Five hole probes are extensively used for measurement of total and static pressures, flow angles, velocity and its components in turbomachinery, and other aerodynamic flows. Their operating range is usually limited to 30–40° depending on the type of the probe head. The chamfer angle of the probe is usually taken around 45°. Recent studies on three hole probes have shown that 30° chamfer angle is desirable for unsteady flow measurements. Hence the present investigation is undertaken to find the optimum chamfer angle of five-hole probes. A special five-hole probe of 9.6 mm head diameter and 3 mm diameter pressure take off tubes was designed and fabricated. The large size of the probe was chosen to minimize machining inaccuracies. The probe chamfer angle was varied from 30° to 60° in 5° steps. For each of the chamfer angles, the probe was calibrated in the range of −30° to +30° in 5° interval and the calibration curves are presented. In addition the sensitivities of the calibration coefficients are determined. It is concluded that five-hole probe with a chamfer angle 30° has large operating range, while five-hole probe with a chamfer angle of 50° has good sensitivity.
In general flow can be analysed by three techniques, namely, flow visualization, computational methods, and measurements of flow parameters. Computational methods are expensive to develop and use. Flow visualization techniques serve only to locate flow regions of interest. Obtaining quantitative data often requires direct measurement of the flow. One such method of direct measurement is by inserting multihole pressure probes into the flow.
Multihole pressure probes have been conveniently used to determine static and total pressures and flow angles in two mutually perpendicular planes (named yaw and pitch planes) in three-dimensional flow fields with suitable calibrations. From these four flow parameters, flow velocity and its three components can be determined. Multihole pressure probes thus combine the means for simultaneous measurement of total, static, and dynamic pressures and flow directions with one instrument. When designing a pneumatic probe that is to be used for flow measurements, the effects of blockage, frequency response, pressure hole size and geometry, the local Mach and Reynolds numbers, and the relative scale of the phenomenon under investigation must be specified.
For measuring three-dimensional flows, multihole probes with four, five, seven, or even higher number of pressure holes strategically placed on aerodynamic bodies such as sphere, hemisphere, and prism can be used. In principle a four-hole probe can measure the four quantities that are required to completely define the flow. However for the sake of symmetry in both yaw and pitch planes, five hole probes are usually employed. When the yaw and pitch angles of the flow exceed the usual operating range of five hole probes, seven hole probes or probes with larger number of holes are employed.
Five hole probes Treaster and Yocum [
The shape of the head of the five-hole probe can vary widely as shown in Figure
Geometry of five-hole probe heads Dominy and Hodson [
Forward facing tubes
Pyramid head with perpendicular holes
Cone head with forward facing holes
Cone head with perpendicular holes
Hemispherical head with forward facing holes
Hemispherical head with perpendicular holes
A 9.6 mm diameter five-hole probe was fabricated with the tip of the probe shaped as truncated cone. The probe was made of a 9.6 mm brass rod with five holes of 3 mm diameter drilled in a + format with a clearance of 0.15 mm between the holes and outer circumference. Five 3 mm diameter stainless steel tubes were fitted tightly into these holes. The diameter of the tubes was reduced slightly by grinding, so that the tubes would fit into the holes. The tubes were silver brazed to the brass body at the rear end with 10–15 mm of the tubes exposed. Plastic tubes were fitted to the exposed ends of the tubes. Among the five tubes, one forward-facing tube was at the centre, two chamfered side tubes were on the horizontal axis, and remaining two side tubes were on the vertical axis as shown in Figure
AutoCAD drawing of Five-hole probe.
The probe support was made up of stainless steel (SS) material. The probe support consisted of a 9.6 mm diameter. SS tube of 400 cm long is attached to a 12.7 mm hexagonal rod. A hole of 9.6 mm diameter was centrally drilled in the hexagonal rod. A small tube was perpendicularly silver brazed to the hexagonal rod. A M3 tapped hole was drilled in the center of the small tube to position the probe and to hold the probe tightly. The probe holder and support are shown in Figure
Probe holder and support.
