SU1205021A1 - Hot-wire anemometer - Google Patents

Description

The invention relates to devices for measuring parameters of vapor-gas environments,

The aim of the invention is to improve sensitivity, accuracy and. increase in speed.

FIG. 1 shows a thermal anemometer, a longitudinal section 5 in FIG. 2 the same cross section; in fig. 3 shows a section in FIG. I (isometric)

The thermoanemometer consists of a heat ™ insulating body 1, having a rectangular gas channel 2, a temperature-sensitive element 3 located in it, and gas inlets 4. Flat metal inserts 5 o are fixed in the gas channel 2. 7, Thermo ™ sensing element 3 contains flat thermistors (for example, two fixed cs sealed with dielectric substrate 9.

Thermoanemometer works as follows.

Thermistors during the passage; deNII of electric current produces heat. The complete heat transfer equation for a heated temperature-sensitive element with a gas flow is

, F, - | 2- ,,

e,

-Go,

 loss x

, -A, heat, discharged

due to heatpro ™

gas conduction

on the case;

te

flow;

thermal losses through the transverse - section of the sensitivity of the element on the body; heat conduction coefficient of the housing;

thermal conductivity coefficient; sensitive element;

area of thermoresistors;

ten

15

2

the cross-sectional area of the temperature-sensitive element; wall thickness; mass flow rate of gas; specific heat of gas

;

yt

distance from thermistor to housing; overheating of thermistors.

The thermo-anemometer operates under the condition that all the heat provided on the thermistors is carried away by the flow, i.e.

20

F

DT

 losses

This condition is fulfilled with an infinite thermal resistance of the body (with a large wall thickness and low thermal conductivity) and with a heat sink through the cross section of the sensing element, much smaller, heme heat sink in gas flows.

..F, .T

where about heat transfer coefficient. This inequality holds true

 Y oi E

45

50

55

40

where D is the thickness of the heat-sensitive element; K is its length.

The indications of the temperature-sensitive element are influenced by thermal fields appearing on the surfaces of the gas channel parallel to the temperature-sensitive element due to the heat transfer by the gas along the channel. In the dynamic mode of the hot-wire anemometer, the gas velocity and its direction change, as a result of which the thermal field of the gas channel walls changes, which leads to measurement errors. When mounted on the surfaces of the gas channel 2 of flat metal inserts 5, the redistribution of thermal fields occurs quickly due to their

low thermal resistance. Therefore, the rate of change of thermal fields is determined by the heat resistance of heat transfer to the gas flow, since the inertia of metal inserts is equal to v

2.:

me

06

where m is the mass of the metal insert with its specific heat capacity; about - Heat transfer coefficient.

In order that the change in the thermal field does not lead to the occurrence of measurement errors, it is necessary that the time of this change be less than the inertia of the temperature-sensitive element. This is possible when the thickness of the metal inserts is half the thickness of element 3. The readings are influenced by the thermal fields of the surfaces of the gas channel, parallel to element 3, located above

ten

2050214

and under them, therefore, metal inserts 5 are located on the surfaces of the g channel and parallel to element 3, are equal to it in area 5 and similar in geometrical form. The thermal balancing device is made in the form of rods installed in the housing in open holes with ability to move. A change in the thermal resistances of the housing over the surfaces of the thermistors is achieved by varying the distance from the metal rods to the surface of the thermistors. A layer of air above the metal rod 7 is involved in the formation of the thermal resistance of the housing, therefore, to eliminate convective heat exchange with the gas flow, the holes 6 in the housing 1 are made through. The rods 7 can be mounted on a threaded connection,

A a

15

20

/ g

2.

 h

FIG. five

Claims (1)

THERMOANEMOMETER containing a thermosensitive element made in the form of metal thermistors placed on a dielectric substrate located in a heat-insulating casing with a gas channel equipped with gas inlets, characterized in that, in order to increase sensitivity, accuracy and speed, the gas channel is made rectangular in section and is equipped with flat metal inserts placed on its walls symmetrically with respect to the plane of installation of the thermosensitive element, in filled in the form of flat thermistors, the metal inserts being equal to the heat-sensitive element ^ in area, similar in geometric shape and half as thick as that, the case is equipped with a thermal balancing device in the form of metal rods mounted to move above the thermistors in through holes, made in the case. Figure 1