The basic principle of the hotline tachometer
Published on:2014-12-16 8:52:38 Hits:

The basic principle of the hotline tachometer 

In analyzing the basic principle of the sensing element to the hotline as a model for the thermal analysis of the film can do the same.

1. Heat balance hotline

Thermal element is the use of the heat balance principle to measure the fluid velocity. Its simple model can be seen in Figure 3-43. Hotline by current heating, its temperature is higher than the ambient temperature. When the surrounding medium is flowing, there will be heat transfer. At steady state, the current is equal to the heat of the hot heating amount of heat of the surrounding medium.

As shown, take a closed thermodynamic system, heat balance equation can be listed in the system:

Q J = Q 0 + Q K + Q f

Where QJ-- current heating heat;

Q 0 - hotline on when the surrounding fluid flow heat transfer;

Q K - hotline to stand thermal heat;

Q f - Hotline radiant heat to the surrounding space.

For such a contained convection, conduction, radiation heat transfer process complex theoretical analysis, if it can be simplified according to the actual situation, there will be greater difficulties.

For the actual hot heat transfer can be made in the following analysis:

① temperature difference between the fluid and the hotline are generally less than 300 ℃, heat radiation can be neglected Qr;

② When the fluid flow rate when more than 0.5m / s, far less than the natural convection forced convection heat transfer, can not consider the effects of natural convection;

③ minimize the hotline in design and manufacture of the probe when the forks of thermal conductivity, so that under certain conditions can be ignored hotline to guide heat forks QK.

Based on the above analysis, heat transfer process can be considered hotline is basically a forced convection heat transfer process. Heat balance equation can be simplified as:

Q J = Q 0 (3-85)

Current on the hot heat for heating:

Q J = R W (3-86)

Where I W - heating current (A);

R W - Hotline resistance ().

Convective heat transfer heat release can be written as:

Q C = h d (T W -T f) (3-87)

Where h-- exile forced on heat transfer coefficient (W / m 2 · ℃);

- Hotline length (m);

T W - hot temperature (℃);

T f - Fluid Temperature (℃).

D-- hot filament diameter.

The QJ, QC two expressions into equation (3-85), we get:

R W = h d (T W -T f) (3-88)

Thus, as long as the right to determine the hotline exile heat transfer coefficient h, you can use the equation entirely sure hotline thermal balance. According to the study of heat transfer, and the introduction of the Reynolds number:

Re = 1

Prandtl number:

P = 1

Stewart efforts color number:

Nu = 1

Three dimensionless parameter relations, the convective heat transfer coefficient and the measured velocity of the fluid, physical properties, etc., can be written as follows guidelines equation:

Nu = f (Re, Pr) (3-89)

Thus, the heat balance relationship can be expressed as the hotline

R W = (T W -T f) f (Re, Pr) (3-90)

2. hotline gram English law

The heat balance equation describes the relationship between the hotline exothermic complete tachometer, but given the absence of a specific relationship, in actual use is inconvenient.

To determine the fluid flow and heat exchange between the specific relationship, the key is to determine f (Re, Pr) specific expression Na =. Britain assumed potential grams and calories from the hotline was taken away without changing the characteristics of the circulation loop position assumed by the hotline, made the following specific relationship:

Nu = 1 + (2 PrRe) (3-91)

On the basis of gram-British relations, the 1946 雷默斯 of gases and liquids through a lot of testing, made the following formula:

Nu = 0. 42Pr 0. 2 +0. 57Pr 0. 33 Re 0. 5 (3-92)

Substituting this into equation (3-90), we get:

RW = (TW -T f) (0. 42Pr 0. 2 +0. 57Pr 0. 33 Re 0. 5) (3-93)

Assuming: A = 0 42 Pr 0. 2.

B = 0. 57 Pr 0.33 () 0. 5

So, the equation can be written as:

RW = (A + Bu 0. 6) (TW -T f) (3-94)

Wherein, A, B is a coefficient from the hot dimensions, fluid properties and flow conditions determined. For a given hotline tachometer and given the measured fluid can be considered A, B is a constant.

In the formula (3-94), flow index of 0.5 u can only set up within a certain range of flow rates, in a more precise sense, it is the fluid velocity changes with the change, it is generally to 1 / m of expressed. According to a recent study showed that in the usual speed range, the hotline run: 1 / m = 0.45 has a better correlation. Other forms of the thermosensitive element, there are a large deviation. Therefore, a more general form of the equation can be written as:

R W = (A + Bu) (T W -T f) (3-95)

This equation is called the hotline tachometer grams English formula, it is hot tachometer basic relationship, written - as a function of the form u = f (I w, T w). That may change in the fluid velocity in the heating current Iw, hot temperature changes in both the anti-Tw manifested.

In actual use, you can put Iw and Tw both a fixed together to form two different ways of working tachometer.

If the use of a constant current source current is kept constant, changes in fluid velocity can be obtained from the hotline temperature change (change in resistance hotline), namely:

Iw = constant u = f (Tw) (3-96)

This is called the hotline of constant work.

If you use the bridge to make the hotline to keep the temperature constant, changes in fluid velocity can be obtained by current changes hotline, that is:

Tw = constant u = f (Iw) (3-97)

This is called the hotline thermostat works.

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