High-temperature superconducting (HTS) Flux pumps are promising devices to maintain steady current mode in HTS magnets or to energize rotor windings in motors and generators in a contactless way. Among different types of flux pumps, the dynamo-type flux pump has been very common due to its simple structure and ease of maintenance... However, understanding the principle of dynamo-type flux pump has been challenging despite the recent progress. For addressing this challenge, the numerical modeling has proved to be an appropriate tool. Modeling is usually fast, precise, cost-efficient and enables to examine some details that is hardly possible via measurements. For this purpose, an efficient numerical model based on Minimum Electromagnetic Entropy Production (MEMEP) method has been used to study the performance of the flux pump in open-circuit mode. This model is the fastest and most efficient model of a flux pump with good agreement with experiments. In addition to the main behavior of the flux pump, the influence of airgap on open-circuit DC voltage of the flux pump has been investigated. This study revealed an important property in HTS flux pumps that with increasing airgap, pumping voltage in superconducting tape does not cease, but only reaches to insignificantly low values, which is not measurable via experiments. Furthermore, the modeling results of open-circuit DC voltage were compared to experimental ones obtained from the article published in 2016 by Bumby \textit{et al.} \cite{bumby2016anomalous} and the results showed good agreement. (read more)