The probe was calibrated in the calibration tunnel available in Thermal Turbomachines Laboratory of Department of Mechanical Engineering. The probe was fixed so that its head is at the center of calibration section to minimise boundary layer and duct wall effects. A photo of the calibration tunnel is shown in Figure
Calibration tunnel.
The calibration device was mounted on the calibration section of the calibration tunnel. The probe was mounted in the central hole of the calibration device with the probe tip at the centre of the axis of the calibration tunnel. The calibration device has provisions to change the yaw angle in the range of ±180 deg. at an interval of 1 deg. and pitch angle of the probe in the range of ±30 deg. at an interval of 1 deg. The probe can be rotated in both clockwise and anticlockwise directions to change the yaw angle with the help of a rotating mechanism fitted onto the calibration device. It is desirable to carry out the calibrations in the yaw and pitch angle range of
Calibration device.
The twenty-channel single selection scanning box (model no. FCO 91-3) and FC012 digital micro manometer manufactured by Furness control Ltd., Bexhill, London were used to measure probe pressure. The scanning box has twenty channels, which were numbered sequentially. The pressures to be measured were connected to the numbered inputs. The outlet channel was connected to the micromanometer. A particular channel was selected manually in the scanning box and its corresponding pressure was read from the micromanometer. The micromanometer used has a resolution of 0.1 mm with a range of ±200 mm of water gauge. The accuracy of the micromanometer is ±0.1 mm of the water column. The output of the scanning box was connected to the micromanometer and it gave reading directly in terms of velocity in m/s or pressure in mm of water gauge. Time constant potentiometer was used to get time averaged pressures.
The calibration of the five-hole probe for the present experimental investigation was carried out in the low-speed calibration tunnel. Free stream velocity of air was maintained at 25 m/s determined from the settling chamber pressure and calibration section wall static pressure. The five pressure tubes of the probe along with the settling chamber wall static pressure and calibration section wall static pressure taps were connected to a scanning box, which enabled to measure multipressures using just one pressure measuring instrument (digital micromanometer in this case). The fluctuating pressure signals were typically averaged over a period of 5 seconds time to allow for conditions to reach steady-state. Using the calibration device, the pitch and yaw angles of the probe were changed by 5° increment in range of ±30°, respectively. After calibration of the probe at one chamfer angle, the probe was removed from the calibration device and the chamfer angle was changed by machining the probe in a lathe by using the tool bit at the desired angle.
The chamfer angle of the probe was varied systematically from 30 to 60 deg, with gradually increasing chamfer angle at an increment of 5 deg.
This range covers most of the usually used chamfer angles. After completing calibration of the probe with a chamfer angle, the probe chamfer angle was changed by machining the probe in a lathe. The accuracy of the probe chamfer angle is checked in Metrology Laboratory, Department of Mechanical engineering, IIT Madras and was found to be within an accuracy of ±0.2 deg.
The pressures measured by the five tubes of the five-hole probe with a chamfer angle of 30° are presented in Figure
Nondimensional probe pressures for chamfer angle of
For the sake of clarity, only pressures at three values of yaw angles, namely, −30°, 0°, and, 30° are presented. The data are presented to see that the data is following the expected trends. Such data for all chamfer angles are plotted to validate the data. However the data are not presented for the sake of brevity.
As expected pressures measured by the central, left, and right holes show parabolic variation with the pitch angle. At
The pressures measured by the five holes, the calibration tunnel settling chamber pressure, and calibration section wall static pressure at different values of yaw and pitch angle are used to define nondimensional calibration coefficient as follows:
The calibration curves are presented as follows for the probe with different chamfer angles:
These calibration curves are presented and discussed in the following sections.
