ABSTRACT

A low specific speed centrifugal compressor powered by an ordinary electric motor (20, 000 rpm) has been designed at Tsinghua University for air systems application of automotive fuel cell engines. Preliminary design characteristics have been concluded. Three-Dimensional Computational Fluid Dynamics (CFD) is used to investigate the flow field of the impeller. The characteristic curve and primary aerodynamic parameters of designed impeller are achieved. The experimental results indicate that the designed low specific speed centrifugal compressor has comparatively high efficiency and wide operating range. The highest efficiency and pressure ratio of the centrifugal compressor is up to 78% and 1.42, respectively. The designed low specific speed centrifugal compressor can meet the requirement of air systems of automotive fuel cell engines. Moreover, the low specific speed centrifugal compressor avoids difficulties of usage of high-speed electric motors in high specific speed compressor.

NOMENCLATURE

C absolute velocity

sN non-dimensional specific speed

Q volumetric flow rate

U rotational velocity

W Relative velocity

h� specific isentropic enthalpy

� Rotational angular speed

INTRODUCTION

Research and development of fuel cell systems for multiple applications has dramatically increased in the past few years. However, the vehicular application of the

fuel cell system leads to a number of unique challenges, such as physical packaging within the vehicle, durability and operation under extreme environmental conditions, and demanding duty cycles that include high peak power requirements and a rapid response time, etc. Pressurizing technology can give higher efficiency, higher power density and better water balance characteristics for the fuel cell system. However, at present there is no mature compressor which can ideally match the air system of fuel cell systems.

The requirements of pressurized fuel cell systems include: compact structure, light weight, oil-free of air, low operate noise, easy maintaining, low cost and high operate efficiency [1]. Nowadays, a variety of compressors are being developed for fuel cell applications. The centrifugal compressor, one of compressors suitable for fuel cell systems, proves to be a superior machine in terms of efficiency, and therefore offers the most promising effect on system efficiency. Moreover, it has the advantages of small volume, low cost, high reliability, etc.

In order to pressurize the air, it is necessary to use some auxiliary power to operate centrifugal compressors since exhaust gas power of fuel cell systems by now is not powerful enough to drive turbines working with centrifugal compressors. As one of the auxiliary powers, a high speed electric motor (around 60000 rpm) can be used to drive centrifugal compressors. A turbocharger type compressor system integrated with a high-speed (>100,000 rpm) motor is being developed by Honeywell[2]. This system makes fuel cell pressurizing air system more compact and has quicker response characteristic. However, usage of high-speed motor causes several serious issues including high cost, complex maintain, low stability, need of special cooling system, etc. Therefore this type of compressor system is hard to be applied into commercialization products [3].

If an ordinary motor (20000rpm) is used, the problems of high speed motors could be avoided. However, if centrifugal compressors are still designed conventionally in this case, pressure ratio can only achieve around 1.05 to 1.1, therefore cannot meet the pressurizing requirements of air systems for fuel cells. So it is necessary to design a centrifugal compressor

2008-01-1692

Research and Design of a Centrifugal Compressor for Fuel Cell Turbocharger

Xinqian Zheng and Yangjun Zhang State Key Laboratory of Automotive Safety and Energy, Tsinghua University

Copyright © 2008 SAE International

with higher pressure ratio on the same condition of rotational speed and mass flow, i.e., to design a low specific speed centrifugal compressor. However, under low specific speed condition, it is a challenge to design a centrifugal compressor with high efficiency and wide operation range characteristic Moreover, there barely is reference regarding this type design.

Compressor design software, Concept, has been used to design and analyze the low specific speed centrifugal compressor [4]. Also, Computational Fluid Dynamics (CFD) software, Numeca, is used to analyze flow behavior inside the compressor. The results show that designed low specific speed centrifugal compressor has a reasonable performance.

COMPRESSOR DESIGN

The main design parameters of automotive fuel cell low specific speed compressor are: design rotational speed 20000 rpm, design pressure ratio 1.4 ~1.45, design flow 0.1~0.15 kg/s, reasonable surge tolerance. Within rotational speed of 20000 rpm for ordinary motors, in order to reach pressure ration of 1.4, there must be some different design characteristics for low specific speed from the conventional centrifugal compressor design, as shown below:

LOW SPECIFIC SPEED DESIGN

Non-dimensional specific speed Ns is given by: 1/ 2

3/ 4s

QN

h�� �

�( � is the rotational speed, Q is the

volumetric flow rate, h� is the specific isentropic enthalpy). Specific speed qualitatively shows work ability of compressors. With same rotational speed, higher specific speed means higher compressor ratio. Specific speed for conventional design of centrifugal compressors is typically around between 0.7 and 1.0 so that compressors can achieve comparatively high efficiency. However, compared to conventional design, low specific speed centrifugal compressors have higher inverse pressure gradient and larger fraction of secondary flow. Therefore profound understanding of flow characteristic inside stage is needed to design a high performance low specific speed centrifugal compressor.

