GB2125556A - Fluid pressure sensor subjected to thermal effect of fluid jets - Google Patents

Abstract

A pressurized fluid is vented via ports 15 from a cell 6 in a turbine blade 3 and the pressure of the fluid in the cell is inferred from the change in the heat dissipation of the probe as measured at 28 as a change of current flowing through the probe 18. This is used to check that the ports 15 are unobstructed. Means are provided for calibrating the apparatus so that the results obtained can be compared with those which would be expected. <IMAGE>

Description

SPECIFICATION Pressure sensor The invention relates to pressure sensors and, more particularly, to such sensors which determine pressure by measuring the rate of heat dissipation of a probe.

In the automated inspection of objects having hollow cavities or cells, it is frequently desirable to measure the pressure of a fluid or gas contained within the cells. As an example, the pressure of a gas contained in a cell and escaping through the perforations in a perforated gas turbine engine blade is measured to determine the existence of any blockages in the perforations. The utilization of pressure probes which are inserted into the cells in such a case is both expensive and time-consuming.

It is an object of the present invention to provide a new and improved pressure sensor.

It is an object ofthe present invention to provide a new and improved pressure sensor which measures the pressure of a fluid contained in a cell without itself entering or contacting the cell.

One form of the present invention measures the pressure of a fluid contained in a cell by venting a portion of the fluid th rough perforations in the cell wall, placing a probe into the fluid path, and measuring the heat dissipation properties ofthe probe.

FIGURE 1 illustrates one embodiment of the present invention.

FIGURE 2 is a plot of one form of a calibration standard to be used in the present invention.

In Figure 1, a gas turbine engine blade 3 contains a cell 6 which is filled with a pressurized fluid such as compressed gas or air provided by a source (not shown). The cell 6 may communicate with another cell 9 by means of a channel 12. The gas escapes from cell 6through ports or perforations 15 and impinges upon a probe 18. The escaping gas is illustrated as arrows 20. in one embodiment, the probe 18 is heated by passing an electric currentthrough it by means of conduits 21 and 23 attached to a source of constant electric power 25. A meter 28 interconnected in conduit 21 gives a measure of the amount of electric power delivered to the probe 18.

The escaping gas 20 impinging upon the probe 18 will eithertransmit heatto the probe 18 or absorb heat from the probe 18, depending partly upon the temperature of the escaping gas 20 as compared with that of probe 18, as well as upon the pressure of the gas contained in cell 6. However, it is preferred thatthe escaping gas 20 be cooler than the probe 18. One reason is that one can then view the heat absorbed by the escaping gas 20 as being equal to the power supplied by the power source 25 at steady-state.

Irrespective ofwhether heat is transmitted or absorbed by the probe 18, a steady-state temperature will be reached in probe 18 which corresponds to the particular gas pressure in the cell 6. These principles are utilized to determine the pressure within a cell 6 in atestblade3asfollows.

The power supplied to probe 18, as indicated by meter 28, is held constant. A calibration standard is generated in which the probe temperatures at steadystate are recorded for different cell pressures. This establishes a series of probetemperature-cell pressure data pairs. Following this, gas at an unknown pressure in a cell 6 in a test blade 3 is vented through perforations 1 to probe 18 as before until the steady-state is reached. The temperature of the probe 18 is again measured. The calibration standard data pairs are then examined to find the pair having a probe temperature closest to that measured for the test cell 6. The pressure associated with the closesttemperature can be taken to be that ofthetest cell 6.

Alternatively, an interpolation between the two pressures associated with the two temperatures bracketing the closest temperature can be used.

In an alternative embodiment, the probe 18 is brought to a predetermined temperature, either by heating as above or otherwise. The probe 18 is placed in the path of the escaping gas 20 and the timetemperature transient of the probe 18 is recorded.

Such a recording is repeated for different gas pressuresto generate another calibration standard. When an unknown pressure isto be measured in atestcell, a temperature-timetransient is likewise recorded for the test cell. Comparison ofthe latter test transient with those in the calibration standard will allow the conclusion thatthe unknown gas pressure in the test cell is that paired in the calibration standard with the transient most closely resembling the test transient.

It is to be noted that one of the operative principles involved in the present invention is the change in heat dissipation in the probe 18when exposedto the gas vented from cell 6. The change can be manifest as a temperature change of the probe 18 and measured as such. Alternatively, it can be measured indirectly as, for example, by increasing the power delivered by the power source 25to the probe 18 until a predetermined temperature is attained. In such a case, the heat dissipated is indicated by the added energy delivered to the probe 18.

