METHOD AND DEVICE FOR MEASURING A FLOW VELOCITY OF A GAS STREAM

Abstract

The invention relates to a method for measuring a flow velocity (v) of a gas stream (14) featuring the steps: (a) time-resolved measurement of an IR radiation parameter (E) of IR radiation of the gas stream (14) at a first measurement point (P1) outside of the gas stream (14), thereby obtaining a first IR radiation parameter curve (E.sub.g1,1(t)), (b)time-resolved measurement of an IR radiation parameter (E) at a second measurement point (P2) outside of the gas stream (14), thereby obtaining a second IR radiation parameter curve (E.sub.g1,2(t)), (c) calculation of a transit time (τ1) from the first IR radiation parameter curve (E.sub.g1,1(t)) and the second IR radiation parameter curve (E.sub.g1,2(t)), in particular by means of cross-correlation, and (d) calculation of the flow velocity (vG) from the transit time (τ1), (e) wherein the IR radiation parameter (E.sub.g1) is measured photoelectrically at a wavelength (g1) of at least 780 nm, and (f) a measurement frequency (f) is at least 1 kilohertz.

Claims

1. A method for measuring a flow velocity of a gas stream, comprising: (a) time-resolved measurement of an IR radiation parameter of IR radiation of the gas stream at a first measurement point outside of the gas stream, thereby obtaining a first IR radiation parameter curve, (b) time-resolved measurement of the IR radiation parameter at a second measurement point outside of the gas stream, thereby obtaining a second IR radiation parameter curve, (c) calculation of a transit time from the first IR radiation parameter curve and the second IR radiation parameter curve, and (d) calculation of a the flow velocity from the transit time, (e) wherein the IR radiation parameter is measured photoelectrically at a wavelength of at least 780 nm, and (f) a measurement frequency is at least 1 kilohertz.

2. The method according to claim 1, wherein characterised in that (i) the gas stream is a stream of a gas mixture that contains a first gas and at least a second gas, (ii) the first gas has a first gas excitation wavelength and (iii) the IR radiation parameter is an irradiance at the first gas excitation wavelength.

3. The method according to claim 2, wherein (i) the second gas has a second gas excitation wavelength and (ii) the method further comprises the following steps: (a) time-resolved detection of a second IR radiation parameter in a form of an irradiance at the second gas excitation wavelength at the first measurement point, thereby obtaining a first irradiance curve, (b) time-resolved detection of the second IR radiation parameter at the second measurement point, thereby obtaining a second irradiance curve, (c) calculation of a second transit time between the first and second irradiance curves, and (d) calculation of the flow velocity from the first transit time and the second transit time.

4. The method according to claim 3Error! Reference source not found., further comprising: filtering out of IR radiation of the gas stream that does not lie within a predetermined first measurement interval of 0.3 μm around the first gas excitation wavelength or a predetermined second measurement interval of 0.3 μm around the second gas excitation wavelength.

5. The method according to claim 1 wherein the IR radiation parameter is measured at a wavelength of at most 15 μm.

6. The method according to claim 1 wherein a temperature of the gas stream is at least 200° C.

7. A device for measuring a flow velocity of a gas stream, comprising: (a) a first IR radiation sensor for the time-resolved measurement of a first IR radiation parameter of IR radiation of the gas stream to obtain a first IR radiation parameter curve, (b) a second IR radiation sensor for the time-resolved measurement of the first IR radiation parameter of IR radiation of the gas stream to obtain a second IR radiation parameter curve, and (c) an evaluation unit designed to automatically calculate a transit time between the first IR radiation parameter curve and the second IR radiation parameter curve, and calculate a flow velocity from the transit time, (d) wherein the IR radiation sensors are photoelectric IR radiation sensors and have a measurement range with a lower cut-off wavelength that is at least 0.78 μm and have a measurement frequency of at least 1 kilohertz.

8. The device according to claim 7, wherein the measurement range has an upper cut-off wavelength that is at most 15 μm.

9. The device according to claim 7, wherein the evaluation unit is configured to automatically conduct a method according to claim 1.

