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1996 SAE International Congress & Exposition

Feb. 26-29, 1996, Detroit, MI

DESIGN OF AN AUTOMOTIVE HVAC BLOWER WHEEL FOR FLOW, NOISE AND STRUCTURAL INTEGRITY

C. Toksoy, M. Zhivov, F. Cutler, & A. Ecer, Technalysis

M. Rayhill, S. Guzy, & R. Vasko, Harrison Division General Motors Corporation

Introduction

Design of a centrifugal blower wheel for automotive air-conditioning is a complicated process for several reasons. Although it is a low cost product, the design of a squirrel-cage type blower wheel, shown in Figure 1, requires a certain level of precision. Wheels with slightly modified geometries may perform quite differently in terms of flow efficiency and noise. Usually, the HVAC system is not designed around a blower wheel; rather the wheel is fitted to a given system. Centrifugal blowers with forward-curved blades are not aerodynamically designed to be the most efficient. Instead, they are chosen for their ability to deliver relatively high flow rates for rather restrictive packaging requirements [1]. For example, in comparison to a mixed flow fan, the resulting flow field is more three-dimensional and complex; resulting in many separated flow regions and noise generation mechanisms.

The objective of the blower wheel design is to provide the best wheel to suit these rather non-ideal conditions. Depending on the specific situation, the design requirements may vary depending on requirements for air flow performance, noise, structural integrity and cost. Another important factor is the expectation in terms of time required to design a new wheel. One has to perform the engineering tasks described here in a relatively short time, even though the problem to be solved is rather complex.

In the present application, we consider a specific design problem. We start with a reference wheel which provides sufficient flow but it is relatively noisy. The problem is defined as reducing the noise level while producing at least the same flow performance. It is also desirable to reduce the cost of the design by using a less expensive material. The packaging is fixed. While the blower wheel and the housing have to be ideally considered as a single unit, the housing design is not to be changed and the new wheel should match the existing housing. The problem described above is a typical design problem for HVAC blower wheels. As mentioned above, packaging requirements usually results in such restricted design conditions.

In the present paper, we describe a design strategy, which involves the implementation of a computational procedure complemented with an experimental testing program. It is demonstrated that by using three-dimensional CFD tools , one can design blower wheels which will provide the desired objective. The use of such tools reduce the cost and time required for testing to reach the desired objective. At the same time, it helps to improve the basic understanding of many complex situations which may occur inside the rotating blower wheel. Many times CFD tools are used to analyze a design which is not the desired one. The strategy here is to use CFD to eliminate rather than analyze the bad designs and come up with a design to meet the specifications.

Objectives

The objectives of the present development were to improve an existing a/c blower wheel along the following directions:

  • Reduce the flow induced noise generated by the blower,
  • Retain and possibly improve the flow performance,
  • Develop a blade shape which lend itself to a less costly pull-up mold design rather than the existing split louver design,
  • Replace a higher strength but expensive material ($2.50/lb.) with a less expensive ($0.70/lb.) material.

More specifically, the design process involved making changes to the blade profile while controlling the flow performance. It also involved increasing the blade thickness for improving strength and allowing the less expensive material to be utilized, while at the same time providing noise reduction which is also sensitive to such changes.

Design Criteria

The design criteria of a blower wheel for improved flow performance and noise characteristics can be summarized as follows:

It is desired to improve the combination of the static pressure rise across the wheel and the pressure rise along the housing. In the case of a blower wheel with forward curved blades, a considerable portion of the pressure rise is developed inside the housing which is tied to the tangential velocity (swirl) introduced by the trailing edge of the blade. Thus, proper evaluation and design of the trailing edges of the blades becomes a critical problem to match an existing housing. We are interested in evaluating the deviation angles between the blade trailing edge and the flow exiting the blade. This region is a source of flow losses and noise as well as determining whether the housing matches the wheel properly. Of course we are also interested in understanding the matching between the inlet and the leading edges of the blade, as well as the velocity distribution within the blade passage. We are concerned about the pressure distribution along the blade and flow attachment along the entire blade, flow circulation regions and the variations within the three-dimensional flow field from the pressure side to the suction side. Large variations in the circumferential direction will cause unsteadiness in the wake of the wheel exit resulting in loss of performance and generation of noise.

In this particular project, we are trying to increase the blade thickness at areas of high stresses. As we change the geometry, we are also concerned about improving flow performance. In most cases, noise and efficiency are directly related. However, when making design improvements, the priority was given to reduce the noise rather than increasing the efficiency. It will be shown that the final design is both more efficient, quiet and less expensive to manufacture at the end of the process.

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