It is also to be noted that an extensive calibration standard may not be necessary for some purposes. It may be desired to merely determine whetherthe cell pressure is above or below a threshold, in which case the heat dissipation ofthe probe is examined to determine whether it is above or below a corresponding dissipation value.

It is also to be noted that different pressures within cells 6 and 9 as well as within perforations 15will exist at different points such as points 29,30,31, and 32 therein. This is partially because the pressures within a flowing fluid will exhibit a gradient pattern. Consequently, in a strict sense, the pressure actually measured in generating the calibration standard should be that at one fixed point, such as point 30. The pressure then inferred to exist in the test cell will be the pressure at that point also. Of course, the pressure gradients may be sufficiently small thatthe pressure throughoutthe cell, as well as within the perforations (such as at position 31), may be treated as constant and uniform.Accordingly, the term pressure herein is used in a generalized sense as referring to the pressure at the selected fixed point orto an approximate cell pressure indicative of an overall average.

It is also to be noted that the temperature of the probe 18, whether heated or not, will increase or decrease depending upon the relative initial temperature of it as compared with the temperature of the escaping gas 20. Accordingly, theterm "dissipation" is taken as having both a negative and a positive sense. That is, positive heat dissipation implies the giving off of heat, while negative heat dissipation implies the absorption of heat.

It is also noted thatthe comparison of the measured heat dissipated in the case of the test cell with the calibration standard can be done byan interpolation means such as a computer.

It is also to be noted that the probe 18 must occupy substantially the same position with respect to blade 3 during all measurements so that substantially the same heat-exchange relationship occurs during these measurements. Further, the shapes ofthe cells and the perforations therein should be similar.

As an example, the following implementation of one form ofthe invention has been undertaken. All of the following numerical data are approximate. Aflat copper probe shown in phantom outline 18A in Figure 1 and measuring 1.75 inches long x 0.25 inches wide x 1116 inch thick was positioned in the path of gas which isto escape through ports (commonly called "gill holes" in this case) shown as dashed circles 15A. The ports 15A were contained in a blade taken from the high pressure rotor of a gas turbine engine. The probe 18Awas positioned such that the distance between probe 18A and the gill holes was .07 inches.

Electric power was delivered to the probe 1 8A at the rate of 45 watts. Gas pressure was applied to cell 6 by connecting cell 9 to a plenum (not shown) having a controlled pressure. This caused gas, which had a temperature of 71 degrees F,to entercell 6 through passages such as channel 12 and to escape through ports 15Aandto impinge onto probe 18A. The temperatureof probe 18Awas measuredwhen a steady-state was reached.At this time, the gas pressure was measured at three locations in cell 6, namely, nearthe bottom ("root"), nearthe middle ("pitch"), and near the top ("tip"). The pressurewas measured by removing the probe 18A for access two holes 15A and then inserting a pressure sensor through selected holes 1 5A located nearthesethree locations. A hypodermic needle connected to a pressure gauge served as such a pressure sensor. This procedure was repeated for different gas pressures to obtain a series of data forthe blade 3. Three other series of data were obtained forthree other blades of threedifferentshapes bysimilarproceduresand some ofthe data are listed in Table 1.

TABLE I STEADY-STATE CELL PRESSURE, PLENUM PRESSURE, PROBE TEMP., INCHES OF MERCURY BLADE INCHES OF MERCURY DEGREES F.* ROOT PITCH TIP No. 1 12.3 419 S.t 4.5 4.9 16.7 379 7.6 6.5 6.9 19.9 356 9.0 7.8 8.3 24.0 336 11.2 9.8 10.3 28.2 319 13.7 11.8 12.5 No. 2 12.5 396 5.0 4.9 4.7 16.0 367 6.6 6.4 6.1 20.0 340 8.5 8.2 7.9 24.0 323 10.5 10.3 9.7 No. 3 12.0 456 4.8 4.2 4.4 16.0 416 6.8 5.8 6.1 20.0 387 8.4 7.6 8.0 24.0 364 10.3 9.4 10.0 No. 4 12.0 412 4.8 4.7 4.4 16.0 376 6.7 6.6 5.9 20.0 351 8.6 8.5 7.6 24.0 333 11.0 10.6 9.3 * Power delivered to probe was 45 watts in all cases.

The steady-state probe temperature in the data of Table 1 is plotted in Figure 2 as a function of the minimum ofthethree cell pressures (root, pitch and tip) with which it is associated. The symbol for blade number one is a circle; for number two, a cross; for numberthree, a square; and for number four, a triangle. Interpolation lines 60A, 60B, 60C and 60D are drawn to provide one type of a calibration standard.

