, and the ratio of wall thickness to building width. For any water table depth greater than the building width, the flux is found to be within 10% of the flux without a water table.

A comparison between measured and calculated heat losses through a slab-on-ground floor

Abstract The time-dependent theory of the thermal coupling between a solid floor and the ground, previously developed by the author, is compared for the first time with experiment, using heat losses measured from the insulated floor of a small house over a period of two years. A new expression for the time-dependent heat losses from an arbitrary central region of the floor is obtained and compared with measurement, and measured temperatures at the surface of the concrete floor (under the insulation) and at 1 m depth are also compared with the theory. Because previous work has shown that for an assumed typical ground conductivity of 1.4 W m−1 K−1 the predicted average heat loss is considerably higher than that measured, the measured average values have been added to the calculated time-dependent components for the purposes of comparison. Agreement with experiment is good, particularly for the total heat losses, and the sensitivity of the calculations to the ground thermal properties, which are often unknown, is fortunately not large.