quantitative visualization of flow inside an automotive ...-0.2-0.1 0.0 0.0 0.1 0.2 0.3x [m] y [m] v...

Quantitative visualization of flow inside an automotive HVAC module with varying temperature control mode

Ho Seong Ji, Young Il Jang and Sang Joon Lee

Department of mechanical engineering Pohang University of Science and Tehnology (POSTECH)

Hyojadong, Namgu, Pohang, 790-784 R. O. KOREA

[email protected], [email protected], [email protected] http://efcl.postech.ac.kr/

Abstract: - Air flow inside an automotive HVAC module was visualized using a high-resolution PIV technique with varying the temperature operation mode. The PIV system consists of a 2-head Nd:YAG laser(125 mJ), a high-resolution CCD camera(2K x 2K), optics and a synchronizer. A real automotive HVAC module was used directly as a test model. Some casing parts of the HVAC module casing were replaced with transparent windows for capturing flow images with laser light sheet beam illumination. Time-averaged velocity fields were measured for different temperature control modes. Flow characteristics of the air-conditioned air flow inside the automotive HVAC system were evaluated from the PIV data. Key-Words: HVAC, PIV, Temperature baffle, Flow visualization 1 Introduction An automotive HVAC (heating ventilating and air conditioning) system as a device for controlling temperature, humidity, air flow and the internal air ventilation for passenger compartment is very important to improve comfort of passengers. To improve the performance of HVAC system and occupant thermal comfort, detail understanding of the flow characteristics in the compartment is required. Recent advances in computational fluid dynamics and experimental diagnostic techniques make easy the analysis of climatic environment within vehicles. In automotive HVAC module thermal equipments, the temperature baffle control the thermal condition of automotive compartment by changing the direction of air-conditioning flow.

Because of high-speed flow from a sirocco fan and complicated flow pathway, the flow structure inside the HVAC module has three-dimensional flow characteristics. Since the complicated flow path including temperature control baffles, curved flow pathway and heat exchanger influence the flow characteristic, it is not easy to measure the flow inside the HVAC module. Therefore, most of previous studies for HVAC module were carried out using numerical simulation (1, 2, 5). Some experimental studies for internal flow of HVAC module were carried out using point-wise measurement techniques such as hot-wire anemometer, 5 hole probe and LDV. Some researches investigated the air mixing through the T-junction (3) and developed a cylindrical HVAC (4).

Several experimental studies focused on the aero-acoustic noise around sirocco fans. There are limited data on quantitative flow information inside the HVAC module. Nowadays, PIV (Particle Image Velocimetry) has been used widely as a reliable velocity field measurement technique. In this study, we investigated the flow inside the real HVAC module using a PIV technique.

2 Experimental setup and methods Instantaneous velocity fields of flow in a real automotive HVAC unit were measured with a high-resolution PIV technique. The PIV system used in this study consists of a high-resolution CCD camera (2K x 2K), cylindrical lens, a dual-head Nd:Yag laser, and a delay generator. Figures 1, 2 show the schematic diagram and photograph of the experimental setup with laser light sheet illumination, respectively. The maximum pulse repetition rate of dual-head Nd:YAG laser is 15 Hz and its energy output is 125 mJ per pulse. Since the laser pulse has a short pulse width of about 7 ns, the imageds of high-speed air flow were captured clearly. In order to synchronize the dual-head Nd:YAG laser and the 2K × 2K CCD camera, a delay generator (Stanford DG535) was used. The time interval Δt between two laser pulses was also controlled using the delay generator. During the time interval Δt, some particles move in and out of the laser light sheet.

5th WSEAS Int. Conf. on FLUID MECHANICS (FLUIDS'08) Acapulco, Mexico, January 25-27, 2008

ISSN: 1790-5117 Page 202 ISBN: 978-960-6766-30-5

A B C D

A B C D EXT/GATE

BNC

Delay generator

Optics & Cylindrical lens

2 Head Nd:YAG laser (125mJ)

DC Power supply

Blower unit

A B C D

A B C D EXT/GATE

BNC

A B C D

A B C D EXT/GATE

BNC

Delay generator

Optics & Cylindrical lens

2 Head Nd:YAG laser (125mJ)

DC Power supply

Blower unit

Figure 1 Schematic diagram of PIV experimental setup

Figure 2 Photograph of laser light sheet illumination

Figure 3 Photograph of the HVAC module tested in this

study Therefore, it is needed to adjust thickness of the laser light sheet appropriately and reduce the time interval as short as possible. In addition, the time interval Δt depends mainly on the maximum particle displacement in the

(a) Measurement sections in x-y plane

x

z

y

x

z

y

(b) Coordinate system Figure 4 Measurement sections and coordinate system

interrogation window. In this experiment, the laser light sheet was about 1mm thick.

