Experimental investigation on static/dynamic characteristics of a fast-response pressure sensitive paint

Ruiyu LI,Limin GAO,*,Tianlong ZHENG,Guanhua YANG

aSchool of Power and Energy,Northwestern Polytechnical University,Xi’an 710072,China

bCollaborative Innovation Center of Advanced Aero-Engine,Beijing 100083,China

1.Introduction

Surface pressure measurements are critical to aerodynamic testing in a wind tunnel.However,traditional pressure measurementmethods like pressure taps and electronically scanned pressure transducers are poor in spatial resolution and would introduce aerodynamic interference in wind tunnel testing.An optical technique using Pressure-Sensitive Paint(PSP)is a preferred method to measure the distribution of surface pressure on an aerodynamic model.Compared withconventional pressure measurement methods,the PSP technique provides a relatively simple and inexpensive method to obtain full- field pressure images of interested surface.Images obtained using the PSP technique have a high spatial resolution and low aerodynamic interference.Thus,the PSP technique has been considered a promising technique in the field of aerospace research.1

PSP has been extensively used in steady flow conditions2–4with a response time of approximately 1–10 s,5which is inconsistent with unsteady measurements.Consequently,several developments have been made to extend the application of PSP to unsteady flows.The USA,6,7Russia,8the UK,9,10Japan,11–13and China14,15have exerted extensive efforts to develop Fast-response Pressure Sensitive Paints(FPSPs).

Dynamic calibration methods have been developed to determine the response time and response frequency of FPSPs.In response time calibration,a step in pressure is required.Sakaue and Sullivan16measured the response time through a successive change of pressure caused by a normal shock wave in a shock tube,and observed the binder thickness and temperature dependencies for the response time.Meanwhile,the solenoid valve has also been used to generate a pressure change for PSP response time calibration.17,18In response frequency calibration,a periodic change in pressure is required.Resonance tubes,13,19pulsating jets,20and fluidic oscillators7are capable of determining the applicable frequency range of PSP.Previously published articles indicated that the minimal response time has been pushed to 34.8 μs,21thereby satisfying the requirement of unsteady flow measurements.

The Chinese Academy of Sciences(CAS)has developed a fast-response PSP,and its practical application has also been confirmed by testing in a circular cylinder measurement.22However,the characteristics of the FPSP are still poorly elucidated.In the current study,the static and dynamic characteristics of the FPSP,including temperature sensitivity and response time,are analyzed to serve as the basis for an experiment on unsteady surface measurement.

2.Measurement principle

PSP techniques are based on photoluminescence(which includes both fluorescence and phosphorescence)and the oxygen-quenching characteristics of a few polymer probe molecules.23There are two primary methods for acquiring the pressure data of a PSP:intensity method and lifetime method.The intensity-based(radiometric)method is based on the dependence of the intensity on pressure,and it employs continuous illumination and records the intensity of luminescence from a paint.It has been widely used because of its low requirement of measuring equipment and simple principle.The lifetime method is based on the dependence of the luminescent lifetime on pressure.Due to its high demand for the performance of equipment such as the light source strobe frequency,it is difficult to be applied in continuous measurement.Therefore,the intensity-based method is finally used in this paper.The pressure and luminescent intensity can be modeled by a simplified form of the Stern-Volmer relation,which can be expressed as24where Irefrepresents the intensity of luminescence at the reference,and I represents the test conditions.Their pressure distributions are represented by P and Pref,respectively.A(T),B(T),and C(T)are temperature-dependent calibration coefficients.

3.Characteristics of fast-response PSP

The static and dynamic characteristics of the PSP must be confirmed before applying the fast-response PSP technique to an unsteady flow field measurement.In particular,the static characteristics refer to the temperature and pressure dependencies,while the dynamic characteristics include the response time and frequency.Limited by experimental equipment,we do not study the frequency characteristic of the paint.A brief description of the paint and these characteristics will be presented in this section.