The
Minimum and maximum values and sensitivity of
Chamfer angle | 30° | 40° | 50° | 60° |
---|---|---|---|---|
Minimum value | −4.050 | −6.00 | −16.200 | −7.500 |
Normalized value | 1.000 | 1.481 | 4.000 | 1.852 |
Maximum value | 2.380 | 3.000 | 6.200 | 10.700 |
Normalized value | 1.000 | 1.261 | 2.605 | 4.496 |
Total difference | 6.430 | 9.00 | 22.400 | 18.200 |
Normalized value | 1.000 | 1.400 | 3.484 | 2.830 |
Sensitivity at |
0.083 | 0.140 | 0.293 | 0.293 |
Normalized value | 1.000 | 1.687 | 3.530 | 3.530 |
Sensitivity at |
0.058 | 0.083 | 0.121 | 0.107 |
Normalized value | 1.000 | 1.431 | 2.086 | 1.845 |
Sensitivity at |
0.104 | 0.134 | 0.362 | 0.021 |
Normalized value | 1.000 | 1.288 | 3.481 | 0.202 |
Minimum and maximum values and sensitivity of
Chamfer angle | 30° | 40° | 50° | 60° |
---|---|---|---|---|
Minimum value | −3.220 | −5.800 | −12.100 | −5.200 |
Normalized value | 1.000 | 1.801 | 3.758 | 1.615 |
Maximum value | 3.020 | 5.400 | 15.000 | 8.000 |
Normalized value | 1.000 | 1.788 | 4.967 | 2.649 |
Total difference | 6.240 | 11.200 | 27.100 | 13.200 |
Normalized value | 1.000 | 1.795 | 4.343 | 2.115 |
Sensitivity at |
0.081 | 0.108 | 0.211 | 0.140 |
Normalized value | 1.000 | 1.333 | 2.605 | 1.728 |
Sensitivity at |
0.057 | 0.077 | 0.090 | 0.099 |
Normalized value | 1.000 | 1.351 | 1.579 | 1.737 |
Sensitivity at |
0.107 | 0.179 | 0.438 | 0.120 |
Normalized value | 1.000 | 1.673 | 4.093 | 1.121 |
Calibration curves:
From Figure
The
Minimum and maximum values and sensitivity of
Chamfer angle | 30° | 40° | 50° | 60° |
---|---|---|---|---|
Minimum value | 0.000 | 0.000 | 0.000 | 0.000 |
Maximum value | 2.180 | 3.500 | 10.000 | 6.000 |
Normalized value | 1.000 | 1.606 | 4.587 | 2.752 |
Total difference | 2.180 | 3.500 | 10.000 | 6.000 |
Normalized value | 1.000 | 1.606 | 4.587 | 2.752 |
Sensitivity at |
0.025 | 0.097 | 0.301 | 0.162 |
Normalized value | 1.000 | 3.880 | 12.040 | 6.480 |
Sensitivity at |
0.014 | 0.030 | 0.052 | 0.080 |
Normalized value | 1.000 | 2.143 | 3.714 | 5.714 |
Sensitivity at |
0.058 | 0.095 | 0.321 | 0.141 |
Normalized value | 1.000 | 1.638 | 5.534 | 2.431 |
Sensitivity at |
0.037 | 0.040 | 0.104 | 0.111 |
Normalized value | 1.000 | 1.081 | 2.811 | 3.000 |
Sensitivity at |
0.015 | 0.019 | 0.023 | 0.029 |
Normalized value | 1.000 | 1.267 | 1.533 | 1.933 |
Sensitivity at |
0.059 | 0.086 | 0.289 | 0.074 |
Normalized value | 1.000 | 1.458 | 4.898 | 1.254 |
Minimum and maximum values and sensitivity of
Chamfer angle | 30° | 40° | 50° | 60° |
---|---|---|---|---|
Minimum value | −0.620 | −0.900 | −1.500 | −2.800 |
Normalized value | 1.000 | 1.452 | 2.419 | 4.516 |
Maximum value | 0.480 | 0.700 | 0.900 | 0.500 |
Normalized value | 1.000 | 1.458 | 1.875 | 1.042 |
Total difference | 1.100 | 1.600 | 2.400 | 3.300 |
Normalized value | 1.000 | 1.455 | 2.182 | 3.000 |
Sensitivity at |
0.017 | 0.025 | 0.033 | 0.043 |
Normalized value | 1.000 | 1.471 | 1.941 | 2.529 |
Sensitivity at |
0.018 | 0.018 | 0.03 | 0.011 |
Normalized value | 1.000 | 1.000 | 1.667 | 0.611 |
Sensitivity at |
0.018 | 0.019 | 0.008 | 0.003 |
Normalized value | 1.000 | 1.056 | 0.444 | 0.167 |
Sensitivity at |
0.016 | 0.017 | 0.022 | 0.062 |
Normalized value | 1.000 | 1.063 | 1.375 | 3.875 |
Sensitivity at |
0.016 | 0.023 | 0.041 | 0.031 |
Normalized value | 1.000 | 1.438 | 2.563 | 1.938 |
Sensitivity at |
0.019 | 0.031 | 0.050 | 0.047 |