10. The device according to claim 7, further comprising (a) a pipe for conducting the gas stream, wherein the first IR radiation sensor and the second IR radiation sensor are arranged to detect IR radiation from the pipe, or (b) an outflow or through-flow opening, wherein the first IR radiation sensor and the second IR radiation sensor are arranged to detect IR radiation of the gas stream flowing out of the outflow opening.

Description

[0037] In the following, the invention will be explained in more detail by way of the attached figures. They show

[0038] FIG. 1 a device according to the invention for conducting a method according to the invention according to a first embodiment, and

[0039] FIG. 2 a device according to the invention for conducting a method according to the invention according to a second embodiment.

[0040] FIG. 3 depicts a device according to the invention for conducting a method according to the invention according to a third embodiment.

[0041] FIG. 1 shows a furnace 10 in which a gas stream 14, in this case in the form of an exhaust gas stream, is produced by combustion or other exothermic processes or external heat supply of a fuel by means of a burner 12. A temperature T of the gas stream 14 is above T=1400° C., for example. As in the present case, the furnace 10 can be a device for heating a metal bath or a glass bath 16. The furnace may also, for instance, be part of a power plant or cement plant. A furnace, power plant or cement plant with a measurement device according to the invention is also a subject of the present invention. The gas stream 14 runs through a pipe 18.

[0042] FIG. 1 also depicts a measurement device 20 for measuring a flow velocity vG of the gas stream 14. The flow velocity vG is the average flow velocity which, when multiplied with a cross-sectional area A of the pipe 18, gives the volumetric flow of gas. In the present case, the pipe is circular, so that the cross-sectional area results in A=πD.sup.2/4.

[0043] The measurement device 20 comprises an IR radiation sensor 22.1 and a second IR radiation sensor 22.2. The first IR radiation sensor 22 is arranged to detect a first IR radiation bundle 24.1 that spreads through a measuring line 25.1.

[0044] If a schematically depicted molecule 26.1 situated in the first IR radiation bundle 24.1 emits an IR photon 28 which moves in the first IR radiation bundle 24.1 towards the first IR radiation sensor 22.1, it reaches a sensor element 30.1 in the form of an InAsSb photodetector, which subsequently generates a voltage. The photovoltage U.sub.1 generated by the sensor element 30.1 thus depends on the irradiance of the radiation falling on the sensor element 30.1. The sensor element 30.1 is arranged at a distance from the pipe 18.

[0045] The measuring line 25.1 does not protrude into the pipe 18, thereby largely preventing the creation of additional turbulence.

[0046] The sensor element 30.1 has a measurement range M=[λ.sub.min, λ.sub.max] with a lower cut-off wavelength λ.sub.min and an upper cut-off wavelength λ.sub.max. In the present case, λ.sub.min=0.78 μm and λ.sub.max=5.3 μm.

[0047] The IR radiation sensor 22.1 measures an IR radiation parameter curve E.sub.g1,1(t) as a function of the time t with a measurement frequency f.sub.mess of at least 1 kHz, in the present case of f.sub.mess=16 kHz. It is favourable if the measurement frequency f.sub.mess is a maximum of 1 MHz. The analogue raw data is converted into digital values by an analogue-digital converter of the radiation sensor 22.1. The bit depth of the sampling is 8 to 24, preferably 16 bit.

[0048] The second IR radiation sensor 22.2 is designed to measure radiation from an IR radiation bundle 24.2 that spreads in a second measuring line 25.2. The IR radiation of the second IR radiation bundle 24.2 comes, for example, from a second molecule 26.2. The first IR radiation bundle 24.1 extends along a first straight line G1; the second IR radiation bundle 24.2 extends along a second straight line G2. The two straight lines G1, G2 are at a measuring distance d from one another. As depicted in the present case, they preferably run parallel to one another.

[0049] The measuring distance d is preferably at most 500 millimetres, for example 350±50 millimetres.

