In general, the experiments confirmed that different flow and
operating conditions resulted in various heat transfer characteristics
which basically characterized as so-called ‘‘normal’’, ‘‘deteriorated’’
and ‘‘improved’’ heat transfer regimes [16]. It is very
difficult to provide the exact definitions of these heat transfer
regimes. Nevertheless, many criteria were mentioned in the open
literature [2], and a frequently-used one was proposed by Koshizuka
et al. [26] as the following: