JPH0521487B2 - - Google Patents

Description

[Detailed description of the invention]

(Technical Field) The present invention relates to a method for simultaneously measuring flow velocity in a pipe and dirt on the inner surface of a pipe. (Prior art and its problems) Conventionally, condenser pipes, heat exchanger pipes, and other pipes in which a certain fluid such as cooling water is allowed to flow, have their inner surfaces contaminated by contact with the fluid being caused to flow. For example, Japanese Patent Application Laid-open No. 1983
As shown in Japanese Patent No. 26809, etc., there is a method using a steady-state dirt meter that estimates the dirt coefficient based on the temperature difference between two positions under constant heater power. This method using a steady-state fouling meter involves heating the outer circumferential surface of the pipe to be measured over a predetermined length with a heater. The difference between the temperature of the fluid on the side and the temperature of the fluid on the side: ΔT is calculated.In this case, when the power applied to the heater is constant and the flow rate inside the pipe is constant, the temperature difference: ΔT is calculated as Estimation of such contamination is made based on the influence of contamination. In addition, for other measurement methods using a transition type stain meter that directly calculates the internal heat transfer coefficient from the cooling curve of a heater block (made of thick-walled copper) attached to the outer circumferential surface of the pipe to be measured, We are attempting to diagnose contamination based on the fact that the cooling curve of the block is affected by contamination on the inner surface of the tube when the flow rate in the tube is constant. However, when measuring contamination using these conventional contamination meters, it is necessary to know the flow velocity inside the pipe, so a flow meter is installed separately in the system, and these two devices are used for the first time. ,
It is now possible to determine the dirt coefficient or dirt diagnosis. Therefore, separately providing such a flow meter complicates the overall dirt measurement system and also becomes a factor in increasing costs. (Solution Means) The present invention has been made against the background of the above, and aims to provide a method that can measure flow rate and contamination at the same time. When simultaneously measuring the flow rate of the fluid in the pipe to be measured and the dirt on the inside of the pipe, two heating elements with different lengths are arranged in the axial direction of the pipe to be measured to form two types of heating sections. While the fluid is flowing through the pipe, each of the heating elements heats the outside surface of the pipe, and the temperature of the outside surface of each pipe in the heating section where the heating elements are installed and the temperature sufficiently upstream of the heating elements are determined. The heat transfer rate is determined from the difference between the obtained upstream fluid temperature and the pipe outer surface temperature in each heating section, and the heat transfer rate is calculated based on the dirt thickness and flow rate. Since it is a function of By determining and solving both of these relational expressions simultaneously, the flow velocity of the fluid in the pipe and the contamination on the inside of the pipe were determined. (Specific configuration/example) By the way, when a flow inside a pipe is locally heated from outside the pipe, it is recognized that there is a functional relationship between the amount of heating, the temperature of the pipe wall, the flow rate of the fluid, the physical property values, etc. However, in addition to this, the inventors' numerical analysis revealed that the characteristics of heat transfer with respect to flow velocity differ depending on the length of the heating section. In the present invention, by utilizing such characteristics, that is, by heating sections of two different lengths arranged in one pipe, the flow rate and dirt in pipes such as condensing organs and heat exchanger pipes can be reduced. They succeeded in simultaneously taking measurements during operation. Therefore, we will clarify the measurement principle for simultaneously measuring the flow rate (flow velocity) and dirt in the pipe below, but first, we will explain the meanings of the symbols used here, their units, and subscripts. They are listed below. Symbols Di, Do: Inner diameter and outer diameter of the pipe (m) eu: Relative error in flow velocity [=(u _cat − u _p )/u _p ]
(−) e _R : Relative error of contamination coefficient [=(R _Fcat −R _Fp )/
R _Fp ] (-) G: Mass flow rate (Kg/s) L: Length of heating section (m) Nu _p : Nusselt number (ignoring solid wall thickness) (-) Q: Amount of heating (w) q: Heat flux (W/m ² ) R _F : Fouling coefficient (=δ _F /λ _F ) (m ² k/w) R ~ _F : Dimensionless staining coefficient (-) R ~ _w : Dimensionless pipe wall Thermal resistance (-) Re: Reynolds number (-) T _f : Fluid temperature well upstream of the heating section (°C) Ti: Temperature of the slime inner surface (°C) Two: Average temperature of the tube outer surface (°C) Two ^* : Primary heat flow Average temperature on the outer surface of the tube (℃) Twi: Average temperature on the inner surface of the tube (℃) u: Average flow velocity (m/s) α _p : Heat transfer coefficient (ignoring solid wall thickness) [w/( ^m2・k] ΔTε: Measurement error of (Two−T _f ) (℃) Θ: Dimensional temperature difference (−) Θ ^* : Dimensional temperature difference when heat flow is one-dimensional (−) δ _F : Slime Thickness (μm) ε _p : Measurement error of heating amount Q (-) λ _F : Thermal conductivity of swim [w/(m・k)] λ _f : Thermal conductivity of fluid [w/(m・k) ] λ _w : Thermal conductivity of the tube [w/(m・k)] ν: Kinematic viscosity (m ² /s) Subscript C: When clean F: When slime is attached L: Long heating section (L heater) S: Short heating section (S heater) O, ca: True value and calculated value, respectively. First, in Figure 1, the amount of heating and the average temperature on the outer surface of the pipe are measured, and the flow rate and contamination inside the pipe are measured at the same time. A physical model of the method is shown, in which the flow in a pipe with a mass flow rate G with a sufficiently developed turbulent velocity distribution is axially symmetrical from the outside of the pipe, and is electrically uniform with a heat amount Q over a length L section. Here, the heat supply is assumed to be a constant amount of heat per unit time.Then, the thermal conductivities λ _w and λ _f of the tube wall and the fluid, and the fluid The physical properties are constant, and the material and dimensions of the pipe, heating section, and type of fluid are given.Also, the thickness of the dirt adhesion layer (slime) is δ _F , and the thermal conductivity is λ _F. but,
Actually, the measurement item is the dirt coefficient R _F =δ _F /λ _F. Electrical insulating films are thin and have low thermal conductivity, so