Some casing parts of the HVAC model were replaced with transparent poly-carbonate to minimize the reflection of laser beam and to acquire clear particle images. Figure 3 shows the modified HVAC module for PIV experiment. To simulate real operation conditions, the duct system used air path to the passenger compartment was attached at the exit of the HVAC module to simulate real condition. Therefore, the pressure load could be controlled nearly the same as real operation condition using real duct system. The CCD camera was place perpendicular to the laser light sheet. A vertical plane aligned with the flow direction was illuminated with the laser light sheet passing through a cylindrical lens located in front of the measurement section. As tracer particles, atomized olive oil droplets were generated from two Laskin nozzles. The olive oil droplets with a diameter of about 1-2 ㎛ give good traceability for high velocity fluctuations of the flow. In order to obtatin accurate instantaneous velocity field data, the time interval between two adjacent particle images was adequately adjusted using a delay generator.


ISSN: 1790-5117 03 ISBN: 978-960-6766-30-5

x [m

]

z [m]

zx

x [m

]

z [m]

zx

zx

(a) Cool-vent mode

x [m

]

z [m]

x [m

]

z [m]

(b) Warm mode Figure 5 Mean velocity fields in x-z plane

Figure 4 shows the measurement sections in x-y

plane for the PIV experiments to measure the whole flow inside the HVAC module and coordinate system used in this study. The field of view for each section is 150 x 150 mm2. To evaluate the whole velocity fields inside the HVAC module, velocity field data at five measurement sections combined together using a mathmatical method. In order to reduce possible errors in the combining procedure, each measurement section was overlapped about 20% with the adjacent section as shown in Fig. 4(a). As shown in Fig. 4(b), the x, y and z axes are represent as the direction of exit flow of HVAC module, downstream flow beyond the heater exchanger and exit flow of blower unit, respectively. For measuring instantaneouse velocity fields of the flow inside the HVAC module, a 50mm standard lens was attached in front of the CCD camera to capture flow images. 3 Results and discussions At each measurement section, 400 image frames were captured consecutively. The interrogation window size

ωω

(a) Cool-vent mode

(b) Warm mode Figure 6 Vorticity distributions in x-z plan

was 64 x 64 pixels with 50% overlapping. From each particle image pair, the corresponding velocity field was calculated using a cross-correlation PIV algorithm. After removing spurious error vectors, the several hundreds of instantaneous velocity fields were ensemble-averaged to get the time-averaged flow statistics.

Figure 5 shows the time-averaged mean velocity field in x-z plane in front of the evaporator core. The flow structure in the x-z plane seems to be influenced by the exit shape of blower unit, the temperature control mode and duct shape of HVAC unit in front of the evaporator core. Figure 5 (a) shows the mean velocity field for the cool-vent mode condition. For the cool-vent mode, the flow passes only the evaporator core and moves forward to duct system directly. Then the flow from the blower moves straight along the +x direction. The flow near the exit of the blower unit has high flow speed, but the flows in the regions of z=0.09 and 0.17 do not have high speed. The exit of blower unit has supporting elements to prevent deformation of blower unit. The exit flow seems to be influenced by this structure. The maximum velocity is about 9m/s near the exit region of blower unit. Figure 5 (b) shows the time-averaged mean velocity field for the warm mode condition. For the warm mode, the flow passes the evaporator core and the heater core.


ISSN: 1790-5117 Page 204 ISBN: 978-960-6766-30-5

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(b) Warm mode Figure 7 Mean velocity fields in x-y plane

The maximum velocity at the exit of blower unit is about 6m/s. But, the maximum velocity was reduced about 30%, compared to the cool-vent mode condition. This means that the heat exchanger plays a role of blockage element or flow resistance, and then the flow loss momentum in the warm mode.

