Similarity laws for pumps are only applicable to incompressible fluids, and they can’t be directly applied to the extrapolation of compressor characteristic maps. But by varying the exponent of the relative rotational speed ratio to reflect the influence of compressibility, improved similarity laws can be applied to compressors where compressible fluid is considered. This paper proposes a new similarity law for compressor map extrapolation to low speeds by using the two lowest available speed lines and proposes a method of calculating the exponent. A comparison between calculated and real compressor performance of a compressor indicated that this exponent extrapolation method is simple, generic and has acceptable accuracy. The obtained low speed lines of compressors can be applied in gas turbine performance simulations to estimate the starting process of gas turbine engines.

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