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In this paper, the subject of mathematical model is a series of math expressions, which is used to calculate different regions' volume fraction and analyze flow characterization in multi-strand tundish. But research about mathematical model for multi-strand tundish is few, and so far, there has been no acknowledged math model for multi-strand tundish to describe its flow characteristic. If Sahai's model, which is originally proposed for the case of single-strand tundish (proposed in reference[10], and this model is widely used in the world), is applied to describe flow feature in multi-strand tundish, the calculation results would be unreasonable. Based on the data of water-model experiment results, the sum of each strand's dead region's volume fraction is bigger than 100 %, and this obviously doesn't agree with reality; and the value of dead region's volume fraction is calculated to be minus according to mathematical simulation results data in another case. What's more, Sahai's model does not propose the standard of plotting the RTD-curve, and this makes scholars around the world can't achieve consensus of views about plotting RTD-curve. And the model doesn't consider the bypass flow and can't calculate its volume fraction, but bypass flow is critical to tundish metallurgy. And through Sahai's model, the calculation result of plug flow region's volume fraction is also not reasonable, because the model doesn't well describe the essence of plug flow. So these suggest that it is not reliable to apply Sahai's single-strand tundish model to multi-strand tundish case. Then a new model is attempted to propose in this paper for your discussion. In the new model, the standard of plotting RTD curve is definitely proposed, and relative calculation method is also proposed; and the feature of dead region is carefully studied and the model proposes a new method to calculate its volume fraction, and the calculation formula about its volume fraction can be adjustable according the actual demand; what's more, the new model considers the bypass flow and proposes a method to calculate its volume fraction for the first time, and then volume fraction of plug flow region, backmix flow region, dead region and bypass flow can be calculated and obtained at the same time; and this new model can better capture the deviation of reality flow pattern from ideal plug flow pattern, and reflects the feature of plug flow.