Condensation heat transfer in microgravity conditions.

In the present paper, a thorough review of the experimental and numerical studies dealing with filmwise and dropwise condensation under microgravity is reported, covering mechanisms both inside tubes and on plain or enhanced surfaces. The gravity effect on the condensation heat transfer is examined considering the results of studies conducted both in terrestrial environment and in the absence of gravity. From the literature, it can be inferred that the influence of gravity on the condensation heat transfer inside tubes can be limited by increasing the mass flux of the operating fluid and, at equal mass flux, by decreasing the channel diameter. There are flow conditions at which gravity does exert a negligible effect during in-tube condensation: predictive tools for identifying such conditions and for the evaluation of the condensation heat transfer coefficient are also discussed. With regard to dropwise condensation, if liquid removal depends on gravity, this prevents its application in low gravity space systems. Alternatively, droplets can be removed by the high vapor velocity or by passive techniques based on the use of condensing surfaces with wettability gradients or micrometric/nanometric structuration: these represent an interesting solution for exploiting the benefits of dropwise condensation in terms of heat transfer enhancement and equipment compactness in microgravitational environments. The experimental investigation of the condensation heat transfer for long durations in steady-state zero-gravity conditions, such as inside the International Space Station, may compensate the substantial lack of repeatable experimental data and allow the development of reliable design tools for space applications.

Coupling radiative, conductive and convective heat-transfers in a single Monte Carlo algorithm: A general theoretical framework for linear situations.

It was recently shown that radiation, conduction and convection can be combined within a single Monte Carlo algorithm and that such an algorithm immediately benefits from state-of-the-art computer-graphics advances when dealing with complex geometries. The theoretical foundations that make this coupling possible are fully exposed for the first time, supporting the intuitive pictures of continuous thermal paths that run through the different physics at work. First, the theoretical frameworks of propagators and Green's functions are used to demonstrate that a coupled model involving different physical phenomena can be probabilized. Second, they are extended and made operational using the Feynman-Kac theory and stochastic processes. Finally, the theoretical framework is supported by a new proposal for an approximation of coupled Brownian trajectories compatible with the algorithmic design required by ray-tracing acceleration techniques in highly refined geometry.

About the Role of Falling Droplets' Sweeping in Surface Renewal during Dropwise Condensation.

To determine the heat transfer coefficient during dropwise condensation, two models are necessary: a heat transfer model through a single drop and a model of drop-size distribution. To model the distribution of the drop size, most studies dissociate the drop population into two distinct parts. A semiempirical model is then used to evaluate the drop-size distribution of "large" drops (i.e., typically greater than few micrometers), while the drop-size distribution of "small" drops is modeled using a statistical approach based on population balance. Currently, no accurate data are available to validate this latter distribution. In the present study, the statistical approach is compared to an individual-based numerical approach. This numerical approach takes into account the entire lifecycle of a drop: nucleation, growth, coalescence, and disappearance by sweeping of moving drops (jumping droplets are not considered in this paper). The drop-size distributions of large drops obtained thanks to this model are very similar to those obtained from the classical law in all configurations studied. Nevertheless, the distributions of "small" drops are notably different between the two types of modeling. In the configurations considered in the present study, an analysis of the main hypotheses used in the statistical approach (in particular, the assumption of a constant removal characteristic time τ irrespective of the range of drop size) revealed that the main mechanism for surface renewal is not the sweeping of the surface by moving drops. From these results, a modification of the statistical model is proposed and discussed.

Heat transfer intensification in an actuated heat exchanger submitted to an imposed pressure drop.

This paper is dedicated to the analysis of the improvement of heat transfer and the reduction of the pressure losses induced by the use of an active exchanger of millimeter size in a cooling loop. For pressure conditions imposed at the terminals of such a mini-channel whose upper wall is deformed by a progressive sinusoidal wave and for low Reynolds numbers (Re < 1000), we study the influence of the deformation parameters on the thermo-hydraulic performance of the exchanger (flow, heat transfer). The mechanical power applied to the deformed wall is connected to these parameters as well as to the pressure difference imposed by the external pump. The overall performance increases slightly with the value of the mechanical power up to a critical value for a given wall corrugation. Nevertheless, overall performance is up to 2 orders of magnitude higher than conventional static corrugated channels.