It is a typical irreversible process of flow and heat transfer in the microchannel and it is necessary to reduce the irreversibility to improve effective utilization of energy in the heat transfer process. The entropy generation rate and transport efficiency of thermal energy are derived from the first and second law of thermodynamics

which indicate that the effective utilization of thermal energy increases with the decrease of net temperature gradient of fluid in the micro heat sink. A new micro heat sink with complex structure is proposed and simulated based on the previous study. The effect of structure on entropy generation rate and transport efficiency of thermal energy is also analyzed. The results reflect that the less the net temperature gradient of fluid

the less irreversibility and the more uniform the temperature of bottom in the heat sink. Based on the definitions of thermal enhancement factor

entropy generation rate and transport efficiency of thermal energy

it is reasonable to use thermal enhancement factor to evaluate the comprehensive performance of micro heat sinks

while the other two are used to measure the irreversibility and level of utilization in the heat transfer process. In short

the evaluation criteria is provided by the first law of thermodynamics

while the essence of heat transfer enhancement is indicated by the second law of thermodynamics

giving a complete thermodynamic theory for optimizing micro heat sinks.