ONE-DIMENSIONAL DESIGN

Figure 1 shows the meridian plane of the low specific speed centrifugal compressor design.

Fig. 1 Meridian plane of centrifugal compressor

design

Inducer of impeller uses comparatively small radius of curvature and adopt zero-incidence design to be suitable for low flow rate work condition of fuel cell systems. Elongate blade design is required to meet the demand of high pressure ratio within low rotational speed and low flow rate. Because of low specific speed characteristic, properly widen impeller exit blade height in order to decrease relative speed inside impeller passage, thereby, reduce secondary flow effect. Moderately big impeller radius is needed to generate high tangent velocity of exit, therefore, make compressors able to output high pressure ratio under low rotational speed. But the maximum impeller radius is limited by the requirement of compact design of fuel cell systems.

Diffuser is used to converts kinetic energy of air exiting from impeller into static pressure. Automotive centrifugal compressor normally uses vaneless diffuser due to wide operation range and small volume. Air flows almost as a logarithmic curve inside of vaneless diffuser. At the exit of low specific speed impeller, tangential velocity of air is much higher than radial velocity of air along impeller blade, which indicates that absolute velocity of airflow inside vaneless diffuser is quite close to tangential direction (see Fig.2). As a result, the distance air flows in the vaneless diffuser greatly increases so that frictional losses are high and therefore back flow occurs easily. In order to reduce these losses, short vaneless diffuser is required in this case.

Because of low radial velocity of air and low flow rate, small diameter of volute is adopted for low specific speed centrifugal compressor.

Fig. 2 Velocity triangle of exit of designed impeller

C) Volute design:

PERFORMANCE PREDICTION

Use Compal module of Concept NREC to predict performance of whole compressor stage including impeller, vaneless diffuser, and volute. The results are shown in Fig.3 and Fig.4 indicating that the stage pressure ratio reaches 1.5 with efficiency of 76 % at rotational speed of 20000 rpm, which meets design requirements.

Fig. 3 Pressure Ratio-Flow Rate performance curve

of designed stage

Fig. 4 Efficiency-Flow Rate performance curve of

designed stage

IMPELLER THREE-DIMENSIONAL DESIGN

Use Axcent module of Concept to design the compressor three-dimensionally. Because blades are designed narrow and with small curvature radius, comparatively big blade wrap angle is used to reduce load on unit area of blades and also decrease the velocity difference between pressure face and suction surface to control the formation of secondary flow, therefore improve flow characteristic and reduce divergence loss. Adopt split blades to lessen flow block effect. Use shortest axial length of impeller in order to decrease the volume. 40 backward angle is applied to raise efficiency and widen steady operation range.

FLOW ANALYSIS

A CDF software NUMECA is used to simulate three dimensional flow field of designed centrifugal compressor. Use Navier-Stokes equation as control equation and Spalart-Allmaras model as turbulence viscosity. Because flow velocity inside low specific speed centrifugal compressors is low, volute slightly affects flow performance. Therefore, to save amount of simulating calculation, only one of the blade flow passages needs to be calculated instead of all-around impeller flow passages with volute based on rotational symmetry assumption. For the simulation, mesh one blade flow passage with 30, 0000 nodes, define stagnation temperature, stagnation pressure, velocity direction, and static pressure of impeller inlet, define flow rate of impeller exit and give periodic boundary condition circumferentially.

Simulation is taken at different rotational speed of 20000 rpm (design rotational speed), 18000 rpm, and 16000 rpm. Performance curves of designed impeller are plotted in Fig.5 and Fig.6 showing that the highest efficiency and pressure of designed centrifugal compressor (without considering volute) are 83 % and 1.6 separately. This indicates that designed centrifugal compressor has fairly wide and flat operating characteristic, which satisfies the requirement of wide operation range for fuel cell systems.

0.00 0.05 0.10 0.15 0.20 0.250.0

0.3

0.6

0.9

1.2

1.5

1.8

20000 r/min 18000 r/min 16000 r/minPr

essu

re R

atio

Flow Rate / kg/s

Fig. 5 Pressure Radio-Flow Rate performance curve

of designed impeller (including diffuser)

0.00 0.05 0.10 0.15 0.20 0.250

10

20

30

40

50

60

70

80

90

20000 r/min 18000 r/min 16000 r/min

Eff

icie

ncy

(%

)

Flow Rate / kg/s

Fig. 6 Efficiency-Flow Rate performance curve of

designed impeller (including diffuser)

Figure 7 and Figure 8 illustrate static pressure distribution on pressure surfaces and suction surfaces of main blades and splitter blades, showing that loads on pressure surface and suction surface are mostly the same so that it avoids miscibility loss at exit of main blades and splitter blades. The static pressure distribution curves are mainly smooth except that there is a little peak value exchange of static pressure in the beginning of the curves due to flow stagnation effect from beginning edge of blades. As a whole, static pressure distribution is reasonable.

