Research of the hydraulic resistance coefficient of sunny air heaters with bent pipes during turbulent air flow

Цитирование: Абдукаримов, Б. А. Исследование коэффициента гидравлического сопротивления cолнечных воздухонагревателей с изогнутыми трубками при турбулентном потоке воздуха / Б. А. Абдукаримов, А. А. Кучкаров // Журн. Сиб. федер. ун-та. Техника и технологии, 2022, 15(1). С. 14–23. DOI: 10.17516/1999-494X-0370

design [33] and relatively high efficiency [34].The general direction of work on the creation of air solar collectors is to find ways to reduce heat loss to the environment [35], intensify heat transfer on the absorber [36] and reduce the cost of pumping air through the collector [37, 38] by reducing the hydraulic resistance of the collector.

Device characteristic

A model of a solar water heater with a concave tube with low hydraulic resistance (Fig. 1) was developed; the device has a length l = 1200 mm, width = 400 mm, height h = 62 mm. This solar air heater has metal tubes in the working chamber with a small heat capacity and concave shape and is built in a checkerboard pattern. The length of each pipe is l = 150 mm. The average duct distance is l = 60 mm. At the base of the duct, a concave shape is given in two rows, this geometric figure has a depth h = 2 mm and a width = 15 mm. The geometric shape attached to the manifold ducts is internally concave along the outer surface of the channel and vice versa. When using a solar air heater, the inlet and outlet pipes are located at d = 15 mm when using a spray gun.

Fig. 1. General schematic view of a solar air heater with concave tubes. Here1-air outlet, 2-window, 3-black metal surface (absorber), 4-air duct, 5-channels for air inlet, 6-case

Pressure loss analysis on the device

This solar air heater works in two ways:

• –    Blowing air into the inside of the collector;

• –    Air intake from the manifold.

During operation by the method of inflating air, diagonal inlet and outlet nozzles of the device are used.

In the case of air intake, each channel uses its own separate channels for air intake.

The channel diagram is a checkerboard shape that covers the entire airflow of the collector through the chamber.

The working chamber of the heater has a concave geometric shape in the air pipes (Fig. 2). air striking these figures forms a vortex motion.

The depth of the concave shape should not exceed h = 2 mm, the diameter of the tube (Fig. 3).

The coefficient of hydraulic resistance in the air pipes of a solar air heater is determined as follows:

Fig. 2. Circular motion on the concave parts of the air duct

Fig. 3. General view of the air duct

Fig. 4. Schematic view of the steps of a concave air duct

^со=^Г 1 +

100(lgRe-4.6)(l-^1)1-65'

exp C^)0'3

DVn

exp

25(1-^)¹³²" л_^л0.75 - ^^Dvn J

Here ξ fl is the hydraulic resistance coefficient of a smooth air pipe; Re is the value characterizing the air flow regime, dvn is the diameter of the smaller part of the pipe, Dvn is the largest diameter of the pipeline, t is the number of steps in the pipe (Fig. 4).

The hydraulic resistance coefficient ξ fl of a smooth air tube, as mentioned above, is determined as follows:

o. 0,316 I M \ ^fl ™ Де⁰-²⁵⁴ \ji _t)

Here Re is the value characterizing the air flow regime, μ is the dynamic viscosity of air at certain temperatures, and μ st is the dynamic viscosity of heated surface air. n – the ratio of dynamic volatility to temperature differences is 1/3.

The Reynolds number is determined as follows:

Re = — .                                                        (3)

V

Here is V – speed, d – diameter, ν – kinematic viscosity of air. The dynamic viscosity of air is determined by the following:

Р = р ^^ (— ) .                                            (4)

'О т + С \T₀J

Here μ is the dynamic viscosity in (Pa) at a given temperature T ; μ 0-control viscosity in (Pa) at some control temperature T 0; T -set temperature in Kelvin; T 0 control temperature in Kelvin; Sutherland С -constant for the gas whose viscosity is to be determined.

The pressure loss on the device is determined by the following:

Ah = 2 f * .                                                     (5)

Here Σξ is the total hydraulic resistance coefficient, v is the average velocity, and 9 is the free fall velocity:

^5=6п+^ .                         (6)

Theoretical analysis of experimental results

The concave tube solar heater was mainly tested at five different speeds, and at each speed experiment, the main parameters were obtained, including the air speed, the temperature of the heated air from the heater, and the temperature of the absorber and concave tubes (Fig. 5–9, Table 1–5).

Fig. 5. The process of working a solar heater with concave tubes

Table 1. The results were obtained at five different speeds from a concave-tube solar air heater (14.08.2019)

Inlet air speed m/s	Outlet air speed m/s	Time for experience	Outdoor temperature oC	Heated air temperature oC	Absorber Surface temperature oC	Tube temperature oC
3.86	3.68	1330–1400	34	73	84	82
5.2	4.78			72	84	81
6.4	5.84			71	83	81
7.88	6.23			70	82	80
8.2	6.55			68	81	79

Fig. 6. Reynolds number versus hydraulic resistance coefficient, ^= f(Re)

Table 2. The dependence of the Reynolds number on the coefficient of hydraulic resistance

Re	5111	6766	8271	9559	9993
5 fl	0.036	0.033	0.031	0.03	0.0295
^ co	0.2	0.194	0.19	0.187	0.185
Σξ	0.236	0.227	0.221	0.217	0.214

Fig. 7. Reynolds number versus speed, v=f(Re)

Table 3. Reynolds number versus speed

Re	5111	6766	8271	9559	9993
V m/s	3.77	4.99	6.1	7.05	7.37

Fig. 8. Reynolds number versus pressure loss ∆h=f(Re)

Table.4 Reynolds number versus pressure loss

Re	5111	6766	8271	9559	9993
Δ h m	0.16	0.28	0.41	0.54	0.59

Fig. 9. The dependence of the Reynolds number on the temperature jf the heated air, t = f(Re)

Table 5. Reynolds number versus temperature of heated air

Re	5111	6766	8271	9559	9993
t oC	73	72	71	70	68

Conclusion

By replacing common pipes in the working chamber of a solar air heater with pipes with a concave geometric shape, a decrease in the hydraulic resistance coefficient and pressure loss without reducing the heat transfer coefficient of the device was achieved.