3.1.Description of fast-response PSP

The fast-response PSP used in the current study is developed in the China Academy of Aerospace Aerodynamics(CAAA)and the Institute of Chemistry Chinese Academy of Sciences.The PSP is excited by a generated light of an LED array centered at 365 nm(approximately 20 nm full width at half maximum),which is the optimal excitation wavelength.The emission light of the PSP is from 600 nm to 700 nm.The plate colors of the PSP sample before and after excitation are shown in Fig.1,in which an orange plate color is observed before excitation and a fluorescein red after excitation.By attaching a luminophore to the porous binder,the contact area between the luminophore and an oxygen molecule is substantially increased,thereby decreasing the response time.25

3.2.Static characteristics

Pressure changes are correlated with the changes of temperature.To ensure the accuracy of a measurement,the effect of a temperature change on fast-response PSP results must be considered.Temperature sensitivity ST,which is expressed in%/100 K,is defined to analyze the temperature effect intuitively,which is expressed in Eq.(2).It is a mean value of luminescent intensity in the temperature range of T1to T2.I1and I2are the intensity of luminescence at T1and T2respectively.High temperature sensitivity indicates high temperature errors,which is not what we expect.

Another important static characteristic that should be considered is pressure sensitivity Sp,which is expressed in%/100 kPa and has a very similar definition to temperature sensitivity,and their expressions are shown as follows:

Fig.1 Colors of fast-response PSP before and after excitation.

High pressure sensitivity represents a substantial change in the luminescence intensity under the same pressure variation,thereby indicating a considerably high PSP precision.

3.3.Dynamic characteristic–response time

Response time τ represents the sensitivity of the fast-response PSP,which is influenced by the binder thickness h and the mass diffusivity of oxygen in a PSP layer Dm;24thus,the following expression is provided:

Measuring the dynamic response time requires a pressure step signal,and a schematic of the calibration is presented in Fig.2.A pressure jump from Pstartto Pendwithin a short time is observed.The experimental results in Section 5.2 indicate that the response time behaves similarly to a first-order dynamic system.Thus,the response time can be defined as the consumption of time when the output value reaches 63.2%of Pend.In addition,a rise time t95,which is defined as the time required to reach the final pressure level within a range of±5%,is used in this study.

4.Experimental setup

Two sets of calibration rigs were designed and installed to study the characteristics of the fast-response PSP.The details of the calibration rigs will be described in this section.

4.1.Calibration rig for temperature dependency study

Fig.2 Response time determination.

Fig.3 Photo of the static calibration system.

The temperature dependency study is based on a static calibration system we installed which is shown in Fig.3.This system comprises an excitation light source,an air source,a CCD camera,and a calibration device.The calibration device integrates TESCOM pressure regulators with a control accuracy of 0.1%FS,a TEC temperature controller with a control accuracy of 0.1 K,and an integrated cooler and heater.All the components of the calibration system were placed in a dark box to increase the Signal-to-Noise Ratio(SNR)by reducing the effect of the environmental light.The pressure in the calibration device can vary from 5 kPa to 300 kPa,and the temperature can vary from 0 to 80°C,thereby satisfying the calibration demands.

4.2.Rig for response time calibration

The calibration chamber used for the investigation of response time is shown in Fig.4.The calibration chamber generates a pressure jump from a vacuum to the atmosphere.Before conducting a test,the pressure in the calibration chamber was set to a negative value using a vacuum pump.After stabilizing the pressure,a pressure step was generated by opening the valve.

A dynamic pressure sensor with a maximum respond frequency of 500 kHz was installed on the calibration chamber backboard to monitor the pressure variation.A PSP sample was also installed on the inside surface of the backboard and at the same location where the sensor tip was installed to minimize the error caused by different positions.Data were collected using a Photo Multiplier Tube(PMT),which can convert a weak optical signal to a voltage signal.A red filter((650±25)nm)was placed in front of the PMT to block other spectral bands for enhancing the SNR.A high-speed dynamic data acquisition system was used for data collection and the sampling frequency can reach up to 40 MHz.A synchronous data acquisition of the PMT and the dynamic pressure sensor is achieved by using a phase-locking trigger system.Note that the reference voltage applied on the PMT,which was used to control the output voltage amplitude,must be the same in the dynamic and static calibrations.

Fig.4 Calibration rig for the response time study.