Fig. 7 Static pressure of main and splitter blade

Fig. 8 Static pressure of main and splitter blade

Figure 9 illustrates the absolute velocity vector of air at exit of designed impeller. It shows absolute velocity vector of airflow is almost parallel to tangential direction of impeller since tangential velocity is much bigger than radial velocity of air (effect from high pressure ratio at low flow rate), which perfectly supports the linear dimensional analysis before shown in Fig.2. Therefore, it strengthens the design of short vaneless diffuser for low specific speed centrifugal compressors in order to reduce friction loss inside the diffuser.

Fig. 9 Absolute velocity vectors near exit of the

impeller

EXPERIMENTS

After optimization design, the centrifugal compressor is manufactured. In order to obtain the experimental performance conveniently, the designed centrifugal compressor is driven by a turbine just like an experiment for a turbocharger. The turbine is driven by the pressured air which is hot up in combustion. The total pressure p� is measured by

total pressure rake with 0.2% inaccuracy. Total temperature

T � is measured by thermocouple with inaccuracy 0.5 .Mass flow m� is measured by vortex flowmeter with inaccuracy 1%.

The experimental results including pressure ratio, efficiency contours, power contours, corrected mass flow and corrected rotation speed are plotted in Fig. 10 and Fig. 11. The pressure ratio � is the ratio of absolute outlet pressure

2P� divided by absolute inlet pressure 1P� . The efficiency

contours in Fig. 10 is defined as 1

2 1

( ) 11

k k

t tT T

��

�

(1)

The corrected mass flow is defined as

1

1

100

298t

cort

T kPam m

K P� � � (2)

The corrected shaft speed is defined as

1

298cor

t

Kn n

T� (3)

Then, the power contours in Fig. 11 are calculated by the following formula

2 1( )cor p t tP m C T T� � (4)

Figure 10 shows pressure ratio and efficiency contours of the centrifugal compressor with low specific speed. The highest pressure ratio of the centrifugal compressor is up to 1.42. The maximum efficiency ring is 78%, which is close to the efficiency of conventional centrifugal compressor. Figure 11 shows pressure ratio and power contours of the centrifugal compressor. Power consumption of compressor is important for performance of full cell. Power consumption of compressor is below 5kW for most conditions.

Corrected mass flow /(Kg/s)

Pres

sure

Rat

io

Fig. 10 Experimental performance map of

centrifugal compressor: pressure ratio & efficiency

contours.

Corrected mass flow /(Kg/s)

Pres

sure

Rat

io

1 kW

3 kW

5 kW

4 kW

2 kW

Fig. 11 Experimental performance map of

centrifugal compressor: pressure ratio & power

contours.

CONCLUSIONS

A low specific speed centrifugal compressor for automotive fuel cell systems is designed in this project. This centrifugal compressor can be driven by an ordinary electric motor, therefore has advantages of low cost, easy manufacture, long life length to be fit for low flow rate condition.

The experimental results show that designed low specific speed centrifugal compressor reaches efficiency of 78 % and pressure ratio of 1.42, which basically fulfils the pressurizing requirements of fuel cell systems.

ACKNOWLEDGE

The authors would like to thank for Weidong Xing, Junyue Zhang, Jizhong Zhang, and other researchers in National Key Lab. of Diesel Engine Turbocharging Tech. This project has been supported by FuYuan Turbochargers CO., LTD (Weifang, Shandong, China).

REFERENCES

1. Vine, A. J., Thornton, W. E., Pullen, K. R., et al, 2005, “Low Specific Speed Turbocompressors,” International Conference on Compressors and their Systems, London: Springer 225-234.

2. Mark, G., 2004, “Cost and Performance Enhancements for a PEM Fuel Cell Turbocompressor,” DOE Hydrogen, Fuel Cells & Infrastructure Technologies Program Review Presentation, Honeywell Systems, Systems & Services, Philadelphia.

3. Chen, Q. S., Qi, Z. N., 2001, “Technology Challenge and Prospect of Fuel Cell Vehicle,” Automotive Engineering, 23(6): 361-364. (in Chinese)

4. Japikse, D., 2006, “Centrifugal Compressor Design and Performance,” Vermont: Concepts ETI, Inc.