[0050] The photovoltages U.sub.1, U.sub.2 generated by the respective sensor elements 30.1, 30.2 are directed to an evaluation unit 32. The photovoltage U.sub.1 is a measure of an irradiance E.sub.1 measured by the sensor element 30.1 and constitutes an IR radiation parameter. The irradiance E.sub.2 is measured by the second sensor element 30.2 and is also time-dependent.

[0051] The evaluation unit 32 calculates a transit time τ as the time at which the cross-correlation function R.sub.E1,E2(τ′)=E.sub.1.Math.E.sub.2(τ′) reaches its maximum, wherein .Math. is the operator symbol for the cross-correlation.

[0052] If a local concentration c of a first gas g1, such as methane, water, carbon dioxide, carbon monoxide, sulphur trioxide, sulphur dioxide or nitrous oxide, fluctuates in the exhaust gas stream 14, this results in a change in irradiance E.sub.g1,1 when the corresponding fluctuation moves through the area of the first IR radiation bundle 24.1. Spatial inhomogeneities of the concentration remain largely constant over the measuring distance d, thereby resulting in similar curves of the respective irradiances E.sub.g1,1(t) and E.sub.g1,2(t) on first sensor element 30.1 and the sensor element 30.2.

[0053] Black-body radiation emanating from a wall 34 in the pipe 18 does not disturb this measurement. If, for example, H.sub.2O is selected as a first gas, it has a first gas excitation wavelength λ.sub.g1 of 3.2 μm. In this case, it is beneficial if the IR radiation sensors 22.1, 22.2 have a measurement interval of M=[λ.sub.1−0.3 μm, λ.sub.g1+0.3 μm].

[0054] If, as provided for by a preferred embodiment, a second gas g2 is selected, whose second gas excitation wavelength λ.sub.g2 does not lie in the measurement interval M for the first gas g1, the degree of measurement accuracy can often be increased. For instance, carbon dioxide can be used as a second gas, whose second gas excitation wavelength is λ.sub.g2=4.27 μm.

[0055] FIG. 2 schematically depicts a jet engine 36 on which the measurement device 20 is arranged in such a way that the gas stream 14, which in this case leaves the jet engine 36 through an outflow opening 38, is measured.

[0056] FIG. 3 schematically shows part of an electric arc furnace 40 with a melting chamber 42 in which steel scrap is melted by means of an electric arc between electrodes 43.1, 43.2, 43.3, thereby creating a metal bath 16. On the right-hand side of the image is an enlargement of the area outlined with a dashed line. Exhaust gases produced by melting form the gas stream 14 and are discharged through the pipe 18. The pipe 18 has an annular gap 44 through which air 46 can also enter the pipe 18. In order to measure the gas stream 14, the measurement device 20 is arranged on the gap side of the pipe 18.

REFERENCE LIST

[0057] 10 furnace [0058] 12 burner [0059] 14 gas stream [0060] 16 metal bath [0061] 18 pipe [0062] 20 measurement device [0063] 22 IR radiation sensor [0064] 24 IR radiation bundle [0065] 25 measuring line [0066] 26 molecule [0067] 28 IR photon [0068] 30 sensor element [0069] 32 evaluation unit [0070] 34 wall [0071] 36 jet engine [0072] 38 outflow opening [0073] 40 electric arc furnace [0074] 42 melting chamber [0075] 43 electrode [0076] 44 annular gap [0077] 46 air [0078] λmin upper cut-off wavelength [0079] λmax lower cut-off wavelength [0080] λ.sub.g1 first gas excitation wavelength [0081] λ.sub.g2 second gas excitation wavelength [0082] τ transition time [0083] A cross-sectional area [0084] c concentration [0085] D diameter [0086] d measuring distance [0087] E irradiance [0088] E(t) IR radiation parameter curve [0089] f.sub.mess measurement frequency [0090] f.sub.g1 first gas excitation wavelength [0091] f.sub.g2 second gas excitation wavelength [0092] M measurement interval, measurement range [0093] vG flow velocity [0094] T temperature [0095] t time [0096] U.sub.1 photovoltage