Heat-transport measurements for ethane in a turbulent liquid-vapor two-phase state.

Below the critical point (CP) at $P_c,T_c$ liquid and vapor co-exist along a line $T_\phi(P)$ in the temperature-pressure plane. When a fluid at $P < P_c$ is heated from below and $\Delta T = T_b - T_t$ ($T_b$ and $T_t$ are the temperatures at the bottom and top of the sample respectively) straddles $T_\phi$, then liquid can condense at the top and drop to the bottom. This process will contribute strongly to the effective conductivity $\lambda_{eff}$ of the sample. We measured $\lambda_{eff}$ using ethane close to but {\bf below} the CP along various isobars using a constant $\Delta T$ and varying $T_m$ = $(T_t+T_b)/2$. For $T_t > T_\phi$ the sample was in the single-phase vapor region and $\lambda_{eff}$ exceeded the pure conduction value because the sample underwent turbulent convection. As $T_m$ was decreased so that $T_t$ entered the two-phase region, we found that the heat transport was enhanced further, but that the enhancement did not start until $T_t$ reached a critical value $T_t^c < T_\phi$. At that point a meta-stable boundary layer at the sample top was assumed to have reached a sufficient thickness for nucleation of liquid to occur. For $T_t < T_t^c$ the heat transport increased continuously and linearly with decreasing $T_t$. When $T_t$ decreased sufficiently, $\lambda_{eff}$ reached a maximum where it was an order of magnitude larger than in the single-phase state.