It is assumed that the heat flow in the tube axis direction can be ignored. Therefore, the temperature is the average temperature on the outside surface of the tube.
Consider Two, the average internal temperature of the slime Ti, and the temperature T _f on the upstream side of the fluid. Also, here, the average is
It means time average. Under such circumstances, assuming that the heat flow is one-dimensional in the radial direction, the temperature difference Two ^* − T _f will be expressed by the following equation (hereinafter, the temperature in this case will be denoted by ^*) . ). (Two ^* −T _f ) _F = Q/πDiL{Di/2λ _w nDo/Di+
δ _F /λ _F +1/α _p }...(1) Here, α _p is the convective heat transfer coefficient when the solid wall thickness is ignored, and (Ti−T _f ) is taken as the temperature difference. be. When this equation (1) is made dimensionless by dividing it by (Q/πλ _f L), the following equation is obtained. Θ _F ^* = (Two ^* −T _f ) _F /Q/πλ _f L=R ~ w + R ~ _F +
1/Nu _p ...(2) However, R~ _w = λ _f /2λ _{w o} Do/Di...(3) R~ _F = λ _f /DiR _F = λ _f /Di δ _F /λ _F ... (4) Nu _p = α _p Di / λ _f ...(5) By the way, the dimensionless temperature Θ _F in the actual case cannot be expressed as in equation (2) because there is heat flow in the direction of the tube axis. . However, considering that the solid wall becomes Θ _F ^* in the limit when it becomes small, we assume the form of Θ _F as follows. Θ _F = (Two−T _f ) _F /Q/πλ _f L=Θ _F ^* −ξ _F (Re, R
~ _F )...(6) Here, Re=uDi/ν...(7) Also, ξ _F includes the effects of the material and dimensions of the pipe, the length of the heating section, the type of fluid, etc. Yes, but
Here, we will consider the case where the material and dimensions of the pipe and the fluid are given, so for now we will consider it as a function of Re, R ~ _F. Note that for Re, the influence of cross-sectional area reduction due to slime adhesion is considered only for u. The above equation (6) becomes as follows in the case of a clean pipe with no slime adhesion (R _f 〓=0). Θ _c = Θ _c ^* −ξ _c (Re) = (R ~ _w + 1/Nu _p ) − ξ _c (Re) ...(8) In other words, ξ _F is ξ _c when R ~ _F = 0 Therefore, we assume that it can be decomposed as follows. ξ _F (Re, R ~ _F ) = ξ _c (Re) + ξ _F ′ (Re, R ~ _F )
...(9) The functional form of Nu _p with respect to Re can be obtained based on the numerical solution. Therefore, given R _{~ w} and R~ _F , Θ _F ^* and Θ _c ^* can be calculated [Equation (2)], and then the functional form of Θ _c (Re) from the numerical solution can be obtained [Equation (8)].
Furthermore, from the numerical solution Θ _F and the obtained Θ _F ^* , ξ _F [Equation (6)], ξ _c
From ξ _F , we can find the functional form of ξ _F ′. If the long heating section (L heater) and short heating section (S heater) are expressed by subscripts L and S, the following equation can be derived from equations (6) and (2) above. . Θ _FL −(R〜 _W +R〜 _F +1/Nu _pL )+ξ _FL =0……(10) Θ _FS −(R〜 _w +R〜 _F +1/Nu _pS )+ξ _FS =0……(11) Therefore , for these equations (10) and (11), measure T _f and T _wp and Q of the two types of heating sections installed in one pipe, and substitute them into Θ _FL and Θ _FS . This results in a simultaneous equation of Re, R~ _f , and by solving it, the desired flow velocity and dirt coefficient can be obtained. The functional form of ξ _c is expressed by the following formula, ξ _c = A ₀ + A ₁ {10/nRe} ⁿ ...(12) And the coefficients A ₀ , A ₁ and index n in this formula (12) For example, the value of A brass pipe or Ti pipe is
When L/Di=4 (L heater) and L/Di=0.25 (S heater), it is given by Table 1 below.

