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State-of-the-art light-emitting diodes (LEDs) are made from high-purity alloys of III-V semiconductors, but high fabrication cost has limited their widespread use for large area solid-state lighting. Here, efficient and stable LEDs processed from solution with tunable color enabled by using phase-pure 2D Ruddlesden-Popper (RP) halide perovskites with a formula (CH (CH ) NH ) (CH NH ) Pb I are reported. By using vertically oriented thin films that facilitate efficient charge injection and transport, efficient electroluminescence with a radiance of 35 W Sr cm at 744 nm with an ultralow turn-on voltage of 1 V is obtained. Finally, operational stability tests suggest that phase purity is strongly correlated to stability. Phase-pure 2D perovskites exhibit >14 h of stable operation at peak operating conditions with no droop at current densities of several Amperes cm in comparison to mixtures of 2D/3D or 3D perovskites, which degrade within minutes.

Turbulent drag reduction by polymer additives in a channel is investigated using direct numerical simulation. The dilute polymer solution is expressed with an Oldroyd-B model that shows a linear elastic behaviour. Simulations are carried out by changing the Weissenberg number at the Reynolds numbers of 4000 and 20 000 based on the bulk velocity and channel height. The onset criterion for drag reduction predicted in the present study shows a good agreement with previous theoretical and experimental studies. In addition, the flow statistics such as the r.m.s. velocity fluctuations are also in good agreement with previous experimental observations. The onset mechanism of drag reduction is interpreted based on elastic theory, which is one of the most plausible hypotheses suggested in the past. The transport equations for the kinetic and elastic energy are derived for the first time. It is observed that the polymer stores the elastic energy from the flow very near the wall and then releases it there when the relaxation time is short, showing no drag reduction. However, when the relaxation time is long enough, the elastic energy stored in the very near-wall region is transported to and released in the buffer and log layers, showing a significant amount of drag reduction.

Turbulent heat transfer to CO2 at supercritical pressure flowing in heated vertical tubes is investigated using direct numerical simulation at the inlet Reynolds number Re0=5400, which is based on inlet bulk velocity and tube diameter. Temperature range within the flow field covers the pseudocritical region, where very significant fluid property variations are involved. Both upward and downward flows are considered. The wall temperature distribution shows well-known heat transfer deterioration characterized by the localized peak in upward flows, while no such anomaly is observed in downward flows. The deterioration occurs at the region where turbulence is attenuated significantly, and is followed by the enhancement with restoration of turbulence caused by complicated interactions with a buoyancy effect. Further investigation of turbulence statistics indicates that ρux″ux″¯, ρux″ur″¯, and ρux″h″¯ are significantly affected by their respective buoyancy production terms due to ρ′ux′¯, ρ′ur′¯, and ρ′h′¯ which are proven to be significant in vertical supercritical flows. Combined with the deformation of mean velocity profile into an M-shaped one in upward flow, ρ′ux′¯ becomes negatively correlated from a certain downstream region so that ρux″h″¯ undergoes a very complicated transition changing both in sign and magnitude, causing severe impairment of heat transfer in upward supercritical flows.