Normalized value | 1.000 | 1.632 | 2.632 | 2.474 |
Calibration curves:
From Figure
However this trend is broken when chamfer angle is 55 deg. For this and higher chamfer angles, the side holes are more parallel to the streamlines. Hence they sense more static pressure. Also the variation of the side hole pressures may not be much as the yaw and pitch angles increase beyond a certain range. It is quite likely that different definition of
The
Calibration curves:
From Figure
The pressure probes used for fluid flow measurements should satisfy two conflicting requirements. They are sensitivity of the probe and operating range of the probe. These conflicting requirements depend on the chamfer angle of the probe head. The present section examines the sensitivity of the probe measurements with yaw and pitch angles. This can be done in two ways: one by examining the sensitivity of the pressures measured by the probe with yaw and pitch angles and the other by examining the sensitivity of the calibration coefficients of the probe with yaw and pitch angles. The second approach is taken in the paper. Ideally the sensitivity has to be determined at all combinations of yaw and pitch angles. While this is possible, it is cumbersome to present and interpret. Hence the sensitivity of various coefficients are examined at three values of yaw and pitch angles, namely,
Sensitivity of calibration coefficients.
Larger value of sensitivity of
From Figure
The following major conclusions are drawn from the present experimental investigation on the effect of chamfer angle on the calibration curves of five hole probes. The value of the calibration coefficients increases as the chamfer angle increases. This trend is observable up to the chamfer angle of 50 deg. For this chamfer angle, the calibration coefficients show maximum values. This is a desirable trend, as the sensitivity of the calibration coefficients increases, more accurate values of interpolated values of yaw and pitch angles and total and static pressures can be obtained. However the useful range of the probe is limited to about 30°. If the flow to be measured is expected to have large changes in yaw and pitch angles, probe with a chamfer angle of about 30° is desirable. Probe with this chamfer angle has large useful range. Sensitivity of static pressure coefficient is nearly independent of probe chamfer angle. For the chamfer angles of 55 deg. and higher values, alternate techniques given at the end of Section
Pitch coefficient (defined in text)
Static pressure coefficient (defined in text)
Yaw coefficient (defined in text)
Total pressure coefficient (defined in text)
Probe dynamic head (Pa) (defined in text)
Mean pressure (Pa) (defined in text)
Pressures measured by bottom, centre, left, right and top holes of the probe (please see Figure
Total pressure (Pa)
Static pressure (Pa)
Yaw Angle (deg.)
Pitch Angle (deg.)
Chamfer Angle (deg.).
The authors declare that there is no conflict of interests regarding the publication of this paper.
The authors would like to thank Mr. P. Perumal of Thermal Turbomachines Laboratory for fabricating the probe and other technical and administrative staff of Thermal Turbomachines Laboratory, Department of Mechanical Engineering, IIT Madras, for their help. The authors would like to thank the reviewers for their suggestions, which improved the quality of the paper substantially.