Figure 6 shows the vorticity contour in the x-z plane in front of the evaporator core. Figure 6 (a) shows the vorticity contour for cool vent mode. As the flow goes downstream, the vorticity increases. At the location of x=0.095 and 0.165, the flow has counter-rotating vortex structure due to turbulent shear flow from the blower unit. Even though the vorticity distributions have similar feature, the vorticity for warm mode is smaller than that for cool-vent mode.

Figure 7 shows the mean velocity fields in the x-y plane. For the cool-vent mode, the air flow moves toward the duct system directly through evaporator core as shown in Fig. 7(a). The temperature baffle seems to isolate the flow path toward the heater core. Due to complicated flow path from the evaporator to the connecting duct, the flow has stagnation in the region A. From this result, we can conjecture that heat exchanger does not have good performance.

Evap. Core

Htr Core

De-frost

Ventilation

floorTemp. door

Evap. Core

Htr Core

De-frost

Ventilation

floorTemp. door

(a) Cool-vent mode

(b) Warm mode Figure 8 Comparison of flow path inside

the HVAC moducle The maximum velocity was about 5.7 m/s in front of

the duct system due to narrow flow path to the duct. Comparing the velocity between before and after the evaporator core, the flow speed was decreased by the presence of evaporator which works as flow resistance or back-pressure loader. The flow through the evaporator is closely related with performance of heat exchanger and flow resistance. The flow passing through the evaporator core can be observed from x=0.1 to x=0.24, the part of evaporator (x=0.24~0.32) might influence the performance of heat exchanger. These flow phenomena are matched with the results of Fig. 5 (a).

In the warm mode, the air flow moves toward the duct system through the evaporator and heater cores as shown in Fig. 7(b). The flow path from the evaporator to the heater core is aligned to the x direction. Then the flow just near the evaporator is inclined toward the heater core along the flow path. The maximum velocity is about 6.4m/s at the location of (0.24, -0.11) due to narrow flow path. Flow stagnation regions are formed in the regions of sudden flow-path change (region B and region B`) near the heater core. From these results, the performance of


ISSN: 1790-5117 Page 205 ISBN: 978-960-6766-30-5

heat exchanger would be reduced by this stagnation region. The flow beyond the heater core moves toward the duct system by passing the narrow flow path. Based on these results, we illustrated the flow path schematically in Fig. 8.

4 Conclusions The velocity fields of flow inside a HVAC module were measured with varying the temperature operation mode and the PIV results sere summarized as follows; 1) Flow characteristics of a real automotive HVAC

module under real operation condition were evaluated through PIV measurements.

2) The flow structure inside the HAVC module was quite different depending on the temperature operation mode. For the cool-vent mode, the velocity from the blower unit move toward the connecting duct system directly through the evaporator core. In this case, momentum loss due to the heat exchanger is not so high, compared to the warm mode. On the contrary, for the warm mode, since the air flow from the blower unit passes through the heat exchanger, the flow loss was more than that of the cool-vent mode. The performance of heat exchanger seemed to be influenced by the region of stagnation flow due to complicated flow path.

3) The present experimental results can be used to validate with numerical predictions and to improve the performance of automotive HVAC module in the initial design stage.

Acknowledgements This work was supported by Automobile Core Basic Technology Development Project from Ministry of Commerce, Industry and Energy, KOREA. References: [1] Aroussi A., Abdul Ghani S.A.A., and Rice E., PIV

measurement and numerical simulation of airflow field in a road vehicle HVAC cowl box, SAE technical paper, 2001, 2001-01-0294

[2] Bennett L., Dixon C.W.S., and Watkins S., Modeling and testing of air flow in a HVAC module, SAE technical paper, 2002, 2002-01-0506

[3] Hirota M., Asano H., Nakayama H., Asano T., and Hirayama S., Three-dimensional structures of turbulent flow in mixing T-junction, JSME International Journal Series B, 2006, Vol. 49, No, 4, 1070-1077

[4] James L., Ken H., and Karim N., Design and development of a cylindrical HVAC case, SAE technical paper, 2004, 2004-01-1385

[5] Shojaee M.H., Tehrani F.P.H., Noorpoor A.R., and Adili M.R., Analysis of vehicle passenger compartment HVAC using simulation, SAE technical paper, 2004, 2004-01-1505


ISSN: 1790-5117 Page 206 ISBN: 978-960-6766-30-5

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