The calibration standard can be used, for example, in connection with a test blade 3 identical in shape and location of ports 15Ato blade number one as follows: The procedure of positioning and heating probe 18A, applying gas pressure to cell 3 and measuring the steady-state probe temperature is repeated as in the case of blade number one. However, the cell pressure is determined by reference to the calibration standard: assuming that the steady-state probe temperature is measured as 390 degrees F, the cell pressure deemed to be associated with thattempera- ture, as determined bytheinterpolation line 60A, for blade number one is 5.1 inches of mercury. This is shown by dashed lines 65A and 65B.Deviation of this pressure from a predetermined pressure can indicate the absence or blockage of some of the ports 1savor some other blade anomaly.

A pressure sensor has been disclosed which senses the pressure of a fluid in a cell indirectly by means of sensing the heat dissipation of a probe present in the path of fluid escaping from the cell. The dissipation is compared with prior heat dissipation cell pressure data pairs obtained in the establishment of a calibration standard in which fluid at different pressures was vented onto the probe in the same manner and the respective heat dissipation were recorded. An alternative embodiment is disclosed in which an unheated probe is used and time-temperature tran sients in both the establishment ofthe calibration standard and in the measurementofan unknown pressure are utilized ratherthan steady-state temperatureofthe probe.

The invention is not limited to the embodiments described above. Numerous substitutions, omissions,and modificationswhich areobvioustothose skilled in the art can be made without departing from the true spirit and scope of the invention as defined in the following claims.

Claims (10)

1. A method of measuring the pressure of a fluid in a cell, comprising the steps of: venting a portion of the fluid through atleastone= port in the cell, inducing a change in the rate of heat dissipation of a probe with the vented fluid, measuring the dissipation rate.

2. A method in accordance with claim 1 and further comprising the step of comparing the measured rate with a calibration standard.

3. A method in accordance with claim 1 in which the probe is heated byan external source.

4. A method of measuring the pressure of a fluid contained in a perforated cell of defined shape, comprising the steps of (a) establishing a calibration standard by performing at leastthefollowing steps: (i) venting fluid contained in such a cell at a known pressure through some ofthe perforations, (ii) inducing a change in the rate of heat dissipation of a probe with the vented fluid, (iii) measuring the dissipation rate, (iv) repeating steps (a), (b), and (c) above for a selected number of repetitions using different known fluid pressures in the cell; (b) measuring an unknown fluid pressure in such a cell by performing at least the following steps:: (i) venting fluid through some of the perforations in the cell, (ii) inducing a change in the rate of heat dissipation ofthe probe with the vented fluid, (iii) measuring the dissipation rate, (iv) comparing the measured rate of (b) (iii) with the prior measured rates in the calibration standard.

5. Apparatus for measuring the pressure of a fluid escaping through perforations in a cell, comprising: probe means located in a predetermined position in the path of the escaping fluid for establishing a heat exchange relationship therewith; measuring means coupled to the probe means for measuring heat dissipated bythe probe; interpolation means coupled to the measuring means for deducing the pressure from the measurementofthe heat dissipated.

6. Apparatus in accordance with claim 5 in which the interpolation means utilizes heat dissipationpressure data points obtained from priorcelltests.

7. Apparatus in accordance with claim 5 in which the interpolation means generates a signal when the deduced cell pressure reaches a predetermined value.

8. A method of measuring the pressure of a gas contained within a perforated cell in a turbine engine compressor blade comprising the steps of (a) establishing a calibration standard by performing at leastthefollowing steps: (i) venting gas contained at a known pressure in the cell of a reference blade through some of the perforations, (ii) placing a probe in a predetermined position in the path ofthevented gas for establishing a heat exchange relationship therewith, (iii) measuringtheheatdissipated bytheprobe, (iv) repeating steps (i), (ii), and (iii) above for a selected number of repetitions using different known gas pressures in the cell; (b) measuring an unknown fluid pressure in the perforated cell of a blade substantially identical to the reference blade by performing at least the following steps: : (i) Venting gas th rough some ofthe perforations, (ii) establishing substantially the same heat exchange relationship of step (a) (ii) between the probe and the vented gas, (ii) measuring the heat dissipated bythe probe, (iv) comparing the presently measured heat exchange of step (b) (iii) with the measured values of the calibration standard.

9. A method of pressure measurement substantiallyas hereinbefore described with referencetothe drawing.

10. Apparatus for measuring pressure substantial ly as hereinbefore described with reference to and as illustrated in the drawing.