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New technologies

The Visualization of Inner Flow inside Air Conditioning System with PIV

Yousuke Saitou* Ryota Nakajima**

AbstractThe PIV (Particle Image Velocimetry) system visualizes airflow velocity and vector at multiple points. In the PIV, fine image capturing is required with such factors as adoption of highly photogenic particles capable of accurately tracing the airflow, generation of high-energy light emission, selection and precise adjustment of appropriate image capturing hardware, and state-of-the-art image processing.This paper explains those factors solved in the PIV system construction and airflow visualization results in our centrifugal blower product of an automotive climate system followed by correlation with a digital simulation result.

Key Words : Blower, Air conditioning, Visualization/ Image processing

1. Introduction A demand for miniaturization of the centrifugal blower unit in air-conditioning systems has been rising to pro-duce a comfortably wide open space in automotive cab-ins. However, downsizing the unit decreases the airflow rate, and if fan revolution is raised to compensate this, it would raise the noise level. Thus, the miniaturization is in need of a solution, either by advancing the blower performance to equalize with conventional size blower units or by preventing noise generation. Both of the blower performance and the noise genera-tion depend on the airflow conditions. To optimize the airflow during product design phases, it is essential to comprehend the airflow inside the blower unit. As a technology to visualize the internal airflow, we introduce the PIV system and its application case in this report.

2. Principle of PIV Analysis To trace the invisible airflow, tracer particles are fed into the flow from upstream of the observation field. When the field is irradiated with laser light, the parti-cles in the airflow reflect with a certain brightness. The camera consecutively captures the reflected light images while synchronizing the shutter opening timing with the light irradiation (Fig. 1). Then, brightness of the particle reflection is compared in a pair of the consecutive images. By calculating the correlation function from those images, airflow velocity vectors are mapped (this is called vector processing) (Fig. 2).

Fig. 1 PIV analysis flow

Fig. 2 Direct cross-correlation method

* Global Technology Division, Test Fundamental Technology Group**CAE MBE Group

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CALSONIC KANSEI TECHNICAL REVIEW vol.13 2017

3. System Construction and Clear Image Capture

Calculation accuracy of the tracer particle velocity vec-tor depends on the particle substance, camera sensor/lens, light source, and other influential factors. The fol-lowing sections explain details of these factors.3.1. TracerParticleSubstance

In order to track the fluid behavior, the inflow particles must accurately trace the airflow. To achieve the accura-cy, the external disturbance must be considered. The particle sedimentation occurs due to the specific gravity difference from the fluid. If the sedimentation ve-locity is too fast, the particles deviate vertically from the airflow axis, resulting in an inaccurate velocity vector. In light of the vertical components on fluid dynamics, the equation of particle motion holds as follows (1):

From the equation above, the sedimentation velocity can be derived as follows (2):

This equation expresses that the sedimentation velocity is proportional to the square of the particle diameter. By another external disturbance, the particles tend to radially drift from the rotational center due to the cen-trifugal force. The equation (3) shown below expresses that the drift-ing radial velocity, generated by the centrifugal force, is proportional to the square of the particle diameter as in the sedimentation velocity.

These equations signify that the small diameter particles have less external disturbances. However, reflectance,

counted as an observability index, is also proportional to the reflection area on the particle and to the square of the particle diameter. It is evident from the fact that changes of the particle diameter generates contradictory effects on the reflectance against the sedimentation ve-locity and the drifting radial velocity. In other words, too small particle diameter causes insufficient reflectance that brings difficulty in the airflow observation. In addition, when the particle reflects the laser light, the light scattering intensity tends to increase at the oppo-site of the light incidence angle. Because large scattering intensity may blur captured images, it is necessary to select a suitable particle substance (Fig.3). To meet these requirements, we adopted a refractory particle substance called “DEHS (Di-2-Ethylhexyl-Seb-acat)” which has a sufficiently small radius, as well as advantages in the reflectance and scattering intensity at the airflow velocity (30 m/sec) in general centrifugal blower units.

Fig. 3 Light scattering intensity

3.2. CameraSensorandLens For fine image capturing, it is necessary to optimize characteristics of each optical instrument. Fundamentally, once the frame rate (frames per sec-ond) is adjusted to finely track the particle behaviors, the maximum number of valid pixels is determined by the capability of processing photoelectrically converted data. To maximize the valid pixel numbers, projection of the particle images on the sensor elements must be optimized by adjusting the optical magnification of the lens and the size of the actual view field through the lens. For the adjustment, the actual view field size can be derived based on the relation between distance from the object plane to the camera lens and the focal length of the camera (Fig. 4).

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The Visualization of Inner Flow inside Air Conditioning System with PIV

Fig. 4 Layout of optical instrument

For clear image capturing with the selected optical instrument, sufficient light intensity is required for the lens incidence. The light can be intensified by opening the iris diaphragm. However, if the iris is opened too wide, the light in-tensity exceeds the acceptable exposure level where the element can sense. Opening the iris also widens the blur area around the focal plane (airy disc) due to diffraction of the intense light through the lens. In other respects, the camera focus may not be precisely adjusted due to the shallow depth of field. Furthermore, when the object plane is moved near or far side, it is even more difficult to adjust the focus. Taking these matters into account, we always cautious-ly adjust the iris opening to minimize the blur area while securing a certain depth of field and an adequate light intensity (Fig. 5 and 6).

Fig. 5 Iris condition of the lens

Fig. 6 Sample images

3.3. LightSource As a light source, single wavelength laser is used to pre-vent an impact of chromatic aberration. We adopted the 532 nm wavelength Nd:YAG laser which stably produces high-energy light and allows easy adjustment of pulse irradiation timing.

