This paper reviews some new understanding of heat transfer deterioration (HTD) in strongly heated vertical pipe flows recently established with the aid of direct numerical simulation (DNS). First, a new perspective of flow laminarisation due to non-uniform body force (such as buoyancy) is discussed, which has been established based on an isothermal flow. In contrast to the common understanding, it is shown that applying such a force does not cause changes to the key turbulence characteristics (including mixing) when the pressure force is kept unchanged, that is, when the analysis is based an Equal Pressure Gradient reference framework. The so-called flow laminarization, which is implicitly based on an Equal Flow Rate reference, can now be explained using the reduction of a newly-defined Apparent Reynolds Number (ARN). When applied to strongly heated air flow in a pipe, the ARN is used to produce a Reynolds number/Grashof number map to display the flow laminarisation (severe HTD) and recovery regimes, which agrees well with DNS data. Furthermore, the ARN predicts the presence of a bi-state region where the flow may be turbulent or laminar-like (corresponding to mild or severe HTD). This finding can be used to explain some of the large scatters in experimental Nusselt numbers in the strong HTD region. Next, analysis is made of a strongly-heated supercritical fluid flow in which severe HTD occurs due to complex reasons. The application of the ARN led to the establishment of a unified explanation for HTD mechanisms due to the variations of thermal properties, flow acceleration due to heating and buoyancy, all of which are treated as pseudo-body forces. The theory also explains the effect of spatial flow development on turbulence using pseudo-body forces. Finally, it is demonstrated how the ARN can be used to predict friction factor and Nusselt number as well as turbulent shear stresses and temperature profiles in laminarised HTD flows.