Developing an effective deflector for duct natural ventilation

. Information is given on increasing the available gravitational pressure of natural duct ventilation by increasing the use of wind flow energy. The scheme and operation of the developed deflector with a vertical finned cylindrical surface based on the CAI deflector are described. The laboratory bench scheme developed and patented for evaluating the efficiency of various types of deflectors is presented. Thermo-anemometer was used in experimental studies as instrumentation TKA-PKM (52), as well as a wing type anemometer. The effectiveness of the developed deflector was evaluated using the mathematical method of planning and processing of experiments. Scientific and experimental investigations were carried out on the basis of an orthogonal composite plan of the first order. A complete factorial experiment was carried out in CFE 23. A regression equation adequately describes the influence of the design and process parameters on the air velocity in the natural ventilation duct at the significance level α = 0.05. Experimental studies have confirmed that it is possible to increase the degree of utilization of wind stream energy when it flows over a vertical finned cylindrical surface in comparison with a smooth cylindrical surface. It is shown that air velocity in exhaust duct can be increased up to values of 3.35 m/s.


Introduction
Natural ventilation vents are designed and installed in residential buildings.The indoor climate, i.e. the heat, air, and humidity conditions, depends on its efficiency.It has a direct impact on people's well-being and health.The problem of enhancing the effectiveness of natural duct ventilation has recently become more acute in connection with increasing heat protection of the enclosing structure of the building (installation of plastic windows, construction of buildings with ventilated facades, etc.) and is an urgent issue.In duct systems of natural ventilation, the supply of external air and removal of polluted internal air is performed through special ducts provided in the building structures, or through attachable ducts at the expense of the available gravitational pressure [1 -3].

Gravitational pressure P
 is determined by the equation [4]: where h -vertical distance from the center of the ventilation opening in the room to the air outlet (shaft), m; Н ρ -the density of outside air at tout = 5 о С, kg/m 3 ; В ρ -indoor air density, kg/m 3 ; g -acceleration of gravity -9.81 m/s 2 .
The gravity pressure can obviously be increased by increasing the use of wind energy, i.e., by improving the known and developing efficient devices -special ventilation nozzles: deflectors mounted at the mouths of exhaust shafts.

Main text
The CAI air deflector is widely used in Russia, its diagram is shown in Figure 1 [4,5] The deflector can have a cylindrical or rectangular cross-section.The purpose of the main parts of the CAI deflector ventilation nozzle is shown below.The deflector is attached to the exhaust air duct (exhaust shaft) by means of a spigot 1. Diffuser 2 is designed as a truncated cone.The lower part of the cylindrical bulb sits on top of the ventilation duct leading through the roof of the building.The diffuser slows down the air velocity and increases the static pressure.Hood 5 is an upper protective cap attached to the diffuser by three studs (clutch 4).It prevents penetration of precipitations and debris into the ventilating duct.Casing 3 is designed as a hollow cylindrical shell.The shell is connected to the diffuser by three stays (clutch 4).The plane of the casing dissects the air flow and creates a low-pressure area inside the cylindrical shell, which increases the available pressure in the ventilating duct and increases the exhaust air velocity in the duct ventilation, i.e., the draught.
In our opinion, in the deflector CAI wind energy is not fully utilized.To increase the available pressure, it is proposed to retrofit the deflector CAI.
The cylindrical surface of the deflector's shell, when it is flowed around by the wind flow, allows to use only a part of the flow energy when it is blown off (descending) near the upper and lower bases.The velocity vector of the wind flow is perpendicular to the cylinder shell surface, i.e., parallel to the plane of its bases.Therefore, increasing dynamic pressure on the cylindrical shell reduces static pressure on its end faces by a smaller value, as follows from Bernoulli's equation [4], i.e., there is a reserve of increased ejection velocity of internal air coming from natural ventilation channel.
To improve the efficiency of the deflector for extracting polluted air from the premises in the system of natural ventilation in the process of operation, it is proposed to change the external shape of the surface of the deflector.To achieve the stated technical result, we made a decision: on the outer surface of the cylindrical shell on its perimeter to place a corrugated band, forming vertical grooves and channels by joining of ribbed equilateral triangular elements, the depth of which is 10...15 % of the branch pipe diameter [2].
Scheme of the developed deflector for exhaust of contaminated air is shown in   This design increases the velocity of the wind jet flowing towards the end sections of the cylindrical shell in the deflector CAI, due to an increase in its flow rate due to a simultaneous decrease in flow rate in the tangential direction from the corrugated baffle strip.Increasing the velocity of the wind flow will increase the dynamic pressure, correspondingly decreasing the static air pressure at the end sections of the baffle casing by the same amount according to Bernoulli's equation [3]: where РS, Рd -static pressure, dynamic flow pressure, respectively.Pa; ρ -air density, kg/m 3 ; v -wind speed, m/s; Рcomp.-complete flow pressure, Pa.
A greater reduction in static pressure at the end sections of the body of the proposed deflector creates a greater vacuum at the mouth of the exhaust stack (ventilation riser), which will increase the linear velocity of air in it, respectively, the volume flow rate of air extracted from the premises.

Methodological part
Experimental studies to assess the effectiveness of the deflector were carried out on a laboratory stand, its scheme is shown in Fig. 3   The experimental set-up consists of a vertical cylindrical draft tube 1, diameter D =100 mm, which is fixed to the cross piece 2. The cross-arm has screw holes at the ends for studs 3. The studs allow adjusting the height of the bottom of the pipe above the fixed support (floor).The pipe ends with flange 4, which serves for fixing the experimental deflector 5. To assess the effectiveness of the deflector mounted on a cylindrical tube, an artificial wind pressure is created.The wind is simulated by means of a radial fan positioned at the same height as the deflector.The wind velocity at the surface of the deflector, as well as the velocity of ejected air stream in the exhaust stack 1 -v, m/s were measured with the help of combined electronic device -thermo-anemometer.TKA-PKM (52), as well as with a wingtype anemometer.
Experiments were carried out using the mathematical method of planning and processing experiments [5].
A first-order plan has been implemented CFE 2 3 (table ).The factors chosen were: X1 -wind speed (4 -10 m/s); X2 -distance from the deflector to the fan (0.4 -1.2 m); Х3 -height of protrusions (15 -30) mm.As a response function -Y is the assumed air velocity in the exhaust stack, m/s.
where Fest -estimated F -attitude, F0.05(4;8) -tabular value of the Fisher criterion at a significance level of 0.05 and degrees of freedom of 4 and 8 [5].
The analysis of equation (3) shows that the air velocity in the duct (exhaust stack) was significantly influenced by the wind speed and the depth of the vertical ducts.At the same time, the distance from the deflector to the ventilator in the selected variation intervals had no significant effect on the response function.With increasing wind speed, the air velocity in the duct increases between selected variables, from 1.81 m/s to 3.35 m/s, which is higher than that of the CAI baffle plate [1].

Conclusion
Based on scientific and experimental research performed, an effective construction of a deflector with a corrugated external surface for natural duct ventilation has been developed and tested under laboratory conditions.The developed deflector is recommended for implementation in the housing and communal complexes of the Russian Federation and others regions.

Table 1 .
Planning matrix CFE 23and the results of its processing.