During the dynamic characteristics study,the calibration system,including the calibration chamber,the LED light source,and the PMT,was covered by a black-out cloth to prevent the influence caused by the ambient light.Fig.4(a)shows the photo of the experimental site.

5.Results and discussion

5.1.Results of temperature dependency study

Seven conditions with different temperatures T,that is,25°C,30 °C,40 °C,50 °C,65 °C,70 °C,and 80 °C,are selected to test the temperature dependency of the fast-response PSP.The pressure varies from 40 kPa to 150 kPa,with a step of 10 kPa under each temperature condition.A pressure of 40 kPa is selected as the reference pressure Prefin this study.The effects of temperature on pressure sensitivity and intensity are observed in this section,and a pressure calibration equation considering the temperature effects is also presented.

To quantitatively investigate the effect of temperature on the pressure sensitivity,Spvarying with temperature at 150 kPa is shown in Fig.5.The pressure sensitivity is shown to decrease below 30 °C or above 50 °C.On one hand,the collision frequency of molecules between luminescent and oxygen molecules increases with increasing temperature because of an increase of energy transference between molecules,thereby facilitating an improvement in the pressure sensitivity.On the other hand,high temperature can easily cause a deactivation of luminescent molecules,thereby preventing them to participate in the oxygen-quenching process,which leads to a decrease in the pressure sensitivity.When temperature increases from 25°C to 50 °C,an increase in the molecular thermal motion is the predominant factor;thus,the pressure sensitivity increases.Meanwhile,when the temperature is above 50°C,inactive luminescent molecules become the main factor,thereby decreasing the pressure sensitivity.From 25 °C to 50 °C,the effects of these factors tend to be balanced,so Spremains the same.In summary,the effect of temperature on the pressure sensitivity is caused by the interplay of these factors.The preceding analysis indicates that the temperature insensitive range is from 30 °C to 50 °C.For a high-precision measurement experiment,the effect of temperature below or above this temperature range must be considered.

Fig.5 Pressure sensitivity at different temperatures.

To analyze the effect of temperature on intensity,the temperature sensitivities at three typical pressure conditions,which are below,around,and above the environmental pressure,are also shown in Fig.6.According to the figure,we can see that the intensity decreases slowly in the temperature range of 25–70 °C which is caused by an increase in the thermal motion of molecules,while it decreases sharply above 70°C due to the deactivation of luminescent molecules.It can be concluded that this kind of PSP should not be applied above 70°C.

The calibration curves at different temperatures are illustrated in Fig.7.Based on our analysis,the result at a temperature of 80°C should be neglected.The results are in a nearly linear relation(Iref/I=A（T）+B（T）P/Pref)at temperatures from 25 °C to 70 °C,and the calibration coefficients A(T)and B(T)at different temperatures are given in Table 1.According to the temperature calibration curves in Fig.8,we indicate that Iref/I is seen to be linearly related to temperature.Therefore,the least square method is used to fit A(T)and B(T)based on Table 1,and the equations are given in Eq.(4).Finally,the pressure calibration equation considering the temperature impact is fitted as given in Eq.(5).

Fig.6 Temperature sensitivity at different pressures.

Fig.7 Pressure calibration curves at different temperatures.

Table 1 Calibration coefficients A(T)and B(T).

Fig.8 Temperature calibration curves at different pressures.

5.2.Results of dynamic characteristic

Unlike in the previous study,the data collection device used in the current dynamic experiment is a PMT.The PMT provides an extremely rapid response and demonstrates high sensitivity.However,the SNR is considerably low because of the high sensitivity of the PMT to a light signal and the inevitable influence of stray light in the experiment.Thus,the SNR determines the minimum pressure level or resolution that can be detected by the testing system using the same signal collection device.