3.4. SynchronizationofCameraShutterandLaserIrradiationTimings

If the shooting objects move while the camera shutter opens in consecutive exposures, motion blur appears in captured images. To prevent the motion blur, extremely fast shutter speed is required in the order of nanosec-onds. However, such fast shutter speed and exposure are impossible due to the camera capability. To achieve ex-ceptionally short exposure, alternatively, the PIV system flashes the laser light for just a short moment during the shutter open period and thereby captures clear images without motion blur (Fig.7).

Fig. 7 Synchronized timing of camera and laser3.5. CasingMaterialandFabrication

In order to irradiate the observation objects with suf-ficiently intense light and to capture the object images without any distortion, the casing of the blower unit must be highly transparent and less refractive. Therefore, the casing was formed by cutting and grind-ing acrylic (methacrylate polymer) blocks with cemented carbide end milling cutters so that air-bubbles are not mixed in injection molding.

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CALSONIC KANSEI TECHNICAL REVIEW vol.13 2017

Since the outer surface of the casing does not affect airflow, unnecessary convexity was cut to reduce light refraction and polished to meet a total transmittance target. With application of the PIV system constructed through the studies so far, we visualize the airflow in experiment as described from the next section.

4. Airflow Visualization A centrifugal blower unit used in an automotive air-con-ditioning system was experimented to visualize airflows around the nose and between the blades for validation of the PIV system.

4.1. AirflowaroundNose

(1)Apart from the Nose (2)Around the Nose

Nose

Blower outlet

Observed area (1) Observed area (2)

Fig.8 Observed area

Time progress（Δt=10-4sec）

Velo

city

[m

/s]

030

Fig.9 Vector mapping result apart from the nose

Time progress（Δt=10-4sec）

Velo

city

[m

/s]

030

Fig.10 Vector mapping result around the nose

Velo

city

[m

/s]

030

Fig.11 Averaged vector mapping around the nose

4.1.1.VectorMappingResultofAirflowaroundNose Fig. 8 shows two areas around the nose where the airflow vectors were mapped. The vector map away from the nose (Fig. 9) shows that the air uniformly flows toward the blower outlet at fast velocity. On the other hand, the map near the nose (Fig. 10) shows occurrences of large turbulences with indication of significant vector changes in each frame. Since the airflow must be statistically identified for anal-ysis of the blower efficiency, the turbulent vectors were averaged as follows.4.1.2.AveragedVectorMapping

Fig. 11 shows the averaged turbulent vectors. From Fig. 10, the tendency of the airflow behavior cannot be entirely identified because the vectors change over time. In this respect, averaging the significantly changing vectors clarifies the airflow behavior. This statistical identification allows the derivation of the optimum nose shape design that satisfies the blower performances.

4.2. AirflowbetweenBlades

Observed area

Fig.12 Observed area

Blade

Nose

LE

TE

Positive pressure surface

Suction pressure surface

Fig.13 Flow between blades before PIV processing

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The Visualization of Inner Flow inside Air Conditioning System with PIV

4.2.1.VectorMappingResultofAirflowbetweenBlades

Velo

city

[m/s

]

015LE

TE

Fig.14 Flow between blades with vector mappin

Secondly, a vector map was created from the capture of the airflow between the rotating blades (Fig.12 and 13). The created vector map (Fig. 14) indicates that the airflow velocity is slow at the Leading Edge (LE) and fast at the Trailing Edge (TE). Since the vectors were derived from observations at fixed coordinates, angular velocity vectors of the rotat-ing blades are also included in the airflow velocity in the map, causing difficulty in identifying the accurate airflow behaviors. Therefore, the blade rotating velocity was ex-cluded from the airflow velocity, and it was converted to the vector map in the blade coordinate system.4.2.2.EffectofRotationalVelocityExclusion

View on Rotating CoordinateView on Fixed Coordinate

Fig.15 Comparison of the view on each coordinate

On the view of fixed coordinates (Fig. 14 on the left), airflow vectors are oriented along the blade shape. After the blade rotating velocity is excluded from the original image, a vortex appears in the airflow vectors between the blades (Fig. 14 on the right). Such precise analysis, in-cluding the airflow between the blades, allows designing optimum blade shapes.

5. Comparison with Simulation Result The derived PIV results were compared with the CFD simulation results. In this comparison, we focused on the vortex caused between the blades. Lattice Boltzmann method was used for the calculation, and the momentum are applied to the airflow by the

blade rotation for simulation of actual products. As a boundary condition, the airflow rate measured in testing was specified at the blower outlet.

PIV Result CFD Result

Fig.16 Comparison between PIV and CFD

Both of the PIV measurement result and the CFD sim-ulation result (Fig. 16) show that vortexes are caused on the suction pressure surfaces of the blades. This comparison concludes that the PIV measurement result accurately correlates with the CFD calculation result, and both of the results accurately visualize the real phenomenon.

6. Concluding Remarks(1) The fluid behaviors can be precisely visualized with

the PIV system once a suitable optical system is se-lected and the images are appropriately processed on the objectives.

(2) We validated that the PIV measurement result cor-relates with the CFD calculation result, and both of the results are accurate.

(3) We will continuously develop high-performance cli-mate blower units by utilizing such fluid visualization technologies.

References(1) Toshio Kobayashi and other authors: Handbook of

Particle Image Velocimetr, Chapter 2 to 5, Journal of the Visualization Society of Japan (2002)

(2) Jun Sakakibara: PIV Seminar (basic), The 19th Visual-ization Frontier, Visualization Society of Japan (2015)

(3) Edmund Optics Japan Co., Ltd.: Imaging Resource Guide,http://www.jst.go.jp/SIST/handbook/sist02/index.ht

YousukeSaitou RyotaNakajima