[Table] Also, the functional form of ξ _F ′ is shown by the following formula, ξ _F ′ = B ₁ R _F + B ₂ R _F ² ...(13) And B ₁ and B ₂ in this formula (13) are , is approximated by the following formula: B ₁ = b ₁₀ + b ₁₁ nRe + b ₁₂ (nRe) ² B ₂ = b ₂₀ + b ₂₁ nRe + b ₂₂ (nRe) ² ...(14), and the coefficient in this formula (14) is A
When L/Di=4 or 0.25 for brass tubes and Ti tubes, the results are as shown in Table 2 below.

[Table] Accordingly, in the present invention, based on the above measurement principle, for example, as shown in FIG. The S heater 4 has a short length (for example, 1/4 times the inner diameter of the tube), and the S heater 4 has a long length in the tube axis direction (for example,
Four times the inner diameter of the pipe) L heaters 6 are arranged to form two types of long and short heating sections, and a constant power (heat flux: q) is loaded to each of the S heaters 4 and L heaters 6. to detect the average temperature on the outer surface of the tube in each heating section, and to detect the average temperature on the outside surface of the tube in each heating section,
The temperature of the fluid sufficiently upstream of the heater 4, L heater 6), for example, the pipe 2 to be measured of the fluid flowing through it.
Detect the inlet temperature of and calculate the respective temperature difference (ΔT _S ,
ΔT _L ). Note that the temperature of the fluid sufficiently upstream of the heating section can be detected over time with a suitable thermometer, or the temperature of the pipe wall when not heated can be used as the fluid temperature. The average temperature (time average) on the outside surface of the tube in these heating sections can be easily detected from the electrical resistance value of the thermocouple or heater itself installed approximately at the center of each heating section, and the The amount of heat input in the section, in other words, the heat flux q _S , q _L
can be determined from the amount of current applied by the DC power supplies 8 and 10 to the respective heaters 4 and 6. Then, ΔT _S , ΔT _L and Q _S detected above,
Using _QL , the flow velocity inside the tube to be measured 2 and the contamination (coefficient) on the inside surface of the tube can be calculated simultaneously by the microcomputer 12 or the like from the above-mentioned equations (10) and (11). In addition, in the figure, 13 is a heat insulating material. Incidentally, according to the method of the present invention, the relationship between the heat transfer rate (h=q/ΔT), flow rate, and fouling coefficient (R _F ) for an aluminum brass tube (outer diameter: 25.3 mm, wall thickness: 1.24 mm) was calculated. The obtained results are shown in FIG. As a result of this experiment, the measurement accuracy was found to be good, with a flow rate of ±5% and a dirt coefficient of ±13%. In addition, in this experiment, the heater was
A 7.5 μm polyamide film was used as the insulating layer in contact with the outer surface of the tube, a 10 μm Ni foil was layered on top of that, and a 35 μm polyamide film was layered on top of that, and the S heater 4 had a length of 6 mm. In addition, the length of the L heater 6 is approximately 100 mm, and the average temperature of the outer surface of the tube in each heating section is calculated from the current flowing and the voltage detected by the mV meter connected to both ends of the heater. Estimated from the calculated electrical resistance of the heater. Further, the amount of heat q input to each heater was 20000 W/m ² . (Effects of the Invention) As is clear from the above explanation, the contamination diagnosis method using heat transfer that has been studied in the past applies the well-developed turbulent heat transfer characteristics, and it is possible to apply it to an actual plant. However, the present invention takes advantage of the fact that heat transfer characteristics vary depending on the heating length. As a result, it has the characteristics of being able to measure the contamination coefficient and flow rate at the same time, and the sensor can be made relatively small, making it possible to diagnose the contamination status of actual plants in real time, and The cost of countermeasures against pollution in plants can also be significantly reduced. Further, according to the present invention, since the fouling coefficient can be directly measured in real time, the measurement accuracy is higher than indirect methods such as measuring the pressure loss coefficient of the sample tube. The cost can be reduced compared to methods such as performance comparison using instruments.

[Brief explanation of drawings]

FIG. 1 is an explanatory diagram showing a physical model for explaining forced convection heat transfer characteristics in a locally heated pipe, and FIG. 2 is a cross-sectional explanatory diagram showing a system for implementing the present invention. , and FIG. 3 is a graph showing the relationship between the heat transfer rate h, the fouling coefficient _RF , and the flow rate determined in the example. 2...Tube to be measured, 4...S heater, 6...L heater, 8, 10...DC power supply, 12...microcomputer, 13...insulation material.

Claims (1)

[Scope of Claims] 1. A method for simultaneously measuring the flow velocity of a given fluid and the dirt on the inside of the pipe in a pipe through which the fluid flows, which comprises using two heating elements having different lengths in the axial direction of the pipe to be measured. These heating elements are arranged to form two types of heating zones, and while the fluid is flowing through the tube, heating is performed from the outside surface of the tube by these heating elements, and the heating zone in which these heating elements are installed is heated. By detecting the temperature of each tube outer surface and the temperature of the fluid sufficiently upstream of these heating elements, the heat transfer rate can be calculated from the difference between the obtained upstream fluid temperature and the tube outer surface temperature in each heating section. Also, since the heat transfer rate is a function of the dirt thickness and flow velocity, find the relational expression between the fluid temperature on the upstream side, the pipe outer surface temperature of one heating section, and the dirt thickness and flow velocity, which are each unknown. On the other hand, by determining a similar relational expression for another heating section and solving both relational expressions simultaneously, the in-pipe flow velocity and the tube inner surface contamination of the fluid are determined. Simultaneous measurement method for surface stains.