Several tests have been performed to determine the resolution in different pressure conditions.To obtain the minimum detected pressure(PMinDetected)at a certain pressure(Pcertain),we increase the pressure with a variable step ΔP based on Pcertain,which means to set the pressure in the calibration chamber as Pcertain+ΔP, Pcertain+2ΔP, as well as Pcertain+3ΔP,respectively and collect the pressure by both the PMT and the sensor for 5 s after the pressure keeps stable.An averaged pressure aveP is obtained by processing the PMT results using the time average method.If the averaged pressure increases linearly,we define ΔP as PMinDetected;if not,increase ΔP until the PMT results meet a linear relation.The data processing method is given in Fig.9(a),and the results are shown in Fig.9(b).The symbols,which represent the minimal pressure changes that can be detected by the PMT,are the experimental test results.A fitting curve was used for convenience.For example,the pressure jump must be above 2.85 kPa if Pendis equal to the atmospheric pressure.By decreasing the pressure,the luminescence intensity increases,thereby enhancing the SNR of the PMT and lowering the minimum pressure change.

The red lines in Fig.10 represent a luminescent intensity signal or the original signal.The pressure step process is difficult to determine because of the low SNR.Thus,a suitable filter method must be carefully selected.Common filtering methods that are used to determine a step signal are the classic FIR,Butterworth,median,and wavelet filters.All these filtering methods are used on the same pressure step signal to determine the optimum signal(see Fig.10).The blue lines in the figures are the filtering results.Although the FIR and Bathurst filters can suppress high-frequency oscillation well,they cause a smoothing effect at the pressure step,thereby distorting the original signal.Compared with the FIR and Bathurst filters,the median and wavelet filters can successfully determine the step signal without distortion.By comparing the median and wavelet filters,we observe that the latter can better suppress high-frequency oscillation.Thus,the wavelet filter is selected as the final filtering method,and the wavelet base function used in this section is Haar because its shape approximates the signal.The pressure signal obtained by the dynamic pressure sensor had a high SNR,and was not processed using any filter method.

Fig.10 Original signal processing results with different filtering methods(U:output voltage;t:time).

Three experiments with different pressure steps were conducted to test the response time of the fast-response PSP.The pressure step was set at 66.5,74.5,83.5 kPa to environment pressure which is 97.5 kPa at a constant temperature of 25°C.Meanwhile,the sampling frequency was set at 10 kHz.

Fig.11 Step responses of PMT and sensor with different pressure jumps.

Table 2 Response time and settle time of PSP under differen pressure jump.

The luminescent intensity signal obtained by the PMT can be related to the pressure value by using the Stern-Volmer relationship at 25°C.Thereafter,the wavelet filter was applied to process the pressure signal.The results are shown in Fig.11,in which the dotted red and solid green lines represent the t pressures measured by the sensor and the PSP,respectively.Meanwhile,Δt represents time consumption,which is measured by the sensor,of the pressure jump.To exclude the influence of time consumption of the opening of the solenoid valve,the settle time t95and response time τ are defined as the time difference between the PMT and the sensor.Table 2 shows the results.The settle time is nearly four times as large as the response time,which is consistent with a first-order response system.The response time decreases with a large pressure jump,and the response time is below 0.016 s when the pressure jump is under 40 kPa.

6.Conclusions

Surface pressure measurements are critical to aerodynamic testing in a wind tunnel.The development of a fast-response PSP can generate substantial surface data using simple procedures and instrumentations.In this study,two calibration devices were used for the analysis of the static and dynamic characteristics of a fast-response PSP.The conclusions are as follows:

(1)The self-builtcalibration system can successfully acquiretemperature sensitivity and responsetime measurements.

(2)The fast-response PSP can be used normally from 25°C to 80°C,but the effects of temperature have varying extents in different temperature ranges.From 30°C to 40°C,the PSP is highly insensitive to temperature.The influences of temperature must be considered to obtain accurate measurements in this temperature range.

(3)The dynamic calibration device with a solenoid valve can achieve a pressure jump within a millisecond order.The resolution is determined by the SNR ratio of the PMT.The measurement results show that the response time of the FPSP decreases with a large pressure variation,and the response time is below 0.016 s when the pressure variation is under 40 kPa.

Acknowledgments

The authors express their sincere gratitude to Mrs.Chen Liusheng,a researcher in the Institute of Chemistry at the Chinese Academy of Sciences,for providing the fast-response pressure sensitive paint used in this study.This study was cosupported by the ‘111 Project”of China(No.B17037),Innovation Foundation for Doctor Dissertation of Northwestern Polytechnical University of China(No.CX201713),and the National Natural Science Foundation of China (No.51476132).

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