Analysis of sensitive components in the basic working principle, to the hotline as a model, the hot-film can do the same analysis.

1Hotline between the heat balance

Thermal devices are used to measure the thermal balance principle of fluid speed. This shows that the simple model shown in Figure 3-43. Current hotline for heating, the temperature is higher than the surrounding medium temperature. When the surrounding medium flows, there will be heat transfer. In a stable state, the current on the hotline around the heating medium heat equivalent to the heat dissipation.

figure, from thermodynamics closed system, which can list the system of thermal balance equation:

Q J = Q 0 + Q K + Q f

-in QJ - current heating heat;

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

QK - Support Hotline to the thermal heat;

Q f - hotline to the space around the heat radiation.

to contain such a convection, thermal conductivity, radiation heat transfer the complex process of theoretical analysis, not based on the actual situation can be simplified, there will be more difficult.

hotline for the actual conditions of heat transfer can be made following analysis:

hotline between the fluid and temperature difference generally less than 300 , can ignore the heat radiation Qr;

when the fluid flow rate of greater than 0.5 m / s, far less than natural convection heat transfer forced convection heat transfer, can not consider the impact of natural convection;

in the design and manufacture probe when the hotline to minimize cross-thermal conductivity, so under certain conditions can be ignored hotline on cross-calorie derivative of the QK.

accordance with the above analysis, a hotline that can heat transfer process is essentially a forced convection heat transfer process. Heat balance equation simplifies to:

Q J = Q 0 (3-85)

current on the heat for heating the hotline:

Q J = R W (3-86)

where I W - heating current (A);

R W - hotline Resistance ().

heat transfer of heat release can be written:

QC = hd (TW-T f) (3-87)

-in h - the forced exile of coefficient (W / m 2 );

- hotline length (m);

T W - hotline temperature ();

T f - fluid temperature ().

D - hot wire diameter.

will QJ, QC-on behalf of the two expressions (3-85), available:

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

so, as long as the hotline to determine the coefficient of thermal exile h, will be determined completely on the use of the hotline between the heat balance. According to the heat transfer research and the introduction of Reynolds number:

Re = 1

Prandtl number:

P = 1

few hard-GOLD:

Nu = 1

three dimensionless parameters, the coefficient of heat transfer fluid and measured the speed, physical properties, such as the relationship between the guidelines equation can be written as follows:

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

a result, the thermal balance between the hotline can be expressed as

RW = (TW-T f) f (Re, Pr) (3-90)

2. Hotline grams of the British law

the heat balance equation describes the hotline gun integrity of the heat, but did not give details of the relationship, when in actual use is inconvenient.

to determine the fluid flow and heat for the specific relationship between, the key is to identify Na = f (Re, Pr) the specific expression. According to the British grams hotline potential circulation and heat from the assumption that the hotline was taken away not to change the circulation of the assumption, made the following specific relationship:

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

grams in the Sino-British relations on the basis of 1946 through the ramus of the large number of gases and liquids test, proposed the following formula:

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

this generation-in-(3-90), available:

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

if established: A = 0. 42 Pr 0. 2

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

then, can be written-on:

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

-in, A, B by the hotline size, fluid-flow conditions and the coefficient. For a given gun of the hotline and given the fluid under test, it that the A, B is a constant.

in-(3-94), the flow rate of only 0.5 u the index to a certain velocity of the establishment, in a more accurate sense, it is as fluid and changing the speed of change, the general 1 / M in the form said. According to a recent study showed that commonly used in the speed range, speaking on the hotline: 1 / m = 0.45 a better relationship. Other forms of thermal components have greater deviation. Therefore, on-the more general form can be written:

RW = (A + Bu) (TW-T f) (3-95)

called the hotline on the type of gun grams of the British formula, it is of fundamental hotline gun relationship, written - as function of the form u = f (I w, T w). That the velocity of change in the heating current Iw, anti-Tw hotline between the temperature changes reflected.

in actual use, can be both Iw and Tw in a fixed, and thus two different working methods of the gun.

if the use of constant current source to maintain the current constant, the temperature can change from the Hotline (hotline changes in the value of resistance) are fluid speed of change, that is:

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

This is called the hotline of the constant current methods of work.

bridge so that if the use of the hotline to maintain a constant temperature, through the hotline can be changes in the current velocity of change, that is:

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

This is called the hotline of the thermostat working methods.